Temperature Dependence and Impedance Characteristics of Hybrid Solar Cells Based on Poly(phenylene vinylene): ZnO Nanoparticles With Added Surfactants

A recent study has shown that the use of a semiconducting surfactant in hybrid bulk heterojunction solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxyl)-1,4-phenylene vinylene] and zinc oxide significantly increases their performance. To further study the mechanism underlying the improved performance resulting from the addition of a semiconducting surfactant, we compared the temperature and light intensity dependence of the current voltage characteristics and impedance spectra of devices that used no surfactant, an insulating oleic acid (OA) surfactant, or a semiconducting 2-(2-ethylhexyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinoline-6,7-dicarboxylic acid (BQ) surfactant. The temperature and light intensity dependence data enabled us to extract the activation energy for different devices, and we measured the junction resistance through impedance spectroscopy. The device that used BQ had significantly lower activation energy (9–12 meV) than that without surfactant or oleic acid (17–21 meV), thus indicating improved charge transport and consequently an increased short-circuit current. Furthermore, the impedance characteristics showed that the injection resistance, when the BQ surfactant was used, was lower than that when no surfactant or oleic acid was used. These experiments provide further evidence that the use of semiconducting surfactants is essential to improve the performance of hybrid bulk heterojunction solar cells.