ABSTRACT
The recent swift surge of hybrid metal–halide perovskites for photovoltaic applications is revolutionizing the field of the renewable energies thanks to an unprecedented increase of the performance of perovskite-based photovoltaic devices, from 3.8% in 2009 (Kojima et al. 2009) up to the current approximately 20% (Burschka et al. 2013; Im et al. 2011; Jeon et al. 2015; Lee et al. 2012; Liu et al. 2013; Zhou et al. 2014). However, if this class of materials seems to hold for the attainment of clean, renewable energy from the sun, it also presents many open issues. The positive optical (De Wolf et al. 2014; Kitazawa et al. 2002) and physical properties (Stranks et al. 2013; Xing et al. 2013; Wehrenfennig et al. 2014a,b) of hybrid metal–halide perovskites in fact are paralleled by very unusual phenomena, which do not find similar analogues in the traditional inorganic semiconductors. In 2014, Snaith and co-workers reported on an unusual hysteretic effect in the measured current–voltage (J–V) curve of the CH3NH3PbI3–xClx perovskite (Snaith et al. 2014). These authors observed a different photocurrent with respect to the direction of the potential scan, from zero to open-circuit voltage or vice versa, together with a dependence of the photocurrent on the scanning rate and the device architecture (Snaith et al. 2014). Later studies reported on many similar effects, where 89the response of the material showed a non-steady-state, time-dependent response, generally depending on the illumination conditions and external parameters, such as the presence of moisture in the atmosphere and/or the presence of electric fields.
