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C: Energy Conversion and Storage; Energy and Charge Transport

Cyclopentadithiophene and Fluorene Spiro-Core Based Hole Transporting Materials for Perovskite Solar Cells Rana Nakar, Francisco Javier Ramos, Clement Dalinot, Pablo Simon Marques, Clément Cabanetos, Philippe Leriche, Lionel Sanguinet, Marwan Kobeissi, Philippe Blanchard, Jerome Faure-Vincent, François Tran-Van, Nicolas Berton, Jean Rousset, and Bruno Schmaltz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05931 • Publication Date (Web): 24 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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The Journal of Physical Chemistry

Cyclopentadithiophene and Fluorene Spiro-Core based Hole Transporting Materials for Perovskite Solar Cells Rana Nakar,a F. Javier Ramos,b,c,d Clément Dalinot,e Pablo Simon Marques,e Clément Cabanetos,e Philippe Leriche,e Lionel Sanguinet,e Marwan Kobeissi,f Philippe Blanchard,e Jérôme Faure-Vincent,g François Tran-Van,a Nicolas Berton,a Jean Rousset,b,d Bruno Schmaltza* a

Laboratoire de Physico-Chimie des Matériaux et des Electrolytes pour l’Energie (PCM2E),

EA6299, Université de Tours, Parc Grandmont, Tours 37200, France b

IPVF, Ile-de-France Photovoltaic Institute (IPVF), 30 Route Départementale 128, 91120

Palaiseau, France c

CNRS, Ile-de-France Photovoltaic Institute (IPVF), UMR 9006, 30 route départementale 128,

91120, Palaiseau, France d

EDF R&D, 30 Route Départementale 128, 91120 Palaiseau, France

e

MOLTECH-Anjou, UMR 6200, UNIV Angers, CNRS, SFR Matrix, 2 bd Lavoisier, 49045

ANGERS Cedex, France f

Laboratoire Rammal Rammal, Equipe SOA, Université Libanaise, Section V, Nabatieh, Liban

g

Université Grenoble-Alpes, CEA, CNRS, INAC-SyMMES, F-38000 Grenoble, France

Abstract New three-dimensional hole-transporting materials (HTM) based on either 9,9’spirobifluorene (SBF) or Spiro-[cyclopenta[1,2-b:5,4-b’]dithiophene-4,9’-fluorene] (SDTF) core have been synthesized. All three HTMs, namely SBFCz2, SDTFCz2 and SDTCz2F, are end-capped with two peripheral 3,6-dimethoxydiphenylaminyl-carbazole (CzDMPA) units. The HTMs behave as molecular glasses with glass transition temperature (Tg) close to or higher than that of the reference HTM Spiro-OMeTAD. Thermal and opto-electronic properties strongly depend upon the nature of the bridging core unit between the two CzDMPA units. The two fluorene-bridged molecules SBFCz2 and SDTFCz2 exhibit similar properties. On the contrary, SDTCz2F, where the CzDMPA units are bridged to the cyclopentadithiophene ring, displays lower HOMO/LUMO energy levels and smaller band gap. Upon doping of the HTM layer in perovskite solar cells (PSC), in spite of a much lower hole mobility, SDTCz2F leads to the highest power conversion efficiency (16.4% compared with 14.5% and 14.3% respectively for SBFCz2 and SDTFCz2). 1 ACS Paragon Plus Environment

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Introduction Among the whole photovoltaic (PV) technologies, hybrid solar cells based on organicinorganic metal halide perovskite have become one of the most promising topics in materials science in the past few years 1. Following the seminal work of Miyasaka et al. in 2009 on a perovskite-sensitized solar cell fabricated using a liquid electrolyte, perovskite PV was triggered by the 2012 report on the 9.7% efficient solid-state perovskite solar cell (PSC)2, where a perovskite was formed by spin-coating of high concentration precursor solution 3. Organic-inorganic metal halide perovskite solar cells have made impressive progress in a very limited period of time with maximum power conversion efficiencies (PCEs) now beyond 25% 4,5

