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.