Improving the conductivity of PEDOT:PSS hole transport layer in polymer solar cells via copper(II) bromide salt doping.

Copper(II) bromide (CuBr2) salt has been applied to dope poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole transport layer (HTL) in polymer solar cells (PSCs), improving dramatically the conductivity of PEDOT:PSS film and consequently the device power conversion efficiency (PCE). Under the optimized doping concentration of CuBr2 of 10 mmol·L(-1), PCE of the CuBr2:PEDOT:PSS HTL-incorporated BHJ-PSC device based on poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5- (4',7'-di-2-thienyl-2',1',3'- benzothiadiazole) (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) reaches 7.05%, which is improved by ∼20.7% compared to that of the reference device based on pristine PEDOT:PSS HTL (5.84%) and represents the highest PCE for PCDTBT:PC71BM-based PSC devices without an electron transport layer (ETL) reported so far. The dramatic improvement of the conductivity of PEDOT:PSS film is interpreted by the weakening of the Coulombic attractions between PEDOT and PSS components. The work function of CuBr2:PEDOT:PSS slightly increases compared to that of the undoped PEDOT:PSS as inferred from scanning Kelvin probe microscopy (SKPM) measurements, contributing to the improved PCE due to the increases of the open-current voltage (Voc) and fill factor (FF).