Workshop summary

Solar Cells, 7

(1982

- 1983)

3 - 22

3

Workshop summary

The Commercial Photovoltaics Measurements Workshop was held on July 27 - 29, 1981, in Vail, CO [1]. The purpose o f the meeting was twofold: (1) to examine the status o f photovoltaic measurements, data and standards development and (2) to identify gaps in the measurements technology that impede the widespread utilization of photovoltaics. The intent was also to make more visible the importance of a reliable measurements technology in permitting (1) a better understanding and control of photovoltaic materials, processes and devices, (2) improved productivity, (3) a fair and accurate comparison o f photovoltaic products and (4) greater user confidence in and acceptance o f photovoltaics. To ensure that the Workshop was aimed at relevant measurement needs, the topics covered were selected on the basis o f inputs from visits to a cross section of the U.S. photovoltaics industry. The results of these visits are included in the introductory paper to the Workshop. Although this Workshop is the first to be aimed specifically at the measurement needs o f the photovoltaics industry, it does follow a series of workshops and meetings [2 - 6] that were concerned with measurements and were sponsored by various parts of the National Photovoltaic Program of the U.S. Department o f Energy (DOE). Over 80 representatives from industry, national laboratories, U.S. Government, universities and elsewhere participated in the discussions and presentations of the Workshop. In particular, 11 companies involved with the manufacture or development o f commercial and near-commercial photovoltaic devices were represented.

1. Overview On the basis of the comments and discussions at the Workshop, it is clear that there is a consensus that standard methods o f measurement are important. They are a necessary part of doing business. They are the comm o n language at the buyer-seller interface. It is also clear that despite the extensive work in measurements, data and standards development that has been conducted (mostly with the support of the National Photovoltaic Program and other programs of the U.S. Federal Government), a great deal still needs to be done. As the photo© Elsevier Sequoia/Printed in The Netherlands

voltaics c o m m u n i t y has gained in experience and sophistication, it has begun to recognize that a reliable measurements technology is important to its ability to function effectively and with confidence. The industry has also begun to recognize that a larger number of "measurement tools" are needed than were at first anticipated and that more will be needed as the industry prepares for the high volume production expected in the near future. The following are some examples of the measurement needs identified. The high production of the near future will create new requirements on test and process monitoring techniques and on equipment for the rapid acquisition, processing and analyzing of a variety of data. The test methods used to characterize the starting silicon material are deficient to the extent that they cannot yet be used with confidence to determine whether the material can be used to fabricate satisfactory solar cells. The accuracy of cell and module o u t p u t measurements will remain in question while better methods are developed to measure the spectral response of solar cells, to match the spectral responses of test and reference devices and to measure the spectral distribution of solar simulators. Many people feel that module certification will ultimately be needed, e.g. to satisfy codes and other regulations such as those promulgated by the U.S. National Electric Code. Before a certification can be established, however, difficult questions need to be answered about the selection of criteria for certifications and then about the validity o f the test methods used to determine compliance with these criteria. Finally, to satisfy adequately the requirements for the optimization, design, sizing and operation of photovoltaic systems, significant advances are needed in the accuracy, completeness and timeliness of solar data. The Workshop also showed that the photovoltaics community, as represented by the attendees, looks to the U.S. Federal Government to continue to play a significant role in measurements development. A number of aspects of this role for the U.S. Government's photovoltaics program were identified. One is to provide recognized centers of measurement expertise so that the industry and testing laboratories can rely on them to verify and confirm the validity of their measurements. A specific example of this service was identified in a unanimous vote at the evening session on the role of the U.S. Government. The vote called for the U.S. Government's program to provide a referee function to certify the measurement validity of any testing laboratory that would undertake to provide the c o m m u n i t y with calibrated reference cells. Another identified aspect of this role is to support, in particular, the development of needed measurements technology that is of such an extensive or broad nature that any one manufacturer or group cannot reasonably divert sufficient resources to do the job. This aspect would apply, for example, to produce a new or to refine an existing test m e t h o d that requires significant research, development and analysis. Two other specific examples were identified by the attendees: the development and collection of field experience with photovoltaic products and the nation-wide collection of solar data. The attendees in a unanimous vote during the same evening

session passed another resolution that the U.S. Government should not only continue but also expand its solar data collection activities. Another aspect of the role of the U.S. Government is to support standards development and to promote information and technology transfer. An example of the promotion of technology transfer is the sponsoring of meetings such as this Workshop, thereby providing a neutral ground for industry to meet. This Workshop, for example, allowed a large segment of the community to assess the value of global calibration of reference cells and to be advised of a new and portable instrument for measuring the currentvoltage (I-V) characteristic curve of arrays, an instrument need identified earlier in the Workshop.

2. Discussion session summaries The following are summaries of the nine discussion sessions which were prepared by either the chairman of the session or the secretary of the meeting.

