Achieving Band Gap Reduction and Carrier Lifetime Enhancement in Metal Halide Perovskites via Mechanical Stretching.

Strain engineering has become an efficient way to tune the optical and electronic behaviors of metal halide perovskites as a result of their unique structure-dependent optoelectronic characteristics. In this work, we show that the band gap can be reduced and, meanwhile, the carrier lifetime is increased by simply stretching the MAPbI3-xClx perovskite thin films. The narrowed band gap and prolonged carrier lifetime are beneficial for the photovoltaic actions, indicating that 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 a narrower band gap. Consequently, the trap states near VBM can be radiative as the trap energy levels become closer to the VBM, resulting in a prolonged carrier lifetime. This work brings huge opportunities to control the optoelectronic properties of metal halide perovskites through mechanical stress toward optoelectronic applications.