Effects of Oxide Contact Layer on the Preparation and Properties of CH3NH3Pbl3 for Perovskite Solar Cell Application

In perovskite solar cells, oxide electron transport layers (ETL) and their interface with the organometal trihalides are key to achieve efficient and stable devices. In the present work we investigate ZnO and TiO2 ETLs and their influence on the preparation of CH3NH3PbI3 film by two different techniques. In the one-step technique, a solution is used for the deposition of a precursor layer which is dripped and subsequently annealed. In the two-step sequential technique, a PbI2 precursor layer is converted into perovskite. We show that, on ZnO, the annealing treatment of the one-step deposited layer is optimum for a duration time of only 2 min. This duration is much less critical for the TiO2 underlayer. Long annealing times produce the degradation of the pigment and formation of PbI2. It is also shown that the one-step technique gives better results for the sensitization of smooth oxide underlayers whereas the two-step one must be utilized for rough or structured underlayer sensitization. The best solar cell performances were achieved by combining a low-overvoltage electrodeposited ZnO layer, a planar architecture, and a perovskite layer prepared by a one-step deposition-dripping route. A maximum overall conversion efficiency of 15% was measured for the ZnO-based perovskite solar cell. Cell impedance spectra have been measured over a large applied voltage range. Their analysis, using an ad-hoc equivalent circuit, shows that charge recombinations are reduced for the one-step perovskite and that a better interface with the oxide is produced in that case.