The potential of polycrystalline silicon for large scale terrestrial photovoltaic device application is well recognized. It shares with single-crystal silicon numerous desirable physical and chemical properties, while also showing great promise of reduced cost by circumventing many of the complex and energy intensive steps associated with growth of single crystals. These facts have prompted many scientists to investigate various forms and shapes of polycrystalline silicon. The results involving SIS, MIS and p/n junction devices will be reviewed. Results will also be presented to show how grain boundaries play a dominant role in determining the electrical and photovoltaic properties of polycrystalline silicon.

Various photovoltaic loss-mechanisms associated with defects and grain boundaries (g.b.'s) in polycrystalline silicon have been experimentally studied. Analysis was carried out on two types of substrates/cells viz. Wacker Silso and laser crystallized RTR ribbons. Solar cells were fabricated on selected regions of the substrates and their characteristics (V_OC, I_SC, F.F.) related the substrate structure. Mechanisms related to photovoltaic losses are divided into two categories: electronic and physical. Parameters describing electronic loss mechanisms, such as changes in minority carrier diffusion length, dark current and local photo-current losses were measured and their dependence on density and type of defects was determined. A variety of analytical techniques were used for this study. These include I-V characterization of solar cells, I-V characterization of g.b.'s and light intensity dependences of some material parameters. Loss mechanisms associated with physical effects are defect-defect and impurity-defect interactions. It is shown that physical effects such as impurity segregation and defect annihilation can lead to significant loss/gain in photovoltaic characteristics.

One of the indicators which determine a material's potential for use as a solar cell is the minority carrier diffusion length (L_D) of the material. To determine L_D a surface photovoltage (SPV) technique is used. This method is dependent upon an accurate knowledge of the optical absorption coefficient as function of wavelength. Most of the work to date in this area has involved Czochralski grown material which has resulted in two empirical models for the absorption coefficients. However, with the advent of new growth processes, for example ribbon growth and cast silicon, it became necessary to measure the absorption coefficients of these materials before a SPV measurement could be performed. The results for the absorption coefficients for various types of silicon sheet material are compared to those previously used in the two models. The resultant effect upon the diffusion length is also discussed in detail.

Opto-electronic properties of the various silicon forms were obtained from the solar cell performance. Performance parameters included the photovoltaic characteristics, minority carrier diffusion length, spectral response and small light spot scanning. Self consistent results were obtained from the various measurement techniques, which can provide useful information on sheet quality, as a potential for low cost terrestrial applications, and areas where the sheet formation methods can be improved.

We discuss the effect of oxides on the forward and reverse bias J-V characteristics in a-Si:F:H MIS type devices. Data are also presented for the variation of the open circuit voltage with the work function of the Schottky contact. We conclude that the density of interface states is low.

Many N⁺P INJ P silicon solar cells made with silicon from different growth techniques have current-voltage relations of the form: I = I₀ [exp(qV/AkT) - 1] where the quality factor A is non-integral, is >1 and shows a temperature dependence. The dark forward characteristics of such cells have been measured over a range of temperatures and the behavior of the factor A derived from them. Lack of agreement with previous models has led to the development of a new model in which N⁺ conduction electrons tunnel to deep levels near that side, these levels being due to junction contamination by impurities. Electron recombination then occurs with holes thermally assisted into the junction from the P side. This mechanism involves increased I₀ values over those for diffusion diode processes and thus reduces the cell power conversion efficiency.

The current status of non-silicon photovoltaic solar cells is discussed including the identification of current technical and economic issues and future research directions for potential high efficiency low cost technologies. This review covers such advanced materials as CdS/Cu₂S, CdS/CuInSe₂, and GaAs homojunction and heterojunction devices; such emerging materials as InP, Zn₃P₂ and CdTe; and liquid junction electrochemical photovoltaic cells. An attempt is made to compare the current relative status of these various technologies and to indicate their near term potential where possible.

The ultimate expected efficiencies of thin film amorphous solar cells may be raised by using amorphous alloys of germanium and silicon. To date, no single measurement technique has appeared that directly and decisively identifies and characterizes the key photovoltaic related phenomena occuring in solar cell grade amorphous semiconductors. In this presentation, an ultra high speed electrooptical approach to the measurement of the optical and electrical properties of amorphous semiconducting thin films is described. Results of measurements performed to date on amorphous germanium alloys are presented and the implications of the data regarding solar cell applications are discussed.

The development of a polycrystalline, thin-film solar cell utilizing a heterojunction structure based upon N-type CdS and P-type CuInSe₂ semiconductor materials is described. The cell, prepared entirely by vacuum deposition and sputtering techniques onto inexpensive substrates, has potential applications as a low-cost mass produced device for photovoltaic power generation systems. A device efficiency of 7.5% under simulated AM-1 illumination is reported. Material and device properties pertinent to the development of the high efficiency cell are reviewed. The electrical, optical, and structural properties of the deposited thin-film materials are described. Results of detailed cell characterization using a variety of electrical, optical, and thermal measurements are presented and analyzed in terms of a photovoltaic cell model dominated by interface state recombinations. Finally, the projected, realistically achievable performance of this thin-film cell is discussed.

