This paper studied the molecular organization of aromatic organic semiconductors at a graphene-organic heterointerface. The heterointerfaces were designed to resolve the fundamental issues of planar heterojunction organic solar cells, such as the limited exciton diffusion length, optical absorption and interfacial energetics at organic-inorganic and organic-organic interfaces. The quasi-epitaxial growth of a p-type small-molecular organic semiconductors on graphene at anodic interface promoted the π orbital overlap in the direction of charge and exciton transport, as well as the new appearance of several high intensity vibronic optical transitions at transverse electromagnetic waves.
Sae Byeok Jo1, Hyun Ho Kim1, Hansol Lee1, Boseok Kang1, Seongkyu Lee1, Myungsun Sim1, Min Kim1, Wi Hyoung Lee2, Kilwon Cho*1Show Affiliations
- Department of Chemical Engineering,Pohang University of Science and Technology, Pohang 790-784,Korea
- Department of Organic and Nano System Engineering,Konkuk University, Seoul 143-701, Korea
Photon harvesting in organic solar cells is highly dependent on the anisotropic nature of the optoelectronic properties of photoactive materials. Here, we demonstrate an efficient approach to dramatically enhance photon harvesting in planar heterojunction solar cells by using a graphene–organic heterointerface. A large area, residue-free monolayer graphene is inserted at anode interface to serve as an atomically thin epitaxial template for growing highly orientated pentacene crystals with lying-down orientation. This anisotropic orientation enhances the overall optoelectronic properties, including light absorption, charge carrier lifetime, interfacial energetics, and especially the exciton diffusion length. Spectroscopic and crystallographic analysis reveal that the lying-down orientation persists until a thickness of 110 nm, which, along with increased exciton diffusion length up to nearly 100 nm, allows the device optimum thickness to be doubled to yield significantly enhanced light absorption within the photoactive layers. The resultant photovoltaic performance shows simultaneous increment in Voc, Jsc, and FF, and consequently a 5 times increment in the maximum power conversion efficiency than the equivalent devices without a graphene layer. The present findings indicate that controlling organic–graphene heterointerface could provide a design strategy of organic solar cell architecture for boosting photon harvesting.
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