| Summary: | 碩士 === 國立清華大學 === 化學工程學系 === 104 === Use of composite active layers comprising a novel conjugated polymer PTB7-Th and modified C70 (PC71BM) has been shown to give significantly improved power conversion efficiency (PCE) of organic solar cells, but the corresponding reason behind this improvement remains unclear. Here we utilize the inherent contrast difference in X-ray and neutron reflectivity, along with the measurement of simultaneous grazing incident small/wide-angle X-ray scattering and atomic force microscopy to reveal structural characteristics of blend films spin-cast from chlorobenzene solutions containing different levels of 1,8-di-iodooctane (DIO). Films spin-cast from non-DIO solutions have a surface layer enriched in PTB7-Th while there are voids (translating to ca. 10% porosity) near the substrate interface. Under the polymer-rich skin, there are large PC71BM-rich oblate nanodomains (with equatorial diameter 2B ≈ 160 nm and polar diameter 2A ≈ 18 nm) among smaller nanodomains (with 2B ≈ 23 nm and 2A ≈ 5 nm). Minor presence of DIO (up to 3%) in the spin solution not only eliminates the polymer-rich skin layer and the porosity near the substrate interface, but also suppresses the formation of PC71BM-rich large nanodomains in favor of fractal aggregation (with fractal dimension Df ≈ 2.2) of the small PC71BM-rich ellipsoids. The relative intensity of the (100) reflection of PTB7-Th crystallites also increased via DIO-modification. This improved homogeneity in the active-layer structure results in enhanced power-conversion efficiency for polymer solar cells of conventional design.
In inverted-type solar cells, ZnO serves as electron transport layer, but with porosity. Due to these pores, the PTB7-Th and PC71BM may penetrate into these substrates during the coating process of active layer. We have therefore adopted anomalous scattering to solve for the 4-component composition profile in the ZnO substrate layer. Based on this new procedure, PC71BM was found to penetrate deeper into the ZnO substrate than PTB7-Th. Combining the results above, we attribute the higher PCE of inverted-type solar cells to the presence of a PTB7-Th-rich surface layer for better hole transport and the penetration of PC71BM into the ZnO substrate for enhanced electron transport.
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