Strategies for High Performance Perovskite Solar Cells

In this paper we report on systematic studies conducted for the improvement in both the device structure and the materials quality of perovskite based solar cells (PSCs). We have incorporated TiO2 nanorods, of length around 350-400 nm, in the device structure. Such structures were grown by solvothermal technique directly on the glass/FTO substrates. Characterization by femtosecond transient absorption (fs-TA) spectroscopy indicates that the incorporation of TiO2-nanorod array (NA) greatly enhances the collection efficiency of the photo-generated carriers due to substantial reduction in carrier diffusion distance. To improve the crystallinity of the perovskite films we performed systematic studies on cryoassisted growth of the material. The technique eliminates the need for environmentally harmful anti-solvents and enables decoupling of the nucleation and crystallization phases by inhibiting chemical reactions in the precursor films rapidly cooled by immersion in liquid nitrogen. Furthermore, the technique leads to uniform precipitation of precursors due to the supersaturation condition in the residual solvents at cryogenic temperature resulting in highly uniform coverage of the films. Systematic characterization of the films by low-frequency noise and photothermal deflection technique indicate significant in the trap density of the films which is attributed as the main underlying reason for the observed improvement in the power conversion efficiency of the device. A high efficiency of 21.4% is achieved for our champion device.