2.1. Measurement equipment needs discussion session summary (Bob Sanderson, Arizona State University, Tempe, AZ) Several measurement needs were identified by representatives of photovoltaics manufacturers. Many of the suggestions received from the group related to the cell production process. The computer compatibility of instruments monitoring the production process was mentioned as being necessary to facilitate computer analysis of the process. Equipment to automate more fully the production line needs development. Finally, high speed waferhandling equipment and equipment to inspect the mechanical integrity of finished modules automatically are needed. Measurement equipment needs relating to the performance evaluation of photovoltaic devices were also identified. There is a need for solar simulators which more closely match the air mass 1.5 (AM1.5) spectral distribution of the Sun. Equipment to evaluate simulator spectral distributions quickly and easily is also needed. Specialized service representatives would be of benefit to photovoltaics manufacturers to repair complex solar simulators. Several obstacles to the development of specialized equipment for the photovoltaics industry were identified. A principal obstacle to the industry is that the cell production process is a low value-added process, so it is difficult to justify large expenditures on automated process monitoring equipment. Much equipment for automated production lines has been developed, but cost remains a major obstacle to its widespread use. Also, the photo-

voltaics industry is a relatively small market for items such as simulators, making it difficult to justify the training and maintaining of specialized service representatives.

2.2. Interaction with customers o f photovoltaic products session summary (Ghazi Darkazalli, Solar Power Corporation, Woburn, MA ) Recommendations and remarks from the session attendees ranged from the general to the specific. The following is a summary of their comments. 2.2.1. Potential customers In order to improve communication, two potential photovoltaics markets should be recognized: (a) domestic and (b) international. Both markets consist of two sectors, U.S. Government and private. Each sector may require specific systems designs and sizes such as small size systems, residential and intermediate systems and large or industrial systems. Each market and systems size demands different levels of communication between the customer and the supplier. 2.2.2. Problems and solutions (1) The customer is always right and industry should provide special designs, solutions and education to customers. (2) Methods should be developed to protect the customer when photovoltaic components do n o t meet manufacturer's specifications. (3) The U.S. Government should continue its support until the photovoltaics industry and market are mature. (4) The customer must be educated. The Jet Propulsion Laboratory (JPL) plans to school the user on quality and trouble-shooting procedures in the fiscal year 1982 (according to W. Bishop, JPL). The d o c u m e n t on interim performance criteria [7] has been developed to provide a valuable resource for the photovoltaics community. It identifies pertinent performance needs and expectations for photovoltaic systems that can assist in their equitable specifications and procurements. The document also provides valuable information related to performance and safety that can be used in systems design and application. (5) Customers feel that a photovoltaics handbook needs to be developed by industry covering the following areas: subsystem components; systems performance; installation; monitoring and trouble shooting. (6) Systems cost is extremely high; therefore, there is a need to insure systems against a catastrophy. (7) There is a need to establish long-term solar data. (8) With regard to photovoltaics standards, the following mixed views were presented: suppliers should develop their own standards; customers could use standards to distinguish the quality and reliability of products; customers could use standards in bid requests for proposals; standards might be fixed and completed before the industry has matured.

(9) Systems failure might be caused by the customer adding loads to the system or b y improper wiring. (10) There is a need for installation and maintenance manuals. (11) The safety of photovoltaic components should be addressed.

2.3. Quality control discussion session summary (Harry A. Schafft, National Bureau o f Standards, Washington, DC) 2.3.1. Session presentations The opening remarks b y the four presenters began with a description o f an approach, adaptable to photovoltaics, of stressing specially designed test structures to evaluate the quality and reliability of integrated circuits and thereby, for example, to evaluate the implications for the reliability of process or device design changes. The approach o f designing test structures to be sensitive to specific failure mechanisms can also be used to obtain quantitative estimates o f the life o f devices if it is integrated with traditional life and stress tests and with an appropriate reliability model. The remarks continued with an analysis of test structures designed to be sensitive to critical process steps by being able to measure sheet and contact resistances for use in the development of CdS solar cells. One o f the key points made is the importance o f such analyses for ensuring that the capabilities and limitations o f any test structures used are understood. A specific example o f using a test structure to monitor a critical process step in the manufacturer of CdS panels followed. Another key point made was that the test structure provided information about the process that could not have been obtained in any other way. Finally, some o f the reasons w h y a process line can become inoperative were discussed. In the course o f the discussion, the importance of achieving a full understanding of all the critical process steps and then controlling them (essentially by external meters and regulators) was stressed by the last presenter. The need to keep simple the measurements technology used on the production line was also emphasized.

2.3.2. Discussions Comments from both discussion groups indicated that test structures are used, at least by some people, in a limited way to understand cell processing steps. The feeling of the groups was that the use o f test structures is most useful and cost effective in the device development stage of the product. Most participants did not see the use of test structures as a process monitoring tool during the production phase; it was felt to be t o o expensive to use them in this way. It was suggested that, because cells are relatively simple structures, such sophisticated measurement tools as test structures are n o t needed after a process understanding has been achieved. When the process line does go awry, some participants said that they can determine the cause o f the problem by simply examining the I - V characteristics of the cells. One of the presenters disagreed, saying that all the possible contributing causes of any anomalies observed in the I - V characteristic curves

