Abstract
Through the incorporation of a thiophene functionality, a novel solution-processable small organic chromophore was designed, synthesized and characterized for application in bulk-heterojunction solar cells. The new chromophore, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)acrylonitrile) (coded as AS2), was based on a donor–acceptor–donor (D–A–D) module where a simple triphenylamine unit served as an electron donor, 1,4-phenylenediacetonitrile as an electron acceptor, and a thiophene ring as the π-bridge embedded between the donor and acceptor functionalities. AS2 was isolated as brick-red, needle-shaped crystals, and was fully characterized by 1H- and 13C-NMR, IR, mass spectrometry and single crystal X-ray diffraction. The optoelectronic and photovoltaic properties of AS2 were compared with those of a structural analogue, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-(diphenylamino)phenyl)-acrylonitrile) (AS1). Benefiting from the covalent thiophene bridges, compared to AS1 thin solid film, the AS2 film showed: (1) an enhancement of light-harvesting ability by 20%; (2) an increase in wavelength of the longest wavelength absorption maximum (497 nm vs. 470 nm) and (3) a narrower optical band-gap (1.93 eV vs. 2.17 eV). Studies on the photovoltaic properties revealed that the best AS2-[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)-based device showed an impressive enhanced power conversion efficiency of 4.10%, an approx. 3-fold increase with respect to the efficiency of the best AS1-based device (1.23%). These results clearly indicated that embodiment of thiophene functionality extended the molecular conjugation, thus enhancing the light-harvesting ability and short-circuit current density, while further improving the bulk-heterojunction device performance. To our knowledge, AS2 is the first example in the literature where a thiophene unit has been used in conjunction with a 1,4-phenylenediacetonitrile accepting functionality to extend the π-conjugation in a given D–A–D motif for bulk-heterojunction solar cell applications.
Highlights
The development of renewable energy technologies is pivotal for accommodating the ever increasing energy demands of the modern society
In our own studies of small molecule chromophores and charge transport materials based on a variety of D–A combinations, we have reported examples of successful solution-processable BHJ
We report the design, facile synthesis and characterization of the optoelectronic and photovoltaic properties of two small organic chromophores, AS2 and AS1, and their direct comparison
Summary
The development of renewable energy technologies is pivotal for accommodating the ever increasing energy demands of the modern society. The D–A–D design in particular is one of the most promising and successful modules based on which various donor and acceptor units have been explored for high-performance solution-processable photovoltaic devices. We report the design, facile synthesis and characterization of the optoelectronic and photovoltaic properties of two small organic chromophores, AS2 and AS1, (shown in Figure 1), and their direct comparison Both materials are based on a D–A–D structural motif where a triphenyl-amine functionality has been chosen as a common donor at both ends of the central acceptor unit, 1,4-phenylenediacetonitrile (PDA), so as to get symmetrical AS2 and AS1.
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