Dual-site synergistic passivation for CsPbBr₃ perovskite solar cells with record 1.707 V V_oc and 11.23% efficiency

Abstract

Achieving high-quality perovskite films with minimal defect states is critical for the development of efficient and stable perovskite solar cells (PSCs). In this work, we introduce diphenylphosphine (DPP) additives into the PbBr₂ precursor solution to passivate defects and enhance device performance. By systematically varying the carbon chain length of DPP molecules, we reveal that phosphorus atoms coordinate with undercoordinated Pb²⁺ ions in the perovskite lattice, effectively reducing defect densities and improving film quality. The experimental results demonstrate that 1,3-bis (diphenylphosphino) propane (DPPP) with optimal carbon chain length achieves significantly improved defect passivation efficiency through enhanced lattice parameter matching with perovskite structures, whereas excessively long chains hinder performance due to steric effects. The best doping amount of DPPP with the ideal carbon chain length yields a power conversion efficiency (PCE) of 11.23% and a record open-circuit voltage (V_oc) of 1.707 V. Furthermore, unencapsulated CsPbBr₃ PSCs with DPPP exhibit exceptional thermal and humidity stability, maintaining more than 90% of the initial performance after 1800 h under 80 °C or 80% relative humidity.

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Yuanyuan Zhao

Yuanyuan Zhao is a professor in the College of Energy Storage Technology at Shandong University of Science and Technology in Qingdao, China. She obtained her PhD from Ocean University of China. Her research interests cover perovskite solar cells and electrocatalytic hydrogen production. To date, she has published over 30 papers in international journals, including Advanced Energy Materials, Nano Energy and Carbon Energy, as well as a book chapter. Additionally, she received the second prize for the Guangdong Province Science and Technology Award in China. She has authorized 6 national invention patents (China) and has led six research projects.

Introduction

Perovskite solar cells (PSCs) with a certified efficiency of 27.0% have attracted considerable attention due to their simple solution processing and excellent photovoltaic performance.¹ However, the thermal degradation properties of organic–inorganic hybrid perovskite materials raises concerns about the long-term stability of PSCs.^2,3 Incorporating inorganic components (Cs⁺ cations) instead of organic ones improves the thermal stability of inorganic perovskite materials (CsPbX₃, X = I, Br, or mixed halogens).⁴ In recent years, CsPbBr₃ has emerged as a promising alternative material for next-generation photovoltaic technology, transparent solar cells, and high-voltage electronic devices, owing to its high carrier mobility and durability under harsh environmental conditions, such as moisture, heat, light, and oxygen.^5–7

The highest reported power conversion efficiency (PCE) of CsPbBr₃ PSCs has reached 11.23%,⁸ but it is still significantly lower than the theoretical efficiency limit, mainly due to severe charge recombination in the film. Most of the charge recombination occurs at the grain boundaries (GBs) in the perovskite film, where shallow states near the perovskite valence band (VB) edge limit hole extraction and transfer, and provide sites for uncoordinated ionic defects, resulting in severe charge recombination.^9–11 Fabricating high-quality perovskite films with large grain size and minimal GBs is an effective strategy to reduce charge recombination and improve device performance.

Adding functional additives to the precursor solution can passivate uncoordinated ionic defects, enabling the fabrication of high-quality perovskite films.^12,13 Lewis base molecules comprising phosphorus (P),¹⁴ nitrogen (N),¹⁵ sulfur (S)¹⁶ and oxygen (O)¹⁷ have shown promise in passivating defects by providing electrons to uncoordinated Pb²⁺ ions, forming coordination covalent bonds. Theoretical calculations suggest that the binding strength with Pb²⁺ ions follows the order: P > N > S > O.¹⁸