. Nevertheless, even if these solar cells demonstrate potentiality, they still suffer from

stability issues. The envisioned strategies to address this concern include perovskite composition tuning, device encapsulation, interface engineering and use of protective holetransporting materials (HTMs) 6–13. On one hand, methyl ammonium lead iodide (MAPbI3) has been the most widely investigated organic-inorganic metal halide perovskite. However, the latter undergoes a phase transition at 55°C 14, detrimental for photovoltaic applications 15. Replacing MA by formamidinium (FA) could be advantageous due to its wide absorption spectrum 16. Nevertheless, FAPbI3 is known to crystallize at room temperature into a yellow photo-inactive phase 6. Furthermore, it is noteworthy that using purely inorganic cesium- and lead-based perovskites (CsPbBr3) is not favorable 17 due to incompatible energetic level, despite a good thermal stability 18. Therefore, mixing cation and halides in the design of the perovskite turns out to be a critical parameter to fabricate solar cells and achieve high efficiencies. Hence, Grätzel et al. reported a PCE up to 21.1% using a triple Cs/MA/FA cation mixture perovskite with high thermal and humidity stability 19. Also, it was shown that a small amount of cesium is sufficient to effectively inhibit the unwanted yellow phase, thus allowing the preparation of pure and stable perovskite films with improved processing reproducibility, which is one of the key requirements for costefficient large-scale manufacturing. On the other hand, charge extracting interface layers also play a critical role in device efficiency and stability 20,21. In order to achieve high efficiency, the fabrication of perovskite solar cells usually requires an organic hole transporting material (HTM). In fact, the best PCEs are presently reached with Spiro-OMeTAD or spirofluorene derivatives HTMs 22,23. However, the design of more efficient and affordable alternatives to Spiro-OMeTAD is extremely 2 ACS Paragon Plus Environment

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challenging and draws lots of research efforts24, using for instance thiophene-rich cores nitrogen-containing HTMs

27,28

or triphenylamine derivatives

29,30

25,26

,

. Nevertheless, using spiro-

core into the design of new HTMs is still an on-going concern and shows promising power conversion efficiencies

31,32

. Noteworthy, Spiro-[cyclopenta[1,2-b:5,4-b’]Dithiophene-4,9’-

Fluorene] (SDTF) core derivatives deserve particular attention because of their ease of tunability by molecular engineering and promising power conversion efficiencies in perovskite-based solar cells

33

. Beyond appropriate energy levels and charge transport

properties, the interface between perovskite and HTM seems to play an important role to increase the efficiency of charge transfer. For instance, it was shown in previous studies that the presence of methoxy functions on the chemical structure of HTM induces a better binding between the HTM and the perovskite, thus improving the quality of the interface

34,35

. The

sulfur atoms contained in the SDTF core may also bring additional thiophene-iodide interactions 33. In this context, we report herein the synthesis of three new molecular glasses based on threedimensional SBF and SDTF derivatives core end-capped with carbazole derivatives arms (Scheme 1).

Scheme 1. Chemical structure of the target HTMs SBFCz2, SDTFCz2, SDTCz2F and the reference SpiroOMeTAD compound.

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These new molecules have been designed in order to introduce as many methoxy functions as in the reference Spiro-OMeTAD HTM. The specificity of this design is i) to avoid a 4-fold difficult chemical coupling and ii) to leave one side of the Spiro-core free of any coupling. Regarding the carbazole derivative precursor, namely 3,6-dimethoxydiphenylamine-carbazole (CzDMPA), the latter has already been widely used in perovskite solar cells and already showed high efficiencies, and is therefore an adequate unit to introduce methoxy groups 36–41. Hence, the three reported molecules were designed to contain eight peripheral methoxy groups. Moreover, among those synthesized materials, two are Spiro[cyclopenta[1,2-b:5,4b’]dithiophene-4,9’-fluorene] (SDTF) derivatives (SDTFCz2 and SDTCz2F) and the third one is based on 9,9′-spirobifluorene (SBFCz2) to assess the impact of the π-conjugated spiro central core on the thermal, electronic and therefore photovoltaic properties. One can also note that the difference between SDTFCz2 and SDTCz2F lies in the location of the CzDMPA moieties, respectively on the fluorene and the cyclopentadithiophene part. These materials have then been evaluated through the fabrication of Cs0.05(MA0.166FA0.833)0.95Pb(Br0.166I0.833)3 triple cation perovskite solar cells. Results and discussion The common synthetic route to SBFCz2, SDTFCz2 and SDTCz2F is depicted in Scheme 2.

Scheme 2. Synthetic route to SBFCz2, SDTFCz2, SDTCz2F.

