High dielectric constant donors and acceptors for organic photovoltaics

Abstract

Organic solar cells bear the potential to develop a long-term technology that is environmentally clean and economically promising for large-scale power generation. Thanks to higher absorption coefficient, organic materials can be incorporated as thin films in organic photovoltaic (OPV) devices leading to a 10-fold reduction in the consumption of the material as compared to their inorganic counterparts. The dielectric constant is an essential parameter for efficient solar cells. Current organic materials for organic photovoltaics have low dielectric constants in the range of 2 to 4. This imposes an efficiency limiting factor in changing light into free charge carriers making them not competitive to their high dielectric inorganic counterparts, like Si which has a dielectric constant of ~12. It is feasible to design more efficient organic solar cells by introducing high-dielectric-constant organic donors and/or acceptors leading to next generation OPV. With this project we want to characterize the performance of organic semiconductors with high dielectric constant for photovoltaic applications. We focus the research on the PEO-PPV and PTEG-1, these materials have triethylene glycol (TEG) side chains which increase their dielectric constant. In previous studies has been seen that PEO-PPV has a dielectric constant of 6 at low frequency ranges (f<MHz), in order to have a more complete information of this material, we analysed the dielectric constant at optical frequency range by ellipsometric spectroscopy. Regarding to the performance of single component solar cells, PEO-PPV shows a short circuit current ten times higher than MEH-PPV which is a polymer with a similar backbone. The higher dielectric constant of PEO-PPV with respect to MEH-PPV can be considered as a possible reason. However we inspected absorption spectrum, mobility of charge carriers and presence of impurities as other possible reasons for enhancement of the photocurrent. Bulk heterojunction solar cells were also performed with several combinations of donors and acceptors: both donor and acceptor with high-ℇ, high-ℇ donor with a low-ℇ acceptor, and low-ℇ donor with a high-ℇ acceptor. Despite a higher dielectric constant for different combinations, the efficiency of the devices was lower with respect to conventional donor acceptor blends with lower dielectric constant. A thorough optimization should be performed on the device design and morphology of the blends. Trying different contacts, solvents and structure of the device were attempts that we did to this end. It is important to remark that the investigation for these materials has just started, so there are still plenty of possibilities to optimize the devices and obtain high efficiencies of solar cells.