A comprehensive comparison of transition metal oxide MoO3 and non-transition metal oxide GeO2 in solar cells

Solution-processed metal oxide materials including transition metal oxide MoO3 and non-transition metal oxide GeO2 have been studied systematically here and they are directly dissolved in the deionized water. Aqueous solution-processed metal oxide material films are proposed utilizing the weak solubility in water. Solution-processed transition metal oxide MoO3 and non-transition metal oxide GeO2-based devices both showed the desired photovoltaic performance. Most importantly, non-transition metal oxide GeO2 was a highly promising interface layer for solar cells. Our results demonstrate the significant role of interfaces in improving the conditions of relaxation time and electron–hole pairs. Such GeO2-based devices exhibit an average energy conversion efficiency of 7.78% (the best is 8.16%), which is even higher than the device with the commonly used MoO3 as hole extraction layer (7.51%). Our results demonstrate the significant role of interfaces in improving the photovoltaic performance.