Enhanced flexibility and light utilization for high-efficiency all-air-processed carbon-based perovskite solar cells

The hole transport material (HTM)-free carbon-based perovskite solar cells (C-PSCs) are widely recognized as competitive candidates towards practical applications. However, the poor mechanical connection at perovskite/carbon interface and the low light transmittance of flexible substrates limit the fabrication of high-efficiency flexible HTM-free C-PSCs. In this paper, we develop flexible HTM-free C-PSCs with enhanced flexibility and light utilization by implanting carbon nanotubes (CNTs) into perovskite film and carbon electrode to boost the connection at perovskite/carbon interface as well as by coating a polymethyl methacrylate (PMMA) antireflection layer to enhance light utilization of the polyethylene naphthalate/indium tin oxide (PEN/ITO) substrates. The CNTs are proven to not only enhance the flexibility of the devices, but also facilitate charge extraction and transfer at perovskite/carbon interface. The average transmittance of the PEN/ITO substrates increase by 3.63% after the incorporation of PMMA antireflection layer. As a result, the flexible HTM-free C-PSC with an architecture of PMMA/PEN/ITO/SnO2/MAPbI3/carbon presents a power conversion efficiency (PCE) of 14.44%, which is the highest PCE for flexible HTM-free C-PSC devices. Moreover, this flexible PSC retains more than 84% of the original PCE after bending 1000 cycles at 7 mm bending radius attributed to the enhanced connection at the perovskite/carbon interface.