Among π-conjugated polymer donors for efficient bulk-heterojunction (BHJ) solar cell applications, poly(benzo[1,2-b:4,5-b′]dithiophene- thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers yield some of the highest open-circuit voltages (VOC, ca. 0.9 V) and fill-factors (FF, ca. 70%) in conventional (single-cell) BHJ devices with PCBM acceptors. In PBDTTPD, side chains of varying size and branching affect polymer self-assembly, nanostructural order, and impact material performance. However, the role of the polymer side-chain pattern in the intimate mixing between polymer donors and PCBM acceptors, and on the development of the BHJ morphology is in general less understood. In this contribution, we show that ring substituents such as furan (F), thiophene (T) and selenophene (S)-incorporated into the side chains of PBDTTPD polymers-can induce significant and, of importance, very different morphological effects in BHJs with PCBM. A combination of experimental and theoretical (via density functional theory) characterizations sheds light on how varying the heteroatom of the ring substituents impacts (i) the preferred side-chain configurations and (ii) the ionization, electronic, and optical properties of the PBDTTPD polymers. In parallel, we find that the PBDT(X)TPD analogs (with X = F, T, or S) span a broad range of power conversion efficiencies (PCEs, 3-6.5%) in optimized devices with improved thin-film morphologies via the use of 1,8-diiodooctane (DIO), and discuss that persistent morphological impediments at the nanoscale can be at the origin of the spread in PCE across optimized PBDT(X)TPD-based devices. With their high VOC ∼1 V, PBDT(X)TPD polymers are promising candidates for use in the high-band gap cell of tandem solar cells. © 2014 American Chemical Society.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-C1-015-21
Acknowledgements: The authors acknowledge financial support under Baseline Research Funding from King Abdullah University of Science and Technology (KAUST). E.H., CR., J.L.B., and M.D.M.c.G. acknowledge financial support by the Center for Advanced Molecular Photovoltaics (CAMP) (Award KUS-C1-015-21) made possible by KAUST. The authors thank KAUST Analytical Core Laboratories for mass spectrometry, SEC measurements and elemental analyses, and Sandra Seywald (MPIP-Mainz, Germany) for additional SEC measurements. The authors thank the Advanced Imaging and Characterization Laboratories at KAUST for technical support. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource user facility, operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. P.K.S. is grateful to the University Grants Commission, New Delhi, India for a research fellowship.
ASJC Scopus subject areas
- Materials Chemistry
- Chemical Engineering(all)