Inter- and intra- component processes essential for interfacial electron transfer dynamics

Summary, in English

Interfacial electron transfer is one of the key processes in the novel type of photovoltaic cells-dye sensitized solar cells (DSSC). By means of transient absorption spectroscopy we have studied the dynamics of the initial photo-induced processes in dye-sensitized nano-porous thin films of wide-band gap semiconductor microcrystallines, which represent the main photoactive part of DSSC. In the most efficient combination of dye and semiconductor, RuN3 (Ru(4,4 prime -dicarboxy-2,2 prime -bipyridine)₂(NCS)₂) attached to a nanocrystalline TiO₂ film we have resolved two phases of the photo-induced electron injection that occur on distinctly different time scales. The primary electron injection occurs on sub-hundred fs time scale from a non-thermolized excited state of the dye in competition with intra-molecular energy relaxation processes. The rest of the injection proceeds significantly slower from a quasy-thermolized excited state. The dynamics of the slower part of the injection is essentially non-exponential and can be approximated by a three-exponential process with 1,10, and 50 ps time constants. These two stages of electron injection are separated by a "silent" time interval of 0.1-0.5 ps. We believe that these distinct "modes" of electron injection are different not only by the relative energetics and coupling of the donor and acceptor states of the dye and semiconductor respectively but also by essentially different types of intra-molecular excited state dynamics that influence the injection process. This view is supported by our studies of electron injection dynamics in photoactive RuN3 sensitized SnO2 porous film, in TiO₂ films sensitized by a modified RuN3 dye as well as by the results on the RuN3 photo-induced dynamics in solution.