Dr Lukasz Baisinger
email: lukasz.baisinger@tu-dresden.de

José María

Andrés Castán

Pablo Simón Marqués

Dr Giacomo Londi

Prof. Dr Clemens Göhler

Carsten Deibel

Prof. Dr David Beljonne

Dr Clément Cabanetos

Philippe Blanchard

Dr Johannes Benduhn

Dr Pablo Simón Marqués

Prof. Dr Donato Spoltore

Karl Leo
email: karl.leo@tu-dresden.de

J M Andrés Castán

Reducing non-radiative voltage losses by methylation of push-pull molecular donors in organic solar cells

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Over the past few years, organic solar cells (OSCs) have seen a rapid increase in their performance, approaching other thin-film technologies: Recent progress in the development of non-fullerene acceptors (NFAs) allowed to push the power conversion efficiencies (PCEs) above 18%. [1] While the best performing OSCs now have external quantum efficiency (EQE) and fill factor (FF) values both exceeding 80%, [2,3] the open-circuit voltage (V oc ) is still lagging behind other thin-film technologies. Even in cases where the optical gaps are comparable, the substantial contribution of non-radiative losses in devices based on organic molecules limits the performance of organic photovoltaic devices (OPV). [3][4][5] Therefore, to further increase PCEs and thus market competitiveness of OSCs, the nature of those non-radiative voltage losses needs to be thoroughly understood.

Overall, the V oc of OSCs is dictated by several factors: the optical gap (E g opt ) of the lightabsorbing materials, the energy of the intermolecular charge-transfer (CT) state at the donoracceptor interface (E CT ), radiative voltage losses from the CT state (V r ), and non-radiative voltage losses (V nr ). The relation between those factors and the V oc of the device can be expressed as: [6][7][8] where q is the elementary charge, k B the Boltzmann constant, T the temperature, N CTC the density of CT states, G CT is the generation rate and EQE EL the external electroluminescence (EL) quantum efficiency. According to the assumptions of the detailed balance theory and reciprocity relation between emission and absorption, this formula connects the electrical properties of the V oc to the molecular CT state parameters. At 0 K, the V oc of the device is limited by E CT while at higher temperatures additional radiative and non-radiative recombination, represented by the two logarithms in Equation (1), further reduce the maximum achievable V oc. The term E CT on the right-hand side of the Equation (1) is the energy required to convert photogenerated, strongly bound electron-hole pairs (excitons) into CT states and is therefore an energy loss mechanism. The energy difference between the lowest excited singlet state of either the donor (D) or the acceptor (A) and their corresponding CT state is referred to as driving force. For a long time, it was believed that a sizable driving force is necessary to achieve efficient exciton splitting and thus a good device performance. Recent studies point to the contrary, where efficient charge carrier separation is characterized by a high internal quantum efficiency (IQE) and FF, even when the driving force is reduced down to 10 meV. [9][10][11] Following the detailed balance theory introduced by Shockley-Queisser, the V oc of an ideal solar cell depends on the light intensity, the band-gap of the semiconductor, and temperature of the device. [12] In an ideal scenario, the EQE of the device is a step-function where the EQE of photons with energies above the band-gap is equal to unity. All recombination events in the device are occurring radiatively from the lowest excited state to the ground state, and the emission spectrum is equal to the product of black-body spectrum and absorption. In a real device, the edge of the EQE spectrum is instead broadened due to the nature of the transitions in organic semiconductors and additional absorption of the CT state, which further increases V r , leading to the radiative limit for the open-circuit voltage V rad :

where J sc rad is the short-circuit current density calculated from the integrated product of the EQE and AM1.5g solar spectrum and J 0 rad is the upper radiative limit of the dark current obtained by integrating product of the EQE and black-body spectra. Radiative recombination is the reverse process to absorption and is thus an unavoidable, fundamental loss channel; [START_REF] Würfel | Physics of Solar Cells: From Basic Principles to Advanced Concepts[END_REF] it is however possible to minimize those losses, for example by reducing the interface between donor and acceptor molecules [START_REF] Vandewal | [END_REF] or by carefully selecting materials with reduced electron-phonon coupling or improved exciton dissociation [15] .

The V oc of the real device is always lower than the radiative limit due to non-radiative voltage losses, often being the dominant non-geminate recombination process, and is directly related to the radiative efficiency EQE EL : [8] where k r and k nr are the radiative and non-radiative recombination rates, respectively. Previous studies have shown that this process is inherent to organic molecules, inversely proportional to the cell's effective gap (E CT ) and mediated by carbon-carbon vibrations. [16] Despite the observation of a clear trend following the so-called energy gap law, certain devices show significant deviations, even when E CT is constant. These deviations from the energy gap law could be explained by taking into account additional parameters of the CT state, such as the oscillator strength f of the electronic transition or the reorganization energy . [16,17] In this study.

we investigate the influence of those molecular parameters on the performance of OSCs. For this purpose, we use a molecular model system allowing a systematic variation of CT state properties.

Vacuum-processed OSCs based on well-defined small conjugated molecules constitute an excellent system to study structure-properties relationships. [18,19] In this context, small pushpull D--A molecules where an electron-donating block is connected to an electron-accepting group through a -conjugated spacer have been widely investigated as promising donor materials in OPV. [20][21][22][23][24][25] For example, high-efficiency vacuum-processed OSCs using DCDCPB and its thiophenic analog DTDCTB donors were reported, showing exceptionally long exciton diffusion lengths (for the molecular structures see SI, Figure S1). [26][27][28] In these push-pull molecules, a benzothiadiazole (BTZ) -spacer connects the dicyanovinyl (DCV) electronacceptor block to two different arylamine-based donor blocks exhibiting two methyl groups at the para positions of the two external phenyl rings. As shown by the group of K-T. Wong, the presence of methyl groups leads to lower oxidation potentials, higher extinction coefficients and OSCs with higher PCE. [29] On the other hand, some of us showed that replacing one of the external phenyl rings of triphenylamine (TPA) in push-pull molecule TPA-T-DCV by a methyl group, directly connected to the nitrogen atom (MPA-T-DCV), leads to an improved photovoltaic performance of TPA-T-DCV mainly due to a significant increase in hole-mobility. [30][31][32] By combining these two different approaches, we here describe the synthesis and characterization of four novel push-pull molecules (Figure 1). They display a BTZ -spacer connecting a DCV electron-acceptor block to four different arylamine-based donor blocks. First, a N-methyl-N-phenylaniline and a N-methyl-N-thienylaniline are used leading to push-pull molecules MPA and MTA, respectively, allowing to study the impact of the thiophene ring on the electronic properties. Finally, a methyl group is introduced at the para position of the external phenyl ring of the donor blocks, resulting in derivatives Me-MPA and Me-MTA, respectively. We investigated the performance of these push-pull molecules as donor materials with C 60 as acceptor in OSCs. Using a fully-evaporated model system, we focused on how methylation at the para position of the external phenyl ring of the arylamine donor block affects the V oc of the devices.

Results and Discussion

Donor synthesis and characterization

The synthesis of the benzenic (MPA) and thiophenic (MTA) push-pull molecules are depicted in scheme 1 and scheme 2, respectively. 4-Bromo-N-methyl-N-phenylaniline derivatives 1a [33] and 1b were transformed in boronic acid (2a) and pinacol ester (2b), respectively. Compound 1b was effectively prepared by a Buchwald-Hartwig amination between N,4-dimethylaniline and 1bromo-4-iodobenzene using [1,1'-biphenyl]-2-yldicyclohexylphosphane as ligand. Compounds were then engaged in a Pd-catalyzed cross-coupling reaction with 7bromobenzo[c][1,2,5]thiadiazole-4-carbaldehyde [34] affording aldehydes 3a and 3b. A subsequent Knoevenagel condensation with malononitrile led to MPA and Me-MPA (Scheme 1). The electrochemical properties of the target molecules were analyzed by cyclic voltammetry (CV) using 0.1 M of Bu 4 NPF 6 in CH 2 Cl 2 (see SI, Figure S24). All data are summarized in Table S1 and the electrochemical potentials are expressed vs. the ferrocene/ferrocenium couple (Fc/Fc + ).

As expected for push-pull D--A systems, the UV-Vis absorption spectra of the investigated molecules in solution (in CH 2 Cl 2 ) mainly exhibit an intense and broad absorption band in the visible range attributed to an intramolecular charge transfer (ICT) from the donor to the electron-withdrawing moiety (Figure 2). While the MPA derivatives show an ICT band ranging from 450 nm to 700 nm, the absorption spectra of the thiophenic counterparts are bathochromatically shifted up to 800 nm with absorption maxima around 660 nm. Interestingly, whereas the introduction of methyl groups in Me-MPA and Me-MTA produces a slight red-shift of the ICT band (ca. 8-9 nm), it significantly increases their molar extinction coefficients indicating better absorption properties than for MPA and MTA, respectively. UV-Vis absorption spectra of thin films are shown in Figures S25-S28. The highest occupied molecular orbital (HOMO) energy levels of the push-pull molecules were estimated on thin-films from the ionization energy measured by photoelectron spectroscopy in air (PESA, see SI Figures S29-S32). These data are summarized in Table S2.

Device performance

OPV devices were built comprising 30 nm donor:acceptor (D:A) bulk-heterojunction (BHJ) active layer with 1:1 wt% D:C 60 ratio in the blend. All devices were prepared in a p-i-n (standard) architecture, with 3 nm MoO 3 and 8 nm BPhen as hole-and electron-transporting layers, respectively. From the EQE spectra, shown in Figure 3a, it can be clearly seen that a redshift in the absorption is achieved upon changing the central phenyl ring with a thiophene. As a result, more photons are harvested from the lower-energy part of the solar spectrum and the shortcircuit current density (J sc ) is increased, as shown in the current density-voltage (J-V) characteristics in Figure 3b. The photovoltaic fingerprints of the studied devices are summarized in Table 1.

