F2, F3, F4, A1, A1 F1

Fair

Carole Goble
email: carole.goble@manchester.ac.uk

Sarah Cohen-Boulakia

Stian Soiland-Reyes

Daniel Garijo

Yolanda Gil

Michael R Crusoe

Kristian Peters

Daniel Schober

Related to other papers in this special issue 29 (p285); 8 (p78) Addressing FAIR principles

Keywords: Computational workflow, Reproducibility, Software, FAIR data, Provenance

Computational workflows describe the complex multi-step methods that are used for data collection, data preparation, analytics, predictive modelling, and simulation that lead to new data products. They can inherently contribute to the FAIR data principles: by processing data according to established metadata; by creating metadata themselves during the processing of data; and by tracking and recording data provenance. These properties aid data quality assessment and contribute to secondary data usage. Moreover, workflows are digital objects in their own right. This paper argues that FAIR principles for workflows need to address their specific nature in terms of their composition of executable software steps, their provenance, and their development.

INTRODUCTION

In data intensive science, e-infrastructures and software tool-chains are heavily used to help scientists manage, analyze, and share increasing volumes of complex data [START_REF] Atkinson | Scientific workflows: Past, present and future[END_REF]. Data processing tasks like data cleansing, normalisation and knowledge extraction need to be automated stepwise in order to foster performance, standardisation and re-usability. Increasingly complex data computations and parameterdriven simulations need reliable e-infrastructures and consistent reporting to enable systematic comparisons of alternative setups [START_REF] Deelman | The future of scientific workflows[END_REF][START_REF] Peters | PhenoMeNal: Processing and analysis of metabolomics data in the cloud[END_REF]. As a response to these needs, the practice of performing computational processes using workflows has taken hold in different domains such as the life sciences [START_REF] Cohen-Boulakia | Workflows for computational reproducibility in the life sciences: Status, challenges and opportunities[END_REF][START_REF] Garijo | Workflow reuse in practice: A study of neuroimaging pipeline users[END_REF][START_REF] Shade | Computing workflows for biologists: A roadmap[END_REF], biodiversity [START_REF] Mathew | A semiautomated workflow for biodiversity data retrieval, cleaning, and quality control[END_REF], astronomy [START_REF] Freudling | Automated data reduction workflows for astronomy: The ESO Reflex environment[END_REF], geosciences [START_REF] Duffy | Designing a road map for geoscience workflows[END_REF], and social sciences [START_REF] Turner | Workflows for quantitative data analysis in the social sciences[END_REF]. Workflows also support the adoption of novel computational approaches, notably machine learning methods [START_REF] Berthold | KNIME: The Konstanz Information Miner[END_REF], due to the ease with which single components in a processing pipeline can be exchanged or updated.

Generally speaking, a workflow is a precise description of a procedure -a multi-step process to coordinate multiple tasks and their data dependencies. In computational workflows each task represents the execution of a computational process, such as: running a code, the invocation of a service, the calling of a command line tool, access to a database, submission of a job to a compute cloud, or the execution of data processing script or workflow. Figure 1 gives an example of a real workflow for variant detection in genomics, represented using the Common Workflow Language  open standard [START_REF] Amstutz | Data Intelligence 119 FAIR Computational Workflows Specification. Common Workflow Language working group[END_REF].

Computational workflows promise support for automation that scale across computational infrastructures and large datasets while shielding users from underlying execution complexities such as inter-resource incompatibilities and software dependencies. From an execution perspective, workflows are a means to handle the work of accessing an ecosystem of software and platforms, managing data, securing access, and handling heterogeneities. From a reuse and reproducibility perspective, the automated methods can be packaged and ported across computational platforms, easing how we can create and execute workflows in different environments and across the diverse expertise levels of users. From a reporting perspective, they are a means to specify and document the experiment design and report the methodology: accurately recording the data inputs, parameter configurations and history of their runs and the provenance of their output data products [START_REF] Cuevas-Vicenttín | Scientific workflows and provenance: Introduction and research opportunities[END_REF]. The provenance of a result (i.e., why and how a given result has been obtained by an analysis) enables the comprehension, comparison and verification of multiple results, and hence facilitates the exchange, standardisation, trust and reusability of those results.

The rise in the use of workflows has been accompanied by a range of diverse systems by which they can be implemented. At one end of the spectrum reside ad-hoc scripts (shell code, Python, Java, etc.) and interactive notebooks which provide an intuitive interface to quickly interact with the analysis results (e.g., Jupyter  , RStudio  , Zeppelin  ). At the other end are Workflow Management Systems (WfMS) that provide  http://commonwl.org/.  https://jupyter.org/.  https://www.rstudio.com/.  https://zeppelin.apache.org/.

FAIR Computational Workflows

Figure 1. A workflow for detecting variants in genome sequences  . The workflow is specified in the Common Workflow Language, displayed using the CWL Viewer. Any CWL compliant WfMS execution engine should be able to execute this standardized workflow description and obtain the same results independent of their underlying infrastructure, for instance using Toil  executed on SLURM, or REANA  on Kubernetes.

 https://w3id.org/cwl/view/git/5a67e91727c8eb44afff27f9e4774eafef579c58/detect_variants/detect_variants.cwl.  http://toil.ucsc-cgl.org/.  https://reana.readthedocs.io/.

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a feature-rich infrastructure for the definition, set-up, execution and monitoring of a workflow. Some WfMS are aimed at general applications (e.g. KNIME  [START_REF] Berthold | KNIME: The Konstanz Information Miner[END_REF]) whilst others have been adopted by specific communities with specialised features and component collections (e.g. Nipype  for neuro-bioimaging [START_REF] Gorgolewski | Nipype: A flexible, lightweight and extensible neuroimaging data processing framework in python[END_REF]). Different aspects of FAIR principles will apply across this range of implementation choices.

WfMS can roughly be divided into two types, namely coarse-grained, with a prime focus on chaining locally hosted or distributed tools (e.g. Galaxy  [START_REF] Afgan | The Galaxy platform for accessible, reproducible and collaborative biomedical analyses: 2018 update[END_REF], KNIME [START_REF] Berthold | KNIME: The Konstanz Information Miner[END_REF], Taverna  11 [START_REF] Wolstencroft | The Taverna workflow suite: Designing and executing workflows of Web Services on the desktop, web or in the cloud[END_REF]) and fine-grained focusing on optimising computational resources over Distributed Computing Infrastructures (DCI) or High Performance Computing (HPC) for applications (e.g. Pegasus  12 [START_REF] Deelman | A Workflow Management System for Science Automation[END_REF], SnakeMake  13 [START_REF] Köster | Snakemake-a scalable bioinformatics workflow engine[END_REF], Nextflow  14 [START_REF] Di Tommaso | Nextflow enables reproducible computational workflows[END_REF], Dispel4Py  15 [START_REF] Filguiera | A Moreno dispel4py: A Python framework for dataintensive scientific computing[END_REF]) and cloud-based container orchestration (Kubernetes  16 ). Many WfMS mix the two kinds [START_REF] Moreno | Galaxy-Kubernetes integration: Scaling bioinformatics workflows in the cloud[END_REF]. All WfMS aim to handle common cross-cutting concerns on behalf of the workflow execution. Concerns include: resource scalability (optimisation, concurrency and parallelisation), secure execution (of tools in their environment, monitoring and fault handling), tracking (process logging and data provenance tracking) and data handling (secure access, movement, reference management). WfMS vary in how their users interact with them, for example by ABIs/APIs, scripting or command lines, or a GUI using "drag, drop and linking". WfMS may execute over HPC or geographically distributed clusters, cloud environments across systems, or even from desktops. They consequently vary in their mechanisms to prepare their components to become executable steps and must manage portability and dependencies on the infrastructure used to run them.

Computational workflows are composed of modular building blocks that have been prepared with standardised interfaces to be linked together and run by a computational engine. Thus, the key characteristic is the separation of the workflow specification from its execution. Capturing the control flow order between components explicitly exposes the dataflow and data dependencies between the inputs and outputs of the processing steps. This explicit separation is fundamental to supporting workflow comprehension, design modularity, workflow comparisons and alternative execution strategies. Scripting environments tend to interleave data and computational processes, although systems such as YesWorkflow [START_REF] Mcphillips | YesWorkflow: A user-oriented, language-independent tool for recovering workflow information from scripts[END_REF] provide the means to annotate existing scripts with special comments that reveal their hidden computational modules and dataflows. Interactive notebooks make the distinction when organized appropriately by defining their dataflow in the form of interactive computational cells; i.e., input and output variables are explicit in each cell, data dependencies are explicit on each cell, and the steps are executed in order. Notebooks can also be used as "meta workflows" when steps run a script or a WfMS. • FAIR data: Properly designed workflows contribute to FAIR data principles, since they provide the metadata and provenance necessary to describe their data products and they describe the involved data in a formalized, completely traceable way.

• FAIR criteria for workflows as digital objects: Workflows are research products in their own right, encapsulating methodological know-how that is to be found and published, accessed and cited, exchanged and combined with others, and reused as well as adapted.

These two aspects are explored in the rest of the article. References to FAIR principles [START_REF] Wilkinson | The FAIR guiding principles for scientific data management and stewardship[END_REF] are given in brackets.

FAIR DATA FOR AND FROM WORKFLOWS

Computational workflows are enablers of automated data processing. For automation to be most effective the data the workflows act on should be FAIR: unique resolution of identifiers, explicit data organisations, structures and semantics, machine-readable licenses and access permissions, high data quality and so on. FAIR data would enable a WfMS to automatically make informed choices from the phase of the workflow design (e.g., by suggesting tools fitting data features when several alternative tools could be considered for a given data analysis step) to the phase of workflow execution (e.g., by validating the data against a step's expected type). A WfMS needs to be able to access precise information on data origin, the way of accessing it, and a set of associated metadata, which is ideally described by established vocabularies and computerinterpretable semantics.

