New semiconductor materials for efficient organic photovoltaics
High efficiency and low cost are crucial to ensuring wider adoption of solar cell technology. As of 2013, organic photovoltaics' inefficiency and stability problems combined with their promise of low cost and increased efficiency made them a popular field in solar cell research. Wide-bandgap organic semiconductors have high potential to fulfil expectations for high efficiencies. In particular, organic polymers with wider bandwidth than the commonly used inorganic silicon are highly desirable complementary absorbers in tandem solar cells. Within ECOCHEM (Development of low band gap conjugated polymers by ecofriendly synthetic methodologies for high performance organic photovoltaics), scientists successfully synthesised highly stable wide-bandgap conjugated polymers that hold great promise for industrial-scale manufacturing of flexible large-area devices. Efforts were focused on developing alternating copolymers consisting of repeating units of electron donor and electron acceptor monomers. Thus, scientists developed semiconductor polymers containing indacenodithiophene or indacenodithienothiophene acting as electron-rich monomers and quinoxaline, thienopyrrolodione or difluorobenzothiadiazole acting as electron-deficient monomers. Until now, poly (3-hexylthiophene) has been the most commonly used wide-bandgap polymer for tandem organic solar cells. However, massive production of devices based on this polymer is often impeded by time-consuming fabrication processes. All copolymers developed by ECOCHEM scientists exhibited large bandgaps, similar to that of poly (3-hexylthiophene). Yet, the kind of electron-deficient unit was found to largely affect the optical properties of the indacenodithiophene and indacenodithienothiophene copolymers. The energy conversion efficiency of inverted solar cell architecture based on semiconductor polymers containing indacenodithienothiophene and quinoxaline was found to be equal to that of poly (3-hexylthiophene). Such large-bandgap copolymers also proved to be ideal candidates for use in the top cell of tandem structures. Having the ability to be easily processed in the air and being compatible with roll-to-roll methods, such large-bandgap copolymers are paving the way for exciting innovations in large-area flexible solar cells. However, applications not only span solar cells, but also energy-efficiency power electronics, biosensors and solid-state lighting.
Keywords
Semiconductor, organic photovoltaics, solar cell, wide-bandgap, roll-to-roll, conjugated polymers