New heterostructures for perovskite solar cells and investigation of the stability of methylammonium lead iodide layers
The project is to establish the band line ups for perovskite materials in combination with II-VI semiconductors and transparent conducting oxides (TCO) in order to identify and make alternative structures for planar PV devices. Presently, the best perovskite devices use TiO2 electron transport layers and organic hole conductors. However, there is no known reason why other semiconductors should not be used as heterostructure partners, and this is what makes this research novel.
Our lab has a very wide range of materials, and we expect that it will be possible to fabricate working devices from materials chosen on band line-up criteria, via the Anderson model. The semiconductors that will be investigated as junctions with perovskite materials will include wafers of silicon, gallium arsenide, gallium phosphide, gallium antimonide, indium phosphide, indium arsenide, indium antimonide and thin films of zinc oxide, zinc sulphide, zinc selenide, zinc telluride, with both p- and n-type doping where available. The aim is to use UPS and inverse photoemission spectroscopy (IPES) to determine the band shapes and positions for filled and empty states respectively.
Devices will be prepared using spin-coating technique for the deposition of the organic-inorganic hybrid perovskite, in particular methylammonium lead iodide (MAPI), deposited on different wafers or sputtered thin films. Contacts will be finally deposited on the cell by thermal evaporation, choosing suitable metals for Ohmic behaving contacts. Once made, the devices will be the subject of extensive characterisation to determine the properties and the efficiency limiting effects, for example by using XRD, optical spectroscopy, I-V, current transport measurements, C-V and admittance.
In parallel, a project concerning perovskites stability will be undertaken, monitoring the material composition over one month and under different ambient conditions. Degradation of perovskite materials is thought to be caused by UV-radiation and the presence of water and/or oxygen. However, the mechanism of the degradation has not been studied widely. The aim is therefore to separate the effects of each degradation factor on the materials’ structure. The perovskite layer will be exposed to separate and combined degradation conditions and then the structure will be analysed using XRD, ellipsometry, VIS and IR spectroscopy. The study will be performed on perovskite layers obtained using different deposition processes: the one step solution process and the two step interdiffusion process, which will be both spin-coated layers.