Combined experimental and theoretical study of bulk-heterojunction organic photovoltaics

Organic photovoltaics (OPVs) possess several interesting properties compared to conventional photovoltaics, such as easy processing and flexibility. The low efficiencies and limited lifetimes of such devices however prevent their commercial applications. Bulk-heterojunction (BHJ) solar cells, where the active layer is a bicontinuous composite of donor and acceptor phases, are a possible solution. As a donor phase a conjugated, light-excitable polymer is required, while the acceptor is most often a fullerene derivative. 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 from two different perspectives: the formed nanomorphology of the active layer, and the charge transfer on a molecular scale.

Due to the limited lifetime of generated excitons, the active layer morphology is of critical importance in BHJ OPV's. The active layer needs to have a phase-separated nm-size morphology in order to effectively charge separate the generated excitons at the donor/acceptor interface. 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 two fast-scanning calorimetry techniques, such as Rapid Heat-Cool Calorimetry (RHC) and Fast Scanning Differential Chip Calorimetry (FSDCC).
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 ground state DFT calculations under periodic boundary conditions (PBC) and, as excited states are involved, time-dependant DFT (TD-DFT) using the CAM-B3LYP functional to study the orbital overlaps of the states required for charge transfer.