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Vandewal, Koen
Publications (10 of 12) Show all publications
diva2:656978
Open this publication in new window or tab >>Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells
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2014 (English)In: energy and environmental science, ISSN 1754-5692, Vol. 17, no 1, p. 361-369Article in journal (Refereed) Published
Abstract [en]

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 favorable morphology and optimal interface energy level offset ensures efficient exciton separation and charge generation. The structure-property relationship demonstrated in this work is 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.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2014
Keywords
organic solar cell, fullerene, conjugated polymer, charge transfer state
National Category
Natural Sciences Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-99424 (URN)10.1039/c3ee42989j (DOI)000329550700025 ()
Available from: 2013-10-17 Created: 2013-10-17 Last updated: 2015-05-29Bibliographically approved
diva2:642883
Open this publication in new window or tab >>Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene-Quinoxaline Copolymer
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2013 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 6, p. 806-814Article in journal (Refereed) Published
Abstract [en]

The side-chain architecture of alternating copolymers based on thiophene and quinoxaline (TQ) is found to strongly influence the solubility and photovoltaic performance. In particular, TQ polymers with different linear or branched alkyloxy-phenyl side chains on the quinoxaline unit are compared. Attaching the linear alkyloxy side-chain segment at the meta- instead of the para-position of the phenyl ring reduces the planarity of the backbone as well as the ability to order. However, the delocalisation across the backbone is not affected, which permits the design of high-performance TQ polymers that do not aggregate in solution. The use of branched meta-(2-ethylhexyl)oxy-phenyl side-chains results in a TQ polymer with an intermediate degree of order. The reduced tendency for aggregation of TQ polymers with linear meta-alkyloxy-phenyl persists in the solid state. As a result, it is possible to avoid the decrease in charge-transfer state energy that is observed for bulk-heterojunction blends of more ordered TQ polymers and fullerenes. The associated gain in open-circuit voltage of disordered TQ:fullerene solar cells, accompanied by a higher short-circuit current density, leads to a higher power conversion efficiency overall. Thus, in contrast to other donor polymers, for TQ polymers there is no need to compromise between solubility and photovoltaic performance.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2013
Keywords
polymer solar cell; solubility; side-chain geometry; backbone twisting; charge-transfer state; thiophene-quinoxaline copolymer
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-96487 (URN)10.1002/aenm.201201019 (DOI)000319888000017 ()