Hybrid bulk heterojunction solar cells : blends of ZnO semiconducting nanoparticles and conjugated polymers

In hybrid polymer photovoltaics, conjugated polymers are combined with wide bandgap metal oxide semiconductors like TiO2 or ZnO. Reported max. power conversion efficiencies (PCE) at AM1.5G conditions for a hybrid polymer bulkheterojunction device are up to 1.6 %. In this paper we report on the current-voltage characteristics of bi-layer devices consisting of TiO2 and a conjugated polymer. Several polymers with different optical bandgap were studied. The max. External Quantum Efficiency (EQE) of the devices is comparable, but the PCE differs considerably (0.2-0.5%). The differences can for a large part be explained by the differences in optical bandgap of the polymers. It is shown that a low band gap is beneficial for the short circuit current, but does not automatically result in a high PCE as relative shifts of the HOMO (HOMO) energy levels of the polymers reduce the open circuit voltage (Voc). The calcns. show that a PCE up to 19 % can be achieved using the max. possible Voc and a fill factor of 80%. Judicious engineering of material combinations is required to achieve such a power output, and it expresses the need for a continuing search on potentially low cost, efficient metal oxide/polymer BHJ structures.