Herein, a carbazole linker, bearing two 4,4’-dimethoxydiphenylamine groups, is smartly used in a convergent synthetic strategy to introduce height methoxy functions on spirofluorene core derivatives with only two Buchwald coupling reactions (the first one being to prepare the carbazole-based precursor, and the second one to afford the target HTM). For comparison, the 4 ACS Paragon Plus Environment

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synthesis of the reference Spiro-OMeTAD compound requires four cross-coupling reactions 42

. Hence, SBFCz2 has been synthesized in fast one-step reaction between the commercialy

available precursor N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9-H-carbazole-3,6-diamine and brominated intermediate SBFBr2 under microwave conditions at 150°C for 2h. In parallel, SDTF derivatives bearing bromine atoms only on the fluorene moiety (SDTFBr2) or on the cyclopentadithiophene moiety (SDTBr2F) have been synthesized and similarly connected to the described carbazole precursor. All molecules show good solubility in common organic solvents such as chlorobenzene, toluene, tetrahydrofuran, dichloromethane. The three compounds were thus easily characterized by 1H/13C nuclear magnetic resonance (NMR) and mass spectra analysis, in agreement with their respective structures (cf. supporting information, Figure S1-S6).

1,0

Normalized absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SBFCz2 SDTFCz2 SDTCz2F

0,8

0,6

0,4

0,2

0,0 300

350

400

450

500

Wavelength (nm)

Figure 1. UV-Vis. absorption spectra of HTMs diluted in THF (solid line) and HTM thin films on glass substrate (dashed line).

The normalized UV-Vis absorption spectra of SBFCz2, SDTFCz2, SDTCz2F in THF solution and in thin film (spun cast on glass substrate) are plotted in Figure 1. Absorption bands appearing in the spectral region between 250 and 500 nm can be assigned to the π-π* transitions of the CzDMPA moiety 43,44. As shown in Figure 1, all the molecules in solution exhibit two 5 ACS Paragon Plus Environment


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maximum absorption bands around 300-310 nm and 375-390 nm. The shorter wavelength band is assigned to the π–π* transition of the carbazole and diphenylamine moieties, while the second band can be attributed to internal charge transfer on the CzDMPA units 45. As expected, replacing the fluorene spacer by the cyclopentadithiophene leads to a bathochromic shift in the case of SDTCz2F, characteristic of the lower resonance energy of thiophene rings

46

. In the

solid state, the spectra broaden and exhibit a bathochromic shift due to π-π stacking interactions. It is noticeable that the redshift is more pronounced for SDTCz2F, suggesting that the CzDMPA moieties induce stronger intermolecular packing when they are connected to the thiophenic part of the molecule. Optical band gap (Eg) calculated from the onset absorption band gives values between 2.59 eV and 2.76 eV (Table 1). Time Dependent Density Functional Theory (TD-DFT) calculations performed at the B3LYP/6-31G(d,p) level of theory indicate that the optical bandgap of molecules SBFCz2 and SDTCz2F can be ascribed to the HOMO/LUMO transition (Figure 2 and Table S1). Concerning compound SDTFCz2, as HOMO and LUMO are localized on independent conjugated systems, the related oscillator strength is low, and the first excited state is dominated by the HOMO to LUMO+1 transition.

Figure 2. HOMO, LUMO and LUMO+1 Molecular Orbitals obtained from DFT calculations at B3LYP/6-31G (d,p) level.

Cyclic voltammetry (CV) has been widely used to elucidate the redox behavior of π-conjugated molecules and to estimate their energy levels. The cyclic voltammograms are shown in Figure 6 ACS Paragon Plus Environment

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3. On the anodic sweep, all new compounds exhibit two reversible oxidations waves assigned to the formation of the stable radical cation and dication species 47. The HOMO energy levels were estimated from the half-wave oxidation potential of the first oxidation waves, considering Fc+/Fc couple at -5.1 eV versus vacuum 48. The calculated HOMO values are -5.18 eV for both SDTFCz2 and SBFCz2, -5.20 eV for Spiro-OMeTAD, and -5.25 eV for SDTCz2F. In order to probe the energy levels in solid state, photoemission spectroscopy in air (PESA) measurements were also performed on thin films (Figure S10). The measured work functions (-5.20 eV, -5.26 eV and -5.29 eV for SBFCz2, SDTFCz2 and SDTCz2F, respectively) are in good agreement with the HOMO levels determined in solution using CV. These energy levels seem to be well suited for charge transfer from the perovskite. On the other hand, it is noteworthy that LUMO levels (Table 1) are high enough to prevent electron back-transfer from the cathodic electrode to the perovskite layer

49

. The HOMO stabilization of SDTCz2F,

demonstrated by the higher oxidation potential, is in good agreement with DFT calculations (Figure 2) and can be related to the geometry of the molecule. Indeed, the high dihedral angle (65°) obtained for SDTCz2F (Figure S8) will likely result in weaker electron donating effect of the cyclopentadithiophene moiety compared to the fluorene one, thus reducing electron density on the carbazole moiety, on which the HOMO is mainly localized.

