Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
Increasing molecular weight (Mw) of conjugated polymers as electron donor has been reported to improve solar cell performance in a number of polymer: PCBM bulk heterojunction systems, the reason of which has been attributed to the change in film morphology as a result of changing molecular weight. However, further study correlating such morphological change with photo-physics of operating devices, more specifically the bimolecular recombination kinetics of photo-generated charge carriers, is scarce. Here, bimolecular recombination in a low bandgap polymer DT-PDPP2T-TT: PCBM bulk heterojunction solar cell is found reduced (Langevin prefactor decreasing from 0.27 to 0.07) when the polymer Mw is increased from 33kDa to 80kDa. The bimolecular recombination in the high and low Mw DT-PDPP2T-TT: PCBM devices were compared using charge extraction using a nanosecond switch, combined with charge extraction by linearly increasing voltage and bulk generation time-of-flight techniques. The study revealed a high charge carrier mobility of 2.39×10-3 cm2V-1s-1 in high Mw device, which is over an order of magnitude higher than that in the low Mw device. The bimolecular recombination coefficient normalized to mobility is four times lower in the high Mw device compared to the low Mw device. The reduced bimolecular recombination, along with the fast charge carrier transport due to the high mobility, explains the high performance achieved in the high Mw device with active layer thickness of 250 nm. Such molecular weight dependence of reduced-bimolecular recombination could be explained by the presence of energetic barrier between the amorphous and crystalline phase of the polymer: PCBM blend, which could be tuned by increasing the molecular weight of the donor polymer. Grazing-incident wide-angle X-ray scattering (GIWAXS) and transient absorption spectroscopy will be performed to examine the hypothesis.
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International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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