Determination of the optical gap

To investigate voltage losses, E g opt for each material is determined, setting the upper limit for the V oc when assuming only radiative recombination. Here, we use the definition of E g opt , as described by Rau et al. [7] , which is directly determined from the measured EQE spectrum of the device. The derivative of the low-energy edge of EQE is interpreted as the distribution of optical gap energies P(E g ) and E g opt is defined as the mean peak energy of this distribution:

where the integration limits a and b are chosen such that P(E g ) is approximately equal to 50% of its maximum value. Since this quantity is calculated from the photovoltaic EQE spectrum, it is therefore directly related to the device performance and is the most suitable choice when investigating voltage losses of the device. [7,35] The calculated E g opt values for every device are included in Table 1. The overall short-circuit current is only slightly increased due to simultaneous decrease of the absorption in the blue-green region of the spectrum. Replacement of the phenyl ring (MPA) with thiophene (MTA) leads to a decrease in the V oc , which is in agreement with higher-energy HOMO levels, as estimated from PESA measurements. On the other hand, the V oc is increased for both methylated donors by roughly 50 mV, which could be explained by (1) decrease of E CT or (2) reduced voltage losses. In the next paragraph, detailed analysis of the energetic landscape and voltage losses are discussed to explain this observation.

Charge-transfer state properties

In OSCs, the CT state is the lowest absorbing and emitting state and therefore its properties have crucial impact on the V oc and overall performance of the device [8,36] . To better understand the energy losses in the studied devices, the low-energy tail of the EQE spectrum is measured sensitively over several orders of magnitude and fitted with Equation (5) [36] to extract CT state properties, as shown in Figure 4. This sensitively measured EQE spectrum will be referred to as EQE PV .

The product of EQE PV (E)•E is referred to as reduced EQE PV ; f equals IQE PV N CT 2df σ (d is the thickness of the light-absorbing layer and f σ the dipole coupling of the CT state to the ground state). Due to the reciprocity relation between emission and absorption of the CT state [36] , EL spectra can also be used to extract E CT and  CT :

Similarly, here the term EL•E -1 is referred to as reduced EL spectrum. It is important to note that both Equation (5) and (6) differ only by the prefactor f and f I and the sign of  CT . Setting f I = f allows to normalize both reduced spectra such that their crossing point energy is equal to the energy of the CT state. A summary of all obtained electronic parameters is listed in Table S3. It is worth pointing out that E CT for both devices in each pair is similar, especially in the case of devices MTA and Me-MTA, where the energy difference is only 4 meV. The grey curve corresponds to the EQEPV calculated from the EL spectra by assuming room temperature for the black-body spectrum. The analysis proves that the spectra are in accordance with the reciprocity relation between emission and absorption.

Investigation of the voltage losses

Since E CT is similar, the increase in the V oc for the methylated molecules can be connected to decreased voltage losses. Using E CT extracted from the fits to the EQE PV and EL spectra and E g opt , we computed the driving force E CT = E g opt -E CT to be nearly identical for both devices in each pair. The V oc in the radiative limit, calculated using Equation (2), is also similar. Now, it is possible to quantify the amount of both radiative and non-radiative voltage losses by: Electronic parameters of each device and their corresponding voltage losses are presented in Table 2. We find that the radiative limit for the V oc is similar in each pair and the radiative voltage losses change negligibly between the devices. On the other hand, the non-radiative voltage losses are reduced by approx. 50 mV for the devices based on methylated donor molecules. V nr was obtained using two independent techniques, using Equation (8) with parameters obtained from the EQE PV spectrum and directly from the EQE EL measurements (see SI, Table S4). Both methods show the same trend of decreasing V nr upon methylation of the donor molecule.

Table 2. Electronic parameters and corresponding voltage losses for the studied devices.

Device, donor E CT [eV]

E CT [eV] V rad [V] V oc [V] V r [V] V nr a [V] V nr b [V] MPA 1

Recombination dynamics

In order to gain further insight into non-radiative voltage losses, we perform transient photovoltage (TPV) decay measurements to study the recombination dynamics. All measurements are conducted within the small perturbation regime. Photovoltage decays are measured under open-circuit conditions, where no external charges flow and all photogenerated charges must eventually recombine within the device. The photovoltage decay is a well establish technique for probing the lifetime of excess charge carriers generated by a laser pulse in an optically biased device. [37,38] In our model system, we compare two sets of devices, each of which is characterized by almost identical optical gap. This implies that the number of photogenerated charge carriers should be equal between devices in each group, ensuring that the photovoltage decay resembles the recombination dynamics. Exponential decay time constants are obtained by fitting measured photovoltage transients with a monoexponential decay function (Figure S34) and were subsequently plotted as a function of the V oc (Figure S35), which allows to extract the lifetime  of free charge carriers, as shown in Figure 5. The lifetimes for each device are obtained from a fit via Equation (9):

where  is related to the temperature T, the diode ideality factor n id and the recombination order m:

As a result, the recombination time constant  0 for each device can be obtained. [38] The fitting parameters are summarized in Table S5. The decay times of free charge carriers extracted from TPV measurements confirm that the recombination is slower in devices with lower non-radiative voltage losses. The corresponding charge carrier lifetimes are higher by an order of magnitude. Similar values of  for both groups of devices (see SI, Table S5) indicate the same recombination order and diode ideality factors for both devices in each group. [START_REF] Vandewal | [END_REF]38] Figure 5. Lifetimes extracted from the TPV measurements.

Computational Methods

According to Azzouzi et al., [17] there are several molecular parameters that can be responsible for the reduction of non-radiative voltage losses, mostly related to the properties of the CT state at the intermolecular D:A interface. In our set of devices, we can rule out the influence of E g opt and E CT to be responsible. On the other hand, reorganization energies and the oscillator strength of the transition are still possible parameters accountable for the reduction of V nr. In order to determine which parameters are the most relevant for reducing non-radiative voltage losses, we perform quantum-chemical calculations at the density functional theory (DFT) level to assess the intramolecular reorganization energy  upon charging the two donors, MTA and Me-MTA, and the C 60 acceptor. For the C 60 molecule, we find a  -of 89 meV, while the two donors show similar relaxation energies  + of 137 meV for MTA and 135 meV for Me-MTA. We then perform time-dependent DFT (TDDFT) calculations on representative D:C 60 pairs. Namely, as shown in Figure 6, a cluster of C 60 molecules was built around a single MTA (Me-MTA) donor and each D:C 60 pair is optimized at the DFT level, using the same level of theory as above. Full TDDFT calculations were performed on each ground-state optimized molecular configuration in the evaluation of E CT and the oscillator strength f of the first electronic transition. Table 3 summarizes the results obtained for all the MTA:C 60 and Me-MTA:C 60 investigated configurations, respectively. We note that the CT state energies coming from TDDFT calculations are corrected by the total reorganization energy  =  + +  -. 3, as this provides the largest difference in oscillator strength between MTA and Me-MTA donors. The corresponding 3D transition dipole density surfaces are plotted in Figure 7. Figure 7 shows that the presence of the additional methyl group on the aromatic ring tends to bend the molecule over the C 60 acceptor in order to maximize  interactions. This results in a more intimate interaction of the donor molecule with C 60 that translates into a slightly larger contribution from the aromatic amine moiety of the donor to the transition density (seen as the green isosurface in Figure 7). The larger wave function overlap manifested by the transition density distribution turns into a larger oscillator strength and, according to the model proposed by Azzouzi et al., [17] accounts for reduced non-radiative voltage losses in OPV devices based on methylated versions of the molecular donors.

Conclusion

In conclusion, we study a model system based on novel push-pull, D--A molecules, used as donors in OSCs with C 60 as acceptor. We find that adding methyl groups to the donor molecule increases the V oc of the device. Utilizing highly-sensitive measurement techniques, we analyze the properties of the CT state within the devices and calculate the corresponding voltage losses.

While the amount of radiative voltage losses remains unaffected by the modification of the donor, the non-radiative voltage losses are reduced by about 50 mV. Studies of the TPV decays reveal that for methylated molecules, the recombination rate is slower. From quantum-chemical calculations, we conclude that main reason for the reduction of non-radiative voltage losses is an increase in the oscillator strength of the electronic transition, associated with a more intimate interaction at the molecular scale between donor and acceptor molecules.

Experimental Section

UV-Vis

UV-vis spectra were recorded in dichloromethane or as thin-films on indium tin oxide (ITO) substrates using a PERKIN ELMER 950 spectrometer.

Cyclic voltammetry

Cyclic voltammetry was performed using a BIOLOGIC SP-150 potentiostat with positive feedback compensation in 0.10 M Bu 4 NPF 6 /CH 2 Cl 2 (HPLC grade). Experiments were carried out in a one-compartment cell equipped with a platinum working electrode (2 mm diameter) and a platinum wire counter electrode. A silver wire immersed in a 0.01 M solution of AgNO 3 in CH 3 CN was used as reference electrode and checked against the ferrocene/ferrocenium couple (Fc/Fc + ) before and after each experiment. The potentials were then expressed vs. Fc/Fc + .

Photoelectron spectroscopy in air

Photoelectron spectroscopy in air (PESA) measurements were recorded using a Riken Keiki PESA spectrometer (Model AC-2); samples were spin-cast over ITO substrates and irradiated with a laser beam with power settings of ca. 20 nW; the electron detector allows to measure the ionization potential operating in atmospheric air.