Research domains such as the life sciences have developed open ontologies, vocabularies and services for data interoperability (I1, I2, I3) and identifier resolution (F1, A1). Efforts such as the Breeding API (BrAPI)  17 standardise the interface for exchanging data between applications and the EDAM ontology  18 [START_REF] Ison | EDAM: An ontology of bioinformatics operations, types of data and identifiers, topics and formats[END_REF] precisely specify the input and output of tools executed in a workflow (see FAIRsharing.org [START_REF] Sansone | The FAIRsharing Community FAIRsharing as a community approach to standards, repositories and policies[END_REF] for examples). Formalised and fine grained annotation of data is still considered costly to produce. Consequently, a significant amount of workflow processing still deals with metadata wrangling, format transformations and identifier mapping [START_REF] Garijo | Common motifs in scientific workflows: An empirical analysis[END_REF].

Workflows are in turn key contributors to FAIR data compliance. In a world of expanding and diversifying processing tools and computational operating environments, they encode standardised data practices and capture formal computer-interpretable provenance data. The workflow specification itself can be thought of as a recipe to produce a data product that exposes the extent of the effort made to make the data FAIR and it is formal enough to be validated against emerging FAIR indicators [START_REF] Wilkinson | A design framework and exemplar metrics for FAIRness[END_REF]. Determining whether the data produced by a workflow is FAIR is not straightforward and requires concrete criteria, which should be provided by both the FAIR indicators and the workflow specification.

The combination of FAIR data and FAIR tools within a supportive FAIR e-infrastructure would significantly aid the operation and quality assurance of workflows. Machine processable metadata describe the inputs, outputs and performance of tools and the underlying resources for running workflows and managing results. Examples include annotations on tools and libraries (e.g. Bio.tools  19 , Bioconductor, CRAN, PyPI) and on software containers (e.g. Biocontainers  20 ). Standardised specifications on handling data formats and executables, automated handling of tool dependencies, versioning and other explicit metadata on computational resource-needs would aid harmonisation of successive software tools execution and efficient job scheduling and data movement throughout different FAIR e-infrastructures.

For data generation a standardised workflow specification and automated execution contributes to transparency, reproducibility, analytic validity, quality assurance and the attribution and comparison of results. Well-designed workflow management systems can automate the production of metadata descriptions of data products (F2, I2, I3, R1.3) and the deposition of data in searchable resources (F4). Identifiers, licensing and access present interesting challenges in workflow execution:

• Identifiers (F1, F3, A1): concerns include the propagation of identifiers through the workflow, tracking data attribution [START_REF] Groth | FAIR data reuse -the path through data citation[END_REF] and the minting of identifiers for large numbers of intermediate results. Minids [START_REF] Chard | I'll take that to go: Big data bags and minimal identifiers for exchange of large, complex datasets[END_REF] are proposed as light-weight identifiers to unambiguous name, identify and reference research data products. Identifiers can then optimise data exchange by reference and reduce unnecessary or insecure data movements. Workflows need to move data references through their engines and not the data itself. • Licensing (R1.1): As workflows often access and combine data from different sources, data licenses need to be respected, honoured and propagated, as do licenses on the software used by the workflow tasks. Combining licenses is particularly tricky and can impact on the ability to license the workflow itself or its data products, often neglected or delayed because of this challenge. • Data access (A1.1, A1.2): Single sign-on for workflow constituents requires harmonised Authentication and Authorization Infrastructure (AAI) propagation through the different tasks, which may be hosted by different service providers using different systems.

Workflows intrinsically provide precise documentation of how the data has been generated (R1.2). The details of every executed process together with comprehensive information about the execution environment used to derive a specific data product is retrospective provenance, either observed by the WfMS or disclosed to it by the computational task itself. A great deal of work has focused on provenance tracking of computational workflows [START_REF] Herschel | A survey on provenance: What for? What form? What from?[END_REF], leading to standardisation efforts such as the W3C PROV model and ontology [START_REF] Khan | Sharing interoperable workflow provenance: A review of best practices and their practical application in CWLProv[END_REF] (I1, I2). Challenges remain: provenance standards have yet to be fully embraced by WfMS, there are still shortages of provenance processing tools. Automated provenance collection can be too finegrained and too detailed to be of service to researchers [START_REF] Alper | LabelFlow framework for annotating workflow provenance[END_REF], indicating a need for ontological abstractions. So, the computational steps are themselves unFAIR. Although open source tools allow us to inspect procedures, many codes are proprietary software or opaque boxes (especially those only available as

FAIR Computational Workflows

run-time binaries or representing deep learning algorithms) that do not disclose the link between their inputs and outputs, breaking the provenance lineage of data. Steps in coarse-grained workflows are often wrapped applications with buried sub-workflows and manual (not tracked) steps within. Data resources and tools do not always report basic metadata such as their version or licence in a standardised, machine interpretable way. Bioschemas  21 aims to get such metadata marked-up in resources in a lightweight way. A greater problem is unFAIR service provision, whereby the components change their interfaces without notice, breaking the workflows that use them. Given their data focus, the FAIR principles are chiefly focused on the availability of metadata rather than the quality of service of the databases, tools and the e-infrastructures in which data can exist.

FAIR CRITERIA FOR WORKFLOWS AS DIGITAL OBJECTS

The initial FAIR criteria have been envisioned for data. As workflows are digital objects in their own right, it is natural to draw an analogy with data and to try to apply the FAIR Principles to them. The majority of workflows are not yet registered in specialised repositories or are stored in software repositories indistinguishable from other software. Conventions for naming workflows still have to be devised (F1). Workflows vary in the quality of their documentation, as happens with software, described using proprietary or native programming languages.

Researchers have been actively exploring ways for workflows to be FAIR. Workflow registries and repositories typically cater for specific WfMSs, such as KNIMEHub  22 for KNIME and nf-core  23 for Nextflow Life Science pipelines, to support findability and accessibility (F4), with description and metadata associated with deposited workflows (F2) and in some cases persistent, unique identifiers (F1). Access is typically baked into the workflow systems (A1), for example only Galaxy workflows are available in dedicated Galaxy installations such as Workflow4Metabolomics  24 . Others such as WINGS  25 provide means to export workflows augmented with semantic representations as Linked Data [START_REF] Garijo | Abstract, link, publish, exploit: An end to end framework, for workflow sharing[END_REF]. Workflow findability in repositories has been studied [START_REF] Starlinger | Re)Use in Public Scientific Workflow Repositories[END_REF] alongside workflow similarity [START_REF] Starlinger | Similarity search for scientific workflows[END_REF] where workflows are compared based on their metadata and structure. For workflows to be accessible in the same way as data, they need to be archived and cited just as data is archived and cited using citation metadata [START_REF] Smith | FORCE11 Software Citation Working Group. Software citation principles[END_REF]. In the schema.org mark-up used by citation infrastructures such as Datacite, terms indicate data that is derived from other data. Ideally there should also be terms indicating the software or service used to perform a transformation, harmonised with workflow provenance. myExperiment  26 [START_REF] De Roure | The design and realisation of the myExperiment Virtual Research Environment for social sharing of workflows[END_REF] is an attempt at a WfMS agnostic repository, pioneering approaches for workflow finding, sharing and publishing with licenses. It credits authors when workflow designs were reused or [START_REF] Belhajjame | Using a suite of ontologies for preserving workflow-centric research objects[END_REF] that allows for bundling of all the artefacts associated with an investigation or piece of research into one whole that can also be cited. Figure 1 workflow's description files and links to executable containers and data files can be downloaded in a Research Object zip-based bundle along with citation metadata and assigned a DOI on deposit. The European Open Science Cloud for Life Sciences  28 has started work to build a workflow registry (F4) using the CWL standards with Research Objects federated (I3) with registries for tools (bio. tools) and containers (Biocontainers).

Several attempts have been made to standardise workflow descriptions in order to aid discoverability (F1) and enable interoperability (I1). The Interoperable Workflow Intermediate Representation [START_REF] Plankensteiner | IWIR: A language enabling portability across grid workflow systems[END_REF] was proposed as a common bridge for translating fine-grain workflows between different languages, independent of the underlying distributed computing infrastructure [START_REF] Terstyanszky | Enabling scientific workflow sharing through coarse-grained interoperability[END_REF]. The Workflow Description Language  29 and the Common Workflow Language [START_REF] Amstutz | Data Intelligence 119 FAIR Computational Workflows Specification. Common Workflow Language working group[END_REF] are recent community efforts to describe workflows. The CWL open standards are used to describe workflows and command-line tool interfaces in a way that makes them portable and scalable across a variety of software and hardware environments and runnable by other CWLcompliant engines. This last point is critical. As descriptions of processes workflows inherit properties of FAIR data, but as executable processes they inherit properties of software. Workflows as processes challenge the FAIR principles by their structure, forms, versioning, executability, and reuse.

Structure. Workflows are often inherently composite. Their components can be workflows in a nested, fractal way, i.e., (interdependent or sets of) sub-workflows that can be executed as part of complex workflows (see Fig. 1). The distinction between a workflow and its component steps is blurred [START_REF] Haendel | FAIR-TLC: Metrics to assess value of biomedical digital repositories: Response to RFI[END_REF]. FAIR principles can thus be applied simultaneously on multiple levels. To render composite workflows and sub-workflows findable relies also on the findability of the involved tools and data types as researchers often use these as search attributes. FAIR properties on the components -metadata, licensing, author credit, access authorization and so on -propagate to the workflow level and may be incompatible. Fundamentally, how we identify, cite and credit composite, multi-authored objects is an open question [START_REF] Katz | Transitive credit as a means to address social and technological concerns stemming from citation and attribution of digital products[END_REF].

Forms. When we speak of a FAIR workflow what do we mean? A workflow can be a CWL specification with test or exemplar data; an implementation of that design in a WfMS; an instantiation of that implementation ready to be run with input data and parameters set and computational services spun up; a run result with intermediate and final data products and provenance logs [START_REF] Garijo | Abstract, link, publish, exploit: An end to end framework, for workflow sharing[END_REF]. Workflow-centric Research Objects attempt to create a metadata framework to capture and aggregate each form, but each may have different FAIR criteria.

Versioning. Like software, workflows are living artefacts to be maintained, updated, and eventually deprecated. The code components, the WfMS itself and the underlying computational infrastructures they run on evolve and change. The evolution of a workflow is a form of provenance (R1.2) that tracks any alteration of an existing workflow, resulting in another version that may produce the same or different results [START_REF] Casati | Workflow evolution[END_REF]. Moreover, to make methodological variants, workflows can be recycled and repurposed: cloned, forked, merged and dramatically changed. Workflow repositories such as nf-core 23 embrace this software nature, building on top of collaborative development environments such as github that natively support versioning as well as testing and validation. Thus, FAIR principles have to address versioning and "fixivity"; that is, the need to snapshot a workflow and its dependencies to fix its reproducible state and associate a persistent identifier.

