Publication date: 1st July 2014
Among the many desirable properties of the hybrid halide perovskite semiconductors, the one that interests us the most in our search for a viable high voltage cell is the high values of VOC relative to the semiconductor bandgap that can be obtained. For this reason, we have concentrated on CH3NH3PbBr3 with a bandgap of 2.3 eV. We have considered two main factors in maximizing the VOC of cells based on this perovskite: The nature of the selective charge extraction or nucleation layers and the nature of the perovskite itself. The most important selective contact for attaining high VOC in our case is the hole conductor. The VOC was found to scale with the depth of the hole conductor HOMO level in most cases. The nature of the CH3NH3PbBr3 was also important. This material tends to form large crystals that do not cover the substrate well. Addition of small amounts of Cl (through PbCl2) to the Br perovskite was found to improve the cells, as was well-known for the corresponding iodide perovskite, given values of VOC >1.5 V and with improved photocurrents and efficiencies compared to the pure Br perovskite. An in-depth study of the effect of Cl showed that the CH3NH3Pb(Br,Cl)3 formed smaller and better covering crystals than the pure bromide due to nucleation of the perovskite on the relatively insoluble (compared to the corresponding higher halides) PbCl2. More recent efforts are directed towards lowering the bandgap of the CH3NH3Pb(Br,Cl)3 to the range of 1.8 - 2 eV, which would be more ideal for use as the high voltage part of a tandem cell.
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