Molecular doping of low-bandgap-polymer:fullerene solar cells: Effects on transport and solar cells

We show how molecular doping can be implemented to improve the performance of solution processed bulk heterojunction solar cells based on a low-bandgap polymer mixed with a fullerene derivative. The molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into blends of poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) via co-solution in a range of concentrations from 0% to 1%. We demonstrate that the hole conductivity and mobility increase with doping concentration using field-effect measurements. Photoinduced absorption (PIA) spectroscopy reveals that the polaron density in the blends increases with doping. We show that the open circuit voltage and short circuit current of the corresponding solar cells can be improved by doping at 0.5%, resulting in improved power conversion efficiencies. The increase in performance is discussed in terms of trap filling due to the increased carrier density, and reduced recombination correlated to the improvement in mobility.

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► We use molecular doping to improve the efficiency in polymer:fullerene solar cells.
► We p-dope PCPDTBT and PCPDTBT:PCBM blends at concentrations between 0% and 1% with F4-TCNQ.
► Molecular doping with F4-TCNQ improves the hole conductivity, mobility and polaron densities in PCPDTBT.
► JV characteristics show improved photocurrent and efficiency.
► We demonstrate molecular doping is a simple way to improve device performance.