High performance novel flexible perovskite solar cell based on a low-cost-processed ZnO:Co electron transport layer

In this work, high quality uniform and dense nanostructured cobalt-doped zinc oxide (ZnO:Co) films were used as electron-transport layers in CH₃NH₃PbI₃-based planar heterojunction perovskite solar cells (PSCs) on a flexible conductive substrate. Highly photo catalytically active ZnO:Co films were prepared by a low cost hydrothermal process using the aqueous solution of zinc nitrate hexahydrate, hexamethylenete-tramine and cobalt (II) nitrate hexahydrate. ZnO:Co films were deposited on indium tin oxide (ITO) covered polyethylene terephthalate (PET) flexible substrates. The growth was controlled by maintaining the autoclave temperature at 150 °C for 4 h. The CH₃NH₃PbI₃ layer was deposited on the ZnO:Co films by spin coating. Spiro-OMeTAD was employed as a hole-transporting material. The structural, morphology and optical properties of the grown ZnO nanostructures were characterized by X-ray diffraction (XRD), field-emission scanning electron microcopy (FESEM), energy-dispersive X-ray spectrometry (EDX), ultraviolet-visible (UV-Vis) and photoelectrochemical propriety. XRD spectra showed that both ZnO and ZnO:Co nanorods had a hexagonal wurtzite structure with a strong preferred orientation along the (002) plane. The surface morphology of films was studied by FESEM and showed that both the pure and Co-doped ZnO films had hexagonal shaped nanorods. In the steady state, the ZnO electrode gave a photocurrent density of about 1.5 mA/cm². However, the Co-doped ZnO electrode showed a photocurrent density of about 6 mA/cm², which is 4-fold higher than that of the ZnO electrode. Based on the above synthesized Co-doped ZnO films, the photovoltaic performance of PSCs was studied. The Co-doped ZnO layers had a significant impact on the photovoltaic conversion efficiency (PCE) of the PSCs. The latter was attributed to an efficient charge separation and transport due to the better coverage of perovskite on the nanostructured Co-doped ZnO films. As a result, the measured PCE under standard solar conditions (A M 1.5G, 100 mW/cm²) reached 7%. SCAPS-1D simulation was also performed to analyze the effect of the co-doped ZnO thin film on the corresponding solar cell performances.