Electrodeless Determination of the Trap Density, Decay Kinetics, and Charge Separation Efficiency of Dye-Sensitized Nanocrystalline TiO2

Abstract

We have studied photoinduced charge separation in a bare, 3.4 μm thick layer of nanocrystalline (“nc”) anatase TiO₂ and an nc-TiO₂ layer coated with free-base 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (H₂TPPC) using the electrodeless flash-photolysis time-resolved microwave-conductivity technique (FP-TRMC). Photoconductivity transients, resulting from the formation of mobile, conduction band electrons in the semiconductor have been measured on excitation with 3 ns pulses of UV (300 nm) and visible (410−700 nm) light. The product of the yield of formation of mobile charge carriers, φ, and the sum of their mobilities, Σμ, has been determined from the maximum conductivity for light intensities varying from ∼10¹² to ∼10¹⁶ photons/cm²/pulse. For the bare nc-TiO₂ layer at 300 nm and the coated layer at all wavelengths, φΣμ initially increased with increasing intensity, reached a maximum, and eventually decreased at high intensities. The initial increase is attributed to the gradual filling of (surface) electron trapping sites. This effect was absent when the samples were continuously illuminated with background irradiation at 300 nm with an intensity of 6 × 10¹³ photons/cm²/s (40 μW/cm²), thereby presaturating the trapping sites prior to the laser pulse. The trap-free mobility of electrons within these 9 nm nanoparticles is estimated to be 0.034 cm²/Vs at 9 GHz. The eventual decrease in φΣμ at intensities corresponding to an electron occupancy of more than one electron per particle is unaffected by background illumination, and is attributed to a decrease in μ due to electron−electron interactions within the semiconductor particles. The photoconductivity action spectrum of the coated nc-TiO₂ layer closely followed the photon attenuation spectrum in the visible of the porphyrin, with a charge separation efficiency per absorbed photon of 18% at the Soret band maximum. The after-pulse decay of the photoconductivity showed a power law behavior over a time scale of nanoseconds to several hundreds of microseconds, which is attributed to multiple trapping and detrapping events at chemical or physical defects within the semiconductor matrix.