The photovoltaic performance of organic− inorganic hybrid films based on regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and ZnO nanorods depends strongly on the quality of the interfaces between the organic polymer and the inorganic nanostructures. A thin, solutionprocessed TiO 2 interlayer was introduced on the surface of ZnO nanorods to improve the interface between ZnO and P3HT. In order to understand the effects of the TiO 2 interlayer on the initial photophysical processes after photoexcitation, femtosecond time-resolved fluorescence dynamics of the hybrid ZnO nanorods/P3HT (ZnO/P3HT) films with or without TiO 2 interlayer coating were comparatively studied using a fluorescence up-conversion technique. Because of the wellaligned P3HT nanodomains in these annealed hybrid films, energetically downhill excitation energy transfer was found to be the dominant process in the subps time scale with an excitation at 400 nm. The fluorescence decay times for the ZnO−TiO 2 /P3HT film were significantly extended, which is likely due to the improved interface quality with fewer traps and defects when TiO 2 interlayer was introduced. Moreover, ultrafast anisotropy decay and a dramatic decrease in residual anisotropy at the emission of 660 nm were observed for the ZnO−TiO 2 /P3HT film in comparison to both P3HT pristine film and the ZnO/P3HT film. This suggests a change in the disorder of energy, the local field, and an efficient energy-transfer process, resulting in a faster exciton reaction (dissociation) within the ZnO−TiO 2 /P3HT film. Solar cells fabricated using TiO 2 -coated ZnO nanorods and P3HT showed efficiencies of up to 0.76%, a 58% improvement over the TiO 2 -free equivalent devices.
ABSTRACT:The photovoltaic properties and exciton decay dynamics of three polyoxometalate (POM)-containing hybrid rod-coil diblock copolymers (HDCPs), PS-Mo6-PT1-3, are studied. Single-component photovoltaic cells of PS-Mo6-PT2 and inverted solar cells based on ZnO nanorod arrays/PS-Mo6-PT1-3 are fabricated showing power conversion efficiencies only up to 0.055%. To understand the poor photovoltaic performance, femtosecond fluorescence up-conversion technique is used to study the exciton decay dynamics of all three HDCPs. Drastically different fluorescence dynamics of the three HDCPs are observed in dilute solutions, which is attributed to the different extent and different type of interpolymer association depending on the P3HT rod block length and the cluster loading ratio. While both cation-mediated POM cluster association and P3HT-P3HT p-stacking contribute significantly to PS-Mo6-PT2 aggregation, the aggregation of PS-Mo6-PT1 and that of PS-Mo6-PT3 is driven predominantly by cluster association and p-stacking, respectively. In conjunction with the high residual polarization anisotropy, it is concluded that charge transfer from P3HT excitons to POM clusters in all three HDCPs is inefficient. An improved system with direct p-conjugation between the POM clusters and the rod block addressing this issue has been proposed.
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