Understanding the photoexcited carrier dynamics in organic photovoltaic (OPV) cells helps us to improve their power conversion efficiency (η) for practical use because some breakthroughs in the recent decade have been based on bulk-heterojunctions (BHJs) between donor (D) and acceptor (A) materials. The BHJ plays a role of increasing the D-A interface area involving photocarrier generation regions, and thus the number of photocarriers increases at the interfaces [1,2]. Although the BHJ can improve the η of OPV cells by up to ca. 8%, the improving rate of η does not significantly increase [3-5]. This is presumably because there are still major unclear points for BHJ as follows: (1) the nanostructure of the BHJ schematic illustration has been shown) and (2) the reproducibility of BHJ fabrication. Figure 6.1 schematically illustrates typical structures of (a) a D-A hetero doublelayered OPV cell and (b) a BHJ-OPV cell. As shown in Figure 6.1b, because the BHJ is fabricated by coevaporation of donor and acceptor materials or by spin-coating conductive polymers, the D-A interface thus formed is too complex to analyze and reproduce the nanostructure compared to that of a double-layered heterojunction [2,6]. These facts make it difficult to discuss the photoexcited carrier dynamics at the BHJ D-A interface. Accordingly, the reason why the BHJ improves the η has been unclear so far.
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