The Sonic Composition of the City

Jean-Paul Thibaud

In The Auditory Culture Reader. Michael Bull & Les Back (Eds.) Berg Publishers, Amsterdam, 2003, p. 329-341 A common phenomenon now marks the ordinary experience of city dwellers : walking through the city to music. Geared with headphones, the walkman listener strolls along and takes in the musical scenery on his/her way. A kind of tuning in is created between his ear and his step. New sonic territories are composed in the course of this mobile listening experience.

As the body moves in sync to the music, the listener transforms the public scene and provides a new tonality to the city street. His footsteps seem to say what his ears may be trying to hide.

How does music with headphones mobilize the walker's gait? What does this contemporary form of urban mobility stand for? How should we consider this micro-ecology of musical navigation?

Using a walkman in public places is part of an urban tactic which consists in decomposing the territorial structure of the city and recomposing it through spatio-phonic behaviors. Double movement of deterritorialization and reterritorialization. This new urban nomad is here and there at the same time, transported by the secret rhythm of his walkman and in direct contact with the place he's walking through. The bounce in his step, the variations in his stride and the unexpected change of his daily route explain at times his imaginary drift but always brings him back to where he started. « All music of the body is music of the body of the earth » affirms Daniel [START_REF] Charles | Flux de marche avec piétinement[END_REF]. Musicalization of the step and puttering of sonic rhythms go together.

We see him fully absorbed, lost in his sonic universe, whereas just one more step, a scream, or a glance is all it takes to bring him back into contact with his surroundings. We mustn't be mistaken, the walkman-listener is not entirely cut off from the urban environment.

His being rooted in the urban space leans more towards an instability of percieved forms. The a delinquent practice consisting of « living not in the margins but rather in the thresholds of social codes that are foiled and displaced » [START_REF] De Certeau | The Practice of Everyday Life[END_REF].

Using a walkman is part of a process of derealization of urban space which depends on the technical mediation of spatio-phonic behaviors. As a transposable object, the walkman offers itself as one of the most advanced stage of musica mobilis. It involves mobile listening. As an involvement shield, the walkman « momentarily allows us to position ourselves outside the social theatre » [START_REF] Kouloumdjian | Le walkman et ses pratiques[END_REF]. It is part of secret listening. As a schizophonic object, the walkman develops « the separation between an original sound and its electro-acoustic reproduction » [START_REF] Schafer | Les mobilisations de l'auditeur-baladeur : une sociabilité publicative[END_REF]. It participates in listening of elsewhere. Thus, as a manager of sensorial channels, it questions the relationship between the dweller and his environment and enables new modes of experiencing the city.

The hidden sides of the city

The walking listener reveals the hidden sides of the city according to three movements.

The first movement : from visible to audible. By affirming the importance of sound as an essential feature of his urban experience, the walkman user brings us to reconsider the visual paradigm that predominates in the social sciences (Bull, 2000). It is less a question of adopting a new point of view but rather a question of listening to the voices of the city. The city is also to be heard and not only to be seen. In this respect, listening with headphones on is like a wonderful decoding instrument of the urban sonic environment. The walking listener uses it not only to protect himself from the sonic aggressions of the city but also to filter and enhance the events which give the place its meaning. Depending upon the places and what is happening in them, the sound volume of the walkman is used in order to be able to listen to or to mask conversations, bells ringing, children screams, traffic noise, etc. What comes out of the urban rumour are the sounds that signify the place, those which qualify the space with a singular voice. Thus, this selective listening enables a hierachization of everyday sounds and a decomposition of the urban soundscape. A new figure of the city dweller may be in the midst of being born. Instead of the eye-oriented flâneur dear to Walter Benjamin, mustn't we substitute the discriminating listening of the musicalized passer-by ?

Second movement : from perception to action. Using a walkman is not limited to reintroducing the audible in our experience of urban space. It also reveals everyday sounds as closely related to social practices. There is no such thing as pure perception, free from the course of action and practical activities in which the perceiving subject is involved. If the walkman voices are superimposed to those of the city, the steps of the city dweller also interfere with what he hears : « the walkman brings with him a mixture of music and body and the walkman user invents the art of coordination on a daily level in order to settle into the space he occupies, a 'short' circuit. » [START_REF] Hosokawa | The Walkman Effect[END_REF]. A few more feet are sometimes enough to pick up the reception of a radiophonic station, a sudden swaying may affect the music of the walkman starting to lack energy, arriving at a noisy crossroad makes the longawaited favorite song suddenly inaudible. Walking is also a sonic action which mobilizes the step as much as the ear. The passer-by with headphones navigates through several worlds at once, the one in which he hears and the one in which he walks. Passer-by par excellence, he orchestrates « a synthesis and a coordination between two heterogeneous series » (Deleuze, 1969) : entanglement between the ear and the step.

Third movement : from private listening to public secret. The walkman phenomena presents a particular heuristic interest in the sense that it also reveals the taken for granted of everyday life. « Breaching experiment » Harold Garfinkel (1967) would say. Making observable what usually goes unnoticed, the walking listener provides partial access to the mystery of appearance. By establishing a disjunction between the visible and the audible, a disturbance of the human sensorium and a form of strangeness in everyday life, it questions the evidence of the « perceptive faith » (Merleau-Ponty, 1991). Thus, between the fully explicit and overt behavior and the complete secret, the walkman user provides a whole range of intermediary opérations : dancing pace which escapes the understanding of others, incongruous movements and gestures which only make sense to the listener, speech with strange voices while listening to the music, etc. What are the minimal conditions for a space to be public ? The problematic nature of these phenomena can be explained by the fact that they render public -by not following them -the social codes from which the expressive order of the body constitutes a shared signifying system. In other words, these kinds of intersubjective discordance uncover the unsuspected background of social life [START_REF] Schafer | Les mobilisations de l'auditeur-baladeur : une sociabilité publicative[END_REF].

These preliminary remarks bring us to consider the way public places are experienced. It requires thinking in terms of thresholds, knots, and configurations. A constant reminder of inbetween practices. Thresholds when the walkman user leaves his house : interpenetration of private and public. Knots when he makes use of his senses : interferences between the ear, the step and the built environment. Configurations when he moves around the city to music : interstice of lived itineraries.

Doors, bridges, and interchanges

The transition from one's home into the public realm represents a special moment in walkman use. Many walkman users have specific daily habits in the street. They carry out a series of operations which use a very precise and regular order. Public places are marked by an access ritual characterized by a repeated ordering of thresholds enabling the walkman user to fit into the rhythm of the street.

Doors.

The door of the house may also function as a sonic door. On one hand, the door serves as a transition between two places of distinct status ; on the other hand, as an intermediary between two kinds of listening experience. Leaving one's home is often synonymous with turning on the walkman and coming home is equated with turning it off. It is paradoxical that access to public space is associated with the beginning of private listening whereas entering the private realm is associated with disponibility and openness towards the surroundings. What is the relationship between coming through the door and turning on the walkman? Where specifically is the threshold which marks the change in listening? The exit door serves as a transitional space between getting the walkman ready and turning it on: « I usually put my headphones on when I walk out the door or they are already positioned on my head and all I have to do is press a button ». Listening to headphones begins outside the home.

The distinction established between putting on the headphones and starting to actually listen leads to a differentiation between leaving one's home and entering a public space. Closing the door to one's home is still considered a private act even if the person is physically outside. « Turning on » the walkman can be understood in two ways : as the moment one begins to listen to the walkman and also as the beginning of the walkman user's voyage. Hearing the music marks the moment that the individual admits that he has entered into a public space.

The threshold between public and private is defined less in terms of physical access (stepping out the door) and more in terms of a change of perceptual orientation. We are dealing here with a « paradoxical territorialization » in the sense that this approach to the public realm implies restricted sonic accessibility combined with an increased visual exposure. This exiting ritual reveals the differentiated status that we give to speech according to the type of space we find ourselves in. The contact which joins the city dweller to his domestic space must be maintained up until the last moment, in other words, up until the door is closed : « When I'm outside, I lock the door, turn on my walkman, ensure that all is working well, that everything is hooked up. Sometimes I turn it on before but it is difficult to say goodbye to the person I live with ». This description validates the hypothesis according to which the walking listener changes his perceptual orientation once he left his house. Whereas it seems appropriate to remain sonically accessible in domestic places, being available in public places seems less necessary. A first type of threshold can be revealed: we call « sonic door » this border point where the contact with the family life has still to be maintained or on the contrary can be interrupted. In other words, the sonic door deals with the phatic function of communication. It involves the possibility to shift our modes of attention towards others.

