Harvey Flaisher

Photoelectrochemical reduction of carbon dioxide in aqueous solutions on p-GaP electrodes: an a.c. impedance study with phase-sensitive detection

Abstract The mechanism of the photoelectrochemical reduction of water and CO2 on p-GaP electrodes in aqueous solutions was studied using photocurrent spectroscopy with chopped light excitation, revealing both a photo-anodic sub-bandgap response and a photo-cathodic current due to the bandgap response. A.c. power curves were recorded for p-GaP in pH 7 phosphate buffer electrolyte using modulated irradiation and a lock-in amplifier. At a light modulation frequency of 150 Hz, CO2 bubbling effected a 15% increase in the photo-cathodic current, relative to that in an Ar flushed solution. This effect disappeared at a light modulation frequency of 1560 Hz. Ruthenium pretreatment of p-GaP resulted in a doubling of both the cathodic photocurrent and the photovoltage. Pretreatment of p-GaP by dipping in a solution of CuSO4 caused a marked enhancement in both the dark current and photocurrent at reverse bias. In preparative reduction of aqueous bicarbonate, using light chopped at 22 Hz and −1.08 V between the p-GaP and a stainless steel counter electrode separated by a cation selective membrane, the production of formic acid was determined. The efficiency of conversion of the incident light energy to the Gibbs energy of formation of the formic acid produced amounted to 0.3%, both with bare and with Cu treated p-GaP.

Quantitative separation of mechanisms for power dissipation in solar cells by photoacoustic and photovoltaic measurements

Photoacoustics is used as a calorimetric method in conjunction with electrical measurements to determine which mechanisms are involved in the conversion of most of the absorbed radiation to thermal energy in (mainly Si p‐n) solar cells. The major mechanisms that are identified and quantified include local cooling, near the junction of the cells. Quantification is made possible by the use of a model for internal energy fluxes in a photovoltaic cell, which takes into account the different spatial distributions of heat generated by photogenerated and injected carriers. The experimental results agree well with calculations based on the model also in the case of thin‐film CdS/CuInSe2 cells.

Thin-film CdSe: photoluminescence and electronic measurements

The electronic properties of thin CdSe films prepared through physical vapor transport were investigated using photoluminescence (PL) and electronic measurements. The films were studied at each of the main preparation steps, i.e., evaporation, annealing, etching, and finally photoetching. At 3 K two distinct donor‐acceptor (DA) transitions at 1.75 and 1.70 eV were found in the photoluminescence spectra in addition to deep states at about 1.55 eV at 20 K. These DA transitions which are produced mainly during the evaporation might be associated with group VII and with alkali metal impurities. After each preparation step the DA transitions change their intensities. It is shown that photoetching of the films leads to a removal of the deep centers, while the 1.75 eV transition is blue shifted. In contrast with single‐crystal CdSe the intensity of the PL increases after photoetching. The results of the PL are consistent with the electronic measurements. They are explained in terms of a previously published model.

A Pronounced Cation Effect on Performance and Stability of Cd‐Chalcogenide/Polysulfide Photoelectrochemical Cells

Variation of the alkali cation in aqueous polysulfide solutions is found to have a dramatic effect on Cd-chalcogenide photoelectrochemical cell (PEC) stability and conversion characteristics. The cation species, although not directly involved in the electrochemical reactions, is important since it is the predominant ionic species in solutions.

Correlation of acoustically detected thermal waves with injected and photogenerated currents in a photovoltaic cell

Acoustic detection of thermal waves emanating from a photovoltaic cell are employed to quantitatively distinguish between two power dissipation mechanisms, namely, space-charge thermalization of photogenerated carriers and recombination of injected carriers in the base region. The method can aid in quantitative calorimetric investigations of power-loss mechanisms in photovoltaic devices.

Electrothermal measurements: A calorimetric method to examine power dissipation in photovoltaic devices

Power loss mechanisms in illuminated and nonilluminated photovoltaic cells are investigated by an ac calorimetric method. In this electrothermal (ET) method, periodic temperature changes, caused by periodic electrical excitation, are sensed as a function of external parameters, e.g., applied voltage. ET signals on crystalline silicon and thin‐film CuInSe2/CdS solar cells are measured as a function of applied voltage. The results are compared to those obtained from an energy balance model. Inherent to the model is the occurrence of injected carrier cooling. The ability of ET measurements to separate different power loss mechanisms is discussed and compared to that of conventional photothermal ones (e.g., photoacoustic).

Power Dissipation Mechanisms in Photovoltaic Cells

Fi.um (photo)acoustic and related measurements we can infer the relative contributions of several of the mechanisms that contribute to the total heat dissipation in a solar cell, including local cooling effects.

Computer Simulation of the Photoacoustic Signal of Photovoltaic Cells

Abslrucf-Photoacoustic measurements provide calorimetric means for characterizing a photovoltaic cell. How photoacoustic measurements performed at low audio frequencies can be applied to the study of energy loss mechanisms in a solar cell is shown. Results from a computer simulation of the photoacoustic signal of a photovoltaic device are compared with the experimental data. The advantages of working at higher irradiation frequencies are discussed. Finally, results of the acoustic signal obtained when an ac voltage is applied to a photovoltaic cell are presented.

Investigation into the photocurrent quadrature signal of photoelectrochemical cells

It is well known that for certain photoelectrochemical cells, the flat‐band voltage calculated from the photocurrent onset is actually false and can differ from the true flat‐band voltage by several hundred millivolts. We have observed this phenomenon for the cadmium selenide/aqueous polysulfide, as well as for other photoelectrochemical cells. In addition, we have observed a phase shift in the photocurrent signal at the voltage where the false onset occurs and we attribute this shift to the electronic filling of surface states. A kinetic model is used in conjunction with an equivalent circuit in order to verify this hypothesis. Results of the theoretical model agree satisfactorily with experimental data. Experimental guidelines for increasing the reliability of photocurrent onset measurements are presented.

Photoconductivity of Rh(I) aryldiisocyanide polymers and oligomers

Abstract We have shown that stacked-layer. three dimensional Rh(1) organometallic coordination polymers having bifunctional bridging aryl diisocyanides and interlayer Rh—Rh interactions demonstrate photoconductive properties. Thin films of these materials can be prepared from dilute solutions containing small oligomers. These have revealed photoconductivities in the visible and near infrared regions.