Device preparation

Devices are prepared by vacuum thermal evaporation in a high vacuum system with a base pressure of <10 -7 mbar (Kurt J. Lesker Ltd.). All layers of materials are evaporated onto a glass substrate with a pre-structured ITO contact (Thin Film devices), cleaned in subsequent ultrasonic baths with NMP, deionized water and ethanol and ultraviolet ozone cleaning system. Prior to evaporation, all organic materials are purified by vacuum gradient sublimation. Metal contacts are deposited through a shadow mask, to form a top contact and overlap with the bottom ITO contact, thus defining the device area of 6.44 mm 2 . To avoid degradation due to external conditions, all devices are encapsulated with a glass cavity glued on the substrate together with a moisture getter. All provided materials are tested as donors against C 60 fullerene acceptor (CreaPhys GmbH), incorporated in a p-i-n (standard) device architecture, using molybdenum oxide (MoO 3 ) (Sigma Aldrich) and bathophenanthroline (BPhen) (Lumtec), as hole-and electron-transporting layers (HTL and ETL), respectively. The structure of a complete device is: ITO / MoO 3 (3) / BHJ (30, 1:1 wt%) / C 60 (15) / BPhen (8) / Ag (100) Numbers in brackets represent the thicknesses in nanometers of the respective layer.

Current-Voltage characteristics

Current-density-Voltage (J-V) characteristics are measured with a source-measurement unit (Keithley 2400, Keithley Ltd.). The sample is illuminated by a sun simulator (16S-150V.3, Solar Light Co.) with intensity adjusted such as to generate the current corresponding to an AM1.5g solar spectrum (100 mWcm -2 ), calibrated with silicon photodiode (S1337, Hamamatsu, calibrated by Fraunhofer ISE).

External Quantum Efficiency spectra

External quantum efficiency spectra are measured using light from a xenon lamp (Apex 150 W Xenon Arc Lamp, Newport Oriel) and a monochromator (Cornerstone 260 1/4m, Newport Oriel). Signal from the device is pre-amplified and recorded with a lock-in amplifier (SR 7265 Lock-In amplifier, Signal Recovery) with 0.2 s time constant. The reference spectrum is taken with a silicon reference diode (S1337, Hamamatsu, calibrated by Fraunhofer ISE).

Sensitive EQE PV measurements

To measure the sensitive EQE PV spectra, light from the 50 W halogen lamp is chopped at 143 Hz and fed into the monochromator (Cornerstone 260 1/4m, Newport Oriel). The output beam is focused onto the OSC under short-circuit condition. Current from the solar cell is pre-amplified (DHPCA-100, FEMTO Messtechnik GmbH) and analyzed with lock-in amplifier (Signal Recovery, 7280 DSP), coupled with the chopper. Time constant of the lock-in is set in the range of 0.5-2.0 s in order to resolve low-current signals and the pre-amplification is set accordingly. Final spectra are calculated by dividing the obtained photocurrent by incident photon flux, measured with calibrated silicon and indium gallium arsenide photodiodes.

Transient photovoltage measurements

TPV decays are obtained by connecting the OSC to a high-impedance (1 MΩ) input of the digital oscilloscope (DPO7354C, Tektronix Inc.). The V oc of the device is adjusted with a bias light, produced by a high-power LED (LED Engin, Osram Inc.), controlled by an external power supply (EA-PS 3032-10B, EA Elektro-Automatik GmbH). An additional light pulse, generated by picosecond Nd:YAG laser (PL2210, Ekspla), is focused on the device and disturbs the steadystate creating a voltage spike, whose decay is measured at the oscilloscope.

Electroluminescence

Relative electroluminescence spectra are recorded by liquid N 2 cooled InGaAs-and silicon-CCD-cameras (Princeton Instruments PyLoN:IR and Spec-10:100) on the exit slit of a 500 mm spectrograph (Princeton Instruments Acton 500i). Both cameras are calibrated prior to the experiment using a quartz-tungsten-halogen irradiance standard. A source-measure-unit (Keithley 2634B) is used to apply constant forward driving currents to the OSC. All spectra are recorded multiple times while driving currents are varied over 2 orders of magnitude to exclude heating effects or additional emission pathways.

Electroluminescence Quantum Yield

Absolute radiative quantum yields are determined by placing a 1 cm 2 large silicon photodiode (Thorlabs FDS1010) in close proximity (ca. 3 mm) to the solar cells emissive area. While the solar cell is held at a constant bias voltage (with the injection current measured by a Keithley 2000 DMM), the diode's photocurrent is measured with a Keithley 480 Picoammeter. The emission yield is then calculated from the photocurrent and the spectral overlap between the photodiode's EQE and the solar cell's electroluminescence spectrum, and includes a geometric mismatch factor based on the assumed Lambertian emission profile and reflection at the metal back electrodes.

Computational Methods

The effect of intramolecular structural relaxation upon charging was considered by calculating the intramolecular reorganization energy of an isolated C 60 , MTA and Me-MTA molecule. Specifically, as described elsewhere [39,40] , the intramolecular reorganization energy , averaged on the charging and discharging processes, was computed as follows:

where stands for the charge on the molecule and , indicate the equilibrium geometries of neutral and charged states. Geometries were optimized using a range-separated hybrid functional as B97X-D [41] , along with the 6-31g(d) basis set, keeping the donors molecular soft degrees of freedom (dihedral angles) frozen, as to account only for the contribution from highfrequency modes, as described elsewhere [39,40] . Donor:C 60 pairs were optimized at the groundstate DFT level using the same level of theory as described above. Energies of the CT state and oscillator strengths were computed with TDDFT calculations employing the same functional and the 6-31g(d,p) basis set. In addition, the range-separation parameter ω was optimally tuned in the presence of a polarizable continuum model (PCM) [42] setting the macroscopic dielectric constant at  = 3.00. DFT and TDDFT calculations were carried out with Gaussian16 software [START_REF] Frisch | Gaussian 16 Rev. C[END_REF] .

Figure 1 .

 1 Figure 1. Chemical structures and short names of the novel push-pull donor--acceptor molecules used in this study. The donor building blocks are highlighted in green.

Scheme 1 .

 1 Scheme 1. Synthesis of MPA-derived push-pull molecules.

Scheme 2 .

 2 Scheme 2. Synthesis of MTA-derived push-pull molecules.

Figure 2 .

 2 Figure 2. UV-Vis transmission spectra of investigated donor molecules in CH2Cl2.

Figure 3 .

 3 Figure 3. (a) EQE spectra and (b) J-V characteristics of the studied devices based on push-pull molecules (solid lines), measured at simulated AM1.5g conditions. The dashed lines represent J-V characteristics measured in the dark.

Figure 4 .

 4 Figure 4. Reduced EQEPV and EL spectra of devices based on a-b) MPA and Me-MPA and c-d) MTA and Me-MTA donor molecules, respectively. The dashed lines represent fits to the experimental EQEPV and EL spectra using equations (5) and (6), respectively. Emission and absorption of the CT state are connected by the reciprocity relation and therefore both spectra can be used to obtain ECT and CT. The intersection point of those two curves equals ECT.

Figure 6 .

 6 Figure 6. Different representations of the cluster of C60 molecules (cue balls in grey) around the MTA donor. Eight different D:C60 configurations were chosen for TDDFT calculations.

Figure 7 .

 7 Figure 7. a, b) Optimized D:A structures (configuration no. 2) and c, d) positive (green) and negative (blue) part of the transition dipole density surfaces for MTA:C60 and Me-MTA:C60, respectively.

Table 1 .

 1 Photovoltaic fingerprints of investigated devices (30 nm donor:C60, 1:1 weight ratio).

	Device, donor	J sc [mA cm	-2 ]	V oc [V]	FF [%]	E g	opt [eV]

Table 3 .

 3 Energy

of the CT state (ECT) and oscillator strength (f) for eight MTA:C60 and Me-MTA:C60 configurations. Configuration MTA:C

60 Me-MTA:C 60 E CT [eV] f E CT [eV] f

	1	1.42	0.035	1.40	0.032
	2	1.46	0.009	1.51	0.067
	3	1.47	0.009	1.50	0.067
	4	1.45	0.008	1.51	0.011
	5	1.46	0.010	1.53	0.006
	6	1.45	0.008	1.45	0.022
	7	1.47	0.009	1.52	0.011
	8	1.46	0.053	1.45	0.049

On average, all the investigated configurations show a similar E CT (1.46 eV for MTA:C 60 pairs and 1.48 eV for Me-MTA:C 60 ones) in agreement with experimental data. Most interestingly, we observed a significant increase in oscillator strength in the case of the methylated molecules (the average f value increases from 0.018 in MTA:C 60 pairs to 0.033 in Me-MTA:C 60 pairs). It is interesting to have a closer look at configuration no. 2 in Table

Acknowledgements

This work was supported by European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement number 722651 (SEPOMO). J.B. acknowledges the DFG project VA 1035/5-1 (Photogen) and the Sächsische Aufbaubank through project no. 100325708 (InfraKart). Computational resources in Mons were provided by the onsortium des uipements de alcul ntensif ( ) funded y the onds de la echerche Scientifi ues de elgi ue ( S -S) under rant o 11 as well as the ier-1 supercomputer of the ed ration allonie-Bruxelles, infrastructure funded by the Walloon Region under Grant Agreement No. 1117545. D.B. is a FNRS Research Director.

Conflicts of interests

Authors declare no conflict of interests. Keywords: organic photovoltaic; organic solar cells; voltage losses; non-radiative voltage losses; push-pull molecules

Entry for the Table of Contents

Structural tuning to improve efficiency: Modifications of the molecular structure of organic molecules allows to influence the performance of OPV devices. We find that addition of methyl group in the para position of the external phenyl ring leads to increase in V oc . With quantum-chemical calculations we attribute this effect to the increase of the oscillator strength of the electronic transition between the donor and acceptor.