Bridges. Spatial transitions of the walking listener are not always expressed by such radical changes of perceptive attitude. Sonic continuity can be sought in the passage of one place to another. Listening with headphones establishes a « sonic bridge » between domestic and public espaces : « I close the door before listening to the music unless there's a good song on the radio, but then I locate it on my walkman as soon as I turn it on to be able to continue listening to the song ». This is what may be referred to as a « phonic deterritorialization » of the urban environment in the sense that the walking listener neutralizes the sonic delimitations between domestic and public spaces by establishing a continual listening between the two spaces. This musical link tends to neutralize the sonic identity of the places. The spatial decontextualization of listening with headphones creates a sonic bridge between the interior and exterior. We call sonic bridge the listening device which encourages the reaching of a material limit without requalifing the sonic space of the listener. Contrary to the sonic door which marks the reaching of a threshold through modifying the sound context, the sonic bridge establishes an auditory continuity independent of the places traveled through. It is part of a double movement of retrospection (the past listening affects the sonic program to come) and anticipation (turning on the walkman occurs before the walkman listener's stroll outside).

Sometimes even a momentary overlap is produced between the radiophonic domestic diffusion and diffusion of the same program by the walking listener : « Sometimes I listen to King FM at home. If the radio is always on and I put King FM on my walkman, it produces amplification of the music. It's almost like when you are in a cave ». Creating a resonance between different sonic emissions makes the listener incapable of developing a sonic place marker. Acoustical characteristics of these sonic productions being practically identical, no clue makes it possible to differentiate one from another. This confusion of diverse sonic sources constitutes an interesting example of neutralization of the territorial power of sound.

Interchanges. A third type of threshold consists in adjusting the sound volume of the walkman in terms of movement, modulating the listening conditions according to the sonic context of the street : « When I go out, I adjust the volume of my walkman so it is neither too loud nor too low, because there could be noises from a car or something like that. So I need to adjust the volume in harmony with the situation ». Becoming attuned to the situation consists in stabilizing the level of intensity between the sounds of the walkman and the one of the urban environment. Regulating the volume of the walkman makes it possible to maintain the preponderance of electro-acoustic productions without necessarily masking urban noises. The listener is looking for the minimal intensity gap between these two sonic sources : « neither too strong nor too weak » we were told. We are referring in this case to « sonic interchange » in the sense that a reversal of the foreground-background relationship could occur at any given moment : outside voices, perceived as sonic background, can suddenly mask the musical emission and show up in the foreground. This is what happens in particular when the walking listener enters a public space. Adjusting the volume consists in stabilizing the foreground-background relationship and attempting to maintain the same balance of intensity throughout the walking route. We witness a « sonic reterritorialization » of the urban space in the sense that sonic urban occurrences are recomposed in terms of musical dynamics. While researching audible thresholds, the walking listener creates a new sonic organization of the street based on a relative mastering of sonic urban masks.

As soon as we no longer limit accessibility of free circulation of the body in space, we may pose a question about public sensorial instrumentation. By upholding the existence of a plurality of transitions to public places, the walking listener makes the « in-between » an essential category of structuring urban territory. We must then « start in the middle, from the middle, enter and exit, neither begin nor finish » (Deleuze, 1980).

The knots of the urban environment

Listening to headphones creates powerful and complex links with the characteristics of the urban milieu. The walking listener enters into a relationship with the urban environmentsonic, built and visual -by experiencing situations of interference, in other words, situations in which different types of space-time connect.

Particularly intense urban noises -road noise, sirens, warning signals -punctuate the flow of the musical audition and conditions the degree of autonomy of listening with headphones.

Acoustic permeability of headphones doesn't fully predict the total intrusion of voices of the city in the listener's perceptive field. When a sonic parasite is imposed, several solutions are imaginable. It is either possible to rewind the tape and play the piece again in which listening was momentarily interrupted: « If there is an intense noise in the street and I can't hear the music, I put the song on again ». The other possibility is to turn up the volume: « When there's a great deal of noise around, I make the walkman louder ». Or resignation may be explained as a waiting period for calm to occur: « If I turn up the volume and I still can't hear, I just have to wait ». In any case, people are not always annoted to this situation of sonic interference. It may also be sought after by the user or may be somewhat beneficial. Certain sonic occurrences are not just parasitic ; they provide informational value which may be momentarily significant in terms of musical audition. The listener voluntarily goes from listening to headphones to listening to the city by intentionally increasing the porosity of his earphones: « The other day, I lowered the volume because I wanted to hear the bells, I wanted to know the exact time, so I lowered the volume to hear the chimes ». Whatever the tactic applied, the walkman user is situated within two simultaneous sonic worlds. We are referring in this case to an « interphonic knot », in other words, the point of convergence between two sonic spaces of a different nature -that of the walking listener and that of the street.

The built environment also interferes with listening to the radio. Buildings are sometimes an obstacle to the propagation of Hertzian waves and limit the reception of radio programs.

The architecture of spaces and the height and form of edifices become audible through the use of headphones: « It's hard to pick up a radio station in the city because of all those huge and tall buildings. Buildings are a sort of screen ». A transduction occurs between the material urban forms and the perceived sonic forms, the visible becomes audible. Depending on the particular case, the listener momentarily loses the station that he's in the midst of listening to or picks up two at a time. It is remarkable that walking listeners are capable of localizing zones of hertzian disturbance so precisely. On their walking route, they identify radiophonic territories in the city and acquire their own knowledge. Different individuals, unknown from each other, make us aware of the same urban phonography. The town center is often evoked in similar terms: « When you are in the center, reception is not very good so you have to find a radio station which gets better reception », « It depends on the radio station I want to listen to, it's really bad in the city center ». Regarding the lake in the city center, « Reception of the radio station Bellevue is better on this side of the lake, you lose the station when you are on the other side », « On one side of the lake, I listened to two stations at once, but I moved the tuner and I lost one of them ». The position of the listener in regards to the built environment is significant in terms of the conditions of radiophonic reception. We are referring in this case to a « topophonic knot », in other words, the interference point between media listening and architectural space. This invisible urban side brings us closer to the experience of Fernand Deligny (1980), who when retracing the daily walking patterns of autistic children, revealed the existence of « chevêtres », common areas to all children without previous disclosure of these places. « Communal » relationship occurs when the walking listeners are linked to each other by a community of behavior and territory which has not been outwardly expressed.

Finally, listening to headphones establishes strange connections between the visual and the musical landscapes. Depending upon the music listened to, the city takes on variable tones and moods. The attention paid to the elements of the visual landscape seems to depend in particular upon the musical style: « Sometimes when I listen to classical music while I'm walking, I notice the trees and nature surrounding me, much more than I normally would.

Listening to rock and roll makes me notice the cars that pass by… ». These cultural associations between sound and image are undoubtedly largely influenced by stereotypes found in film and television productions. This extract illustrates a phenomenon of aesthetization of the visual environment through music. Many authors have studied this significant trait of urban visual culture and have insisted on the particular role that the walkman plays in this evolution. Ian Chambers (1986) considers the walkman an aesthetic device of the city, Paolo [START_REF] Prato | Music in the streets : the example of Washington Square Park in New York City[END_REF] calls it a « spectacularization process » and Claus-Dieter [START_REF] Rath | La "Sociologie des sens" de Simmel revue à l'époque de la télévision. Georges Simmel, la sociologie et l'expérience du monde moderne[END_REF] affirms the birth of the city-screen by referring to the role that the walkman plays in this regard. The objects seen are not apprehended in an equivalent and neutral manner. They are subjected to choices and visual reconfigurations in terms of headphone listening: « There is often a coincidence between what I listen to and what I see, so I can see things that are a bit more elementary, occasionally a bit clearer, and something reaches me ». The walking listener is particularly sensitive to the connection established between the eye and the ear, to the « rhythmic harmony between the music, the listener's body, and the exterior environment » (Cave and Cotton, 1984). We are alluding then to the « visiophonic knot » to designate this research of a convergence point between the audible and the visible, a discovery of synchronized intersensorial sequences.

The walkman should not be reduced to the status of a simple object of the environment among many: it is interposed between the user and his milieu rather than situated at a distance from the relationship. This analysis is close to the definition of « quasi-objects » provided by Michel Serres: « This quasi-object is not an object, but it is one nevertheless, since it is not a subject, since it is in the world, it is also a quasi-object, since it marks or designates a subject which without it wouldn't be » [START_REF] Schafer | Les mobilisations de l'auditeur-baladeur : une sociabilité publicative[END_REF]. The walkman is a third term to be situated at the level of the man-milieu relationship, it is on one hand inseparable from the perceived subject (of the user) and nonetheless distinct from him. The knots we have just referred to must not be confused with the « center » of the « city center » where city dwellers would come to be together but rather as a « pluralist aesthetics of the situation » [START_REF] Schafer | Les mobilisations de l'auditeur-baladeur : une sociabilité publicative[END_REF] which connects the diverse socioecological components of passing places.

The interstice of lived itineraries

Rather than uncover urban territories based on the spatial and visual order of the city, the walking listener makes it possible to identify several navigational practices. The musical walking patterns of the walking listener could be synthesized through a typology of strolls.

Each stroll can be regarded in two ways, first as an art to master the act of getting around, and secondly, as a means to reveal the complex relationship between listening and moving. If both listening to music and walking involves time, the typology that we are offering underlines various kinds of spatio-phonic procedures. We may distinguish six types of walking practices which often overlap.

