Arm modulation of triarylamines to fine-tune the properties of linear D–π–D HTMs for robust higher performance perovskite solar cells

Abstract

Most organic hole-transport materials (HTMs) toward efficient perovskite solar cells (PSCs) thus far still rely on methoxytriphenylamine, which limits the photovoltage and decrease the stability of PSCs. However, alternative donors with better performance are scarce. Herein, we employ the synergistic strategy to modulate the properties of triarylamines (TAAs) by introducing different functional groups on their outer/inside arm. It is found that the arm modulation of asymmetrical TAAs can balance well the hole transport, HOMO level, film quality and stability of the studied HTMs M140 and M141. The photovoltaic performances of devices with doped and dopant-free M140 and M141 have been investigated. Encouragingly, the doped M141 device featuring the N-(9,9-dimethyl-9H-fluoren-2-yl)-N-(4-methoxyphenyl)spiro[fluorene-9,9′-xanthen]-2-amine (MP-F-SFX) asymmetrical TAA achieves a significantly boosted power conversion efficiency (PCE) of 20.74%, accompanied by long-term stability in ambient air. This work not only provides a promising donor candidate (MP-F-SFX), but also reveals an effective way of using rationally designed asymmetrical triarylamines to fine-tune the properties of linear D–π–D HTMs.

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1. Introduction

Organic–inorganic metal halide perovskite solar cells (PSCs) have gained enormous attention during the past few years because of their excellent optoelectronic properties, low cost and high efficiency, which make them promising candidates for next-generation photovoltaic devices.^1–3 PSCs hold considerable promise in photoelectric energy conversion; their power conversion efficiencies (PCEs) have increased rapidly due to the improvement of perovskite composition, optimization of transport layers, and passivation of interfaces.^4–13 Recently, an enormous amount of effort has been devoted to hole-transport materials (HTMs) for further enhancing the efficiency and stability of halide PSCs.^14–59 HTMs not only efficiently extract holes from the perovskite active layer but also suppress charge recombination at the anode, which makes them a crucial part of state-of-the-art n–i–p-type PSCs.

HTMs can be divided into four main categories: inorganic HTMs, polymers, metal–organic compounds and organic molecular HTMs.⁶⁰ In the case of organic HTMs, 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) is the most commonly used organic HTM for PSCs, in which record-high efficiencies have been achieved. However, Spiro-OMeTAD suffers from a low hole mobility in its pristine form because of its propeller-like structure due to the sp³-hybridization atom linkage.¹⁴ In addition, its complex synthetic process significantly limits the suitability for large-scale industrial application of Spiro-OMeTAD in PSCs. Small-molecule alternatives with high hole mobility have been extensively investigated.^60–65 Among them, most of the linear small molecule HTMs contain the core and the methoxy-substituted triphenylamine (OMe-TPA), in which the two methoxy groups are very beneficial for hole transport. However, the methoxy moieties significantly upshift the HOMO level and decrease the glass transition temperature (T_g) of HTMs, which limits the open-circuit voltage (V_OC) and reduce the device stability.^49,66,67 In this respect, a balanced point between the advantage of hole transport and the disadvantages of V_OC loss and low stability must be determined.

Thanks to their tunable functionalized structures, it is attractive to optimize the properties of HTMs through the design of triarylamines (TAAs), which can be modulated by rational design of their outer and inside arms to obtain HTMs characterized by desired properties. Even though the effectiveness of the asymmetrical strategy has been demonstrated in optimizing bis(4-methoxyphenyl)amine for HTMs,^{57,66,68–70} asymmetrical TAAs have rarely been reported in linear D–π–D HTMs and the influence of the outer arm and inside arm (Fig. 1) of asymmetrical TAAs on the optoelectronic properties and hole transport of linear D–π–D HTMs remains to be addressed.

Fig. 1 Chemical structures of asymmetrical TAAs, M140, M141 and M130.

Herein, we report two new linear D–π–D HTMs (M140 and M141) based on asymmetrical TAAs, i.e. N,N-bis(4-methoxyphenyl)spiro[fluorene-9,9′-xanthen]-2-amine (DMP-SFX, Fig. 1) and N-(9,9-dimethyl-9H-fluoren-2-yl)-N-(4-methoxyphenyl)spiro[fluorene-9,9′-xanthen]-2-amine (MP-F-SFX). To the best of our knowledge, both asymmetrical TAAs have never been evaluated in linear D–π–D HTMs. Breaking the symmetric structure of triphenylamine by replacing the benzene unit with a dimethylfluorene or SFX to form M140/M141 enabled us to deeply understand how the outer and inside arms of TAAs affect the photophysical, electrochemical, and thermal properties and hole mobilities as well as the photovoltaic performance of HTMs.

