Thieno[3,2-b]thiophene and triphenylamine-based hole transport materials for perovskite solar cells

Heterocyclic compounds have played significant roles in achieving high performance as hole transport materials (HTMs) for perovskite solar cell (PSC) applications. Various studies have focused on the development of fused heterocyclic conjugated structures for hole transport materials. In this report, three novel π-extended conjugated materials (M1-M3), based on thieno[3,2-b]thiophene (TT) and 4,4′-dimethoxytriphenylamine [TPA(OMe)₂], were designed and successfully synthesized via Palladium (0) catalyzed Suzuki coupling reaction. Their optical, electrochemical, and thermal properties were investigated by UV-Vis, fluorescence, cyclic voltammetry, and thermal analysis. The materials were utilized as hole transport materials in p-i-n architecture perovskite solar cells, which displayed performances of open-circuit voltage (V_oc) as high as 1,050 mV, a maximum short-circuit current (J_sc) of 16,9 mA/cm², a maximum fill factor (FF) of 29.3%, and a power conversion efficiency (PCE) of 5.20%. This work demonstrated that thieno[3,2-b]thiophene and TPA(OMe)₂-based structures are promising cores for high-performance hole transport materials in perovskite solar cell architecture.

1 Introduction

Organic perovskites have received significant attention over the past two decades due to the utility of solar energy (Peak et al., 2016; Liu et al., 2017; Zhang et al., 2017). Regarding their performances, perovskite solar cells (PSCs) highly depend on hole-transporting materials (HTMs), which play a crucial role in hole extracting and transferring (Lv et al., 2015; Wu et al., 2018; Sharma et al., 2021). As a general approach, current is generated in PSCs through the flow of photogenerated electrons (e⁻) from the perovskite layer to the electron transport layer (ETL) and holes (h⁺), and to the hole transport layer (HTL) (Xu et al., 2020; Afanasyev et al., 2021).

Organic hole conductors essentially include two categories: (i) small molecular and (ii) polymeric materials (Turksoy et al., 2003; Ertas et al., 2004; Akman et al., 2020; Chen et al., 2021; Farokhi et al., 2022). Small molecular HTMs have some advantages over polymers such as defined molecular structure, definite molecular weight, easy purification, and good batch-to-batch reproducibility (Lin et al., 2020; Pan et al., 2020; Tavasli et al., 2022; Yildiz et al., 2017; M’Baye et al., 2007). Among them, TAA-based compounds are the most widely used HTMs in PSCs due to their excellent charge transport capabilities (Isci et al., 2020a; Isci, 2020b; Isci, 2021a; Isci, 2022a). Furthermore, the presence of phenylamines in the molecular structure of organic polymers confers unique mechanical features (Wang et al., 2016; Isci et al., 2022b; Farokhi et al., 2022; Quezada-Borja et al., 2022). Thienothiophenes have been attractive small, conjugated compounds for synthesizing semiconducting polymers and are widely used as building blocks for organic materials as they have electron-rich, flat, rigid, and good electron delocalized skeleton, prolonged molecular conjugation, intermolecular S…S interactions, and chemical stability (Ozturk et al., 1995; Saygili et al., 2001; Osken et al., 2012; Cinar et al., 2015; Isci et al., 2021b; 2022d). Thieno[3,2-b]thiophene, in particular, is the most conjugated and stable isomer of TTs, having interesting electronic and optical properties. It has been examined and used in various energy-based organic electronic applications by our group such as OLED (Isci et al., 2020a; 2022a), OFET (Amna et al., 2022), electrocatalyst (Isci et al., 2022d), capacitor (Topal et al., 2021), and memory (Gunturkun et al., 2022).

In this study, we report the synthesis and characterization of three novel 4,4′-dimethoxytriphenylamine [TPA(OMe)₂] and thieno[3,2-b]thiophene (TT)-based conjugated materials, having different electronic substituents such as C₆H₁₃, 4-PhOMe, and Ph. Their photophysical and electrochemical properties were investigated by UV-Vis spectroscopy, emission spectroscopy, and cyclic voltammetry. Moreover, they were used as small molecules in hole transport materials (HTMs). M1-M3 displayed remarkable properties such as mega Stokes shift of 5,067 cm⁻¹, optic band gaps changing between 2.80 and 2.67 eV, and theoretical band gaps around 2.35 eV, which are consistent with the experimental results. The maximum fill factor reached 32.4% and the maximum Voc was measured to be 1,050 mV in device fabrications. This work demonstrated that the introduction of a TT core is a promising strategy for achieving high performance of HTMs toward PSCs.

