Uncovering the Out-of-plane Nanomorphology of Organic Photovoltaic Bulk Heterojunction by GTSAXS

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Organic photovoltaics (OPVs) have gained much attention due to their potential to offer low-cost, high-performance and flexible devices. To cope with the intrinsic strong exciton binding energy and short carrier diffusion length of organic semiconductors, OPVs usually employ the bulk heterojunction (BHJ) device structure, in which organic p-type (donor) and n-type (acceptor) semiconducting materials are mixed in the active layer to create rich donor/acceptor (D/A) interfaces for exciton dissociation.

The record power conversion efficiency of OPV has been reaching over 19% with the morphology optimization of BHJ active layer. However, the quantitative determination of out-of-plane nanoscale phase structure of BHJ is still under great challenge. The thin film device structure breaks the symmetry into the in-plane (IP) direction and out-of-plane (OOP) direction with respect to the substrate, leading to an intrinsic anisotropy in the bulk morphology. Due to the charge transport in active layer is mainly along OOP direction, the quantitative characterization of OOP phase morphology should be of great importance to the novel materials development and further optimization of OPV efficiency.

Fig. 1 Summary of this work. a The experimental geometry comparison of GTSAXS, GISAXS and RSoXS. b The illustration of the capability and the approximation of each technique to quantitatively extract the in-plane and out-of-plane nanomorphology. c GTSAXS 2D pattern and corresponding OOP linecuts of thin film based on P3HT: PC71BM.

In this work, we utilized an X-ray scattering technique, Grazing-incident Transmission Small-angle X-ray Scattering (GTSAXS), to uncover this new morphology dimension. The measurement method of GTSAXS on OPV BHJ thin film was formalized systematically. This technique was firstly implemented on the model systems based on fullerene derivative (P3HT:PC71BM) and non-fullerene systems (PBDB-T:ITIC, PM6:Y6), which demonstrated the successful extraction of the quantitative out-of-plane acceptor domain size of OSC systems.

The detected in-plane and out-of-plane domain sizes show strong correlations with the device performance, particularly in terms of exciton dissociation and charge transfer. Proper domain size along OOP direction can simultaneously enhance the Jsc and FF of OPV devices.

Overall, we have successfully applied GTSAXS to determine the OOP nanomorphology of OPV BHJ thin film for the first time, which should be able to provide a new avenue for the future novel photovoltaic materials development and OPV device optimization.

For more information, please refer to our recent publication in Nature Communications, “Uncovering the Out-of-plane Nanomorphology of Organic Photovoltaic Bulk Heterojunction by GTSAXS” Nat. Commun., 2021, 12, 6226.

Xinxin Xia

PhD candidate, The Chinese University of Hong Kong