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Synergistic effect of polymer and small molecules for high-performance ternary organic solar cells
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Significant progress has been achieved in the production of bulk heterojunction organic solar cells (OSCs) based on binary active layer composed of donor-acceptor (D-A)-type polymers or small molecules as donors and PCBM as acceptor. D-A-type polymer exhibits several advantages, such as good film-formation property and high absorption capability. However, the crystallinity of D-A polymers is generally not very good enough to obtain high transporting ability. Therefore, in the polymer solar cells, for improving the device performance, it is need to blend small amount of diiodooctane (DIO), chloronaphthalene (CN) or other additives to tune the active layer’s crystallinity and morphology. Compared to polymers, small molecules have several advantages such a well-defined structures, high intrinsic carrier mobility, no batch-to-batch variation, and ease of synthesis, purification, and modification. Generally, small molecules have higher mobility and more prone to long range-order than polymers, which are always getting high fill factors in solar cells. The problem is the phase separation of small molecular film with PCBM is big and induced negative influence on the device performance. In order to tune the crystallinity, Chen Y et al reported blending a small amount of polydimethylsiloxane (PDMS) as additive or plasticizer, achieving a better phase separation and a higher performance. However, from DIO to PDMS, the most additives reported are inert in the film, which will become traps for charge separation and transport when blending slightly overdose of them. This sensitivity and disadvantage seriously limit the blending method in organic solar cells application, especially for large area or industrial fabricated procedure.

As an approach for further increasing the power conversion efficiency (PCE) compared binary solar cells with inert additives, ternary concept is a similar and most potential way to increase the absorption breadth of solar cell using multiple donor components with different (ideally complementary) absorption features. Ternary blends based on two donor components and one acceptor component (or one donor and two acceptors) provide a potentially effective route to pursuing of high short-circuit current density (Jsc) and consequently high efficiencies exceeding the theoretical limits for binary blend solar cells could be possible without sacrificing the simplicity of a single active-layer processing step.


In this study, a new ternary OSC is fabricated, which contains a D-A-type polymer and a novel small molecule.The obtained optimal device exhibits high efficiency (8.40 %), with Voc of 0.97V, Jsc of 12.17mAcm-2, and FF of 71.23%. The D-A polymer containing benzo[1,2-b:4,5-b’]dithiopheneand thieno[3,4-c]pyrrole-4,6-dione groups (named as PBDTTPD-HT) is selected because its Voc is as high as 1.0V, and a novel small molecule is designed and synthesized with high crystallinity in the ternary system. A synergistic effect of polymer and small molecules is observed for the first time.The small molecules increase the crystallinity of the donor phase, where as the polymers modify the D-A interface. The PCE of the ternary OSCs (8.40 %) is higher than that of binary systems based on small molecules (7.48 %) or polymers (6.85 %). The results suggest that appropriate design of the developed ternary system could offer multiple benefits and synergistic effects such as modified film morphology, improved charge carrier mobility, and promoted exciton dissociation at the D-A interface. The GIWAXS  experiments were carried out at 1W2A-SAXS station of BSRF.


Yajie Zhang, Dan Deng, Kun Lu*, Jianqi Zhang, Benzheng XiaYifan Zhao, Jin Fang, Zhixiang Wei*, Synergistic Effect of Polymer and Small Molecules for High-Performance Ternary Organic Solar Cells, Advance Materials, 2015, 27, 1071-1076.

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