Orientation control of two-dimensional perovskite (CH3(CH2)3NH3)2(CH3NH3) n-1PbnI3n+1 (n=2) thin films by thermal annealing

It has been attempted to preferentially orientate Pb-I layers in two-dimensional (2D) organic-inorganic hybrid perovskite thin films (CH3(CH2)(3)NH3)(2)(CH3NH3)Pb2I7 perpendicular to substrates only by thermal annealing after spin coating of a reagent solution for improvements in the energy-conversion-efficiency of solar cells. It is found from X-ray diffraction measurements that the ratio of diffraction intensity from the (202) plane to that from the (060) plane becomes larger in thermally annealed (50 degrees C-135 degrees C) samples. This indicates that the Pb-I layer tends to grow perpendicular to the surface of the substrate. In particular, the ratio has reached 8.2, which is larger compared with the ratio of 2.7 for the randomly oriented powder sample, for the sample prepared on SnO2 substrates. Such (202) oriented films seem to contribute to improvements in the energy-conversion-efficiency of tandem-type solar cells utilizing the 2D perovskite thin films as an active layer of the top cell.