Structure–property correlations of tetraphenylbenzidine-based self-assembled monolayers for perovskite and organic solar cells†

Wei Wang‡ , Yao Xu‡ , Baobing Fan , Hui Li , Zhengwei Hu , Houji Cai , Yi Zhang , Haiyang Zhao , Yazhong Wang , Wenkai Zhong *, Fei Huang * and Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, Guangdong Basic Research Center of Excellence for Energy & Information Polymer Materials, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. E-mail: wkzhong@scut.edu.cn; msfhuang@scut.edu.cn

First published on 23rd June 2025

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1. Introduction

Perovskite solar cells (PSCs) and organic solar cells (OSCs) have emerged as cost-effective photovoltaic techniques, offering unique advantages in solution processability, mechanical flexibility, and tunable optoelectronic properties.^1–9 In particular, the use of self-assembled monolayers (SAMs) as hole transport layers (HTLs) has enabled remarkable device efficiencies, with power conversion efficiencies (PCEs) exceeding 26% for p–i–n type PSCs and 20% for OSCs.^10–15 Such breakthrough stems from the atomic-level thickness control and tailorable interfacial properties of SAMs.^16,17 Despite these successes, the development of universal SAM platforms suitable for both PSCs and OSCs remains challenging.

Conventional SAMs design strategies focus on three modular components: anchoring groups for ITO binding, π-conjugated linkers for controlling film thickness and uniformity, and functional heads for regulating energy alignment.^18,19 Carbazole-based derivatives, such as Me-4PACz and 2PACz, have dominated SAMs research, achieving enhanced hole extraction through HOMO level optimization.^20–23 Yet, their planar molecular structures predispose them to detrimental aggregation during solution processing, leading to inhomogeneous interfacial dipoles and incomplete defect passivation.²⁴ Alternative SAM designs by introducing donor–acceptor architectures or acridine motifs have been proposed. While promising, these approaches are often tailored specifically for mixed-cation perovskites and have yet to demonstrate general applicability across different photovoltaic systems.^25,26 Therefore, there remains a strong need for more versatile SAMs that can simultaneously offer appropriate energy-level alignment, low optical losses, and thermal and morphological stability.^27–29

In this work, we present a series of novel N,N,N′,N′-tetraphenylbenzidine (TPD)-based SAMs, namely 6PA-TPD, 6PA-TPDO, and 6PA-TPDF, designed to explore the divergent needs of PSCs and OSCs. TPD was selected as the core unit due to its chemically tunable energy levels, high hole mobility, and thermally stable arylamine framework.^30–33 The substituent engineering with methoxy groups or fluorine atoms allows for modulating molecular geometry and dipole moments, highest occupied molecular orbital (HOMO) levels, work functions (WF), surface morphology, and wetting property.³⁴ By correlating such properties with the photovoltaic performance in both OSCs and PSCs, we uncover key design rules and performance trends, revealing how different substitution patterns influence interfacial energetics, morphology, and device efficiency. Our findings offer valuable insights into the development of universal SAMs for high-performance, solution-processed solar cells.

2. Results and discussion

The synthetic routes for the three SAM molecules, 6PA-TPD, 6PA-TPDO, and 6PA-TPDF, are outlined in Fig. 1a and follow a concise three-step sequence. The synthesis begins with a Buchwald–Hartwig amination between N⁴,N^4′-diphenyl-[1,1′-biphenyl]-4,4′-diamine and a hexyl-functionalized aryl bromide to construct the central TPD core (compound 1). This is followed by a phosphonation reaction to form the phosphonate ester (compound 2), and subsequent hydrolysis yields the phosphonic acid-functionalized target molecule (compound 3). By introducing different substituents, hydrogen, electron-donating methoxy groups, or electron-withdrawing fluorine atoms, on the starting diamine, we obtained three SAMs: 6PA-TPD, 6PA-TPDO, and 6PA-TPDF, respectively. This substitution strategy enables systematic tuning of the various molecular properties for photovoltaic applications. Detailed synthetic procedures (Schemes S1–S5, ESI†) and full characterization data, including ¹H, ¹³C and ³¹P nuclear magnetic resonance (NMR) as well as high-resolution mass spectrometry (HRMS), are provided in the ESI† (Fig. S9–S27).

The thermal stability of SAMs was evaluated by thermogravimetric analysis (TGA) (Fig. 1b). The results reveal that decomposition temperatures (T_d, defined as the temperature at 5% weight loss) for 6PA-TPD, 6PA-TPDO, and 6PA-TPDF were determined to be 375 °C, 308 °C, and 237 °C, respectively, indicating sufficient thermal robustness for use in photovoltaic applications. The UV-vis absorption spectra of the SAMs in THF solution and as thin films on indium tin oxide (ITO) substrates were shown in Fig. 1c. All SAMs in solution show similar absorption profiles with onset wavelength at ∼400 nm and absorption peak at ∼350 nm. The absorption in thin films has red-shift of absorption peaks compared to their solution states. The appearance of Urbach tails suggests the disorder in molecular packing and energetic landscapes in thin film state.

