Showing papers in "Progress in Photovoltaics in 1996"

Silicon solar cells

Abstract: The key attributes for achieving high-efficiency crystalline silicon solar cells are identified and historical developments leading to their realization discussed. Despite the achievement of laboratory cells with performance approaching the theoretical limit, commercial cell designs need to evolve significantly to realize their potential. In particular, the development of cell structures and processes that facilitate entirely activated device volumes in conjunction with well-passivated metal contacts a nd front and rear surfaces is essential (and yet not overly challenging) to achieve commercial devices of 20% efficiency from solar-grade substrates. The inevitable trend towards thinner substrates will force manufacturers to evolve their designs in this direction or else suffer substantial performance loss. Eventually, a thin-film technology will likely dominate, with thin-film crystalline silicon cells being a serious candidate. Present commercial techniques and processes are in general unsuitable for t hin-film fabrication, with even greater importance placed on the achievement of devices with entirely activated volumes (diffusion lengths much greater than device thicknesses), well-passivated metal contacts and surfaces and the important inclusion of li ght trapping. The recent achievement of 21.5% efficiency on a thin crystalline silicon cell (less than 50 μm thick) adds credibility to the pursuit of crystalline silicon in thin films, with a key attribute of this laboratory cell being its extremely good light trapping that nullifies the long-term criticism of crystalline silicon regarding its poor absorption properties and correspondingly perceived inability to achieve high-performance thin-film devices. For low-cost, low-quality polycrystalline sil icon material, the parallel-multijunction cell structure may provide a mechanism for achieving entirely activated cell volumes with the potential to achieve reasonable efficiencies at low cost over the next decade.

21.5% Efficient thin silicon solar cell

Abstract: Although many calculations since the early 1980s have predicted that high performance in thin crystalline silicon cells is feasible, performance levels demonstrated in the past have been quite modest. Using a self-supporting silicon membrane, experimen tal energy conversion efficiency above 20% is described for the first time for a silicon cell of less than 50 μm thickness, with efficiency up to 21.5% independently confirmed for a 47-μm thick device. The cells demonstrate a better ability to tra p light internally within their structure than any previously measured device. They also demonstrate the surface passivation benefits of the recently described parallel multijunction thin-film silicon cell approach.

Scaling-up of CIS technology for thin-film solar modules

Abstract: The compound semiconductor Cu(Ga,In)Se2 (CIGS) and related compounds have demonstrated their high potential for high-efficiency thin-film solar cells up to levels approaching 18%. The Center for Solar Energy and Hydrogen Research (ZSW) and the University of Stuttgart (IPE) are working together on CIS upscaling. On the module basis, ZSW is collaborating with Phototronics Solartechnik GmbH/Putzbrunn as a manufacturer for a-Si modu les supporting module technology. With the aim of developing high-volume fabrication technologies, all the laboratory deposition techniques suitable for highest device performance are applied now also on the module level to avoid physical and chemical effects that could limit device performance to a low level. All film deposition techniques are developed for high-vacuum in-line fabrication on a large area, except for the buffer layer of CdS, and monolithic integration is realized by patterning steps. A key issue for the development of modules is upscaling of the CIGS absorber deposition. Films of CIGS are prepared by simultaneous thermal evaporation of the elements. Modules are prepared on substrate areas of 7 × 7, 10 × 10 ??? 30 × 30 cm2. Actual results of modules of these sizes are shown.

Spatially resolved analysis and minimization of resistive losses in high-efficiency Si solar cells

Abstract: This paper presents an improved method for measuring the total lumped series resistance (Rs) of high-efficiency solar cells. Since this method greatly minimizes the influence of non-linear recombination processes on the measured Rs values, it is possible to determine Rs as a function of external current density over a wide range of illumination levels with a significantly improved level of accuracy. This paper furthermore explains how resistive losses in the emitter, the base, the metal/silicon contacts and the front metal grid can be separately determined by combining measurements and multidimensional numerical simulations. A novel combination of device simulation and circuit simulation is introduced in order to simulate complete 2 × 2 cm2 PERL (‘passivated emitter and rear locally-diffused’) silicon solar cells. These computer simulations provide improved insight into the dynamics of resistive losses, and thus allow new strategies for the optimization of resistive losses to be developed. The predictions have been experimentally verified with PERL cells, whose resistive losses were reduced to approximately half of their previous values, contributing to a new efficiency world record (24.0%) for silicon solar cells under terrestrial illumination. The measurement techniques and optimization strategies presented here can be applied to most other types of solar cells, and to materials other than silicon.

