Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)
Publication date: 5th February 2015
Mixed-halide hybrid perovskites such as CH3NH3Pb(BrxI1-x)3 are a promising family of photovoltaic absorber materials that have achieved power conversion efficiencies of over 18%. By varying the halide composition, the optical bandgap can be tuned over the range 1.6-2.3 eV, making this family of materials a suitable candidate for both single-junction solar cells as well as the large bandgap absorber of a tandem solar cell. However, reports of mixed CH3NH3Pb(BrxI1-x)3 devices with higher bromine content have so far not been able to achieve the increase in open circuit voltage that may be expected from the larger bandgap of these materials. We observe from in-situ XRD measurements that bromine-rich (0.2<x<1) CH3NH3Pb(BrxI1-x)3 and other mixed-halide perovskites undergo a structural transformation under photoexcitation, segregating into two distinct perovskite phases over the course of minutes. The optical properties are also changed by this transformation resulting in the appearance of intense photoluminescence and absorption features from a lower bandgap phase. The perovskite reverts back to its original structure and starting optical spectra when left in the dark. We perform photoluminescence and cathodoluminescence spectral mapping to image the phase segregated domains under light soaking. We suggest that photoexcitation induces halide segregation, resulting in iodide-rich domains that act as traps and pin the open circuit voltage at a lower potential. The kinetics of this process have a similar temperature dependence to the slow transient behavior of hysteretic solar cells which is suggestive of halide migration playing a prominent role in perovskite photovoltaic hysteresis. The occurrence of halide migration is expected to have serious implications to the long term stability of perovskite photovoltaics and other optoelectronic devices.
Time evolution (in 5 second increments) of the photoluminescence (PL) spectra of CH3NH3Pb(BrxI1-x)3 for x = 0.4 during visible light soaking.
Hoke, E.T. et al.; Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 2015, 6, 613-617.
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