Publication date: 1st July 2014
Very recently, a new class of material based on hybrid mixed halide perovskite emerged as a promising solution for photovoltaics. Used first as light antenna in the “dye-sensitized” configuration it has been recently demonstrated that they can also operate extremely well (with η > 16%) in planar heterojunction architecture. The variety of configurations in which they have been used along with the high performances demonstrated, cast the doubt on the fundamental physics governing the photovoltaic mechanism, in particular regarding the nature of the elementary photo-excitations, i.e. whether free charges or bound excitons are generated and their dynamics with a special attention to exciton formation and charge generation. With this aim we address the photophysical properties of both CH3NH3PbI3 and the CH3NH3PbI3-xClx. By temperature dependent linear absorption measurements we estimate the exciton binding energy to be about 50 meV. Using this value we model the ionization processes using Saha equation leading to an evaluation of the branching ratio between free carriers and excitons. We find that at equilibrium there is a predominant fraction of free charges under photovoltaic operating regime. Finally, since non-equilibrium dynamics can also affect the device operation, we investigate the photoinduced dynamics at ultrafast timescale by temperature-dependent ultrafast transient absorption spectroscopy. The results demonstrate that the operation mechanism of this material at room temperature is fundamentally governed by free charges and not excitonic species.
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