Pablo Simón Marqués

José María

Andrés Castán

Sylvie Dabos-Seignon

Sébastien Richeter

Ahmad Mehdi

Sébastien Clément

Philippe Blanchard

Clément Cabanetos

Synthesis, Characterization and use of a POSS-Arylamine Based Push-Pull Octamer

The first synthesis of an original octamer consisting of a central polyhedral oligomeric silsesquioxane (POSS) cage decorated with electro-and optically active arylamine-based push-pull building-blocks is reported herein. Once fully characterized, the potential of the new compound was evaluated as donor material in bulk heterojunction organic solar cells. With promising power conversion efficeincies, good processability and film forming properties, such multimer architectures turns out to be potential candidates to bridge the gap between small molecules and polymers

Introduction

Within the past generation, organic photovoltaics (OPV) has attracted considerable research attention as evidenced by the rapid and constant growth in efficiencies from less than one percent to more than 17% in single junction organic solar cells (OSCs). 1 This leap resulted from the synergistic optimization of both the device stack and the nature of the active layer. 2,3 Considering that the latter is usually composed of an electron donor blended with an electron acceptor, it turned out that controlling the morphology of this organic thin film is a key parameter to achieve high efficiencies. [START_REF] Gaspar | Materials[END_REF][START_REF] Guerrero | [END_REF] Therefore, the development of strategies to regulate the self-assembly properties of these photoactive materials through chemical engineering is generally not straightforward and commonly relies on purely empirical trial-and-error processes. Hence, several approaches are usually considered involving the use of solvent additives, post-deposition thermal/solvent annealing and/or the incorporation of nanoparticles. [6][7][8] Several studies have reported that incorporating carbon nanotubes or nanoparticles such as silver, gold and silica nanoparticles can improve the morphology of the active layer. [9][10][11][12] Recently, theoretical studies have notably shown that introducing poly(3-hexylthiophene) (P3HT)-grafted silica nanoparticles with appropriate grafting density and molecular weight of P3HT chains can lead to the formation of an active layer with suitable characteristics (crystallinity of P3HT, domain size, elimination of isolated islands of (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM)) and thus, to the improvement of photovoltaic performances. 13 In this respect, cage-like silsesquioxanes, usually called polyhedral oligomeric silsesquioxanes (POSS), consisting of a silicon-oxygen cubic core with a corona of organic substituents, are promising materials to be integrated in OPV system. 14 Due to their nanometric size (1 to 3 nm), they are often considered as the smallest functionalized silica nanoparticles. 15 Their migration and aggregation behavior in a matrix can be easily tuned by varying the nature of the organic substituents to obtain the desired nanostructure. 16,17 For instance, Morgan and collaborators have reported the positive impact on the phase segregation of a P3HT:PCBM blend by adding specifically tailored polyhedral oligomeric silsesquioxanes (POSS) based nanoparticles. 18 More recently, Gong et al. have also shown that end-functionalizing P3HT chains with POSS results in the formation of two-dimensional conjugated polymeric crystals where P3HT chains are sandwiched between two POSS layers, allowing to improve chargecarrier mobility and the power conversion efficiency (PCE) up of 40% compared to one-dimensional (1D) P3HT blended with PC61BM. 19 On the other hand, the POSS core can also facilitate the electron injection thus improving the current density of light emitting devices when grafted to semiconducting polymers. 20,21 Hence and as a step forward, we report herein the decoration of a POSS derivative by an optically active push-pull molecule and the use of this original structure as electron donor material in bulk heterojunction organic solar cells (Figure 1). 22 We indeed recently showed that multimeric architectures can combine the advantages of both small molecules and polymers, namely a limited batch-to-batch variation due to their well-defined structure and good film forming properties.

Materials and Methods

All reagents and chemicals from commercial sources were used without further purification unless specified. Solvents were dried and purified using standard techniques.

Octavinyl POSS 1 (97%) was kindly provided by Sikemia (Montpellier, France). POSS 2 and compound 5 were prepared according to reported methods. 22,24 Microwave assisted reactions were performed in the cavity of a Biotage Initiator+ system in sealed reactors. Flash chromatography was performed with analytical grade solvents using ALDRICH silica gel (technical grade, pore size 60 Å, 230-400 mesh particle size). Flexible plates ALUGRAM Xtra SIL G UV254 from MACHEREY-NAGEL were used for TLC. Compounds were detected by UV irradiation (BIOBLOCK SCIENTIFIC). NMR spectra were recorded either on a BRUKER AVANCE III 300 spectrometer ( 1 H, 300 MHz and 13 C{ 1 H}, 75 MHz) or on a BRUKER AVANCE III 500 ( 1 H, 500 MHz, 13 C{ 1 H}, 125 MHz and 29 Si{ 1 H}, 99 MHz) spectrometer. Chemical shifts in ppm are calibrated to TMS on the basis of the relative chemical shift of the residual non-deuterated solvent as an internal standard and coupling constants J in Hz. UV-Vis absorption spectra were recorded with a PERKIN ELMER 950 spectrometer. Matrix assisted laser desorption/ionisation time-of-flight (MALDI-ToF) mass spectra were recorded on a Rapiflex TOF/TOF instrument (Bruker, Wissembourg, France). High resolution mass spectrometry (HRMS) was performed with a JEOL JMS-700 B/E or a JEOL Spiral-TOF JMS3000. Cyclic voltammetry was performed using a BIOLOGIC SP-150 potentiostat with positive feedback compensation in 0.10 M Bu4NPF6/CH2Cl2 (HPLC grade).

Experiments were carried out in a one-compartment cell equipped with a platinum working electrode (2 mm of diameter) and a platinum wire counter electrode. A silver wire in a 0.01 M solution of AgNO3 in CH3CN was used as reference electrode. The potentials were then expressed vs. the ferrocene/ferrocenium couple (Fc/Fc + ). Photoelectron spectroscopy in air (PESA) measurements were recorded using a Riken Keiki PESA spectrometer (Model AC-2) with power settings of 20 nW. Final compounds were purified on a JAI size-exclusion recycling HPLC equipped with 2 and 2.5H columns mounted in series. Compound 3: Cyanoacetic acid (408 mg, 4.8 mmol), 2 (502 mg, 0.4 mmol), EDC (1.01 g, 5.3 mmol) and DMAP (673 mg, 5.5 mmol) were placed in a Schlenk tube under an inert atmosphere and anhydrous dichloromethane (5 mL) was added. The reaction mixture was then heated under reflux for 6 h. Once at room temperature, the organic layer was washed with HCl (1 M) (2 x 10 mL), water (2 x 10 mL), dried with MgSO4 and the solvent was removed under reduced pressure using a rotary evaporator to afford 616 mg of compound 3 as a yellow oil (85% yield). IR (neat):  = 2083 cm -1 (C≡N), 1737 cm -1 (C=O), 1179 cm -1 (Csp 3 -O). 1 H NMR (500 MHz, DMSO-d6): δ 4.26 (t, 3 JH,H = 7.0 Hz, 16H), 4.03 (s, 16H), 2.79 (t, 3 JH,H = 7.0 Hz, 16H), 2.66 (m, 16H), 1.04 (m, 16H). 13

EA-POSS:

A microwave flask was charged with POSS 3 (90 mg, 50.2 μmol), aldehyde 5 (355 mg, 0.6 mmol) and CHCl3 (10 mL) before being sealed and deaerated by means of argon bubbling. Then two drops of triethylamine were added, and the mixture was irradiated in the microwave reactor at 120 °C. After 2 h of reaction, the crude was cooled down, the solvent removed under vacuum and the solid was triturated in MeOH and filtrated. Thereafter, the resulting mixture was injected in a recycling preparative HPLC. The major fraction was isolated, concentrated and the red solid was recrystallized in a CH2Cl2/MeOH (1:9) mixture affording 200 mg of the target compound as a red solid (63% yield). Additionally, 82 mg of aldehyde 5 were recovered through the HPLC purification. IR (neat): ν = 3026 cm -1 (Csp-H, Ar), 2963-2852 cm -1 (Csp 3 -H), 2220 cm -1 (C≡N), 1722 cm -1 (C=O), 1589-1432 cm -1 (C=C, Ar), 1184 cm -1 (Csp 3 -O), 1051 cm -1 (Csp 3 -N). 1 H-NMR (300 MHz, CDCl3): δ 8.20 (m, 8H), 7.63 -7.50 (m, 40H), 7.46 (d, 3 JH,H = 8.6 Hz, 32H), 7.39 (m, 48H), 7.31 (d, 3 JH,H = 7.2 Hz, 16H), 7.21 (d, 3 J H,H = 3.9 Hz, 8H), 7.13 (m, 32H), 7.09 -7.03 (m, 16H), 4.43 (t, 3 J H,H = 7.2 Hz, 16H), 2.89 (t, 3 J H,H = 7.2 Hz, 16H), 2.80 (t, 3 J H,H = 8.2 Hz, 16H), 1.15 (t, 3 JH,H = 8.4 Hz, 16H). 13 Device characterization and testing: Indium tin oxide (ITO) pre-coated glass slides of 24×25×1.1 mm with a sheet resistance of RS = 7 Ω/sq were purchased from Visiontek Systems LTD. The substrates were washed with a diluted Deconex® 12 PA-x solution (2% in water) and scrubbed using dishwashing soap before being cleaned by a series of ultrasonic treatments in distilled water (15.3 MΩ cm -1 ), acetone and isopropanol for 15 min each. Once dried under a steam of nitrogen, a UV-ozone plasma treatment (UV/Ozone ProCleaner Plus, Bioforce Nanosciences) was performed for 15 min. An aqueous solution of poly(3,4ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS; Ossila), filtered through a 0.45 µm RC membrane (Millex®), was spun-cast onto the patterned ITO surface at 6000 rpm for 40 s before being baked at 140 °C for 30 min. Then, blends of EA-POSS and PC61BM or PC71BM at different weight to weight ratios were dissolved in chloroform at a total concentration of 10 mg mL -1 , stirred at 30 °C for 30 minutes and spun-cast at 1500 rpm onto the PEDOT:PSS layer. Finally, devices were completed by the successive thermal deposition of Ca (7 nm) and aluminium (100 nm) at a pressure of 1.5 × 10 -6 Torr through a shadow mask defining six cells of 27 mm 2 each (13.5 mm x 2 mm). J-V curves were recorded using a Keithley 236 source-measure unit and a home-made acquisition program. The light source was an AM1.5 Solar Constant 575 PV simulator (Steuernagel Lichttecknik, equipped with a metal halogen lamp). The light intensity was measured by a broad-band power meter (13PEM001, Melles Griot). Atomic Force Microscopy (AFM) experiments were performed using the Nano-Observer microscope from CS Instrument. The topographic images were obtained in tapping mode. Images were processed using Gwyddion SPM data analysis software. Optimized blends were spun cast on the above described PEDOT:PSS modified ITO substrates. Gold electrodes (100 nm) were subsequently and thermally evaporated under a vacuum of 1.5 x 10 -5 Torr, through a shadow mask defining actives area of 10 mm², 5 mm², 1.5 mm² and 0.8 mm² per substrates. Hole mobilities h were evaluated using the Mott-Gurney law, ie, JSCLC = (9/8)0re(V 2 /d 3 ) where r is the static dielectric constant of the medium (r = 3) and d, the thickness of the active layer.