The HOMO energy levels of SAMs were estimated via electrochemical cyclic voltammogram (CV) measurements (Fig. S1, ESI†). The lowest unoccupied molecular orbital (LUMO) levels were calculated based on the optical bandgap and HOMO levels. The HOMO of 6PA-TPD, 6PA-TPDO and 6PA-TPDF were −5.20 eV, −5.06 eV and −5.21 eV, respectively; the LUMO were −2.24 eV, −2.17 eV and −2.35 eV, respectively (Fig. 1d). Detailed data are summarized in Table S1 (ESI†). As anticipated, the presence of methoxy leads to a shallowing of the HOMO energy levels, while the introduction of fluorine atoms results in a deepening of HOMO levels. Nevertheless, the HOMO levels of the SAMs are slightly shallower than those of common donor polymers such as D18 or PM6 (∼−5.5 eV) for OSCs, which can facilitate hole extraction due to the favorable downhill energy offset. For p–i–n type PSCs, where effective hole extraction and interfacial energy alignment are critical for minimizing hysteresis and improving stability, these SAMs offer appropriate energetics to interface with the perovskite valence band (∼−5.4 eV). Therefore, these energy levels suggest that all three SAMs are suitable as HTLs in both OSCs and p–i–n type PSCs.

To gain insight into the structure–property relationship of the SAM molecules, density functional theory (DFT) calculations were performed at the B3LYP-D3(BJ)/6-31G(d,p) level.³⁵ These simulations provide key information on the molecular geometry, dipole moment, frontier molecular orbitals, and electrostatic potential (ESP) distributions of 6PA-TPD, 6PA-TPDO, and 6PA-TPDF. The calculated dihedral angles of the central diphenyl cores reveal differences in molecular planarity: 39.0° for 6PA-TPD, 40.2° for 6PA-TPDO, and 35.6° for 6PA-TPDF (Fig. S2, ESI†). The enhanced planarity in 6PA-TPDF can be attributed to the electron-withdrawing nature of the fluorine substituents, which diminish the electron density around the nitrogen atoms and thus lower the torsional strain in the conjugated backbone. The dipole moments and ESPs further reflect the impact of substituents on the electronic structure. 6PA-TPD and 6PA-TPDO exhibit relatively high dipole moments of 5.12 D and 5.15 D, respectively, while the symmetric and planar structure of 6PA-TPDF results in a lower dipole moment of 4.08 D (Fig. 2a–c). The ESP maps reveal the spatial charge distribution, showing strong electronegative regions near the phosphonic acid anchoring groups in all three molecules (Fig. 2d–f). In particular, 6PA-TPDF has ESP with symmetric distribution but less negativity through the TPD core, which is consistent with its reduced dipole moment. Such differences in dipole moment and ESP distribution are expected to influence molecular packing and intermolecular interactions, thereby affecting film morphology and the work function (WF) modulation of the underlying electrode.^36–38

Fig. 2g–i shows the frontier molecular orbital distributions of the SAMs. For 6PA-TPD and 6PA-TPDO, the HOMOs are localized primarily on the triphenylamine moiety on one side of the TPD core, while the LUMOs are situated on the opposite side, indicating a strong charge-transfer character due to the twisted backbone (Fig. S3, ESI†). This spatial separation of frontier orbitals can reduce carrier delocalization and help suppress charge recombination in p–i–n type PSCs.²⁴ In comparison, 6PA-TPDF shows more delocalized HOMO and LUMO extending across the entire TPD unit, including the fluorinated aryl groups. The presence of fluorine not only increases the planarity of the molecule but also leads to a more uniform electron cloud distribution, which could facilitate charge transport pathways across the heterointerface in devices.

Kelvin probe force microscopy (KPFM) was used to evaluate the surface contact potential difference (CPD) and corresponding WF shifts induced by SAM modification (Fig. 3a–c). Calibration was first performed on a gold reference (WF = 5.2 eV), yielding a baseline CPD of −0.502 eV (Fig. S4, ESI†). Using this calibration, the CPD values of ITO modified with 6PA-TPD, 6PA-TPDO, and 6PA-TPDF were determined to be −0.264 eV, −0.321 eV, and −0.024 eV, respectively, corresponding to WFs of 4.962 eV, 5.019 eV, and 4.722 eV. These results were further confirmed by scanning Kelvin probe microscopy (SKPM), showing consistent trends (Fig. S5 and Table S2, ESI†). Meanwhile, all the films show uniform potential across the probed area, indicative of good anchoring for SAMs on ITO. Such results indicate that the WF of ITO/6PA-TPDO is deeper than that of pristine ITO (4.998 eV), which may be due to the increased higher dipole moments of 6PA-TPDO as demonstrated previously.

