The adage "America's business is business" certainly applies to the development of photo-voltaics as part of this country's mixed but balanced energy sources.

The photovoltaic (PV) industry is operating within two technology groups: those based on single crystal materials and those based on thin films. The former is substantially mature, while the latter relatively new and developing rapidly. Industry's interests are driven by market response in a range of applications which are progressively broadening. Lower cost and higher performance are the keys to industrial development. These qualities are emerging in the thin film technology spectrum.

The desire for high solar cell efficiencies has been a strong factor in determining the course of recent silicon crystal growth research efforts for photovoltaics. This review, therefore, focuses on single-crystal, dislocation-free ingot growth methods (Czochralski growth, float zoning, and cold crucible growth) and on sheet growth technologies, generally multicrystalline, that have achieved moderately high (>13.5%) laboratory-scale efficiencies. These include dendritic web growth, growth from capillary dies, edge-supported pulling, ribbon-against-drop growth, and a recent technique termed crucible-free horizontal growth. Silicon ribbon crystals provide a favorable geometry and require no wafering, but they contain defects that limit solar cell performance. Growth processes, their current status, and cell efficiencies are discussed. Silicon material process steps before and after crystal growth are described, and the advantages of silicon are presented.

Low-cost, high-efficiency solar cells are the key to large-scale applicability of photo-voltaic systems. Recent calculations done by DOE/EPRI suggest that flat plate modules will require at least 15% efficiency at a cost of -50c/watt in order to economically compete with conventional energy sources. Greater than 18% efficient solar cells will therefore be required to achieve this goal because once the cells are placed inside the module, there are additional losses due to reflection from the glass, inefficient packing factor, and mismatch in the cell properties. The purpose of this paper is two-fold; firstly, to review the approaches that have recently produced >18% efficient silicon cells and, secondly, to address some of the remaining issues and challenges that can improve our understanding further and lead to even higher efficiency silicon cells.

Concepts for obtaining practical solar cell modules with one-sun efficiencies up to 30% at air mass 1 are now well understood. Such high-efficiency modules would utilize multibandgap structures. To achieve module efficiencies significantly above 30%, it will be necessary to employ different concepts, such as spectral compression and broad band detection. A detailed description of the concepts for the design of high-efficiency multibandgap solar cells is given.

The possibility of obtaining power conversion efficiencies in excess of 30 percent from practical multijunction concentrator solar cells has stimulated research on several approaches to achieve this goal. This paper will review the important material and fabrication issues associated with each viable approach with the aim of helping to provide additional perspective in assessing this research.

On the basis of a simple energy-cost model, we estimate that photovoltaic (PV) concentrator systems will require sunlight concentration ratios in the hundreds in order to meet the cost goals for large-scale power generation. We address the design requirements, presented by these high concentrations, for cells and packaged-cell devices to meet the heat transfer needs, to achieve acceptable electrical and optical losses, and to provide mechanical stability against environmentally induced stresses. Ohmic and optical loss considerations provide a basis for contact grid design. Heat transfer and mechanical stability considerations help define cell-package design options and can present conflicting requirements which may require trade-off evaluations.

A review is given on recent progresses of amorphous silicon device technologies and its applications to solar cell. Firstly, some unique advantages of amorphous silicon as a new electronic material are pointed out with demonstrating some tangible examples in the live technology. Secondly, some topics in the R&D efforts on new film deposition trials related to high processing rate are introduced. Then, approaches to improve photovoltaic performances utilizing the optical and carrier confinement effects with new amorphous materials such as amorphous SiC, SiGe and micro-crystalline Si are demonstrated. In the final part, progress of the a-Si solar cell performances is also summarized and discussed.

Amorphous silicon (a-Si) based alloys have attracted a considerable amount of interest because of their applications in a wide variety of technologies. However, the major effort has concentrated on inexpensive photovoltaic device applications and has moved from a laboratory curiosity in the early 1970's to viable commercial applications in the 1980's. Impressive progress in this field has been made since the group at University of Dundee demonstrated that a low defect, device quality hydrogenated amorphous silicon (a-Si:H) 12 material could be produced using the radio frequency (r.f.) glow discharge in SiH4 gas ' and that the material could be doped n- and p-type.3 These results spurred a worldwide interest in a-Si based alloys, especially for photovoltaic devices which has resulted in a conversion efficiency approaching 12%. There is now a quest for even higher conversion efficiencies by using the multijunction cell approach. This necessitates the synthesis of new materials of differing bandgaps, which in principle amorphous semiconductors can achieve. In this article, we review some of this work and consider from a device and a materials point of view the hurdles which have to be overcome before this type of concept can be realized.

The characterization necessary to account for the physical properties and device performance of any semiconducting material is presented. This can be divided into five major categories. The results of intensive studies of hydrogenated amorphous silicon thin films are reviewed in detail. It is concluded that many important characteristics of these commercially important materials are still not known with any degree of certainty.

CuInSe2 has unique optical and electronic properties which make it a prime candidate for low-cost high efficiency thin-film polycrystalline solar cells. Within a decade of the first experiments with thin-film solar cells efficiencies had exceeded 10% and already pre-commercialization efforts are underway. The status and prospects of single junction CuInSe2 based solar cells are reviewed and the potential extension into multijunction configurations considered.

Electric power generated by cadmium telluride (CdTe) polycrystalline thin-film solar cells may potentially be cost-competitive with power from conventional sources such as oil, natural gas, coal, and nuclear. The polycrystalline CdTe solar cell technology has recently achieved reported efficiencies of over 10% in several laboratories, and large-area cells can be deposited by several potentially low-cost methods. This paper describes cadmium telluride material properties, electronic properties (especially doping and contacting), CdTe alloys and their potential usefulness, solar cell structures in which CdTe can be an important component, and the status of several efforts to optimize the performance of polycrystalline CdTe solar cells deposited by potentially low-cost technologies.

The work described in this paper includes some aspects of the long-range, high-risk research being conducted at the Solid State Photovoltaic Research Branch at SERI. The areas covered include: amorphous silicon, high efficiency, multifunction solar cells, CdS/CuInSe2 thin films, crystal growth research and solid state theory. In each of these areas specific research projects which have long-range implications are highlighted in this review.