Progression towards high efficiency perovskite solar cells via optimisation of the front electrode and blocking layer

The effects of a fluorine doped tin oxide (FTO) electrode, titanium dioxide (TiO_2-x) blocking layer (BL) and perovskite (methyl ammonium lead triiodide) preparation on the overall properties of the photovoltaic cells have been studied. The FTO electrode was deposited by atmospheric pressure chemical vapour deposition (APCVD) and the hole blocking layer by spin coating, atomic layer deposition (ALD) or sputtering. We have shown the importance of obtaining uniform thin films of FTO, with low sheet resistance to aid the formation of pin hole free uniform TiO_2-x blocking layers and hence well adhered, perovskite layers. The optimal BL thickness was 20 nm, while thicker films gave decreased shunt resistance and thinner a greater number of pin holes through the layers. We also showed that the conformal nature of ALD and magnetron sputtering, along with their increased uniformity control over spin coating again improved cell efficiency. The main improvement comes for the smaller R_oc, attributed to an improved electrical transport through particularly the sputtered TiO_2-x blocking layer. After identifying the optimised parameters, all the properties were combined to fabricate large solar cells (1 cm²) yielding power conversion efficiencies beyond 16%.