Bifunctional Graphene Oxide Hole-Transporting and Barrier Layers for Transparent Bifacial Flexible Perovskite Solar Cells

A thin graphene oxide (GO) layer has been prepared on 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 conventional nontransparent devices with an ITO top electrode using radio frequency (RF) sputtering. The GO layer with high transmittance in the visible range exhibits excellent hole-extracting capabilities from the perovskite layer. Scanning electron microscopy (SEM) and atomic force microscopy (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 poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-based counterparts. The enhancement in stability becomes much more apparent for 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 a bifunctional GO HTL exhibits a high efficiency of 9.34% (cf. 11.39% for the glass substrate and 12.31% for the nontransparent/glass substrate) and good long-term stability.