High voltage and efficient bilayer heterojunction solar cells based on an organic-inorganic hybrid perovskite absorber with a low-cost flexible substrate.

A low temperature (<100 °C), flexible solar cell based on an organic-inorganic hybrid CH3NH3PbI3 perovskite-fullerene planar heterojunction (PHJ) is successfully demonstrated. In this manuscript, we study the effects of energy level offset between a solar absorber (organic-inorganic hybrid CH3NH3PbI3 perovskite) and the selective contact materials on the photovoltaic behaviors of the planar organometallic perovskite-fullerene heterojunction solar cells. We find that the difference between the highest occupied molecular orbital (HOMO) level of CH3NH3PbI3 perovskite and the Fermi level of indium-tin-oxide (ITO) dominates the voltage output of the device. ITO films on glass or on the polyethylene terephthalate (PET) flexible substrate with different work functions are investigated to illustrate this phenomenon. The higher work function of the PET/ITO substrate decreases the energy loss of hole transfer from the HOMO of perovskite to ITO and minimizes the energy redundancy of the photovoltage output. The devices using the high work function ITO substrate as contact material show significant open-circuit voltage enhancement (920 mV), with the power conversion efficiency of 4.54%, and these types of extra-thin planar bilayer heterojunction solar cells have the potential advantages of low-cost and lightweight.