In this study, we investigated the impact of different Lewis base molecules, including bis(diphenylphosphino)methane (DPPM), 1,3-bis(diphenylphosphino)propane (DPPP), and 1,5-bis(diphenylphosphino)pentane (DPPPE), on defect passivation. These molecules possess a comparable structural framework but differ in the length of the carbon chain between the two phosphorus atoms. The carbon chain length has a significant influence on the spatial matching between the two P groups and the adjacent Pb²⁺ ions. Specifically, DPPP, with an intermediate chain length and a P–P distance (∼5.62 Å) closely matching the Pb–Pb distance in the CsPbBr₃ lattice (∼5.95 Å), enables the simultaneous coordination of two adjacent Pb²⁺ ions. This dual coordination effectively reduces the density of undercoordinated Pb²⁺ ions defects and promotes the formation of larger, more uniform grains with fewer boundaries. In contrast, DPPM and DPPPE have mismatched P–P distances (3.05 Å and 8.19 Å, respectively), limiting their ability to bridge two adjacent Pb²⁺ ions. As a result, the performance of CsPbBr₃ PSCs improved with increasing carbon chain length from DPPM to DPPP, achieving optimal device characteristics with DPPP. However, further extension of the chain length in DPPPE diminished the coordination effectiveness, slightly compromising the device performance. Specifically, the DPPP-treated CsPbBr₃ PSC device achieved a PCE of 11.23% and a record open-circuit voltage (V_oc) of 1.707 V. Furthermore, the unencapsulated CsPbBr₃ PSC with DPPP exhibited remarkable thermal and humidity stability over 1800 hours in ambient air at 80 °C or 80% relative humidity.

Experimental

The ESI† provides experimental details, covering raw materials, preparation methods, measurements and characterizations of CsPbBr₃ PSCs.

Results and discussion

The electrostatic potential (ESP) distributions of DPPM, DPPP and DPPPE were calculated using density functional theory (DFT), as shown in Fig. 1a. These molecules exhibit electron-rich regions near the P atoms, which may facilitate the passivation of positively charged defects (uncoordinated Pb²⁺ ions) in the perovskite films (Fig. 1b). The distances between the two P atoms of DPPM, DPPP and DPPPE are 3.05 Å, 5.62 Å and 8.19 Å, respectively, while the distance between two adjacent Pb²⁺ ions in the CsPbBr₃ perovskite lattice is about 5.95 Å.¹⁹ The molecular size of DPPP suggests it can achieve dual-site Pb²⁺ ions passivation. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that, compared to the control CsPbBr₃ film, the Pb 4f peaks of the CsPbBr₃ film doped with DPPM, DPPP, and DPPPE exhibited a shift, with the binding energy decreasing by 0.05 eV, 0.10 eV, and 0.05 eV, respectively (Fig. 1c). This shift indicates a change in the density of the surrounding electron cloud,²⁰ suggesting a strong interaction between DPPP and Pb²⁺ ions through a Lewis acid–base interaction.²¹ The energy dispersive X-ray spectroscopy (EDS) mapping image shows the uniform distribution of P from DPPP in the CsPbBr₃ film (Fig. S1, ESI†). Fourier transform infrared spectroscopy (FTIR) analysis demonstrated that the stretching vibration peak of the P–C bond²² shifted to a higher wavenumber, indicating a chemical interaction between the P donor and the Pb²⁺ ion (Fig. 1d and e).

Fig. 1 (a) Electrostatic potential image of DPPM, DPPP and DPPPE. (b) Schematic diagram of the interaction mechanism between diphenylphosphines (DPPs) and the CsPbBr₃ lattice. (c) XPS Pb 4f spectra of perovskite films with and without the addition of DPPs. (d) FTIR spectra of pure DPPP powder, pure CsPbBr₃ perovskite film and CsPbBr₃ perovskite film with the addition of DPPP. (e) Local magnification of FTIR.

