Synergistic surface modulation with isotropic 2D GA₂PbI₄ and Lewis base enhances efficiency and stability of perovskite solar cells†

Peiquan Song‡ ^a, Lina Shen‡ ^a, Lingfang Zheng ^a, Enlong Hou ^a, Peng Xu ^a, Jinxin Yang ^a, Chengbo Tian ^a, Zhanhua Wei *^a, Xiaguang Zhang *^b and Liqiang Xie *^a
aXiamen Key Laboratory of Optoelectronic Materials and Advanced Manufacturing, Institute of Luminescent Materials and Information Displays, College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China. E-mail: lqxie@hqu.edu.cn; weizhanhua@hqu.edu.cn
^bKey Laboratory of Green Chemical Media and Reactions, Ministry of Education, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, College of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, 453007, China. E-mail: zhangxiaguang@htu.edu.cn

First published on 2nd August 2024

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Liqiang Xie

Dr Liqiang Xie received his BS from Xiamen University in 2012 and his PhD from Xiamen University in 2017 under the supervision of Prof. Bing-Wei Mao and Prof. Zhong-Qun Tian. He joined Huaqiao University in October 2017. He is now an associate professor at the Institute of Luminescent Materials and Information Displays of Huaqiao University. His current research focuses on single-junction perovskite solar cells and perovskite/silicon tandem solar cells. He has published over 50 peer-reviewed papers in scientific journals such as Nature Communications, Journal of the American Chemical Society, and Advanced Materials.

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Introduction

Perovskite solar cells (PSCs) have emerged as revolutionary contenders in the field of photovoltaics, offering facile fabrication processes and impressive power conversion efficiencies (PCEs) of over 26%,¹ and are considered a promising solution to address energy shortages and greenhouse effects. However, during the formation of polycrystalline perovskite films, the occurrence of inherent defects (interstitial/uncoordinated ions, substitutions, dangling bonds, and grain boundaries) could lead to significant non-radiative charge recombination. This phenomenon detrimentally affects the key parameters of PSCs, including open-circuit voltage (V_OC), fill factor (FF), and short-circuit current density (J_SC), resulting in reduced efficiency and making the devices highly susceptible to degradation in adverse environmental conditions such as moisture, heat, illumination, and electric fields. Therefore, overcoming this challenge is essential for achieving optimal device performance.

The rational utilization of competent interfacial passivators is critical for improving the device performance and operational stability, which are related to defects/traps, carrier dynamics, interface energy-level alignment, and ion migration. Organic interfacial passivation materials can be divided into neutral molecules and organic salts. Neutral molecules including butylamine (BA)² and phenethylamine (PEA)³ can directly react with 3D perovskite to form low-dimensional perovskites. However, the passivation effect of neutral molecules on perovskites is more homogeneous than that of organic salts and is not suitable for a variety of interfacial defects. Passivation by organic salts manifests in various states, including coordination with Lewis acids (such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)⁴ and iodopentafluorobenzene (IPFB)⁵) or bases (molecules contain CO,⁶ PO⁷ and SO⁸ group), and the formation of 2D/3D heterogeneous junctions.⁹ This approach was proved to be more effective in regulating interfacial imperfections, resulting in superior solar cell performance.

Lewis acids and bases have strong coordination with charged defects due to their electron-deficient properties and lone-electron pairs, respectively, which can neutralize charged defects to form acid-base complexes.¹⁰ In addition, the formation of a 2D perovskite at the perovskite/hole-transporting layer interface using alkyl (or phenyl) ammonium salts such as n-butylammonium iodide (BAI),¹¹n-octylammonium iodide (OAI),¹² and pentafluorophenylethylammonium iodide (FEAI)¹³ was a commonly adopted approach in surface defect regulation. Surface treatment with these salts, which results in the in situ formation of more stable 2D perovskite, effectively diminishes the point defects and dangling bonds, thereby diminishing carrier non-radiative recombination on the surface. Moreover, the amino groups in these salts can form hydrogen bonds with iodide ions and coordinate with the Pb²⁺ interstitials,¹⁴ further adjusting the band alignment¹⁵ and increasing the quasi-Fermi level splitting,¹⁶ thereby achieving improvements in V_OC and FF. Additionally, the formed 2D perovskite exhibited a higher ion migration activation energy,¹⁷ effectively blocking ion migration at the interface. Therefore, the formed 2D/3D structure further enhanced the stability of the device.¹⁸

