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Laboratory safety procedures for processing II-VI and related compounds for thin film photovoltaics
Solar Cells, 19 (1986-1987) 225-236
225
LABORATORY SAFETY PROCEDURES FOR PROCESSING I I - V I AND RELATED COMPOUNDS F O R THIN FILM PHOTOVOLTAICS*
STEVENS. HEGEDUS, J. D. MEAKIN and B. N. BARON
Institute of Energy Conversion, University of Delaware, Newark, DE 19716 (U.S.A.) J. A. MILLER
Safety Division, Department of Public Safety, University of Delaware, Newark, DE 19716 (U.S.A.) (Accepted January 31, 1986)
Summary The Institute of Energy Conversion (IEC) at the University of Delaware has been fabricating and evaluating thin film solar cells from I I - V I and related compounds for over a decade. Materials of interest have included CdS, (CdZn)S, CdTe, (CdHg)Te, CuInSe~, ZnS, ZnSe and ZnO. In this paper, we describe specific safety-driven procedures which have evolved for material handling, operating and cleaning of equipment, as well as environmental and biological monitoring. The emphasis of the paper is based on our experience with small-scale batch and unit operations-scale depositions of CdS and (CdHg)Te by evaporation in an academic research and development laboratory. The toxicology and techniques for monitoring environmental levels of cadmium and mercury are described.
1. Solar cells based on I I - V I and related compounds A distinguishing feature of photovoltaic research on thin film s of comp o u n d semiconductors is that each research group must produce its own semiconductor films and very frequently must also synthesize its own starting material. This is in marked contrast to work on any of the commercially available single~rystal wafers where the research can focus on junction formation and device analysis. As a consequence, much of the research directed towards cell development is actually devoted to film preparation. The different deposition techniques used serve to distinguish the various research efforts. In this paper we will review the procedures used at the Institute of Energy Conversion (IEC) to ensure the safe production and *Paper presented at the SERI Photovoltaics Safety Conference, January 16-17, 1986, Lakewood, CO, U.S.A. 0379-6787/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
226 utilization of II-VI and related compounds. Most experience has been gained with physical vapor deposition (PVD) of thin film c o m p o u n d semiconductor cells with a focus on the production at the laboratory scale of experimental devices. However, some unit-operations pilot-scale work will be discussed. Much of the early work in thin film photovoltaics was carried out on the CdS/Cu2S and CdS/Cu2Te heterojunctions [1]. Within the last few years there has been a rapid growth in the use of the chalcopyrite I-III-VI 2 compounds, principally CuInSe2, with the elements sulfur, selenium and tellurium from Group VI and aluminum, gallium and indium from group III. Formation of the heterojunction solar cell is most commonly made with CdS or (CdZn)S. Less extensively investigated are the IIa-V 2 compounds such as ZnaP 2. The III-V compounds are under intensive investigation but will not be included in this paper as they are dealt with elsewhere in this conference. The commonly used I I - V I compounds evaporate congruently and PVD is probably the most widely used film-growth process. Relatively unsophisticated equipment can be used to grow films by PVD. The techniques used at IEC for laboratory scale PVD of CdS have been described in refs. 2 - 4 . Growth techniques related to PVD which also use the c o m p o u n d as a starting material include hot-wall [5] and close-spaced vapor transport [6] and flash evaporation [7]. Because of the enclosed nature of the growth equipment, the safety concerns for all these techniques center on the load and unload cycles, the periodic cleaning procedures and the exhaust venting which are described in the following sections. Conventional sputtering or reactive sputtering share many of the features of PVD. The reactive materials are well enclosed and safety issues center on the load/unload steps. However, the reactive sputtering for CdS and CuInSe 2 involve the highly toxic gases H2S and H2Se and appropriate handling and venting procedures have been described by Thornton [8]. A specific safety issue recently arose with the addition of mercury to CdTe films during the growth process to form (CdHg)Te. In recognition of the volatile nature of elemental mercury, specific handling procedures will be described which were developed to ensure total containment at all stages of source loading and substrate loading and unloading.