Figure 3. Cyclic voltammetry of HTMs in dichloromethane with 0.1 M TBAP (scan rate 50 mV.s-1).

The thermal properties of the synthesized compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Data are summarized in Table 1. High thermal stabilities were recorded for the three molecules as shown by their degradation 7 ACS Paragon Plus Environment


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temperatures (Td, at 5% weight loss), estimated around 400°C for SBFCz2 and SDTFCz2, and 266°C for SDTCz2F (Figure S7). The glass transition behavior of the new molecules was investigated using DSC under nitrogen at a heating rate of 20°C min-1 (Figure 4). All materials exhibit a molecular glass behavior without crystallization and melting peaks (Tg between 112°C and 145°C). SDTCz2F exhibits the lower Tg (112°C), which is comparable with the Tg of the Spiro-OMeTAD reference compound around 120°C 50. SDTFCz2 and SBFCz2 both show higher Tg of ca. 145°C and 136°C, respectively.

Figure 4. DSC curves of HTMs (second heating/cooling cycle) at a scan rate of 20°C/min under N2.

Conscious that one major limitation of the perovskite solar cell technology deals with the limited air stability, the use of a hydrophobic HTM layer to limit water diffusion turns out to be a definite advantage 51. Hence, contact angles (α) of deionized water on the surfaces of the three HTMs films were assessed (Figure S9). Thin films of SBFCz2 and SDTCz2F give much higher water contact angles of 104 and 105° respectively, in comparison to the SpiroOMeTAD film (85°). Therefore, replacing Spiro-OMeTAD by SBFCz2 or SDTCz2F would efficiently limits the water diffusion into the perovskite layer, thus potentially improves moisture stability of the overall cell. Considering SDTFCz2 film, the contact angle is fairly lower (94°) than SDTCz2F film. The difference in hydrophobic properties of both HTMs, designed from the same SDTF core could be explained by the specific orientation of the molecules relative to the substrate 52. Regarding the charge transport properties, organic field effect transistors (OFETs) were fabricated to evaluate the hole mobility. Figure S11 shows the plot of the square root of drain current versus 8 ACS Paragon Plus Environment

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gate voltage for OFETs prepared with SBFCz2, SDTCZ2F, CDTFCz2, in the saturation regime (Vds =-80V). As summarized in Table 1, a modest hole mobility of ca. 6.2×10-6 cm2.V-1.s-1 was estimated for SBFCz2. By replacing the orthogonal fluorene with the cyclopentadithiophene, the hole transport properties clearly improve by nearly an order of magnitude since a mobility of 2.4×10-5 cm2.V-1.s-1, similar to that of Spiro-OMeTAD (2.5×10-5 cm2.V-1.s-1), was measured for SDTFCz2 53 in the same conditions. However, involving the cyclopentadithiophene in the π-conjugated backbone (SDTCz2F) has a negative effect on hole mobility since a decrease of almost three orders of magnitude was recorded for SDTCz2F compared to its structural analogue SDTFCz2. Table 1. Summary of optical, thermal and electrochemical properties of HTMs.

SBFCz2 SDTFCz2 SDTCz2F SpiroOMeTAD

TD (°C)a

Tg (°C)b

λmax (nm) c

Egopt (eV) d

HOMO/LUMO (eV) f

Mobility (cm².V-1.s-1) g

442 402 266

137 145 112

377 378 387

2.76 2.74 2.59

-5.18/ -2.42 -5.18/ -2.44

6.2 × 10-6 2.4 × 10-5

-5.25/ -2.66

8.5 × 10-8

-

120

385

2.99

-5.20/-2.21

2.5× 10-5

a

TGA data recorded at a heating rate of 10°C. min-1.b DSC data recorded under an N2 atmosphere at a heating rate of 20°C.min-1.c UV-Vis spectral data measured in THF solution.d Optical band gap (Egopt calculated from the onset of thin films UV-Vis absorption spectra using the relation Egopt =1240/λonset.f HOMO value determined from the half-potential of the first oxidation wave of CV measurement EHOMO (eV) = - (5.1+E1/2/Fc (eV) ), and LUMO value calculated from the HOMO value and optical band gap (Eg) using the relation ELUMO= Eg + EHOMO ,g Mobility values measured by OFET in saturation regime.

