Simultaneous Determination of Quantum Efficiency and Energy Efficiency of Semiconductor Photoelectrochemical Cells by Photothermal Spectroscopy

Abstract

During a photoelectrochemical reaction only a portion of the light energy absorbed by the semiconductor (CdS or TiO2 single crystal) is utilized in the electrode reaction. The unused portion of energy is expended through various mechanisms as heat. Therefore by monitoring temperature changes within the photoanode as a function of electrode potential and light intensity, information concerning the efficiency of the process can be obtained. Experimental results are presented and interpreted using a model for the energy balance within the system. This permits the determination of the quantum and energy efficiencies simultaneously without the need to calibrate the light source.A number of photoelectrochemical cells based on semiconductor electrodes for photoelectrosynthesis (e.g,, the photodecomposition of water) and the conversion of solar energy to electricity have ,been reported (!-15). The major factors determining the efficacy of these semiconductor electrode ceils are the quantum efficiency for electron flow (i.e., number of electrons flowing/number of photons absorbed), and the power conversion efficiency (i.e., chemical or electrical power output/input radiant power). It is not uncommon to find near unity quantum yields for electron flow at a sufficiently high bias (positive for the case of n-type semiconductors) during irradiation with greater than bandgap energy light for many semiconductors. However, even when high quantum efficiencies have been obtained, the power efficiencies were much lower (10-15). For example, for photooxidation at a CdS single crystal anode, Wrighton et al. (15) reported that with an Se 2-solution the maximum monochromatic power efficiency obtained was 3.4% with a maximum quantum efficiency of 49%. Such findings demonstrate that the major part of the light energy absorbed by the semiconductor is not used for the photo-assisted oxidation but rather is converted to heat energy probably via radiationless transitions within the conduction band of the semiconductor (i.e., when the photon energy is in excess of the bandgap energy) or electron-hole recombination processes. Consequently, thermal measurements of the semiconductor electrode during etectroIysis can aid in the determination of the efficiencies and perhaps in the elucidation of the mechanism of the cell processes.We recently described a new spectroscopic method called Photothermal Spectroscopy (PTS) (16). This technique involves placing a thermistor on or in close proximity to a sample and measuring temperature changes (i.e., thermistor resistance changes) during sample irradiation with monochromatic light. This technique can be easily adapted to cases when the sample is a semiconductor electrode (17). Cahen (18) has also shown that similar measurements with photoacoustic spectroscopy can be used to determine the efficiency of solid-state photovoltaic devices. We report in this paper measurements of the conversion of light energy to chemical and/or electrical energy at * Electrochemical Society Active Member. ~ Electrochemical So...

The mechanism of the desulfurization of benzenethiol by nickel (110)

“…The binding energy corresponding to the molecular intermediate was 284.7 eV, which is comparable to the binding energy of molecular benzene on Ni(l 10), previously measured at 284.5 eV. 4 As the sample was annealed, the intensity of the carbon peak dropped continuously. The peak also shifted toward lower binding energy, and above 400 K, the remaining carbon on the surface gave rise to a peak at 283.7 eV, which is comparable to carbidic carbon on Ni(110).4…”

“…The spectrum reported previously for the phenyl or benzyne species is comprised of two major losses at 748 and 3050 cm"1 and two very weak losses at 1178 and 1460 cm:1. 4 The spectrum obtained from a low exposure to benzenethiol has three visible losses at 743,1136, and 3050 cm'1. The absence of the mode at 1460 cm"1 is presumably due to the fact that it is a very weak mode, and perhaps slight differences in orientation reduce its intensity.…”

“…Steric crowding clearly does affect the saturation coverage since the maximum sulfur coverage achieved for mmethylbenzenethiol is even lower than benzenethiol itself.17 The saturation coverage of benzene on Ni(110) is 0.25 ML and corresponds to a very tightly packed layer of benzene molecules lying with the ring parallel to the surface. 4 The comparatively high coverage of benzenethiol compared to that of benzene indicates that the aromatic ring on the benzenethiol is not parallel to the surface. The strong intensity of the C-H stretching modes in the vibrational spectra confirms this conclusion, since forbonded benzene those modes are essentially absent4 due to the dipole selection rule.…”

Excited-state processes of relevance to photoelectrochemistry

“…(1) Implicit in this argument is the assumption that the ratio of 0r to (j>m is independent of potential. A complementary technique, photothermal spectroscopy (PTS), has recently been employed to monitor the related conversion of light to heat with potential (13). For a ZnO:Cu-based PEC, 4>x was determined by PTS, by PL quenching, and by a direct measurement (photocurrent resulting from measured input power); good accord was found in comparing the three techniques (U).…”

Application of photothermal spectroscopy to in-situ studies of films on metals and electrodes

“…Since the detector is not affected by scattered or reflected light, problems associated with changes in refractive index (/( )) are minor. Electrochemical applications to the determination of efficiencies of photoelectrochemical cells and solid state photovoltaic cells also are possible (25).…”