Engineering topics
Publications and source records attributed to Spitzer, M. B..
A review of the status of InP cell efficiency and of approaches to the reduction of cell cost is presented. The use of heteroepitaxial techniques such as InP-on-GaAs and InP-on-Si is discussed along with the use of chemical and mechanical techniques for removal and recovery of the substrate. The efficiency ultimately obtainable with designs made possible by such an approach is calculated.
Thin-film AlGaAs-GaAs double-heterostructure concentrator cells were fabricated which exhibit total-area conversion efficiencies as high as 23.5 percent AM0 at 100 suns, 25 C. This is one of the best space concentrators measured to date at NASA and is designed for a thin-film cell without a prismatic coverglass. This solar cell structure consists of a GaAs/AlGaAs film less than 5 micron thick mounted to a glass cover/superstrate, with coplanar back-side contacts. The coverglass is not prismatic. The CLEFT process, a method for mechanically separating epitaxial layers from their substrate, is used to process these cells into thin films. The advantages of single-crystal GaAs are thereby retained, while reducing weight and cutting cost by allowing for substrate reuse. Thin-film cells also have better thermal management capabilities and can be stacked for use in tandem structures. Cell fabrication and performance are described, and directions for further improvements are identified.
The development of advanced GaAs concentrator solar cells, and in particular, the use of CLEFT (cleavage of lateral epitaxial films for transfer) processes for formation of thin-film structures is reported. The use of CLEFT has made possible processing of the back, and cells with back surface grids are discussed. Data on patterned junction development are presented; such junctions are expected to be useful in back surface applications requiring point contacts, grating structures, and interdigitated back contacts. CLEFT concentrator solar cells with grids on the front and back surfaces are reported here; these cells are 4 microns thick and are bonded to glass covers for support. Air mass zero efficiency of 18.8 percent has been obtained for a CLEFT concentrator operating at 18.5 suns.
The fabrication of highly efficient indium phosphide solar cells by metalorganic chemical vapor deposition is reported. Since InP can be annealed at temperatures at low as approximately 100 C and the annealing occurs under solar illumination itself, this accomplishment is relevant to space power systems. A total area air mass zero efficiency of 17.9 percent and an air mass 1.5 efficiency of 20.4 percent are reported. Electrical characterization identifying loss mechanisms is made, and a shallow homojunction design is discussed along with possible improvements.
On-going development of indium phosphide solar cells for space applications is presented. The development is being carried out with a view towards both high conversion efficiency and simplicity of manufacture. The cell designs comprise the ion-implanted cell, the indium tin oxide top contact cell, and the epitaxial cell grown by metal organic chemical vapor deposition. Modelling data on the limit to the efficiency are presented and comparison is made to measured performance data.
This paper reports the development of a flat-plate module based on highly efficient ion-implanted silicon solar cells. The cells developed in this work are 53 sq cm in area and have exhibited AM 1.5 efficiencies of over 18 percent. The use of back surface reflectors to reduce module operating temperature is discussed. A comparison of various approaches to cell design is made, and module test data for these designs are presented. A comparison of float zone and Czochralski silicon is made. Module efficiency of over 15 percent (at 25 C) is reported.
This paper reviews ongoing research aimed at the attainment of highly efficient silicon solar cells. The importance of low-recombination highly-doped n(+) and p(+) regions and the manner in which such regions are fabricated are discussed. Theoretical light-trapping considerations are combined with experimental reflectance data to show that high quantum efficiency may be obtained from thin (100-micron) cells. The principal finding of this work is that thin solar cells with conversion efficiencies of over 20 percent may be fabricated if recombination at the front and back metal/silicon interfaces is reduced. Large-area cells (53 sq cm) with an efficiency of 18 percent are reported.
Success was reported in meeting the long-sought goal of manufacturing a crystalline silicon module with efficiency of 15% or better. Encapsulated cell efficiencies as high as 17.6% were obtained.
A development status evaluation is made for high efficiency Si solar cells, with emphasis on the suppression of the deleterious effects of surface recombination. ZnS(0.9)Se(0.1) and GaP are identified as candidates for the reduction of surface recombination. Attention is given to methods developed for the deposition of heteroepitaxial compounds designed to block minority carrier transport to the Si solar cell surface without interfering with the majority carrier flow.
Pulsed excimer laser annealing was successfully performed using a 50 w laser. Both polished and texturized cells were tried, however, there are serious problems with nonuniformity on texturized cells. A number of cells were produced and compared to diffusion furnace annealed cells. There was no clear economic advantage in using an excimer laser and there was a small penalty on average efficiency. The conclusion was that the excimer laser anneal process must be able to produce superior cells to be considered as a viable process option.
