Annealing- and doping-free hole transport material for p-i-n perovskite solar cells with efficiency achieving over 21%

Hole transport materials (HTMs) play an important role in perovskites solar cells (Pero-SCs) which can greatly affect the stability and power conversion efficiency (PCE). While most of the efficient HTMs need doping (which results in poor stability) or the treatment of thermal annealing (which increases the complexity of the device fabrication process). In this work, we synthesized a small molecule HTM DFBT-PMTP based on 3,3'difluoro-2,2' bithiophene and applied it as HTM without the need for doping and thermal annealing in p-i-n Pero-SCs with the structure of ITO/HTM/MAPbI(3-x)Cl(x)/C60/BCP/Ag. The PCE of the corresponding devices achieves a high value of 21.23% (19.8% for the PTAA-based reference ones), with an open-circuit voltage of 1.17 V and a high fill factor of 82.28%, which is to date among the highest values for the p-i-n MAPbI(3-x)Cl(x)-based Pero-SCs. Interestingly, the PCE of the devices declined to 18.65 and 15.5%, while the annealing temperature of DFBT-PMTP increased to 100 and 150 degrees C, respectively. And the reasons for this abnormal phenomenon have been investigated including the intrisinc properties, the energy levels of MAPbI(3-x)Cl(x) on top and the decices performance of the DFBT-PMTP films without annealing and annealed at 100 and 150 degrees C, respectively. The results revealed that the precise molecular structure design for the HTMs can make huge changes in the corresponding photovoltaic properties and fabrication process due to the effects on the top perovskites.