Spontaneous heterointerface modulators for perovskite solar cells†

Perovskite solar cells (PSCs) have attracted significant attention owing to their prominent photovoltaic (PV) performance. Their multi-layered structure facilitates efficient charge separation in i-type-like perovskite photoabsorbers when sandwiched with a p-type hole transport material (HTM) and n-type electron transport material (i.e., n–i–p or p–i–n structures), enabling high PV performance. Since heterointerfaces are prone to creating defects that act as carrier traps, modulating them in PSCs is crucial. In this review, the nascent yet potent spontaneous heterointerface modulation techniques, and effectiveness of spontaneous heterointerface modulators (SHMs) are highlighted. SHMs are used as additives for precursor solutions of PSC components (e.g., perovskite precursor solutions and HTM solutions) and can bypass the conventional treatment of heterointerface modulations; SHMs can increase process efficiencies of PSC fabrications. Additionally, SHMs can effectively modulate buried interfaces, reducing the risk of defect formation caused by atmospheric contamination. Furthermore, several SHMs serve additional benefits that the conventional step-by-step heterointerface modulation cannot attain. In particular, alkyl-primary-ammonium-based ionic liquids (RA–TFSIs), which are a novel series of ionic liquids designed for PSC applications and used as HTM additives, are highlighted. During the deposition of HTM solutions containing RA–TFSIs on perovskite layers, RA cations spontaneously passivate the perovskite layers, generating unique benefits, which can be tuned via the molecular structure of RA cations. In addition, emerging RA–TFSIs have shown promise beyond PSCs, as deviation of their components from the currently major ionic liquids opens avenues for diverse applications that have remained unexplored. Therefore, this review will guide further development of materials science.