Progress on the development of a high efficiency module is described. The effort includes development of high efficiency cells using crystalline silicon wafers from float zone silicon. Module-size cells, 53 sq cm in area, were fabricated with efficiency of 18%. Operating temperature reduction is also studied.
Recent advances in silicon solar cell processing have led to attainment of conversion efficiency approaching 20%. The basic cell design is investigated and features of greatest importance to achievement of 20% efficiency are indicated. Experiments to separately optimize high efficiency design features in test structures are discussed. The integration of these features in a high efficiency cell is examined. Ion implantation has been used to achieve optimal concentrations of emitter dopant and junction depth. The optimization reflects the trade-off between high sheet conductivity, necessary for high fill factor, and heavy doping effects, which must be minimized for high open circuit voltage. A second important aspect of the design experiments is the development of a passivation process to minimize front surface recombination velocity. The manner in which a thin SiO2 layer may be used for this purpose is indicated without increasing reflection losses, if the antireflection coating is properly designed. Details are presented of processing intended to reduce recombination at the contact/Si interface. Data on cell performance (including CZ and ribbon) and analysis of loss mechanisms are also presented.
The results of a new research on the use of controlled dopant profiles and oxide passivation to achieve high open circuit voltage V sub oc in silicon solar cells is presented. Ion implantation has been used to obtain nearly optimal values of surface dopant concentration. The concentrations are selected so as to minimize heavy doping effects and thereby provide both high blue response and high V sub oc ion implantation technique has been successfully applied to fabrication of both n-type and p-type emitters. V sub oc of up to 660 mV is reported and AMO efficiency of 16.1% has been obtained.
This paper reports the development of a flat-plate module based on highly efficient ion-implanted silicon solar cells. The cells are 53 sq cm in area and have exhibited AM 1.5 efficiencies of over 18 percent. The use of back surface reflectors to reduce module operating temperature is discussed. A comparison of various approaches to cell design is made, and module test data for these designs are presented. Module efficiency of 14 percent (25 C) is reported.
The fabrication of high efficiency solar energy conversion modules (13% efficiency at NOCT - Nominal Operating Cell Temperature) is discussed, with emphasis placed on reducing NOCT, since reduced operating temperature improves both efficiency and module lifetime.
The results of heteroepitaxial growth of GaAs and GaAlAs directly on Si are presented, and applications to new cell structures are suggested. The novel feature is the elimination of a Ge lattice transition region. This feature not only reduces the cost of substrate preparation, but also makes possible the fabrication of high efficiency monolithic cascade structures. All films to be discussed were grown by organometallic chemical vapor deposition at atmospheric pressure. This process yielded reproducible, large-area films of GaAs, grown directly on Si, that are tightly adherent and smooth, and are characterized by a defect density of 5 x 10(6) power/sq cm. Preliminary studies indicate that GaAlAs can also be grown in this way. A number of promising applications are suggested. Certainly these substrates are ideal for low-weight GaAs space solar ells. For very high efficiency, the absence of Ge makes the technology attractive for GaAlAs/Si monolithic cascades, in which the Si substrates would first be provided with a suitable p/n junction. An evaluation of a three bandgap cascade consisting of appropriately designed GaAlAs/GaAs/Si layers is also presented.
The status of an ongoing DOE program to develop an AM1 photoelectric module with 15 percent conversion efficiency at normal heating temperatures, is reviewed. Emphasis is given to the efforts of a private company to develop a high efficiency module which also has high durability in normal operating conditions. The main design options considered are: high efficiency modules; large area modules; and optimized module fabrication techniques. The design of an automatic system for encapsulating module stacks is described.
The objective of this program is the investigation and evaluation of the capabilities of the ion implantation process for the production of photovoltaic cells from a variety of present-day, state-of-the-art, low-cost silicon sheet materials. Task 1 of the program concerns application of ion implantation and furnace annealing to fabrication of cells made from dendritic web silicon. Task 2 comprises the application of ion implantation and pulsed electron beam annealing (PEBA) to cells made from SEMIX, SILSO, heat-exchanger-method (HEM), edge-defined film-fed growth (EFG) and Czochralski (CZ) silicon. The goals of Task 1 comprise an investigation of implantation and anneal processes applied to dendritic web. A further goal is the evaluation of surface passivation and back surface reflector formation. In this way, processes yielding the very highest efficiency can be evaluated. Task 2 seeks to evaluate the use of PEBA for various sheet materials. A comparison of PEBA to thermal annealing will be made for a variety of ion implantation processes.