Influence Of Stress On Optoelectronic Characteristics Of Flexible Perovskite Solar Cells

Flexible perovskite solar cells(F-PSCs)have shown great potential application towards future wearable power sources with increasing power conversion efficiency(PCE)and mechanical stability.Strain engineering has become an efficient way to tune the optical and electronic behaviors of metal halide perovskites due to their unique structure-dependent optoelectronic characteristics.In this study,we found that the mechanical stability of F-PSCs is also influenced by the way of mechanical bending.The F-PSCs exhibit better mechanical stability with concave bending as compared with the ones with convex bending.Further study based on J-V characteristics and impedance spectra indicate that the series resistance(R_s)of the device has a larger increase,which results in the degradation of photovoltaic performance after convex bending.Furthermore,it is revealed that the conductivity decrease of perovskite film is responsible for the increased R_s,consequently leads to poorer mechanical stability after convex bending.Our findings may attract the attention of developing the standard of characterizing mechanical stability in the flexible perovskite-based devices.In this work we show,the band gap can be reduced and meanwhile the carrier lifetime is increased by simply stretching the perovskite thin films,both valid for 3D and 2D perovskites.The narrowed band gap and prolonged carrier lifetime are beneficial for the photovoltaic actions,indicating mechanical stretching can be a simple and efficient way to achieve photovoltaic property optimization of stretchable perovskite-based devices.Furthermore,Raman spectra show that the Pb-I bond length is shortened with mechanical stretching,which increases the valence band maximum(VBM)through orbital coupling,leading to narrower band gap.Consequently,the trap states near VBM can be radiative as the trap energy levels get closer to the VBM,resulting in prolonged carrier lifetime.This work brings huge opportunities to control the optoelectronic properties of metal halide perovskites through mechanical stress towards optoelectronic applications.