Rational molecular design of multifunctional self-assembled monolayers for efficient hole selection and buried interface passivation in inverted perovskite solar cells

Abstract

Self-assembled monolayers (SAMs) have been widely employed as the bottom-contact hole-selective layer (HSL) in inverted perovskite solar cells (PSCs). Besides manipulating the electrical properties, molecularly engineering the SAM provides an opportunity to modulate the perovskite buried interface. Here, we successfully introduced Lewis-basic oxygen and sulfur heteroatoms through rational molecular design of asymmetric SAMs to obtain two novel multifunctional SAMs, CbzBF and CbzBT. Detailed characterization of single-crystal structures and device interfaces shows that enhanced packing, more effective ITO work function adjustment, and buried interface passivation were successfully achieved. Consequently, the champion PSC employing CbzBT showed an excellent power conversion efficiency (PCE) of 24.0% with a high fill factor of 84.41% and improved stability. This work demonstrates the feasibility of introducing defect-passivating heterocyclic groups into SAM molecules to help passivate the interfacial defects in PSCs. The insights gained from this molecular design strategy will accelerate the development of new multifunctional SAM HSLs for efficient PSCs.

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Introduction

Organic–inorganic hybrid perovskite solar cells (PSCs) are considered to be one of the most exciting advancements in emerging photovoltaic technologies with a transcendent power conversion efficiency (PCE) currently reaching 26.1%.^1–3 Apart from engineering the perovskite compositions^4,5 and crystallization kinetics,^6–10 the key to further enhancing the performance and stability of PSCs lies in delicate engineering of the interfaces¹¹ between the perovskite absorber and contact interlayers.^12–15 In inverted PSCs, the use of ultrathin self-assembled monolayers (SAMs)^16–18 that can efficiently extract holes from perovskite to the anode as the hole-selective layer (HSL) has become increasingly popular.^19–21 Unlike PTAA or other conventional organic hole-transporting materials,²² SAMs offer new opportunities as an economic, scalable, and stable HSL in inverted PSCs due to the extremely low material consumption, simple fabrication, and feasibility of molecularly tailoring their chemical structures.^23,24

SAM molecules based on functionalities of electron-rich conjugated backbones and phosphonic acid anchoring groups have been successfully adopted in high-performance inverted PSCs.^25–29 The electron-rich conjugated backbones should have a suitable highest occupied molecular orbital (HOMO) energy level and a suitable dipole moment pointing toward the ITO substrate for effectively tuning the substrate work function (WF).^18,30–32 Therefore, having the SAM HOMO energy level well-aligned with the perovskite valence band maximum (VBM) and controlling the molecular dipole orientation of the SAM with respect to the substrate are critical in designing new SAMs.^33,34 In addition, the structural tunability of the SAM backbone enables the introduction of functional groups that are capable of passivating defects of perovskites. This allows the integration of effective hole selectivity and passivation of buried interfaces in PSCs on one SAM molecule.

Asymmetric conjugated molecules usually have large molecular dipole moments,³⁵ but their dipole moments tend to cancel each other in the aggregated state by forming a quasi head-to-tail packing motif.³⁶ However, as SAM formation is templated by the substrate, the orientation of asymmetric SAM molecules can be fixed to avoid canceling the dipoles. Thus, asymmetric conjugated molecules having strong molecular dipole moments can potentially improve the performance of SAM HSLs. Our recently reported carbazole-based asymmetric conjugation-extended SAM molecule CbzPh,²⁷ which was used as a HSL in inverted PSCs, exhibited superior performance to the symmetric carbazole-based SAM molecule 4PACz. This is attributed to the enhanced stacking and molecular dipole of the extended asymmetric conjugation, and their synergistic effect further alleviates the dipole elimination brought about by the disorderly oriented molecular dipole moments in the aggregated state. More recently, Wang et al. down-shifted the HOMO energy level of CbzPh through bromination,³⁷ making it suitable as a HSL for both wide-bandgap PSCs and organic solar cells, which also demonstrates the potential of SAM molecules based on an asymmetric conjugated backbone. Therefore, it is promising to further develop high-performance asymmetric SAM molecules through such a rational molecular design strategy.