Executability. Workflows are executable objects. To be interoperable and reusable they need to be portable, encapsulating all their runtime dependencies. Lightweight container-based virtualisation solutions to distribute software (e.g. Docker  30 , Singularity  31 ) and platform independent software packaging and distribution (e.g. Conda  32 ) revolutionise workflow reusability. Nevertheless, workflows and the software tools they use are time limited objects whose active lifespan is dependent on that of their components and WfMS as much as on their scientific relevance. Consequently CWL addresses both explicit support for containerised execution and the lifting of workflow descriptions from the WfMS or application it is embedded in so that it may be runnable in other CWL-compliant engines, even when no longer executable in its native form.

In workflow e-infrastructures (e.g. in local workstations or cloud environments), resource limitations need to be defined by the workflow. The stability of the execution environment needs to be covered by the infrastructure and its components. Security and access control issues are crucial as workflow systems often execute codes in shared distributed resources.

Reuse. Reusing workflows turns out to mean different things depending on the purpose of the reuse. Workflow redo/re-run re-executes the exact same workflow, data, parameter settings and tools with the aim to re-create identical results for testing the robustness of the process. Workflow replication allows minor changes, usually in the workflow environment and/or parameter settings, but the results are expected to be the same. Workflow reproduction aims to retain the same analysis but with variations to the means (steps) or data to test the robustness of the results. Workflow reuse may use all or part of the original workflow with new data, with a possible different aim in workflow repurposing/recycling [START_REF] Garijo | Abstract, link, publish, exploit: An end to end framework, for workflow sharing[END_REF][START_REF] Wroe | Recycling workflows and services through discovery and reuse[END_REF]. Regardless of intent, the workflow user must be confident that the expected results are generated. R1 is fostered by robust software practices [START_REF] Artaza | Top 10 metrics for life science software good practices [version 1; peer review: 2 approved[END_REF][START_REF] Taschuk | Ten simple rules for making research software more robust[END_REF][START_REF] Da Veiga Leprevost | On best practices in the development of bioinformatics[END_REF] that entail:

• Proper testing of the computational workflow and its modules as well as the software tools that are invoked during workflow runtime;

FAIR Computational Workflows

• Validation of parameters to preclude workflow failure and faulty or unsafe results. The formulation of parameters must therefore be FAIR and must include documentation and explanation of their purpose and range definitions (testing of parameter ranges). The BioCompute Object specification [START_REF] Alterovitz | Enabling precision medicine via standard communication of HTS provenance, analysis, and results[END_REF] emphasises detailed representation and validation of parameters for regulatory approval of reusable computational pipelines for precision medicine.

These thoughts lead to two conclusions. First that treating FAIR workflows as data artefacts only goes so far. Their characteristics as software artefacts means that they should also be subject to appropriate FAIR principles for software, incorporating best practices for software maintainability, maturity and computation reproducibility [START_REF] Stodden | Enhancing reproducibility for computational methods[END_REF]. Second, that the individual parts, forms, versions and execution environments of a workflow need to be FAIR by themselves, leading to complex interdependencies which need to be covered by additional FAIR metrics aligned to their nature. Generally a compromise has to be found between the amount of work users have to invest in annotating their workflow and the amount of metadata needed for verification by FAIR indicators. Automatic annotation strategies and annotation tool support will be crucial to ease the burden of workflow developers.

CONCLUSIONS

Computational workflows capture complex multi-step methods that require FAIR practices in order to be properly published, findable, accessed, cited, reused, and combined with others. FAIR principles for data, and for software, are generally applicable, but need to be extended in order to address the processual nature of workflows. Consequently new FAIR indicators will also need to be developed. A framework for FAIR workflows will enhance reproducibility, quality and transparency of the data generated, but also of the processing path that lead to the data results. When used to document the provenance of new data products, workflows become a powerful component for FAIR data practices and provide new capabilities such as better findability, ideally based on the intrinsic methods used to generate data. The FAIRification of workflows along all these lines will pave the way for trustable data with the added value of being ready for secondary data reuse and exploitation by third parties.

AUTHOR CONTRIBUTIONS

C. Goble (carole.goble@manchester.ac.uk, corresponding author) and D. Schober (Daniel.Schober@ipbhalle.de) initiated the effort and conceived the paper. C. Goble co-ordinated and led the writing, and edited the manuscript. C. Goble, S. Cohen-Boulakia (cohen@lri.fr), S. Soiland-Reyes (soiland-reyes@manchester. ac.uk), D. Garijo (dgarijo@isi.edu), Y. Gil (gil@isi.edu), M.R. Crusoe (michael.crusoe@gmail.com), K. Peters (Kristian.Peters@ipb-halle.de), D. Schober all contributed to the concepts, arguments, and written text. All reviewed the text. D. Schober and M.R. Crusoe contributed examples.

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  11 https://taverna.incubator.apache.org/.  12 https://pegasus.isi.edu/.

 https://www.knime.com/.  https://nipype.readthedocs.io/.  https://galaxyproject.org/.   13 https://snakemake.readthedocs.io/en/stable/.  14 https://www.nextflow.io/.  15 https://pypi.org/project/dispel4py/.  16 https://kubernetes.io/.

   21 http://bioschemas.org/.  22 https://hub.knime.com/. Downloaded from http://direct.mit.edu/dint/article-pdf/2/1-2/108/1893377/dint_a_00033.pdf by École polytechnique user on 18 January 2024 , and packages workflows into collections and with other digital objects such as associated data files and publications. The work laid the foundations for workflow based Research Objects  27

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 23 https://nf-co.re/.  24 https://workflow4metabolomics.org/.  25 http://www.wings-workflows.org/.  26 http://myexperiment.org/. repurposed

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  Validation of interoperability claims that tests workflow replication on different platforms;  30 https://www.docker.com/.  31 https://sylabs.io/singularity/.  32 https://conda.io/. from http://direct.mit.edu/dint/article-pdf/2/1-2/108/1893377/dint_a_00033.pdf by École polytechnique user on 18 January 2024

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 17 https://brapi.docs.apiary.io/.  18 http://edamontology.org/. Downloaded from http://direct.mit.edu/dint/article-pdf/2/1-2/108/1893377/dint_a_00033.pdf by École polytechnique user on 18 January 2024

 19 http://bio.tools/.  20 https://biocontainers.pro/. Downloaded from http://direct.mit.edu/dint/article-pdf/2/1-2/108/1893377/dint_a_00033.pdf by École polytechnique user on 18 January 2024

 27 http://researchobject.org/.  28 http://www.eosc-life.eu/.  29 https://software.broadinstitute.org/wdl/. Downloaded from http://direct.mit.edu/dint/article-pdf/2/1-2/108/1893377/dint_a_00033.pdf by École polytechnique user on 18 January 2024

ACKNOWLEDGEMENTS

Carole Goble acknowledges funding by BioExcel2 (H2020 823830), IBISBA1.0 (H2020 730976) and EOSCLife (H2020 824087). Daniel Schober's work was financed by Phenomenal (H2020 654241) at the initiation-phase of this effort, current work in kind contribution. Kristian Peters is funded by the German Network for Bioinformatics Infrastructure (de.NBI) and acknowledges BMBF funding under grant number 031L0107. Stian Soiland-Reyes is funded by BioExcel2 (H2020 823830). Daniel Garijo, Yolanda Gil, gratefully acknowledge support from DARPA award W911NF-18-1-0027, NIH award 1R01AG059874-01, and NSF award ICER-1740683.

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FAIR Computational Workflows

We propose that FAIR Principles apply to workflows, and WfMSs, in two major areas:

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FAIR Computational Workflows

Data Intelligence

FAIR Computational Workflows


Spiro-Ometad Xavier Sallenave

Mona Shasti

Elham Halvani Anaraki

Dmytro Volyniuk

Vidas Juozas

Grazulevicius

Shaik M Zakeeruddin

Abdollah Mortezaali

Michael Grätzel

Anders Hagfeldt

Gjergji Sini

Interfacial and Bulk Properties of Hole Transporting Materials in Perovskite Solar Cells: Spiro-MeTAD versus

Keywords: dft, energy disorder, interfaces, solar cells, perovskites, interaction energy

Two spiro-MeTAD compounds (1 and 2) were synthesized, characterized by experimental and quantum mechanical methods, and used as hole transporting materials (HTMs) in perovskite solar cells (PSC). The new compounds differ from spiro-OMeTAD only by the presence of methyl substituents as compared to methoxy groups. This modification results in absorption-band blue shift by ~20 nm as compared to spiro-OMeTAD, increased glass transition temperature for 2, and reduced ionization potentials by 0.02-0.12 eV. Compared to the time-of-flight hole mobility of 9.10 - 4 cm 2 /Vs measured for spiro-OMeTAD in vacuum-deposited layers, hole mobilities larger by five times were obtained for spiro-MeTAD compounds, ranging between 2-5.10 -3 cm 2 /Vs. Despite this improvement, J-V measurements in PSCs resulted in power conversion efficiency (PCE) of 17.2% and 17.05% for 1 and 2 HTMs, respectively, as compared to 19.24 % for spiro-OMeTAD. These results point to the dominant importance of the perovskite/HTM interfacial properties as compared to HTM hole-transport properties in the bulk. The impact of additives on the hole mobility of HTMs and on the PCEs is also discussed. Given that improved hole-mobility and energy-level alignment are the main targets of the current research efforts in this domain, our results alert on the necessity to give priority to improving perovskite-HTM interaction properties.

INTRODUCTION

Efficient and low cost solar-energy harvesting by means of photovoltaic devices constitutes one of the main objectives for present research. Among different solar cell concepts, dye sensitized solar cells (DSSC) and perovskite based solar cells (PSC) have shown increasing power conversion efficiencies (PCE) recently reaching 14.3% 1 and 25.2% 2 , respectively. In both types of solar cells, the prevalently used hole transporting material (HTM) is 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). While several phenomena jointly determine the solar cell performances, here we focus on the impact of the HTM properties in PSC.