Auger electron and atomic absorption spectroscopies (AES and AAS) have been used to measure compositions of Cu₂S/ZnxCd_1-xS heterojunctions. A Zn rich region near the Cu2S-ZnCdS interface is verified by both techniques. For Cu2S films grown by aqueous ion-exchange the amount of Zn enrichment at the interface increased with Cu₂S film thickness. Zn, Cd, Cl and 0 were detected by AES on the Cu₂S surface, with the Zn and Cd extending into the Cu₂S bulk. Amounts of Zn and Cd measured in the Cu₂S by means of AAS are less than about 0.1 atomic percent.

The purpose of this paper is to present an extended summary of the oral presentation given at the 24th SPIE Symposium. Details of the rationale for the study, methods, procedures, and results are available.1 The object of this work was to prepare cuprous sulfide films by sulfiding copper in a mixture of hydrogen sulfide and hydrogen, to characterize the films, and to establish that solar cell device grade cuprous sulfide could be prepared by a method that permits characterization of the pure material before, during, and after reaction in degrading environments. Copper films, deposited in vacuum onto a substrate suspended from a quartz ultramicrobalance, were sulfided in mixtures of hydrogen sulfide and hydrogen until the composition cuprous sulfide was reached. With the gravimetric apparatus, the optical transmittance (T) and reflectance (R) were also obtained in situ at intervals during the deposition and sulfiding processes. The sulfided films were characterized by SEM, ISS, ESCA, SIMS, diffraction, profilometry, resistivity, and Hall techniques after removal from the balance. Between 70 and 94°C and with hydrogen/hydrogen sulfide ratios between 11 and 39:1, copper films can be sulfided to cuprous sulfide in 30-36 hours to produce cuprous sulfide films 100 to 400 nm thick. The SEM data show the films are smooth enough to deduce the optical constants from the T and R data. The carrier mobility and optical properties were found to be comparable with those reported for device grade cuprous sulfide. Composition in depth profiles show only copper and sulfur can be detected in the films, except for a trace of surface oxygen. The well-characterized cuprous sulfide films were found to be stable up to 100°C when heated in 13.3 kPa of oxygen, and after a slow reaction up to ca. 225°C, formed cuprous oxide and cupric sulfate at 300°C.

The basic principles of electrochemical photovoltaic cells are reviewed, with emphasis on the semiconductor/electrolyte interface. Systems for direct conversion of solar to electrical energy are compared with those designed for production of chemical fuels. Recent results are presented as examples. The characteristics of electrochemical solar cells are compared with those of solid state devices.

Experimental results on stabilization against photo-induced corrosion of n-Si and n-GaAs in contact with electrolytes are given. Photocorrosion is examined on silicon using voltammetry on GaAs using a rotating ring-disk technique. The most extensive testing of stability was done for n-Si in a solution of N,N,N',N'-tetramethyl-p-phenvlenediamine in methanol and for n-GaAs in an aqueous solution of Fe(II) EDTA. Multiple waves were observed for the oxidation of several organic compounds on illuminated n-Si. To explain this auasi-metallic behavior a model based on an intervening thin surface oxide is postulated. In the case of n-GaAs the influence of mechanical surface damage, pH of the solution and redox couple used was studied. It is found that surface defects areativ enhance the susceptibility of the GaAs to photo-induced corrosion.

Various planar configurations of the luminescent solar concentrator (LSC) are discussed including: the uniformly doped, the stacked plate, the thin film, and the multilayered film LSC. Radiation which is lost from the luminescent plate by falling within the critical angle for total internal reflection is examined in terms of the above configurations. A ten parameter efficiency profile has been developed to better evaluate the collector performance and to allow direct comparison with other photovoltaic devices. A collector efficiency of 3.2% is reported and the feasibility of 8-10% efficient devices discussed. Several optical distribution factors have been defined and experimentally evaluated for the luminescent plate and associated photovoltaic cell. These factors have been used to calculate a plate-to-cell optical coupling coefficient for two commercially available solar cells. LSC's based on organic and inorganic phosphor systems are compared and the problem of dye stability discussed.

A type of solar concentrator for photovoltaics utilizing light pipe trapping of luminescence is described. Total collector efficiencies of 3. 2% have been measured, and efficiencies of 10% appear theoretically possible. The photodegradation lifetime of the dyes presently used is about one year under optimal conditions.