Herein, we report two novel asymmetric SAM molecules designed through substituting the fused benzene ring on CbzPh with benzofuran and benzothiophene, obtaining CbzBF and CbzBT, respectively (Fig. 1). Their asymmetric structure ensures enhanced molecular dipole moments, and extended conjugation facilitates stronger intermolecular interactions to realize ordered and dense assembly of SAMs. Moreover, the introduction of lone-pair-rich, Lewis-basic heteroatoms of oxygen and sulfur can effectively passivate the uncoordinated Pb²⁺ at the buried interface in the PSC.³⁸ Based on the above-mentioned rational asymmetric molecular design, CbzBF and CbzBT have successfully achieved more efficient hole extraction and effective buried interface passivation, significantly improving the performance of the derived devices compared to those based on CbzPh. A champion PCE of 24.04% with improved device stability could be achieved by applying CbzBT as a HSL in an inverted PSC.

Fig. 1 Molecular structures, calculated dipole moments and HOMO energy levels of CbzPh, CbzBF, and CbzBT.

Results and discussion

The electrochemical properties of CbzPh, CbzBF and CbzBT were studied by cyclic voltammetry (CV) (Fig. S2†). The HOMO energy levels of CbzPh, CbzBF and CbzBT are estimated to be −5.48 eV, −5.46 eV, and −5.50 eV, respectively, which are nearly identical. This could facilitate a better cascade alignment between the SAM HOMO energy level and the VBM of the perovskite, ensuring efficient hole extraction from perovskite to the SAM. The lowest unoccupied molecular orbital (LUMO) energy levels of CbzPh, CbzBF and CbzBT were estimated to be −2.19 eV, −2.02 eV, and −2.10 eV, respectively, based on their optical bandgaps (Fig. S1†). These values are significantly higher than the conduction band minimum (CBM) of perovskites, thereby effectively blocking the electrons from reaching the anode. These results were further corroborated by density functional theory (DFT) computation (Fig. S3†), where the calculated HOMO energy levels for CbzPh, CbzBF and CbzBT were −5.49 eV, −5.50 eV and −5.54 eV, respectively, agreeing well with those obtained from the CV measurements.

The gas-phase molecular dipole moments were also calculated by DFT (Fig. S4†), among which CbzBF (2.48 D) and CbzBT (2.64 D) both show larger dipole moments than CbzPh (2.04 D). It is notable that a larger molecular dipole pointing toward the substrate is favorable for down-shifting the ITO WF. As shown by UPS measurements (Fig. S5†), the WF of CbzBF- and CbzBT-modified ITO substrates is −5.08 eV and −5.27 eV, deeper than that modified with CbzPh (−4.92 eV). The surface morphology and potential of SAMs deposited on the ITO substrates were represented by Kelvin probe force microscopy (KPFM). As shown in Fig. S6,† there is no obvious change in the surface morphology of ultra-thin SAM-modified ITO and bare ITO, but their surface contact potential difference (CPD) is significantly improved compared with bare ITO. Compared to the CPD of −111.4 mV for bare ITO (Fig. S7†), significantly increased values of 206.1 mV, 363.2 mV and 669.6 mV were observed for CbzPh/ITO, CbzBF/ITO and CbzBT/ITO, respectively, confirming that CbzBF and CbzBT with larger molecular dipole moments can down-shift the ITO WF more effectively, which is consistent with the results of UPS. This should facilitate more efficient hole-extraction at the interface by minimizing the energy offset to achieve a higher fill factor (FF) and open-circuit voltage (V_oc) in the corresponding PSC.^26,27,33

To gain deeper understanding into the molecular packing of CbzPh, CbzBF, and CbzBT in a condensed phase, single crystals suitable for XRD analysis of SAM π-scaffolds were obtained by slow evaporation from a solution in acetone. Although the packing of SAM π-scaffolds in their single crystals could differ from the packing pattern of substrate-supported SAM molecules, it could provide us valuable information for understanding and identifying the intrinsic intermolecular interactions in different π-extended carbazoles for establishing a comparison. This also excludes the interruption of strong intermolecular interactions from phosphonic acid anchoring groups, such as hydrogen bonding. The structures of π-scaffolds of CbzPh, CbzBF, and CbzBT are highly planar, and the twisting angles between the extended aryl group and the carbazole core are 4.27°, 2.29°, and 2.02°, respectively. For the π-scaffold of CbzPh (7H-benzo[c]carbazole), a large longitudinal slip of 5.48 Å indicates insufficient π–π interactions (Fig. 2a), resulting in lack of close-contact interaction between two parallelly neighboring molecules. This may prevent CbzPh from forming an ordered assembly on the substrates. For the π-scaffold of CbzBF (5H-benzofuro[3,2-c]carbazole) and CbzBT (5H-benzo[4,5]thieno[3,2-c]carbazole), the relative slip distances between two adjacent parallel molecules are 5.33 Å and 5.29 Å, respectively (Fig. 2b and c). Moreover, the fused benzofuryl or benzothienyl moiety extends the conjugation length longer than phenyl, which enables stronger CH⋯π and π–π interactions between neighboring molecular π-planes. Such improved π–π interactions of CbzBF and CbzBT provide sufficient π–π overlap to facilitate the formation of more ordered and dense assemblies of the corresponding SAM molecules on the ITO substrate.

