Abstract
The secret to the success of mixed bulk heterojunctions (BHJs) in yielding highly efficient organic solar cells (OSCs) could reside in the molecular structures at their donor/acceptor (D/A) interfaces. In this study, we aimed to determine the effects of energy and charge cascade structures at the interfaces by using well-defined planar heterojunctions (PHJs) as a model system. The results showed that (1) the charge cascade structure enhanced VOC because it shuts down the recombination pathway through charge transfer (CT) state with a low energy, (2) the charge cascade layer having a wider energy gap than the bulk material decreased JSC because the diffusion of the excitons from the bulk to D/A interface was blocked; the energy of the cascade layers must be appropriately arranged for both the charges and the excitons, and (3) molecular intermixing in the cascade layer opened the recombination path through the low-energy CT state and decreased VOC. Based on these findings, we propose improved structures for D/A interfaces in BHJs.
Highlights
Recent years have seen the development of solution-processed organic solar cells (OSCs) based on mixed bulk-heterojunction (BHJ) structures that have yielded power conversion efficiency of over 10% with high external quantum efficiency (EQE) values of over 80%1–3
A phenyl C61 butyric acid methyl ester (PCBM) acceptor was spin-coated onto an electrode/electron transport layer, and the interlayer and P3HT films were successively transferred onto the PCBM layer by contact film transfer (CFT)
The device structure was ITO/ZnO/PCBM (28 nm)// interlayer//P3HT (47 nm)/MoOx (7.5 nm)/Ag (70 nm), where //denotes the interface created by the CFT method
Summary
Recent years have seen the development of solution-processed organic solar cells (OSCs) based on mixed bulk-heterojunction (BHJ) structures that have yielded power conversion efficiency of over 10% with high external quantum efficiency (EQE) values of over 80%1–3. To understand how the very high photocurrent generation observed in the mixed BHJ has been obtained, one must consider the efficient flow of the photogenerated excitons to the charge separation interfaces between the domains[8,9,10] Well-defined energy and charge cascade structures at the donor/acceptor interfaces have been constructed in planar heterojunctions (PHJs)[11,12,13,14,15,16] These studies have been motivated by interest in either using PHJs as models of the interfaces in mixed BHJs, or pursuing the high efficiency of PHJ-type OSCs. It has been reported that the energy cascade toward the charge-generation interface helps the interface collect more excitons[10]. The functions of the intermixed layers at the D/A interfaces are discussed, and the discussion is extended to intermixed layers in the mixed BHJ
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