2. Toxicology The development and production of solar cells from I I - V I materials involves various metals and their compounds. The toxicological properties of these compounds are not fully understood; however, detailed literature is available on the characteristics of exposure to the elemental forms such as cadmium and mercury [9, 10]. There can be significant biochemical differ-
227 ences between the element and its compounds, e.g. cadmium is water soluble and therefore easily absorbed by h u m a n tissue whereas CdS is insoluble. Potential exposure of workers to cadmium and its compounds can occur in m a n y operations in the manufacturing of CdS solar cells. The toxicity rating for cadmium compounds on the scale of 0 to 3 is 3, indicating that a short exposure to small quantities may cause death or permanent injury [9]. Routes of entry of concern are by ingestion and inhalation with the lungs, liver and kidneys confirmed as target organs [9, 11]. The very high oral toxicity of this metal is offset by the violent emetic action caused by its ingestion [9]. Inhalation of cadmium dust or fumes can cause dryness of the throat, coughing, headache, chest constriction, dyspnea and vomiting [9, 10]. These symptoms are usually delayed, so that even fatal concentrations can be inhaled without sufficient warning [9]. The toxicity of cadmium is partly because of its affinity for combining with sulfhydryl-containing enzymes [ 11 ]. Cadmium compounds including CdS are listed as carcinogens of the connective tissue, lungs and liver in test animals [9, 10, 12]. Accordingly, great care must be taken to minimize exposure to even very low levels of CdS. Mercury and its salts have been confirmed to be general cellular poisons and protein precipitants owing to the release of Hg 2+ ions [11]. The insidious chronic form of mercurialism has been well documented throughout history. The classical symptoms include gingivitis, stomatitis, excessive salivation and metallic taste accompanied by behavioral changes and tremors [13]. Exposure to high concentrations by the respiratory route can cause pneumonitis, bronchitis, chest pains, dyspnea and coughing [11, 13]. Classical manifestations such as tremors may also be present. Oral, respiratory and dermal exposure are all important routes of entry [ 11 ]. The distribution of mercury in tissues and organs exhibits significant interspecies variation with accumulations in the kidney, liver, myocardium, intestines, upper respiratory tract, oral mucosa, testis, skin, bone marrow and the placenta [13]. The kidney accumulates the highest concentrations in most cases [13]. Because of the proven toxicity of cadmium and mercury and the nature of the various procedures used in solar cell technologies, it is necessary to monitor the exposure of workers using classical industrial hygiene methods.
3. Monitoring the workplace 3.1. Cadmium The evaluation of the potential exposure of workers to cadmium consisted of the following: (i) a review of materials and methods; (ii) inspections o f facilities; (iii) air and surface monitoring; (iv) biological screening. Identification of the individuals at risk on the basis of their daily operations was the first step to be taken. These workers and their areas of
228 operation were then inspected for blatant problems such as improper techniques and housekeeping. As an example, the source material CdS is a bright yellow powder. Housekeeping problems can be easily identified by visual observation of deposits. Air and surface monitoring for cadmium were conducted in three separate ways. Area air samples were used most frequently in locations of high potential exposure. An area sample is defined by positioning the sampling train in a fixed location. A personal sample collected by the sampling train being worn by the worker. Surface or swipe samples were also taken in locations of high concentration such as the CdS loading box, the evaporator and doorways to the laboratory. 3.2. Materials and m e t h o d s
Air samples were collected on 0.8 pm mixed cellulose ester filters, 37 mm in diameter, closed faced, using personal-sampling pumps calibrated to a flow rate of 2 1 min-1. Swipe samples were collected on distilled water moistened Fisher brand quantitative filters (catalog 9-790-4C) 9 cm in diameter. One floor tile (1 f t : ) was wiped once in one direction in a back and forth motion followed by wiping a second time perpendicular to the first wipe. Both air and swipe samples were analyzed for cadmium by atomic absorption spectroscopy using a National Institute for Occupational Safety and Health (NIOSH) physical and chemical analysis method (P and CAM) 173 [14] by an independent American Industrial Hygiene Association (AIHA) certified laboratory. Some representative sampling results for airborne cadmium are shown in Table 1 and results for surface
TABLE 1 Airborne cadmium concentrations measured from a small bell-jar type PVD CdS system Sample description
Cadmium level (mg m-3)
Personal sample during cleaning of PVD system Conventional vacuum exhaust during cleaning Area sample in room during cleaning of PVD system with conventional vacuum cleaner Area sample in room with PVD system open, no vacuum cleaner TLV for airborne Cd (8 h exposure) [ 15]
0.025 0.280 0.019 0.0002 0.050
229 TABLE 2 Surface-deposited cadmium concentrations measured from a small bell-jar type PVD CdS system Location
C a d m i u m level (rag ft -2)
Floor tile in front of CdS loading box Floor tile in front of PVD system Floor tile at door to CdS room
0.140 0.480 0.280
3.3. Mercury Owing to the very recent introduction of mercury to form (CdHg)Te in our laboratory, the study of potential exposures to mercury has not been as extensive as the sampling and analysis for cadmium b u t essentially the same approach is being followed. A detailed review of the proposed experimentation with (CdHg)Te was conducted initially to identify potential risks. Qualitative air samples for mercury were collected on mercury vapor detector tubes using a universal sampling pump. Samples were taken in the room and from the exhaust of the evaporation pump during a (CdHg)Te deposition. In all cases, results were below the detection limits of the tubes (0.05 mg m -z) which coincides with the TLV.