Fig. 2 The molecular packing patterns in single crystals of the π-scaffolds of (a) CbzPh (7H-benzo[c]carbazole), (b) CbzBF (5H-benzofuro[3,2-c]carbazole), and (c) CbzBT (5H-benzo[4,5]thieno[3,2-c]carbazole).

The SAMs were grown on ITO substrates by spin-coating their solutions in isopropanol (IPA), followed by annealing at 100 °C on a hot plate for 15 min, and the surface of the SAM-coated ITO substrate was rinsed with IPA to ensure removal of residual SAM molecules. To ensure the successful functionalization of CbzPh, CbzBF and CbzBT on ITO substrates, we characterized the surface atomic composition of these SAM-coated ITO substrates using high resolution X-ray photoelectron spectroscopy (HR-XPS). The characteristic signals of P 2p and S 2p electrons demonstrate the successful anchoring of CbzPh, CbzBF, and CbzBT on the ITO surface (Fig. S8–S11†). Moreover, a semi-quantitative analysis of the elemental contents by HR-XPS was conducted using previously reported methods^39,40 to identify the molecular density and regularity in SAMs. It was found that CbzBF and CbzBT have improved coverage factors⁴¹ compared with CbzPh (Table S3†), indicating the formation of denser and more ordered monolayers of CbzBF and CbzBT. This can be attributed to the more pronounced π–π interactions between their own molecules of CbzBF and CbzBT, as revealed by single crystals of their π-scaffolds. The formation of denser assemblies and more ordered packing of monolayers can synergize with the larger molecular dipoles to effectively tune the ITO WF, consistent with the UPS and KPFM measurements.

In inverted PSCs, the SAM-modified substrate plays a critical role in providing a crystallization template for the perovskite absorber, and a suitable non-wetting surface should facilitate the formation of a uniform and compact perovskite film.^42–44 To assess the surface wettability, contact angle measurements were performed on the SAM-modified ITO substrates. The contact angles of water droplets on CbzPh-, CbzBF- and CbzBT-modified substrates were measured to be 77.6°, 79.4°, and 86.6°, respectively (Fig. S13†), which can all serve as suitable non-wetting surfaces to facilitate perovskite crystallization.^27,43,45 Moreover, the introduction of heteroatoms usually improves the hydrophilicity of the surface, but we found that after the introduction of heteroatoms, the contact angles of CbzBF and CbzBT become even larger than that of CbzPh. This is attributed to the enhanced intermolecular interactions resulting from further extended conjugation that promotes the growth of CbzBF and CbzBT into denser monolayers,⁴⁶ just as the trend observed in single crystals, also echoing the HR-XPS results.

The SEM images of the perovskite films atop the CbzPh-, CbzBF- and CbzBT-modified substrates are shown in Fig. 3a. The morphology of perovskite films grown on different substrates showed significant differences, in which larger grain sizes were observed on the surface of CbzBF- and CbzBT-supported perovskites than that supported by CbzPh (Fig. 3a). The X-ray diffraction (XRD) of perovskite films deposited on different SAMs is shown in Fig. 3b. The perovskite phase does not exhibit any detectable peak shift or broadening, but they show different (110) crystal face diffraction intensity in the order of CbzBT > CbzBF > CbzPh, and different PbI₂ diffraction intensity in the order of CbzBT < CbzBF < CbzPh (Fig. S14†), indicating better perovskite film quality after introducing sulfur and oxygen heteroatoms to passivate the defects. The perovskite films with a higher PbI₂ content are known to reduce device stability owing to the formation of Pb⁰, which can act as a non-radiative recombination center.⁴⁷ The improved film quality could also reduce the interfacial trap density and alleviate charge recombination to enhance the FF and V_OC of devices.^48,49

Fig. 3 (a) The SEM images, (b) XRD patterns (here * denotes the diffraction peak of PbI₂ and # denotes the diffraction peak of ITO), (c) PL spectra and (d) TRPL spectra (the direction of incident light comes from the perovskite side) of perovskite films deposited on CbzPh, CbzBF and CbzBT HSLs.

