Perovskite solar cells (PSCs) have shown remarkable progress in the last decade, with certified power conversion efficiencies (PCEs) approaching 26%. This advancement is attributed to composition engineering, crystal growth modulation, and surface passivation. Surface passivation plays a vital role in reducing trap-state density and suppressing non-radiative recombination, ultimately improving device performance. However, conventional passivation agents often suffer from poor conductivity, hindering charge carrier transport. This trade-off between defect passivation and charge transport necessitates the development of new passivation materials with efficient charge extraction and transmission capabilities. Previous studies have employed various materials like 2D perovskites, inorganic compounds, and polymers as passivating agents, but these often exhibit low conductivity, limiting fill factor (FF) improvement despite ultrathin layer thicknesses. To address this, researchers have explored semiconducting polymers to replace conventional insulator-based passivation agents, achieving higher FFs through flat-band alignment. Other approaches have focused on modifying passivation layer distribution and creating charge transport pathways within the layer. This work presents a novel mixed organic halide salt system for BSPT of RbCl-doped FAPbI3, aiming to enhance both defect passivation and charge transport. Specifically, it investigates the synergistic effects of blending 4-tert-butylphenylmethylammonium iodide (tBBAI) with phenylpropylammonium iodide (PPAI), and their impact on PSC performance.

The literature extensively documents the use of various surface passivation strategies for improving PSC performance. Phenethylammonium iodide (PEAI) has been shown to reduce defect density and suppress non-radiative recombination. Other studies have explored the use of two-dimensional (2D) perovskites, inorganic compounds, and polymers as passivation agents. While these materials effectively passivate defects, their low conductivity often limits charge transport within the device. This limitation has been a major bottleneck in improving the fill factor (FF) of PSCs, which has lagged behind the improvements in short-circuit current density (Jsc) and open-circuit voltage (Voc). Previous research has demonstrated the advantages of using semiconducting polymers as passivation agents to improve charge transport. Modifying the passivation layer distribution and creating charge transport pathways within the layer have also been shown to enhance device performance. These existing studies provide a foundation for the present work, which seeks to overcome the limitations of previous approaches by using a binary synergistic post-treatment method.

The researchers employed a modified two-step method to deposit the perovskite film, followed by spin-coating of the passivation layer. The device structure consisted of FTO/SnO2/perovskite/passivation layer/Spiro-OMeTAD/Au. The BSPT method involved blending tBBAI and PPAI in isopropanol (IPA) and spin-coating the mixture onto the perovskite surface without further annealing. Various characterization techniques were used to investigate the effects of the BSPT method. Grazing-incidence X-ray diffraction (GIXRD) and conventional X-ray diffraction (XRD) were used to analyze the crystallization quality of the passivation layer. Grazing-incidence wide-angle X-ray scattering (GIWAXS) was employed to study the molecular orientation and packing. X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemical composition. Ultraviolet photoelectron spectroscopy (UPS) determined the energy band structure. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the surface morphology. Steady-state photoluminescence (ssPL), time-resolved PL (TRPL), transient reflection spectroscopy (TRS), and PL mapping were conducted to characterize the surface defect density. All-atom molecular dynamics (AA-MD) simulations were used to investigate the interaction between tBBAI and PPAI molecules. Finally, the device performance was evaluated using current-voltage (J-V) characteristics, external quantum efficiency (EQE), and maximum power point (MPP) tracking. Additional device characterizations such as Mott-Schottky analysis, thermal admittance spectroscopy (TAS), electrical impedance spectroscopy (EIS), transient photocurrent (TPC), and open-circuit voltage decay (OCVD) were performed to gain a deeper understanding of the charge carrier dynamics.

The BSPT method led to several key improvements in the PSCs. GIXRD and XRD results showed enhanced crystallinity and improved molecular packing in the BSPT passivation layer compared to using PPAI alone. GIWAXS measurements revealed a more ordered molecular orientation in the BSPT layer, suggesting improved charge transport. XPS analysis indicated a high Pb:I ratio in the BSPT sample, suggesting complete filling of iodine vacancies at the perovskite surface. UPS analysis demonstrated a more suitable energy band alignment for hole extraction after BSPT treatment. AFM and SEM imaging revealed a smoother and more uniform surface morphology in the BSPT samples, suggesting better coverage and reduced leakage current. ssPL, TRPL, and TRS measurements confirmed a significant reduction in surface defect density after BSPT, leading to longer carrier lifetimes. The AA-MD simulations further supported the experimental findings, showing stronger interaction and improved molecular packing between PPAI and tBBAI. Consequently, the BSPT PSCs exhibited a significantly improved fill factor (FF) of 84%, reaching a certified PCE of 26.0%. The devices demonstrated excellent stability, retaining 81% of their initial PCE after 450 hours of maximum power point tracking. Device characterizations such as Mott-Schottky analysis, TAS, EIS, TPC, and OCVD further confirmed the improved charge separation, reduced trap density, faster charge extraction, and suppressed recombination observed in the BSPT devices.

The results demonstrate that the BSPT method effectively addresses the trade-off between defect passivation and charge carrier transport in PSCs. By using a synergistic blend of tBBAI and PPAI, the passivation layer simultaneously enhances defect passivation and improves charge transport properties. The improved crystallinity, ordered molecular packing, and suitable energy band alignment all contribute to efficient hole extraction and transfer, leading to improved device performance. The high PCE and stability achieved with the BSPT method highlight its potential for developing high-efficiency and stable PSCs for practical applications. The significantly improved FF compared to control devices, coupled with increased Voc and similar Jsc, underscores the effectiveness of the BSPT strategy in optimizing charge carrier management. The exceptionally low ideal factor of 1.09, calculated from Voc-light intensity relationship, signifies remarkably suppressed SRH recombination. The superior performance of BSPT devices compared to control devices highlights the significant advantages of this strategy.

This research successfully demonstrates a novel binary and synergistic post-treatment (BSPT) method for creating highly efficient and stable perovskite solar cells. The method uses a carefully chosen blend of organic halide salts to create a passivation layer with enhanced crystallinity, ordered molecular packing, and a favorable energy band alignment. The resulting devices exhibit a record-certified PCE of 26.0% and exceptional stability, maintaining 81% of their initial PCE after 450 hours of continuous operation. This approach presents a significant advancement in PSC technology, paving the way for further optimization and the development of more efficient and stable devices for practical applications. Future research could explore different combinations of organic halide salts and further optimize the BSPT method to achieve even higher efficiencies and longer lifetimes. Investigating more stable hole transport layers and improved encapsulation strategies would be crucial steps towards commercialization.

While the study demonstrates exceptional performance and stability, several limitations warrant consideration. The operational stability, although high, still needs to be further improved to meet commercial standards. The use of Spiro-OMeTAD as the hole transport layer, known for its limited stability, is a potential bottleneck. Furthermore, the initial rapid degradation observed in the thermal stability tests points to the need for a more thermally stable passivation agent. The study predominantly focuses on the normal n-i-p planar PSC structure; exploring the applicability of BSPT in other PSC architectures would be valuable. Although the use of humidity during the perovskite thermal annealing is mentioned as critical, the detailed control procedure for humidity is not thoroughly described, which may affect the reproducibility of the perovskite film quality.