Ultrafast Electron Transfer and Decay Dynamics in a Small Band Gap Bulk Heterojunction Material

Abstract

Bulk heterojunction materials comprising bicontinuous networks of electron donor and acceptor components sandwiched between electrodes with different work functions (e.g., indium-tin oxide, ITO, and a lower work-function metal such as Al) have yielded the best results to date for organic polymer-based solar cells. In such bulk heterojunction materials, ultrafast photoinduced electron transfer occurs at the interface between the polymer (donor) and the fullerene (acceptor) and results in efficient charge separation. Provided that the back transfer rate is sufficiently slow, the photo-generated mobile positive and negative carriers can be collected at the electrodes; electrons at the lower work function electrode and holes at the higher work function electrode. Using this approach, power conversion efficiencies of 5 % have been demonstrated in bulk heterojunction materials comprised of poly(3-hexylthiophene), P3HT and the [6,6]phenyl-C61 butyric acid methyl ester fullerene derivative, PCBM. Although the quantum efficiency is high within the absorption spectrum, the band gap of this conjugated polymer is too large (the solar radiation spectrum extends into the infrared). Thus, the quest for higher solar cell efficiencies has focused on smaller band gap polymers designed and synthesized to improve the harvesting of solar radiation. Here we describe the results of time-resolved spectroscopic studies of the small band gap semiconducting copolymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, made of alternating electron-rich and electron-deficient units and in bulk heterojunction materials comprising PCPDTBT and PCBM. The molecular structures of PCPDTBT and PCBM are shown in Scheme 1. Because carrier recombination prior to collection at the electrodes would limit the photovoltaic power conversion efficiency, the goal of our stud-