Interfacial electron transfer (ET) from sensitizer/dyes to semiconductor nanocrystalline thin films studied by ultrafast infrared spectroscopy

Motivated by both applied and fundamental interests, interfacial electron transfer (ET) dynamics from molecular and polymeric adsorbates to inorganic semiconductor (SC) nanocrystalline thin films have been investigated by ultrafast transient absorption spectroscopy. In molecular systems, injection rate as a function of semiconductor matrix and bridge distance between donor and acceptor was examined by choosing proper dyes and semiconductors. For the polymeric composites, investigation focuses on two types of conjugated polymers, polyphenylenevinylene and polythiophene derivatives.; ET dynamics from Ru complexes to ZnO, Nb2O5 and SnO2 films were investigated and compared with the previous study on TiO2. All injection kinetics are shown to be biphasic, with ultrafast (<100 fs) and slow (on ps or longer timescale) components, corresponding to injection from unthermalized and thermalized excited states of dye molecules respectively. A much faster injection rate from unthermalized excited states is observed for d-type semiconductor (Nb2O5 and TiO 2) than that for s-type semiconductor (ZnO and SnO2) likely due to orders of magnitude higher density of states in the former.; ET rate in the short-bridge limit as a function of methylene and phenyl bridge length was measured by testing ReCnA (n = 1--5) and RuPn (n = 0--2) dyes, respectively. The injection rate of ReCnA/SnO2 exhibited an exponential dependence on bridge length for n = 3--5 with a decay constant (beta) of 1.0/CH2. Deviation from the exponential dependence was observed for n = 1 and 2. The transfer rate from RuPn series to SnO2 films decays exponentially as a function of spacer length with beta of 0.3 A-1 at pH 2 and 0.44 A -1 on dry film.; Photoinduced electron injection dynamics from the conjugated polymers were studied as well. The ET from MEH-PPV to SnO2 and TiO 2 was found to occur within timescales of 800 and <100 fs; the faster injection rate to TiO2 is also attributed to orders of magnitude higher accepting density of states in TiO2 than in SnO2. The injection from polythiophenes to SnO2 occurs on 140 fs timescale, faster than that of MEH-PPV/SnO2, probably due to stronger binding in polythiophene with SnO2.