A series of alternating oligothiophene (nT)-isoindigo (I) copolymers (PnTI) were synthesized to investigate the influence of the oligothiophene block length on the photovoltaic (PV) properties of PnTI:PCBM bulk-heterojunction blends. Our study indicates that the number of thiophene rings (n) in the repeating unit alters both polymer crystallinity and polymer-fullerene interfacial energetics, which results in a decreasing open-circuit voltage (Voc) of the solar cells with increasing n. The short-circuit current density (Jsc) of P1TI:PCBM devices is limited by the absence of a significant driving force for electron transfer. Instead, blends based on P5TI and P6TI feature large polymer domains, which limit charge generation and thus Jsc. The best PV performance with a power conversion efficiency of up to 6.9% was achieved with devices based on P3TI, where a combination of a favorable morphology and an optimal interfacial energy level offset ensures efficient exciton separation and charge generation. The structure-property relationship demonstrated in this work would be a valuable guideline for the design of high performance polymers with small energy losses during the charge generation process, allowing for the fabrication of efficient solar cells that combine a minimal loss in Voc with a high Jsc. © 2014 The Royal Society of Chemistry.
|Original language||English (US)|
|Number of pages||9|
|Journal||Energy Environ. Sci.|
|State||Published - 2014|
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: We thank the Swedish Research Council, Swedish Energy Agency, VINNOVA, Chalmers Areas of Advance Materials Science, NANO and Energy for financial support. CM thanks Formas and the Chalmers Areas of Advance Energy and Nano-science and Nanotechnology for funding. We further acknowledge financial support from the National Science Foundation, the Center for Advanced Molecular Photovoltaics (Award no. KUS-C1-015-21) made by the King Abdullah University of Science and Technology (KAUST) and the Department of Energy, Laboratory Directed Research and Development funding, under contract DE-AC02-76SF00515. SE is grateful to the National Science Foundation for financial support in the form of a Graduate Research Fellowship.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.