The XPS spectra of the buried interface of perovskite films peeled off from the ITO/SAM substrate also confirmed the passivation effect of CbzBF and CbzBT on Pb²⁺, because there was a shift in the Pb 4f peak of perovskite films spin-coated on ITO/CbzBF and ITO/CbzBT compared with those on ITO/CbzPh (Fig. S12†). The decrease in Pb²⁺ binding energy is due to the sharing of lone pair electrons by oxygen or sulfur atoms on CbzBF and CbzBT with Pb²⁺,^50,51 which reiterates our achieved buried interface passivation through rational SAM molecular structure design.

Based on these encouraging findings, CbzPh, CbzBF, and CbzBT SAMs were applied as HSLs for fabricating inverted PSCs to evaluate their effectiveness with a device configuration of ITO/SAM/perovskite/C₆₀/BCP/Ag (Fig. 4a). The reference device based on CbzPh showed an average PCE of 19.36% with a V_oc of 1.06 V, short-circuit current density (J_sc) of 23.58 mA cm⁻², and FF of 76.56% under 1-sun illumination (AM 1.5G and 100 mW cm⁻²). The average PCE of the devices based on CbzBF and CbzBT was significantly improved to 21.18% and 21.65%, respectively, due to the significantly enhanced FFs to 82.00% and 82.71%, and V_oc to 1.08 V and 1.09 V, respectively (Table 1, Fig. 4b, c and S15†). Notably, the champion PCE of the devices based on CbzBT was as high as 22.08% without using any additive and top surface passivation treatment, with an impressively high FF of 84.20%. The significantly enhanced FF should be benefited from the enhanced SAM intermolecular π–π interactions promoting more ordered assembly, increased molecular dipoles enabling better ITO WF regulation, and introduced Lewis basic heteroatoms passivating the buried interface. Moreover, the hole-only devices with an architecture of ITO/SAM/perovskite/MoO₃/Ag were fabricated for space charge-limited current (SCLC) measurements. The turning point of the ohmic region and trap-filling region of the curve revealed the trap-filled limit voltage (V_TFL), which were determined to be 0.873 V, 0.607 and 0.597 V for CbzPh-, CbzBF- and CbzBT-based devices, respectively (Fig. S16†). The lower V_TFL showed the lower trap density of devices based on CbzBF and CbzBT as the trap density is directly proportional to the V_TFL. The dark current and light intensity-dependent V_oc values of PSCs based on CbzPh-, CbzBF-, and CbzBT-modified substrates were measured (Fig. S16†). The CbzBT-based device shows the smallest dark current and the highest ideality factor, both of which indicate that a densely packed CbzBT monolayer could effectively reduce current leakage and interfacial recombination to improve the V_oc, FF and stability of the corresponding PSCs.

Fig. 4 (a) Device architecture of the inverted PSCs; (b) current density–voltage (J–V) characteristics; (c) box plots of the PCEs of the PSCs employing SAM-modified substrates of CbzPh, CbzBF and CbzBT; (d)–(f) device characteristics of (d) J–V sweeps, (e) EQE and (f) stabilized power output (SPO) of the champion device employing the CbzBT HSL.

Table 1 Detailed photovoltaic parameters of PSCs based on CbzPh, CbzBF and CbzBT. Average data were obtained from 16 cells

HSL	V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)
CbzPh	1.08	23.80	78.42	20.22
	1.06 ± 0.02	23.58 ± 0.56	76.56 ± 1.27	19.36 ± 0.54
CbzBF	1.09	24.00	83.04	21.72
	1.08 ± 0.01	23.78 ± 0.29	82.00 ± 0.55	21.18 ± 0.28
CbzBT	1.10	23.93	84.20	22.08
	1.09 ± 0.01	23.91 ± 0.22	82.71 ± 0.84	21.65 ± 0.23