The success of spiro-OMeTAD is supposed to reside at least on the good energy level matching between HOMO of this HTM with that of several dyes and of the perovskite absorber. However, this semiconductor suffers from low hole mobility of roughly 10 -6 -10 -5 cm 2 /Vs in the amorphous phase, 3 reaching only ~10 -3 cm 2 /Vs in single crystals, 4 which constitutes an important drawback for its use in high efficiency photovoltaic devices. This issue has motivated intensive research efforts in order to replace spiro-OMeTAD by more efficient HTMs, which dominance has indeed been already overcome. [5][6][7] However, a general mechanism that could explain this success has not been identified so far, due to multiple differences between the new molecules and spiro-OMeTAD.

Consequently, a seemingly random optimization process is still required with every new HTM, rendering their further development unpredictable, cost intensive, and inefficient.

As a means to render more efficient the research for new HTMs, a detailed analysis of the pros and cons of spiro-OMeTAD properties is needed. While detailed analyses on the hole mobility of amorphous spiro-OMeTAD are missing, interesting insights come from two recent studies: (i) a theoretical upper-limit hole mobility of roughly 4.10 -3 cm 2 /Vs was found along one crystallographic direction in spiro-OMeTAD single crystals, 4 which is due to large electronic couplings along this direction, whereas vanishingly small electronic couplings as low as only a few meV were found along the other directions. Geometrical deformations, strong dipole moments, and steric hindrance were found to constitute the bottlenecks preventing spiro-OMeTAD from high hole mobility.

Accordingly, very small electronic couplings and hole mobilities can be expected with regard to the hole mobility of spiro-OMeTAD in the amorphous phase. (ii) Important energy disorder in hole transport levels was found in amorphous spiro-OMeTAD, which, in the frame of a Gaussian distribution, was characterized by a standard deviation (-parameter) of 0.16 eV. 8 The origin of the important  value of spiro-OMeTAD can in part stem from the distribution of dipole moments ranging between 0-5.3 Debye 4 generated by the presence and random orientations of eight methoxy groups. Indeed, methoxy substituents in triphenylamine compounds were found to reduce the hole mobility as compared to the non-substituted one, due to the enhancement of the dipole moment and their disorder. [9][10][11][12][13] On the other hand, several methoxy-and methyl substituted triphenylamine-based compounds were found to transport holes faster than non-substituted ones in spite of important dipole moments, 14,15 with better effect in the case of methyl substituents. 16 Detailed analysis of these contradictory results pointed to the competition between two factors playing important roles for the hole mobility: 16 (i) the permanent molecular dipole moments impacting negatively the hole transport, and (ii) the strength of intermolecular interaction. The latter factor was found to strongly reduce the intra-and intermolecular geometrical randomness and their contribution on the energy disorder. 16 Interestingly, it was found that the methyl substituents are able to enhance the intermolecular interactions, hence to reduce the geometrical disorder, without modifying the permanent dipole moments of the parent molecules. On the contrary, while the methoxy groups enhance the intermolecular interactions more importantly as compared to methyl groups, they increase the dipole moments of the parent molecules, thus destroying part of the beneficial effect stemming from the stronger intermolecular interactions.

Motivated by these results, and aiming to obtain new HTMs exhibiting improved hole transport properties, but very similar structural, electrochemical, and optical properties as compared to spiro-OMeTAD, here we present two new molecules called spiro-MeTAD-1 and spiro-MeTAD-2, reported hereafter as 1 and 2, differing from spiro-OMeTAD by only replacing the methoxy groups with methyl groups linked in different positions and different numbers. The basic idea was to reduce the energy disorder (value of -parameter) by removing the detrimental effect of the dipole moments induced by the methoxy groups. We show that the new compounds indeed exhibit smaller , higher hole mobility by at least five times as compared to spiro-OMeTAD, while simultaneously preserving similar electronic and structural characteristics as the original molecule. Interestingly, the use of these new compounds in the device structure of FTO/compactTiO 2 /meso-porousTiO 2 /perovskite/HTM/Au results in PCE of 17.2% and 17.05% for 1 and 2-based devices, respectively, as compared to 19.2% for spiro-OMeTAD. In view of the better transport properties found for the new HTM compounds as compared to spiro-OMeTAD, the slightly smaller PCEs of the new HTMs point to the dominant importance of the perovskite/HTM interfacial properties.

In spite of the slightly smaller PCE of the new HTMs as compared to spiro-OMeTAD, their use in PS solar cells seems very promising. Last but not least, the new compounds are less costly than spiro-OMeTAD with respect to laboratory procedure, which may result in decreased cost efficiency of their use in PV devices.

The paper is organized as follows: we firstly focus on the intrinsic and bulk properties of the new compounds, after which we compare their performances in perovskite solar cells with respect to spiro-OMeTAD.

EXPERIMENTAL SECTION

2.1. Materials. Synthesis. 1-iodo-3,5-dimethylbenzene (98%), Copper iodide CuI (>99%), potassium phosphate K 3 PO 4 (97%), lithium amide LiNH 2 (95%) were purchased from Alfa Aesar.

Sodium tert-butoxide (97%), palladium acetate Pd(OAc) 2 (98%), tri-tert-butylphosphine P(tBu) 3 (98%) were purchased from Aldrich. 2,2',7,7'-tetrabromo-9,9'-spirobifluorene (>98%) and p,p'ditolylamine (>97%) were purchased from TCI Chemicals. DMF and toluene were dried by distillation from calcium hydride CaH 2 and sodium metal, respectively. Column chromatography and TLC were performed on silica gel 60 (230-400 mesh, Merck) and silica gel 60 F254 plates (Merck), respectively.

Devices. PbI 2 and PbBr 2 were purchased from TCI Chemicals. Cesium iodide CsI was purchased from abcr GmbH. FAI was purchased from Dyesol. Anhydrous DMF and DMSO were purchased from Acros Chemicals.

2.2.

Characterizations. 1 H and 13 C NMR spectra were recorded on a Bruker DPX-400 NMR spectrometer in 5 mm tubes with CDCl 3 as solvent in all cases. The 1 H and 13 C chemical shifts were referenced to solvent peaks, 7.26 and 77.2 ppm, respectively. High-resolution mass spectroscopy was performed by the CNRS IMAGIF in Gif-sur-Yvette. Thermogravimetric analysis (TGA) was carried out on a TA Instrument Q50 TGA under argon flow at a heating rate of 20°C/min. The temperature of thermal degradation (T d 5% ) was measured at the point of 5% weight loss. Differential scanning calorimetry analysis (DSC) was performed on a TA Instruments Q100 calorimeter, calibrated with indium and flushed with argon. Samples were scanned from 0 °C to 200 °C at a heating rate of 20°C/min then rapidly cooled to 0 °C (quenching) and heated at the same rate to 200 °C. UV-Vis measurements were performed at room temperature in spectrometric-grade THF solutions and on glass subtrates using a Jasco (V-570) spectrometer. Cyclic voltamperometry (CV) was carried out using an Autolab (AU128N, FRA2) electrochemical analyser with Pt disc used as working and counter electrodes and Ag wire as pseudo-reference. The three electrodes were immersed in a CH 2 Cl 2 solution with 0.1 M tetrabutylammonium hexafluorophosphate (n-Bu 4 PF 6 )

and 10 -3 M 1 or 2 derivatives. Ferrocenium/ferrocene (Fc + /Fc) redox potential was measured in order to calibrate the pseudo-reference electrode.

Ionization potentials and hole mobility measurements.

To obtain ionization potentials (IP PES ) of the compounds in solid-state, photoelectron emission spectra were taken under similar setup of electron photoemission spectrometry in air (PESA) as it was described elsewhere. 17 The vacuum-deposited films onto fluorine doped tin oxide glass substrates were fabricated for PESA measurements. The samples under electric field of 10 ) and CuI (171 mg, 0.9 mmol) under argon. The reaction mixture was heated at 130 °C for 24 h. After cooling, the solution was filtered under celite and the solid washed with CH 2 Cl 2 (100 mL). The organic phase was washed with water (4×25 mL) and dried with MgSO 4 .

After evaporation of solvent, the residue was purified by column chromatography (SiO 2 ) with cyclohexane/ CH 2 Cl 2 (1/1) as eluent to obtain white solid (740 mg, 75%). 1 H NMR (CDCl 3 )  6.71 (s, 4H), 6.60 (s, 2H), 2.29 (s, 12H). 13 C NMR (CDCl 3 )  General procedure for the synthesis of 1 and 2. To a solution of 2,2',7,7'-tetrabromo-9,9'spirobifluorene (0.5 mmol) in dry toluene (15 mL) were added diphenylamine 3 or 4 (2.5 mmol), sodium tert-butoxide (288 mg, 3 mmol) and Pd(OAc) 2 (6 mg, 0.025 mmol). The solution was degassed with argon for 30 min. Then P(tBu) 3 (10 mg, 0,05 mmol) was added and the reaction mixture was refluxed (115 °C) for 12 h. After cooling, the solution was poured into 30 mL of water.

The aqueous layer was extracted with ethyl acetate (2×30 mL).The organic phase was washed with brine (3×20 mL) and water (2×20 mL) and dried with MgSO 4 . After evaporation of solvent, the residue was purified by column chromatography (SiO 2 ) with cyclohexane/ CH 2 Cl 2 (7/3) as eluent.

1: white solid, 85% yield. compact layer was deposited on FTO via spray pyrolysis using O 2 as carrier gas at 450 °C from a precursor solution of titanium diisopropoxide in anhydrous ethanol with the molarity of 0.2 M.

Then, a mesoporous TiO 2 layer was deposited by spin coating for 15 s at 5000 rpm with a ramp of 2000 rpm/s, using a 30 nm particle paste (Dyesol) diluted in ethanol with weight ratio of 1:6. After the spin coating, the substrates were sintered at 450 °C for 30 min under dry air flow.

The perovskite composition used for this study is Cs 0.1 FA 0.9 Pb(I 0.9 Br 0.1 ) 3. The perovskite precursor solution was prepared by dissolving of FAI, PbI 2 , CsI and PbBr 2 powder in anhydrous DMF:DMSO 4:1 (v:v) and the molarity of solution is fixed 1.35 M. This solution was kept at 70 °C for 30 min.

