Charge transport dynamics of polymer solar cells under operating conditions: Influence of trap filling

The authors report time-of-flight measurements on photovoltaic blends of poly(3-hexylthiophene) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2('),2(')-diyl) (F8TBT). Through the use of an additional illumination source to provide a background current density, the influence of the charge density and trap population on hole and electron dynamics is studied. For holes, little change in the transit time is observed with this background illumination, whereas for electrons, a clear electron transient is observed with background illumination that is not observed without. Under solar illumination the electron trap states are sufficiently populated such that charge transport in all-polymer blends may not be as unbalanced as previously thought.