2 Materials and methods

All the reagents purchased from Sigma-Aldrich and Acros Organics were used without further purification, and the solvents used in the syntheses were of technical grade. Column chromatography was performed with ≤0.063 μm silica gel for purification. ¹H and ¹³C NMR spectra were recorded on Varian 500 and 126 MHz spectrometers, respectively. Proton and carbon chemical shifts were reported in parts per million downfield from tetramethylsilane (TMS). FTO glasses, cut in 2.5 cm² × 2.5 cm² (OPV-FTO22-15, 14Ω/□), were purchased from OPV Tech. Hydrochloric acid (37%), 4-tert-butylpyridine (TBP, 98.0%), isopropyl alcohol (HPLC Grade, 99.9%), acetonitrile (ACN, technical grade, 99.8%), chlorobenzene (anhydrous, 99.8%), titanium isopropoxide {Ti[OCH(CH₃)₂]₄, 97%}, and cesium iodide (CsI, 99.999%) were acquired from Merck. Lead iodide (PbI₂, 99.99%) was purchased from Tokyo Chemical Industries. Methylammonium bromide (MABr, >99.5%), lead bromide (PbBr₂ 99.999%), and formamidinium iodide (FAI, >99.5%) were obtained from Lumtec. Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.0%) and DMF (anhydrous, >99.5%) were purchased from Acros Organics. DMSO (>99.7) and Spiro-OMeTAD {2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene} were bought from Merck and Borun New Material Technology, respectively.

3 Result and discussion

3.1 Design and synthesis

The monoketones (3-5), TTs (6-8), some of the dibrominated TTs (9, 11), and 4,4′-dimethoxytriphenylamine [TPA(OMe)₂] were synthesized following our previous reports (Isci et al., 2020a; Isci, 2021a). Initially, the synthesis of the core units, thieno[3,2-b]thiophenes 3-5, was conducted starting from 3-bromothiophene (1). The monoketones, 3-5, were constructed in a one-pot three-step reaction: (i) lithiation of 3-bromothiophene (1) with n-butyllithium at −78°C, (ii) additions of elemental sulfur, and then (iii) α-haloketones. The yields were calculated to be 55%, 90%, and 85%, respectively. Their ring closure reactions were conducted in the presence of polyphosphoric acid (PPA) in refluxing chlorobenzene to give 6-8 in 87%, 85%, and 80% yields, respectively. The dibrominated TTs 9-11 were obtained through selective dibromination of 6-8 using NBS at −10°C in DMF in 85%, 80%, and 90% yields, respectively. Their Suzuki coupling reactions with boronated 4,4′-dimethoxytriphenylamine produced the target functionalized thienothiophenes M1-M3 in 55%, 60%, and 65% yields, respectively (Scheme 1).

SCHEME 1

SCHEME 1. Synthetic route of the materials M1-M3.

3.2 Optical properties

Photophysical properties (UV-Vis absorption and emission spectra) of the compounds were investigated in THF at room temperature (Table 1; Figure 1A). The maximum π-π* absorption wavelengths (λ_max) of the compounds M1-M3 were measured to be 401, 389, and 404 nm, respectively. The absorbances of the TTs showed increasing bathochromic shift from hexyl (-C₆H₁₃) to dimethoxytriphenylamine para substituents, indicating that high electron donating dimethoxytriphenylamine substituted M3 exhibited the higher bathochromic shifts of π-π* transitions, compared to weak electron-donating hexyl (-C₆H₁₃)-substituted M2. M1, having 4-MeOPh, which is a weaker electron donor than dimethoxytriphenylamine but a stronger donor than hexyl, had absorbance between M2 and M3. From the absorption spectra, the onset maximums of 458, 445, and 464 nm were observed for M1-M3, respectively. Then, the optical band gaps of the polymers were determined to be 2.71 (M1), 2.80 (M2), and 2.66 eV (M3).

TABLE 1

TABLE 1. Optical and electronic data of M1-M3.

FIGURE 1

FIGURE 1. (A) UV-vis absorption spectra and (B) emission spectra of M1-M3 in THF.

The fluorescence measurements of the materials were performed in THF (Table 1; Figure 1B). They displayed emission maxima of 492 (M1), 477 (M2), and 508 nm (M3). A similar trend, i.e., the longest emission with M3 (540 nm), was observed, which could be due to the presence of more electron-donating units compared to the others, providing the system with a more efficient charge-transfer property. The Stokes shifts ranged from 88 (M2) to 104 (M3) nm. Mega Stokes shift (>100 nm) of M3 indicated a fast relaxation from the excited state to the ground state owing to intramolecular charge transfer between TT and 4,4′-dimethoxytriphenylamine groups.