The perovskite solution was spin coated in a two steps program at 2000 and 6000 rpm for 10 and 20 s, respectively. During the second step, 200 μL of chlorobenzene was poured on the spinning substrate 10 s prior to the end of the program. The substrates were then annealed (usually at 100 °C) for 1 h in a dry box. The substrates were cooled down for few minutes and a spiro-OMeTAD solution (74 mM in chlorobenzene) was spin coated at 4000 rpm for 20 s. Spiro-OMeTAD was doped with Li-TFSI salt and TBP. The molar ratio of additives for spiro-OMeTAD was: 0.5, and 3.3 for Li-TFSI, and TBP, respectively. For spiro-MeTAD labelled as 1 and 2, the molarity of solution was chosen as 37 mM with the same molar ratio of additive. We also investigate the effect of molar ratio of the additives. For spiro-MeTAD labelled as 1-2 and 2-2, the molar ratio of additives was selected as: 1 and 6.6 for Li-TFSI, and TBP with the same solution molarity of 37 mM, respectively. Finally, 70-80 nm of gold top electrode was thermally evaporated under high vacuum.

A spectrophotometer (Varian Cary5) was used to measure UV-Vis absorption spectra of the samples. Current-voltage characteristics are measured by a digital source meter (Keithley 2400)

with voltage scan rate of 10 mV/s using a 450 W xenon light source under AM 1.5G. IPCE spectra were measured under constant white light bias with an intensity of 10 mW/cm 2 through a Gemini-180 double monochromator.

2.6. Computational Details. Density functional methods (DFT) 20 by employing the B3LYP 21,22 and B97X-D 23 functionals in conjunction with the 6-31G* basis set were used in this study.

Symmetry unconstrained geometry optimizations were carried out on the isolated molecules and on four type of dimers constructed for each of compounds spiro-OMeTAD and 1 (Annex I, SI).

Frequency calculations for the isolated molecules and for the dimers indicated in all cases presence of only positive frequencies, thus assuring that the obtained geometries correspond to minimum points on the potential surfaces of individual molecules or dimers. The intermolecular interaction energy calculations were performed with the B97X-D functional and the same basis set. The interaction energies calculated on the bases of the total energies of the isolated molecules and of th e dimers were subsequently corrected for the zero point energy (ZPE), and for the basis set superposition error (BSSE) following the counterpoise method. The B3LYP functional was used for the rest of the calculations. The optical properties of the monomers and dimers were calculated by means of the time dependent density functional theory method (TDDFT). [24][25][26][27][28] The vertical ionization potentials (IP) were calculated as energy difference between neutral and cation radical species at the neutral state geometry. The intramolecular reorganization energies ( i ) were calculated in the frame of the potential surface method. 29 The electronic couplings between the molecular orbitals were calculated according to the direct interaction method. 30 All calculations were performed with Gaussian 09 program. 31 A more detailed description on the choice of the theoretical method and the theoretical model compounds and dimers is given in Annex I in the Supporting Information (SI).

RESULTS AND DISCUSSION

Synthesis and thermal characteristics

The synthetic route for the studied compounds is outlined in scheme In particular, compound 2 shows a significant increase in T g (143 °C) compared to spiro-OMeTAD (110 °C), which may lead to better thermal stability of the device over time.

Characteristics of pristine compounds

Geometries and frontier orbitals of isolated compounds

The geometries of compounds 1 and 2 obtained in the absence of medium effects ("gas phase") are

shown in Figure 1a. The geometry of spiro-OMeTAD is also shown for comparison. Except for the substitutions on the diphenylamine moieties, quite similar global geometries were obtained.

(b)

The HOMOs and LUMOs of spiro-OMeTAD and compounds 1 and 2 also show quite similar shapes, with the dominant contribution coming from the spirofluorene moieties (Figure 2). Both HOMO and LUMO levels of spiro-OMeTAD are higher than those of compounds 1 and 2 (Table 2), which is due to the -donor effect of the methoxy groups in the former case. Nevertheless, this effect is more pronounced in the case of HOMOs due to their important delocalization over the peripheral di-phenyl moieties, thus resulting in smaller HOMO-LUMO gap in the case of spiro-OMeTAD as compared to compounds 1 and 2. It is worth noting that the delocalization of the HOMOs over both fluorene moieties (Figure 2) is expected to be strongly affected by the geometrical deformations occurring in the real materials, 4 in which case HOMO localization on only one of the arms is to be expected.

Impact of substituents on the strength of intermolecular interactions

The replacement of methoxy groups of spiro-OMeTAD by one or more methyl groups is expected to influence the strength of intermolecular interactions in the bulk of these materials. Indeed, changes in the intra-and intermolecular geometrical randomness, directly impacting the energy disorder and hole mobility, have already been reported. 16 Additionally and importantly, the methyl vs methoxy substituents are expected to impact on the perovskite-HTM interfacial interactions.

In order to obtain some insight on these aspects, four model dimers were optimized for spiro-OMeTAD and compound 1 (Figure 2). The average dissociation energy in the case of 1 is larger than for spiro-OMeTAD (23 kcal/mol and 12 kcal/mol respectively) indicating that the intermolecular interactions are strengthened upon methyl substitutions. In the following we show that the stronger intermolecular interactions for compounds 1 and 2 as compared to spiro-OMeTAD impact several properties, such as polarization energies in the bulk of these materials, hole transport properties, and characteristics of layer-deposition from solutions. obtained at the B97XD/6-31G* level ("gas phase"). For clearer representation, one molecule of each dimer is highlighted in yellow. Note that in each dimer type, the intermolecular interactions in the case of spiro-OMeTAD and of compound 1 involve strictly equivalent molecular moieties, which allows for a pertinent comparison between the corresponding dissociation energies.

Physical and optical properties

Cyclic voltammetry (CV) was employed to investigate the redox properties and to estimate the ionization potentials (IP SS ) of compounds. The ionization potentials were additionally determined by photoelectron spectroscopy in air (IP PES ). The results of these measurements on compounds 1 and 2 and spiro-OMeTAD are shown in Figure 3 and in Table 2 3. All compounds exhibit similar optical behaviour in dilute solution, with a strong absorption band located in the UV spectral domain (380-400 nm). It should be noted that the absorption peaks of the two compounds 1 and 2, both in solution and in the solid state, are narrower than those of spiro-OMeTAD allowing a better transparency of the HTM layer.

The optical band gaps (E g opt ) of 1-2 and spiro-OMeTAD were estimated from the edge of the first absorption peak in solution (Table 3). The smaller bandgap of spiro-OMeTAD as compared to compounds 1 and 2 seems to correlate with the similar trend of the corresponding HOMO-LUMO gaps (see Figure 1).

Photoluminescence spectra of HTMs in chlorobenzene are shown in Figure 4c. The trend of the emission peaks at 420 nm for spiro-OMeTAD [START_REF] Arora | [END_REF]34 and at 407 nm for compounds 1 and 2 is in line with the similar trend of the absorption onset wavelengths and with the one between the corresponding HOMO-LUMO gaps. It is worth noting that the hole mobility found for the vacuum-evaporated "home-made" spiro-OMeTAD is at least 1-2 orders of magnitude larger than values found in literature for spin coated layers (4•10 -5 cm 2 /Vs and 1.7•10 -6 cm 2 /Vs from SCLC 3 and FET 35 measurements, respectively) and only one order of magnitude smaller than the FET value of 1.3•10 -3 cm 2 /Vs found for single crystals of spiro-OMeTAD. 35 While these comparisons suggest enhanced degree of ordering in our layers obtained by vacuum evaporation, the pertinent information remains that the transport properties of the methyl-containing compounds 1 and 2 are improved as compared to spiro-OMeTAD. In order to obtain some insight on the reasons for the observed improvements, temperature dependent TOF hole-mobility measurements were performed, and different intra-and intermolecular properties for compounds 1, 2 and spiro-OMeTAD were calculated. Additionally, four model dimers were optimized for each compound (Figure 2) and the HOMO-HOMO electronic couplings between adjacent molecules were calculated.

The energetic disorder parameter (σTable 4) was deduced within the Gaussian disorder model, according to the following equation: 36

] ) ( exp[ ] ) 3 2 ( exp[ 2 1 2 2 2 0 E C         (1)
Here, σ is the standard deviation of the hopping site manifold, Σ is the positional disorder, C is a constant, and ˆ= σ/kT. S2,S3, and S4 (SI) and only selected numerical results are given in Table 4. The important result is that the energetic disorder parameter σ decreases importantly from 0.106 eV to 0.084 eV in the order spiro-OMeTAD > 2 = 1 (Table 4), which is consistent with the increasing hole-mobilities in the same order.

Details on different methods are given in the Figures

Interestingly, the largest   value, which characterizes the intramolecular charge transport properties, was obtained for spiro-OMeTAD (0.36 cm 2 /Vs, Table 4). Given the similar electronic couplings shown in Table 4, the larger   of spiro-OMeTAD simply translates to lower intramolecular reorganization energies  h as compared to 1 and 2 compounds.

However, the highest σ and the lowest mobility  values found for spiro-OMeTAD suggest that the differences in the hole mobilities of these compounds are dominated by disorder phenomena. Indeed, the almost zero dipole moments of the methylated compounds (Table 4) indicate absence of the detrimental effect from the dipole-moment disorder. This effect is in strike contrast to the large dipole moments as high as 5.3 Debye calculated for spiro-OMeTAD, which has been found to negatively impact the charge-transport properties of organic semiconductors. [9][10][11][12] Additionally, stronger intermolecular interactions were calculated for the 1 compound as compared to spiro-OMeTAD (Table 4), which effect has been shown to reduce the geometrical contribution to the disorder parameter of 1 and 2 compounds as compared to methoxy-substituted ones. 16 The global hole mobility improvement for methyl-substituted compounds wrt spiro-OMeTAD results consequently from a delicate balance between two opposite factors, the positive effect being dominant: (i) detrimental increase in intramolecular reorganization energy ( h ), and (ii) positive impact from the decrease in energy disorder parameter (), in turn stemming from the decrease in dipole moment.

