The presence of the anomalous photocurrent hysteresis in many organometal trihalide perovskite solar cells was recorded by many groups. Sometimes an unreasonable fill factor (FF) larger than unit was observed, and in many cases the efficiency measured in devices with photocurrent hysteresis can be seriously overestimated. The mystery photocurrent hysteresis has become one major hindrance for the accurate power conversion efficiency (PCE) measurements. Thus understanding and elimination of the photocurrent hysteresis is an essential challenge for the whole perovskite photovoltaic research society now. Three possible mechanisms for the photocurrent hysteresis were proposed previously.
Ferroelectric photovoltaic effect.
The migration of ions.
The charge trapping and detrapping effect caused by a large defect density in perovskite bulk or at the surfaces.
In this talk, we will report our carefully analysis on the three mechanisms by test the ferroelectricity and ion drift effect. We identified a large density of trap states near the top surface and grain boundaries of the perovskite thin films as the origin of photocurrent hysteresis. We also introduced a simple and effective method to eliminate the notorious photocurrent hysteresis by depositing a phenyl-C61-butyric acid methylester (PCBM)/C60 double fullerene layer on perovskites2. The photocurrent hysteresis completely disappeared with optimum passivation, meanwhile the PCE doubles from 7~8% to 15~17%.
To verify our scenario, a series of characterizations like photoluminescence, thermal admittance spectroscopy, transient photocurrents and impedance spectroscopy were carried out for devices with and without the double fullerene layer. The results demonstrated the excellent passivation effect of PCBM/C60 by reducing the density of surface traps in a complementary way and explain the huge improvement of PCE as the enhancement of the electronic transport properties and the decrease of the surface charge recombination.
1 Snaith, H. J. et al. Anomalous Hysteresis in Perovskite Solar Cells. The Journal of Physical Chemistry Letters 5, 1511-1515 (2014).
2 Yuchuan Shao, Zhengguo Xiao, Bi Cheng, Yongbo Yuan & Jinsong Huang. Elucidating the Origin and Elimination of Photocurrent Hysteresis by Fullerene Passivation in CH3NH3PbI3 Planar Heterojunction Solar Cells. under review (2014).