Heterojunction Formed via 3D-to-2D Perovskite Conversion for Photostable Wide-Bandgap Perovskite Solar Cells

Metal-halide perovskites have emerged as a significant point in the realm of photovoltaic (PV) devices due to their outstanding attributes such as feasible bandgap tunability, extended carrier diffusion lengths and a high tolerance to defects. Single-junction perovskite solar cells (PSCs) have achieved an impressive certified power conversion efficiency (PCE) of 26.1%, close to the efficiency levels achieved by crystalline silicon solar cells.<br/>All-perovskite tandem solar cells, which consist of a wide-bandgap (WBG, ~1.8 eV) perovskite top cell paired with a narrow-bandgap (NBG, ~1.2 eV) perovskite bottom cell, offer the potential for higher efficiency than the SQ limit of single-junction solar cell while maintaining the benefits of low-cost solution-processing. In the past few years, the PCE of all-perovskite tandem solar cells has already surpassed that of single-junction PSCs, positioning themselves as a promising next-generation photovoltaic technology. Despite the significant advancements in PCE, instability issues still hinder the commercialization of all-perovskite tandem solar cells.<br/>The WBG subcells, which alloy a high content of bromide (~40%), suffer from severe light-induced halide segregation under continuous illumination, thereby limiting the operational lifetime of tandem PSCs. Various strategies have been employed to address this challenge, such as passivating the interface, relaxing lattice strain, modifying compositions and developing new transport layers. Such efforts have, to some extent, mitigated light-induced halide segregation. Nevertheless, understanding the fundamental mechanism of phase segregation in full device structure is still a matter of ongoing investigation.<br/>In this presentation, we will reveal that charge accumulation accelerates the halide segregation in WBG PSCs under illumination, originating from the mismatch of energy levels and high trap density at contacting heterointerface with the electron transport layer (fullerene, C₆₀). This prompts us to design the perovskite/C₆₀ interface with facilitated charge extraction and durable interfacial contact.<br/>The construction of an intermixed 2D/3D heterostructure, achieved by spin-coating long-chain ammonium ligands dissolved in isopropyl alcohol onto the surface of a 3D perovskite, has been widely utilized to optimize contact heterointerfaces in PSCs. However, when applied to the MA-free and Cs/Br-enriched WBG perovskites, recognized for their better photostability and thermal stability, this technique yields inconsistent results due to intricate surface chemistry with increased activation energy for surface reconstruction growth.<br/>Herein, we will show a generic 3D-to-2D perovskite conversion approach: depositing a well-defined MAPbI₃ thin layer by a hybrid evaporation/solution method, followed by its complete transformation into a 2D structure via a long-chain ammonium ligand. This approach overcomes the composition-dependent surface sensitivity of conventional solution-processing strategies and enables the preferential growth of wider dimensionality (n ≥ 2) on the MA-free WBG perovskite layer. The resulting 2D/3D heterostructure effectively accelerates the charge extraction at the perovskite/C₆₀ interface and thereby suppresses light-induced halide segregation. We achieved a champion PCE of 19.6% and an impressive <i>V_oc</i> of 1.32 V in 1.78-eV WBG PSCs. Meanwhile, the devices exhibited superior operational stability – maintaining 95% of their initial PCE after 1000 h operation at the maximum power point (MPP). These enabled us to realize an impressive PCE of 28.1% in monolithic all-perovskite tandem solar cell. Encapsulated tandem devices retained 90% of their initial performance following 855 h of operation at the MPP under full one-sun illumination in ambient conditions. This promising operating stability (longest <i>T₉₀</i> recorded to date) combined with high PCE represents a crucial step toward the practical application of all-perovskite tandem solar cells.