Investigation of Metal and Metal Oxide Intermediate Layers in Organic Multi-Junction Solar Cells

Organic bulk-heterojunction (BHJ) solar cells comprising a blend of electron-donors (conjugated polymers) and electron-acceptors (fullerene derivatives) are promising alternative, renewable energy sources offering the possibility for low-cost, solution-based processing. To enhance solar cell performance and power conversion efficiencies, transmission and thermalization losses can be reduced by stacking sub-cells with complementary absorption behavior to obtain multi-junction solar cells. In this work, blends of various small and wide bandgap polymers (PCPDTBT, PSiF-DBT, PCDTBT, F8T2) and fullerene derivatives ([60]PCBM, [70]PCBM, ICBA) were evaluated with regard to their performance in organic PEDOT:PSS-free BHJ solar cells comprising a molybdenum(VI) oxide hole-transport layer and copper electrodes modified with a titanium interfacial layer. The focus was on thermally deposited metal (Al, Ag, Au, Cu, Ti) and metal oxide (TiO_x, ZnO, MoO₃) intermediate layers (IML) for organic multi-junction solar cells. In addition, recombination layers were modified via metal (Ca, Ti) and alkali-metal compound (LiF, Cs₂CO₃) interfacial layers to investigate the effect on the photovoltaic performance.