Chemical reaction analysis of copper indium selenization

Abstract: A chemical reaction analysis of the selenization of copper indium layers to form copper indium diselenide is presented. Time-progressive selenization reactions were carried out in a laminar flow tubular reactor in a dilute H2Se atmosphere at 400°C. Copper, indium and copper-indium thin films were reacted for 1–60 min. The reacted films are analyzed by x-ray diffraction and atomic absorption spectrophotometry to identify the chemical species present in the reacted films. A reaction network for film formation is proposed and data from time-progressive selenizations were analyzed to obtain species composition profiles. Rate expressions are postulated and a mathematical model for the selenization is developed. The behavior of the model is compared with the experimentally determined species compositions to obtain specific reaction rate constants.

Recycling strategies to enhance the commercial viability of CIS photovoltaics

Abstract: Recycling of photovoltaic (PV) modules at the end of their useful life is important for the success of commercializing PV technologies. There are economic, regulatory and technical issues related to recycling; these are addressed here in a case study o f recycling CuInSe2 (CIS) PV modules, which is focused on: economics of the use and re-use of materials; regulations on environmental disposal and waste handling; logistics and economics of recycling and disposing of products by industries face d with comparable environmental issues; and a workable program of the PV industry of the future. The main conclusions are that there are potential regulatory hurdles but these can be overcome by paying attention to module design elements. In the case of l arge installations, collection of decommissioned PV modules is feasible with current infrastructure, whereas collection from small remote installations is problematic. Collecting and consolidating used PV modules will be simplified if modules are returned to the manufacturer or to a contracted recycling center as ‘products’ destined for refurbishment and/or re-use. The projected cost of recycling CIS PV modules ranges from 0 to 0.08 US$ W−1, depending on the specific methods, participants and regulations of recycling.

Evaporated Te on CdTe: A vacuum‐compatible approach to making back contact to CdTe solar cell devices

Abstract: A commonly used process for forming low-resistance contacts to thin-film p-type CdTe involves the formation of a Te layer by etching the CdTe film in a concentrated mixture of nitric and phosphoric acids. The authors compare evaporated Te back contacts with ‘control’ back contacts formed by the usual etching process, and demonstrate that evaporating Te onto a CdTe thin film is a viable process for forming a low-resistance contact. The best efficiency achieved for a CdTe solar cell made with an evaporated Te back contact is 12.1%, whereas the efficiency of the device made with the control back contact was 11.9%. The evaporation process offers numerous advantages over acid etching, most notably vacuum compatibility amenable to large-scale production of CdTe solar cell modules.

Extraction of individual‐cell photocurrents and shunt resistances in encapsulated modules using large‐scale laser scanning

Abstract: Traditional laser scanning of encapsulated thin-film modules yields an optical beam-induced current (OBIC) signal for each cell that is proportional to the product of the cell's shunt resistance and photocurrent. Multiple laser chopping frequencies, ho wever, if judiciously chosen, can separate the photocurrent and shunt resistance of the individual cells even when electrical access is only available across the entire module. This method of extraction makes no assumptions about the forward bias behavior of the cells, requires no time-consuming shading of cells and has the potential to be made semi-automatic. The method has yielded accurate results for CuIn(Ga)Se2, CdTe, and single-junction amorphous Si modules. Additional measurement at forwa rd bias can be useful for separation of the photocurrent and shunt resistance of cells with very low OBIC signals.

