The Role of Oxygen Exposure on the Performance of All-Vapor-Processed Perovskite Solar Cells With CuPC Hole Transport Layers

Abstract

Methylammonium lead iodide (MAPbI ₃ ) perovskite solar cells were made using an all-vapor process, including two-step close space vapor transport-processed MAPbI ₃ absorber and evaporated copper phthalocyanine (CuPC) hole transport layer (HTL). N-i-p solar cells fabricated entirely in a nitrogen glovebox had poor performance due to s-shaped J-V curves and fill factors (FF) $< $ 45%. Solar cells exposed to dry air in a desiccator for seven days, or to O ₂ flowed into the evaporator during CuPC deposition, had significantly improved performance with reduced or eliminated s-shaped behavior and improved FF up to 72%. Co-planar conductivity measurements show that exposure to dry air, deposition with oxygen, and MoO _x capping layers all increase the conductivity of the CuPC HTL. Drift-diffusion simulations show that increasing hole concentration consistent with oxygen doping effects can explain the J-V behavior of the solar cell. Solar cells using spiro-OMeTAD HTLs achieved similar Power Conversion Efficiency but higher V _oc up to 1.01 V. Drift-diffusion simulations show that the V _oc difference can be explained by differences in doping density and valence band position between spiro and CuPC.