CELIV hole mobilities. The electric field dependences of CELIV hole mobilities in spin-coated layers of pure spiro-OMeTAD and spiro-MeTAD compounds are shown in Figure 5. Similar global trends wrt TOF results can be observed, with hole mobilities increasing in the order spiro-OMeTAD < 1 < 2.

In view of the large utilization of additives in perovskite solar cells, we performed hole mobility measurements of spiro-OMeTAD, 1, and 2 compounds in spin-cast layers with-and without additives. The results shown in Figure 5 indicate hole-mobility improvement for spiro-OMeTAD and 1 compounds, but missing impact in the hole mobility of 2. While the results for spiro-OMeTAD and 1 compounds correspond to the common-knowledge based expectations, the missing sensitivity of 2 hole-mobility wrt the presence of additives is intriguing. We suspect these differences to correlate with the increasing intermolecular interaction energies, probably making the HTM-additives mixture less homogeneous in the case of strong HTM-HTM interactions.

Device characteristics

A schematic energy level diagram of the solar cell device is illustrated in As shown in figure S7, the Scanning electron microscopy (SEM) images in cross-section view reveal that roughly 126-148 nm thick HTL has covered the perovskite film in devices based on new compounds while the thickness of spiro-OMeTAD is roughly 190-196 nm in control devices.

According to owing larger hole mobility, this thickness is appropriate for hole extraction process in new compounds-based devices, as proven in J-V characteristics. 5. The control sample with spiro-OMeTAD as HTM showed a power conversion efficiency (PCE) of 19.2% and 16.9% in backward and forward scan direction, respectively, with an open circuit voltage (V OC ) of 1.128 V, a short current density (J SC ) of 22.86 mA cm -2 and a fill factor (FF) of 74.6% under reverse scan conditions. For 1-based devices we observe a small hysteresis with V OC of 1.109 V, J SC of 22.57 mA/cm 2 , FF of 68.7% resulting in a PCE of 17.2% in backward scan. Spiro-OMeTAD-, 1-, and 2-based devices show hysteresis which can be due to inappropriate ambient atmospheric condition during the film deposition. Although the highest PCE of 18.0% in backward voltage scan can be obtained for devices labelled as 2-2 with V OC of 1.124 V, J SC of 22.86 mA/cm 2 and FF of 70.2%, the hysteresis effect is increased as well with increasing concentration of additives (molar ratios of additives in 2-2 being two times larger as compared to 2). Increasing the amount of additives has thus a negative impact on the photovoltaic As a final and general comment, we observe that the trends observed in the PV performances of the studied compounds are at odds wrt the larger hole mobility of 1 in presence of additives as compared to both other compounds (Figure 5), thus pointing to other dominant factors.

Additionally, these trends could hardly be related to differences in the corresponding HOMO levels, as the IP SS or IP PES values for 1 and 2 compounds vary by only 0.02 eV and by a maximum of only 0.04 eV with spiro-OMeTAD. Impact of the additives in the HTM layer morphology, or differences in interfacial properties could thus be in play, such as, amongst others, eventual energy level-shifts, the quality and strength of perovskite-HTM contact and interactions, polarity mismatch, etc.

Conclusions

The new spiro-MeTAD HTMs labelled as 1 and 2 were synthesized and applied in perovskite solar cells with the aim to outperform the spiro-OMeTAD based ones by means of improved hole mobility. Indeed, the results obtained for the pure HTM indicate stronger intermolecular interactions, smaller polarity, and larger hole mobility for 1 and 2 as compared to spiro-OMeTAD.

However, the spiro-MeTAD-based PSC devices showed slightly inferior performances, with the best PCE of 17.2% and the lowest hysteresis obtained for 1-based devices, as compared to 19.2%

for spiro-OMeTAD -based devices. Given the better transport properties but slightly smaller performances of 1 and 2 compared to spiro-OMeTAD, these results comfort the idea that interface properties dominate the performances of PV devices in general as compared to HTM bulk properties. Our results suggest that the smaller PV performances of spiro-MeTAD compounds stem from less efficient perovskite → HTM hole transfer. The striking similarity of the new compounds with regard to the molecular shape, frontier orbital space-distribution, and energy levels of spiro-OMeTAD allows highlighting the dominant impact of the nature of the terminal groups (methyl versus methoxy) in the interaction between HTM-perovskite layers at their interface.

The influence of additive concentration in the HTM on the perovskite solar cell performance was investigated and showed important differences between the spiro-MeTAD-versus spiro-OMeTADbased perovskite solar cells. Results showed that for 2-2-based devices, efficiency reached to 18.0% in comparison with PCE of 19.2% in spiro-OMeTAD devices, but increasing additives concentration has a negative effect on hysteresis. Accordingly, the inclusion of additives in HTMs using the new spiro-MeTAD compounds would need further and careful optimization. As such, the similarity of PV performances of the new spiro-MeTAD compounds wrt spiro-OMeTAD suggests promising PV properties for these methylated spiro compounds, which could become even more attractive if their lower laboratory cost as compared to spiro-OMeTAD were to be similarly observed at the industrial level. Larger light transparency found for the new compounds and higher

Figure 1 .

 1 Figure 1. (a) Geometries of spiro-OMeTAD (with orientation of methoxy groups arbitrary chosen)

Figure 2 .

 2 Figure 2. Geometries of four dimer types constructed with spiro-OMeTAD and compound 1,

Figure 3 . 1 < 2 . 1 and 2 . 1 > 2

 3121212 Photon energy (eV)

Figure 4 .Table 3 .Figure 5

 435 Figure 4. UV spectra of (a) dilute THF solutions (normalized at Abs = 1), and (b) in solid state

Figure 5 .

 5 Figure 5. Electric field dependencies of hole-drift mobility of compounds 1, 2 and spiro-OMeTAD

Figure 6 .

 6 After light absorption by the perovskite layer, photogenerated electrons and holes are moved to the photoanode (TiO 2 layer) and hole conductor layer, respectively. The HOMO level of HTMs should be higher than the valence band of the perovskite layer for hole extraction. As shown in CV results, the HOMO levels of both spiro-MeTAD compounds 1 and 2 are a little deeper than spiro-OMeTAD.

Figure 6 .

 6 Figure 6. Schematic energy level diagram of the solar cell device. The ionization potentials of spiro-OMeTAD is as reported in Ref 32

Figure 7

 7 Figure 7 shows photoluminescence spectra of perovskite-HTM systems for different HTMs

Figure 7 .

 7 Figure 7. The PL spectra of the samples. The notations 1 and 1-2 correspond to two

performance of 1 - 2

 12 and 2-2 -based devices. As shown, hysteresis is increased while PCE is decreased compared to 1 and 2 -based devices. Interestingly, the performances of spiro-OMeTAD PSCs remain basically unaffected when the additive amounts are increased by the same amount (two times).

Figure 8 .

 8 Figure 8. J-V curves of the best cells recorded with reverse (from V OC to J SC ) scan direction.

Figure 9 . 1 -

 91 Photovoltaic parameters from different devices in backward direction are shown in Figure9. 1-

Figure 9 .

 9 Figure 9. Statistical plot of J SC , V OC , FF and PCE of different devices.

Figure 10 .

 10 Figure 10. Incident photon-to-current efficiency (IPCE) curves for HTM.

Figure 11

 11 Figure11shows the maximum power conversion efficiency as function of time from maximum

Figure 11 .

 11 Figure 11. Maximum power conversion efficiency devices: (a) after device preparation (b) after 2

  Pd(OAc) 2 , P(tBu) 3 , toluene, 115 °C, 12 h.)It is worth noting that following the same synthesis path, the yields of compound 1 and 2 are roughly 20-30% larger as compared to that of spiro-OMeTAD. This is due to the smaller amount of solvent and shorter time needed for the separation of the new compounds by chromatographic methods as compared to spiro-OMeTAD. While we are not able to evaluate the impact of this aspect at the industrial level, we believe that the same trend might be observed in both laboratoryand industrial synthesis conditions.

1. p,p'-ditolylamine 3 is a commercially available product. Diphenylamine intermediate 4 was synthetized by a Cu-catalysed amination of 1-iodo-3,5-dimethylbenzene. Spiro derivatives 1 and 2 containing substituted methyl functional groups were synthesized by the Buchwald-Hartwig amination reaction, carried out between 2,2',7,7'-tetrabromo-9,9'-spirobifluorene and the respective bis(methylphenyl)amines containing p-Me or m,m-Me substituents (3 or 4 respectively). The two spiro-MeTAD compounds were purified by column chromatography and characterized by NMR spectroscopy ( 1 H and 13 C) and mass spectrometry. All the analytical data were consistent with the proposed structures. The two compounds have a good solubility in all of the common organic solvents. + Scheme 1. Synthesis of compounds 1-2 (Reagents and conditions: sodium tert-butoxide, Thermal properties of 1 and 2 were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under argon atmosphere. The thermal decomposition (T d ) and the glass transition (T g ) temperatures are summarized in

Table 1. 1 and 2 showed high thermal stabilities with T d 5% values superior to 400 °C, which are high enough for applications in optoelectronic devices. Confirmed by DSC analysis, compounds 1 and 2 are revealed as amorphous materials. All compounds exhibit high glass transition temperatures (T g ) ranging from 96 to 143 °C.

Table 1 .

 1 The thermal characteristics of spiro-OMeTAD and compounds 1 and 2.

	Molecules	T d	5% (°C)	T g (°C)
	1		450	96
	2		443	143
	spiro-OMeTAD		424	110

Table 2 .

 2 . Ionization potentials (IP SS ) (and electron affinities (EA SS )) from cyclic voltammetry (and

	optical band gap E g opt ), ionization potentials from photoelectron emission spectra (IP PES ), and
	HOMO ( HOMO ) and LUMO ( LUMO ) energies (B3LYP/6-31G* level in "gas" phase) corresponding

to compounds 1, 2 and Spiro-OMeTAD.

Table 4 .

 4 Hole transport parameters of compounds 1, 2 and spiro-OMeTAD. The hole mobilities are approximate values taken at the electric field of 3.6 . 10 4 V/cm. Theoretical parameters calculated for

	compounds 1, 2 and spiro-OMeTAD at the B3LYP/6-31G* and B97WD/6-31G* levels in gas
	phase are also given, such as HOMO and LUMO energies, intramolecular reorganization energies
	( h ), HOMO-HOMO electronic couplings (t H-H ) deduced from a selected set of optimized dimers
	(average over 4 dimers for each compound, Figure 2), and dipole moments of isolated compounds.
	Compound	μ	σ	μ 0	 h	t H-H	Dip. Mom.

