3 research outputs found

    Dependence of Exciton Diffusion Length on Crystalline Order in Conjugated Polymers

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    Exciton diffusion in organic semiconductors is crucial to the performance of organic solar cells. Here, we measured the exciton diffusion length in poly­(3-hexylthiophene) (P3HT) as a function of the crystalline order using spectrally resolved photoluminescence quenching (SR-PLQ) techniques. The crystalline order in the P3HT films, characterized according to the mean crystal size and normalized crystallinity, was varied by changes in thermal treatment temperatures. The exciton diffusion length increased from 3 to 7 nm as the mean crystal size increased more than twice and the crystallinity increased by a factor of 6. A higher crystalline order improved the spectral overlap and reduced the distance between chromophores, enhancing Förster-mediated exciton diffusion. The higher crystalline order also lengthened the conjugated segments and reduced the energetic disorder, producing favorable condition for exciton hopping

    Boosting Photon Harvesting in Organic Solar Cells with Highly Oriented Molecular Crystals <i>via</i> Graphene–Organic Heterointerface

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    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 <i>V</i><sub>oc</sub>, <i>J</i><sub>sc</sub>, 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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