Robert B. Hall

The design and fabrication of thin-film CdS/Cu 2 S cells of 9.15-percent conversion efficiency

Thin-film polycrystalline CdS/Cu2S cells with energy conversion efficiencies in sunlight of up to 9.15 percent and areas of ∼1 cm2have been developed. The improvement over previously achieved efficiencies is due to the development of techniques to separately measure and minimize fill factor losses. Specific design and fabrication changes based on a detailed quantitative analysis of the cell operation, were introduced to correct series resistance, shunt conductance and field effect losses. Further increases in efficiency can be expected from the development of a planar junction thin-film CdS/Cu2S cell.

Thin‐film polycrystalline Cu2S/Cd1−xZnx S solar cells of 10% efficiency

Polycrytalline, thin-film Cu/sub 2/S/Cd/sub 1-x/Zn/sub x/S heterojunction solar cells with conversion efficiencies of 10% have been prepared on Cd/sub 1-x/Zn/sub x/S with 0.1< or =x< or =0.2. Light-generated currents of up to 26 mA/cm/sup 2/ (prorated to 100 mW/cm/sup 2/) have been achieved, comparable to the best observed in Cu/sub 2/S/CdS cells of the same design. The improved performance for Cu/sub 2/S-based devices is as a consequence of the higher open-circuit voltage achieved with the addition of zinc.

The design and fabrication of high efficiency thin film CdS/Cu2S solar cells☆

Abstract Thin film photovoltaic cells of CdS / Cu 2 S which exhibit conversion efficiencies in excess of 9% have been designed and fabricated. Specific cell designs are prepared from an analysis of optical and electronic loss mechanisms operative in the cell. Material and engineering modifications to the fabrication process are then made to minimize specific energy conversion losses. The present cell design consists of five thin film layers which are sequentially prepared on a copper substrate 35 μm thick. In addition to the material control required for each component layer, the electrical, chemical, mechanical and topological compatibilities at the interfaces between each adjoining layer must be assured to achieve the desired cell performance. Our present analysis shows that a fully optimized solar cell based on a CdS/Cu 2 S junction will have a practical conversion efficiency limit of about 11%. It is anticipated that practical conversion efficiencies of 14–15% can be achieved utilizing a (CdZn)S/Cu 2 S junction designed to produce the maximum open-circuit voltage possible using Cu 2 S as the absorbing layer. Present cell results which incorporate this design are presented.

Thin-film polycrystalline Cu/sub 2/S/Cd/sub 1-x/Zn/sub x/ S solar cells of 10% efficiency

Polycrytalline, thin-film Cu/sub 2/S/Cd/sub 1-x/Zn/sub x/S heterojunction solar cells with conversion efficiencies of 10% have been prepared on Cd/sub 1-x/Zn/sub x/S with 0.1< or =x< or =0.2. Light-generated currents of up to 26 mA/cm/sup 2/ (prorated to 100 mW/cm/sup 2/) have been achieved, comparable to the best observed in Cu/sub 2/S/CdS cells of the same design. The improved performance for Cu/sub 2/S-based devices is as a consequence of the higher open-circuit voltage achieved with the addition of zinc.

Reactive sputtered copper indium diselenide films for photovoltaic applications

Single phase chalcopyrite CuInSe2 coatings have been deposited by reactive cosputtering from Cu and In planar magnetron sources operated in an Ar+H2Se working gas. Effective sputtering yields from the conditioned Cu and In targets were approximately 0.7 and 0.5 atoms/unit charge, respectively. Sputtering rate, H2Se injection rate, and H2Se and H2 partial pressure measurements were consistent with the overall reaction Cu+In+2H2Se→CuInSe2+2H2. The formation of near‐stoichiometric coatings appears to be aided at elevated temperatures by a reemission mechanism which removes excess In. Photovoltaic devices formed by evaporating CdS onto the sputtered CuInSe2 yielded short circuit currents of about 33 mA/cm2 and efficiencies of about 4%.

