A new strategy for buried 2D/3D heterojunctions in perovskite solar cells
Metal halide perovskite solar cells (PSCs) are promising as high-efficiency, low-cost photovoltaics; however, their efficiency and stability are often compromised by high defect densities at grain boundaries and interfaces. To mitigate these issues, long-chain ammonium salts are introduced to the surface of three-dimensional (3D) perovskites to construct 2D/3D heterostructures, enabling effective chemical and field-effect passivation. Previous studies have mainly integrated 2D/3D heterostructures into the perovskite bulk or at its upper surface to improve device performance. Nevertheless, 2D/3D perovskite engineering at the buried interface remains challenging, because the pre-deposited 2D perovskite layer would be dissolved during subsequent 3D perovskite processing, while 2D perovskites introduced as additives are also difficult to selectively assemble at the buried interface. Moreover, achieving controllable 2D/3D perovskite heterojunction at the buried interface with well-defined dimensionality, orientation, and energy-level alignment has become a key challenge, and related studies remain scarce. Previous ligand-based methods for constructing buried 2D/3D heterojunctions suffer from weak interfacial interactions, leading to undesirable ligand diffusion into the 3D perovskite bulk and non-uniform distribution at the interface. Recently, Jen et al. constructed localized 2D/3D perovskite heterojunctions at the buried interface by leveraging the Lewis acid-base interaction between the –NH3+ group of the oleylammonium iodide (OAmI) ligand and a sulfur-functionalized self-assembled monolayer (SAM). The rationally designed SAMs featuring Lewis-basic sulfur atoms (CbzBT-B) are able to anchor the ligands and thereby facilitate the growth of localized 2D perovskite phases. Besides, De Wolf et al. added 4-hydroxybenzylamine (HBzA) into the 2PACz solution, where an acid-base reaction between the HBzA amine and the phosphonic acid group (–PO(OH)2) of 2PACz forms a robust ionic bond. This interaction improves HBzA anchoring on the ITO surface and facilitates the formation of a 2D/3D heterojunction at the buried perovskite interface. However, the intrinsic packing density and uniformity of SAMs limit ligand anchoring and the subsequent growth of 2D perovskites. Therefore, achieving a well-defined buried 2D/3D heterojunction requires tightly confining ligands to the charge-selective contact, particularly for scalable PSCs. In the recent work by Wang et al., an in situ solid-state ligand-exchange strategy is proposed to form a 2D perovskite layer exclusively at the SnO2/perovskite interface, without introducing undesired 2D-phase contamination into the 3D perovskite bulk. Owing to the binding affinity between the –SH and SnO2, thioglycolic acid (TGA) is first introduced during the synthesis of SnO2 nanoparticles to obtain TGA-capped SnO2 nanoparticles, thereby enhancing the adhesion of alkylamine molecules on the SnO2 surface. Subsequently, various alkylamines were anchored onto the SnO2-TGA nanoparticles via an acid-base reaction between the –NH2 and –COOH groups. Therefore, OAm is immobilized on the SnO2 surface through ionic bonding with TGA. During the subsequent thermal annealing of perovskite, ion exchange occurs between OAm-TGA and FAI, leading to the formation of 2D/3D perovskite heterojunctions. In situ photoluminescence (PL) spectroscopy is employed to elucidate the crystallization kinetics of perovskite films on the SnO2-TGA-OAm substrate. At the initial stage of ethyl acetate antisolvent dripping, both the control and target samples exhibit a rapid increase in PL intensity. In the subsequent period, however, the target sample undergoes a prolonged stage of continuous PL increase, whereas the control shows persistent PL decay, indicating that the SnO2-TGA-OAm substrate effectively modulates the crystallization process, promoting the formation of high-quality perovskite films with reduced defects.