Correlation between the temperature dependences of short-circuit current and carrier mobility in P3HT:PCBM blend solar cells

Thermal annealing has been widely used to improve device performances of organic solar cells with regioregular (RR) poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction blends. Especially, short-circuit current density (Jsc) of the thermally-annealed device is significantly increased compared to that of the non-annealed one. The Jsc is proportional to the product of the carrier mobility and the number of photogenerated carriers which depends on the photocarrier generation efficiency and carrier recombination lifetime. Therefore, the enhanced Jsc implies that the thermal annealing can increase either the mobility and/or lifetime of the photogenerated carriers. In order to understand which parameter is more affected by thermal annealing, we compared the temperature dependence of the Jsc and carrier mobility of P3HT:PCBM (1:1, weight%) blend solar cells. The carrier mobility, measured from a time-of-flight photoconductivity (TOF-PC) measurement, increases from about 10-5 cm2/Vs to the order of 10-4 cm2/Vs as the temperature increases from 300 K to 360 K and then saturates above 360 K up to 400 K. This behavior is very similar to the temperature dependence of the current density of the P3HT:PCBM solar cell devices with the same blend ratio. Therefore, this correlation indicates that the thermal annealing increases the carrier mobility by improving morphological order of the blend film and thereby enhances the Jsc of the P3HT:PCBM blend solar cells.