The impact of incorporating distinct diphenylphosphine (DPP) molecules into the PbBr₂ precursor solution on the resulting PbBr₂ film was investigated. Through top-view scanning electron microscopy (SEM), we observed that there were no significant differences in the morphology and porosity of the PbBr₂ films (Fig. S2, ESI†). However, upon further cross-sectional SEM (Fig. S3, ESI†), we found that the DPP additives significantly increased the number of small pores in the PbBr₂ films. These pores facilitate the complete phase transition of lead bromide to CsPbBr₃, as this phase transition process involves a volumetric expansion.²³ The impact of distinct DPP molecules on the surface morphology of perovskite films was investigated using SEM. After the incorporation of DPP molecules, a slight increase in grain size was observed in the perovskite film (Fig. S4, ESI†). The cross-sectional SEM image (Fig. 2b) reveals the increased grain size of the monolayer-aligned perovskite layer with a thickness of 550 nm, which is consistent with the top-view SEM.

Fig. 2 (a) Top-view SEM and (b) cross-sectional SEM of perovskite films with and without the addition of DPPs. (c) XRD spectra of perovskite films with and without the addition of DPPs. (d) Comparison of the relative XRD peak intensities at the (100), (110) and (200) reflections of perovskite films with and without the addition of DPPs.

X-ray diffraction (XRD) analysis showed that the introduction of DPP molecules did not result in the emergence of new peaks, indicating they did not enter the perovskite lattice (Fig. 2c).²⁴ Rietveld refinement²⁵ of the XRD patterns using pseudo-Voigt peak profiles²⁶via the FullProf software^27,28 confirmed that the addition of DPP molecules suppresses the formation of impurity phases, with DPPP exhibiting the most pronounced suppression effect (Fig. S5, ESI†). In the control group, the proportion of impurity phases, including CsPb₂Br₅ and, PbBr₂, was relatively high at 25.98% and 3.39%, respectively. After the addition of DPP molecules, the proportion of CsPb₂Br₅ and PbBr₂ decreased to 18.68% & 2.20%, 15.06% & 0.83%, and 17.26% & 1.30% for DPPM, DPPP, and DPPPE, respectively. This reduction in impurity phases corresponds to the improvement of the quality of the perovskite films. However, due to the intrinsic limitations of the multi-step deposition process, PbBr₂-rich regions persist at the buried interface, making it difficult to completely remove the residual PbBr₂ and CsPb₂Br₅. Interestingly, earlier research has shown that CsPb₂Br₅-containing CsPbBr₃ PSCs have performed noticeably better than individual CsPbBr₃-tailored PSCs,^29,30 mostly due to the indirect-bandgap of CsPb₂Br₅, which suppresses electron–hole recombination.³¹ In comparison to the control group, the introduction of DPP molecules demonstrated a discernible enhancement effect on the (110) peak of perovskite (Fig. 2d), resulting in higher conductivity and enhanced charge carrier mobility.^32,33 The UV-vis curves indicate that the addition of DPP does not alter the absorption characteristics (Fig. S6, ESI†). The reduced Urbach energy (E_U) after the addition of DPP suggests a lower defect density in the film (Fig. S7 and Table S1, ESI†).

Atomic force microscopy (AFM) demonstrated that the surface roughness of the perovskite films added with DPP molecules exhibited a slight reduction in comparison to the control (Fig. 3a), with the addition of DPPP resulting in the minimal surface roughness (R_a = 10.9 nm). Kelvin probe force microscope (KPFM) is employed to ascertain the contact potential difference (CPD). As illustrated in Fig. 3b, the CPD of the control perovskite film was 1.23 V, whereas the CPD of the DPPM, DPPPE, and DPPP treated perovskite films decreased to 1.21 V, 0.814 V, and 0.7 V respectively. The decrease in CPD indicates an increase in the work function, which suppresses halide migration and enhances the stability of PSCs.³⁴ Ultraviolet photoelectron spectroscopy (UPS) also verified the increase in work function of the DPP-treated perovskite films (Fig. S8, S9 and Table S2, ESI†). Confocal photoluminescence (PL) mapping and steady-state PL revealed that the PL intensity of the DPP-treated perovskite films is enhanced and more concentrated (Fig. 3c–e), suggesting the defect states are filled, which is conducive to charge transfer.³⁵ The average carrier lifetime (τ_ave) of the DPPP-treated film reached 1.87 ns, significantly longer than the 0.62 ns of the control film (Fig. 3f and Table S3, ESI†), attributed to the effective passivation of defects. Accordingly, the DPP-treated CsPbBr₃ PSCs have a slower V_oc decline and a longer electron lifetime (τ_n) than the control PSCs (Fig. S10, ESI†), indicating a decrease in charge recombination.^36,37

