Back interface engineering by 2D layered N-Ti3C2 in low-cost carbon based all-inorganic hole transport layer free perovskite solar cells

The mismatched energy level alignments, back interface defects in the perovskite/carbon back interface, as well as low conductivity of carbon counter electrode are the primary factors for the low photovoltaic behavior of allinorganic hole transport layer (HTL)-free perovskite solar cells (C-PSCs) using carbon counter electrode-based. Herein, 2D layered Ti3C2 and N-Ti3C2 are proposed to form Ti3C2@carbon and N-Ti3C2@carbon counter electrodes for modifying the perovskite/carbon back interface. First, the appropriate work function of Ti3C2 can optimize the energy level alignments. Second, nitrogen doping can enhance the conductivity of Ti3C2 which can lower the total resistance of the device. Third, the specific 2D layered structure Ti3C2 and N-Ti3C2 can reduce back interface defects. Therefore, the perovskite/carbon back interface shows improved charge extraction and transport capabilities disclosed by photoluminescence measurements. Consequently, the power conversion efficiencies (PCE) of the C-PSCs are enhanced to 7.56 % from 5.94 % by using Ti3C2@carbon counter electrode, and further to 8.91 % by using N-Ti3C2 @carbon counter electrode, indicating remarkable PCE improvements of 27.3 % and 50.0 %. Moreover, the Ti3C2@carbon and N-Ti3C2@carbon-based C-PSCs show remarkable stabilities under ambient conditions for 30 days. This work is expected to provide a feasible path to optimize the back interface of the all-inorganic C-PSCs.