Outstanding performance of electron-transport-layer-free perovskite solar cells using a novel small-molecule interlayer modified FTO substrate

Abstract Organic-inorganic halide perovskites have become promising materials for the next generation of photovoltaic devices on account of the preeminent optoelectronic characteristics and boundless potentialities. Perovskite solar cells (PSCs) normally require a complex structure design with an electron transport layer (ETL) to provide a built-in electric field and depress the probability of carrier recombination. Nonetheless, the construction of a proper ETL is not cost-effective, which precludes the practical commercialization of the PSCs. In this respect, a simplified ETL-free PSC is successfully fabricated by inserting a facile and efficient 1-[N-(2-Hydroxyethyl)-4′-piperidyl]-3-(4′-piperidyl) propane (PPPDE) small-molecule thin interlayer between the FTO substrate and perovskite film. Compared with the bare FTO-based PSCs, this surface engineering can prominently ameliorate the photovoltaic performance parameters, yielding an extraordinary PCE of up to 19.71% with a ca. 31% efficiency enhancement, which can be attributed to the optimized interface energy-level alignment with a barrier-free contact, the elevated charge transfer and collection as well as the suppressed electron-holes recombination at the FTO/perovskite interface. Synchronously, the long-term air stability is also improved with mitigated J–V hysteretic behavior due to the larger grain size of perovskite and the suppressed defect-induced degradation. Furthermore, an augmented performance of 15.87% was achieved for the flexible PPPDE embedded ETL-free PSC fabricated on ITO/PEN substrates via a low-temperature deposition process. The demonstrated PPPDE interlayer presents a brand-new strategy that can simplify the cell configuration and improve the photovoltaic performance of ETL-free PSCs.