cannot be factored out. The response from the floor was that perhaps test structures could be useful as a diagnostic tool. While it was not mentioned specifically, this would necessitate the development of a baseline of test structure data to establish a reference point from which to determine which of several processes may be out of control. It was pointed out that, even if test structures were used as a process monitoring tool, information could then only be obtained after the metallization step. Some participants felt that this would be too late to learn that the line was producing " j u n k " . Subsequent discussions indicated that it is not always necessary to wait until after the metallization step to obtain important process information. One of the presenters mentioned using a structure to detect strain in the silicon lattice which could be used to determine whether the junction breakdown voltage of the circuit would be satisfactory. Another participant mentioned the use o f a test structure which was used to monitor the junction implant and anneal steps for making a solar cell. These test structures are eventually covered by the metallization and so do not reduce the active area o f the cell. Yet another participant mentioned using temporary contacts in conjunction with laser scanning to m o n i t o r steps in the process prior to metallization. If these discussions were taken one step further, we might even begin to consider that a sufficient degree of innovation in the design and application of test structures is all that is needed to be able to capitalize on this approach for solar cells. It was pointed out that, even though a product from the production line may appear to be satisfactory, it is not necessarily possible to know whether it will work reliably at some time in the future. There seemed to be general acceptance of the approach of using test structures for reliability monitoring. Indeed, the approach might be as useful at the module level as on the cell level. Just as test structures could be designed on the cell to test cell level failure mechanisms, special minimodules could also be designed to explore the module design for sensitivity to module level failure mechanisms. The failure most often mentioned during these discussions is that due to metallization contact failure. There was considerable interest expressed in a test structure to provide information about metallization quality. There was mention of the traditional accelerated stress and life tests used to obtain information about product reliability. Some measure of dissatisfaction was expressed with these tests. Nevertheless, it was pointed out that such tests are needed to obtain failure information. Such information is required to identify relevant failure mechanisms before test structures can be designed to determine the product's sensitivity to these mechanisms. As a final note, two presenters emphasized again the need to analyze test structure designs and test conditions, whether they are for obtaining process or reliability information. Without a full understanding of these structures, the parameter that is in fact measured may not be the one that was intended to be measured.

2.4. Silicon characterization discussion session summary (R. O. Bell, Mobil Tyco Solar Energy Corporation, Waltham, MA) There is probably no such single material as "solar-grade" silicon that is appropriate for all crystal growth and cell fabrication procedures. The only way really to characterize feed silicon is by sampling from the batch, growing a test piece of material and fabricating it into cells. This is not t o o onerous for high volume production when the solar-grade batches are in the multikilogram or larger range. Some preliminary screening can be done b y chemical analysis for impurities and by a diffusion length measurement (usually by surface photovoltage (SPV)) on the starting material. If, for example, m o l y b d e n u m or titanium were present in "substantial" quantities, this should be grounds for rejecting the material. The use of diffusion length measurements was somewhat more controversial with some participants feeling that it was a good tool, b u t with others believing the correlation with cell performance to be t o o p o o r to be useful. Diffusion length measurements are routinely made by the SPV technique. It was stressed by a number of the participants that with silicon material there is no such parameter as the "diffusion length". There are several reasons for this. (1) Any material processing, especially thermal processing, can alter the diffusion length. (2) The diffusion length obtained b y various techniques is an average over both the surface and the bulk of the material. (3) The number obtained by SPV, while called a diffusion length, is not the number that a device physicist would want to use in theoretical calculations o f the cell performance. The effective diffusion length (as some people would call it) is, however, a very valuable figure o f merit. For example, a plot o f cell efficiency versus effective diffusion length produces a smooth curve even when widely differing materials are used, as long as the cell processing is constant. A number of different techniques for characterizing silicon were discussed. These primarily included (a) SPV, (b) spectral response, (c) dark I - V curves, (d) deep level transient spectroscopy (DLTS), (e) Zerbest plots and (f) laser scanners. (a) No general dissatisfaction with the experimental procedure for SPV measurements was expressed, although generally the method of barrier formation was n o t that outlined in ref. 8. The major problem has to do with its interpretation. In many cases, the presence o f a back-surface field, a thin sample and areal or bulk inhomogeneities will affect the measured diffusion length. (b) The measurement of spectral response is another optical technique that, when properly interpreted, can be used to evaluate material lifetime. Similar information as in the SPV technique is available. (c) The analysis of dark I - V curves can give valuable data about lifetimes in silicon. The diffusion current (the n = 1 region) varies inversely with

10 base diffusion length wherever the recombination current (n = 2 region) is determined by the space charge region lifetime characteristics. Rather careful analysis and measurement as a function of temperature and voltage are necessary to obtain the most from this technique. (d) DLTS can be used to look at energy levels and concentrations of recombination centers. It has been applied to materials doped with metallic impurities such as titanium or molybdenum. Its use on materials such as sheet silicon is not as clear cut. (e) A Zerbest plot, which is a technique applied to metal-oxide-semiconductor structures to measure the generation lifetime, has been used. Good correlation with cell performance was found but the meaning of the way in which this lifetime relates to device physics is not clear. (f) Brief mention of several other evaluation techniques for lifetime or diffusion length was made including laser or IR scanning, electron-beaminduced current, open-circuit voltage decay and photoconductive decay. A brief discussion of resistivity measurements occurred. Spreading resistance, four-point probe, Hall or Van der Pauw and eddy current are possible methods. The technique chosen depends on why the measurement is undertaken. Although multiple metallic impurities apparently do not interact when present in silicon, other elements such as oxygen or carbon have very complex behavior. In some cases, oxygen is detrimental when present during growth and in others just the opposite occurs. This is a fertile field for more study. No universal technique for the identification o f impurities is available. While the development of such a technique is highly desirable, the group sessions were unable to give an answer as to how this should be done. Many evaluation techniques are available, but it is their proper interpretation which is probably the most difficult aspect.

