(a) Short-circuit current density (JSC), (b) open-circuit voltage (VOC), (c) fill factor (FF), and (d) power conversion efficiency (PCE) of HC(NH2)2PbI3 perovskite solar cells based on TiO2 nanohelices. Time-limited photocurrent response at different chopping frequencies of (e) 4 Hz, (f) 10 Hz and (g) 50 Hz. 530 nm monochromatic beam generated from a 75 W Xenon lamp was used as the light source. Reproduced from ref. 38 with permission from the Royal Society of Chemistry.