Atomic force microscopy (AFM) was used to examine the surface morphology of the electrodes (Fig. 3d–f). The surface root-mean-square (RMS) roughness decreased upon SAM deposition, from 5.94 nm for pristine ITO to 1.35 nm, 1.54 nm, and 3.48 nm for ITO/6PA-TPD, ITO/6PA-TPDO, and ITO/6PA-TPDF, respectively. To quantify the uniformity of the surface morphology, power spectral density (PSD) analysis was performed (Fig. S6, ESI†). The 6PA-TPDF sits highest, indicating it has the largest overall roughness, in good agreement with its RMS. The PSD roll-off from the low- to high-frequency regions marks a characteristic correlation length (R_g), which was extracted by unified fit (Fig. S7 and Table S3, ESI†).³⁹ Both 6PA-TPD and 6PA-TPDO have large correlation lengths of 108 nm and 120 nm, respectively, representing gently varying surfaces. In contrast, the 6PA-TPDF shows a smaller value of 39 nm, indicative of finer, more short-range topographic features.⁴⁰

Surface morphology can well correlate with the wetting behavior of the solutions used for film deposition, which in turn influences crystallization and the resulting film quality.⁴¹ To quantify this effect, we measured contact angles of water and ethylene glycol (EG) on each SAM-modified ITO surface and calculated surface energies via the Owens–Wendt–Rabel–Kaelble method (Fig. 3g, h and Table S4, ESI†). The surface energy decreases in the order 6PA-TPDO (49.9 mJ m⁻²) > 6PA-TPD (34.7 mJ m⁻²) > 6PA-TPDF (32.3 mJ m⁻²). The high surface energy of 6PA-TPDO represents the most hydrophilic interface, which is beneficial for spreading polar solvents, including the DMF:dimethyl sulfoxide (DMSO) mixtures used for perovskite precursor solutions, leading to controlled nucleation during film formation.^42–44

To evaluate photovoltaic performance of SAMs as HTLs in p–i–n type PSCs, devices with structures ITO/SAM/Al₂O₃/Cs_0.05FA_0.95PbI₃/PEAI/PCBM/BCP/Ag were fabricated (Fig. 4a, left). The current density–voltage (J–V) curves are shown in Fig. 4b, while the key metrics including open-circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (FF) and PCE are summarized in Table 1. The device employing 6PA-TPD shows a PCE of 22.42%, with a V_OC of 1.08 V, J_SC of 24.66 mA cm⁻², and an FF of 84.17%. Replacing 6PA-TPD with 6PA-TPDO led to a higher PCE of 23.18%, due to slight improvements in both J_SC and FF. However, the 6PA-TPDF device shows a notable reduction in FF (68.14%), resulting in a significantly declined PCE of 17.28%. External quantum efficiency (EQE) spectra confirmed the accuracy of the J_SC measurements (Fig. 4c), with integrated current densities (J_EQE) of 23.60, 23.72, and 22.45 mA cm⁻² for 6PA-TPD, 6PA-TPDO, and 6PA-TPDF devices, respectively, closely aligning with the values in J–V curves and validating the effective photon-to-current conversion.

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To study photovoltaic performance of the SAMs in OSCs, devices with structure ITO/SAM/PM6:BTP-eC9:L8-F/PDINOH/Ag were fabricated (Fig. 4a, right). The J–V curves are presented in Fig. 4e, with corresponding photovoltaic parameters summarized in Table 2. Interestingly, the OSC performance displays an inverse trend compared to PSCs for 6PA-TPDO and 6PA-TPDF. The 6PA-TPD based device shows a PCE of 16.19% (V_OC = 0.856 V, J_SC = 27.17 mA cm⁻², FF = 69.60%). The use of 6PA-TPDO as the HTL leads to a pronounced reduction in V_OC with 0.652 V, resulting in a sharp drop in PCE of 11.84%. The 6PA-TPDF device achieved the highest PCE among the three, exhibiting a PCE of 18.27%, with simultaneously improvements in J_SC and an FF compared to the 6PA-TPD device. EQE spectra confirm the similar J_SC observed in J–V curves (Fig. 4f). Additionally, these SAMs were tested in binary OSCs using the PM6:Y6 blend as the active layer (Fig. S8 and Table S5, ESI†), which shows a similar trend in PCE for different SAMs. Such results further verify the broader compatibility of 6PA-TPDF as a high-performance HTL across different active layer systems.