Conformal films for light-trapping in thin silicon solar cells

Abstract: A light-trapping structure (10) comprises a base material (11) having at least a first face (12) textured with a texture (13) of period T and comprised of facets (14, 15, 16, 17...). Laid on or otherwise attached to the facets of the texture (13) is a film (18) which is continuous and sufficiently thin that it conforms in its profile to the texture (13). The film (18) will thus be shaped in conformance with the texture (13) to have a period T. Ideally the film (18) is of substantially constant thickness (t). In use according to a preferred embodiment of the invention the base material (11) is light reflective, at least at the facets (14, 15, 16, 17) comprising the texture (13) whereby light rays such as light ray (19) treat the facets as a mirror and are thereby reflected according to the laws of light reflection from a surface so as to be reflected off other facets of the texture (13). Where the refractive index of the film (18) is different from the refractive index of the layer on top of it, internal reflection can occur at the boundary between these layers. In the case of a thin film silicon solar cell the base material (11) can comprise a substrate of glass to which a thin film (18) of doped silicon is applied. Additional films (not shown) can be applied on top to create a multi-layer thin film silicon solar cell with the various layers doped appropriately and the whole covered in a glass encapsulant (20). In one form the texture (13) is three-dimensional as defined in the specification.

Solar cell efficiency tables (version 8)

Abstract: Updated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and several new entries since January 1996 are briefly described. (author)

Theoretical comparison of conventional and multilayer thin silicon solar cells

Abstract: Thin solar cells based on low-quality silicon are assessed for a range of possible material parameter values and device structures. Device thickness is freely optimized for maximum efficiency for a range of doping densities and numbers of junctions, le ading to results differing markedly from previous investigations. Modelling of conventional and multilayer structures in this paper indicates little difference in efficiency potential on low-lifetime ( 15%) are possible given adequate light trapping. Conventional structures (single and double junction cells) are superior if excellent light trapping is assumed. Thicker multilayer structures are advantageous in the case of poor light trapping or surface passivation. In an optimized cell in low-quality silicon, increasing the number of junctions allows a high current to be maintained, but at the cost of a reduced voltage and fill factor caused by increased junction recombination. Formidable pra ctical difficulties are likely to be encountered to realize the theoretical performances discussed.

Uniform pyramid formation on alkaline-etched polished monocrystalline (100) silicon wafers

Abstract: Pyramidal texturing of monocrystalline silicon using alkaline etchants depends strongly upon the initial condition of the wafer surface and upon etching parameters. Texturization of polished wafers is often incomplete, with non-textured areas arising to yield high values of reflectance. A new technique is introduced for uniform pyramid formation on polished wafers. Nitrogen is used to expel dissolved oxygen in the etch solution, since it has been observed that oxidizing agents act to encourage polished etch surfaces.

Solar cell efficiency tables (version 7)

Abstract: Updated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and several new entries since July 1995 are briefly described.

Comparison of p-n junction and inversion-layer silicon solar cells by means of experiment and simulation

Abstract: Inversion-layer solar cells can be fabricated on crystalline silicon in a time- and energy-efficient way. In this article we experimentally investigate inversion layer cells of the type developed in the 1980s at the University of Erlangen. The best cell has an independently confirmed one-sun efficiency of 15.7%, the highest reported to date for this simple cell technology. In order to gain insight into the performance-limiting mechanisms, these cells are compared to p-n junction cells fabricated on identical substrates. Subsequently, the impact of the most important emitter parameters on the performance of both cell types is determined by means of two-dimensional numerical modelling. These simulations reveal that inversion-layer cells can principally produce the same efficiencies (> 23%) as p-n junction cells, provided the emitter parameters are properly adjusted and the front contact is of a sufficiently high quality. Therefore, a research project is presently under way at ISFH aiming at an improvement of inversion-layer cell efficiency above 18%. The basis for these new cells is the fact that silicon nitride films deposited at higher temperatures (∼400°C) demonstrate strongly improved passivation properties compared to the present 250° C silicon nitride films.

Solar cells with mesh‐structured emitter

Abstract: The mesh-structured emitter solar cell (MESC) is introduced as a novel solar cell processing scheme. By the formation of inverted pyramids or microgrooves on a wafer with a homogeneous heavy phosphorus diffusion, a mesh of highly conducting emitter lines is formed. Using this technique, the lateral conductivity of the emitter can be increased, keeping the emitter dark saturation current at a low level. The high phosphorus surface concentration results in a low contact resistance even for screen-printed contacts. Thus, this technique is ideal for solar cells with screen-printed contacts, because the finger spacing of the front contact can be extended, resulting in smaller shadowing losses. Also the processing scheme of high-efficiency solar cells can be simplified, because the formation of the surface texturization and the locally deep diffused emitter can be combined in one step. The first cells with a mesh-structured emitter, evaporated front contacts and local ohmic rear contacts have shown efficien ies up to 21.1%. Lifetime test structures have been used to determine a low dark saturation current of 58 fA cm−2 for the mesh-structured emitter, although the structure is not yet optimized.