Table 5 .

 5 Photovoltaic parameters of different HTM based-devices under backward and forward

	scan direction.
	HTM	Scan	J SC (mA/cm 2 )	V OC (V)	PCE (%) FF (%)	Integrated
		direction					Jsc (IPCE)
	Spiro-	Rev	22.86	1.128	19.24	74.6	22.04
	OMETAD	For	23.19	1.085	16.99	67.5

Table 6 .

 6 Maximum power point tracking (MPPT) for HTM devices for initial measurements and after 2 months.

	HTMs	Spiro-	1	1-2	2	2-2
		OMeTAD
	MPPT(%)	18.26	15.076	12.68	14.79	15.33
	MPPT(%) After	17.93	15.01	------	14.43	-----
	2 month

glass transition temperatures in the case of compound 2 as compared to spiro-OMeTAD could additionally constitute valuable advantages for their use in perovskite solar cells.

ASSOCIATED CONTENT

Supporting


domain: spi phys

text:
Christine Rousselle
email: christine.rousselle@univ-orleans.fr

Christine Mounaïm-Rousselle

C Hespel

P Bréquigny

Specific Challenges for Ammonia Engines

Despite several trials of ammonia-powered vehicles in the past, no vehicles are currently on the road. Theoretical studies in the 1960s highlighted the various technological challenges to be overcome. Now that the world is aware of the need to move towards a low-carbon society, the cards have been reshuffled: not only ammonia is considered as one of the vectors of clean electricity for the future, but it can also be a fuel in its own right, even if its fundamental combustion characteristics are not advantageous. Some various aspects ranging from control of injection, in the gaseous or liquid phase, ignition and combustion itself, to pollutant emissions and their possible post-treatment are summarized.

Introduction

In despite of some successful stories about ammonia engines, no ammonia-powered vehicles are currently on the road. In fact, during the 60s', the theoretical studies concluded that ammonia, due to its pure combustion characteristics, is so different in comparison to hydrocarbon fuels that it would induce too many changes and development to be implemented in a vehicle. But since these last 6 years, it becomes more evident that ammonia is not only an excellent hydrogen carrier due to its higher volumetric energy density (as 15.6 MJ/L for liquid ammonia, compared to 5.6 MJ/L at 70 MPa, for compressed hydrogen), but also a possible carbon-free fuel for internal combustion engines. Ammonia engines could be not only for road vehicles but also for marine transport, rail and for genset and auxiliary power units and even aircrafts. In 2014, researchers from Oxford University announced that ammonia is a clean and secure hydrogen-containing energy source, proposing the idea' to use it directly in engines' [2]. Ammonia is an ideal fuel since it is not a greenhouse gas and its complete combustion forms only N2 and H2O [3].

First, ammonia, containing 17.8% by weight of hydrogen, can be stored in the liquid phase at approximately 9 bar at 20°C or -33°C at ambient pressure. Its high autoignition temperature and research octane number (RON=130) guarantee no occurrence of knock phenomenon if it is used without other fuels. But in the other hand, it limits its use in current engines. As suggested in [4], a compression ratio of 35:1 has to be used to obtain auto-ignition of ammonia. But as underlined in Kurien and Mittal [5], higher compression ratios (>35:1) are impractical in the case of conventional engines since the permissible compression ratio in real world applications is in the range from 12:1 to 24:1 Therefore, to enhance the ignition, spark plug and/or preor local mixing with other fuel is needed. In the case of premixed combustion, the low laminar flame speed of ammonia [6] also induces the requirement of boosted conditions or promoting its burning with other fuels. It is therefore evident that ammonia cannot be considered suitable as a drop-in replacement for conventional fossil fuel. Development of combustion strategies to optimize ammonia performance in engines is necessary. From the past, many studies focused on ammonia sharing with other fuels without reaching level higher than 90% of energy fraction. These results remain interesting as it could be convenient for practical uses to have 'flex-fuel' engine, in case of no sufficient availability of ammonia. Moreover, in the longer perspective, a 100% sustainable fuel supply is desired to fully decarbonize the transport and energy sectors. Therefore, in this review, will be considered recent studies where at least 98% of energy from ammonia is used as fuel.

Therefore, the development of Internal combustion engines with ammonia as the main fuel follows mainly 3 directions: -to consider conventional SI Spark ignition engine). From recent studies, it is stated that addition of small quantities of H2 can strongly enhance the stability of NH3 combustion [7,[START_REF] Mounaïm-Rousselle | Special Issue Ammonia as an Energy Carrier[END_REF] and can reach similar efficiency than natural gases engines.

-to consider compression ignition engine by 2 ways [5,[START_REF] Dimitriou | [END_REF]: (i) adding a spark plug to help the combustion ignition even if the compression ratio is more about 16-18; (ii) igniting ammonia charge with high reactive fuel (hydrocarbons), as Diesel, biodiesel, DME… A summary of various challenges from ammonia/air mixture preparation, with gaseous or liquid injection, ignition and combustion itself, to pollutant emissions and their possible post-treatment are addressed in the following.

Injection of ammonia in engine

As for any fuel, the injection process is one key parameter to optimize local air/fuel mixture inside the combustion chamber, to enhance combustion development and limit the pollutant emission itself [10]. In the case of ammonia, no universal solution seems to come offas the potential of different injection systems remain to be studied. Therefore, as schematized in Figure 1, all configurations could be chosen in liquid or gaseous form, port fuel or direct injection, split or not. This becomes more complex when the injection of one other fuel as combustion booster has to be done. In the past, different studies used classical port-fuel gaseous injectors as is, but . It was only recently concluded that the dynamic of the ammonia gaseous jet is similar to gaseous methane [11]. The main difference will be at the end the local air/fuel ratio as indicated in Table 1 due to the strong different stoichiometric air/fuel ratio. . Therefore, even if the choice of gaseous direct injection could be justified, nthe flow rate to ensure sufficient ammonia amount is not usual.

Since this type of injection is mainly for spark ignition engine, the laminar flame speed and the interaction with turbulence will be the major properties that drive the combustion process. As concluded in [START_REF] Lhuillier | Proceedings of the Combustion Institute[END_REF], as the laminar flame speed is very low in the case of classical SI engine conditions, the turbulence flow affects a lot the combustion process. One consequence is the difficulty to stabilize the combustion for low load as well as for low and high rpm as in [START_REF] Mounaïm-Rousselle | Special Issue Ammonia as an Energy Carrier[END_REF].

The advantage of direct liquid injection compared to port fuel is not only to help the filling of the engine but also to vaporize as fast as possible the fuel inside the chamber by breaking up the fuel spray into droplets. Then, the vaporization of droplets is greatly significant for the spatial distribution of fuel vapor/air, the ignition, and the combustion itself. Generally, the use of high-pressure injection systems favors the atomization in small droplets which enhances the liquid-air interface, the vaporization, and the fuel/air mixing to avoid NH3 trap. However with ammonia, atomization can be mainly driven by flashboiling process which requires to be more studied before being controlled for engine applications. Some recent studies started to focus on ammonia liquid spray as for example in [START_REF] Pelé | [END_REF]. As it can be seen in Figure 2, the examples of liquid spray morphology for ammonia in comparison for gasoline indicate the challenge to ensure optimum fuel/air mixture inside the combustion chamber.

Figure 2: example of liquid spray shapes for ammonia and gasoline with a Gasoline Direct Injector (120 b injection pressure). Some first CFD modelling indicated the difficulties to predict well ammonia vaporization process and especially the droplet size to help engine design [14]. First results in SI engine provided as conclusion that injection strategies (late, early and split injections) need also to be more explored before choosing a right one [15].

In the case of Reactivity Controlled Compression Ignition, i.e. the ignition of ammonia combustion by local distribution of a more reactive fuel, as in Figure 1 (right), most of recent studies considered premixing of ammonia with air in the intake as in [16]. For that, current common-rail injector for the reactive fuel is used even if to control small quantities, very low injection pressure is required, resulting some difficulties to control efficiently the amount of pilot fuel. A recent study evaluated the dual-fuel concept with premixed ammonia and direct-injected n-heptane but by using a gasoline direct injector to investigate the possibility of running a CI-engine with higher ammonia-energy share than previously seen. The GDI injector allows small, quick injections with rapid atomization of the pilot fuel [l7]. It is really hard to predict what best solution will be chosen in the future as it is dependent to the engine architecture, the operating conditions and the pollutant emissions trade-off. Other interesting alternatives are to directly inject ammonia near the reactive fuel as recently in [18].

Combustion in ICE -Spark ignition

In the case of premixed combustion, the ignition tool and the laminar-turbulence interaction are key parameters to favor the combustion development itself. The laminar flame speed of ammonia is approximately 1/5th of gasoline at stoichiometric and ambient conditions. Therefore, the ammonia combustion requires larger spark advance and yields to longer combustion durations compare to a conventional gasoline fuel. Therefore, the flow dynamics between the spark ignition and the end of combustion has more effect on flame development itself. From a combustion standpoint, turbulence ahead of the flame front tends to increase the flame surface area and increases the fuel consumption rate. As highlighted in [START_REF] Pelé | [END_REF], the turbulence enhances more ammonia flame than ammonia/hydrogen or ammonia/methane ones. This is due to the low value of laminar flame speed, which induces the flame development in the 'broken reaction zone' of the Turbulent Combustion regime diagram as indicated in Figure 3 (blue points). However, as indicated in [19,[START_REF] Won | THIESEL 2022 Conference on Thermo-and Fluid Dynamics of Clean propulsion Powerplants[END_REF], using diesel engine architecture with the addition of spark-plug is interesting as neat ammonia can be in a wider range of operating conditions (low load, low and high speed), despite of the low level of turbulence in diesel combustion chamber, due to the swirl motion. This is totally different to what it can be observed with same engine running with 'classical fuel' as gasoline, where in fact the spark increases locally the temperature, inducing one strong peak of pressure following by another main heat release rate. This is in fact considered as two-steps combustions. In the case of ammonia, the heat release rate evolves exactly as in current SI engine. The increase of compression ratio has no sufficient impact on laminar flame speed (less than 10% of improvement) to explain the stability of combustion. This is due to the turbulence level and vortices size which affect more the ammonia flame.

