Photoinduced Charge Transfer in Hybrid Systems of CuInS2 Nanocrystals and Conductive Polymer

Colloidal CuInS2 nanocrystals are a promising alternative to toxic cadmium or lead chalcogenide nanocrystals that are widely studied as absorbing material in hybrid solar cells. Photovoltaic devices with colloidal CuInS2 nanoparticles suffer, however, still from low performance. The present study focuses on a detailed investigation of charge transfer as an elemental process involved in the energy conversion process. Therefore, the excited state properties and the process of charge transfer in CuInS2 (CIS) nanocrystal/polymer composites were studied by applying quasi-steady-state photoinduced absorption (PIA) and steady-state photoluminescence (PL) as well as time-resolved photoluminescence (PL) spectroscopy. The excited state dynamics of our systems was studied using time-correlated single photon counting. We examined two different composites, namely, CuInS2 nanocrystals combined with either poly(3-hexylthiophene) (P3HT) or poly [2,6-(4,4-bis-(2-ethylhexyl)-4H-cylopenta[2,1-b;3,4-b'] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Optical absorption and emission spectra of these hybrid material systems exhibit luminescence quenching and polaronic photoinduced absorption indicating photoinduced charge transfer. By systematic variations of the composition of the films, the material ratios favoring efficient charge transfer were determined.