7328
J . Phys. Chem. 1993,97, 7328-1330
Photocarrier Transport in Colloidal Ti02 Films R. Ktinenkamp,' R. Henninger, and P. Hoyer Hahn- Meitner Institut Berlin, Glienicker Strasse 100, 1000 Berlin 39, Germany Received: February 17, 1993; In Final Form: April 19, I993
We have studied the transport kinetics in colloidal Ti02 films using time-resolved photoconductivity techniques. The transport process in these films is dispersive. In depleted films, trapping lifetimes are of the order of microseconds and typical drift lengths are less than 100 nm. Excess electrons induced either by doping, charge transfer, optical pumping, or electrical injection can easily saturate the trap states in these films and thereby induce drastic changes in the trapping kinetics. Decay times can then reach the millisecond range, which results in correspondingly longer transport distances.
Over the past years semiconductorcolloidshave been prepared from a wide range of materials,' and some of these have successfully been deposited as structurally stable thin films. The particle size can be chosen in a wide range, the lower limit being determined by a tendency toward agglomeration. Small grain films may appear amorphous in X-ray analysis, but in more sensitive techniques atomic ordering can often be traced down to the smallest cluster sizes. These small grain films show quantum size effects in the optical spectra. Due to the loose packing that can be achieved in some fabrication methods, the surface area of the colloidal films can be several thousandfold that of the geometric projection. Structural modification, quantum size effects, and large surface-to-volume ratios make these films interesting for many electrooptical applications and for use as chemical sensors,2 light-activated catalyst^,^ solar cells,4v5 and other devices. Recent work has shown promising results for some of these applications. Many of these requirecharge transfer across the colloidal films; hence, studies of the transport properties, Le., carrier ranges, lifetimes, and mobilities, are of high importance. Here we report time-resolved photoconductivity studies on colloidal Ti02 films with the aim to elucidate the fundamental features in the transport kinetics. An intriguing change in the transport properties is induced by the filling of deep traps by excess carriers. We show that for injection through contacts, optical pumping, interface charge transfer, and doping, trapping lifetimes can be altered over several orders of magnitude. It is concluded from this study that specific operation conditions can result in transport properties greatly different from those at equilibrium. Colloidal Ti02 solutions were prepared from titanium isopropoxide (30 mL) and 2-propanol (10 mL), which were slowly added to cooled water (500 mL) applying vigorous stirring. Nitric acid was added to give a pH value of 1. Boiling for 12 h allowed the organic components to evaporate and led to a crystallization of Ti02 particles. This colloidal solution was spin-coated onto InSnOz-covered glass substrates, which were subsequently heated for 5 min at 450 "C. Coating and drying were repeated several times until films of 1-pm thickness were obtained. Finally, the samples were baked at 450 OC for 30 min. This preparation method gives similar films as reported in ref 5 . Transmission electron microscopy and X-ray analysis indicate the films to consist of 40-60-nm-diameter anatase crystallites. The films are highly porous; adsorption experiments show the surface area to be -400 times larger than that of evaporated Ti02 films. The films are structurally stable below 650 O C when a phase transformationto rutile and agglomeration to larger grains occurs. The conductivityof the films at 330 K is typically 10-9 (Q cm)-l at room temperature with an activation energy of 1.6 eV, indicating that the Fermi level is near midgap. For the photoconductivity measurement semitransparent front contacts
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0022-3654/93/2091-1328%04.00/0
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CURRENT
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VOLTAGE (V) Figure 1. Current-voltage characteristics of InSn02/Ti02/Pt sample. Insert: current response to forward bias step voltage of 3 V.
were fabricated by vacuum (
