Effect of layer number on flexible perovskite solar cells employing multiple layers of graphene as transparent conductive electrodes

Recently, excellent performance of perovskite solar cells has led to increasing interest in their flexible/portable power applications. Providing reliable flexible transparent conductive electrodes (TCEs) is believed to be crucial for maximizing the performance of flexible perovskite solar cells. Here, we first report efficient and reliable ultra-flexible p-i-n-type CH3NH3PbI3 perovskite solar cells employing multiple layers of graphene as anode TCEs. Through the variation of the layer number (Ln) of graphene TCEs, the solar cells are shown to be optimized at Ln = 2, where the power-conversion efficiency (PCE) exhibits 13.35 and 13.94% for forward and reverse scans, respectively with almost no hysteresis in the current density-voltage curves. These results originate from the Ln-dependent trade-off correlation between the structural, optical, and electrical properties of the solar cells, resulting in largest external quantum efficiency at Ln = 2. The PCE is maintained at ∼90% of its original value after 1000 bending cycles even at a bending radius of 2 mm, overwhelming bending stability against the bending deformation.