Electron injection process is a key process for realizing high performance solar cells (Fig.1). Thus, we have studied electron injection process in dye-sensitized nanocrystalline semiconductor films so far by using transient absorption (TA) [1] and time-resolved microwave conductivity (TRMC) methods. We estimated the absolute value Φ of injection efficiency [2] and studied the effect of sensitizer dyes [3], semiconductors [4] and excitation wavelength [5]. We examined here the effect of the additives, 4-tert-butylpyridine (TBP) and lithium ion, which are known to markedly improve the performance of solar cell devices.
For black-dye/TiO2 film in air, electron injection occurred in the picosecond time range with relatively low efficiency (F=0.4). Efficiency was improved by immersion of the film in acetonitrile (F=0.65). By adding 4-tert-butylpyridine (TBP), decrease of efficiency was observed, owing to an increase in the energy level of the conduction band. Similar tendency was observed in N3/TiO2 films. The decrease of injection efficiency can be explained using the model considering heterogeneity of free energy change for electron injection [3].
We gratefully acknowledge financial support from the New Energy and Industrial Technology Development Organization (NEDO).

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[3] R. Katoh, et al., J. Phys. Chem. B 106 (2002)12957.
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[5] R. Katoh, et al., C. R. Chimie 9 (2006) 639.