The Solar Cell Test Facilities at Rockwell International are described. The facilities enable the characterization of cells in the dark and under a variety of illumination conditions. A basic automated measurements system has been built around an Analog Devices MACSYM II minicomputer. Computerized dark I-V and C-V tests allow automatic determination of barrier height, diffusion potentials, doping profiles, diode n-factors, and extrapolated reverse saturation currents. The same system is used to measure the illuminated I-V characteristics and determine I_SC, V_OC , F.F., and locate the maximum power point. The facility also contains a computerize spectrophotometer which performs simultaneous measurements of either absolute spectral response and reflectivity or transmissivity and reflectivity. Concentrator solar cells can be characterized at up to 200X insolation using a chopped, concentrated Schoeffel solar simulator. A unique, high concentration test facility utilizing sunlight has also been built.

This paper reports on the design, fabrication and testino of a "dense array" consisting of 16 overlapped linear modules each containing 16 contiguous 1 cm x 1.25 cm GaAs concentrator cells in a row. The overlapping is done so that only active cell area is presented to the concentrated sunlight. The array presents a frontal area of 320 cm² and is designed to yield a system output of ~230 volts at 20-25 amps at 1000 SUNs. The results of tests carried out at Sandia's Central Receiver Test Facility at concentrations up to 1500 SUNs and temperatures to 185° C will be presented.

Varian Associates is engaged in development and pilot production of a wide variety of compound semiconductor photovoltaic devices. The various facets of this work are described. Included is pilot production of AlGaAs/GaAs solar cells, various spectrum splitting devices and approaches, and a monolithic integrated GaAs solar cell array.

The promise of multicolor (multiple bandgap) solar converters for achieving conversion efficiencies in excess of 25% in space or 30% in terrestrial applications has been widely discussed. We describe current results of efforts to reach these performance goals with two cell monolithic stacks containing an intercell ohmic contact. Basic requirements for the successful fabrication of high efficiency tandem structures are defined, and the apparent practical limitations on the formation of such multiple bandgap solar cells by presently available techniques are identified. Experimental tandem or "cascaded" structures made by M0-CVD, LPE and MBE growth techniques in various materials combinations are described and current performance results are presented.

Applications of complementary surface analysis techniques (AES, SIMS, XPS) to solar cell device problems are discussed. Several examples of device interface and grain boundary problems are presented. Silicon, gallium arsenide and indium phosphide based devices are reviewed. Results of compositional and chemical analysis are correlated directly with EBIC measurements performed in-situ on identical sample areas. Those are, in turn, correlated with resulting photovoltaic device performance. The importance of microanalysis to the solution of critical device problems in the photovoltaics technology is emphasized.

This paper reviews several aspects of optics involved in the design of solar cells.

Standard measurements of photovoltaic parameters are typically analyzed by modeling the solar cell as a two-terminal device with spatially uniform characteristics. These measurements give the average response of the cell, but provide little information on local features and flaws which are important in determining cell quality. In this paper we describe an instrument which measures and displays the response of the solar cell to a precisely positioned spot of HeNe laser light. By scanning the spot across the cell surface we can create a map of the spatial variation in response of the cell. This map allows us to isolate flaws in cell contact integrity, locate open top surface grid lines, and evaluate fundamental junction performance. The system is useful for identifying and locating changes in the cell as it progresses through various experiments (e.g. stability studies). The laser scanner system is designed to be flexible and can accomodate different types of solar cell materials and a wide range of spot and scan sizes. We describe several modes of operation of the equipment, and present results from two photovoltaic materials (CdS/Cu₂S and Zn₃P₂) which demonstrate the capabilities of the system. Finally we discuss some of the proposed future uses of the system.

The conductivity of a thin film on a metal plate can be calculated from measurements of the Q of the cavity of a Fabry-Perot interferometer and the thin film if the cavity without the film has been previously measured. The theory for the conductivity which shows a cubic dependence on the thickness of the film has been developed. The measurement system at 93 gigahertz is described. Experimental results are presented.

Simultaneous measurements using several types of remote sensing techniques have been analyzed and combined to yield data sets on the optical properties of clouds and aerosols. The parameters observed include (1) extinction at nine wavelengths in the 0.3-to-10.0 μm wavelength range as measured by solar radiometers, (2) the backscatter coefficient and depolarization as a function of range as measured by a pulsed ruby lidar, (3) the angular scattering function out to 8° from the sun as measured by an aureole photometer, and (4) the visual characteristics and identification of sky conditions obtained through time-lapse sky photography. The data are sorted according to the type of cloud or aerosol present as determined by lidar backscatter information (height profile and depolarization characteristics) and visual identification. Some results on optical properties are presented including the wavelength dependence of aerosol extinction, and preliminary data on angular scattering in the near-forward direction and wavelength dependence of extinction due to several types of clouds. Practical applications of the data include their use in design of optical propa-gation links and solar energy systems, and as input to computer models of atmospheric transmission.