Figure 3 -difference of ammonia flame regime between current Spark Ignition engine and Diesel engine ignited by a spark-plug (from conditions in [START_REF] Lhuillier | Proceedings of the Combustion Institute[END_REF]19]). Recently, different turbulence dynamics have been tested using different piston shapes [START_REF] Won | THIESEL 2022 Conference on Thermo-and Fluid Dynamics of Clean propulsion Powerplants[END_REF]. The conclusions are not so evident as even if high turbulent kinetics energy is needed to enhance sufficiently ammonia flame, unburnt ammonia can be obtained also because of the piston shape.

-Ignition by pilot reactive jet In the case of pilot fuel ignition, a minimum CR around 15 seems to be sufficient from previous studies to guarantee a good ignition. It is surely dependent on the cetane number of the pilot fuel [START_REF] Niki | [END_REF], but remains one promising solution to retrofit diesel engines, usually used in ships, gensets and construction machines by slightly modifying the intake port to supply ammonia. But most of previous studies are not focused on high ammonia energy share as in [15]. The example of pressure trace presented in Figure 4 between RCCI and Spark ignition for the same engine (Compression ratio 16) indicates that clearly for this case, the more reactive fuel is ignited before allowing the combustion of the premixed ammonia/air mixture. This combustion mode is interesting as it is possible to run at leaner conditions, down to 0.5 of equivalence ratio for this case with a high indicated ammonia combustion efficiency. However the range of operating conditions and different injection strategies are so wide, as for spark-ignition engines, that it is difficult to provide a clear vision of the best strategies . In addition, as emphasized in [16], the more viscous the pilot fuel, the more difficult very small injections are and the larger the spray droplets are.,

Combustion imaget in RCCI Combustion image SI mode Red, spark ignition,
Blue, decane pilot ignition Figure 4 -Different combustion developments: ammonia/air equivalence ratio 0.9 -decane : 2.2% energy share Recently some studies started to consider prechamber igniter as another way to help ammonia combustion in engine as for example in [23,24], but the question remains what the best solution is to fill the prechamber with ammonia, hydrogen, cracked mixtures etc.

Pollutants and post-treatment

If one considers ammonia combustion equation, without no carbon content fuel, no CO or CO2.

As underlined in [START_REF] Kobayashi | Proceedings of the Combustion Institute[END_REF], the emissions are additional challenges of NH3/air combustion. Indeed, if the combustion is complete, no unburnt NH3, and as a function of local equivalence ratio, temperature and pressure, NOx and N2O could be or not important. The relationship between NOx and ammonia slip is not monotonous: NOx emissions tend to be produced at high combustion temperatures (for CI engines) or lean equivalence ratio (for SI engines), and unburnt ammonia at low temperatures or rich equivalence ratio respectively. No "ideal" temperature level eliminating both emission species were determined as underlined in [START_REF] Dimitriou | [END_REF]. Moreover, emissions of unburnt ammonia can occur during unstable combustion conditions or also due to fuel trapping, especially in the case of retrofitted CI engine with portfuel injection of ammonia. In Figure 5 [20], a scheme of emissions evolution at the exhaust for ammonia SI engine is suggested as a function of air/fuel ratio. Therefore, for post-treatment systems, 2 strategies are possible: to run in slightly rich conditions with unburnt ammonia and H2 (due to internal cracking) as underlined in [19,[START_REF] Reggeti | [END_REF] or slightly lean conditions with NOx but small quantities of NH3. In the case of RCCI engine, one main question is about CO2 emissions due to the reactive fuel. But it has to be considered that in the case of ammonia combustion as a function of the operating conditions N2O specie can be also produced. Even if the level is low (around 50 ppm), its global warning potential is 265 times that of CO2 with a lifetime in the atmosphere of over 100 years . The comparison of ammonia CI engines, ignited by a small decane pilot injection or a spark plug shows that the level of N2O emissions is similar (less than 50 ppm) without any visible effect of the equivalence ratio. Moreover, in Figure 6, N2O level was converted in CO2 equivalence. Therefore, even if decane contributes to enhance CO2 emissions, CO2 emissions are in the same order of magnitude for both spark and pilot ignition for lean mixture up to the stoichiometry where N2O and CO2 are slightly increased in the case of RCCI condition because of the increase of diesel energy share to guarantee engine stability.

Spark ignition in Diesel engine

Conclusions

Even if there are several announcements from companies to deliver ammonia engines soon, the studies about ammonia engines remain sparse, especially for direct injection configurations or pilot reactive ignition. Moreover, most of studies are more and more done by CFD modelling. Unfortunately due to the lack of knowledge as ammonia is a particular fuel, it is not possible until now to design engines with ammonia by only considering classical injection, atomization, combustion and kinetics modeling. The development of optimized post-treatment requires accurate predictions of pollutant species. But as underlined in [27], not only NH3 oxidation chemistry presents still many open issues, especially at high-pressure as in engines, but also the sensitivity of NH3 to other chemistries as DeNOx, NO-NO2 loop, and N2O. Last the addition of fuel promoters has also a direct effect on the oxidation chemistry and needs more understanding to predict ICE combustion.

Figure 1 :

 1 Figure 1: different configurations of ammonia injection with other fuels for SI or CI engines.In the past, different studies used classical port-fuel gaseous injectors as is, but . It was only recently concluded that the dynamic of the ammonia gaseous jet is similar to gaseous methane[11]. The main difference will be at the end the local air/fuel ratio as indicated in Table1due to the strong different stoichiometric air/fuel ratio. . Therefore, even if the choice of gaseous direct injection could be justified, nthe flow rate to ensure sufficient ammonia amount is not usual.Since this type of injection is mainly for spark ignition engine, the laminar flame speed and the interaction with turbulence will be the major properties that drive the combustion process. As concluded in[START_REF] Lhuillier | Proceedings of the Combustion Institute[END_REF], as the laminar flame speed is very low in the case of classical SI engine conditions, the turbulence flow affects a lot the combustion process. One consequence is the difficulty to stabilize the combustion for low load as well as for low and high rpm as in[START_REF] Mounaïm-Rousselle | Special Issue Ammonia as an Energy Carrier[END_REF].The advantage of direct liquid injection compared to port fuel is not only to help the filling of the engine but also to vaporize as fast as possible the fuel inside the chamber by breaking up the fuel spray into droplets. Then, the vaporization of droplets is greatly significant for the spatial distribution of fuel vapor/air, the ignition, and the combustion itself. Generally, the use of high-pressure injection systems favors the atomization in small droplets which enhances the liquid-air interface, the vaporization, and the fuel/air mixing to avoid NH3 trap. However with

  Current SI engineNH 3 (+ xFuel) + air H 2 0 + N 2 (+ CO 2 +CO) (NOx + N 2 O+NH 3 )

Figure 5 :

 5 Figure5: comparison of emissions from ammonia SI engines and Natural gas ones[START_REF] Won | THIESEL 2022 Conference on Thermo-and Fluid Dynamics of Clean propulsion Powerplants[END_REF].In the case of RCCI engine, one main question is about CO2 emissions due to the reactive fuel. But it has to be considered that in the case of ammonia combustion as a function of the operating conditions N2O specie can be also produced. Even if the level is low (around 50 ppm), its global warning potential is 265 times that of CO2 with a lifetime in the atmosphere of over 100 years . The comparison of ammonia CI engines, ignited by a small decane pilot injection or a spark plug shows that the level of N2O emissions is similar (less than 50 ppm) without any visible effect of the equivalence ratio. Moreover, in Figure6, N2O level was converted in CO2 equivalence. Therefore, even if decane contributes to enhance CO2 emissions, CO2 emissions are in the same order of magnitude for both spark and pilot ignition for lean mixture up to the stoichiometry where N2O and CO2 are slightly increased in the case of RCCI condition because of the increase of diesel energy share to guarantee engine stability.

Figure 6 :

 6 Figure 6 : comparison of CO2 eq. emissions from pilot diesel ignition versus spark ignition in ammonia CI engines.

Table 1 :

 1 comparison of different fuels properties

	Fuel	NH 3	H 2	CH 4	C 8 H 18	C 12 H 24
	air/fuel stoichio	3.57	2.38	9.52	59.50	88.06
	mass air/fuel ratio	6.06	34.32	17.16	15.05	15.11
	energy (KJ/Kg air) 3078.04 3494.46 2913.75 2823.92 2812.71

Acknowledgments

We are thankful to R. Pelé, A. Dupuy, phd and master students who assisted the research and B. Moreau and Y. Haidous for technical assistance for experimental setups.


Sarah Cohen-Boulakia

Keywords:

FAIR Scientific Workflows: Status, Challenges and Research Opportunities

Sarah Cohen-Boulakia Professor Université Paris-Saclay Biography Sarah Cohen-Boulakia is a full Professor at the Laboratoire de Recherche en Informatique at Universite Paris-Sud/Paris-Saclay. She holds a Ph.D. in Computer Science and a habilitation from the same University. She has been working for fifteen years in multi-disciplinary groups involving computer scientists and biologists of various domains. She spent two-years as a postdoctoral researcher at the University of Pennsylvania, USA and 18 months at the Institute of Computational Biology (IBC) of Montpellier, France. Dr. Cohen-Boulakia's research interests include provenance and design of scientific workflows, reproducibility of scientific experiments, integration, querying and ranking in the context of biological and biomedical databases. She currently co-animates a National working group on reproducibility of scientific experiments and she is involved in the European Research Infrastructure ELIXIR (https ://www.elixir-europe.org/).

Summary

Computational workflows describe the complex multi-step methods that are used for data collection, data preparation, analytics, predictive modelling, and simulation that lead to new data products. They can inherently contribute to the FAIR data principles: by processing data according to established metadata; by creating metadata themselves during the processing of data; and by tracking and recording data provenance. These properties aid data quality assessment and contribute to secondary data usage. Moreover, workflows are digital objects in their own right. In this talk we will discuss FAIR principles for computational scientific workflows. We will highlight remaining challenges to address the workflows specific nature in terms of their composition of executable software steps, their provenance, and their development.