Effects of Aging and Annealing on the Density of Trap States in Organic Photovoltaic Materials

The concentration of charge carriers in bulk heterojunction solar cells is limited by recombination processes whose rates may be affected by trapping in deep localized states within the band gap. Trapping reduces carriers mobility and causes dispersive charge transport with negative impact on device performance. Aging and thermal annealing may cause variations of the trap state distribution in the photoactive materials. Variable-temperature light-induced electron spin resonance (LESR) measurements are one method for revealing the distribution of deep trap states (DOS). The LESR signal intensity is in fact proportional to the concentration of trapped charges that survive recombination long enough for ESR detection at the chosen experimental temperature. We used LESR to study the effect of aging and thermal annealing on drop-casted blend films composed of regioregular poly(3-dodecylthiophene-2,5-diyl) (rrP3DDT) as charge donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor materials. The trapped carriers concentration reduced slightly after aging and strongly after annealing; lower recombination rates after aging tended to be restored after annealing. We ascribe the variations in charge concentration to two effects: (1) a decrease in charge generation efficiency due to blend demixing causing less exciton splitting and charge separation; and (2) the presence of oxygen contaminant acting as additional trap states. The experimental DOS can be modeled by Gaussian curves whose parameters (average and standard deviation) are influenced by aging and annealing. Aging shifts the average binding energy to higher values and increases Gaussian width (i.e., energetic disorder). Annealing tends to restore the average energy to the original value, while curve width is invariant or increases. We conclude that the effects of aging and annealing on the DOS are due to the presence of reversible oxygen contamination and to the changes in the blend film microscopic morphology. Both are favorably modified by thermal annealing, although some (apparently irreversible) features of the energetic disorder deserve further investigation.