FWO travel credit for a long stay abroad at the TU Drësden Duitsland (R-7573)

Conjugated polymers and small molecules, based on alternating electron-donating (D) and electron-accepting (A) building blocks have led to state-of-the-art organic solar cell materials, governing efficiencies beyond 10%. Unfortunately, the connection of D and A building blocks via cross-coupling reactions does not always proceed as planned, which can result in the generation of side products containing D-D or A-A homocoupling motifs. Previous studies have reported a reduced performance in polymer and small, molecule solar cells when such defect structures are present. A general consensus on the impact of homocouplings on device performance is, however, still lacking as is a profound understanding of the underlying causes of the device deterioration. The intimate mixing of two or more conjugated molecules, results in the formation of a so-called charge transfer (CT) state, which is a crucial intermediate step in terms of charge separation and has been directly linked to the open-circuit voltage VOC. By measuring the CT state energy (ECT) and corresponding reorganization energy (lamda), the effect of homocoupling motifs on the energetic CT environment is investigated by using temperature dependent sensitive external quantum efficiency measurements. Furthermore, the temperature dependence of lambda is investigated for vacuum deposited low donor content small molecule solar cells to verify whether lambda inherently limits the photovoltaic performance

Keywords:charge transfer state, organic solar cells, reorganization energy

Disciplines:Ceramic and glass materials, Materials science and engineering, Semiconductor materials