Effect of residual catalyst on solar cells made of a fluorene-thiophene-benzothiadiazole copolymer as electron-donor: A combined electrical and photophysical study

The effects of residual catalyst impurities (palladium) on the hole mobility of a fluorene-thiophene-benzothiadiazole copolymer (poly{4′-(9,9-bis(2-ethylhexyl)fluoren-2-yl)-2′,1′,3′-benzothiadiazole-7,7′-diyl]-co-2′-(9,9-bis(2-ethylhexyl)fluoren-2-yl)thien-7,5′-diyl]}) (PFB-co-FT), as well as on its photovoltaic and photophysical response when blended with 6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), are investigated. Two samples of the copolymer, only differing for the Pd content (9 and 3360 ppm), are considered and compared. The transport of positive carriers is characterized by a lower mobility and a higher dispersion in the Pd-rich PFB-co-FT sample. The photovoltaic parameters of PFB-co-FT:PCBM solar cells show a significant dependence on the residual catalyst impurities, attributed to a different concentration of trap states. Variations in charge mobility and trapping induced by impurities was confirmed also by ESR (Electron Spin Resonance) experiments: an increased concentration of trapped charges in the presence of a higher level of metal impurities was revealed by light induced ESR, while the variation of polaron mobility correlates with the lifetime variation of the photogenerated PCBM triplet state detected by time-resolved ESR. All experimental evidences point to a strong effect of Pd impurities on the transport properties of charge carriers.