Charge Carrier Generation and Extraction in Hybrid Polymer/Quantum Dot Solar Cells

Here we investigate charge carrier generation and extraction processes in hybrid polymer/nanocrystal solar cells by means of time-resolved optical and photoelectrical techniques. We addressed the role of both poly(3-hexylthiophene) and colloidal arenethiolate-capped PbS quantum dots, which constitute the hybrid composite nanomaterial, in the photoinduced processes most relevant to device operation by changing the compositional ratio and applying chemical and thermal postdeposition treatments. The carrier generation processes were found to be wavelength-dependent: excitons generated in the polymer domains led to long-lived weakly bound charge pairs upon electron transfer to PbS nanocrystals; whereas charge carrier generation in the nanocrystal domains is highly efficient, although effective separation required the application of external electric field. Overall, charge carrier generation was found efficient and almost independent of the strength of applied electric field; therefore, competition between separation of electron-hole pairs into free carriers and geminate recombination is the major factor limiting the fill factor of nanocomposite-based solar cells. Device efficiency improvements thus require faster interfacial charge transfer processes, which are deeply related to the refinement of nanocrystal surface chemistry.