The « route » consists in choosing a musical itinerary which goes from one place to another. This itinerary proposes two variants. On one hand, when a route is taken regularly, daily, as for example, the path to work, the walking listener rediscovers it each day by changing the music he listens to. The same route may be heard in different ways. On the other hand, an occasional or exceptional itinerary could be listened to with the same sonic program as that of another route. The walking listener appropriates the space he walks through in terms of his familiarity with the musical pieces. Thus, the route superposes a double temporalityone of which is walking around and the other is the music. This diversifies or, to the contrary homogenizes, the perception of the urban environment.

The « stride » consists in minimizing occasions for stopping and in encouraging continuity of the walk. Not saying « hello » to an acquaintance on the street, limiting the possibility of speaking to others, neutralizing visual stimulation in the street and synchronizing one's steps to musical rhythms are just some of the tactics allowed when listening to headphones. The stride gives priority to musical rhythms, provides partial autonomy to the urban temporality and restricts occasions for diversion offered by the street. The walkman user's step remains regular in the sense that certain « exterior » sonic events are still out the user's perceptive realm. The stride substitutes the musical listening temporality over the one of the public environment.

The « gait » consists in establishing variations of walking speeds, and modulating the step according to the music. These variations are set off either by particularly intense sonic occurrences coming from the street (acceleration of the walk to escape acoustic parasites and reestablish quality headphone listening) or changes in musical tempos. In the second case, the variation is expressed by an acceleration or slowing down of the walk and can be voluntarily sought by the walking listener (programming the gait in particular of joggers). The act of listening may concern the urban soundscape or the musical programs on the walkman, but in both cases the gait transposes the rhythmic qualities of sonic time at the level of the walking expression of the listener.

The « style » consists of composing figures through the help of gestures, putting movements into form in a way that makes them aesthetic. The walking listener decomposes the music he listens to in rhythmic sequences and in sonic flows and makes up for it through gestures. By reacting in this way, he provides an original form of getting around -sort of a inbetween walking and dancing -and thus metamorphoses the place traveled through just as a result of his presence. The street becomes an urban scene in which an improvised choreographic scene takes place more or less exhibited and remarked. The style configures the urban space by partially defunctionalizing the act of getting around and by establishing an intertwining of the bodily rhythms of the passer-by and the musical rhythms he hears.

The « detour » consists in increasing the travel distance in regards to the most direct route going from one place to another. It can be revealed through various practices : avoiding noisy areas which prevent headphone listening, choice of a route with the least amount of social visibility which enables the dance-walk, backtracking when the walkman user gets lost in his music. Whether freely chosen or not, the detour traces a winding itinerary in terms of the expressive and perceptive behavior of the walkman user or in terms of his awareness of his surrounding social environment. By indirectly or directly taking paths linked to headphone listening, the walking listener retraces his own urban cartography, he demonstrates it as he maps it out.

The « shortcut » consists in shortening effectively or subjectively the time of the walk, in taking the shortest route to get from one point to the next. While it may be contrary to the detour, it does not necessarily involve contrary elements. The walking listener may of course decide to go home by the most direct route when the battery of his walkman is low, but he can also be so absorbed in listening that he doesn't notice the time spent for his stroll. Forgetting part of the trip subjectively reduces the duration of the trip and offers a time-saving element in regards to commuting time. By concentrating on the experience of the walkman listener's sonic time, the shortcut miscounts the clocklike time in favor of musical dynamics. This typology makes it possible to specify the resources available to the walkman listener to show others how he appropriates and redefines urban territory in terms of headphone listening. Through his body expressions and through his organization of itineraries, the passer-by renders public and accessible to others these ways of navigating through the city.

He recounts his musical perception of the urban space in his walks like Stillman, in the City of Glass by Paul Auster, draws the letters from a coded message through the movement of his steps…

The sensibilization of urban life

The walkman functions like a technological device which introduces the senses within the urban territory. The thresholds, knots, and arrangements it creates need to be apprehended phenomenologically, according to the variable combinations of visibility and audibility. To conclude, we wish to show -based on musical nomadism -how new modes of appearance and behavior in public are constituted.

A spacing regulating device. The use of the walkman in urban environments is linked to the art of behavior which sets up intervals and regulates spacing. Headphones are what we could call a « primary earshell », in other words, an envelope which becomes part of the listening condition of the passer-by, but can be also taken off anytime. The walking listener filters the information delivered by the urban environment and plays with the perceptual orientation possibilities offered to him. He can adopt diverse points of view and listening not entirely based on the position of his body in space but also on the modalities of headphone listening. This technical mediation guaranties a diversity of perspectives, a moving off centre of the self. It orchestrates diverse types of distancing by operating a separation of the eye and the ear. This active reconfiguration of perspectives by the walkman listener contributes to the possible creation of a common public space : « the common world is over once we view it from only one point of view, when it is only allowed to present itself from a single perspective » (Arendt, 1961).

A mobile operator of configurations. Through his meanderings, his stride and his gait, the walking listener shapes the urban space without necessarily providing a reason for his actions.

By keeping the content of his headphone listening a secret, he hides elements of a pragmatic context which would enable the observer to give meaning to the walkman listener's behavior.

But again, by demonstrating diverse types of instability of perceived forms, the walking listener questions the expressive power of appearance. It is not really a question of revealing perceptible forms but rather of showing how these forms are made and then unmade : morphogenesis when gestures and steps adjust to musical rhythms, anamorphosis when sonic emissions are filtered by headphones…The difficult problem of the relationship between sensoriality and intelligibility is introduced. The walking listener instantly invites us to respond to Karl Mannheim's already ancient request : « the most obvious task that we must accomplish is to be able to think at the very base of frames of mobile reference ».

  precarious balance it creates between what he hears and what he travels through, between what he sees and what he listens to, between what he percieves and what he expresses, reveals

BIBLIOGRAPHY

  ARENDT, H. (1961) Condition de l'homme moderne. Paris, Calmann-Lévy BULL, M. (2000) Sounding Out the City. Oxford, Berg CAVE, F. et COTTEN, C. (1984) Investigation préliminaire d'une modification volontaire de l'environnement sonore et social. L'exemple du « walkman ». Paris, Ministère de l'Environnement CHAMBERS, I. (1986) Popular Culture. The metropolitan experience. London and New York, Methuen & Co.


Junseop Byeon

Ho Cho

Junke Jiang

Jihun Jang

Claudine Katan

Jacky Even

Jun Xi
email: jun.xi@xjtu.edu.cn

Mansoo Choi
email: mchoi@snu.ac.kr

Yun Seog Lee

Structural isomer of fluorinated Ruddlesden-Popper perovskite towards efficient and stable 2D/3D perovskite solar cells

Keywords: structural isomer, Ruddlesden-Popper perovskite, 2D perovskite, stability, intermolecular stacking ToC figure

Two types of structural isomers, meta-and para-CF3 groups linked to phenethylammonium iodide (PEAI), were systematically investigated as interface modifiers. Compared to para-CF3-PEAI, the meta-CF3-PEAI based perovskite is characterized by shorter interlayer distances, different surface dipoles and band alignments. The molecular substitution leads in turn to the enhancement of the vertical charge transport and to a superior suppression of non-radiative recombinations. meta-CF3-PEAI as modifier on the 3D perovskite absorber results in an improvement of the power conversion efficiency to 22.4% as well as enhanced stability against humidity and light illumination.