Results & discussion

The synthetic route to prepare the target electro-active POSS EA-POSS derivative is illustrated in Scheme 1. First, a thiol-ene click reaction between the octavinyl POSS 1 and -mercaptoethanol was performed to afford the octa(2-hydroxyethylthioethyl) polyhedral oligomeric silsesquioxane 2 that was subsequently engaged into a Steglich esterification reaction with cyanoacetic acid. In parallel, the complementary intermediate 5 was prepared by a microwave assisted Suzuki reaction between the commercially available boronic arylamine 4 and the 5'bromo-[2,2'-bithiophene]-5-carbaldehyde. Finally, 3 and 5 were engaged in a Knoevenagel condensation, also performed under microwave irradiation, to afford the fully substituted core in a reduced period of time. Instead of performing a conventional purification on silica gel or through a Soxhlet extraction (without guarantying their effectiveness), it was decided to inject the crude into a sizeexclusion recycling HPLC (Figure 2). As depicted in Figure 2, three fractions were successfully collected. While fraction F1 corresponds to compound 5, introduced in excess to favor the reaction, the exact composition of F2 still remains unclear. However, a mass analysis of the major fraction, namely F3, revealed the presence of a single molecular structure with a molecular weight of 6369 g.mol -1 , thus corresponding to the target octamer, namely EA-POSS (see SI). Once isolated with a decent yield after a final recrystallization (63%), the latter was characterized by UV-Visible absorption spectroscopy (Figure 3). As expected, two main bands, characteristic of the push-pull moieties were recorded at ca 350 nm and 494 nm, attributed to -* and internal charge transfer (ICT) transitions respectively. Cyclic voltammetry was then conducted in a 0.1 M solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) solubilized in dichloromethane. It turns out that two oxidation processes, attributed to the successive formation of a stable radical cation and dication on the push-pull derivatives, were observed at Epa1 ≈ 0.45 V and Epa2 ≈ 0.89 V vs. Fc/Fc + (Figure 4). Interestingly, the super imposition of successive cyclic voltammograms of EA-POSS revealed a propensity to form electrodeposited thin films on the surface of the working electrode. Estimated at ca -5.21 eV from the onset of the first oxidation process, its highest occupied molecular orbital (HOMO) level was found to be slightly deeper (-5.58 eV) once spun cast on a glass sheet and measured by photoelectron spectroscopy in air (see SI). These thin films were also used to determine the optical band gap of EA-POSS in solid state (ca 1.96 eV), enabling the estimation of its LUMO level (-3.62 eV) and revealing its electronic compatibility with fullerene derivatives. Consequently, the potential of this decorated POSS derivative as donor material was thereafter evaluated through the fabrication of basic air-processed bulk heterojunction solar cells. To that end, the well-known PC61BM was first used as the complementary electron acceptor material. It turned out that power conversion efficiencies (PCEs) up to 1.4% were reached for a 1:2 donor-acceptor weight-to-weight blend ratio deposited from a chloroform solution (Table S1). Current density-voltage (J-V) curves of the best device are plotted in Figure 5a and the corresponding photovoltaic data are gathered in Table 1. As so often, efficiencies were significantly improved by replacing the PC61BM by the PC71BM. With similar hole mobilities of ca 3.3 x 10 -4 cm 2 V -1 s -1 measured for both blends via the space charge limited currents (SCLC) methods (Figure S13), this difference mainly comes from a better contribution of the C70 derivative to the photocurrent (Figure S14) resulting in higher short circuit currents (Jsc) (7.21 mA.cm -2 vs 4.11 mA.cm -2 ) and therefore PCE (2.50% vs 1.44%). On the other hand, while annealings are usually deleterious for discrete push-pull based bulk heterojunctions solar cells, post thermal treatments were found to be beneficial, to some extent, for both EA-POSS / fullerene blends (Figure 5b and Figure S15-16). With moderate impacts on the open circuit voltage (slight decrease for PC71BM based devices), a mild thermal annealing up to 90°C indeed positively affects both the Jsc and fill-factors (FF). As a result, promising maximum PCEs of 2.35% and 3.41% were reached for the PC61BM and PC71BM based active layers respectively (Table 1).To gain further insights into these features, atomic force microscopy (AFM) measurements were performed on annealed and non-annealed champion devices (Figure 6). Interestingly, as cast active layers both show similar patterns with nanometric cavities of 2-6 nm depth homogeneously distributed over the surfaces. It turned out that annealing the samples induces a complete vanishing of the latter resulting in smooth and homogenous surfaces (root mean square values < 0.5 nm), demonstrating, once again, the drastic, but in this case, beneficial impact of thermal treatments.

O Si

Si

Conclusions

In summary, we report herein the synthesis, characterization and use of the first molecular electro active POSS derivative as donor material in organic solar cells. Decorated with eight simple arylamine-based pushpull building-blocks, promising power conversion efficiencies up to 2.5% and 3.4% were reached once blended with the commonly used PC61BM and PC71BM respectively. Considering that this proof of concept was achieved with an electro active dye limited in term of absorption range in the visible (350-550 nm), the synthetic accessibility and tunability of such octamer architectures clearly open doors to new panchromatic design principles. Moreover, with good solubility, processability and film forming properties, multimer structures contribute in bridging the gap between small molecules and polymers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

23

 23

Figure 1 .

 1 Figure 1. Structure of the push-pull decorated Electro-Active POSS derivative (EA-POSS).

  C { 1 H-NMR (125 MHz, DMSO-d6): δ 164.3, 115.0, 64.6, 29.1, 25.3, 24.3, 12.4. 29 Si{ 1 H}-NMR (99 MHz, DMSO-d6): δ -68.5. MS (MALDI-TOF + , dit +) m/z: 1815.1 [M + ].

Scheme 1 .

 1 Scheme 1. Synthesis of the POSS-containing octamer EA-POSS.

Figure 2 .

 2 Figure 2. Chromatogram profile obtained by UV-vis spectrometric detection after injection of the EA-POSS crude in the sizeexclusion recycling preparative HPLC.

22

 22

Figure 3 .

 3 Figure 3. UV-Vis absorption spectra of EA-POSS in diluted dichloromethane solutions (ca. 10 -5 M).

Figure 4 .

 4 Figure 4. a) Cyclic voltammograms of EA-POSS and b) electrodeposition of EA-POSS upon cycling in the following conditions: 5 mM in 0.1 M Bu 4 NPF 6 /CH 2 Cl 2 recorded at 100 mV.s -1

Figure 5 .

 5 Figure 5. J-V characteristics of the best solar cells based on a blend of EA-POSS with PC 61 BM (red) and PC 71 BM (blue) before (A) and after (B) a thermal treatment at 90°C for 5 min.

Figure 6 .

 6 Figure 6. AFM phase images of optimized blends before (top) and after thermal annealing (bottom).

  , 127.1, 126.8, 125.0, 124.1, 123.6, 123.5, 116.1, 96.7, 65.0, 30.1, 26.5, 13.2. 29 Si{ 1 H}-NMR (59.6 MHz, CDCl3): δ -68.9. MS (MALDI-TOF + , dit + ) m/z: 6368.2 [M+].

	C{ 1 H}-NMR
	(75 MHz, CDCl3): δ 162.9, 148.1, 147.7, 146.6, 146.5,
	146.4, 140.5, 139.9, 136.3, 134.0, 128.9, 128.1, 127.8,
	127.4

Table 1 .

 1 Photovoltaic characteristics of the best bulk heterojunction cells based on EA-POSS blended with the PC 61 BM and PC 71 BM in a 1:2 D:A ratio (8 devices fabricated per experimental condition). Measurements performed under an AM. 1.5 simulated solar illumination (100 mW.cm -2 ).

		Thermal	Voc	Jsc	FF	PCE
	Fullerene	treatment	(V)	(mA cm-2)	(%)	(%)
		None	0.95	4.11	36.67	1.44
	PC61BM	5 min at 70°C 5 min at 90°C	0.96 0.96	4.67 5.41	44.54 44.91	2.01 2.35
		5 min at 110°C	0.92	4.67	41.53	1.78
		None	0.96	7.21	36.05	2.50
		5 min at 70°C	0.93	8.01	43.07	3.20
	PC71BM	5 min at 90°C	0.90	8.31	45.56	3.41
		5 min at 110°C	0.86	4.67	42.38	1.70

Authors thank the MATRIX SFR of the University of Angers. J.M.A.C. and P.S.M. thanks the European Union's Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement No.722651 (SEPOMO). The Région Pays de la Loire is also acknowledged for the Projet «étoile montante SAMOA».


Víctor Tejeda-Orusco

Martin Blais

Clément Cabanetos

Philippe Blanchard
email: philippe.blanchard@univ-angers.fr.