The three compounds described above have been introduced as hole transport material (HTM) in the stacking of a perovskite based solar cell. A SEM image of a cross section of a complete cell terminated by a SBFCz2 presented in Figure 5a, shows that smooth, compact and covering films are obtained on the top of the perovskite layer. The impact of the layer thickness has been tested with fabrication of different devices, varying the rotating speed of the spin coater during the deposition process. The thicknesses have been estimated from the cross section image and range between 100 and 200 nm for 3000 rpm and between 200 and 300 nm for 2000 rpm. Moreover, the influence of the addition of the Li and Co doping agents classically used in the case of Spiro-OMeTAD, has also been explored. The J-V curves of champion cells, terminated with undoped hole transport materials are shown in Figure S14. Due to the low intrinsic hole mobility of the studied compounds the performances of those cells remain quite low but differences can be clearly observed. While the efficiencies measured with undoped SDTCz2F,



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i.e. the molecule that shows by far the lowest hole mobility, remain lower than 1% (not shown), the cells containing undoped SDTFCz2 and SBFCz2 present a modest but clear photovoltaic behaviour (Figure S14), exhibiting average PCE of 2.46% and 8.70%, respectively. Surprisingly, while SBFCz2 does not show the best mobility, this latter allows to reach a higher PCE, up to 9.54% owing to high JSC (18.70 mA cm-2) and VOC (1.02 V) values (the detailed photovoltaic parameters can be found in Table S2). As expected, the doping of HTM leads to a large increase of the cells performances mostly due to a remarkable increase of the fill factor. The best J-V curves measured with the different HTMs for the two tested thicknesses are presented in Figure 5 and the average results over four cells are shown in ESI (Figure S12). In a general manner, devices with the HTM films deposited at 3000 rpm, i.e. thinner ones, show the highest current and consequently the best efficiencies. Thus, PCEs of 14.50 %, 14.26% and 16.40% have been achieved for the champion cells containing SBFCz2, SDTFCz2 and SDTCz2F, respectively (the detailed parameters are summarized in Table S3). In addition, those champion cells exhibit stabilized efficiencies (after 300 s) of 13.3%, 13.6% and 15.0%, respectively (Figure S15). The increase in performance over illumination time is directly related with the increase in maximum power point voltage (VMPP). Interestingly, undoped molecules show much longer stabilization times than doped ones, as evidenced in Figure S16. In the case of doped molecules, a period between 30-60 s is necessary in order to achieve the stabilized efficiency. Although other effects can not be excluded, the HTMs, even when chemically doped, could still need a short time of light preconditioning to be properly photodoped, and so reach the stabilized power output. In contrast, for undoped molecules, the photodoping requirements are much longer in terms of time (around 2 minutes), consistent with the fact that in the absence of chemical doping, the whole doping effect is made through light. It can also be noticed that devices incorporating doped SBFCz2 and SDTFCz2 HTMs reach their highest efficiency in only ~30 s, (similar to doped Spiro-OMeTAD-based ones), while the process takes twice as long (~60 s) with SDTCz2F. This seems to indicate that a stronger photo-doping effect compensates for the much lower hole mobility of SDTCz2F. Independently of the HTM employed, the thinnest films are more efficient, indicating that the HTMs cover homogeneously the perovskite surface and prevent direct contact between the photoactive layer and the gold, a too high thickness being detrimental to the carrier collection. Moreover, the three tested molecules allow an efficient extraction of the holes created under illumination. The performances achieved with the new HTM compounds are promising although they remain lower than those obtained with the SpiroOMeTAD used as reference 10 ACS Paragon Plus Environment

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(11.3%/8.1% (reverse/forward) for an undoped material and 18.0 %/15.2% (reverse/forward) for a doped one, see Table S2 and Table S3, respectively), the major limitations being a lower photocurrent and a higher hysteresis mainly visible on the fill factor (leading to the discrepancy between the J-V curve and the current obtained from the EQE integration presented in Figure S13). This latter has been recently described as an interfacial phenomenon created by an undesirable accumulation of charges between the perovskite layer and the extraction contact 54,55