Introduction

Organic-inorganic hybrid halide perovskites based solar cells (PSC) have demonstrated a power conversion efficiency (PCE) that exceeds 25% from polycrystalline thin-films due to their defect tolerance, long carrier diffusion length, high optical absorption coefficients, and tunable bandgap. 1- 5 Despite the remarkable progress in PCE, there are still challenges remaining before PSCs can be commercialized, mostly related to long-term stability. In general, oxygen and humidity have been considered as the key external sources for degradation. [START_REF] Aristidou | Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells[END_REF][START_REF] Yun | Humidity-Induced Degradation via Grain Boundaries of HC(NH2)2PbI3 Planar Perovskite Solar Cells[END_REF] Even when stored in an inert atmosphere, PSCs undergo degradation under long-term light illumination condition. [START_REF] Domanski | Systematic investigation of the impact of operation conditions on the degradation behaviour of perovskite solar cells[END_REF][START_REF] Li | Microscopic Degradation in Formamidinium-Cesium Lead Iodide Perovskite Solar Cells under Operational Stressors[END_REF] Previous studies have revealed that the instability of PSC originates from trapped charges and ion migration which are relevant to the defects in the perovskite layer, such as those related to interstitial iodide and undercoordinated Pb 2+ . [START_REF] Cho | Investigation of Defect-Tolerant Perovskite Solar Cells with Long-Term Stability via Controlling the Self-Doping Effect[END_REF][START_REF] Motti | Controlling competing photochemical reactions stabilizes perovskite solar cells[END_REF][START_REF] Kwak | An atomistic mechanism for the degradation of perovskite solar cells by trapped charge[END_REF][START_REF] Ahn | Trapped charge-driven degradation of perovskite solar cells[END_REF][START_REF] Wu | Quantifying the Interface Defect for the Stability Origin of Perovskite Solar Cells[END_REF] The migration of defects can easily be stimulated by excess charges, a high electric field, and light illumination, resulting in material decomposition as well as degraded device performances. [START_REF] Motti | Controlling competing photochemical reactions stabilizes perovskite solar cells[END_REF][START_REF] Kwak | An atomistic mechanism for the degradation of perovskite solar cells by trapped charge[END_REF][START_REF] Ahn | Trapped charge-driven degradation of perovskite solar cells[END_REF][START_REF] Byeon | Charge Transport Layer-Dependent Electronic Band Bending in Perovskite Solar Cells and Its Correlation to Light-Induced Device Degradation[END_REF][START_REF] Di Girolamo | Ion Migration-Induced Amorphization and Phase Segregation as a Degradation Mechanism in Planar Perovskite Solar Cells[END_REF][START_REF] Shao | Grain boundary dominated ion migration in polycrystalline organic-inorganic halide perovskite films Energy[END_REF] Thus, mitigating the defect-related degradation of the perovskite layer in PSC is highly desirable to improve the long-term stability. [START_REF] Jeong | Pulsatile therapy for perovskite solar cells[END_REF][START_REF] Niu | Interfacial Engineering at the 2D/3D Heterojunction for High-Performance Perovskite Solar Cells[END_REF][START_REF] Zheng | Managing grains and interfaces via ligand anchoring enables 22.3%-efficiency inverted perovskite solar cells[END_REF] Quasi-two-dimensional Ruddlesden-Popper perovskites (RPP) ((A')2(A)n-1PbnX3n+1, where n is the number of octahedral slab layers; A' is the spacer ligand; A denotes small cations such as CH3NH3 + , CH(NH2)2 + , and Cs + ; and X is halide anion) are emerging due to their enhanced stability against oxygen and humidity . [START_REF] Mao | Two-Dimensional Hybrid Halide Perovskites: Principles and Promises[END_REF][START_REF] Tsai | Highefficiency two-dimensional Ruddlesden-Popper perovskite solar cells[END_REF][START_REF] Xi | Abnormal spatial heterogeneity governing the charge-carrier mechanism in efficient Ruddlesden-Popper perovskite solar cells[END_REF][START_REF] Xi | Alternative Organic Spacers for More Efficient Perovskite Solar Cells Containing Ruddlesden-Popper Phases[END_REF] The periodic intercalation of long spacer ligands into inorganic slabs can passivate defects as well as impede the migration of defects by separating the 3D perovskite layers.

However, long spacer ligands can also reduce electrical conductivity across the 2D perovskite layers, thereby often resulting in a limited PCE when using a pure RPP layer as a light-absorber. 24- 28 Instead, a bilayer configuration of 2D/3D perovskite that integrates a RPP layer in tens of nanometers thickness as an interfacial layer between a 3D perovskite layer and a hole-transporting layer (HTL) has been successfully employed, demonstrating significant enhancements of both the performance and stability of PSCs. [START_REF] Grancini | One-Year stable perovskite solar cells by 2D/3D interface engineering[END_REF][START_REF] Jang | Intact 2D/3D halide junction perovskite solar cells via solid-phase in-plane growth[END_REF][START_REF] Kim | Optimal Interfacial Engineering with Different Length of Alkylammonium Halide for Efficient and Stable Perovskite Solar Cells[END_REF] Most progress in 2D/3D perovskite solar cells thus far has been related to the engineering of the opto-electronic properties of RPPs by changing the type of spacer ligands, which typically comprise conjugated oligomers, aliphatic ammoniums, and single aromatic rings. [START_REF] Thrithamarassery Gangadharan | Searching for stability at lower dimensions: current trends and future prospects of layered perovskite solar cells[END_REF] Among the many types of organic spacers, organic ligands with fluorinated functional groups have shown enhanced hydrophobicity of RPPs, further improving the stability of the 2D/3D PSCs. [START_REF] Liu | Ultrahydrophobic 3D/2D fluoroarene bilayer-based water-resistant perovskite solar cells with efficiencies exceeding 22%[END_REF][START_REF] Zhou | High-Performance Perovskite Solar Cells with Enhanced Environmental Stability Based on a (p-FC6H4C2H4NH3)2[PbI4] Capping Layer[END_REF][START_REF] Zhou | Fluoroaromatic Cation-Assisted Planar Junction Perovskite Solar Cells with Improved VOC and Stability: The Role of Fluorination Position[END_REF] In fact, the fluorinated functional molecules with ammonium terminal have been utilized to take various effects on improving the device performance. For instance of 2D/quasi-2D perovskites, fluorinated molecule, 4-fluorophenethylammonium, is found to control the phase distribution of RPP and enhance charge transport. [START_REF] Zhang | Enhanced Charge Transport in 2D Perovskites via Fluorination of Organic Cation[END_REF][START_REF] Shi | Fluorinated Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells with over 17% Power Conversion Efficiency and Improved Stability[END_REF][START_REF] Hu | Synthetic control over orientational degeneracy of spacer cations enhances solar cell efficiency in two-dimensional perovskites[END_REF] Furthermore, fluorinated molecule,

pentafluorophenethylammonium, efficiently passivates the trap states at surfaces and grain boundaries of 3D perovskite without forming 2D phases, improving thermal stability. [START_REF] Pitaro | Tin-Lead-Metal Halide Perovskite Solar Cells with Enhanced Crystallinity and Efficiency by Addition of Fluorinated Long Organic Cation[END_REF] Hence, designing fluorinated functional spacers plays significant role in formation of 2D/3D structure. [START_REF] Pitaro | Tin-Lead-Metal Halide Perovskite Solar Cells with Enhanced Crystallinity and Efficiency by Addition of Fluorinated Long Organic Cation[END_REF] Several studies have proposed organic cations with multiple fluorine atoms such as trifluoromethyl (CF 3 -) groups linked to an aromatic ring. [START_REF] Cai | Graded 2D/3D (CF3-PEA)2FA0. 85MA0.15Pb2I7/FA0.85MA0.15PbI3 heterojunction for stable perovskite solar cell with an efficiency over 23.0%[END_REF] 2-CF 3 -PEAI (2-trifluoromethylphenylethylamine hydroiodide) can be used as a trap deactivator for perovskite solar cells, improving performance and stability [START_REF] Zhou | 2-CF3-PEAI to eliminate Pb0 traps and form a 2D perovskite layer to enhance the performance and stability of perovskite solar cells[END_REF] . In addition to the category of functional group, the position of the functional group is also found to be crucial because fluorination with different positions in the aromatic ring affects the distribution of electron clouds, which can change the dipole moment and affect the crystal structure of the 2D perovskite layers. [START_REF] Shi | Fluorinated Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells with over 17% Power Conversion Efficiency and Improved Stability[END_REF][START_REF] Qiu | Tuning the Interfacial Dipole Moment of Spacer Cations for Charge Extraction in Efficient and Ultrastable Perovskite Solar Cells[END_REF][START_REF] Liu | Tuning structural isomers of phenylenediammonium to afford efficient and stable perovskite solar cells and modules[END_REF] Moreover, previous studies reported that the stacking and orientational modes of the crystal are influenced by the fluorination position, even with the same chemical formula, resulting in different abilities of the defect passivation and charge transport properties. [START_REF] Zhou | Fluoroaromatic Cation-Assisted Planar Junction Perovskite Solar Cells with Improved VOC and Stability: The Role of Fluorination Position[END_REF][START_REF] Hu | Synthetic control over orientational degeneracy of spacer cations enhances solar cell efficiency in two-dimensional perovskites[END_REF] Therefore, understanding how the isomer structure affects the optoelectronic characteristics of the RPP interface layer can provide a pathway by which to realize the full potential of the RPP interface layer for highly efficient and stable PSCs.

Herein, we systematically investigate the effects of novel steric fluorine-rich groups on the organic ligands of phenethylammonium (PEA) and the assembly of their mono-layered (n = 1)

RPPs as interface layers on 3D perovskite to enhance device performance and stability levels.

Results and Discussion

As Interestingly, the m-CF3 RPP exhibits a larger lattice constant along the out-of-plane orientation than that of the p-CF3 RPP. The different spacings between the layers suggest a difference in the stacking modes originated from the altered direction and the magnitude of the hydrogen bond or van der Waals interaction between the two organic spacers. [START_REF] Hu | Synthetic control over orientational degeneracy of spacer cations enhances solar cell efficiency in two-dimensional perovskites[END_REF][START_REF] Liu | Tuning structural isomers of phenylenediammonium to afford efficient and stable perovskite solar cells and modules[END_REF][START_REF] Wei | Synthesis, optical properties and photostability of novel fluorinated organic-inorganic hybrid (R-NH3)2PbX4 semiconductors[END_REF] In addition, Cross-sectional scanning electron microscopy (SEM) images of indium-tin-oxide (ITO) / RPP / Au samples, prepared by spin-coating of precursors with equimolar concentrations of 1 M, show the formation of layered 2D plates (SI Figure S2a). The m-CF3 RPP sample has a thickness of 1.0 µm, which is thicker the p-CF3 RPP sample's thickness of 0.48 µm.