Thin-film silicon crystal growth on low cost substrates

Abstract Thin films of crystalline silicon are being grown by us on dissimilar substrates for photovoltaic solar cell application. The approach offers the potential to combine the high performance and stability of crystalline silicon with the low cost of thin films. Growth from saturated solution is being used to meet the specific requirements of large grain growth, benign grain boundaries, and long minority carrier diffusion length. For a good quality crystalline overlayer on a substrate, the growth process consists of five steps: (1) wetting, (2) nucleation, (3) non-impinging crystal growth, (4) fill-in crystal growth, and (5) homoepitaxial film growth. Silicon films with crystals that have a width to thickness ratio of greater than 2 to 1 and an overall thickness of 20μm have been grown on steel and quartz substrates. These films have demonstrated the required morphological characteristics for a high performance thin-film polycrystalline silicon solar cell.

Multilayer Ohmic Contacts on CdS

A multilayer technique for evaporation of Ohmic contacts onto CdS is described. The electrical properties of these contacts do not change markedly after vacuum heat treatment up to 350°C. This technique consists of a sequential evaporation of a preparative layer, an active metal and possibly a covering metal. The Ti (preparative)‐Al (active)‐Pt (cover) sequence has been found most successful. All the more than forty evaporations investigated on CdS single crystals, or on evaporated recrystallized layers, showed Ohmic characteristics between 2 mV and 200 V and showed only generation‐recombination noise above (at most) 300 Hz.

Analysis of capacitance‐voltage measurements on heat‐treated Cu2−xS/CdS heterojunctions

The capacitance‐voltage characteristics of a p‐type (metalllike)/n‐type semiconductor junction are described in terms of a simple three‐region space charge in the semiconductor. The assumed space charge consists of a narrow (∼100 A) high‐density space charge at the interface, followed by an extended low‐density space charge (insulating layer), and finally the bulk space charge. The calculations assume that the equilibrium space‐charge density does not change with applied reverse voltage. The electric field at the junction is calculated analytically, and the electrostatic potential is calculated for the case of abrupt junctions between the various space‐charge regions. The results indicate effective barrier‐height lowering as a consequence of interface charges, even in the presence of the insulating layer which dominates capacitance‐voltage measurements. An interpretation is given to the slope and voltage‐axis intercept of (1/C)2‐vs‐V plots for a variety of special cases of the general space‐charge distrib...

Light trapping in Silicon-Film™ solar cells with rear pigmented dielectric reflectors

This paper presents the novel method of using pigmented dielectric reflectors to provide light trapping in thin-film silicon solar cells. This type of reflecting material offers many potential advantages over specular metallic reflectors, including low cost, compatibility with high temperatures common to solar cell processing, and high, broadband and diffuse reflectance. As this is the first time this concept is described, the basic theory of the optical behavior of pigmented materials is presented by connecting the basic material properties of pigment and medium to their light-trapping benefit in thin silicon solar cells. Several general principles leading to maximum light-trapping benefit are identified, and experimental evidence is presented corroborating these general principles. Light trapping is demonstrated in thin silicon solar cells with pigmented dielectric reflectors by measurement and analysis of external quantum efficiency curves. Copyright

Photodecomposition and electronic structure of lead azide

Photodecomposition, optical absorption, and conductivity measurements were performed on thin films of lead azide in order to determine interrelation between electronic structure and chemical instability. Both pure lead azide and thallium and bismuth doped films were investigated. The azide films are prepared by chemical conversion of evaporated metallic films by the vapor of hydrazoic acid and are found to consist of 1 μm2 platelets which lie parallel to the substrate and are optically active. No effects of doping on the Fermi level are observed and we conclude that the dopants are self‐compensated by native defects. The observed changes in ultraviolet and infrared absorption during thermal or photodecomposition can be attributed to the formation of azide vacancies. Thallium doping results in changes in optical properties similar to those obtained on decomposition. The initial step in photodecomposition appears to be the creation of charge‐transfer excitons.