In this work, we compare the effect of incorporating selenophene versus thienothiophene spacers into pentacyclic lactam-based conjugated polymers for organic solar cells. The two cyclic lactam-based copolymers were obtained via a new synthetic method for the lactam moiety. Selenophene incorporation results in a broader and red-shifted optical absorption while retaining a deep highest occupied molecular orbital level, whereas thienothienophene incorporation results in a blue-shifted optical absorption. Additionally, grazing-incidence wide angle X-ray scattering data indicates edge- and face-on solid state order for the selenophene-based polymer as compared to the thienothiophene-based polymer, which orders predominantly edge-on with respect to the substrate. In polymer:PCBM bulk heterojunction solar cells both materials show a similar open-circuit voltage of ∼0.80-0.84 V, however the selenophene-based polymer displays a higher fill factor of ∼0.70 vs. ∼0.65. This is due to the partial face-on backbone orientation of the selenophene-based polymer, leading to a higher hole mobility, as confirmed by single-carrier diode measurements, and a concomitantly higher fill factor. Combined with improved spectral coverage of the selenophene-based polymer, as confirmed by quantum efficiency experiments, it offers a larger short-circuit current density of ∼12 mA cm. Despite the relatively low molecular weight of both materials, a very robust power conversion efficiency ∼7% is achieved for the selenophene-based polymer, while the thienothiophene-based polymer demonstrates only a moderate maximum PCE of ∼5.5%. Hence, the favorable effects of selenophene incorporation on the photovoltaic performance of pentacyclic lactam-based conjugated polymers are clearly demonstrated.
KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: We thank the Chalmers Areas of Advance Materials Science, Energy and Nanoscience and Nanotechnology as well as the Swedish Research Council, the Knut and Alice Wallenberg foundation through a Wallenberg Scholar grant to OI, Formas for funding. O.I., R.K. and M.R.A. further acknowledge the Swedish Energy Agency and R.K and M.R.A. the South Australian government for financial support. A.A. acknowledges SABIC for the Career Development SABIC Chair. Part of this work was done at the Cornell High Energy Synchrotron Source (CHESS), supported by the NSF & NIH/NIGMS via NSF award DMR-1332208.