The origin of the different inter-layer spacing values originated from the fluorination position on the intermolecular interactions and physical properties are investigated by density-functional theory (DFT) simulations. SI Figure S3 shows schematics of the simulated intermolecular ordering for both single-crystalline m-CF3 RPP and p-CF3 RPP, describing the intercalation of bulky organic spacers into the 2D sheets of [PbI6] 4-octahedra. The relative binding energy between the adjacent couple m-CF3 RPP layer (6.82 meV Å -2 ) is calculated to be lower than p-CF3 RPP (10.08 meV Å -2 ), indicating the interlayer binding strength of m-CF3 RPPs is smaller. The DFT calculation results show that the benzene rings within the m-CF3 RPPs have a more favorable stacking mode. [START_REF] Zhang | Enhanced Charge Transport in 2D Perovskites via Fluorination of Organic Cation[END_REF][START_REF] Passarelli | Enhanced Out-of-Plane Conductivity and Photovoltaic Performance in n = 1 Layered Perovskites through Organic Cation Design[END_REF] The differences in the intermolecular stacking modes can originate from the relatively distinct interactions between multiple fluorine (F•••F) groups on the terminals of the benzene moieties (SI Figure S3). Similar to the result from the XRD measurement, the simulated values of (002) plane lattices constant along the layer stacking direction of m-CF 3 (18.57 Å) is greater than that of p-CF3 RPPs (17.88 Å), correlating with the estimated lattice distance result from the XRD patterns.

More importantly, the calculated charge density difference between organic and inorganic regions shows evident hybridization between m-CF3, while it is negligible for p-CF3 (SI Figure S3b). CF3 and p-CF3 RPP, respectively. Using the equation ∆𝐸 $ %&' = -1.809 ×10 [START_REF] Kwak | An atomistic mechanism for the degradation of perovskite solar cells by trapped charge[END_REF] ∆𝑝, 71 the maximum of the valence band m-CF 3 RPP is predicted to lie 0.40 eV higher than that of p-CF 3 RPP. This agrees well with the experimentaltrend of the ionization energies, with a value for m-CF 3 layered perovskite found 0.30 eV smaller than that of p-CF 3 layered perovskite (5.5 eV vs. 5.8 eV). The calculated present dipole moment (1.3 D) of m-CF3 localized in 2D structure is found to be higher than that (0.3 D) of p-CF 3 from structural formula using DFT calculations, which can facilitate the dissociation of photo-generated excitons (SI Table S1). [START_REF] Shi | Fluorinated Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells with over 17% Power Conversion Efficiency and Improved Stability[END_REF] The larger molecular dipole of m-CF 3 may however lead to a smaller surface dipole, since this quantity depends on the alignment of the molecular dipole with the stacking axis perpendicular to the surface. 71 Additionally, as shown in SI Figure S7, the dielectric constant of m-CF 3 RPP is found to be only slightly larger than that of p-CF 3 (4.28 vs 4.02), while the dielectric constant of sole spacers are almost the same. Therefore a large difference in exciton binding energies is not expected, consistently with experimental observations (Table 1).

The carrier transport properties of the RPPs in out-of-plane orientation were measured by using conductive atomic force microscopy (C-AFM). Both m-CF 3 RPP and p-CF 3 RPP samples were prepared by spin-coating precursor solutions of 1 M concentration on ITO-coated glass substrates to apply a voltage bias to RPPs in out-of-plane orientation between an AFM tip and the ITO layer. The PL peaks of the m-CF 3 RPP and p-CF 3 RPP are measured to be at 2.51 and 2.30 eV, respectively. The PL peaks are highly correlated with the exciton peaks (E s1 ) of 2.57 and 2.37 eV from the optical absorbance spectra (SI Figure S2b), which suggests the formation of strongly bonded excitons produced from typical quantum-well-like regimes in the RPP structures. In addition, the continuum edges of the m-CF 3 and p-CF 3 RPPs are estimated to exist at 2.83 and 2.65 eV, respectively (Figure 1e,f, Table 1). The exciton binding energy levels (E b ) of the m-CF 3 RPP and p-CF 3 RPP are 260 and 280 meV, respectively, as determined from the continuum edge by subtracting the exciton peak energy (Table 1). This difference in the E b can also contribute to a lower recombination of photogenerated carriers in the 2D/3D stacked heterostructure, as predicted from the calculated dipole moments localized in the 2D structure.

The m-CF 3 RPP and p-CF by enhancing charge transport out-of-plane direction to the substrate plane. [START_REF] Bouduban | Crystal Orientation Drives the Interface Physics at Two/Three-Dimensional Hybrid Perovskites[END_REF][START_REF] Liu | In Situ Observation of Vapor-Assisted 2D-3D Heterostructure Formation for Stable and Efficient Perovskite Solar Cells[END_REF][START_REF] Zhou | Highly Efficient and Stable Solar Cells Based on Crystalline Oriented 2D/3D Hybrid Perovskite[END_REF] Furthermore, out-ofplane GI-WAXS profiles show that the m-CF3 treatment has a stronger effect on reducing the PbI2 phases and enhancing the 3D phases compared to the p-CF3 case, an outcome also correlated with the GI-XRD measurements (Figure 2c and SI Figure S10). The XRD measurements of the perovskite films also show a slight decrease in PbI2 and an increase in the α-phase of the perovskites after the RPP treatments, suggesting interfacial Pb-related defect passivation (SI

Figure S11

). [START_REF] Cho | Investigation of Defect-Tolerant Perovskite Solar Cells with Long-Term Stability via Controlling the Self-Doping Effect[END_REF] Therefore, the m-CF3 RPP not only benefits the crystallinity of the perovskite film but also facilitates the monolayered (n = 1) RPP formation in c-axis parallel to the substrate with high out-of-plane charge transport

The optical absorbance spectra in Figure 3a demonstrate that both the m-CF3-and p-CF3-treated perovskites show a slight blue shift absorbance onset (4-5 nm) and an increasement in the absorbance intensity in the range of 400-550 nm after the formation of RPPs upon the 3D perovskite films. The higher absorbance intensity of the m-CF3-treated 2D/3D perovskite than the p-CF3-reated specimen is likely due to the formed relatively thicker 2D film treated by m-CF3 in an identical concentration. In addition, the photoluminescence (PL) intensity is increased after the RPP treatment. The perovskite treated with m-CF3 RPP shows the strongest PL emission, suggesting more effective passivation of non-radiative recombination centers (Figure 3b). In addition, the blue shifts in the PL peaks of both m-CF3 and p-CF3 2D/3D heterostructures also suggest filled trap states at the sub-bandgaps. [START_REF] Garai | Surface recrystallized stable 2D-3D graded perovskite solar cells for efficiency beyond 21%[END_REF][START_REF] Shao | Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells[END_REF][START_REF] Menke | Understanding Energy Loss in Organic Solar Cells: Toward a New Efficiency Regime[END_REF][START_REF] Shockley | Detailed Balance Limit of Efficiency of p-n Junction Solar Cells[END_REF][START_REF] Yang | Tailoring Passivation Molecular Structures for Extremely Small Open-Circuit Voltage Loss in Perovskite Solar Cells[END_REF] This correlates with the enhanced charge carrier lifetimes of the 2D/3D heterostructures found in the time-resolved PL (TRPL) measurements (SI

Figure S12

). Both PL measurements highlight the fact that the phase-pure (n = 1) RPPs with m-CF3 more effectively suppress the non-radiative recombination of 3D perovskite than p-CF3.