In the case of utilizing long-chain monovalent organic cations for surface modulation of 3D perovskite, the formed layered 2D perovskites prefer the out-of-plane orientation. The intrinsic van der Waals gap and large spacing distance of the cations in 2D perovskites may hinder charge transport in the vertical direction within the devices, resulting in reduced efficiency.¹⁹ Guanidinium halides were considered effective surface^20,21 and bulk^22–24 passivation materials of perovskites. Their relatively small ionic radius enables the penetration into the perovskite bulk for bulk passivation. Moreover, when forming 2D perovskite at the surface, they exhibited a small interlayer spacing, favoring efficient carrier extraction. Zhang et al.²⁵ employed guanidinium iodide (GAI) and guanidinium chloride (GACl) for the surface and bulk passivation of perovskite, respectively. The non-radiative recombination at the interface was notably diminished. PSCs treated with GAI exhibited an increase in V_OC from 1.09 V to 1.12 V. Despite the notable effectiveness of using this halide guanidinium salt for surface modulation, there remains considerable room for achieving superior performance toward high-performance PSCs. Besides focusing on cations, attention to anions is also crucial. Lewis bases, by providing lone pairs of electrons to coordinate with electron-deficient species on the perovskite surface, demonstrated more effective passivation capability than halide anions.^26,27 Employing (GAAc) to post-treat the perovskite film in inverted (p–i–n) PSCs was proved to be effective in enlarging perovskite grains and leading to the formation of secondary phases, which enabled a device efficiency of 20.1%.²⁸

In this work, we found that GAAc post-treatment for n–i–p PSCs resulted in the formation of an isotropic 2D GA₂PbI₄ phase that is more efficient for charge carrier transport than the traditional anisotropic layered 2D perovskite with a preferred out-of-plane orientation. The formed wide-bandgap perovskite can effectively establish a type I contact with the 3D perovskite surface, efficiently hindering the recombination of electrons and holes at the perovskite/hole-transporting material interface. Furthermore, the Ac⁻ can more effectively bind to iodide vacancies (V_I) and efficiently coordinate with Pb²⁺, inhibiting the formation of Pb⁰ clusters. Compared to GAI and formamidinium acetate (FAAc) surface treatment, the GAAc passivator exhibited a significant synergistic effect. The external quantum efficiency of the device working as a light-emitting diode reached 12.8% under the short-circuit current density, exhibiting minimized non-radiative recombination losses. Ion migration activation energy measurements showed that the GAAc-treated perovskite exhibited much-reduced ion migration. As a result, the optimized device exhibited a remarkable PCE of 25.22% with a high V_OC of 1.20 V. Additionally, when scaled up to a 1 cm² device, the efficiency still achieved 24.18%. Owing to the synergistic effect of the surface-generated 2D perovskite and the passivation by Lewis base (Ac⁻), the devices with GAAc treatment demonstrate a T₈₀ operational lifetime of 2073 hours, representing outstanding operational stability for n–i–p PSCs.

Results and discussion

Surface structure transformation and energy level evolution of perovskite after GAAc treatment

As depicted in Fig. 1a, GAAc was introduced to the surface of perovskite film to handle the well-known imperfections (such as I⁻ vacancies, FA⁺ vacancies, and uncoordinated Pb²⁺, etc.), which are detrimental to the performance and stability of PSCs. We propose that guanidinium (GA⁺) with abundant amino groups can interact with I⁻ by hydrogen bonding, and the lone pair of electrons on the CO of acetate (Ac⁻) provides a better Lewis acid-base coordination by interacting with Pb²⁺, eliminating the undesirable charged centers and decreasing the non-radiative carrier recombination.

