Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors

The recent success of lead halide perovskites is given by their optimal primary optoelectronic properties relevant for photovoltaic and, more in general, for optoelectronic applications. However, a lack of knowledge about the nature of instabilities currently represents a major challenge for the development of such materials. Here we investigate the luminescence properties of polycrystalline thin films of lead halide perovskites as a function of the excitation density and the environment. First we demonstrate that in an inert environment photoinduced formation of emissive sub-band gap defect states happens, independently of the chemical composition of the lead halide semiconductor, which quenches the band-to-band radiative emission. Carrier trapping occurs in the subnanosecond time regime, while trapped carriers recombine in a few microseconds. Then, we show that the presence of oxygen, even in a very small amount, is able to compensate such an effect.