The defect passivation effect of 2D RPPs treatment is further examined by space-charge-limited current (SCLC) measurements to compare actual trap-state density in the 2D/3D perovskite thinfilms. Electron-only types of SCLC devices are prepared with a structure of ITO / SnO 2 / stacked 2D/3D heterostructure perovskite / C 60 / Au. As shown in Figure 3c, the trap-filled limit voltages (VTFL) of pristine 3D perovskite, m-CF3-treated perovskite, and p-CF3-treated perovskite specimens are estimated to be 0.133, 0.113, and 0.121 V, respectively (SI Table S2). The trap density (n t ) can be calculated from the following equation:

V TFL = en t d 3 2εε 4
, where e, d, 𝜀 4 , and ε are the elementary charge, the thickness of perovskite thin-films, the vacuum permittivity, and the dielectric constant of the perovskite thin films, respectively. The estimated n t values of the pristine 3D perovskite, the m-CF3-based 2D/3D heterostructure, and the p-CF3-based 2D/3D perovskites are 1.34 × 10 15 , 1.02 × 10 15 , and 1.17 × 10 15 cm -3 , respectively. In addition, X-ray photoelectron spectroscopy (XPS) of the 2D/3D heterostructures shows that the both RPPs have significantly reduce metallic Pb0 states with new fluorine moieties that form on the surfaces (SI

Figure S13

). It is reported that Pb0 can be generated during fabrication process and act as a nonradiative recombination center. The strong electronegativity of CF3 group could effectively eliminate metallic lead with deep level defect. [START_REF] Zhou | 2-CF3-PEAI to eliminate Pb0 traps and form a 2D perovskite layer to enhance the performance and stability of perovskite solar cells[END_REF] Consistent with previous studies, the SCLC and XPS results indicate that both CF3 isomers with different meta-and para-positions can effectively suppress deep-level traps of Pb0 at the perovskite interface by forming phase-pure (n = 1) RPPs. [START_REF] Wang | A Eu3+-Eu2+ ion redox shuttle imparts operational durability to Pb-I perovskite solar cells[END_REF][START_REF] Alharbi | Methylammonium Triiodide for Defect Engineering of High-Efficiency Perovskite Solar Cells[END_REF] The band alignments of the RPPs in 2D/3D perovskites are examined by using ultraviolet photoelectron spectroscopy (UPS). The Fermi level (EF) and the energy band alignments between the 2D and 3D perovskite layers are shown in Figure 3d To further evaluate the quality of junction and to identify the dominant recombination process in the PSCs, the ideality factors of the PSCs are calculated by extracting the slope of the VOC vs. Ln (light intensity) plot (Figure 4d). [START_REF] Du | Elucidating the Origins of Subgap Tail States and Open-Circuit Voltage in Methylammonium Lead Triiodide Perovskite Solar Cells[END_REF][START_REF] Wu | Efficient methylammonium lead trihalide perovskite solar cells with chloroformamidinium chloride (Cl-FACl) as an additive[END_REF] The PSC treated with m-CF3 RPP showed an ideality factor of 1.30, whereas the PSC treated with p-CF3 RPP showed a value of 1.53, which indicates that the m-CF3-treated PSCs are less limited by trap-assisted recombination. Additionally, the result of electrochemical impedance spectroscopy by fitting Nyquist plot shows the similar results of ideality factors in PSCs. The impedance spectroscopy proved that PSC treated recombination resistance increased in the order of pristine 3D perovskite, p-CF3-treated PSC, and m-CF3-treated PSCs (SI Figure S21). This result also indicates that the m-CF3-treated PSC effectively suppressed recombination losses, resulting in an increase of VOC and the fill-factor (FF). In addition, the built-in potential (Vbi ) values of the PSCs were obtained by a Mott-Schottky analysis using the following equation:

1

C 2 = 2 εε 0 eAN d (V bi -V)
, where C, A and Nd are correspondingly the capacitance, the active area of the PSCs, and the doping concentration. The Vbi values of the devices are estimated by determining the intercept of the linear fitted region from the C -2 vs. V plots. The Vbi values of the pristine PSC, m-CF3-RPP-treated PSC, and p-CF3-RPP-treated PSC are estimated to be 0.95, 1.00, and 0.97 V, respectively (Figure 4e).

The Mott-Schottky plots show that the introduction of the 2D/3D perovskite junction slightly increases Vbi compared with pristine device.

Furthermore, the PSCs treated with m-CF3 RPP exhibit significantly enhanced device stability against humidity, maintaining 85% of the initial PCE after 385 hours at a temperature of 25 °C and relative humidity of 65%, as shown in Figure 5a. The p-CF3-RPP-treated PSCs also shows enhanced stability against humidity, but lower stability compared to the m-CF3 RPP-treated PSCs.

The outcomes of enhanced device stability against humidity can be attributed to the hydrophobicity of the RPP with fluorinated spacer ligands. [START_REF] Zhou | Fluoroaromatic Cation-Assisted Planar Junction Perovskite Solar Cells with Improved VOC and Stability: The Role of Fluorination Position[END_REF][START_REF] Wang | A guide to use fluorinated aromatic bulky cations for stable and high-performance 2D/3D perovskite solar cells: The more fluorination the better?[END_REF] The hydrophobicity of the RPP-treated perovskites are evaluated by measuring the contact angles of water droplets on the films. After the m-CF3 RPP and p-CF3 RPP treatments on top of the 3D perovskite, the contact angles increase from 55.1° to 65.5° and 62.8°, respectively. Furthermore, the optical absorbance spectra of the m-CF3-RPPtreated film showed negligible changes after 150 hours of storage at a relative humidity of 65%, whereas the pristine 3D perovskite showed a significant decrease in the intensity across the entire wavelength range (Figure 5b). Furthermore, the m-CF3-treated PSCs show an improved stability against light illumination, maintaining 78% of the initial PSC even after 1100 hours in a N2 ambient condition under 1-Sun illumination (SI Figure S22). [START_REF] Kim | Optimal Interfacial Engineering with Different Length of Alkylammonium Halide for Efficient and Stable Perovskite Solar Cells[END_REF][START_REF] Liu | Ultrahydrophobic 3D/2D fluoroarene bilayer-based water-resistant perovskite solar cells with efficiencies exceeding 22%[END_REF]

Conclusions

In summary, two types of chemically similar but distinct structural isomers, meta-CF3 and para-CF3 groups linked to PEAI, were systematically investigated as interface modifiers. Compared to p-CF 3 RPP, the m-CF 3 RPP shows a higher interlayer molecule dipole moment, and highly oriented crystal growth with the favorable intermolecular interaction, which may enhance the dissociation of 2D excitons and the out-of-plane transport of the charge carriers to the HTL. Changes in surface dipoles may also explain the shift of band alignments, that shall affect in turn the device properties.

The formation of the phase-pure (n = 1) m-CF3 RPP on 3D perovskite film demonstrates the effective suppression of non-radiative recombination and improved carrier lifetimes owing to the passivation of uncoordinated-Pb related defects on the 3D perovskite surface. As a result, the m-CF3-RPP-treated PSCs exhibited a significantly improved PCE up to 22.4%. Furthermore, the m-CF3-RPP-treated PSCs not only show improved stability against humidity owing to the increased hydrophobicity with additional fluorine functional groups, but also excellent operational stability under 1-Sun illumination. These results suggest that the fabrication of stable PSCs while overcoming the vulnerability to humidity of 3D perovskites through fluorinated RPP spacers.

Moreover, on top of the multiple fluorine atoms on aromatic spacers, we show that the position of fluorinated groups can further improve the humidity stability of devices. This structural isomer strategy can also be applied to other types of RPP ligands, thus opening an effective pathway by which to improve the performance and stability outcomes of PSCs. Device characterization: The J-V curves were obtained by using a class-AAA solar simulator (Sol 3A 64023A, Oriel) with a sourcemeter (Keithley 2400) equipped with an AM1.5G filter. 1-Sun illumination intensity of 100 mW cm -2 was calibrated using a certified reference Si cell equipped with a KG5 window (91150-KG5, Newport). Scan voltage ranged from 0.1 to 1.2 V in forward scan and from 1.2 to 0.1 V in reverse scan. The scan rate was set as 100 mV s -1 . Active area of device was defined by using an aperture mask (0.0729 cm 2 ). All J-V characterization were conducted in a glovebox at room temperature. Humidity stability of perovskite films were performed by using humidity-controlled equipment (Rcom max20). C-AFM results were obtained to examine DC current of 2D RPPs film by using NX10 equipment (Park Systems) with cantilever tip (SD-R30-NCH, Park systems). A voltage bias of 2 V was applied to acquire DC current. The absorbance spectra of perovskite films were measured by using an UV-visible spectrophotometer (Cary 5000 UV-Vis-NIR, Agilent Ltd). A fluoroMax-4spectrofluorometer (Horiba) was used to measure steady-state PL and time resolved PL. A Xe-arc lamp (150 W) and laser with 463 nm wavelength were used as a excitation source for steady-state-and time-resolved-PL measurements, respectively. The XRD spectra were obtained by using D8-Advance (Bruker) with a monochromatic Cu-Kα radiation. GI-WAXS data were acquired using Xeuss 2.0 HR SAXS/WAXS system with an exposure dwell time of 60 s. FE-SEM (Auriga, Carl Zeiss Ltd) was used to acquire top-view SEM images and FE-SEM (Nova Nanolab, FEI) was used to obtain crosssectional SEM images. XPS and UPS were performed by using an electron spectroscopy instrument (Nexsa) with an incident radiation using the He-I peak (21.22 eV). The EQE spectra were obtained from incident photon to current efficiency test (IQE-200B, Oriel). For SCLC measurement, electrochemical workstation (Autolab320N, Metrohm) was used with a voltage range from 0 to 2 V. EIS were conducted by using Electrochemical workstation (Autolab 320N, Metrohm) with a white LED (100 mW cm -2 ) as light source. Applied frequency ranging from 10 0 to 10 6 Hz and bias voltage ranging from 0.0 to 0.8 V with voltage steps of 0.2 V were used during EIS measurement. To evaluate the hydrophilicity of the prepared perovskite film, contact angle measurement (Phoenix-150, SEO) was carried out using 40 μl of water. The operational stability test under 1 Sun illumination was performed using a reliability measurement system (K3600,

EXPERIMENTAL SECTION

Chemicals

McScience) which repeatedly measure the PCE every 5 min.