We demonstrated that the arm modulation of asymmetrical TAAs can balance well the hole transport, HOMO level, film quality and stability of linear D–π–D HTMs. Introducing the SFX unit as the inside arm can effectively downshift the HOMO level of HTMs and thus improve the V_OC of devices. On the other hand, the outer arm not only plays an extra role in preventing HTM aggregation, but also shows positive effects on the energy level, hole transport and molecular stability of HTMs. More notably, the synergistic effect due to the inside and outer arms of MP-F-SFX resulted in a doped HTM characterized by significantly reduced V_OC loss, improved charge carrier mobility and excellent film formability. Accordingly, the champion cell based on doped M141 with the MP-F-SFX donor achieved a PCE of up to 20.74%. These results qualify MP-F-SFX as a potential challenger to OMe-TPA.

2. Experimental

2.1 Design and synthesis

The rationale for developing asymmetrical TAAs is as follows: (1) we retained at least one methoxy at the arms of TAAs for efficient hole-transport. (2) The fused ring structure of dimethylfluorene/spiro[fluorene-9,9′-xanthene] (SFX) is expected to increase the thermal stability and improve the film quality of HTMs. (3) Dithieno[3,2-b:2′,3′-d]pyrrole (DTP) was employed as the core to facilitate intramolecular charge transfer (ICT) in D–π–D HTMs. In addition, OMe-TPA-based M130 (Fig. 1) was evaluated as a reference HTM. The synthetic routes towards the target HTMs are shown in Scheme S1 (ESI†). The final molecules were identified using NMR spectra; these analytical data for all HTMs are in good agreement with the formulated structures. Details of measurements, device fabrication, instrumentation, characterization, and ¹H NMR and ¹³C NMR spectra (Fig. S1–S5) of the new compounds are shown in the ESI.†

3. Results and discussion

3.1. Optoelectronic properties and calculation

The optical properties of M140 and M141 were evaluated by using absorption and emission spectroscopy (Fig. 2a), with the corresponding photophysical data summarized in Table 1. In CH₂Cl₂ solution, the UV-vis absorption spectra of M140 and M141 were similar, displaying absorption peaks (λ_abs) at 446 and 447 nm, respectively. Clearly, the influence of the peripheral N-substituent in the SFX-based triarylamine on the ICT of the HTM is insignificant, which is different from a previous study reported by Seo and co-workers.⁶⁶ They observed a 16 nm shift of the absorption peak when the outer anisole was replaced by the fluorene unit in the peripheral group of spiro compounds. Therefore, the influence of the outer arm on the ICT progress depends on the conjugation strength between two nitrogen atoms. In the cases of M140 and M141, their inside arm governs the optical bandgap as well as absorption properties. The λ_abs value of M140 has a significant bathochromic shift (21 nm) in comparison with M130 due to an extended π-conjugation through the SFX unit compared to the benzene unit. To gain insight into the structural and electronic properties of both the HTMs, density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level (Fig. 2b). The highest occupied molecular orbitals (HOMOs) for M140 and M141 are localized on the whole molecules, while the electron density distributions of their lowest unoccupied molecular orbitals (LUMOs) are mainly located on the SFX-DTP-SFX part of the HTMs. Therefore, both M140 and M141 have a large degree of conjugation, which is an important approach to improve interchain π–π overlapping while obtaining broader absorption.⁷¹

Fig. 2 (a) The UV-vis absorption and emission spectra of HTM solutions. (b) Molecular orbital surfaces of the HOMO and LUMO. (c) DPV curves of HTMs and ferrocene. (d) Energy level diagram of the device.