17.6% efficient multilayer thin‐film silicon solar cells deposited on heavily doped silicon substrates

Abstract: We report new results for multilayer thin-film silicon solar cells deposited onto electronically inert, heavily doped crystalline silicon substrates. The n-p-n-p-n active layers of a total thickness of 17 μm combined with a 15-μm thick p+-type buffer layer were deposited by chemical vapour deposition epitaxially onto a 1019 cm−3 doped Czochralski-grown silicon substrate. The cells fabricated using these layers exhibit an energy conversion efficiency of up to 17.6%, as measured by Sandia National Laboratories, which is the highest efficiency ever achieved for a thin-film silicon cell deposited onto such an electronically inert crystallographic template. An open-circuit voltage of 664.2 mV is also reported, the highest ever for a cell on such substrates.

Grid‐connected amorphous silicon photovoltaic array

Abstract: A roof-mounted photovoltaic (PV) array has been installed at Loughborough University in the UK. The array output is connected through a commercial grid-connected inverter to the University supply network. The system was the third grid-connected PV inst allation to be connected and generating in the UK. It is the first and only grid-connected system using amorphous silicon thin-film PV modules in the UK. The system is fully instrumented to support the research and development that CREST is conducting on grid-connected PV systems, including: inverters; thin-film amorphous silicon and the effects of shading on performance. The paper presented here describes the design, installation and commissioning of the system. Data from the first year of operation along with the results of a preliminary performance analysis are presented and discussed.

Rural photovoltaic electrification programme on the Bolivian high plateau

Abstract: From 1988 to 1993, the Institute of Solar Energy at the Universidad Politecnica de Madrid carried out a programme on the Bolivian high plateau that attained photovoltaic (PV) electrification of 1500 rural households by means of individual Solar Home Systems (SHS). This programme focused on three aspects: domestic electrification, transfer technology and user participation. In the case of domestic electrification, the SHS design presents some special features related to reliability and security, including the use of car batteries with lower than usual electrolyte density. The transfer technology process has been focused o n the fabrication of lamps and charge regulators specifically designed for this programme. At present, good-quality and competitive local products are available in Bolivia. User participation has been structured around a specific organization named ADES ( Asociacion de Electrificacion Solar). Each user has contributed an initial fee of US$ 80 plus a monthly maintenance fee of US$ 1. The results of the programme show the maturity of PV technology for rural electrification projects in developing countries. In our particular case it is also the cheapest way to achieve rural electrification on the Bolivian high plateau.

Simultaneous dopant diffusion and surface passivation in a single rapid thermal cycle

Abstract: In this work, we present results on simultaneous formation of emitter/back-surface field or emitter/surface passivation in a single rapid thermal cycle. We have investigated the diffusion kinetics of dopant elements like phosphorus, boron (from a doped spin-on glass (SOD) film), aluminium (from evaporated films) or aluminium-boron (from an A1-B SOD film). In particular, we have shown that rapid thermal co-diffusion of P and A1 (or A1-B) leads to low sheet resistances, optical emitter profiles and a hig h gettering effect. Furthermore, the possibility of using the remaining SOD films as a surface passivation layer was investigated. Dark saturation current measurements as deduced from the photoconductivity decay technique demonstrate the passivation effec t of the remaining SOD film. The highest efficiency of 12.8% obtained was achieved on SOD oxide-coated solar cells.

