Improved performance of hole-transporting layer-free perovskite solar cells by using graphene oxide sheets as the nucleation centers

Graphene oxide sheets (GOSs) are introduced between indium tin oxide (ITO) and CH3NH3PbI3 in inverted hole-transport layer-free planar heterojunction perovskite solar cells. The concentration of the GOSs is extremely low and they are sporadically covered on the ITO substrates. Combined with the high resistance of these GOSs, they cannot be regarded as an interface layer. However, the GOSs can act as the nucleation sites in the crystal growth process of CH3NH3PbI3 films, which results in dramatically improved morphology and crystallization of the CH3NH3PbI3 films. As a result, the performance of the devices is significantly improved as compared with the reference device. The optimized device shows a power conversion efficiency of 6.62%, which is about 40% higher than the reference devices. This improvement is attributed to the increased charge carrier transporting property and reduced charge carrier recombination in the CH3NH3PbI3 films.