3.4. Biological screening There are several reasons for including monitoring of blood and urine for metals when individuals are occupationally at risk. These are as follows: (i) even with extensive air sampling, exposures to high concentrations of short duration m a y be missed; (ii) for some elements, the b o d y burden of the metal m a y be cumulative; (iii) some individuals could also be subject to non-occupational exposure. Cadmium blood and urine levels have been determined for workers having potential exposures. All samples were collected b y Delaware Medical Laboratories, Newark, Delaware with analyses conducted by Smith Kline Clinical Laboratories, Inc., King of Prussia, Pennsylvania. The results of the tests are recorded in Table 3. These results suggest a trend to lower cadmium uptake with time; however, because of the small number of workers sampled, test-to-test variations and results less than the lower detection limit, this cannot be definitively concluded. Only one set of data has been accumulated on blood and urine mercury levels. The average blood mercury level was 0.4 #g d1-1 (normal range 0 10 #g 1-1) and the average urine mercury value was less than 3/~g 1-1 (normal range 0 - 3 0 #g 1-1). The University of Delaware has recently initiated a medical screening program for employees at high risk from hazardous chemicals such as pesti-
230 TABLE 3 Average c a d m i u m c o n c e n t r a t i o n in b l o o d a n d u r i n e for IEC e m p l o y e e s b y y e a r
Year
1981 1982 1983 N o r m a l range D e t e c t i o n limit
Number o f workers
12 13 5
Average blood cadmium
Average urine cadmium
(/.tg 1-1 )
(/.tg
1.8 2.0 none detected 0-10 5.0
7.0 0.44 none detected 7-22 5.0
1-1 )
cides and heavy metals. This program includes a medical history, annual physical examination and appropriate biological tests. The IEC has been included in this program. From our experience with biological testing for cadmium, we must caution that blood and urine concentrations reported b y the analytical laboratories have n o t been completely reliable and repeatable.
4. Cleaning Cleaning the film-growth equipment is potentially the most hazardous operation. The operator is directly exposed to large quantities and surface areas of deposited material on insides of bell jars, substrate holders and other fixtures inside the system. Mechanical scraping or brushing is preferred for removing the bulk of the deposit. Full personal protection, which includes gloves, goggles, respirator and lab coat, should be worn. If a final chemical etch and rinse is needed, it should be performed under a wellventilated hood. Toxic gases such as H2S, H2Se and H2Te result from the etching in acid of CdS, CuInSe 2 and CdTe respectively. These gases are initially readily detected b y their strong foul odor. However, owing to olfactory fatigue, an exposure to significant levels may occur when the operator is unable to detect the gases. H2Se is particularly toxic with an 8 h, time weighted average (TWA) value of only 0.05 ppm compared to 10 ppm for H2S [15]. Little is k n o w n about the toxicity of H:Te. It is c o m m o n practice to use vacuum cleaners for localized removal of deposited material in the form of flakes and powder. Only equipment fitted with a high efficiency particulate air (HEPA) filter should be used. These filters have 99.7% particle retention efficiency at 0.3 #m particle size. As shown in Table 1, sampling the exhaust of a conventional vacuum cleaner which was being used to remove post-deposition material from a CdS evaporator indicated that a very high level of CdS was being dispersed into the room. Vacuum cleaners must not be used to clean up spills or small
231 droplets of mercury since the heat and air flow atomize and disperse the material promoting vaporization. Mercury spill kits should be available and operators should be familiar with their use. Materials contaminated in the cleaning process, especially with mercury, must be disposed of properly. Solvents should be bottled and labeled whereas solids (rags, tissue paper, etc.) should be placed in sealable bags. These bottles and bags should be set aside for collection by an authorized toxic waste disposal organization.
