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We studied by means of electron spin resonance (EPR) techniques the photoinduced processes in a poly[(9,9-bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]-thiadiazol-4,8-diyl) (F8BT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C-61 (PCBM) blend. Using light-induced EPR (LEPR), we observed the generation of charge-separated states in the blend upon visible light excitation, and we detected the formation of the PCBM excited triplet state by using time-resolved EPR (TREPR) with microsecond time resolution. From spectral simulation of the TREPR spectrum we were able to identify a double pathway for the PCBM triplet generation: the normal intersystem crossing from PCBM excited singlet state (either directly produced by light absorption or populated by singlet-singlet energy transfer from the polymer) and a generation pathway via polaron pairs recombination. The experimental evidence of this process allows to assign the relative order of the energies of the polaron pairs and the triplet states and provides a more complete description of the photophysical processes taking place in this photoactive blen

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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.

Improving the dispersion properties of unfunctionalized fullerenes in photovoltaic blends is a key factor for replacing the commonly employed butyric acid methyl ester derivatives (PBCM and PC 71 BM), with economical and light harvesting advantages. We consider here the effects of a pyrene derivative (PyBB) as dispersant of neat fullerenes (C 60 or C 70 ) in photovoltaic blends prepared with a low energy gap conjugated polymer (PTB7) as electron donor. The morphological and spectroscopic properties in the presence and absence of PyBB were evaluated by AFM, emission fluorescence, and light induced EPR, and the deep trap density of states was calculated in the two cases. The electrical properties of the devices prepared with or without PyBB were investigated and compared. The intimate interaction of fullerene/PyBB aromatic species revealed by the spectroscopic analysis is in agreement with the enhanced dispersion of the three-component blends. At the same time the deep trap density of states is varied, and the mobility of negative charge carriers is reduced in films prepared with PyBB dispersant, frustrating the beneficial effect of PyBB on the blend morphology.