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The contrasting trends observed for our three TPD-based SAMs in OSCs versus p–i–n type PSCs motivated a targeted investigation into their distinct design rules. To uncover the underlying relationships, we analyzed the correlations between various SAM molecular properties, including dihedral angle, dipole moment, HOMO level, WF, RMS roughness, R_g, and surface energy, and key photovoltaic parameters (J_SC, V_OC, FF, and PCE) for each device type. Stronger pairwise correlations are indicated by higher Pearson correlation coefficients (r).

The correlation map in Fig. 4d highlights key structure–property relationships of the three SAMs in p–i–n type PSCs. PCE is almost driven by V_OC (r = +0.79) and FF (r = +0.77). V_OC in turn is dictated by the deeper HOMO (r = −0.79), larger molecular dipoles (r = +0.92), increased backbone planarity (r = +0.86), and higher WF (r = +0.96), which increases V_OC by minimizing interfacial barriers and suppressing recombination. FF, meanwhile, depends primarily on interfacial morphology: smoother films, i.e., low RMS roughness (r = −0.96) and larger lateral correlation lengths, i.e., R_g (r = +0.85) facilitate more uniform charge extraction and lower series resistance. When considered together, the dihedral angle, dipole moment, WF, and R_g all exhibit strong positive correlations with the final PCE. These parameters are interrelated rather than independent. For instance, methoxy substitution maintains a degree of backbone torsion while redistributing the ESP, thereby enhancing the molecular dipole. This increase in dipole moment contributes to optimize surface morphology and leads to a higher WF that promotes charge extraction.

As shown in Fig. 4g, for OSCs, PCE also strongly and positively correlates with both V_OC (r = +0.97) and FF (r = +0.97), and shows virtually no correlation with J_SC (r = −0.25). V_OC exhibits a strong negative correlation with the HOMO level (r = −0.96), implying that reducing the HOMO offset between the SAM and the donor material is critical for maximizing V_OC. Similarly, FF shows a strong negative correlation with the HOMO level (r = −0.86) and a moderate positive correlation with RMS surface roughness (r = +0.77). These results suggest that for performance optimization in OSCs, the molecular design of TPD-based SAMs should prioritize deepening the HOMO level, via the introduction of strong electron-withdrawing groups or extended conjugation, over modifications targeting planarity, dipole moment, or WF.

3. Conclusions

In summary, a series of novel SAMs, namely 6PA-TPD, 6PA-TPDO, and 6PA-TPDF, were developed using a TPD core as the anchoring unit. Using 6PA-TPD as a reference, we systematically investigated how methoxy (6PA-TPDO) and fluorine (6PA-TPDF) substitutions influence key SAM characteristics, including molecular dihedral angle, dipole moment, HOMO level, WF, surface morphology, and surface energy. The photovoltaic performance of these SAMs was evaluated as HTLs in both p–i–n type PSCs and OSCs. In p–i–n type PSCs, the 6PA-TPDO-based device achieved a higher PCE of 23.18%, surpassing the 22.42% from the 6PA-TPD device. In contrast, in OSCs, the 6PA-TPDF-based device delivered the highest PCE of 18.27%, outperforming both 6PA-TPD and 6PA-TPDO counterparts. To uncover structure–function relationships, Pearson correlation analysis was performed, revealing two distinct yet complementary design strategies for TPD-based SAMs: for p–i–n type PSCs, performance enhancements require synergistic tuning of molecular dipole moment, WF, and surface morphology through molecular torsion and electrostatic potential engineering; for OSCs, focusing on deepening the HOMO level is sufficient to simultaneously improve both V_OC and FF. These insights establish practical design rules for tailoring TPD-based SAMs to meet the interfacial requirements for different photovoltaic technologies.

Author contributions

Wei Wang: writing – original draft, methodology, investigation; Yao Xu: investigation, formal analysis, validation; Baobing Fan: investigation, formal analysis, validation; Hui Li: investigation, formal analysis, validation; Zhengwei Hu: software; Houji Cai: software; Yi Zhang: software; Haiyang Zhao: software; Yazhong Wang: formal analysis; Wenkai Zhong: conceptualization, writing – review & editing, funding acquisition; Fei Huang: conceptualization, writing – review & editing, resources, supervision, funding acquisition; Yong Cao: resources, supervision. Wei Wang and Yao Xu contributed equally to this work.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

This work was financially supported by Fundamental Research Funds for the Central Universities (2024ZYGXZR076) and Guangzhou Basic and Applied Basic Research Project (No. 2025A04J3912).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01880c

‡ These authors contributed equally to this work.

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