Charge Injection Rates in Hybrid Nanosilicon-Polythiophene Bulk Heterojunction Solar Cells

The injection time for transfer of an electron from photoexcited dodecathiophene or polythiophene to a silicon nanocrystal (2.2 nm diameter) is calculated by computing the retarded Green's function for the system from the Hamiltonian and Kohn-Sham states produced by density functional calculations. We found that it can be of the order of 10-100 fs if the thiophene chain lies approximately parallel to the silicon surface. However, the electron injection time is 1-2 orders of magnitude longer if the oligothiophene chain lies perpendicular to the silicon surface. A chemisorption interaction between the thiophene chain and the nanocrystal provides a relatively small improvement (decrease) of injection times, much weaker than that achieved by enforcing the parallel arrangement of the chain with respect to the nanocrystal.