Raquel Andreu
email: randreu@unizar.es

Santiago Franco

Jesús Orduna

Beatriz E Diosdado

4H-pyranylidene-based small push-pull chromophores: synthesis, structure, electronic properties and photovoltaic evaluation

Keywords: Push-pull systems, 4H-pyranylidene, small conjugated molecules, organic photovoltaics

The synthesis and electrochemical and optical characterizations of four new 4Hpyranylidene-based push-pull molecules are reported herein as well as their evaluation as donor materials for organic photovoltaics. Studied systems exhibit good absorption properties and appropriate LUMO levels for a photoinduced electron transfer to C 60 . Bilayer organic solar cells fabricated from these new donors and C 60 as acceptor gave photocurrent however with low power conversion efficiencies. As shown by X-ray diffraction and theoretical calculations, titled molecules present a twisted structure which may prevent the formation of suitable - contacts between adjacent molecules hence explaining their low photovoltaic performance.

Introduction

Small -conjugated molecules alternating electron-donating (D) and electron-accepting (A) units with various combinations are of considerable interest as active donor materials [1] as well as non-fullerene acceptors [2] for organic photovoltaics (OPV). In these structures, the most commonly used electron donor blocks include e. b']dithiophene. [1,2] On the other hand, the oxygen-containing γ-pyranylidene unit represents a versatile building block that has been extensively introduced in conjugated systems for applications in the fields of photonics and organic electronics. Its proaromatic character [START_REF] Andreu | 4H-Pyran-4-ylidenes: strong proaromatic donors for organic nonlinear optical chromophores[END_REF] promotes an intramolecular charge transfer (ICT) process in D-A systems through a gain in aromaticity. Interestingly, condensation between pyran-4-one and various CH 2 -activated compounds such as malononitrile, dicyanovinylindan-3-one, indanedione or (thio)barbituric acid is very often used to prepare electron-withdrawing γ-pyranylidene blocks [5] whereas less electrondonating γ-pyranylidene blocks have been reported. [START_REF] Andreu | 4H-Pyran-4-ylidenes: strong proaromatic donors for organic nonlinear optical chromophores[END_REF] Thus, the synthetic versatility provided by this building block allowed the preparation and characterization of a variety of structures, affording materials with interesting properties, such as second-order nonlinear optical (NLO) activity [START_REF] Andreu | 4H-Pyran-4-ylidenes: strong proaromatic donors for organic nonlinear optical chromophores[END_REF][START_REF] Andreu | Aromatic/proaromatic donors in 2-dicyanomethylenethiazole merocyanines: from neutral to strongly zwitterionic nonlinear optical chromophores[END_REF] or two photon absorption,[7] organic light-emitting diodes,[8] good performance as dyes in Dye Sensitized Solar Cells (DSSCs) [START_REF] Bolag | Dye-sensitized solar cells based on novel diphenylpyran derivatives[END_REF] or for the aggregation-induced emission enhancement phenomena. [10] Regarding the field of small molecules acting as donor materials for organic solar cells (OSCs), the 4H-pyranylidene unit has been used as -spacer in combination with acceptor groups, such as dicyanovinyl (DCV) [START_REF] He | Synthesis and Photovoltaic Properties of a Solution-Processable Organic Molecule Containing Triphenylamine and DCM Moieties[END_REF] or (thio)barbituric acids.

[12] However, to the best of our knowledge, the γ-pyranylidene moiety has never been used as electron-donating group D in small conjugated D-A molecules for photovoltaic application, although an heteroquinoid merocyanine dye derived from 2,6-diphenyl-4H-pyranylidene has been recently considered.

[13] It is worth noting that a parent dipyran-based ladder-type building block has been recently used as central donating group D in A-D-A conjugated molecules for efficient nonfullerene acceptors in OPV. [14] On the other hand, the system 2,2',6,6'-tetraphenyldipyranylidene has been used as holetransporting material for hybride perovskite solar cells, [START_REF] Shen | Quinoidal 2,2',6,6'-Tetraphenyl-Dipyranylidene as a Dopant-Free Hole-Transport Material for Stable and Cost-Effective Perovskite Solar Cells[END_REF] and as an efficient hole-collecting interfacial layer in organic solar cells based on poly(3-hexylthiophene):phenyl-C 61 -butyric acid methylester (P3HT:PCBM). [START_REF] Courté | Structural and electronic properties of 2,2',6,6'-tetraphenyldipyranylidene and its use as a hole-collecting interfacial layer in organic solar cells[END_REF] Within this context, and having in mind our experience with the synthesis and characterization of push-pull systems featuring the 4H-pyranylidene fragment for optical applications and DSSCs, [4;6a,c,d;9b,c,e] we present here the synthesis and the electronic properties of four novel small push-pull conjugated molecules. (Fig. 1) In these systems, the exocyclic C=C double bond of a 4H-pyranylidene proaromatic electron-donating unit is functionalized by two thiophene rings end-capped at their 5-positions by one (compounds 1)

or two identical (compounds 2) electron-withdrawing unit(s), namely a DCV (derivatives a)

or a 4-phenyl-2-oxo-2,5-dihydrofuran-3-carbonitrile (derivatives b) unit.

The latter has never been used before in the field of OPV, but gave rise to the preparation of chromophores with an effective polarization and upgraded second-order NLO responses. [START_REF] Moreno-Yruela | D--A compounds with tunable intramolecular charge transfer achieved by incorporation of butenolide nitriles as acceptor moieties[END_REF] The electronic properties of these compounds are discussed with emphasis on structureproperty relationships with the help of theoretical calculations. Moreover, the potential of these derivatives as donor materials in OPV is reported throughout this article. While basic conditions (Al 2 O 3 ) were used for the reaction with malononitrile, [START_REF] Lin | A Low-Energy-Gap Organic Dye for High-Performance Small-Molecule Organic Solar Cells[END_REF] the condensation with 4-phenyl-2-oxo-2,5-dihydrofuran-3-carbonitrile ( 4) was directly performed in refluxing ethanol. [START_REF] Moreno-Yruela | D--A compounds with tunable intramolecular charge transfer achieved by incorporation of butenolide nitriles as acceptor moieties[END_REF] Yields ranged from 26 to 86%.

Scheme 1: Synthesis of 4H-pyranylidene-based push-pull molecules.

Structural characterization by X-ray diffraction.

Single crystals of compound 1a obtained by slow diffusion of hexane into a solution of the corresponding donor in CH 2 Cl 2 at room temperature, were analysed by X-ray diffraction. The X-ray molecular structure of 1a is represented in Fig. 2. In order to distinguish the two thiophene rings of 1a for the discussion, they will be named as S1 and S2 following the labelling shown in Fig. 2. Thiophene S1 is disordered (77% S1C and 23% C1S) due to the free rotation around the C14-C15 single bond. The DCV group of compound 1a adopts a cis conformation (along the C22-C23 bond)

relative to the sulfur atom of the vicinal thiophene S2, as previously found in its analogous compound without the additional thiophene ring (S1) (Compound 5, Fig. 3 (R = H)), [START_REF] Marco | Polarization, second-order nonlinear optical properties and electrochromism in 4Hpyranylidene chromophores with a quinoid/aromatic thiophene ring bridge[END_REF] and in other compounds featuring thiophene moieties end-capped with DCV groups. [START_REF] Moore | Synthesis, Structures and Nonlinear Optical Properties of Novel D--A Chromophores: Intramolecular Charge Transfer from 1,3-Dithiole or Ferrocene Moieties to Polynitrofluorene or Dicyanomethylene Moieties through Conjugated Linkers[END_REF] The extended conjugated π-system in 1a, involving the pyran, thiophene S2 and DCV units, is essentially planar, with an angle of 6.8° between the mean planes of the pyran ring and that of the thiophene S2, and an angle of 9.6° between the planes of thiophene S2 and the DCV unit. However, in contrast to the completely planar conjugated pathway shown by the X-ray structure of compound 5, the presence of the additional thiophene S1 slightly distorts the planarity of the extended -system.

In addition, the S1 thiophene ring is out of the plane of the aforementioned planar -system, the value of the dihedral angle C3-C14-C15-S1C being of 100.7 °. Thus, thiophene ring S1 is almost perpendicular to the conjugated system (Fig. S-12), with angles of 77.2° between the mean planes of thiophene S1 and the pyran rings, and 82.7 ° between the two thiophene moieties S1 and S2.

Comparison of the bond lengths of compounds 1a and 5 together with the standard [START_REF] Allen | Tables of Bond Lengths determined by X-Ray and Neutron Diffraction. Part 1. Bond Lengths in Organic Compounds[END_REF] C=C and C-C bonds in thiophenes derivatives and 2,2',6,6'-tetraphenylbipyranylidene [START_REF] Chasseau | Tétraphényldipyranylidène (DIP Ph 4 )[END_REF] (a 4H-pyranylidene derivative taken as model quinoidal compound) (Fig. 3), allows us to highlight the following features about the ICT in these compounds: (i) comparison of the standard C=C and C-C bonds in an unsubstituted aromatic thiophene with data for systems 1a

and 5 reveals the -electron delocalization along the thiophene ring in these compounds; (ii) the C-O bond is shorter in compound 1a than in 5, leading to a more zwitterionic contribution to the ground state in 1a; (iii) the C 4 -C exo bond for compound 5 is even shorter than the C 4 -C exo one in the model quinoidal derivative. The two later features reveals that whereas for compound 5, the thiophene moiety seems to be the main donor involved in the ICT process towards the acceptor unit, [START_REF] Marco | Polarization, second-order nonlinear optical properties and electrochromism in 4Hpyranylidene chromophores with a quinoid/aromatic thiophene ring bridge[END_REF] the transfer of charge arises from the pyran ring for compound 1a, in agreement with other 4H-pyranylidene--A systems previously reported. [START_REF] Andreu | 4H-Pyran-4-ylidenes: strong proaromatic donors for organic nonlinear optical chromophores[END_REF][START_REF] Marco | Push-pull systems bearing a quinoid/aromatic thieno-[3,2-b]thiophene moiety: synthesis, ground state polarization and second-order nonlinear properties[END_REF] Molecules are stacked along the c axis, with a head-to-tail arrangement (see CCDC-1962297 contains the supplementary crystallographic data for this article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Fig. 3.

