Microwave Reflectance Studies of Photoelectrochemical Kinetics at Semiconductor Electrodes. 2. Hydrogen Evolution at p-Si in Ammonium Fluoride Solution

Abstract

Microwave reflectance methods have been used to study the kinetics of photogeneration of hydrogen on p-Si in acidic fluoride solutions. To estimate the rate constant for interfacial electron transfer, the magnitude of the observed steady-state light-induced microwave reflectivity change has been compared quantitatively with theoretical predictions based on numerical calculation of the electron and hole profiles in the silicon sample. In addition, the transient and periodic microwave reflectivity responses to stepped and sinusoidally modulated illumination, respectively, have been analyzed to obtain information about the electrode kinetics. The time constants for relaxation of the light-induced conductivity derived from this analysis confirm that electron transfer during hydrogen evolution is slow. The apparent inconsistency in the values of the phenomenological electron-transfer rate constants derived from the steady-state and transient or periodic responses can be resolved if it is assumed that hydrogen evolution proceeds via electron capture by protons followed by a slow bimolecular step leading to molecular hydrogen. Surface charging as the result of slow kinetics is expected to lead to band edge unpinning even at low light intensities, and the existence of this effect has been confirmed by transient photocapacitance measurements.

In Situ Detection of Free and Trapped Electrons in Dye-Sensitized Solar Cells by Photo-Induced Microwave Reflectance Measurements

“…technique, have been applied to the characterization of many different semiconductor materials, [5][6][7] as well as in the study of electrochemical [8][9][10][11][12][13][14][15] and solid state junctions. 16 More recently, TRMC has been employed to investigate organic photovoltaic systems 17 and mesoporous films of TiO 2 (bare and sensitized) in the absence of electrolyte and conducting substrate.…”

“…So far the discussion has been limited to the steady state microwave response of the DSC. The relaxation of trapped electrons in the DSC has been characterized previously using frequency resolved near-IR measurements, 29 but the present measurement is the first example of the application of light-modulated microwave reflectance [13][14][15]50,63 to the DSC (see Supporting information for experimental setup). Ideally, the measurement should be made with a small amplitude modulation of the steady state light intensity in order to allow linearization of the response.…”

“…We have demonstrated the utility of the dielectric stack approach previously in microwave reflectance studies of the silicon/electrolyte interface. 11,13,14 The reflectance is calculated using values of the relative permittivity of the TiO 2 layer corresponding to dark and illuminated conditions (the permittivity values for the other components of the cell are given in Table 1 Table 1 for permittivity values and Supporting Information for further details. The calculated reflectance change is negative for the stack configuration used.…”

“…The spacings in the experimental configuration and the position of the short were chosen to optimize the microwave reflectance signal (see the Supporting Information). We have demonstrated the utility of the dielectric stack approach previously in microwave reflectance studies of the silicon/electrolyte interface. ,, The reflectance is calculated using values of the relative permittivity of the TiO 2 layer corresponding to dark and illuminated conditions (the permittivity values for the other components of the cell are given in Table ). Calculation details are given in the Supporting Information.…”

“…Microwave reflectance methods, including the time-resolved microwave conductivity (TRMC) technique, have been applied to the characterization of many different semiconductor materials, − as well as in the study of electrochemical − and solid state junctions . More recently, TRMC has been employed to investigate organic photovoltaic systems and mesoporous films of TiO 2 (bare and sensitized) in the absence of electrolyte and conducting substrate. − In these previous studies, the mesoporous TiO 2 layers were not in contact with a redox electrolyte, which is an essential component of the DSC.…”