Low-temperature solution-processed ionic liquid modified SnO2 as an excellent electron transport layer for inverted organic solar cells

SnO2 with its excellent properties such as high optical transparency, suitable band energy and high election mobility, recently has received special attention from researchers as an outstanding electron transport layer (ETL) for optoelectronic devices. Here, we demonstrate that SnO2 combined with ionic liquid (IL), 1-benzyl-3-methylimidazolium chloride ([BzMIM]Cl), based on low-temperature solution-processed can be an excellent ETL for efficient inverted organic solar cells (iOSCs). Our best performance with P3HT:PC60BM based iOSCs using SnO2/IL as an ETL, has achieved a power conversion efficiency (PCE) of 4.05%, which is the highest reported value so far and was a 38% increase compared to that of SnO2 only (2.94%). Using ultraviolet photoelectrIon spectroscopy, we found that the work function of the cathode decreased significantly from - 4.38 eV to - 3.82 eV for SnO2/IL, an outstanding feature that is necessary for an ideal ETL. Electrochemical impedance spectroscopy studies revealed a significant lower transport resistance and an efficient charge extraction at the interface between photoactive layer and the electrode for the SnO2/IL-based iOSC than for the SnO2 only device. The IOSC devices using SnO2/IL showed excellent long-term stability, with a PCE of similar to 81% compared to the initial value after storage for 2.5 months in ambient conditions. This low-temperature solution-processed SnO2 IL is expected for low-cost, high throughput, roll-to-roll process on flexible substrates for iOSC as well as other optoelectronic devices.