2.5. Solar irradiance data session summary (W. J. Kaszeta, Solavolt International, Phoenix, AZ) 2.5.1. Presentation highlights There are two main uses Of pyranometers and pyrheliometers: measurement for insolation (energy) and irradiance (power). The special needs of each use result in different requirements for calibration and accuracy. Most irradiance measurements are taken on a tilt with the instrument normal to the irradiance; this reduces cosine errors but introduces tilt errors. Some pyranometers have a definite error associated with the use of the instrument on a tilted surface, while others have a minimal error. The total error (uncertainty), due to both calibration and instrument error, for a good pyranometer (such as an Eppley PSP) used for insolation monitoring can be as large as 7%. For other instruments, up to 25% error may occur. Only the better instruments are used by the National Oceanic and Atmospheric Administration (NOAA). All the reported instruments are sold and used for solar energy measurements.

11 Pyranometer measurements of irradiance with an accuracy o f up to 1% can be made if care is taken, including the proper application of temperature correction factors etc. Failure to account for all sources o f deviations can cause significant errors even with the best pyranometers. If proper procedures are not followed, the error can exceed 2%. There is an ongoing international effort covering the intercomparison and calibration o f pyrheliometers. The photovoltaics c o m m u n i t y should recognize this and build its measurements base from this. ASTM Committee E44.02 has completed or is working on many standards pertaining to solar radiation including direct beam and global AM1.5 standard spectra and calibration procedures for pyrheliometers and pyranometers. NOAA is at present operating and will apparently continue to operate for at least the next year the existing 38-station national network for measuring solar radiation. Each location is at present equipped with three basic instruments: a pyranometer, a pyrheliometer and a pyranometer with a shade band for measuring diffuse radiation. Eight U.S. DOE-sponsored university meteorological training sites are operating and supplying additional data on insolation. A combined m o n t h l y report is published by the University of Alabama at Huntsville. Typical activities include the measurement of insolation on tilted surfaces and, in some cases, the measurement of insolation every minute. The University of Alabama at Huntsville has also prepared and will publish " A guide to world insolation data and monitoring networks" [9]. In the Insolation Resource Assessment Program at the SERI considerable effort has been spent on the development and verification of computer programs and instrumentation to quantify accurately the spectral characteristics of insolation. DSET Laboratories Inc. is building a long-term spectral data base under a JPL contract. The initial data will be for New River, AZ, but it is hoped that data for other locations will be collected. 2.5.2. Discussion There was general agreement by the audience that the availability of insolation data, particularly worldwide, is a definite problem for the industry. A good worldwide data base of insolation is needed. Designers need to be able to define the available insolation on a tilted array to within 5% for stand-alone systems design. The present accuracy of solar insolation models used to calculate insolation on a tilted array is about 15%. A temporal resolution of 1 h appears to be satisfactory for insolation data. There is also a need for microclimatic data. The general problem of the design o f the systems located between reporting stations must be dealt with. A representative area for each station should be defined for each season. The U.S. Federal Government should consider increasing the size of the NOAA insolation data network so as to gather more data from areas

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with greater market potential. There exists a need to define the accuracy of the existing insolation data sources such as SOLMET and SOLDAY. An effort should be undertaken to define the accuracy and its effect on systems design. Lower cost spectral instruments are needed. Many participants would like to see a U.S. DOE-sponsored effort to measure spectral data for several areas to determine the time-weighted spectral irradiance resource. Seasonal and annual data are required. The data would be used to optimize the spectral response of cells and to determine regional differences. Those present would like NOAA to be able to provide close-to-real-time insolation data, perhaps based on satellite data corrected with ground-truth data. This could be used to predict systems failure by tracking insolation patterns and taking protective action when unusually low periods of insolation are encountered. More work should be done on using satellite data, which can be used for forecasting and obtaining microclimatic data. There is no formal program in this area. The questions which we should be asking are whether we really are collecting the correct data for photovoltaic systems design and whether there are other measurements that should be made.

2.6. Source for reference cells session summary (R. R. Addiss, Jr., Solar Power Corporation, Woburn, MA ) 2.6.1. The problem The most important parameters to be specified for the photovoltaic products offered for sale are the electrical o u t p u t ratings under standard illumination conditions. In order to provide this, an ability to measure electrical outputs under non-standard illumination conditions by means which allow the results to be related to standard conditions is required. It is generally conceded that the most effective way to do this is to use a properly calibrated reference cell which has a suitable spectral response. In order that the measurements results be uniformly accepted throughout the industry, the calibration o f these reference cells must be performed by an independent impartial reliable organization. This is the only way to ensure that all reference cells in use throughout the industry have their calibration based on the same accepted standard conditions. However, there is no organization to turn to; no such source o f calibrated reference cells exists today. How can this problem be solved?

2.6.2. Discussion objectives The ultimate objective of the discussions is to develop a consensus on a means of establishing a source o f reference cells. To do this we shall have to consider both the technical and the institutional barriers which may exist and what needs to be done to overcome these. We would also like to identify some potential problems that we should be alert to. The standard illumination conditions need to be defined and agreed on. Should these be based on a direct normal spectrum or on a global solar spectrum (or on both)? Sharing