Cost trade between multijunction, gallium arsenide and silicon solar cells

Abstract: Multijunction (MJ),1 gallium arsenide (GaAs) and silicon (Si) solar cells have respective test efficiencies of approximately 24%, 18.5% and 14.8%. Multijunction and gallium arsenide solar cells weigh more than silicon solar cells and cost approximately five times as much per unit power at the cell level.2 A trade is performed for the Tropical Rainfall Measuring Mission (TRMM) spacecraft to determine which of these cell types would have offered an overall performance and price advantage to the spacecraft. A trade is also performed for the multijunction cells under the assumption that they will cost over ten times that of silicon cells at the cell level. The trade shows that the TRMM project, less the cost of the instrument, ground systems and mission operations, would spend approximately $552 000 per kilogram to launch and support1 science in the case of the spacecraft equipped with silicon solar cells. If these cells are exch anged for gallium arsenide solar cells, an additional 31 kg of science can be launched and serviced at a price of approximately $90 000 per kilogram. The 31 kg array weight reduction is shown to derive from the smaller area of the array and hence reductio ns in the weight of the array substrate and supporting structure. If the silicon solar cells are changed out for multijunction solar cells, an additional 45 kg of science above the silicon baseline can be launched and supported at a price of approximately $58 000 per kilogram. The trade shows that even if the multijunction cells are priced over ten times that of silicon cells, a price that is much higher than projected, the additional 45 kg of science are launched and serviced at $180 000 per kilogram. This is still much less than the original $552 000 per kilogram to launch and service the science. Data and qualitative factors are presented to show that these figures are subject to a great deal of uncertainty. Nonetheless, the benefit of the higher efficiency solar cells for TRMM is far greater than the uncertainties in the analysis. © This article is a US Government work and, as such, is in the public domain in the United States of America

Building‐integrated PV activities within the IEA framework: Going from task 16 to task VII

Abstract: Early in 1997 a new task will be launched within the scope of the International Energy Agency (IEA) Photovoltaic Power Systems Implementing Agreement. This new task, Task VII, aims to develop building-integrated photovoltaic (BIPV) technology, consider ing technical viability, economics and aesthetic quality. The task concentrates strongly on the involvement of architects, building engineers and urban planners, and the ability to increase their interest in BIPV. Task VII succeeds Task 16, ‘Photovoltaics in Buildings’ of the IEA Solar Heating and Cooling Program, completed in March 1996. It has proved to be very effective in developing and demonstrating new concepts for building-integrated photovoltaics, with t he international cooperation in the task assisting in advancing and promoting the application of photovoltaics in both residential and commercial buildings. Task 16 played a pioneering role in obtaining the World's attention for BIPV. It is expected that Task VII will provide accelerated development and market acceptance of this promising technology.

Characterization testing of MEASAT GaAs/Ge solar cell assemblies

Abstract: The first commercial communications satellite with gallium-arsenide on germanium (GaAs/Ge) solar arrays is scheduled for launch in December 1995. The spacecraft, named MEASAT, was built by hughes Space and Telecommunications company for Binariang Satellite Systems of Malaysia. The solar cell assemblies consisted of large area GaAs/Ge cells supplied by Spectrolab Inc. with infrared reflecting (IRR) coverglass supplied by Pilkington Space Technology. A comprehensive characterization program was performed on the GaAs/Ge solar cell assemblies used on the MEASAT array. This program served two functions; first to establish the database needed to accurately predict on-orbit performance under a variety of conditions; and second, to demonstrate the ability of the solar cell assemblies to withstand all mission environments while still providing the required power at end-of-life. characterization testing included measurement of electrical performance parameters as a function of radiation exposure, temperature, and angle of incident light; reverse bias stability; optical and thermal properties; mechanical strength tests, panel fabrication, humidity and thermal cycling environmental tests. The results provided a complete database enabling the design of the MEASAT solar array, and demonstrated that the GaAs/Ge cells meet the spacecraft requirements at end-of-life.

Role of Radiation-hard Solar Cells in Minimizing the Costs of Global Satellite Communication Systems.

