Combined experimental and theoretical study of bulk-heterojunction organic photovoltaics

Organic photovoltaics (OPVs) are considered a promising energy source for the future, owing to interesting properties such as easy processing and flexibility. Their main drawback however, is the low efficiencies that can be obtained so far. 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. Due to their high electron affinity and ability to transport charge, fullerene derivatives are the most widespread type of acceptor. As a donor phase a conjugated, light-excitable polymer is required. In this study a poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61 - butyric acid methyl ester (PCBM) blend, the most widespread OPV system, 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, and post-production annealing plays an important role in optimising these two factors.
In this work, post production annealing is therefore studied using several fast-scanning calorimetry techniques, such as Rapid Heat-Cool Calorimetry (RHC) and Fast Scanning Differential Chip Calorimetry (FSDCC. By applying the Avrami model on the isothermal crystallisation of a P3HT/PCBM 1:1 blend, it is found that the crystallisation rate goes through 2 maxima, making it possible to improve the annealing procedure.
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).