In addition to the measurements in solution, absorption properties of the materials spin-coated on ITO were investigated. Solid-state UV maximums were measured to be 429 (M1), 394 (M2), and 468 nm (M3) (Supplementary Figure S1; Table 1). As expected, solid-state absorptions had red shifts compared to their absorptions in THF, signifying an enhancement of π-conjugation and good π-stacking in their film forms.

3.3 Electrochemical properties

Regarding potential applications of the materials as organic electronics, their electrochemical properties have significant as well as optical features. The HOMO and LUMO levels play important roles in charge injection and charge transport of active materials in device fabrications. The electrochemical features were conducted by cyclic voltammetry (CV) measurement in a mixture of dichloromethane/acetonitrile (1:1) in the presence of tetrabutylammonium hexafluorophosphate (TBAPF₆) as an electrolyte at a scan rate of 100 mV·s⁻¹, using Pt wires as counter and working electrodes and Ag wire as a reference electrode (Supplementary Figure S2). The oxidation onset potentials (E_onset) were observed to be 0.65, 0.68, and −0.61 V for M1-M3, respectively. On the basis of the oxidation onset potentials and optical band gaps, the HOMO/LUMO energies of the materials were calculated to be −5.05/−2.34, −5.08/−2.28, and −5.01/−2.34 eV, respectively, using the equations of HOMO = −(E_onset + 4.40) (eV) and LUMO = (HOMO + E_optic) eV (Table 1). With the highest HOMO energy level, M3 with three TPA(OMe)₂ substituents confirmed the presence of the best electron-donating moieties. A higher HOMO level presents increased donor ability. While M2 with weak electron-donating alkyl substituent had the lowest HOMO level, M1 remained between M2 and M3. The results were in good agreement with both theoretically predicted energy gaps and experimental optic band gaps (Table 1).

3.4 Computational chemistry

The structures of the molecules were modeled by using density functional theory (DFT). Geometry optimizations were conducted at B3LYP/6-31G (d, p) level of theory in the implicit THF medium (ε = 7.4257), applying conductor-like polarizable continuum solvent model (CPCM) (Cossi et al., 2003). All computations were performed using the Gaussian’16 program package (Frisch et al., 2016). The optimized geometries and the related frontier orbitals are given in Figure 2.

FIGURE 2

FIGURE 2. Optimized geometries (in THF) and HOMO (below) and LUMO (above) frontier orbitals of (A) M1, (B) M2, and (C) M3 molecules.

The HOMO orbital was found to be delocalized over the main skeleton of the molecule, while the LUMO orbital was mainly located on the TT unit and the adjacent rings. Time-dependent DFT (TD-DFT) calculations were performed at the PBE0 level of theory in the same basis set in solvent media to compute the theoretical band gaps and characteristic wavelengths. The optical band gaps were predicted and calculated from the λ_onset of the UV curves (Table 2). Theoretically predicted optical band gaps were found to be consistent with the corresponding experimental data.

TABLE 2

TABLE 2. The calculated energies of the frontier orbitals, HOMO-LUMO energy gap (Δ), λ_max, λ_onset, and optical band gap values (in THF) for the studied molecules.

3.5 Device fabrications

J-V curves were obtained using a solar simulator equipped with a Keithley 2400 source meter without any prior conditioning. A light source with an AM1.5G filter was used, and 100 mW/cm² intensity of light was maintained during all J-V measurements. Before each measurement, the light power was confirmed using a calibrated silicon reference solar cell with a 4 cm² area. The voltage scan was realized with a fixed rate of 10 mV·s⁻¹ for collecting photovoltaic parameters, and the voltage sweep was set to start from negative to positive during all J-V measurements. The average photovoltaic parameters for the cells were calculated from data collected from four devices for each batch of materials. J_sc values were measured as 12.4, 4.53, and 16.9 mA·cm⁻² for M1-M3, respectively. The higher J_sc value of M3 compared to other HTMs was attributed to a better intramolecular charge transfer between TT and 4,4′-dimethoxytriphenylamine moieties. Considering that M3 had a better intramolecular charge transfer, it was expected to collect photo-induced free charge carriers generated in the perovskite layer more efficiently than M1 and M2 with minimum recombination losses through metal contacts.

Methoxy (-OCH₃) functional groups on the HTMs are responsible for the molecular arrangement as well as intermolecular charge transfer via alignment of the HOMO energy level of the HTM molecules. Although the methoxy group has an electron-withdrawing inductive effect, it can also exhibit electron-donating behaviors depending on the substitution of ortho, para, and meta positions to the phenyl functional group under resonance stabilization which is called the mesomeric effect (Jeon et al., 2014). V_oc values were measured as 980, 630, and 1,050 mV for M1, M2, and M3, respectively, which were found to be consistent with J_sc and PCE values, indicating that charge recombination ratios at the perovskite/HTM interface are lower with M3. The recombination rate is strongly effective on the J_sc values as well as V_oc and FF. Parallel resistance (Rp) is much larger in the case where M3 HTM is used compared to M1 and M2, where the recombination ratio is minimized (Figures 3, 4). Attributing the Voc directly to the difference in HOMO energy levels of the HTMs is thought to be misleading as the distinction between both experimental and calculated HOMO energy levels of the molecules are very close to each other.

