Using pyridal[2,1,3]thiadiazole as an acceptor unit in a low band-gap copolymer for photovoltaic applications
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Date
2015-08-01
Authors
Ibraikulov, Olzhas A.
Bechara, Rony
Chavez, Patricia
Bulut, Ibrahim
Tastanbekov, Dias
Leclerc, Nicolas
Hebraud, Anne
Heinrich, Benoît
Berson, Solenn
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Abstract
Abstract In this report, we explore the optoelectronic properties of a low band-gap copolymer based on the alternation of electron rich (thiophene and thienothiophene units) and electron deficient units (pyridal[2,1,3]thiadiazole (Py)). Initial density functional theory calculations point out the interest of using the Py unit to optimize the polymer frontier orbital energy levels. A high molecular weight (Mn=49kg/mol) solution-processable copolymer, based on Py, thiophene and thienothiophene units, has been synthesized successfully. From cyclic-voltammetry and UV–visible absorption measurements a relatively deep HOMO level (−5.1eV) and an optical band-gap (1.48eV) have been estimated. Charge transport both in horizontal and vertical directions were extracted from field-effect transistors and space charge limited current diodes, respectively, and led to a relatively high in-plane hole mobility in pure polymer films (0.7×10−2cm2V−1s−1). GIWAXS results showed almost identical in-plane lamellar morphologies, with similar average size and orientation of the polymer crystalline domains in both, pure polymer films and polymer:fullerene blends. Also, the gate-voltage dependence of the field-effect mobility revealed that the energy disorder in the polymer domains was not altered by the introduction of fullerenes. The nevertheless significantly higher out-of-plane hole mobility in blends, in comparison to pure polymer films, was attributed to the minor amorphous polymer phase, presumably localized close to the donor/acceptor interface, whose signature was observed by UV–vis absorption. Promising photovoltaic performances could be achieved in a standard device configuration. The corresponding power conversion efficiency of 4.5% is above the value achieved previously with a comparable polymer using benzo [2,1,3]thiadiazole instead of Py as acceptor unit.
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Keywords
Polymer solar cells, Energy disorder, Charge transport, Field-effect mobility, Space-charge-limited current, Morphology
Citation
Olzhas A. Ibraikulov, Rony Bechara, Patricia Chavez, Ibrahim Bulut, Dias Tastanbekov, Nicolas Leclerc, Anne Hebraud, Benoît Heinrich, Solenn Berson, Noëlla Lemaitre, Christos L. Chochos, Patrick Lévêque, Thomas Heiser, Using pyridal[2,1,3]thiadiazole as an acceptor unit in a low band-gap copolymer for photovoltaic applications, In Organic Electronics, Volume 23, 2015, Pages 171-178