Abstract: An analysis embodied in a personal computer program is presented, which quantitatively demonstrates how the availability of radiation hard-solar cells can help to minimize the cost of a global satellite communication system. An important distinction between the currently proposed systems, such as Iridium, Odyssey and Ellipsat, is the number of satellites employed and their operating altitudes. Analysis of the major costs associated with implementing these systems shows that operation at orbital altitudes within the Earth's radiation belts (103–104 km) can reduce the total cost of a system by several hundred per cent,1,2 so long as radiation-hard components, including solar cells, can be used. A detailed evaluation of the predicted performance of photovoltaic arrays using several different planar solar cell technologies is given, including commercially available Si and GaAs/Ge, and InP/Si which is currently under development. Several examples of applying the program are given, which show that the end-of-life (EOL) power density of different technologies can vary by a factor of ten for certain missions. Therefore, although a relatively radiation-soft technology can usually provide the required EOL power by simply increasing the size of the array, the impact upon the total system budget could be unacceptable, due to increased launch and hardware costs. In aggregate, these factors can account for more than a 10% increase in the total system cost. Because the estimated total costs of proposed global-coverage systems range from $1 billion to $9 billion, the availability of radiaton-hard solar cells could make a decisive difference in the selection of a particular constellation architecture.

High and low energy proton radiation damage in p/n InP MOCVD solar cells

Abstract: Indium phosphide p+/n/n+ solar cells, fabricated by metal organic chemical vapor deposition, were irradiated with 0.2-MeV and 10-MeV protons to a fluence of 1013 cm−2. The power output degradation, 1-V behavior, carrier concentration and defect concentration were observed at intermediate points throughout the irradiations. The 0.2-MeV proton-irradiated solar cells suffered much greater and more rapid degradation in power output than those irradiated wit h 10 MeV protons. The efficiency losses were accompanied by larger increases in the recombination currents in the 0.2-MeV proton-irradiated solar cells. The low-energy proton irradiations also had a larger impact on the series resistance of the solar cell s. Despite the radiation-induced damage, the carrier concentration in the base of the solar cells showed no reduction after 10-MeV or 0.2-MeV proton irradiations and even increased during irradiation with 0.2-MeV protons. In a deep-level transient spectro scopy study of the irradiated samples, the minority carrier defects H4 and H5 at Ev + 0.33 and Ev + 0.52 eV and the majority carrier defects E7 and E10 at Ec − 0.39 and Ec − 0.74 eV were observed. Th e defect introduction rates for the 0.2-MeV proton irradiations were about 20 times higher than for the 10-MeV proton irradiations. The defect E10, observed here after irradiation, has been shown to act as a donor in irradiated n-type InP and may be responsible for obscuring carrier removal. The results of this study are consistent with the much greater damage produced by low-energy protons whose limited range causes them to stop in the active region of the solar cell. © This article is a US Government work and, as such, is in the public domain in the United States of America

Hydrogen passivation of N(+)-P and P(+)-N heteroepitaxial InP solar cell structures

Abstract: High-efficiency, heteroepitaxial (HE) InP solar cells, grown on GaAs, Si or Ge substrates, are desirable for their mechanically strong, light-weight and radiation-hard properties. However, dislocations, caused by lattice mismatch, currently limit the performance of the HE cells. This occurs through shunting paths across the active photovoltaic junction and by the formation of deep levels. In previous work we have demonstrated that plasma hydrogenation is an effective and stable means to passivate the electrical activity of dislocations in specially designed HE InP test structures. In this work, we present the first report of successful hydrogen passivation in actual InP cell structures grown on GaAs substrates by metalorganic chemical vapor deposition (MOCVD). We have found that a 2 hour exposure to a 13.56 MHz hydrogen plasma at 275 C reduces the deep level concentration in HE n+n InP cell structures from as-grown values of approximately 10(exp 15)/cm(exp -3), down to 1-2 x 10(exp 13)/cm(exp -3). The deep levels in the p-type base region of the cell structure match those of our earlier p-type test structures, which were attributed to dislocations or related point defect complexes. All dopants were successfully reactivated by a 400 C, 5 minute anneal with no detectable activation of deep levels. I-V analysis indicated a subsequent approximately 10 fold decrease in reverse leakage current at -1 volt reverse bias, and no change in the forward biased series resistance of the cell structure which indicates complete reactivation of the n+ emitter. Furthermore, electrochemical C-V profiling indicates greatly enhanced passivation depth, and hence hydrogen diffusion, for heteroepitaxial structures when compared with identically processed homoepitaxial n+p InP structures. An analysis of hydrogen diffusion in dislocated InP will be discussed, along with comparisons of passivation effectiveness for n+p versus p+n heteroepitaxial cell configurations. Preliminary hydrogen-passivated HE InP cell results will also be presented.