13 experiences o f reference cells should help to identify the elements which should be factored into any program to supply reference cells. 2.6.3. Discussion summary 2.6.3.1. Adequacy o f the calibration standards which are being developed. In its present status, an ASTM standard being developed by Committee E44.09 for the calibration of reference cells based on a direct normal solar spectrum allows a range of atmospheric variables (air mass, water vapor, turbidity) over which the calibration measurements are valid. It does not require that the results be adjusted to specific values of these variables, as currently practised by the U.S. Government laboratories when they calibrate their own reference cells. In effect, no single solar spectrum will be defined as the standard, but rather a range of solar spectra will be allowed. Most attendees thought that this latitude is t o o large and that the atmospheric parameters require more exact specification. A question arose as to how accurate a reference cell has to be. From the lack of response, the audience was apparently not prepared to discuss this. (An accuracy of 5% is not good enough; a goal of at least 1% accuracy is desirable. For example, let us consider two manufacturers who each make a module which outputs the same 10 Wp under the same illumination conditions. One has a reference cell which is calibrated 5% high and the other has a cell calibrated 5% low. Then one manufacturer would specify his module's o u t p u t as 9.5 W while the other would specify 10.5 W for his module. One manufacturer would then enjoy a large competitive advantage in the market-place on the basis of his ability to quote a 10% lower price per watt, even though there would be no difference in the actual price per watt delivered.) A suggestion was made that a standard solar spectrum should be defined analytically such that reference cells could be calibrated by convoluting this standard spectrum with the measured absolute spectral response o f the cell. The standard spectrum could be stored in a computer, for example. The immediate reaction from the audience was that the absolute spectral response cannot be measured accurately enough. (This idea sounds very attractive. Perhaps some investigation of the accuracy attainable would be worthwhile to check the correctness of the perceptions of the audience.) 2.6.3.2. Desirability o f a global standard. No one expressed any negative attitudes toward the establishment of a global method for calibrating reference cells. Most attendees seemed to support the concept in principle but wanted to await the final results of the present DSET study before making a formal commitment. Much discussion centered on the interpretation o f the data obtained to date in this study, which is beginning to indicate that the different calibration number obtained from a global method versus a direct normal method is solely a result o f the different spectral distribution in global sunlight versus direct normal (collimated) sunlight and that the global calibration number is much less sensitive to atmospheric variables

14 such as air mass, water vapor and turbidity than is the direct normal calibration number. To account for this, the idea that spectral bands scattered out o f the direct normal beam by the atmosphere are at least partially recovered in the diffuse c o m p o n e n t of the global radiation was put forward. No data in direct support of this idea were presented, but mention was made of previous work at the Lewis Research Center, National Aeronautics and Space Administration (NASA), which indicated t hat variations in calibration n u m b e r due to the atmospheric variables are reduced by about a factor o f 2 when global rather than direct normal sunlight is used. In the DSET study, only a limited range of atmospheric variables has been sampled so far. It is planned to ex te nd this range considerably before the study is completed.

2.6.3.3. Spectral response. There was some confusion and much concern expressed about the spectral response of reference cells and how it would be matche d to a c om pany's product. Items of concern included how large the variation in the spectral response o f cells o f a given m anufact urer is, how large an error this variation can lead to in measurements o f cell o u t p u t and how the spectral response o f the reference cell should be chosen on the basis o f these variations in p r o d u c t spectral response. One suggestion was to maintain a stable o f reference cells with different spectral responses and to choose the one which most closely matches the cell under test. However, this would mean t hat each cell produced would first have to be checked for spectral response, and this is n o t practical with the technology available at present. F u r t h e r m o r e , n o b o d y seemed to know how to measure the spectral response o f a complete module. A more practical suggestion was to survey the range o f red-to-blue ratios o f the p r o d u c t and to use a reference cell which has the average red-to-blue ratio. In addition, errors can be minimized by using a solar simulator with a spectral distribution as closely matched to the standard solar spectrum as possible. While there was general agreement on these techniques to minimize errors, no one seemed to know how to quantify the a m o u n t o f error which would remain, so no consensus could be reached on whether any of these techniques is good enough. Clearly, some quantitative measure o f mismatch is needed for bot h the spectral response o f cells and the spectral distribution o f solar simulators, t oget her with a procedure to use these "mismatch indices" to determine the uncertainties in cell and module measurements. 2.6.3.4. Packaging. In its present status, an ASTM standard being developed by C o mm i t t e e E44.09 for the physical characterization of reference cells requires an air gap between the cell and cover glass. In earlier reference cells made b y the Lewis Research Center, NASA, this space was filled with an encapsulant which simulates m or e closely the way in which a module is constructed. The decision to eliminate the encapsulant reportedly was made to avoid delamination problems which have been experienced and which effectively destroy t he reliability o f the reference cell. However, experience with unencapsulated reference cells has revealed other problems which have

