Role of Balanced Charge Carrier Transport in Low Band Gap polymer:fullerene Bulk Heterojunction Solar Cells

Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylenevinylene] and 6,6-phenyl C61-butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude.

Hybrid Polymer Solar Cells from Highly Reactive Diethylzinc: MDMO–PPV versus P3HT

The degradation of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene] (MDMO–PPV) during the processing of hybrid organic/inorganic bulk-heterojunction solar cells with zinc oxide (ZnO) from a molecular precursor as acceptor is reported. Upon addition of diethylzinc, the absorption spectrum of MDMO–PPV shifts to the blue, and hole transport through the polymer deteriorates dramatically, indicating a reduction of the conjugation length of the polymer backbone. To prevent polymer degradation through the breaking of trans vinyl bonds, regioregular poly(3-hexylthiophene) (P3HT) is introduced as the electron donor. This system of P3HT and precursor ZnO reveals an unchanged UV–vis absorption profile and zero-field hole mobility with respect to the pristine polymer as well as an improved photovoltaic performance with an estimated power conversion efficiency of 1.4% (AM1.5 global reference spectrum, 1 kW/m2).