Optimized molecular orientation and domain size enables efficient non-fullerene small-molecule organic solar cells
Chang He a
a Institute of Chemistry, Chinese Academy of Sciences (ICCAS), China
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Benidorm, Spain, 2018 May 28th - 31st
Organizers: Emilio Palomares and Rene Janssen
Oral, Chang He, presentation 024
DOI: https://doi.org/10.29363/nanoge.hopv.2018.024
Publication date: 21st February 2018

In bulk-heterojunction organic solar cells (BHJ-OSCs), exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in blend films. Efficient photovoltaic small molecules (SMs) typically possess an acceptor-donor-acceptor (A-D-A) structure that causes intrinsic anisotropy, limiting the control over molecular packing because of the lack of an effective method for modulating molecular orientation. Consequently, the performance of non-fullerene SM organic solar cells (NFSM-OSCs) is currently still lower than that of fullerene-based devices. In this report, we use a group of model compounds, named DRTB-T-CX (X=2, 4, 6 and 8), to demonstrate that adjusting the length of the end alkyl chain can be used to modify the molecular orientation. Through 2D grazing incidence wide-angle X-ray scattering (GIWAXS) characterization, we observed the transition of the molecular orientation in DRTB-T-CX films from edge-on to face-on. Meanwhile, the film comprising the compound with the preferred face-on orientation is found to have enhanced charge mobility and an increased correlation length of π-π stacking, leading to a substantial improvement in the efficiency of the NFSM-OSCs. A top-performance power conversion efficiency (PCE) of up to 11.24% is achieved with the DRTB-T-C4/IT-4F-based device, which is the best performance reported for a state-of-the-art NFSM-OSC. Remarkably, devices based on DRTB-T-C4/IT-4F with active layer thicknesses up to 300 nm can still retain a high PCE of 10% in single-junction solar cells.

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