ABSTRACT
The atomic structure and the related electronic properties of the MOS junction made by electrochemical deposition de¢ne the adsorption spectrum of the photoelectrode and, hence, the practical ef¢ciency of sunlight-induced conversion processes. As a model system, let us analyze the photoelectrochemical behavior of a p-type Si(111) surface onto which Rh particles were photoelectrochemically deposited. As already discussed in the foregoing chapters, p-type Si/oxide/Rh junctions of reduced dimensions are obtained. Figure 6.1a shows the quantum yield recorded at constant incident frequencies in 1 M HClO4 solution. For the investigated frequencies the onset of photocurrent can be observed at potentials more negative than –0.15 V (SCE). This clearly indicates a loss of photopotential of 0.21 eV from a maximum theoretically achievable photopotential of about 0.3 eV. This effect is expected by virtue of a shift of the band edge toward more negative potentials, as observed by capacitance measurements performed on this system (see Figure 5.27). Quantum yield values over 40% are attained, however, with a maximum of 2.3 eV. The dependency of quantum ef¢ciency with the energy of incident photons is shown for different potentials in Figure 6.1b. The quantum yield is calculated as
j h
W h e( ) phη = × ν
ν × (6.1)
The onset of light absorption is observed at photons of energy larger than the band gap. The ef¢ciency increase at larger energies re«ects the reduction of the adsorption length, and hence the length of the diffusion path of light-induced charge carriers in the ¢eld-free region toward the space charge layer. Figure 6.2 shows the dependency of the absorption coef¢cient for Si with the incident wavelength. The adsorption distance is reduced from some tens of millimeters at 1.1 eV up to some hundreds of nanometers at 400 nm. It must be noted that below 1.2 eV the adsorption distance becomes larger than the wafer thickness, usually 300-400 μm, and the charge collection reduces practically to zero.
