Femtosecond Decay and Electron Transfer Dynamics of the Organic Sensitizer D149 and Photovoltaic Performance in Quasi-Solid-State Dye-Sensitized Solar Cells

In this work, the femtosecond upconversion spectroscopy was used for the study of ultrafast decay and electron transfer dynamics of the very promising all-organic sensitizer indoline dye, D149, in inert environments as well as on TiO 2 substrates. The inert environments were toluene and acetonitrile solutions as well as poly(methyl methacrylate) (PMMA) films and Al 2 O 3 nanostructured substrates. These were used to investigate relaxation and aggregation induced decays. TiO 2 substrates were used to probe interfacial electron transfer dynamics. The excitation was at the S 2 band while the dynamics of both S 2 and S 1 bands were detected. The dynamics of the S 2 band in solutions and PMMA film, show a rapid S 2 –S 1 internal conversion, ranging from 0.20 to 0.45 ps depending on the environment. This component becomes <0.10 ps in Al 2 O 3 and in TiO 2 . The dynamics of the S 1 band probe both hot and cold S 1 species. In solutions and PMMA, the hot species relax to cold ones within a few tens of picoseconds while the cold ones decay to the ground state within a few hundreds of picoseconds. In Al 2 O 3, the dynamics become much faster because of aggregation induced quenching. In TiO 2, the S 1 dynamics exhibit three mechanisms of 0.1, 1.3, and 8.6 ps time constants indicating a multiphasic injection mechanism. The electron transfer quantum yield and rate from the S 1 band were calculated, using the time-resolved data, and were found to be 0.90 and 3.82 × 10 11 s –1, respectively. Finally, quasi-solid-state dye-sensitized solar cells, with D149 as sensitizer, were constructed, and their efficiency was found 5.66%. Similar cells, using the ruthenium complexes N719 and black dye as sensitizers, were also constructed for comparison. It was found that D149 showed higher efficiency than black dye but lower than N719.