Journal
ENERGY & ENVIRONMENTAL SCIENCE
Volume 7, Issue 1, Pages 361-369Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c3ee42989j
Keywords
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Funding
- Swedish Research Council
- Swedish Energy Agency
- VINNOVA
- Chalmers Areas of Advance Materials Science
- NANO
- Formas
- Chalmers Areas of Advance Energy and Nano-science and Nanotechnology
- National Science Foundation
- Center for Advanced Molecular Photovoltaics [KUS-C1-015-21]
- King Abdullah University of Science and Technology (KAUST)
- Department of Energy, Laboratory Directed Research and Development [DE-AC02-76SF00515]
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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 (V-oc) of the solar cells with increasing n. The short-circuit current density (J(sc)) 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 J(sc). 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 V-oc with a high J(sc).
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