Fig. 3 (a) AFM image of perovskite films with and without the addition of DPPs. (b) KPFM of perovskite films with and without the addition of DPPs. (c) PL mapping and (d) intensity statistical distribution map of perovskite films with and without the addition of DPPs. (e) PL and (f) TRPL spetra of perovskite films with and without the addition of DPPs. (g) SCLC measurements of electron-only devices.

Space charge limited current (SCLC) measurements were performed to confirm the passivation effect of DPP molecules on perovskite films. Compared to the control group with a trap density (N_t) of 9.330 × 10¹⁵ cm⁻³, the device added with DPPP exhibited a lower N_t of 8.534 × 10¹⁵ cm⁻³ (Fig. 3g and Table S4, ESI†), indicating the passivation of film defects. The J^1/2–V curve (Fig. S11, ESI†) demonstrates a steeper slope for the DPP-treated devices compared to the control device. The DPPP-treated device also exhibited higher electron mobility (μ_e) of 1.0 × 10⁻⁴ cm² V⁻¹ s⁻¹, compared to 8.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ for the control (Table S4, ESI†), confirming the enhancement in electron transport capability. The capacitance–voltage (C–V) curve showed a 96 mV increment in the built-in potential (V_bi) (Fig. S12, ESI†), signifying a heightened driving force for the transport and extraction of photogenerated carriers and the mitigation of carrier recombination.^38–41 The water contact angle of the perovskite film with DPPP increased to 63°, compared to 47° for the control film (Fig. S13, ESI†), indicating DPPP enhances the hydrophobicity of the perovskite surface, thereby improving the stability of the device.⁴²

The DPPP-treated CsPbBr₃ PSC reached a champion PCE of 11.23% and an ultrahigh V_oc of 1.707 V (Fig. 4a, S14–S16 and Table S5–S7, ESI†), which are among the highest reported for CsPbBr₃-based PSCs (Fig. 4b and Table S8, ESI†). The optimized device exhibits reduced hysteresis (Fig. S17 and Table S9, ESI†), which can be ascribed to the enhanced quality of the perovskite film.⁴³ The enhance fill factor (FF) can be attributes to the reduction of charge recombination.⁴⁴ The significant improvement in V_oc suggests that the addition of DPPP effectively reduces the surface defect density of perovskite films, indicating its substantial passivation effect.⁴⁵ The augmented PCE is additionally corroborated by the steady-state power output (Fig. S18, ESI†), the incident photon-to-electron conversion efficiency (IPCE) (Fig. 4c), and statistical photovoltaic data (Fig. S19, ESI†).

Fig. 4 (a) J–V curves of various CsPbBr₃ PSCs. (b) Summary of the representative PCEs for the CsPbBr₃ PSCs. (c) IPCE spectra of various CsPbBr₃ PSCs. Nyquist plots and fitting lines (the inset is the equivalent circuit) of various CsPbBr₃ PSCs under (d) dark and (e) light conditions. (f) Dark J–V curves of various CsPbBr₃ PSCs. Normalized PCE stability of the unencapsulated PSCs under (g) 80 °C, 0% RH and (h) 25 °C, 80% RH conditions. (i) Photostability of the inorganic CsPbBr₃ PSCs under LED illumination (100 mW cm²) at their MPP in the ambient conditions.