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To further understand how the interfacial properties of these SAMs as HSLs affect the charge transfer dynamics, steady-state photoluminescence (PL) (Fig. 3c) and time-resolved PL (TRPL) spectra (Fig. 3d) of perovskite on top of SAM-modified substrates were recorded to evaluate the hole extraction capability of SAMs. The PL intensity and TRPL decay traces of perovskite layers grown on CbzBF and CbzBT are significantly enhanced compared with that grown on CbzPh (Table S4†), reaffirming the reduced defects and suppressed non-radiative recombination at the interface after introducing the oxygen and sulfur heteroatom-containing SAMs and the more ordered assembly of SAMs. They synergistically enhanced the hole extraction capability of CbzBF and CbzBT. Between them, the S atom has higher binding energy with PbI₂; therefore CbzBT should have stronger intermolecular interaction, resulting in higher PL intensity and extended PL lifetime in the CbzBT-supported perovskite film.

To fully exploit the potential of the CbzBT HSL, top surface passivation and light management adopting anti-reflection coating were also applied.⁵² The champion CbzBT-based device could achieve a PCE of 24.04% (V_OC = 1.16 V, J_SC = 24.54 mA cm⁻², and FF = 84.41%) (Fig. 4d), demonstrating the CbzBT HSL's great potential for inverted PSCs. To ensure the data reliability, the external quantum efficiency (EQE) of the device was also measured to calibrate the cell J_SC (Fig. 4e). The integrated photocurrent (24.50 mA cm⁻²) of the champion device matches well with the value obtained from the J–V measurement. The stability of the champion cell was further assessed by tracking the stabilized power output at bias corresponding to the maximum power point (MPP) under continuous AM 1.5G illumination (Fig. 4f). A stabilized PCE of 23.60% was retrieved, which agrees well with the values from the J–V sweeps.

Since the SAM contains a flexible alkyl linker, there are concerns about potential efficiency degradation caused by the SAM's thermal motion at high temperatures. Therefore, the thermal stability of CbzPh-, CbzBF-, and CbzBT-based PSCs was also evaluated under an annealing temperature of 65 °C in a N₂-filled glove box. The devices based on CbzBF and CbzBT HSLs exhibited better thermal stability than those based on CbzPh, retaining 98.9% and 98.2% of their original PCE, respectively, under continuous heating for more than 700 h, which are higher than that of CbzPh based devices (93.3%) (Fig. S17†). The enhanced intermolecular interaction and denser assembly caused by the conjugation extension, as well as the introduced defect-passivating Lewis basic heteroatoms, synergistically enhance the stability.

Conclusions

In summary, we report two novel asymmetric SAM molecules designed by substituting the fused benzene ring on CbzPh with benzofuran and benzothiophene, namely CbzBF and CbzBT, to induce asymmetric conjugation extension and incorporation of lone-pair-rich, Lewis-basic oxygen and sulfur atoms for efficient hole extraction and buried interface passivation. The advantages of CbzBF and CbzBT in tuning ITO WF, optimizing perovskite crystallization, and passivating PbI₂ defects are demonstrated by spectroscopic and interfacial characterization. As a result, the champion device of inverted PSCs employing CbzBT as the HSL could achieve an impressive PCE of 24.04% along with a remarkable FF of 84.41% and improved device stability. Moreover, this work demonstrates the feasibility of introducing passivating groups into high-performance SAM molecules, and insights gained from this molecular design strategy will help accelerate the development of new multifunctional SAM HSLs for more efficient PSCs.

Data availability

The related experimental data are provided in the ESI.†

Author contributions

W. J. conceived the project and synthesized SAM molecules, which was supervised by A. K. Y. J., M. L. conducted PSC devices and interfacial characterization including XPS, SEM, PL and TRPL with the assistance of Y. L. W. J. and F. R. L. wrote the manuscript, and all authors participated in the data analysis and discussions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

A. K. Y. J. acknowledges the sponsorship of the Lee Shau-Kee Chair Professor (Materials Science), and the support from the APRC Grants (9380086, 9610419, 9610492, and 9610508) of the City University of Hong Kong, the TCFS Grant (GHP/018/20SZ) and MRP Grant (MRP/040/21X) from the Innovation and Technology Commission of Hong Kong, the Green Tech Fund (202020164) from the Environment and Ecology Bureau of Hong Kong, the GRF grants (11307621 and 11316422) from the Research Grants Council of Hong Kong, the Shenzhen Science and Technology Program (SGDX20201103095412040), and the Guangdong Major Project of Basic and Applied Basic Research (2019B030302007).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2298041 and 2298042. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc05485c

‡ These authors contributed equally to this work.

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