Hysteresis and Photoinstability Caused by Mobile Ions in Colloidal Quantum Dot Photovoltaics

Organic-inorganic hybrid photovoltaics (PVs) have recently attracted considerable attention as their PV performance has rapidly improved. Abnormal current-voltage (I-V) characteristics or I-V hysteresis, however, were occasionally observed in such systems that hampered the development of the PV technology. Here we study the hysteresis of organic-inorganic hybrid colloidal quantum dot (CQD) PVs by analyzing I-V characteristics upon systematic modulation of organic components of CQDs. We demonstrate that an external bias stress to CQD films transiently prompts redistribution of mobile ions, particularly protons of surface ligands, thus leading to the formation of a transient space-charge region in the CQD films. The variable space-charge region causes I-V hysteresis and photoinstability of CQD PVs, which is closely correlated with the transient behavior of mobile ions. Our findings here could provide significant implications to the understanding of the influence of mobile ions on I-V hysteresis in other organic-inorganic hybrid PVs such as perovskites.