Table 1 Optical properties, redox potentials, mobilities and T_g of HTMs

HTM	λ abs /nm	E g /eV	HOMO/V vs NHE^c	LUMO/V vs NHE^d	Mobility/cm^2 V^−1 s^−1	Conductivity/S cm^−1	T g/°C
a The absorption peaks of M140 and M141 in DCM. b E g is the energy gap between the HOMO and LUMO. c The HOMO was recorded by DPV. d The LUMO was calculated from HOMO − Eg. e These data have been reported in ref. 36.
M140	446	2.55	−5.13	−2.58	2.39 × 10^−4	4.98 × 10^−6	183
M141	447	2.53	−5.16	−2.63	5.60 × 10^−4	5.34 × 10^−6	144
M130^e	425	2.65	−5.08	−2.43	3.71 × 10^−4	3.70 × 10^−6	101

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To evaluate the band alignment of these HTMs with the perovskite, differential pulse voltammetry (DPV) measurement was carried out (Fig. 2c) and the corresponding data are listed in Table 1. The incorporation of dimethylfluorene in the outer arm of M141 resulted in a low-lying HOMO of −5.16 eV, which is slightly deeper than that of M140 (−5.13 eV). In contrast, the HOMO energy level of M130 (−5.08 eV) was upshifted as compared to that of M140 when the inside arm (SFX) was replaced with the benzene unit. Apparently, the incorporation of bulky groups with weak donating capability either in the inside or outer arm in TAAs is beneficial for down-shift of the HOMO level. On the other hand, the LUMO levels of M140 and M141 are calculated to be −2.58 eV, and −2.63 eV, respectively, which are more positive than that of the mixed-perovskite (−3.93 eV).

3.2. Thermal properties

Fig. 3 displays the differential scanning calorimetry (DSC) thermograms of M140 and M141 after two heating–cooling cycles. Only a T_g was recorded during the second cooling for M140 and M141, with T_g values of 144 °C and 183 °C, respectively, indicating the amorphous properties of both HTMs.⁷² Moreover, the T_g values of M140 of M141 are much higher than that of M130 (101 °C, Table 1), suggesting their significantly better morphological stability. Clearly, the T_g values of linear TAA HTMs can be improved by the structural bulk added in the outer and inside arms, leading to higher stability since phase transitions could be avoided during device operation. The decomposition temperatures (T_d) of M140 and M141 were assessed by thermogravimetric analysis (TGA). Both HTMs have a relatively high decomposition temperature around 430 °C, with a weight loss of 5% (Fig. S6, ESI†), indicating their good thermal stability.

Fig. 3 DSC curves of M140 and M141.

3.3. Conductivity and hole mobility of doped HTMs

To investigate the influence of peripheral triarylamines on the final photovoltaic properties of these DTP HTMs, we recorded the conductivity and hole mobility of doped M140 and M141. The two-contact electrical conductivity (Fig. 4a) and space-charge-limited currents (SCLCs, Fig. 4b) were analyzed, and the corresponding data are listed in Table 1. The conductivity of M141 (5.34 × 10⁻⁶ S cm⁻¹) is slightly higher than that of M140 (4.98 × 10⁻⁶ S cm⁻¹), which causes a positive effect on the intermolecular charge transfer. Similarly, the hole mobility of M141 (5.60 × 10⁻⁴ cm² V⁻¹ s⁻¹) is higher than that of M140 (2.39 × 10⁻⁴ cm² V⁻¹ s⁻¹). Recently reports proposed that introduction of 3D electronic structures in HTMs could improve the intermolecular interactions to some extent and thus enhance the transporting channels.⁷⁰ Therefore, it is reasonable for M141 to have a higher conductivity and hole mobility than those of M140 because MP-F-SFX is a more twisted triarylamine than DMP-SFX.

Fig. 4 (a) Conductivity and (b) hole mobility of doped M140 and M141.

3.4. Photovoltaic properties of doped HTMs

The photovoltaic performance of the PSC devices using doped M140 and M141 as HTMs with a device configuration of ITO/SnO₂(with KCl treatment⁷³)/perovskite/HTL/Au was systematically investigated. Doped M130 and Spiro-OMeTAD based PSCs with the same device configuration were employed as references to better clarify the photovoltaic performance. The incident photon-to-current conversion efficiency (IPCE) was measured as shown in Fig. S7 in the ESI.† Compared with the J_SC values from J–V measurements, the integrated currents from the IPCE measurements are slightly lower, a result which can be attributed to the weaker intensities of individual wavelengths.³⁹

Fig. 5 (a) Cross-sectional SEM image of a PSC device with doped M141. Forward and reverse scanning J–V curves of PSCs based on doped M140 (b) and M141(c). (d) J–V curves of the optimized devices with different HTMs. (e) Statistical distribution of PSC performances with doped M140, M141 and M130. (f) Steady-state current densities of the PSCs based on doped M140, M141 and M130.

