Wolfgang Tress from EPFL (Switzerland) gave an Invited Seminarentitled ‘Device Physics of Perovskite Solar Cells’ and hosted by Agustín Mihi
Solar cells based on lead halide perovskites have recently emerged showing a tremendous increase of power-conversion efficiency which exceeded 20 %. In this talk, the device physics of perovskite solar cells is addressed. The focus is on recombination of charge carriers because this process is ultimately limiting open-circuit voltage and fill factor in perovskite solar cells. Different architectures such as planar and mesoporous-TiO2 based devices are presented.
The origin of the open-circuit voltage is discussed based on the reciprocity relation between electroluminescence and photovoltaic quantum efficiency.[1,2] Different recombination mechanisms (radiative, trap-mediated) are investigated and related to the device performance.
Hysteresis in the current-voltage curve is related to recombination as well. It is shown how different prebias voltages influence recombination rates. The results are explained by the mixed ionic and electronic conductivity of the material, where displaced ions change interface and defect recombination. A recently discovered inverted hysteresis and reversible photo-induced degradation mechanisms on the timescale of minutes to hours are put into the framework of ion migration as well.
An outlook is given on strategies aiming for a further improvement of the open-circuit voltage.
 Tress, W. et al. Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination. Adv. Energy Mater. 5, 140812 (2015).  Bi, D. et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Sci. Adv. 2, e1501170 (2016).  Tress, W. et al. Understanding the rate-dependent J–V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field. Energy Environ. Sci. 8, 995–1004 (2015).  Tress, W. et al. Inverted Current–Voltage Hysteresis in Mixed Perovskite Solar Cells: Polarization, Energy Barriers, and Defect Recombination. Adv. Energy Mater. 6, 1600396 (2016).