We investigated the reaction between GAAc and perovskite by X-ray diffraction (XRD). Fig. 1b reveals two additional peaks at 11.5° and 13.0° after GAAc post-treatment. The new diffraction peak indicates the formation of new phases. This phenomenon aligns with previous reports where guanidinium-containing passivation molecules induce the formation of low-dimensional perovskite structures on the surface of perovskite films. To identify the composition of the new diffraction peaks in XRD, we fabricated three types of perovskite films based on their stoichiometric ratios, i.e. GA₂PbI₄, GAFAPbI₄, and GAPbI₃. The corresponding XRD patterns are shown in Fig. S1† and the results indicate that the new diffraction peaks after GAAc treatment belong to the 2D GA₂PbI₄. Furthermore, comparison with the single-crystal diffraction card (ICSD card 92045) confirms the assignment of the two additional peaks at 11.5° and 13.0° to the (021) and (040) planes of the as-prepared 2D GA₂PbI₄ (Fig. S2†).²⁹

The structural transformation of the perovskite surface treated with GAAc was also explored using grazing-incidence wide-angle X-ray scattering (GIWAXS, Fig. 1c and d), showing a distinct diffraction ring at a q-value equal to 0.82 Å⁻¹ (Fig. S3a†) corresponding to the 2D GA₂PbI₄, while the control sample showed no detectable signal in this region. Additionally, azimuthal integration of the diffraction ring (Fig. S3b†) indicated no preferred orientation of GA₂PbI₄, different from previously reported passivators such as BAI, OAI, and PEA to form 2D perovskite with out-of-plane orientations.^12,20,30 Note that anisotropic layered 2D perovskite with an out-of-plane orientation will hinder the charge transport in the vertical direction of PSCs, so the isotropic GA₂PbI₄ is expected to enable more efficient charge carrier transport.

To further comprehend the effect of GA₂PbI₄ formation on the energy levels of perovskite heterostructures, ultraviolet photoelectron spectroscopy (UPS) and ultraviolet-visible absorption spectroscopy (UV-vis) measurements were conducted (Fig. 1e and S4†). The energy level diagram is presented in Fig. 1f. We can observe that GA₂PbI₄ possesses a large bandgap of 2.51 eV and forms type I band alignment with the control perovskite. Due to the built-in voltage in the solar cell and the tunneling effect, the holes can be efficiently extracted by the HTL, and the higher conduction band minimum than the control perovskite makes GA₂PbI₄ act as an electron-blocking layer, effectively preventing the recombination of electrons in the perovskite and holes in the HTL for the GAAc-treated perovskite.

Morphology changes after surface treatment and chemical interactions between GAAc and perovskite

To understand the individual effect of cation and anion of GAAc on the perovskite surface, we compared it with two other molecules, guanidinium iodide (GAI) and formamidinium acetate (FAAc) shown in Fig. S5.† XRD and GIWAXS measurements were performed to investigate the low-dimensional phase in the post-treated perovskite films (Fig. S6 and S7†). Diffraction peaks of 2D GA₂PbI₄ were found in both GAI and GAAc-treated films, whereas no additional diffraction peaks appeared in the FAAc-treated sample. This indicates that the formation of GA₂PbI₄ was triggered by GA⁺, primarily reacting with the residual PbI₂. Scanning electron microscopy (SEM) topographic characterization was carried out to investigate the changes in the surface morphology of perovskite treated with GAI, FAAc, and GAAc. As shown in Fig. 2a–d, compared with the control perovskite film, GA⁺ containing molecules (GAAc, GAI) were found to have a significant influence on the perovskite surface morphology, whereas the FAAc treatment showed almost no change. This suggests that the topography change can be attributed to the formation of 2D GA₂PbI₄ on the 3D perovskite surface, which is consistent with the XRD and GIWAXS results.