Fig. 5a displays the cross-sectional scanning electron microscope (SEM) image of a device based on M141. The thicknesses of the M141 and perovskite films are ca. 175 and 420 nm, respectively. It can be found that M141 forms a dense film on the flat perovskite-absorbing layer. This can be attributed to the twist structure of MP-F-SFX, which could alleviate the aggregation degree of the HTM and thus contributes to the quality of the film. After optimization of the concentration of HTMs (Fig. S8, ESI†), the M140-based device measured with a forward scan showed a PCE of 19.00% with an open circuit voltage (V_OC) of 1.11 V, a short-circuit current density (J_SC) of 23.13 mA cm⁻², and a fill factor (FF) of 0.74 under 1 sun illumination (AM 1.5G, 100 mW cm⁻²) (Fig. 5b and Table 2). In contrast, the devices based on M141 achieved a better performance with a PCE of 20.74%, a V_OC of 1.13 V, an FF of 0.79, and a J_SC of 23.24 mA cm⁻² (Fig. 5c).

Table 2 Photovoltaic performances of PSCs under AM1.5G illumination (100 mW cm⁻²)

HTM	J SC/mA cm^−2	V OC/V	FF	Best PCE/%	Average PCE^a/%
a The average PCE is obtained from 20 devices. b Spiro: abbreviation of Spiro-OMeTAD. c Doped HTMs. d Dopant-free HTMs.
M140 (D)	23.13	1.11	0.74	19.00	18.63
M141 (D)	23.24	1.13	0.79	20.74	20.28
M130 (D)	23.20	1.07	0.75	18.62	18.20
Spiro^b (D^c)	23.26	1.10	0.78	19.95	19.79
M140 (DF^d)	22.99	1.07	0.57	14.00	12.70
M141 (DF)	21.39	1.05	0.50	11.23	11.02

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It should be noted that devices based on both HTMs show negligible hysteresis, which can be attributed to the minimization of interfacial nonradiative recombination losses at the SnO₂ (with KCl treatment)/perovskite. The defect state passivation of perovskite with the K⁺ cations and Cl⁻ anions resulted in a faster electron transfer from perovskite to SnO₂ and a slower recombination rate in the bulk perovskite film.⁷³ Also, the best J–V curves for the cells based on M130 and Spiro-OMeTAD (Fig. 5d) were collected and calculated to obtain the photovoltaic parameters as summarized in Table 2. The best efficiency of M141 is significantly improved compared with that of M130 (18.62%), hence confirming the positive influence of the MP-F-SFX triarylamine on HTM performances.

The performance statistics of PSCs are outlined in Fig. 5e; the average values for M140 and M141 are 18.63% and 20.28%, respectively (Table 2). The greatly improved PCE for M141 is mainly attributed to the highly enhanced FF and V_OC imparted by the change of the peripheral triarylamine, in which the incorporation of fluorene in MP-F-SFX resulted in a 2.3-fold higher hole mobility and 0.03 eV downshift of the HOMO (Table 1). Furthermore, to demonstrate the reliability of the device J–V curves, the stabilized current density was recorded at the maximum power point bias within 150 s for M140- and M141-based devices (Fig. 5f). M141 has a stable PCE output of 20.51% and a photocurrent of 21.15 mA cm⁻², while the stable PCE output is 18.27% and the photocurrent is 19.84 mA cm⁻² for M140.

3.5. Surface morphologies of doped HTMs

The surface morphology of a doped HTM film is concerned with device parameters because a deteriorated morphology usually results in severe recombination and poor charge transport. The morphologies of the perovskite surface and HTM-coated perovskite films were analyzed by atomic force microscopy (AFM, Fig. 6a–c) and scanning electron microscopy (SEM, Fig. 6d and e). The AFM image of the bare perovskite film exhibits a uniform surface with a crystal size in the range of 400–500 nm (Fig. 6a). M141 formed a uniform and pinhole-free film with the lowest root-mean-square (RMS) roughness value of 4.99 nm, verifying its excellent film-forming characteristics, which is in favor of increasing the interface interaction with perovskites and suppressing recombination. The RMS of the M140-coated film (6.73 nm) is larger than that of the M141-coated film (4.99 nm), which is caused by the enhanced crystallization tendency of M140 (see the next section). Similarly, the SEM image of M141 shows an extraordinarily homogeneous and smooth surface without any observable defects and particles (Fig. 6e). In contrast, some pinholes and particles existed in the M140 (Fig. 6d) and M130 counterparts (Fig. 6f).

