Study of Static and Dynamic Disorder in Organic-Inorganic Halide Perovskites

Using two different techniques, Fourier-transform photocurrent spectroscopy (FTPS) and photoluminescence (PL), we have probed the temperature dependence of methylammonium lead iodide (MALI) absorption spectra to be able to distinguish the static and dynamic part of Urbach energy. We extract the Urbach energy as the reciprocal value of the slope of the absorption at the band edge plotted in logarithmic scale. Its value depends on the material disorder and generally correlates well with the loss in the open-circuit voltage (VOC) of optimized cells, compared to their bandgap, for a given photovoltaic technology [1]. When cooling perovskites, we find a strong decrease in their Urbach energy and a slow decrease of their optical band gap energy. We observe that all absorption curves measured at different temperatures intersect at one point called the Urbach focus, which was in our case at 1.52 eV, which represents a general lower limit for the optical band gap in MALI. Fitting Urbach energies from PL measurements we obtain phonon energies of 135 ± 20 cm-1 [2], which implicates that the dynamic disorder of MALI is given by cage vibrations. Finally, we will show and discuss that the density of active static defects in perovskites is very low in comparison to other materials used for solar cells, especially for a solution processed one. These results will help establish practical efficiency limits of perovskite solar cells, as compared to the Shockley-Queisser limit.