The chemical interactions between Ac⁻ and perovskite were investigated using Fourier transform infrared spectroscopy (FTIR). As shown in Fig. 2e, the stretching vibration peaks of the CO bond were observed at 1724 and 1660 cm⁻¹ in FAAc and GAAc salts, respectively. Upon interaction with PbI₂, this vibration shifted to lower wavenumbers (1712 and 1654 cm⁻¹), indicating the coordination with Pb²⁺ by donating the lone pair of electrons of the carboxylate group on GAAc and FAAc. ¹³C nuclear magnetic resonance spectroscopy (¹³C-NMR) was conducted for FAAc, GAAc, and the mixture of the two salts with PbI₂ to further examine the interaction between the Ac⁻ and Pb²⁺. As shown in Fig. 2f, the peaks at 176.58 and 176.35 ppm from Ac⁻ of FAAc and GAAc shifted toward the low field (177.73 and 178.72 ppm) after adding PbI₂, respectively. This shift was attributed to the strong coordination of the lone pair of electrons of CO with Pb²⁺, resulting in a decrease in the electron cloud density of the CO and a decline in the shielding effect. These findings were consistent with the FTIR results, which were expected to effectively passivate the defects of under-coordinated Pb²⁺ ions.⁶

X-ray photoelectron spectroscopy (XPS) was employed to further investigate the interactions between the three types of passivation molecules and perovskite. As shown in Fig. 2g, the control film exhibited two characteristic peaks at 143.36 and 138.46 eV, corresponding to Pb 4f_5/2 and Pb 4f_7/2 orbitals, respectively. After GAI, FAAc, and GAAc treatment, the two peaks shifted to lower binding energies in all three samples. This shift may be attributed to the coordination of Pb²⁺ with the carboxyl group or amino group in the passivators. In addition, there are two additional peaks located at 141.79 eV and 136.80 eV in the control film, which are related to the detrimental deep-level defects of Pb⁰. The peaks disappeared in the post-treated perovskite films. The appearance of Pb⁰ in the control perovskite film may be attributed to the presence of iodide and cation vacancies, which can be transformed into PbI₂ and further be reduced to Pb⁰.³¹ We simulated the perovskite surface with V_I defects and calculated the adsorption energy of I⁻ and Ac⁻ on the surface by density functional theory (DFT) (Fig. 2h). The results show that the binding energy of I⁻ to the V_I surface is −3.599 eV while the binding energy of Ac⁻ to this defect surface is −3.848 eV (Fig. 2i), indicating that passivation materials containing Ac⁻ have stronger interactions with the V_I-rich defective surfaces. This demonstrates that GAAc can effectively regulate the abundant defects in perovskite films, reducing the recombination centers and decreasing the non-radiative recombination in the PSCs.

Modulating surface defects of perovskite films by GAAc treatment

To investigate the influence of the three different salts on the charge carrier dynamics of the perovskite, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were conducted and the results are depicted in Fig. 3a and b. The PL intensity represents the radiative recombination between holes and electrons, and the higher PL intensity indicates lower defect-assisted non-radiative recombination and superior quality of the perovskite films. According to Fig. 3a, the perovskite films treated with GAI, FAAc, and GAAc show an enhancement of PL intensity compared with the control sample. GAAc-treated sample shows the highest PL intensity, which can be attributed to the combined effect of the Lewis base of Ac⁻ and the formed 2D capping layer, simultaneously passivating terminal Pb⁰ and V_I defects on the surface. TRPL results were found to be consistent with the above findings. The average carrier lifetime of perovskite films was calculated by the bi-exponential decay function, and the detailed fitting parameters together with the average carrier lifetime (τ_ave) are shown in Table S1.† Three kinds of passivators all prolonged the carrier lifetimes, and the GAAc-treated perovskite film exhibited a noticeable increment of τ_ave to ∼2 μs. Furthermore, the calculated surface recombination velocity (S) values decrease by one order of magnitude from 2.06 × 10⁻¹ nm s⁻¹ for the control to 2.24 × 10⁻² nm s⁻¹ for the GAAc-treated sample (Fig. S8†), indicating the significantly diminished surface defect density after GAAc healing. The enhancement of perovskite film quality was also supported by the PL mapping images (Fig. S9†).

