Synergistic Engineering of Conduction Band, Conductivity, and Interface of Bilayered Electron Transport Layers with Scalable TiO and SnO Nanoparticles for High-Efficiency Stable Perovskite Solar Cells.

A simple, synergistic engineering of the conduction band (CB), conductivity, and interface of TiO-based bilayered electron transport layers (ETLs) via scalable TiO and SnO nanoparticles processed at low temperature (≤ 100 °C) for regular planar perovskite solar cells (PSCs) was developed. The bottom layer (Lt-TiO:SnO nanocomposite film) was prepared by spin coating from the ethanol suspension of small ground TiO nanoparticles with big ground SnO nanoparticles as the additive. The top C-SnO layer (spin-coated from the concentrated commercial SnO nanoparticles (C-SnO NPs, 20 wt %, 7 nm in size suspended in HO)) can be regarded as an interlayer between Lt-TiO:SnO and perovskite (Psk) absorbers. Bilayered Lt-TiO:SnO/C-SnO ETLs are dense films with a cascade CB, good conductivity, facile electron extraction/transport ability, and a highly hydrophilic surface for depositing high-quality Psk films. Regular planar PSCs based on Lt-TiO:SnO/C-SnO ETLs combined with a (FAI)(PbI)(MABr)(PbBr) absorber and a spiro-OMeTAD hole transporter achieved the highest power conversion efficiency of 22.04% with a negligible current hysteresis. The champion cell lost less than 3% of the initial efficiency under continuous room lighting (1000 lux) for 1000 h (lost 10% after 2184 h) without encapsulation under an inert atmosphere. Four related low-temperature-processed ETLs (Lt-TiO/C-SnO, Lt-C-SnO, Lt-TiO:SnO, and Lt-TiO) were fabricated using the same metal oxide nanoparticle suspensions and studied simultaneously to reveal the function of each metal oxide in the bilayered Lt-TiO:SnO/C-SnO ETLs. In the bottom Lt-TiO:SnO layer, small TiO nanoparticles were needed for making a dense film, and highly conducting big SnO nanoparticles are used to increase the conductivity of ETLs and a handy electron transport path for reducing the charge accumulation and series resistance of the cell. A top C-SnO layer (regarded as an interlayer between Psk and Lt-TiO:SnO) was used to extract/transport electrons facilely, to form a bilayered ETL with a cascade CB, and to create a hydrophilic surface to deposit high-quality Psk films to enhance the photovoltaic performance of the PSCs. This study provides a blueprint for designing good-performance ETLs for high-efficiency, stable regular planar PSCs using various sized nanoparticles prepared in a very simple and low-cost way.