Dual-Functional Enantiomeric Compounds as Hole-Transporting Materials and Interfacial Layers in Perovskite Solar Cells

In this paper, we describe the application of the enantiomeric compounds YLC-1–YLC-4, each featuring a bulky spiro[fluorene-9,9′-phenanthren]-10′-one moiety, as both hole-transporting materials (HTMs) and interfacial layers in both n–i–p and p–i–n perovskite solar cells (PSCs). These HTMs contain an enantiomeric mixture and a variety of core units linked to triarylamine donors to extend the degree of π-conjugation. The n–i–p PSCs incorporating YLC-1(a) exhibited a power conversion efficiency (PCE) of 19.15% under AM 1.5G conditions (100 mW cm–2); this value was comparable with that obtained using spiro-OMeTAD as the HTM (18.25%). We obtained efficient and stable p–i–n PSCs having the dopant-free structure indium tin oxide (ITO)/NiOx/interfacial layer (YLC)/perovskite/PC61BM/BCP/Ag. The presence of the spiro-based compounds YLC-1 and YLC-2 efficiently passivated the interfacial and grain boundary defects of the perovskite and enhanced the sizes of its grains, more so than did YLC-3 and YLC-4. These spiro-based YLC derivatives packed densely and functioned as Lewis bases to coordinate Pb and Ni ions in the perovskite and NiOx layers, respectively. Together, the effects of smaller grain boundaries and defect passivation of the perovskite enhanced the optoelectronic properties of the PSCs. The photoinduced charge carrier extraction in the linearly increasing voltage (photo-CELIV) curves of NiOx/YLC-1(a) showed the faster carrier transport 3.3 × 10–3 cm2 V–1 s–1, which improved the carrier mobility, supporting the notion of defect passivation of the perovskite. The best-performing NiOx/YLC-1(a) device provided a short-circuit current density (JSC) of 22.88 mA cm–2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 80.93%, corresponding to an overall PCE of 20.37%. In addition, the PCEs of the NiOx/YLC-1(a) and NiOx/YLC-4(b) PSC devices underwent decays of only 98.1 and 97.0% of their original values after 41 days under an Ar atmosphere. Thus, these YLC derivatives passivated the NiOx surface and optimized the film quality of perovskites, thereby leading to superior PCEs of their respective PSCs.