15 arisen because o f the air gap. In one case, corrosion o f the metallization was observed "after only 3 days". In another case, moisture condensed on the inside surface of the glass cover plate. These experiences suggest that their use may have to be restricted to indoors only, but this would seriously limit their value. (The elimination of the encapsulant in reference cells raises some concern about their use in global sunlight as specifically provided for in the proposed ASTM standards. Because of the additional optical interfaces between the cover glass and the cell, the reference cell will respond somewhat differently to the diffuse component of global sunlight, which impinges at angles far from normal, than will, for example, a module which has encapsulant between the cells and cover. If this difference is significant, then measurements in global sunlight will give different results from those obtained for the same module when measurements are made in simulator light at nearnormal incidence, even when the spectral response of the reference cell is perfectly matched to the cells in the module. It is not clear that this effect was examined and quantified before the standards were proposed. Unless it can be shown that this effect will always be negligible, the use of unencapsulated reference cells under global conditions should not be allowed. This would seriously limit their usefulness and favors a requirement t h a t reference cells be encapsulated. 2.6.3.5. I n s t i t u t i o n a l concerns. Many questions were raised but few concrete answers were obtained. The main concern was the potential size and profitability of a business supplying calibrated reference cells. The means to approve and m o n i t o r the laboratory providing this service seemed to be of less immediate concern. Many attendees felt that this would not be a large business and therefore might not be profitable. Some participants t h o u g h t this could only be a loss-making business. It was generally assumed that some type of subsidy or guarantee of minimum business would have to be provided to induce a laboratory to seek this business but no one proposed a realistic way to underwrite this. A few suggestions were made concerning procedures which might enhance the possibility o f profitability. For example, a laboratory might be able to relate its reference cell business to other areas o f business it might be in (e.g. the calibration of pyranometers) in order to minimize its investment requirements. The laboratory might elect to produce an inventory of reference cells with a range of spectral response characteristics to match typical cells from various manufacturers. In this way, production could be scheduled in large batches when the required resources were not tied up in other activities. The relative economics o f calibrating cells packaged by the customer versus both packaging and calibrating the cell were discussed but not resolved. Finally, DSET Laboratories announced its plans to provide, on its own initiative, globally calibrated reference ceils beginning in October 1981. This announcement paved the way for a consensus, expressed in a subsequent evening session on the role of the U.S. Government in support of measurements, to petition the U.S. Government to budget funds for establishing a referee function.

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2.7. Cell and module output measurements session summary (Henry Curtis, Lewis Research Center, National Aeronautics and Space Administration, Cleveland, OH) This session was broken into three major discussion topics which were (1) developments in spectral response measurements, (2) the status of standards activities in terrestrial photovoltaics and (3) cell and module measurements. Each discussion topic was introduced by one or more papers, some o f which are published in these proceedings. A compilation o f the general c o m m e n t s during the discussion session follows. The discussion on spectral response measurements centered around bias lighting. The need for bias light during spectral response measurements was generally agreed on; m a ny cells exhibit non-linearities which make the use o f bias light a necessity. The spectral c o n t e n t and intensity level o f the bias light are adequately supplied by an ELH lamp or other similar dichroic filtered tungsten lamps operating near a 1 sun intensity. There was a discussion about unwanted a.c. signal in the d.c. bias light, which is caused by either vibrations or convective heat currents. It was generally felt that careful experimental technique would eliminate this problem. There was also discussion about the merits o f relative and absolute spectral response. It was noted that the National Bureau o f Standards (NBS) has a standard d e t e c t o r package for rental use which will calibrate spectral response equipment absolutely. One o f the main discussion points concerning present standards was the fact that standards are being written which require reference cells or irradiance monitors, while reference cells are not currently generally available. A reliable commercial source of reference cells is definitely needed. It was also r e c o m m e n d e d that standards should be carefully written in anticipation that t h e y will be included in various state codes. Potential problems may arise when U.S. standards are coordinated with those o f o t h e r countries. It was felt in the cell measurement discussions that errors could be kept as low as 1% in a research laboratory setting. This does n o t include errors due to reference cell calibration. Although very accurate cell measurements can be made, the general feeling was that some judgment should be used when requirements for measurement accuracies are stated. For research purposes, single-cell measurements may be required to be accurate to within 1%. However, coarse measurements, taken just to determine whether a cell has a good fill factor, may not need to be so accurate. It was also suggested that measurements procedure standards have a class o f measurements based on accuracy requirements. It was not ed that the use o f t he pulsed technique in cell measurement can lead to errors if the response time o f the cell is o f the same order as the length o f the pulsed illumination.

2.8. Photovoltaic module certification session summary (Gary Nuss, Solar Energy Research Institute, Golden, CO) The purpose o f this session was to explore the need for a certification program for photovoltaic c o m p o n e n t s and/or systems and to discuss what

17 form such a program might take. The form of such a program should answer the who, what, when and h o w questions of the certification of photovoltaic products and of the accreditation of testing laboratories. The Research Development and Demonstration Act o f 1978 (Public Law 95590) calls for the certification of photovoltaic products to guarantee conformance with performance criteria such as the U.S. Secretary of Energy may prescribe. A reasonable photovoltaic product to consider for certification is the module. Just as solar collectors have had to be certified to obtain tax credit for their installation, it can be seen that the same path may be taken for photovoltalc modules, at least for residential applications. To establish a module certification program requires that realistic and practical criteria and tests be selected in such areas as safety and environmental, mechanical and electrical performance. 2.8.1. Session p r e s e n t a t i o n s

Session presentations covered the following subjects: institutional processes and prerequisites for developing a product certification program (Doug Thomas, NBS); experience in the certification o f solar collectors (Dave Waksman, NBS); JPL activities and experiences in module specification applicable to module certification (Ron Ross, JPL); an industry view o f certification (Bob McGinnis, P h o t o w a t t International-Solar Energy Industries Association). The first report covered the elements of a certification program from sample selection to testing, labeling and program administration and discussed the distinctions between "approved" and "accredited" laboratories. Prerequisites for a program were identified, such as criteria for certification, standard test methods, criteria and methods for qualifying testing laboratories and inducements for candidate laboratories to participate in the program. The second report reviewed certification experience in solar heating and cooling. A particular issue that is important to manufacturers is the uniformity of certification procedures; in the case of thermal collectors, multiple certification programs exist with somewhat different test and evaluation procedures. Other issues that have arisen in the thermal programs are the degree of experimental uncertainty, equivalency between o u t d o o r and simulator tests, full load test results versus partial load (actual) conditions, testing costs, the number of test laboratories and the t y p e of test laboratories (private versus quasi-public). All these issues have relevance for photovoltaics certification and accredition. The third report described the J P L - L o w Cost Solar Array ( L S A ) m o d u l e qualification tests and JPL's testing experience. Although some manufacturers view the JPL test program as a certification procedure, it is designed to evaluate the minimum performance requirements for U.S. DOE application experiments. The JPL program examines design and performance requirements including electrical performance and environmental endurance. The number of qualification tests has grown from t w o for LSA block I to eight for block V, in addition to those for characterizing electrical perfor-

