In-depth analysis of defects in TiO2 compact electron transport layers and impact on performance and hysteresis of planar perovskite devices at low light

Properties of the electron transport layer (ETL) are known to influence the performance of lead halide perovskite solar cells (PSCs). But so far very little emphasis has been given on the increased impact of this layer at low light. In this work we compare the effect of thickness and coverage of a TiO2 compact layer on the performance and hysteresis of methyl ammonium lead iodide planar devices tested under 200 lux vs. 1 sun illumination. Standard TiO2 layers are produced with incremental thickness and coverage using sequential spray pyrolysis of a Ti-acetylacetonate precursor (0-50 sprays, 1 spray similar to 1 nm TiO2). Thorough materials characterisation combining FEG-SEM, XPS, and cyclic voltammetry shows that a crystalline, nearly pin-hole free TiO2 layer is achieved by deposition of >15 sprays over small to large areas (0.2 mm(2)-1 cm(2)). Device performance is affected by two main parameters, namely the coverage yield and thickness of the TiO2 layer, especially under 200 lux illumination. A 25 vs. 50 sprays-TiO2 layer is found to provide the best compromise between coverage and thickness and avoid charge recombination at the TiO2/perovskite interface whilst minimizing resistive losses with 11.7% average PCE at 200 lux vs 7.8% under 1 sun. Finally, the analysis of I/V forward vs. reverse scans and open circuit voltage decay data suggests that hysteresis is greatly affected by the capacitive properties of the ETL at low light, whilst other phenomena such as ion migrations may dominate under 1 sun.