Surface recombination influence on photocurrent spectra of organic photovoltaic devices

The photocurrent spectra (PCS)of the ITO (indium tin oxide)/PEDOT:PSS(poly(3,4-ethylenedioxythiophene)- oly( styrenesulfonate))/ P3HT: PCBM( poly( 3- hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester)/Al organic photovoltaic devices(OPVs) with six different thicknesses of P3HT:PCBM layer, ranging from 80 to 345 nm,were measured under monochromatic visible light for the reverse voltage range from -4 to 0V.The measured PCS showed a good correlation with the P3HT:PCBM absorption spectrum for thinner devices, while for thicker OPVs a pronounced red shift in PCS was observed. As this PCS behavior is usually explained by the surface recombination effects at electrode contacts, a drift-diffusion model (DDM), which includes charge carrier surface recombination and thermal injection on the anode and cathode through boundary conditions, was used for the PSC simulations. The experimentally obtained normalized PCS were very well reproduced by the DDM when absorption and charge carrier photogeneration in P3HT:PCBM thin films were assumed to follow the Beer-Lambert law, and surface recombination at electrode contacts was negligible. Further, the effect of surface recombination on normalized PCS was analyzed in detail by reducing the surface recombination velocity (SRV) in the DDM calculations for either majority or minority charge carriers at one of the electrodes. Also, the DDM calculations were conducted for the cases of electron dominated, balanced, and hole dominated transport in the OPVs active layer. It was concluded that only the reduction of SRV for majority carriers at the electrode which extracts faster carriers affects the normalized PCS. For balanced hole and electron transport the surface recombination had no influence on the normalized PCS.

Automated summary

The effect of different thicknesses of the P3HT:PCBM layer on the photocurrent spectra (PCS) of ITO/PEDOT:PSS/P3HT:PCBM/Al organic photovoltaic devices (OPVs) was studied. It was found that for thinner devices, the PCS showed a good correlation with the P3HT:PCBM absorption spectrum, while a pronounced red shift was observed for thicker OPVs. The drift-diffusion model (DDM) simulations showed that reducing the surface recombination velocity (SRV) of majority carriers at the electrode affected the normalized PCS, while surface recombination had no influence on the normalized PCS for balanced hole and electron transport.