18 mance. JPL estimates that for performance tests the required equipment resources can vary from U.S. $ 5 0 0 0 0 to U.S. $300000; for endurance testing, both custom and standard equipment are required at an estimated resource level of U.S. $200 000 - U.S. $500 000. In the fourth report an industry view of certification was presented and several caveats were raised with respect to both the existence and the absence of a formal certification program. Among the concerns cited are the following: that there will be an increase in the design cycle in an effort to satisfy certification requirements, thereby hindering innovation; the high cost of certification, particularly during the evolution of the technology; that certification can disguise differences among products and could minimize the effect of product quality; the definition of what kind of design change necessitates recertification; the establishment of the validity of tests to be imposed in a certification program. The existence of a certification program can benefit the industry, however. Incentives offered by taxing authorities probably will require certification for eligibility; a formal national program can avoid the risk of multiple non-uniform requirements. In addition, certification can help to avoid the imposition of over-stringent specifications. 2.8.2. Discussion s u m m a r y The discussion following the session presentations indicated some confusion regarding the nature of product certification. Certification implies a statistical warranty that a product conforms to particular standards or specifications or that when a product is tested according to a uniformly accepted test m e t h o d it will conform to a manufacturer's performance level specification. Some people equate certification with specifications and/or with product acceptance. Certification programs merely certify that a specific model will perform in a particular way and that all such products of the same model also should perform similarly. Certification does not mean that a faulty product in a particular model line cannot occur. General agreement exists that certification programs are largely for unsophisticated buyers; large systems {with a central station and some intermediate load centers) will be purchased according to user specifications. Also, the impetus for certification is likely to come from taxing authorities which will want to impose certification as a precondition for public financial incentives. Thus, the consensus opinion of the Workshop participants is that certification will be needed for some markets; however, the timing, format and content and the program administration are still uncertain. The establishment of a certification program is dependent on the existence of certain elements such as the following: (1) a market (the need for product certification presupposes that a market exists for the product for which certification may be desirable and that a market exists for the certification program); (2) test methods (uniform test methods must exist and these test methods need to evaluate appropriate performance and safety characteristics); (3) test laboratories (independent qualified laboratories must be willing to undertake the testing program).

19 At the present time, with respect to these requirements, photovoltaics markets do not require product certification, although the thrust of the National Photovoltaic Program and of some sectors of the industry is toward residential applications which are more likely to stimulate a demand for certification. Test methods do exist which can provide a basis for certification, b u t it is still uncertain as to which tests, i.e. which characteristics (if any), b e y o n d the verification of electrical performance are necessary and how adequate these tests are. Finally, few laboratories outside the U.S. Government's national laboratories have the equipment and expertise to conduct the tests that do exist or that may be necessary. In summary, the Workshop consensus recognizes the eventual need and probable benefit of module certification particularly with respect to safety performance. The opinion of the majority was negative with respect to the standardization of sizes, dimensions or a specified level of electrical performance. Opinion was mixed with regard to endurance testing. Manufacturers generally agree that the JPL qualification tests have benefited their technology development and that the tests provide a basis for future certification testing. However, many manufacturers believe that certification must differentiate between applications; not all existing endurance tests are necessary for every application (or every geographic location). Also, the general belief is that the validity of proposed endurance tests must be confirmed before t h e y are imposed in a certification program.

2.9. Role o f U.S. Government in measurements discussion session summary (Steve Hogan, Solar Energy Research Institute, Golden, CO) This session, chaired by J o h n Meakin from the University of Delaware, began with invited remarks by representatives of three manufacturers: Richard Addiss of the Solar Power Corporation, Richard Bell of the Mobil Tyco Solar Energy Corporation and Gary Turner o f ARCO Solar Inc. Addiss cited three activities where the U.S. Federal Government has and can continue to have a major impact. The first is to support special meetings, such as the present Workshop, and standards development work. He mentioned, in particular, the support provided to make it possible for the U.S.A. to undertake the responsibility for the secretariat of the new Technical Committee on Photovoltaics, International Electrochemical Commission. The second activity is in providing at least the initial support necessary to realize a reliable source of certified reference cells for the photovoltaics community. The final activity mentioned is in module testing and qualification that has been conducted by the National Photovoltaic Program of the U.S. DOE. F o r example, the work in developing module acceptance criteria and tests and in accumulating extensive field use data has been very valuable. Without the results o f field tests, which revealed numerous unsuspected failure modes, progress in module design and reliability would have been much slower.

