Suppressing Excess Lead Iodide Aggregation and Reducing N-Type Doping at Perovskite/HTL Interface for Efficient Perovskite Solar Cells

Excess lead iodide (PbI2 ) aggregation at the charge carrier transport interface leads to energy loss and acts as unstable origins in perovskite solar cells (PSCs). Here, a strategy is reported to modulate the interfacial excess PbI2 by introducing π-conjugated small-molecule semiconductors 4,4'-cyclohexylbis[N,N-bis(4-methylphenyl)aniline] (TAPC) into perovskite films through an antisolvent addition method. The coordination of TAPC to PbI units through the electron-donating triphenylamine groups and π-Pb2+ interactions allows for a compact perovskite film with reduced excess PbI2 aggregates. Besides, preferred energy level alignment is achieved due to the suppressed n-type doping effect at the hole transport layer (HTL) interfaces. As a result, the TAPC-modified PSC based on Cs0.05 (FA0.85 MA0.15 )0.95 Pb(I0.85 Br0.15 )3 triple-cation perovskite achieved an improved PCE from 18.37% to 20.68% and retained ≈90% of the initial efficiency after 30 days of aging under ambient conditions. Moreover, the TAPC-modified device based on FA0.95 MA0.05 PbI2.85 Br0.15 perovskite produced an improved efficiency of 23.15% compared to the control (21.19%). These results provide an effective strategy for improving the performance of PbI2 -rich PSCs.

Pressure-Assisted Space-Confinement Strategy to Eliminate PbI2 in Perovskite Layers toward Improved Operational Stability

The existence of the PbI₂ phase in the perovskite film is normally inevitable because of the easy sublimation of the organic component during the crystallization process under a relatively high annealing temperature. However, excess PbI₂ will cause significant degradation on open current voltage (V_OC) and fill factor (FF) under continuous illumination. Here, we developed a pressure-assisted space-confinement (PASC) method to enhance the phase purity of the perovskite film fabricated by the two-step spin-coating method. It was found that high pressure is more conductive to lower the sublimation rate of the organic units, and the space confinement is more favorable for the Ostwald ripening. The combination of them can easily fabricate high-quality perovskite films with large crystal grains and eliminated PbI₂ remnants. As expected, the efficiency of the solar cell was improved from 20.38 to 22.26%; more importantly, the operational stability of the corresponding device had a pronounced improvement, which remains over 85% of its initial efficiency after 500 h maximum power point tracking measurement. Based on this PASC method, a prototype PSC module (PSM) with an active area of 14 cm² was also fabricated reaching an efficiency over 17%.

Enhancing the Efficiency and Stability of Triple-Cation Perovskite Solar Cells by Eliminating Excess PbI2 from the Perovskite/Hole Transport Layer Interface

Metal halide perovskites are promising contenders for next-generation photovoltaic applications due to their remarkable photovoltaic efficiency and their compatibility with solution-processed fabrication. Among the various strategies to control the crystallinity and the morphology of the perovskite active layer and its interfaces with the transport layers, fabrication of perovskite solar cells from precursor solutions with a slight excess of PbI₂ has become very common. Despite this, the role of such excess PbI₂ is still rather controversial, lacking consensus on its effect on the bulk and interface properties of the perovskite layer. In this work, we investigate the effect of removing the excess PbI₂ from the surface of a triple-cation mixed-halide Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite layer by four different organic salts on their photovoltaic performance and stability. We show that treatments with iodide salts such as methylammonium iodide (MAI) and formamidinium iodide (FAI) can lead to the strongest beneficial effects on solar cell efficiency, charge recombination suppression, and stability while non-iodide salts such as methylammonium bromide (MABr) and methylammonium chloride (MACl) can also provide improvement in terms of charge recombination suppression and stability to a moderate extent in comparison to the untreated sample. Under optimized conditions and continuous solar illumination, the MAI- and FAI-treated devices maintained 81 and 86% of their initial power conversion efficiency (PCEs), respectively, after 100 h of continuous illumination (versus 64% for the untreated solar cell with excess PbI₂). Our study demonstrates that eliminating excess PbI₂ at the perovskite/hole transport layer (HTL) interface by treating the perovskite surface with organic salts is a simple and efficient route to enhance the efficiency, and in particular the stability of perovskite solar cells.