Fig. 6 AFM images (the scan size was 5 μm × 5 μm) of a bare perovskite film (a), and doped M140-coated (b) and M141-coated (c) perovskite films. SEM images of doped M140-coated (d), M141-coated (e) and M130-coated (f) perovskite films.

To investigate the interactions between HTMs, the absorption of thin films based on M140, M141 and M130 was studied as shown in Fig. S9 (ESI†). In thin film form, the absorption spectrum of M140 shifts to a short wavelength by approximately 16 nm in comparison with its corresponding spectrum in solution, indicating H-aggregate formation in films. In contrast, the M130-based film exhibits a tiny shift of the absorption peak compared to that of solution. Therefore, the introduction of SFX in triarylamines leads to H-aggregates of HTMs. There are two reasons for the result. First, the spiro structure of SFX does not increase the intermolecular distance due to the nonplanar structure of the triarylamine. Second, the oxygen atom on the SFX may enhance the intermolecular interactions. Moreover, the M141-based film exhibits a small shift of the absorption peak (5 nm) compared to that of solution, although it has an SFX arm. Apparently, the outer arm of M141 results in a big 3D structure, which reduces the aggregates of HTMs.

3.6. Charge transport at the perovskite/HTM interface

Morphological analysis as discussed above implies that the interface contact for the doped M141-based device would be better than that of M140, which is intrinsically linked to the charge transport and charge recombination at the perovskite/HTM interface. The contribution of charge recombination/transport is further evaluated using the steady-state and time-resolved photoluminescence (PL and TRPL) spectra. As presented in Fig. 7a, the PL intensity in the central area of the perovskite is much higher than those of M140, M141 and M130, which indicates very efficient PL quenching for these HTMs. The quenching efficiency (i.e., opposite to the PL intensity) decreases in the order of M141 (94.1%) > M140 (93.0%) > M130 (90.8%). Clearly, M141 bearing the MP-F-SFX donor bridge could quench the PL of the perovskite more efficiently than M140 and M130. The TRPL decay curves in Fig. 7b were fitted with the biexponential function I(t) = A₁exp(−t/τ₁) + A₂ exp(−t/τ₂), where τ₁ and τ₂ are the fast lifetime and long lifetime, respectively. We found that the perovskite films coated with M141 had a shorter τ₁ (2.21 ns) than the samples based on M140 (τ = 3.88 ns) and M130 (τ₁ = 3.63 ns). The results indicate an ultrahigh efficiency in the hole transfer process at the interface between the perovskite and M141, which finally contributes to the high FF value of the M141-based PSC.

Fig. 7 (a) Steady-state PL spectra and (b) time-resolved PL decays of a bare perovskite film, and M140-, M141- and M130-coated perovskite films. (c) Open circuit voltage decay profiles and (b) Nyquist spectra of the studied PSCs.

Meanwhile, fast hole transfer could mitigate the negative effect of charge recombination. To compare the recombination kinetics of the devices based on different HTMs, the open-circuit voltage decay (OCVD) measurement was carried out. Fig. 7c shows the OCVD profiles of devices based on these HTMs. There is very fast voltage decay at the initial stage. After that, the electron recombination at the interface of SnO₂ and the perovskite layer and HTMs starts to contribute to the voltage loss.⁷⁴ The slopes become steeper and steeper for devices based on M141, M140 and M130, indicating that the electron lifetimes are gradually enhanced with the increased bulk of the triarylamine donor.

Charge recombination behavior at the interfaces of PSCs was further investigated using electrochemical impedance spectra (EIS) of the corresponding PSCs in the dark at a potential bias of 0.9 V. The Nyquist plots in Fig. 7d show a semi-circle in the middle-frequency region, corresponding to the charge transport resistance (R_ct) at the perovskite/HTM interfaces. The M141-based device exhibits the largest R_ct (6914 Ω) value compared with the M140- (6046 Ω) and M130-based devices (5704 Ω), suggesting that M141 as a HTM can effectively reduce the charge recombination. The result is consistent with the results obtained from the OCVD measurements.

