Photocurrent generation in organic photovoltaic (OPV) devices is driven by the creation and subsequent separation of excitons into free electrons and free holes. Materials with low exciton binding energies are able to efficiently separate into free charge carriers and limit recombination pathways. The binding energy of an exciton is dependent on a number of factors, such as that described by the Wannier-Mott construct, which states that the exciton binding energy is proportional to the inverse square of the dielectric constant. Additionally, according to the Clausius-Mossotti relation, the dielectric constant of a material can be related to its density, with increased density providing a higher dielectric constant. As a result, it is expected that increasing the dielectric constant of a material will decrease exciton binding energy, reducing the energy losses for exciton separation.
At present, there has been limited development on increasing the dielectric constant of organic semiconductors. However recent work has shown that the inclusion of triethylene glycol monomethyl ether chains can dramatically increase the dielectric constant at low frequency, while maintaining both the optical and electronic properties of their alkylated derivatives. Additionally, it has been shown that the inclusion of these glycolated units has the ability to increase the molecular packing density of the film, enhancing the high (optical) frequency dielectric constant, and thus, homojunction device performance.
Based on these considerations, this presentation will describe a library of novel glycolated materials that have been engineered with the aim of tuning the dielectric constant to decrease exciton binding energy. Additional considerations such as broad absorption within the visible light spectrum, solution processability, and engineered energy levels to optimise the open circuit voltage, to maximise charge generation and extraction will be discussed. Finally, the applicability of dielectric constant manipulation will be reported in the context of homojunction OPV device design and fabrication.
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