Selected bond lengths (Å) for compounds 1a, 5, thiophene and 2,2',6,6'tetraphenylbipyranylidene.

Optical properties

The optical properties of titled compounds have been investigated by UV/Vis spectroscopy in dichloromethane solution. Albeit small, all titled chromophores exhibit intense and broad bands in the visible region (Fig. 4), which can be assigned to an ICT process between the donor and the acceptor moieties.

Comparison of optical properties of compounds that only differ in the acceptor group shows a significant bathochromic shift for b derivatives, in agreement with the more -extended system and the higher electron-withdrawing ability of the furanone moiety. Moreover, b systems present higher molar extinction coefficients () than those of DCV analogues a.

Compared to 5, the additional thiophene moiety, nearly orthogonal to the extended π-system (see section 2.2), leads to a slight hypsochromic shift for 1a. This point might be a consequence of the slight deviation from planarity (see section 2.2) imposed to the conjugated system by the additional thiophene ring, in contrast to the completely planar π system shown by compound 5. [START_REF] Marco | Polarization, second-order nonlinear optical properties and electrochromism in 4Hpyranylidene chromophores with a quinoid/aromatic thiophene ring bridge[END_REF] The introduction of a second acceptor group at the terminal alpha position of the twisted thiophene ring in molecules 2 induces a further blue-shifted absorptions when compared to their analogues 1. (red).

Electrochemical study

The electrochemical properties of the four molecules have been analyzed by cyclic voltammetry in dichloromethane solution in the presence of Bu 4 NPF 6 as the supporting electrolyte. Data are summarized in Table 2, together with those of reference compound 5 in the same conditions. Compound The cyclic voltammograms (CVs) of compounds 1 and 2 show one irreversible reduction peak (involving the acceptor unit) and two reversible oxidation waves (Fig. 5 and S-10). It is important to note that compared to compound 5, the introduction of a thiophene ring in the exocyclic carbon of the 4H-pyranylidene fragment changes appreciably the oxidation behavior of titled compounds 1 and 2 (Table 2).

E ox1 1/2 (V) E ox2 1/2 (V) E red (V) E HOMO (eV) E LUMO (eV) 1a
In this way, compound 5 explained by the more -extended furanone system more prone to stabilize the radical cation and dicationic species (Fig. 5). In addition, the potential difference value ΔE ox 1/2 (1-2) = For conditions, see Table 2.

E ox 1/2 (2) -E ox 1/2 (1)

Theoretical calculations

In order to get further insight into the geometry and electronic properties of the studied compounds, theoretical calculations using DFT (Density Functional Theory) and the CPCM (Conductor-like Polarizable Continuum Model) solvation method were performed.

Concerning the geometry of the studied dyes, (see Fig. S-16 to S-19) calculations show that the push-pull -system of compounds 1 is almost planar. Taking compound 1a as model, it can be inferred that in the calculated structure, the conjugated -pathway deviates more from planarity than in the X-ray structure. Thus, the calculated angle between the pyranylidene unit and the thiophene ring is 18º. Indeed, the additional thiophene ring appears also almost perpendicular to the push-pull conjugated system.

The distortion from planarity shown by theoretical calculations (and also by X-Ray diffraction for 1a) is in agreement with that found in related 4H-pyranylidene derivatives recently described as dyes for DSSCs.

[9e] In that case, the twisted structures prevent the formation of aggregates which is beneficial for DSSCs and, therefore, give rise to an increase of the efficiency of the corresponding cells.

Compounds 2 have a C2 symmetry axis and taking compound 2a as model, the angle found between the main planes of the pyranylidene ring and either the two vicinal thiophene rings is 45 °.

Calculations at the ground state geometry allowed to obtain orbital energies (data are collected in Table 2) and topologies. The calculated values of the HOMO and LUMO levels are consistent with electrochemical results (Table 2). Thus, E HOMO (E LUMO ) values for systems b are higher (lower) than those encountered for their a analogues, in agreement with the less anodic potentials measured for b derivatives and with the superior electron-withdrawing ability of the furanone group. On the other hand, passing from compounds 1 to 2 leads to a decrease in both the HOMO and LUMO levels.

Moreover, according to the calculations, electron densities related to frontier orbitals (see topologies for compounds 1a and 2a in Fig. 6 and for the rest in the Supporting Information) are mainly localized on the 4H-pyranylidene unit and the thienyl ring for the HOMO, and on the thiophene-acceptor block in the case of the LUMO. For systems 1, the additional thiophene unit does not have a significant contribution neither to the HOMO nor to the LUMO. Moreover, according to time dependent DFT calculations (TD-DFT), the first excited state is mainly associated to a one electron HOMO to LUMO transition, and there is a large HOMO-LUMO spatial overlap that accounts for the high molar extinction coefficients ()

(Table 1).

A more complex picture is obtained for compounds 2. The presence of two acceptor moieties, inducing higher symmetry, gives rise to two virtual orbitals (LUMO and LUMO +1) that are close in energy, and therefore, the lowest energy absorption band encompasses two electronic excitations from HOMO to LUMO and from HOMO to LUMO+1 respectively.

Taking compound 2a as model compound, DFT calculations predict the lowest energy absorption at 509 nm with 0.48 as oscillator strength (f) (Fig. 7). This absorption is mainly contributed (94%) from a HOMO to LUMO one electron transition and overlaps to a more intense absorption at 499 nm (f = 1.30) mainly contributed (94%) from a HOMO to LUMO+1 transition.

Electronic properties of thin-films and photovoltaic properties

Thin-films on glass or indium tin oxide (ITO) substrates were prepared by spin-casting solutions of titled molecules in chloroform and then analyzed by UV-vis spectroscopy and photoelectron spectroscopy in air (PESA), respectively.

The absorption maxima of the ICT band for thin-films of 1a, 1b, 2a and 2b are observed at 549 nm, 646 nm, 551 nm and 646 nm, respectively (Fig. 8). Surprisingly, the passage from the solution to the solid state leads to different behaviors. Whereas a bathochromic shift of 20 nm is observed for thin-films of 2b and the optical properties of thin-films of 1b and 2a are weakly affected, thin-films of compound 1a are subjected to a hypsochromic shift of 13 nm compared to solution. In addition, contrary to solutions, there is nearly no difference of absorption maximum of the ICT band between derivatives 1 and 2 suggesting that the impact of the symmetry of molecules 2 on the optical properties in the solid state is really weak. The determination of the onset of absorption at low energy of thin-films of 1a, 1b, 2a and 2b gives an estimation of the optical bandgap E g opt of 1.86 eV, 1.49 eV, 1.84 eV and 1.51 eV, respectively.

Wavelength (nm) 3. Table 3 shows that all titled molecules exhibit photovoltaic properties when used as electron-donor material in bilayer OSCs. However, although these molecules exhibit good absorption properties in the visible spectrum and LUMO levels higher than the one of C 60 (ca.

-4.2 eV), as expected for photo-induced electron transfer, their performance are relatively poor. Our data suggest also that symmetrical extended molecules (2a and 2b) show slightly better photovoltaic performance. The best ones were obtained with the symmetrical compound 2a leading to a power conversion efficiency (PCE) of 0.33 % associated with an open-circuit voltage (V oc ) of 0.57 V, a short-circuit current density (J sc ) of 1.69 mA/cm 2 and a fill factor (FF) of 34 %. The current density vs voltage curves of the best devices for all compounds recorded under illumination are presented in Fig. 10. Interestingly, compound 2a leads to the highest V oc value in agreement with its deepest HOMO level and the largest LUMO (C 60 )-HOMO (Donor) difference. On the other hand, the PV performance of compound 1a are negligible which is consistent with the absence of - intermolecular interactions in its crystal structure, hence probably preventing hole transport that is needed after exciton dissociation at the donor/acceptor interface. In the case of 1b, 2a and 2b showing better PV performance, the more extended -conjugated systems may participate to the development of - intermolecular interactions.

Experimental section

General experimental methods

See Supporting Information.

Device preparation and characterization

See Supporting Information.

Computational details

See Supporting Information.

Starting materials

Aldehydes 1-CHO[9e] and 2-CHO[9e] and acceptor 4 [START_REF] Ford | The Preparation of 2(5H)-Furanones and Dyes Derived from Them[END_REF] was prepared as previously described. Hz, J 2 = 1.2 Hz, 1 H), 7.10 (dd, J 1 = 5.2 Hz, J 2 = 3.5 Hz, 1 H), 7.41 (dd, J 1 = 5.2 Hz, J 2 = 1.2

Synthesis and characterizations

Hz, 1 H), 7.53 (d, J = 4.4 Hz, 1 H), 7.66 (d, J = 0.4 Hz, 1 H). 13 C-NMR (100 MHz, CD 2 Cl 2 ): δ (ppm) 28.0, 28.2, 36.4, 36.7, 72.2, 101.8, 103.8 107.3, 115.2, 116.1, 126.9, 127.3, 127.9, 128.9, 132.1, 137.6, 140.0, 142.8, 149.9, 159.6, 167.0, 168.6 1, 28.2, 36.3, 36.5, 93.8, 101.7, 103.2, 113.2, 116.0, 126.6, 127.2, 128.7, 129.4, 130.0, 132.4, 134.0, 135.4, 136.8, 143.6, 161.6, 166.1, 167.3 9, 30.1, 75.6, 102.7, 105.4, 114.3, 115.1, 129.2, 134.2, 138.9, 139.9, 150.5, 156.7, 168.9 1, 36.5, 97.1, 102.8, 107.0, 112.9, 115.6, 128.4, 129.2, 129.4, 130.1, 132.6, 135.7, 136.5, 136.6, 143.7, 155.8, 162.3, 164.7, 167.7

Conclusions

Four new push-pull systems containing a 4H-pyranylidene moiety as electron-donating block have been designed and synthesized: V-shaped chromophores 2 present a blue-shifted absorption in solution and higher E ox values compared to their linear analogues 1. On the other hand, furanone derivatives b show a lower electrochemical gap and higher  max and  values than for their analogues a with the DCV moiety. The analysis of the crystalline structure of compound 1a by X-ray diffraction has revealed the absence of strong intermolecular interactions due to a thienyl unit out of the plane of the push-pull conjugated segment.

