Spectral response regulation strategy by downshifting materials to improve efficiency of flexible perovskite solar cells

Polyethylene naphthalate (PEN) has been widely employed in highly desired flexible perovskite solar cells (F-PSCs) because of its better chemical stability and higher temperature tolerance. However, the naphthalene ring in the PEN induces poor transmittance in the ultraviolet (UV) region below 380 nm, which significantly lowers the power conversion efficiency (PCE) of F-PSCs. Here, a novel strategy is adopted by introducing UV–visible downshifting material before the PEN substrate to increase the spectral response under the UV region. The PCE of modified F-PSCs increases from 22.19 % to 22.81 % and retains the same stability as that of the control device. The optimized device shows improved photocurrent due to the enhanced spectral response in the UV region. Interestingly, the humidity resistance characteristic of the target device had also improved because of the hydrophobicity of downshifting materials. This novel strategy is distinguishable where downshifting material has been externally employed without altering the internal device architecture, which is also broadly applicable to other types of flexible solar cells.