DFT Calculations: Structure optimizations were performed with DFT calculations by using the Vienna ab-initio simulation package (VASP). [START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF][START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF] The PBE functional within generalized gradient approximation (GGA) was used for all calculations [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] The outermost I 5s, 5p; N and C 2s, 2p; H 1s; Pb 6s, 6p, 5d; F 2s, 2p electrons are treated as valence electrons whose interactions with the remaining ions are modeled by pseudopotentials generated within the projector-augmented wave (PAW) method. [START_REF] Blöchl | Projector augmented-wave method[END_REF] An energy cutoff for the plane-wave basis set is set to 500 eV. A 2 × 4 × 4 kpoint mesh generated by Monkhorst-Pack scheme is used for all relaxation and self-consistent calculations. Energy and force convergence criteria are set as 0.01 meV and 20 meV Å -1 , respectively, to ensure the structure is fully relaxed and converged. To treat the interaction of organic spacer cations, the van der Waals interaction was taken into account with the non-local correlation functional rev-vdW-DF2. [START_REF] Hamada | van der Waals density functional made accurate[END_REF] The positions of the atoms, the volume of crystal cells, and volume shape were allowed to relax during the structural optimization. The following formula could be used to calculate layer binding energy (Eb), 𝐸 & = 6 789 : 1 6 ;<=> (:)

A

, where E iso (n) represent the energy of the unit cell of an isolated slab contains n-layers of RPPs in vacuum, E bulk (n) is the energy of the unit cell of n-layers of RPPs. [START_REF] Jung | A Rigorous Method of Calculating Exfoliation Energies from First Principles[END_REF] A is the in-plane area of the bulk unit cell. The larger E b indicate the stronger binding strength.

The calculations which involved slabs were performed within the DFT as implemented in the SIESTA package with a basis set of finite-range numerical atomic orbitals. [START_REF] Soler | The SIESTA method for ab initio order-N materials simulation[END_REF] Norm-conserving

Troullier-Martins pseudopotentials were used for each atomic species to account for the core electrons. 69 1s 1 , 2s 2 2p 2 , 2s 2 2p 3 , 2s 2 2p 5 , 5s 2 5p 5 , and 5d 10 6s 2 6p 2 were used as valence electrons for H, C, N, F, I, and Pb, respectively. A polarized double-zeta basis set with an energy shift of 200 meV and a real space mesh grid energy cutoff of 300 Ry were used for the calculations. For band structure calculation on top of optimized bulk structures, the GGA-PBE functional combined including spin-orbit coupling (SOC) with on-site approximation are used. The Brillouin zone was sampled with a 4 × 8 × 8 and 4 × 4 × 1 Monckhorst-Pack grid for bulk and slab systems, respectively. For the high-frequency dielectric constant profiles, slabs based on the respective systems were constructed, and an electric field of 0.01 eV/Å was applied along the [001] direction to compute the relaxed electron densities, as described elsewhere. For polarization density profiles, [START_REF] Traore | A Theoretical Framework for Microscopic Surface and Interface Dipoles, Work Functions, and Valence Band Alignments in 2D and 3D Halide Perovskite Heterostructures[END_REF] we used single-point calculations with GGA-PBE functional including SOC on top of rev-vdw-DF2 optimized structures. The fully relaxed bulk structures were used to construct the slabs with three repeated unit cells, as shown in SI Figure S5.

ASSOCIATED CONTENT

Supporting Information

NMR data, additional SEM images, DFT optimized atomistic structures, ADF simulated molecular structure, XRD pattern, XPS, UPS spectra, device performance depending on concentration of organic salt Light stability data (PDF)

  2D phase-pure (n = 1) RPPs with different fluorinated CF3-PEA cations, 2-(3trifluoromethylphenyl)ethanamine and 2-(4-(trifluoromethyl)phenyl)ethanamine, are prepared.Interestingly, despite the similar inorganic framework, two similar RPPs possess different optoelectronic properties and dissimilar intermolecular stacking modes between the adjacent organic cations. Specifically, the 2-(3-trifluoromethylphenyl)ethanamine based 2D structure (n = 1) exhibits enhanced inter-layer hybridization, larger dipole moment, and enhanced out-of-plane conductivity. Accordingly, the out-of-plane charge transport in such 2D structure (n = 1) can be more favorable. By forming highly oriented monolayered RPPs on 3D perovskites, the effects of a structural isomer on their crystallinity and electronic band alignments for hole transfer are investigated to understand how the fluorinated isomers affect suppression of non-radiative losses and improvement of the carrier lifetimes in PSCs. Furthermore, 2D/3D PSCs are fabricated with RPPs, demonstrating an improved PCE as well as enhanced device stability against humidity, retaining 85% of the initial PCE after 385 hours at a relative humidity of 65% without encapsulation.

  a model system for the structural isomer of fluorinated spacer, meta-CF3 and para-CF3 groups anchored to PEAI molecules are prepared to form PEAI-based RPPs. To synthesize were both dissolved in methanol. Aqueous solutions of HI were added dropwise in the precursors to induce an additional reaction with the amine functional group.

Figure 1a shows the

  Figure1ashows the chemical structures of the precipitated iodide salts, in this case meta-CF3

  Slab models were further constructed to study the polarization profiles along the stacking axes (SI Figure S5), the polarization density profiles (SI Figure S6) were calculated to evaluate the surface dipole density 𝑝 of each 2D RPP, equal to 4.84 × 10 -11 C/m and 5.19 × 10 -11 C/m for m-

Figure 1c ,

 1c Figure 1c, 1d and SI Figure S8 show profiles of measured current and surface topology,

  Figure S9). Grazing-incidence wide-angle X-ray scattering (GI-WAXS) measurements were

  Figure S17, and S18, statistical histograms of the device performances after the m-CF3 and p-CF3

  Materials synthesis: All the chemicals were used without further purification. 15 mL of (2-(3trifluoromethylphenyl)ethanamine and (2-(4-trifluoromethylphenyl)ethanamine were firstly dissolved in 200 mL of methanol in a flask in an ice bath, respectively. Then 20 mL of HI aqueous solutions (55 wt%, Sigma Aldrich) were dropped under continuous stirring. After reaction for 6 hr, the solvents of the obtained solutions were removed by vacuum rotary evaporation. The crude precipitates were gradually washed by cold diethyl ether for several times until white powders formed. Finally, both powders were completely dried in vacuum at 65 °C overnight.Device fabrication: Patterned ITO/glass substrates (AMG Inc.) were sequentially sonicated by using acetone, isopropanol, and deionized water. The cleaned substrates were treated by O2 plasma for 5 min. A SnO2 solution were prepared by diluting SnO2 colloidal solution (15% in H2O colloidal dispersion, Alfa Aesar) with deionized water (1:4 of volume ratio). The SnO2 solution were then spin coated at 4000 rpm for 30 s, followed by an annealing at 100 ℃ for 10 min. Before coating perovskite solution, the substrates with SnO2 were treated by UV-ozone for 15 min. A precursor solution of perovskite was prepared by adding 949.47 mg of FAPbI3, 37.814 mg of MAPbBr3, 98.4 mg of PbI2, 13.3 mg of CsCl , 19 mg of MACl in 263 μl of DSMO and 1050 μl of DMF. The perovskite films were fabricated by spin-coating perovskite solution on the SnO2 at 1000 rpm 10 s and 5000 rpm for 25 s. The chlorobenzene (CB) was dripped on the substrate at the end of 10 s.The substrates were annealed at 150 ℃ for 10 min and were subsequently annealed at 100 ℃ for 10 min. The samples were cooled down to room temperature. For interface treatment with RPPs, 3 mg of m-CF3-PEAI in 1ml of IPA solution and 3 mg of p-CF3-PEAI in 1 ml of IPA solution were prepared, respectively. The substrates were pre-heated at 75 ℃ for 5 min. The prepared solutions were poured on the pre-heated substrates at 5000 rpm for 20 s and then annealed 100 ℃ for 10 min. For HTL, Spiro-MeOTAD solution was prepared by dissolving 128 mg of Spiro-MeOTAD in 1.3 ml of chlorobenzene. 50.4 μl of 4-tert-butyl pyridine, 29 μl of Li-TFSI solution (520 mg of Li-TFSI in 1ml of acetonitrile) and 12.6μl of Co-TFSI solution (375mg of FK209 Co-TFSI in 1 ml of acetonitrile) were added into prepared Spiro-MeOTAD solution as a dopant. The HTL was deposited on the perovskite film with 2D/3D heterostructure by spin-coating the Spiro-MeOTAD solution at 3000 rpm for 30 s. The Finally, the substrates were moved to a vacuum chamber for evaporation deposition of 50 nm Au films at a rate of 0.3 Å s -1 . For encapsulation, the samples were moved to a N2-filled glove box and encapsulated using UV-curable resin (Nagase Inc.) and cover glass. For the flexible devices, bare glass substrates were treated with O2 plasma to remove residual organic contaminant and improve wettability. PDMS layers (Dow Corning, Sylgard 184) were spin-coated to a O2 plasma treated glass substrate. The PDMS coated substrates were annealed at 70 °C for 10 min and post-cured for 24 hours at 70°C. ITO / 125 μm-thick polyethylene(naphthalate) (PEN) films (Peccel) were attached to PDMS-coated glass. For fabrication process of SnO2 and perovskite films, the samples were annealed at 135 °C to avoid damage of flexible substrate.

