Proceedings of Perovskite Thin Film Photovoltaics (ABXPV16)
Publication date: 14th December 2015
Solar cells based on CH3NH3PbI3 have been rapidly developed in the past few years, reaching power conversion efficiencies close to 20%. This not only makes organic-inorganic perovskites interesting candidates for photovoltaic applications, but also illustrates their fascinating opto-electronic properties. Although substantial progress has been made in understanding the dynamics of photo-excited charge carriers in perovskites, the temperature-dependent opto-electronic properties of CH3NH3PbI3 as a direct band-gap semiconductor are complex and therefore, not well understood. In this work, we use microwave conductivity to study the both the dark conductivity and the photo-conductance of CH3NH3PbI3 films at temperatures ranging from 80 K to 400 K. From the temperature-dependent dark conductivity, we find that the concentration of dark carriers goes through a broad maximum of 1016 cm-3 in between 180 and 260 K. This is attributed to thermal ionization of intra-band gap states, originating from unintentional impurities in the perovskite. From the change in conductivity upon photo-excitation of the perovskite films, we monitor the charge carrier dynamics at excitation energies of 0.1 to 10 nJ/cm2 (i.e., 1013 to 1015 charges/cm3). Most importantly, we find that when the dopants are unionized (below 180 K), these act as charge carrier traps. Additionally, we determined the temperature-dependent mobility and generation yield of free mobile charges in the orthorhombic, tetragonal and cubic phase. Our data suggest that the tetragonal-cubic phase transition does not affect the free carrier generation yield, whereas this drops in the orthorhombic phase. Furthermore, we find different trends in the photo-conductance between cooling and heating over the tetragonal-orthorombic phase transition (temperature range 120 to 160 K). The relationship between photo-conductance and crystal structure is further discussed from temperature-dependent XRD measurements. Finally, we find that all temperature-induced changes are completely reversible, which is an important conclusion for using perovskites in solar cells.
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