To quantify the electron and hole trap density of perovskite films, the space charge limit current (SCLC) experiments (Fig. S10 and S11†) were performed. From Fig. 3c, it is evident that GAAc treatment resulted in the lowest electron and hole trap densities. This indicates that the combined effect of 2D perovskite of GA₂PbI₄ and Lewis base (Ac⁻) for surface treatment significantly reduces the defect density in perovskite films. Furthermore, UPS was performed to explore the effect of post-treatment on the energy level of perovskite films. A significant change in the work function and valence band is observed in Fig. 3d. The energy-level diagram in Fig. S12† shows an upward shift of the valence band maximum (VBM) after GAAc treatment, which may be related to the coordination with Pb²⁺ at the interface. The higher VBM after GAAc treatment can improve the band matching with spiro-OMeTAD, enhancing the hole transport at the interface. Additionally, Kelvin probe force microscopy (KPFM) was employed to analyze the electrical properties of the perovskite film. Fig. 3e shows that the surface potential of perovskite film significantly changed after being treated by GAI, FAAc, and GAAc. The statistical analysis of the surface potential for four samples is presented in Fig. 3f, we can observe that the control sample exhibits a broader and uneven peak of the potential distribution. This was attributed to the disorderly distribution of defects on the surface, especially at grain boundaries. Upon surface treatment with GAI, FAAc, and GAAc, a narrowing of the full width at half-maximum (FWHM) of the surface potential distribution was observed for all three passivating materials. The GAAc-treated perovskite film exhibited the most concentrated surface potential, which is beneficial to improving the charge transport between the interface between perovskite and HTL. These results further emphasize the superior passivation capability of this dual-functional material.

Temperature-dependent PL spectroscopy was conducted to evaluate the effect of GAAc treatment on the carrier relaxation of perovskite film (Fig. 3g and h). The electron-phonon coupling coefficient (Γ_op) can be extracted by fitting the FWHM of the PL peaks in different temperatures (Fig. S13†). The control sample exhibits a much higher coupling coefficient (Γ_op = 450.8 meV) than the GAAc-treated sample (Γ_op = 280.9 meV). This result implies weaker nonadiabatic coupling between charge carriers and longitudinal phonons of the GAAc-treated perovskite, which is responsible for less energy loss, increased carrier lifetime, and longer diffusion length.³² Therefore, the enhanced photoelectric properties of GAAc-treated perovskite film can be attributed to the formed type I band alignment and the passivation effect of the Lewis base, which suppresses the surface imperfections.