FIGURE 3

FIGURE 3. Current (J)–voltage (V) curves of the solar cells with HTM 1-3 (M1-M3) and Spiro-OMeTAD.

FIGURE 4

FIGURE 4. The energy level diagram of PSC.

Functionalizing the main core of TT with methoxybenzene and 4,4′-dimethoxytriphenylamine moieties was found to be a promising strategy for adjusting HOMO energy levels to be compatible with the HOMO energy level of the perovskite material for extracting holes efficiently (Wu et al., 2018). Also, high LUMO energy levels of HTMs can efficiently act as an electron blocker and prevent short-circuit. Finally, this molecular design strategy is found to be promising for obtaining high J_sc values due to better intramolecular charge transfer.

The current-voltage curves of the devices, fabricated with the given novel hole transport materials, exhibited relatively low fill factors of 32.4%, 25.5%, and 29.3% for M1-M3, respectively, which could be due to high levels of series resistance. This is known to occur with first and higher orders of recombination. The first order of recombination, being surface type and trap-assisted recombinations, is thought to be the primary reason for the relatively low fill factor, given the fact of apparent morphological inconsistencies on the surface of the coated HTM (Wolff et al., 2021). Low parallel resistance is also evident, as it is thought to be due to the defects in the interface of the p-n junction, where the charges are ineffectively separated, which is the interface between the absorber perovskite and the HTM. This causes local short circuits within the device.

As a result of relatively good J_sc and V_oc values, PCE values of 3.94, 0.73, and 5.2, corresponding to M1-M3 were obtained, respectively (Table 3). Comparable device parameters indicate that these novel HTMs can pave the way for forthcoming TT-based molecules as promising candidates for a typical perovskite solar cell architecture.

TABLE 3

TABLE 3. HTM performances of M1-M3.

3.6 Thermal properties

The thermal properties of the M1-M3 were investigated by thermal gravimetric analysis (TGA) at 800°C at a heating rate of 10°C min⁻¹ under an N₂ atmosphere. Although all materials showed high thermal stability with similar degradation profiles, different para substituents on the TT cores led to different thermal behaviors. While the first decomposition temperatures (Td, corresponding to 5% weight loss) of M1 and M2 were measured to be at around 185°C and 167°C, respectively, M3 had a decomposition temperature of 261°C (Figure 5). M3 had relatively better thermal stability compared to M1 and M2, indicating that -TPA(OMe)₂ substituent on the M3 skeleton provided higher thermal resistance, possibly, due to having more aromatic units compared to -PhOMe and -C₆H₁₃ functional groups on M2 and M3, respectively.

FIGURE 5

FIGURE 5. TGA diagram of the materials M1-M3.

4 Conclusion

In summary, three novel thienothiophene (TT) and 4,4′-dimethoxytriphenylamine [TPA(OMe)2]-based conjugated molecules (M1-M3) were successfully synthesized using the Suzuki coupling method. Their photophysical and electrochemical properties were investigated both experimentally and theoretically. M1-M3 were applied as hole transporting materials (HTMs), reaching overall conversion efficiency of 5.2% in p-i-n architecture perovskite solar cells. Additionally, molecular structures, electronic HOMO−LUMO levels, optic band gaps, and device performances were compared with each other. Moreover, experimental optoelectronic analyses were supported by theoretical studies based on density functional theory (DFT) calculations. This study offers a facile approach for developing new HTMs that could boost the PSC performance via molecular engineering using thienothiophene and triphenylamine units (Topal et al., 2022, Isci et al., 2022c).

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

RI: writing-original draft, methodology, formal analysis, and investigation. MU, TY, and AE: investigation, methodology, and data curation. BS: formal analysis and software. CZ and TO: project conceptualization, supervision, and editing. All authors have read and approved the manuscript for publication.

Funding

We thank the Higher Education Council of Türkiye (YOK) and TUBITAK grants to RI (PhD., 100/2000 YOK and 2211A BIDEP/TUBITAK); ITU (Istanbul Technical University), 122Z568 numbered TUBITAK 1001 Project; TY within the Program nr: 2211/C (Grant nr: 1649B032100473); and Unsped Global Lojistik, Türkiye, for the financial support. The computing resources used in this work were provided by the National Center for High-Performance Computing (UHeM) under grant number 1010722021.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2023.1125462/full#supplementary-material

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