Electrochemical impedance spectroscopy (EIS) under dark and one-sun illumination conditions was used to analyze the carrier transport dynamic (Fig. 4d and e). The PSC with DPP-treated exhibited higher charge recombination resistance (R_rec) and a lower charge transfer resistance (R_ct) compared to the control (Table S10, ESI†), indicating a decreased charge recombination rate and an accelerated charge transfer process.^46,47 The J–V curves under dark conditions revealed a lower leakage current density in the DPP-treated devices, further supporting the suppression of charge recombination (Fig. 4f).

The unencapsulated CsPbBr₃ PSC with DPPP exhibited robust thermal and humidity stability, retaining 91.14% and 91.56% of the initial PCE after 1800 hours at 80 °C & 0% RH and 25 °C & 80% RH, respectively, outperforming the control device (Fig. 4g and h). After 600 hours at 80 °C & 80% RH, it maintained 90.32% of the initial PCE (Fig. S20†), which was also better than the control device. Under continuous LED illumination (100 mW cm⁻²), the PCE preservation rate of the DPPP-treated CsPbBr₃ PSC (92.82%) was also significantly higher than the control (67.14%) (Fig. 4i). The initial J–V curve of the unencapsulated PSCs with the above stability test was shown in Fig. S21 (ESI).† These improvements are attributed to the large grain size, high-quality, and effective defect passivation in the DPPP-treated CsPbBr₃ films.

Conclusions

In conclusion, this study proposes an effective defect passivation strategy through the addition of DPP molecules to the PbBr₂ precursor solution. Systematic characterizations demonstrate that the DPP molecules play a pivotal role in the formation of high-quality perovskite films and the inhibition of charge recombination, enabling the preparation of efficient and stable PSCs. The appropriate length of the elastic carbon chain in the DPP molecule is correlated with the adjacent uncoordinated Pb²⁺ ions in the perovskite lattice. The optimal chain length has been observed to increase the grain size of the perovskite film and accelerate electron transfer, while simultaneously inhibiting the non-radiative recombination caused by defects. Ultimately, the CsPbBr₃ device modified with the DPPP achieved a PCE of 11.23% and a record V_oc of 1.707 V. Notably, the unencapsulated CsPbBr₃ PSCs with DPPP exhibit favorable long-term stability under elevated temperatures and high humidity conditions, retaining over 90% of the initial PCE after 1800 hours of testing.

Data availability

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

Author contributions

Yinping Teng: data curation, formal analysis, investigation, resources, software, visualization, writing – original draft, writing – review and editing. Yuanyuan Zhao: funding acquisition, supervision, writing – review and editing. Zhe Xin: data curation, visualization. Liqiang Bian: formal analysis, investigation. Qiyao Guo: validation, visualization. Jialong Duan: validation, visualization. Jie Dou: validation, visualization. Yan Zhang: data curation, visualization. Qiang Zhang: supervision, validation, visualization. Qunwei Tang: conceptualization, funding acquisition, supervision, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledged financial support provided by the National Natural Science Foundation of China (22179051, 62204098, 62304124), Project of Shandong Province Higher Educational Young Innovative Team (2022KJ218), China Postdoctoral Science Foundation (2023M732104), Qingdao Postdoctoral Funding Program (QDBSH20220201002), Postdoctoral Innovation Project of Shandong Province (SDCX-ZG-202303032), Natural Science Foundation of Shandong Province (ZR2024QE036) and Special Fund of Taishan Scholar Program of Shandong Province (tsqnz20221141). Thank Yan Zhang in Analysis and Test Center of College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University for data curation and assessment.

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Footnote

† Electronic supplementary information (ESI) available: SEM, UV absorption spectra, V_oc decay, electron lifetime, J^1/2 curve, Mott–Schottky plots, contact angles, J–V, statistical distributions, steady-state output curve. See DOI: https://doi.org/10.1039/d5ta01594d

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