Bulk-heterojunction organic photovoltaics - A combined experimental and theoretical study -

The main drawback of the organic photovoltaic systems (OPVs) developed to date, is the low efficiencies obtained [1]. So far the highest OPV efficiencies have been found for so-called bulk-heterojunction (BHJ) solar cells, where the active layer is a bicontinuous composite of donor and acceptor phases. A conjugated, light-excitable polymer is used as an electron donor. Fullerene derivatives are the most widespread type of electron acceptor, due to their high electron affinity and ability to transport charge. In this study a poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61 - butyric acid methyl ester (PCBM) blend, one of the most widespread OPV systems, is investigated by both experimental and theoretical means.

An important technological challenge for BHJ OPV's is the morphology of the active layer. Due to the limited lifetime of generated excitons, a nm-size phase separated morphology is required for efficient charge generation at the interface between donor and acceptor. A more crystalline material will also lead to more efficient charge transfer. Because the active layer is essentially a blend material, and the ideal morphology is not the thermodynamic equilibrium state, post-production annealing plays an important role in optimising these systems [2-3].
In this work, post production annealing is therefore studied using several fast-scanning calorimetry techniques, such as Rapid Heat-Cool Calorimetry (RHC) [4] and Fast-Scanning Differential Chip Calorimetry (FSDCC) [5]. These techniques make it possible study the effects of annealing for short times. Surface characterisation is also carried out using conducting atomic force microscopy (C-AFM). Next to the morphology of the active layer, the mechanism of charge transfer and exciton dissociation at the donor/acceptor interface is just as important.
This charge transfer is modelled using ab initio DFT calculations under periodic boundary conditions (PBC). A possible bridge state, allowing charge transfer, was investigated for the excited triplet state of the combined donor/acceptor system. A similar bridge-state was described earlier in literature [6]. Calculations for the excited singlet state, requiring time-dependent techniques, are underway.

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6. Kanaï J., Grossman J.C., Nano Letters, 7, 1967-1972 (2007)