Figure 1 .

 1 Figure 1. (a) Molecular structure of m-CF3 PEAI and p-CF3 PEAI, respectively. (b) XRD spectra

Figure 2 .

 2 Figure 2. (a) GI-WAXS measurements and (b) Azimuthal angle scans for (002) peak of m-CF3

Figure 3 .

 3 Figure 3. (a) UV-VIS absorbance spectra and (b) steady-state PL spectra of pristine 3D perovskite

Figure 4 .

 4 Figure 4. (a) Statistical histogram of J-V characteristics of perovskite solar cells with pristine, m-

Figure 5 .

 5 Figure 5. (a) Stability of perovskite solar cells against humidity without encapsulation (25 ℃,

Figure S3 .

 S3 Figure S3. (a) DFT optimized atomistic structures and (b) charge density difference between the

Figure S4 .

 S4 Figure S4. Electronic band structures of the m-CF 3 and p-CF 3 layered perovskites computed at the PBE level. The x-axis shows the labels of the high symmetry points of the P21/c space group Brillouin zone.

Figure S5 .

 S5 Figure S5. Slab model structures considered for the evaluation of the polarization density profiles along the stacking axes of the m-CF 3 and p-CF 3 layered perovskites. The slabs contain three perovskite layers. The surface layers within the dashed rectangular boxes are allowed to relax while the central bulk-like regions are kept frozen to the positions of the optimized bulk structures. During structural optimization, only the atomic positions were allowed to relax.

Figure S6 .

 S6 Figure S6. Polarization density profiles of the m-CF 3 and p-CF 3 layered perovskites along the stacking axes. 1 The surface dipole densities 𝑝 extracted from the polarization density profiles are equal to 4.84 × 10 -11 C/m and 5.19 × 10 -11 C/m for m-CF 3 and p-CF 3 layered perovskites, respectively. By using the equation ∆𝐸 $ %&' = -1.809 ×10 12 ∆𝑝, the energy of the valence band maximum of the m-CF 3 layered perovskite is predicted to be larger by 0.40 eV than the one of the p-CF 3 layered perovskite.

Figure S7 .

 S7 Figure S7. Dielectric profiles computed from slab models 2 for (a) m-CF 3 and p-CF 3 layered perovskites and (b) the sole spacers m-CF 3 -PEA (red) and p-CF 3 -PEA (blue). In the later case, the nitrogen atom are replaced by carbon to avoid charged unit cells.

Figure S9 .Figure S10 .

 S9S10 Figure S9. Surface SEM images of the pristine 3D perovskite films, m-CF 3 PEAI treated, p-CF 3

Figure S11 .

 S11 Figure S11. XRD spectra of pristine 3D perovskite films, m-CF 3 PEAI treated, p-CF 3 PEAI treated

Figure S12 .

 S12 Figure S12. Time resolved PL spectra of pristine 3D perovskite films, m-CF 3 PEAI treated, p-CF

Figure S13 .Figure S14 . 4 PristineFigure S15 .Figure S16 .Figure S17 .

 S13S144S15S16S17 Figure S13. XPS spectra of (a) Pb-4f core-level spectra, (b) N-1s core-level spectra and F-1s core-

2 )Figure S18 .Figure S19 .

 2S18S19 Figure S18. Device performance histograms of perovskite solar cells before anti-reflective films.

Figure

  Figure S20. J-V curves of flexible device with pristine 3D perovskite, m-CF3 PEAI treated

Figure S22 .

 S22 Figure S22. Operational photo-stability of m-CF3 PEAI treated device with encapsulation under

  Slightly enhanced out-of-plane conductivity of m-CF3 RPP can be expected in comparison to that of the p-CF3 RPP due to the favorable interlayer hybridization of the m-CF3 RPPs. The band structures of m-CF3 and p-CF3 RPP were calculated, and the band gap values were found to be 1.61

	eV and 1.56 eV, respectively (SI Figure S4). While the computed band gap values are smaller
	than the experimental values as expected from DFT calculations at the PBE level , a larger band
	gap for m-CF3 is is nevertheless also predicted by the calculations. The computed band gap values
	are sensitive to the distortions of the perovskite backbone due to the presence of the spacer
	molecules. Indeed, the electronic band gaps of 2D perovskites are affected by both in-plane (b)
	and out-of plane (d) angular distorsions, 26 which amount to 17.9° (19.4°) and 15.3° (12.2°)
	respectively, for p-CF3 (m-CF3). There is therefore a larger in-plane but smaller out-of plane
	angular distorsion for m-CF3 by comparison to p-CF3 resulting however in a net increase of the band
	gap as observed experimentally.

Table 1 .

 1 PL peak position, band gap (Eg), excitonic peak (Es1), continuum edges and exciton binding energies (Eb) driven from steady-state PL and UV-VIS absorbance spectra of RPP films

	Material	PL (eV)	Band edge (eV)	Excitonic peak (eV)	Continuum edge (eV)	E binding (meV)
	m-CF 3	2.51	2.59	2.57	2.83	260
	p-CF 3	2.30	2.39	2.37	2.65	280

Table S2 .

 S2 Capacitance (C), active area, thickness, dielectric constant, trap filled limit voltage and trap density of perovskite with and without organic ammonium iodide treatment.

	Sample	C	Active area	Thickness	ε	VTFL		Nt
		[10	-9 F]	[cm	2 ]	[nm]		[eV]	[10	15 cm	-3 ]
	Pristine	8.06	0.1825	550	27.47	0.133	1.34
	m-CF3	7.64	0.1825	580	27.46	0.113	1.02
	p-CF3	8.20	0.1825	580	29.47	0.121	1.17

Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea * Corresponding authors E-mail: mchoi@snu.ac.kr, jun.xi@xjtu.edu.cn, leeyunseog@snu.ac.kr † J.B. and S.H.C contributed equally to this paper

PEAI treated 2D/3D perovskite films.

Junseop Byeon -Department of Mechanical Engineering, Seoul National University, Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 08826, Republic of Korea Seong Ho Cho -Department of Mechanical Engineering, Seoul National University, Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea Junke Jiang -University of Rennes, ENSCR, CNRS, ISCR, UMR 6226, F-35000 Rennes, France Jihun Jang -Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 08826, Republic of Korea Claudine Katan -University of Rennes, ENSCR, CNRS, ISCR, UMR 6226, F-35000 Rennes, France Jacky Even -Univ Rennes, INSA Rennes, CNRS, Institut FOTON -UMR 6082, F-35000 Rennes, France ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program of the Center for Multiscale Energy Systems, the National Research Foundation (NRF) under the Ministry of Education, Science and Technology, Korea (2012M3A6A7054855, 2022R1A6A3A13065166, 2022M3H4A1A03074093, RS-2022-00154545, and NRF-2020R1C1C1005880) via Institute of Advanced Machines and Design at Seoul National University (SNU) and SNU Creative-Pioneering Researchers Program via Institute of Engineering Research at SNU, and the European Union's Horizon 2020 research and innovation program under grant agreement No. 861985 (PeroCUBE). J. Xi would like to acknowledge the National Natural Science Foundation of China (Grant No. 62205264), and Xi'an Jiaotong University Young Talents Support Program (11302291010704). Katan, C., Composite Nature of Layered Hybrid Perovskites: Assessment on Quantum and Dielectric Confinements and Band Alignment. ACS Nano 2018, 12 (4), 3321-3332. 71 Traore, B.; Basera, P.; Ramadan, A. J.; Snaith, H. J.; Katan, C.; Even, J. A Theoretical Framework for Microscopic Surface and Interface Dipoles, Work Functions, and Valence Band Alignments in 2D and 3D Halide Perovskite Heterostructures, ACS Energy Letters 2022, 7, 349-357.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

Supporting Information

Structural isomer of fluorinated Ruddlesden-Popper perovskite towards efficient and stable 2D/3D perovskite solar cells Junseop Byeon 1,2 † , Seong Ho Cho 1.6 † , Junke Jiang 3 , Jihun Jang 2 , Claudine Katan 3 , Jacky Even 4 , Jun Table S1. VASP simulated molecular structures, dipole moments of m-CF 3 -PEA + and p-CF 3 -PEA + .

The dipole moments are calculated by the VASP code using the PBE functional, considering the mass centers as the origin.