20 Bell also cited the importance o f the U.S. Government's support of standards writing activities, of international standards work and workshops o f this kind. He went on to point out the importance of support in ot her areas such as in measurement methods development, requiring such extensire work that it cannot be justified by any one com pany. Work that will lead to standard m e t h o d s for the calibration of reference cells under direct and global irradiance conditions and work to develop a means to measure the match between spectral response curves were mentioned as examples of the support that has been provided. The U.S. Government also has an i mp o r tan t role in many data acquisition activities such as obtaining solar irradiance data needed by the industry for systems design. Turner stated that U.S. G o v e r n m e n t support has allowed m any photovoltaics research paths to be explored as well as the establishment of an engineering working base which the industry can now profitably use in the development o f its products and markets. He reiterated Bell's remarks in saying that the U.S. G o v e r n m e n t has a unique role in a n u m b e r o f i m port ant activities which require such extensive involvement and have such broad application that no one c o m p a n y can consider undertaking them in any effective manner. Examples of such activities are in environmental and solar irradiance data collection. The discussions t hat followed, while wide ranging, had three recurrent themes: (1) the importance of standards and measurements credibility for doing business, (2) the need for a source of standard reference cells and (3) the importance of insolation and weather data for systems design and operation. Remarks about standards and measurements included the following c o m m e n t s which capture the flavor of the discussions on this topic: "got to have standards to c o n d u c t business", "standards are an i m port ant factor in the growth o f photovoltaics", " t her e is a need for a c o m m o n (measurements) ground", " w e need a basis to compare (products)", and "w e need (measurements) traceability". The t er m "t raceabi l i t y" was m e n t i o n e d repeatedly, generally in the c o n t e x t of a means to establish a level of measurements credibility which is accepted by all. Measurements credibility was of particular concern in discussions on commercially available standard reference cells. Such credibility was seen to come from the involvement o f an unbiased technically capable agent that could reliably verify the measurements capability o f the reference cell supplier. The U.S. Federal G o v e r n e m e n t was seen as being able to serve such a " r e f e r e e " function. Such involvement was seen as a means to p r o t e c t bot h the user and the supplier: the user through the steps taken to ensure accurate calibration and characterization, the supplier through verification by an i n d e p e n d e n t party o f the capability o f his measurements and the soundness of his procedures. U.S. G o v e r n m e n t involvement in this capacity was welcomed by DSET Laboratories Inc. in discussing their plans t o sell reference cells calibrated under global irradiance conditions. The means by which the U.S. G o v e r n m e n t might becom e involved were considered in the discussion session on reference cells.

21

The discussion allowed participants to reiterate their strong support for greater U.S. Government involvement in data collection, particularly for insolation and weather. The future need of being able to forecast insolation was highlighted because o f the importance o f such information in load management by, for example, utilities with photovoltaics power stations. It was felt that the U.S. Government's involvement is justified by the wide use of such data, not only by the photovoltaics industry b u t also by other commercial activities, and that such data is vital for the effective conduct of business in these different activities. It was felt that the U.S. Government is the only organization that can effectively perform this service. Several participants spoke about forming a consortium with other groups that have a need for insolation and weather data in order to speak with a stronger voice in seeking such continued and expanded U.S. Government involvement in data collection. The discussions were capped with two unanimous votes to record the consensus of the Workshop in two areas. The intent of the first vote was to record support for the U.S. Federal Government not only to continue but also to expand its data collection work in solar insolation and weather. The second vote was to emphasize the need for the U.S. Government to serve as or assist in the establishment of a referee organization and thereby to provide measurements credibility for any certified reference cells which are made commercially available to the industry and others.

STEVE HOGAN

Solar Energy Research Institute Golden CO 80401 U.S.A.

HARRY A. SCHAFFT

National Bureau o f Standards Washington DC 20234 U.S.A.

References 1 Proc. Workshop on Commercial Photovoltaics Measurements, Vail, CO, July 27 - 29, 1981, in S E R I Conf. Publ. CP-214-1403 (Solar Energy Research Institute, Golden,

co).

2 Proc. Workshop on TerrestriaIPhotovoltaic Measurements, Cleveland, OH, March 19 21, 1975, in N A S A Tech. Memo. TM X-71802, 1975 (National Aeronautics and Space Administration). 3 Proc. 2nd Workshop on Terrestrial Photovoltaic Measurements, Baton Rouge, LA, November 1976, in N A S A Conf. Publ. CP-2010, 1976 (National Aeronautics and Space Administration); Rep. E R D A / N A S A - 1 0 2 2 / 7 6 / 1 0 , 1976 (Energy Research and Development Administration; National Aeronautics and Space Administration). 4 D. E. Sawyer and H. A. Schafft (eds.), Semiconductor Measurement Technology: Stability o f Thin Film Solar Cells and Materials, N B S - D O E Workshop, in NBS Spec. Publ. 400-58, August 1979 (National Bureau of Standards, U.S. Department of Commerce, Washington, DC).

22 5 Reliability o f Materials for Solar Energy, Workshop Proceedings, Vol. 1, Summary and Recommendations, in SERI Tech. Publ. TP-31-248 (Solar Energy Research Institute, Golden, CO). 6 Proc. Photovoltaic Material and Device Measurement Workshop, Arlington, VA, June 11 - 13, 1979, inSol. Cells, 1 (1980) 113 - 346. 7 Interim performance criteria for photovoltaic energy systems, SERI Tech. Rep. TR-742-654, December 1980 (Solar Energy Research Institute, Golden, CO). 8 Standard test method for minority carrier diffusion length in silicon by measurement of steady-state surface photovoltage, A N S I / A S T M Rep. F391-78, 1978 (American National Standards Institute; ASTM). 9 A guide to world insolation data and monitoring networks, SERI Tech. Rep. TR09119-1, June 1981 (Solar Energy Research Institute, Golden, CO).