Plasmonic interactions in organic solar cells.

Abstract

Various light-trapping strategies have been proposed to improve the low optical absorption in thin-film organic photovoltaic (OPV) cells which results in low power conversion efficiencies. One such method involves incorporating plasmonic metal nanoparticles (MNPs) into OPV cells. However, the underlying physics of how these plasmonic MNPs affect the charge carrier dynamics in the active layers of OPV cells is still unclear, thus limiting potential improvements using this approach. In this study, the effects of periodic ordered plasmonic MNP arrays on the charge carrier dynamics of PCDTBT and PCDTBT:PC60BM were studied using ultrafast optical spectroscopy. Nanosphere lithography was employed as a low-cost and efficient fabrication method for producing relatively large areas (~1cm2) of these periodic MNP arrays. Tuning of the LSPR extinction peak of gold nanoparticles (Au NPs) by altering the MNP shape using thermal annealing was demonstrated. Enhanced optical absorption by thin-films of photoactive PCDTBT and PCDTBT:PC60BM blend was shown, which is attributed to an increase in optical path length due to forward scattering and an increase in the local excitation field strength caused by the embedded periodic Au NP arrays. The increase in optical absorption led to enhanced exciton generation in the PCDTBT system and enhanced polaron formation in the blend system. However, Au NPs in direct contact with the photoactive molecules resulted in metallic quenching of excitons. Incorporating the Au NPs in a thin PEDOT:PSS buffer layer adjacent to the photoactive layers was found to circumvent this problem, and resulted in even larger increases in the exciton and polaron populations in the active layers. The enhanced polaron formation in the blend will be beneficial to OPV cells as more free charge carriers can potentially be extracted as photocurrent.