Thin-film of the four molecules have been analyzed by UV-vis spectroscopy and PESA allowing us to draw the corresponding energetic diagram suggesting a potential interest in OPV. The four small molecules have been then evaluated as donor materials in bilayer OSCs in combination with C 60 . Our preliminary results show a clear photovoltaic effect, however with low power conversion efficiencies. The best PCE value of 0.33 % was obtained for a bilayer OSC based on 2a, associated to low J sc and FF values, 1.69 mA/cm 2 and 34 % respectively. These results suggest limited hole transport properties for 2a as well as for the other titled compounds due to weak - intermolecular interactions.

  g. nitrogen derivatives based on arylamine[3] or carbazole, and thiophene-based conjugated systems derived from oligothiophenes or fused 3,4-dihydro-2H-cyclopenta[2,1-b:3,4-b']dithiophene, silolo[3,2-

Fig. 1 .

 1 Fig. 1. Molecular structures of the target compounds.

Fig. 2 .

 2 Fig. 2. Molecular structure of compound 1a.

  Fig. S-13 to S-15), as generally observed for strongly dipolar compounds.[25] Distance between the planes of two neighbouring molecules is close to 9 Å, likely due to the presence of the cumbersome thiophene S1 rings, almost perpendicular to the conjugated -pathway. This fact prevents suitable - stacking interactions.

Fig. 4 .

 4 Fig. 4. Absorption spectra of compounds in CH 2 Cl 2 : 1a (blue), 1b (green), 2a (black), 2b

[ 20 ]

 20 displays (See Fig.S-9) a single irreversible oxidation peak at 0.82 V corresponding to the formation of the pyrylium radical cation followed by a rapid dimerization reaction leading to a dipyrylium salt. The reverse scan exhibits an irreversible cathodic peak at 0.46 V, assigned to the reduction of the dipyrylium cation leading to 5 after a C-C cleavage reaction. Both observations are consistent with results previously described for other methylenepyran derivatives.[9d,26] For compounds 1a-b and 2a-b, with an additional thienyl ring in the ethylene bridge, two reversible oxidation processes are observed, which can be ascribed to the successive monoelectronic oxidations of the pyranylidene moiety, as previously observed for push-pull platinum complexes with this electron-donating group[27] and for their starting organic material.Compounds 1a-b with one acceptor unit, show lower E ox values than for the corresponding analogues 2a-b with two acceptor units; this fact may be related to the better planarization of the π-conjugated system of mono-functionalized compounds 1, leading to a better stabilization of the radical-cation. This feature is more remarkable for derivatives a than for compounds b. On the other hand, this structural variation does not seem to have such an impact on |E red | values for DCV derivatives whereas for b series, a lower |E red | is found in compound 2b.For a given series (1 or 2), there is a shift of the |E red | values towards less cathodic potentials when changing the acceptor from dicyanomethylene to the 2(5H)-furanone group, indicating the superior electron-accepting ability of the latter. UV-vis study also supports the comparatively weaker electron-withdrawing ability of the DCV group. On the other hand, there is a shift of the E ox values (first and second potentials) towards less anodic potentials when passing from dicyanomethylene (a) to 2(5H)-furanone derivatives(b), this can be

Fig. 5 :

 5 Fig. 5: Cyclic voltammograms of compounds 2a (black) and 2b (red). For conditions, seeTable2.

Fig. 6 .

 6 Fig. 6. Illustration of the HOMO (bottom) and LUMO (top) of compounds 1a (left) and 2a (right).

Fig. 7 .

 7 Fig. 7. Calculated UV-vis spectrum for compound 2a in CH 2 Cl 2 .

Fig. 8 .

 8 Fig. 8. Normalized UV-vis spectra of spin-casted thin-films on glass.

Fig. 9 .

 9 Fig. 9. Energy levels of titled compounds determined in the solid-state by UV-vis spectroscopy and PESA.

a

  After thermal annealing at 100 °C for 10 min.b Speed rate for deposition of the donor: 6000 rpm, C 60 thickness: 25 nm. c Speed rate for deposition of the donor: 4000 rpm, C 60 thickness: 30 nm.

Fig. 10 .

 10 Fig. 10. Current density vs voltage curves of the best bilayer OSC after thermal treatment at 100 °C for 10 min, under white light at 100 mW/cm 2 .

Compound 4

 4 (84.7 mg; 0.44 mmol) was added to a solution of aldehyde 2-CHO (84.6 mg, 0.20 mmol) in absolute ethanol (5 mL). The mixture was refluxed under argon with exclusion of light for 3 days (TLC monitoring). After cooling, the resulting solid was isolated by filtration, washed with cold pentane, cold ethanol and finally with a cold mixture of pentane/CH 2 Cl 2 9:1. A dark green solid was obtained (109 mg; 72%). Mp (°C) 215 (dec.). IR (KBr): ν (cm -1 ) 3064 (Csp 2 -H), 2964 (Csp 3 -H), 2224 (C≡N), 1761 (C=O), 1655, 1613 and 1553 (C=C Ar.). 1 H-NMR (400 MHz, CD 2 Cl 2 ): δ (ppm) 1.22 (s, 9 H), 6.40 (s, 1 H), 6.79 (s, 1 H), 7.05 (d, J = 4.0 Hz, 1 H), 7.46 (d, J = 4.0 Hz, 1 H), 7.65-7.64 (m, 5 H).13 C-NMR (100 MHz, CD 2 Cl 2 ): δ (ppm) 28.

Table 1 :

 1 UV-vis absorption data in CH 2 Cl 2 .

	Compound	 abs , nm	M -1 cm -1
	1a	562	35811
	1b	645	38406
	2a	547	33026
	2b	626	40131
	5[18]	568	46400

Table 1 lists the main optical properties of compounds 1a-b and 2a-b, together with those of reference compound 5[20] for comparison.

Table 2 :

 2 Electrochemical data a and E HOMO and E LUMO values theoretically calculated. b

Table 3 .

 3 Photovoltaic parameters of bilayer OSCs with titled donors and C 60 under AM1.5 conditions (100 mW/cm 2 ). a

	Donor	V oc (V)	J sc (mA/cm 2 )	FF (%)	PCE (%)
	1a b	0.28	0.21	22	0.01
	1b b	0.36	0.90	21	0.07
	2a c	0.57	1.69	34	0.33
	2b b	0.39	1.54	28	0.17

(5-((2,6-di-tert-butyl-4H-pyran-4-ylidene)(thiophen-2-yl)methyl)thiophen-2- yl)methylene)-2-oxo-4-phenyl-2,5-dihydrofuran-3-carbonitrile (1b)

	1.25 (s, 9 H), 5.91 (d, J = 2.2 Hz, 1 H), 6.70 (d, J = 2.2 Hz, 1 H), 6.71 (br s, 1 H), 6.90 (d, J =
	4.2 Hz, 1 H), 6.92 (dd, J 1 = 3.4 Hz, J 2 = 1.3 Hz, 1 H), 7.0 (dd, J 1 = 5.2 Hz, J 2 = 3.4 Hz, 1H),
	7.37 (dd, J 1 = 5.2 Hz, J 2 = 1.2 Hz, 1 H), 7.39 (d, J = 4.2 Hz, 1 H), 7.63-7.61 (m, 5 H). 13 C-
	NMR (100 MHz, CD 2 Cl 2 ): δ (ppm) 28.
	. HRMS (ESI + ) m/z 446.1478
	[M] +• (calculated for C 26 H 26 N 2 OS 2 : 446.1481); m/z 469.1390 [M+Na] + (calculated for
	C 26 H 26 N 2 NaOS 2 : 469.1379)
	5-(Compound 4 (48.3 mg; 0.26 mmol) was added to a solution of aldehyde 1-CHO (93.13 mg,
	0.23 mmol) in absolute ethanol (5.5 mL). The mixture was refluxed under argon with
	exclusion of light for 48 hours (TLC monitoring). After cooling, the resulting solid was
	filtered, washed with cold hexane, cold ethanol and finally with a cold mixture of
	pentane/CH 2 Cl 2 9.5:0.5. A dark green solid was collected (112 mg; 86%).

Mp (°C) 160-162. IR (KBr): ν (cm -1 ) 3038 (Csp 2 -H), 2967 (Csp 3 -H), 2219 (C≡N), 1744 (C=O), 1651, 1597 and 1533 (C=C Ar.). 1 H-NMR (400 MHz, CD 2 Cl 2 ): δ (ppm) 1.12 (s, 9 H),

  . HRMS (ESI + ): m/z 523.1599 [M+H] + (calculated for C 30 H 27 N 4 OS 2 : 523.1621); m/z 545.1432 [M+Na] + (calculated for C 30 H 26 N 4 NaOS 2 : 545.1440).

  . HRMS (ESI + ): m/z 783.1960 [M+Na] + (calculated for C 46 H 36 N 2 NaO 5 S 2 : 783.1958).

Acknowledgments. Financial support from MINECO (CTQ2014-52331R), Gobierno de Aragón-FEDER-Fondo Social Europeo 2014-2020 (E14_17R) and University of Zaragoza (UZ2019-CIE-01) is gratefully acknowledged. The plateau CARMA of the SFR Matrix is acknowledged for the fabrication and characterization of organic solar cells. Martin Blais thanks the University of Angers for his contract as Engineer.

Potential Push-Pull Chromophores: Synthesis, Characterization, TD-DFT Modeling, and Photophysical and NLO Properties. Organometallics 2018;37:2232-44.