Device performance and charge carrier physics characterization

We investigated the influence of GAI, FAAc, and GAAc treatments on the performance of n–i–p PSCs with a structural configuration of FTO/SnO₂/perovskite/spiro-OMeTAD/Ag, where FTO is fluorine-doped tin oxide and spiro-OMeTAD is 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene. The current density–voltage (J–V) curves for the complete PSCs are displayed in Fig. 4a and the main photovoltaic parameters are provided in the inserted table. The control sample has an efficiency of 22.05%, and all three devices with GAI, FAAc, and GAAc treatment exhibit improved efficiencies of 22.64%, 23.58%, and 25.22% respectively. The GAAc-treated device demonstrates the highest PCE compared with the other devices, primarily due to an enhancement in V_OC to 1.198 V, which can be attributed to the synergistic passivation effects of the 2D perovskite and Ac⁻. External quantum efficiency (EQE) spectra of the devices are presented in Fig. S14.† The integrated J_SC values (Table S2†) agree well with the J–V measurement in Fig. 4a (within 5% deviation). Furthermore, the statistics of photovoltaic parameters in Fig. S15† demonstrate a good reproducibility of the performance improvement for the GAI, FAAc, and GAAc-treated devices. The GAAc-treated device obtained a certified PCE of 24.28% with an area of 0.12 cm² and the report is provided in Fig. S16.† Hysteresis index (HI = (PCE_reverse − PCE_forward)/PCE_reverse × 100%) was used to evaluate the passivation effect and ion migration (Fig. 4b and Table S3†). The HI decreased from 3.8% for the control PSC to 3.0%, 3.5%, and 0.7% for GAI, FAAc, and GAAc-treated PSCs, respectively. The steady-state power output (SPO) efficiencies are depicted in Fig. 4c. It can be observed that the SPO values of the control, GAI, FAAc, and GAAc-treated devices were stabilized at 19.74%, 21.71%, 22.56%, and 24.93%, respectively. Notably, the GAAc-treated device maintained a PCE of 24.18% with a V_OC of 1.186 V when the device's active area scaled up to 1 cm² (Fig. 4d).

To comprehend the superb photovoltaic performance of the modified devices, the charge dynamics of the PSCs were investigated. The transient photo-voltage test in Fig. 4e demonstrated that all the passivated devices presented longer V_OC decay lifetimes than the control, which indicates the suppressed charge recombination by GAI, FAAc, and GAAc treatment. Moreover, the relation between V_OC and varied light intensities was also analyzed and the results are presented in Fig. 4f. The ideal factors extracted from Fig. 4f are 2.02, 1.94, 1.71, and 1.48 for the control, GAI, FAAc, and GAAc-treated devices, respectively. This demonstrates that GAAc effectively reduced the trap-assisted recombination, which results in an increment of V_OC and FF. Mott–Schottky plots were provided in Fig. 4g to extract the built-in potential (V_bi), the GAAc-treated device achieved a higher V_bi of 1.010 V than the control (0.854 V), GAI-treated (0.916 V), and FAAc-treated (0.964 V) devices. This indicates the highest charge separation force of the photogenerated carriers in GAAc-treated devices. The trap density of states (tDOS) was calculated from the thermal admittance spectroscopy (TAS) analysis. As shown in Fig. 4h, the modified devices, especially in GAAc-treated devices, showed significantly decreased densities of deep-level trap states in band 3 compared with the control device. The result suggests that the surface modification efficiently suppresses the deep-level defects in the device, which could improve its performance and stability.³³

Next, we analyzed the non-radiative recombination V_OC loss (V_OC-loss_-nonrad) of the PSCs (Fig. 4i). V_OC-loss_-nonrad can be calculated by the function of −ln(EQE_EL)k_BT/q, where k_B is the Boltzmann constant, EQE_EL is the external quantum efficiency of the electroluminescence (EL) when the PSCs were operating as LEDS under an injection current equal to the short-circuit of the PSCs. Fig. S17† shows the EL emission spectrum of the GAAc-treated device at 1.2 V and the inset image shows the device working under this bias. The EQE_EL under short-circuit current for the devices treated by GAI, FAAc, and GAAc were 2.08%, 4.10%, and 12.45%, respectively, showing significant improvement of EQE_EL values compared with the control device (1.02% absolute enhancement). Thus, V_OC-loss_-nonrad values for the control, GAI-treated, FAAc-treated, and GAAc-treated devices were calculated to be 118.30, 99.92, 82.41, and 53.75 mV, respectively. The results suggest that the GAAc treatment is the most effective one in reducing the energy loss caused by non-radiative recombination.

