Electrochemical photovoltaic cells. Project 65021 final technical progress report, April 15, 1979-April 17, 1980

The overall goals of this program have been to experimentally identify semiconductor photoanode/redox couple systems that (a) show promise of meeting acceptable solar efficiencies using polycrystalline materials, and (b) have the potential for use in conjunction with a suitable cathode in which another redox species is available for reduction. Such an overall system allows for the convenient removal, storage, and later discharge of reaction products through the use of porous, flow-through redox electrodes. Increasing solar energy conversion efficiencies and identifying potentially long-life systems were emphasized. Semiconductor/redox couple systems were selected on the basis of the apparent positions of their conduction and valence band energy levels at the interface, together with the semiconductor decomposition potential. Selected redox couples should possess equilibrium potentials that lie negative of the semiconductor decomposition potential and positive of the semiconductor conduction band. Liquid-junction solar cells were characterized using n-MoSe/sub 2/, n-WSe/sub 2/, n-CdSe, and n-GaAs as photoanodes. Results are presented in detail. (WHK

Electrochemical photovoltaic cells. Project 65021 quarterly technical progress report, July 15-October 15, 1979

During the second quarter of this program we have evaluated the photoelectrochemical performance of single-crystal MoSe/sub 2/ and GaAs, polycrystalline CdSe, and cells with RbAg/sub 4/I/sub 5/ solid electrolyte. MoSe/sub 2/ electrode exhibited very good photoresponse in electrolytes containing Br/sup -//Br/sub 2/ redox couple. The photopotential and photocurrent were larger in acid than in alkaline electrolyte. A power conversion efficiency of about 5% was achieved under 200 mW/cm/sup 2/ Xenon light illumination in the acid medium. The catalytic activity of a platinized platinum electrode toward Br/sup -//Br/sub 2/ redox couple was also better in acid electrolyte than in alkaline electrolyte. Higher current densities were obtained in the acid medium. The MoSe/sub 2/ was found to have good stability in acid and alkaline electrolytes; however, it will be necessary to protect the electrolyte against evaporation of the bromine, oxidation by air, or photodecomposition. GaAs was tested in alkaline electrolytes containing Se/sup 2 -//Se/sub n//sup 2 -/, I/sup -//I/sub 2/ or Br/sup -//Br/sub 2/ redox couple and in acid electrolytes containing Fe/sup +2//Fe/sup +3/, Cr/sup +2//Cr/sup +3/, Sn/sup +2//Sn/sup +4/, Cu/sup +1//Cu/sup +2/, Ti/sup +3//Ti/sup +4/, I/sup -//I/sub 2/ or Br/sup -//Br/sub 2/. Large anodic dark currents were observed in solutions containing Sn/sup +2//Sn/sup +4/ or Cu/sup +1//Cu/sup +2/. The large photoeffects found with the other redox couples were usually accompanied by electrode darkening and decrease in photocurrent with time. Polycrystalline electrodes have been made by thermal vacuum evaporation of CdSe on titanium foil or SnO/sub 2/-coated glass. The CdSe still suffered from poor adherence to the substrates and, therefore, poor photoresponse. A solid electrolyte photoelectrochemical cell was fabricated giving photopotentials of about 300 mV at current densities of about 0.1 mA/cm/sup 2/ under 200 mW/cm/sup 2/ Xenon light illumination

Electrochemical photovoltaic cells. Project 65021 quarterly technical progress report, April 15-July 15, 1979

The necessary equipment for evaluation of candidate photoelectrochemical systems has been assembled. Preliminary experimental work has been initiated on the following semiconductor materials: CdS, CdSe, CdTe, GaAs, and MoSe/sub 2/. Single crystals of MoSe/sub 2/ have been evaluated in alkaline as well as acid electrolyte in the presence of a variety of redox couples. Large photocurrents of about 10 to 20 mA/cm/sup 2/ have been observed under illumination with Xenon light (200 mW/cm/sup 2/) in the presence of I/sup -//I/sub 2/, Br/sup -//Br/sub 2/, Cu/sup +1//Cu/sup +2/, and Sn/sup +2//Sn/sup +4/ in the electrolyte. Only small photocurrents of about 1 mA/cm/sup 2/ were seen when the electrolyte contained Sb/sup +3//Sb/sup +5/ or Fe/sup +2//Fe/sup +3/ redox couple. Work has been initiated on the photoelectrochemical properties of polycrystalline materials. Polycrystalline materials were fabricated in the laboratory using vacuum evaporation, sputtering, and electrodeposition techniques. Satisfactory CdSe and CdS photoelectrodes have been fabricated in this way. Heat treatment appears to be a critical step in improving the performance of these electrodes. Preliminary chemical corrosion tests on GaAs, GaP, CdSe, CdTe, MoSe/sub 2/, and MoS/sub 2/ were performed in alkaline and acid electrolytes. Materials that appeared stable after this test were then subjected to a photoelectrochemical corrosion test in electrolyte containing a variety of redox couples. The solid electrolytes, RbAg/sub 4/I/sub 5/ and Na/sub 2/O.11A1/sub 2/O/sub 3/, have been obtained. Photocurrents and photopotentials have been observed in cells of configuration iron oxide, PbI/sub 2/, RbAg/sub 4/I/sub 5//RbAg/sub 4/I/sub 5//C, PbI/sub 2/, and RbAg/sub 4/I/sub 5/

Electrochemical photovoltaic cells. Project 65021 quarterly technical progress report, October 15, 1979-January 15, 1980

During the third quarter of this program, liquid junction devices based upon the semiconductors MoSe/sub 2/, MoS/sub 2/, GaAs, and CdSe have been evaluated. Lifetime testing of MoSe/sub 2/ and MoS/sub 2/ materials in acidic halogen electrolytes at constant current densities of 5 mA/cm/sup 2/ have shown excellent stability to date. For MoSe/sub 2/ single crystals in the electrolyte 1M HBr + 1M Br/sub 2/, short-circuit currents of 63 mA/cm/sup 2/ were achieved with a power conversion efficiency of 6.7% for 200 mW/cm/sup 2/ xenon light illumination. Transient potentiostatic measurements made on MoSe/sub 2/ in this electrolyte indicated little diffusion control, with exchange currents being of the order of 1 to 10 mA/cm/sup 2/. Good photoresponse of MoS/sub 2/ has been observed in 1M HBr + 1M Br/sub 2/. The performance of the natural crystal is comparable to the performance of a single-crystal MoS/sub 2/ in this electrolyte. CdSe thermally evaporated onto porous titanium gave efficiencies of about 4% with 100 mW/cm/sup 2/ xenon illumination. Experimental work was initiated on the dye sensitization of Fe/sub 2/O/sub 3/ and TiO/sub 2/ materials. Of the twelve dyes evaluated, little enhancement of the photoresponse of these materials was noted. Solid-state photoelectrochemical cells have been fabricated, based upon LiI. Cells of the configuration - cond. glass CdSe/LiI + PbI/sub 2//LiI/LiI + C + PbI/sub 2//cond. glass - were fabricated. Photoresponses up to 150 mV were observed