Traps in Regioregular Poly(3-hexylthiophene) and its Blend with [6,6]-Phenyl-C₆₁-Butyric Acid Methyl Ester for Polymer Solar Cells

Thermally stimulated current (TSC) technique is used to characterize traps in the regioregular poly(3-hexylthiophene) (rr-P3HT) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A hole trap in bulk rr-P3HT and an interfacial hole trap located at indium tin oxide (ITO)/rr-P3HT interface are revealed from the TSC measurement. Besides, molecular oxygen (O2) can form a deep electron trap with an onset of detrapping temperature at 225 K in rr-P3HT, in which O2 is located at the main chain region and the detrapping process is induced by chain motions under elevated temperature. In the blend of rr-P3HT:PCBM (1:1 w/w), additional hole trap states are generated in this blend system as compared to those of pure rr-P3HT and PCBM; however, these hole trap states can be reduced by thermal or solvent annealing approaches. Similar to rr-P3HT, a deep electron trap with an onset of detrapping temperature at 250 K can be formed in the blend after O2 exposure. In the case of low PCBM content in the blend (rr-P3HT:PCBM weight ratio of 4:1), an additional electron trap is generated.