Charge Transport and Recombination in Low-Bandgap Bulk Heterojunction Solar Cell using Bis-adduct Fullerene

Charge transport and recombination are studied for organic solar cells fabricated using blends of polymer poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (Si-PCPDTBT) with [6,6]-phenyl-C61-butyric acid methyl ester (mono-PCBM) and the bis-adduct analogue of mono-PCBM (bis-PCBM). The photocurrent of Si-PCPDTBT:bis-PCBM devices shows a strong square root dependence on the effective applied voltage. From the relationship between the photocurrent and the light intensity, we found that the square-root dependence of the photocurrent is governed by the mobility-lifetime (mu t) product of charge carriers while space-charge field effects are insignificant. The fill factor (FF) and short circuit current density (Jsc) of bis-PCBM solar cells show a considerable increase with temperature as compared to mono-PCBM solar cells. SCLC analysis of single carrier devices proofs that the mobility of both electrons and holes is significantly lowered when replacing mono-PCBM with bis-PCBM. The increased recombination in Si-PCPDTBT:bis-PCBM solar cells is therefore attributed to the low carrier mobilities, as the transient photovoltage measurements show that the carrier lifetime of devices are not significantly altered by using bis-PCBM instead of mono-PCBM.