A thin graphene oxide (GO) layer has been prepared on an indium-tin-oxide (ITO)-coated glass and plastic (polyethylene naphthalate; PEN) substrates for the application as a hole transport layer (HTL) of p-i-n type planar perovskite solar cells. Transparent devices can be fabricated by replacing the Ag top electrode of the conventional non-transparent devices with the ITO top electrode using RF sputtering. The GO layer with high transmittance in the visible range exhibits
excellent hole-extracting capabilities from the perovskite layer. SEM and AFM analyses reveal
that the GO HTL uniformly covers the substrate. In addition, the GO-based devices show significantly improved long-term stability compared with the conventional PEDOT:PSS-based counterparts. The enhancement in the stability becomes much more apparent for the flexible devices with the PEN substrate, which has been attributed to the excellent barrier properties of the
GO HTL. As a result, a transparent and flexible p-i-n type perovskite solar cell with the bifunctional GO HTL exhibits a high efficiency of 9.34% (cf. 11.39% for the glass substrate, 12.31% for the non-transparent/glass substrate) and good long-term stability has been demonstrated.