3.7. Device stability of doped devices

The operational stability of the devices in ambient air is in high demand for commercial application, which has a positive correlation with the HTM layer. Contact angle measurements of water droplets on the surfaces of doped M140, M141 and M130 are shown in Fig. 8a–c (the dopants used in the measurements are the same as in the photovoltaic devices). The M141-based film was hydrophobic with a water contact angle of 87.5°. In contrast, the M140- and M130-based films featuring two methoxy groups display lower water contact angles of 82.0° and 73.2°, respectively. The major contribution to the increased water contact observed for the M141-based film arises from its better high-quality film and decreased methoxy groups in the TAA. Therefore, the introduction of the MP-F-SFX triarylamine is beneficial for protection of the perovskite layer against degradation in the presence of moisture.

Fig. 8 The water contact angles of doped M140- (a), M141- (b), and M130-coated films (c) on ITO. (d) The normalized PCEs of PSCs based on M141 and Spiro-OMeTAD stored in ambient air with a humidity of about 30%.

Operational stability data obtained with the device based on M141 are shown in Fig. 8d. The non-encapsulated device based on M141 was stored in ambient air with a humidity of about 30%. The device retains 80% of the initial PCE after storage for 1000 h, which is much better than the reference device based on Spiro-OMeTAD. The PCE of the Spiro-OMeTAD-based device rapidly decreases to around 62% of its initial value after 1000 h under the same conditions. Our analyses here suggest M141's ability to enable PSCs with high stability without sacrificing cell efficiency, a result that can be attributed to the high uniformity and enhanced hydrophobicity of the M141-based film.

3.8. Photovoltaic properties of dopant-free HTMs

To further investigate the influence of asymmetrical TAAs on the performance of HTMs, devices with dopant-free HTMs were fabricated. Fig. S10a (ESI†) presents a comparison of the best J–V curves of PSCs based on dopant-free M140 and M141. Unexpectedly, planar n–i–p structured PSCs based on dopant-free M140 achieve a PCE of 14.00% (Table 2), which is much better than that of M141 (PCE = 11.23%). The statistical distribution of dopant-free PSC performances (Fig. S10b, ESI†) further confirms this conclusion. The trend is contrary to the results based on doped devices. We further recorded the hole mobilities of dopant-free HTMs (Fig. S10c, ESI†). The hole mobility of additive-free M140 (2.25 × 10⁻⁶ cm² V⁻¹ s⁻¹) is higher than that of M141 (1.35 × 10⁻⁶ cm² V⁻¹ s⁻¹). It is well known that dopants such as LiTFSI and TBP are indispensable for Spiro-OMeTAD due to the orthogonal structure of spirofluorene and the propeller-like TPA.¹⁶ M140 and M141, likewise, have bulky propeller-like TAA, which hinders the formation of close π–π stacking. However, MP-F-SFX in M141 is bigger than DMP-SFX in M140, leading to a decreased hole mobility of M141 when the dopants were removed. It should be noted that the PCEs of the dopant-free devices are much lower than those of the doped devices, suggesting that the M140 and M141 HTMs with bulky TAAs still rely on dopants. How to construct efficient dopant-free HTMs with bulky nonplanar TAAs is still a challenge.

4. Conclusions

In summary, we developed two new linear D–π–D HTMs based on asymmetrical TAA donors (DMP-SFX and MP-F-SFX). The arm modulation of the asymmetrical TAAs enables selective control of the energy level, hole mobility, film quality and thermal stability of linear D–π–D HTMs: (1) It is determined that the incorporation of highly conjugated SFX as the inside arm of the TAA makes it difficult for M140/M141 to oxidize, which, in turn, results in deep HOMO levels and thus enhanced V_OCs of devices. (2) The tendency of oriented aggregation (crystallization) of linear D–π–D HTMs, however, is governed by the outer arm of the TAA. Despite the perpendicular spiro structure, the inside SFX arm could not alleviate the HTM aggregation degree. (3) The promising donor MP-F-SFX exhibits the synergistic effects of its low-lying HOMO, improved hole mobility, high glass transition temperature and excellent film formability. Consequently, PSCs based on doped M141 provided an impressive PCE of 20.74% along with negligible hysteresis and good stability. We strongly believe that the design and synthesis of asymmetrical TAAs based on the synergistic strategy will drive the development of linear D–π–D HTMs toward robust higher performance PSCs.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 22075210), the 111 Project (B12015), the Tianjin Natural Science Foundation (18JCYBJC21600, 18JCZDJC97000) and the Key University Science Research Project of Jiangsu Province (20KJA480001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00190f

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