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Donor-п-acceptor conjugated copolymers for photovoltaic applications : tuning the open-circuit voltage by adjusting the donor/acceptor ratio

journal contribution
posted on 2008-02-01, 00:00 authored by Q Peng, K Park, Tong Lin, M Durstock, L Dai
A class of new conjugated copolymers containing a donor (thiophene)−acceptor (2-pyran-4-ylidene-malononitrile) was synthesized via Stille coupling polymerization. The resulting copolymers were characterized by 1H NMR, elemental analysis, GPC, TGA, and DSC. UV−vis spectra indicated that the increase in the content of the thiophene units increased the interaction between the polymer main chains to cause a red-shift in the optical absorbance. Cyclic voltammetry was used to estimate the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) and the band gap (Eg) of the copolymers. The basic electronic structures of the copolymers were also studied by DFT calculations with the GGA/B3LYP function. Both the experimental and the calculated results indicated an increase in the HOMO energy level with increasing the content of thiophene units, whereas the corresponding change in the LUMO energy level was much smaller. Polymer photovoltaic cells of a bulk heterojunction were fabricated with the structure of ITO/PEDOT/PSS (30 nm)/copolymer−PCBM blend (70 nm)/Ca (8 nm)/Al (140 nm). It was found that the open-circuit voltage (Voc) increased (up to 0.93 V) with a decrease in the content of thiophene units. Although the observed power convention efficiency is still relatively low (up to 0.9%), the corresponding low fill factor (0.29) indicates considerable room for further improvement in the device performance. These results provided a novel concept for developing high Voc photovoltaic cells based on donor-π-acceptor conjugated copolymers by adjusting the donor/acceptor ratio.

History

Journal

Journal of physical chemistry B : condensed matter, materials, surfaces, interfaces and biophysical

Volume

112

Issue

10

Pagination

2801 - 2808

Publisher

American Chemical Society

Location

Washington, D.C.

ISSN

1520-6106

eISSN

1520-5207

Language

eng

Publication classification

C1 Refereed article in a scholarly journal

Copyright notice

2008, American Chemical Society

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