. The hysteresis is surely not entirely driven by the nature of the HTM as in our case either

the cells built with the Spiro-OMeTAD show significant a difference between the forward and the reverse scan. But this phenomenon might be related to a less efficient injection of hole and/or the formation of traps at the interface between the HTM and the perovskite. Further studies which are beyond the scope of the article are needed to clearly identify and overpass these limitations. One of the way would be to specifically adapt the deposition process to each tested molecules. To demonstrate the possibility to use these new molecules as HTM, we applied the same procedure for each and further improvements could be done with a careful adaptation of the process parameters or of the doping and precursor solution concentrations.

Figure 5. Cross-sectional Scanning Electron Microscopy image of a complete PSC terminated with a SBFCz2 film deposited at 3000 rpm made by Focused Ion Beam polishing and Scanning Electron Microscopy imaging (a). Reverse (solid line) and forward (dotted line) J-V characteristics for PSC terminated with different doped HTM deposited at 2000 rpm (blue) and 3000 rpm (black). (b) Spiro-OMeTAD reference (green) and SBFCz2; (c) SDTCz2; (d) SDTCz2F.



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Conclusion Three new hole-transporting materials based either on 9,9’-spirobifluorene or spiro[cyclopenta[1,2-b:5,4-b’]dithiophene-4,9’-fluorene]

(SDTF)

as

three-dimensional

spirofluorene derivative core have been synthesized, namely SBFCz2, SDTFCz2 and SDTCz2F. In order to promote hole transport, the core is end-capped with two peripheral 3,6dimethoxydiphenylaminyl-carbazole (CzDMPA), thus each new molecule contain height peripheral methoxy groups, improving perovskite/HTM interface. The HTMs behave as molecular glasses with glass transition temperature (Tg) of 137, 145, and 112°C for molecules SBFCs2, SDTFCz2, and SDTCz2F, respectively, close to or higher than that of the reference HTM Spiro-OMeTAD (120°C). Contact angle measurements on HTM thin films also revealed that the new HTMs are more hydrophobic than Spiro-OMeTAD (beneficial for better cell stability). Thermal and opto-electronic properties have been found to depend strongly upon whether the two CzDMPA units are bridged to a fluorene or cyclopentadithiophene core. The two fluorene-bridged molecules SBFCz2 and SDTFCz2 exhibit similar properties, with HOMO energy levels at -5.18 eV, and moderate hole mobilities of 6.2 × 10-6 and 2.4 × 10-5 cm².V1 -1

.s respectively. On the contrary, SDTCz2F, where the CzDMPA units are bridged to the

cyclopentadithiophene moiety, displays a lower HOMO level (-5.25 eV), smaller band gap, as well as reduced degradation temperature and hole mobility (8.5 × 10-8 cm².V-1.s-1). The stabilization of the HOMO level of SDTCz2F is further confirmed by PESA measurements in solid-state and can be related to the geometry of the molecule as suggested by DFT calculations. Although the low hole mobility of SDTCz2F makes it unsuitable for fabrication of dopant-free PSC, leading to 16.4% PCE upon doping of the HTM layer in triple cation PSC, compared with 14.5% and 14.3% PCE using SBFCz2 and SDTFCz2, respectively. These non-optimized photovoltaic results demonstrate the potential of the three-dimensional SDTF spirofluorene derivative as π-conjugated core in association with carbazole-based peripheral units in order to design efficient HTMs for PSC.

Acknowledgement This work was supported by the French ministry of Higher Education and Research. B. Schmaltz and M. Kobeissi are grateful to the CELEZ project supported by the Région Centre, France and CEDRE project (37335YM) for its financial support. The LANEF framework (ANR-10-LABX-51-01) is acknowledged for its support with mutualized infrastructure. This project has been supported by the French Government in the frame of the program of 12 ACS Paragon Plus Environment

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investment for the future (Programme d'Investissement d'Avenir - ANR-IEED-002-01). Authors thank the MATRIX SFR of the University of Angers and particularly the CARMA platforms for the characterization of organic compounds (PESA). P.S.M. thanks the European Union’s Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement No.722651 (SEPOMO). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxx. General materials and methods; synthetic procedures and characterization, such as 1H NMR, 13C NMR, and mass spectrometry (HRMS); computational details; device fabrication and characterization, such as EQE measurements.

Corresponding Author * E-mail : [email protected]

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