Operational stability and ion migration of the GAAc-treated PSCs

The stability of PSCs is critical to their commercialization. We evaluated the stability of different PSCs under various conditions. First, the statistical data of PSC's shelf stability under a dark environment (25% relative humidity, 25 °C) were depicted in Fig. 5a. The efficiency of all devices exhibited almost negligible decay for 1440 h. Thermal stability tests were performed under 65 °C in the N₂ glove box. We note that a π-conjugated polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5,7-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) was added in spiro-OMeTAD to enhance the stability of the HTL.³⁴ As illustrated in Fig. 5b, the control device experienced a deterioration in efficiency to 65% after 1050 h of heating. In contrast, the efficiency of the FAAc, GAI, and GAAc-treated devices decreased to 79%, 84%, and 93% of their initial values, respectively. Notably, the stability of GAI-treated and GAAc-treated devices surpasses that of the FAAc-treated one, which could be attributed to the effective inhibition of ion migration across the perovskite interface and the enhancement in ion migration activation energy (E_a) due to the formation of the 2D perovskite GA₂PbI₄ at the interface.¹⁷ By conducting temperature-dependent conductivity measurements on lateral devices, we acquired E_a for different samples (Fig. 5c). It reveals that the GAAc-treated film obtained the highest E_a value of 0.79 eV, followed by the GAI-treated sample (E_a = 0.60 eV). Notably, FAAc-treated film exhibited an E_a value of 0.51 eV, higher than the control sample's 0.45 eV. These results indicate that both the 2D GA₂PbI₄ and Ac⁻ contribute to suppressing the ion migration of perovskite in the devices.

Operational stability tests of the devices were tracked at the max power point (MPP) under 1 sun illumination.³³ As depicted in Fig. 5d, the efficiency of the control device decreased to below 80% of its initial efficiency after 66 h. In contrast, FAAc-treated and GAI-treated devices demonstrated significantly reduced efficiency decline, with a T₈₀ lifetime extended to 230 h and 373 h, respectively. Remarkably, the GAAc-treated device showed a substantial improvement in operational stability, with a T₈₀ lifetime of 2073 h. This enhancement is primarily attributed to the synergistic effects of the 2D perovskite and the Lewis base, significantly reducing interface defects and enhancing ion migration activation energy.

Conclusions

In conclusion, adopting GAAc as a surface modifier in PSCs not only enhanced the device efficiency but also significantly improved the stability of the devices. The Ac⁻ acted as a Lewis base and coordinated with Pb²⁺ ions, and the guanidinium cation reacted with the residual PbI₂ and formed an isotropic 2D perovskite, achieving type I heterostructure with the 3D perovskite. This synergistic effect of the cation and anion effectively mitigates the surface defects and reduces the non-radiative recombination at the perovskite/HTL interface. Additionally, UPS measurements demonstrated the improved optoelectronic properties of perovskite film by GAAc treatment, facilitating the charge transfer and reducing the recombination losses. Photovoltaic characterization confirmed a substantially improved efficiency of 25.22% for the GAAc-treated PSCs. Moreover, GA⁺ and Ac⁻ also showed synergistic in enhancing the ion migration activation energy of the GAAc-treated perovskite, thus effectively improving the thermal and operational stability of the device. Remarkably, the GAAc-treated PSCs enabled a T₈₀ operational lifetime of 2073 h under 1 sun illumination. The synergistic effects of 2D GA₂PbI₄ and the Lewis base of Ac⁻ are promising in promoting the development of advanced perovskite solar cell technology.

Data availability

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

Author contributions

L. Xie and Z. Wei supervised the work. P. Song and L. Shen fabricated the devices and analyzed the data. L. Zheng performed FTIR measurements and analyzed the data. E. Hou performed KPFM measurements. P. Xu performed NMR measurements. X. Zhang performed the DFT calculation and analyzed the data. J. Yang and C. Tian contributed to improving the device's performance. P. Song, L. Shen, and L. Xie co-wrote the paper. Z. Wei and L. Xie revised the paper. All authors read and commented on the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22179042 and U21A2078) and the Natural Science Foundation of Fujian Province (2023J06034). We would like to thank Instrumental Analysis Center of Huaqiao University for providing the various tests.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03904a

‡ Authors with equal contributions.

This journal is © The Royal Society of Chemistry 2024