Inverted MAPbI₃ Perovskite Solar Cells Based on Optimized NiO_x Hole-Transporting Material and an Interlayer of Photovoltaic Polymers Exhibit a Power Conversion Efficiency over 21% with an Extended Stability

Five mixing molar ratios, 1:0.6, 1:0.8, 1:1.0, 1:1.2, and 1:1.4, of conventional nickel(II) bis(acetylacetonate) (Ni(acac) 2 ) and a diethanolamine (DEA) stabilizer have been tried for preparing precursor solutions in a sol–gel process for preparing the nickel(II/III) oxide (NiO x ) hole-transporting material (HTM) in inverted methylammonium lead(II) triiodide (MAPbI 3 )-based perovskite solar cells (PVSCs). Considering both the electronic property and the homogeneity of NiO x thin films, a 1:0.8 molar ratio of Na(acac) 2 and DEA in precursor solutions has been found to be optimal for the power conversion efficiency (PCE) up to 19.54% without any dopant in NiO x or the interlayer between NiO x and MAPbI 3 . Three cyano group-containing photovoltaic polymers, benzodithiophenylenevinylenethienothiophenylenevinylene-based pBαCN, diketopyrrolopyrrolequaterthiophene-based P4TDPPCN, and isoindigoquaterthiophene-based P4TICN, were chosen as the interlayer to further improve the PCE of NiO x -optimized PVSCs. The polymer interlayer-containing PVSCs exhibit a PCE as high as 20.09 and 21.43% (with a device active area of 0.04 cm 2 ) for P4TDPPCN and P4TICN, respectively. We have experimentally verified that charge transport resistance ( R ct ), charge recombination resistance ( R rec ), and trap density ( n trap ) are all reduced with the polymer interlayer in PVSCs. The contact of the MAPbI 3 perovskite to NiO x becomes better with a polymer interlayer in between. All of these enable the FF of PVSCs approaching 80%. Interestingly, the photovoltaic polymers provide additional photocurrent for the MAPbI 3 PVSCs with a high short-circuit current density ( J SC ) over 24 mA/cm 2 . While MAPbI 3 failed to deposit on the pBαCN polymer interlayer due to a serious nonwetting issue, it was relatively problem-free for P4TDPPCN and P4TICN . In this report, we provide chemical insight to explain the successful fabrication of the perovskite material on top of the P4TDPPCN or P4TICN polymer interlayer.