5. Personal-protection equipment Many educational and governmental institutions as well as private corporations have policies governing the use of protective eyewear and respirators, which should be viewed as minimum requirements. A variety of eye, breathing and skin protection devices should be available in the laboratory, including full-face shields for splash protection. Respirators must be fit-tested for each individual instead of assuming that they fit properly. Some individual facial features, such as a narrow bone structure, may make it impossible to fit everyone with one brand of respirator. The correct filter cartridge should be used to match the hazardous situation, i.e. organic mists and vapors, etc. (It should be noted that there are currently no cartridges available which are rated as being effective for filtering mercury vapor.) Training sessions should be held so that employees understand the differences between various eye and breathing-protection equipment to enable them to select the appropriate unit for each operation. For example, they should know when to use a properly fitted respirator with filter cartridges instead of a disposable dust mask; when a face shield is needed in addition to safety glasses. This education is a vital role of the Safety Committee, which will be discussed later. Properly fitted respirators with the appropriate filters, eye-protection, gloves and lab coats should be worn during the loading of source material, post
6. Safety-related reactor design and operation features Procedures and equipment must be designed to confine the source materials and deposited materials to either the source loading area or the film-deposition equipment. Source loading should take place in glove-boxes or other sealable containers. Special handling systems, such as spillproof bottles, are needed with mercury. The mercury evaporation source should be tightly sealed during transportation from the loading area to the evaporator. Extreme care is
232
required to prevent spills in re-filling the source bottle in the ventilated glove-box. Standard safety features should be incorporated in all deposition systems to minimize potential hazards owing to high temperatures, explosion or implosion, radiation, etc. For example, glass bell-jar vacuum systems should have fine mesh or solid metal screens to contain glass in the event of an implosion. Roughing pumps should be in a well-ventilated area, preferably vented directly out of the building. When appropriate, the exhaust should be passed through a scrubber. After a deposition, the reactor should be flushed with an inert gas and cycled back down to a mild vacuum ( ~ 1 0 0 pm Hg) to reduce the concentration of any hazardous reactants. After going through this cycle at least once, the system should be opened slightly (i.e. the bell-jar should be raised a few centimeters) and vacuum applied around the opening to further remove any airborne material while venting the vacuum to an external exhaust system. Any exhaust system which may carry toxic aerosols or vapors should be tested with a smoke bomb or other m e t h o d to ensure adequate airflow. Other work stations which share c o m m o n ductwork should be monitored for back~treaming. Air-flow patterns on the roof should be checked under different wind conditions to ensure there is no cross,contamination of building air intake from the exhaust stacks. Extra precautions have been taken to ensure safe handling of mercury. The mercury level in the exhaust from an evaporator used to deposit (CdHg)Te was monitored with calibrated detector tubes. The mercury source was at 90 °C. The system had a cold-trap directly under the base plate. No mercury was detected in the exhaust under sampling conditions which had a lower limit of detectability of 0.05 mg m -3 indicating that all of the unreacted mercury was being condensed on the cold trap.
7. Unit operations experimentation A key unit operation in the manufacture of CdS-based thin film photovoltaic modules is deposition of CdS. To achieve low manufacturing costs it is desirable to continuously deposit onto a moving substrate [16]. However, the design of commercial scale equipment for continuous deposition of CdS requires information that cannot be readily obtained from laboratory scale experiments. Unit operations experimentation is an integral part of process research and development in the chemical industries. It provides the quantitative information and experience that is needed for efficient translation of scale from the laboratory to commercial production. Unit operations experiments on the continuous deposition of CdS were performed from June 1980 to December 1981 at the University of Delaware using a reel-to-reel vacuum coater which is described elsewhere [17]. Depositions were carried out with substrate throughputs ranging
233
from 180 c m 2 h -I to 600 c m 2 h -I at film growth rates from 0.5/~m min -I to 2/~m rnin-i. Runs yielding up to 3000 c m 2 of 25/~m thick CdS have been carried out at throughputs of approximately 400 c m 2 h -I. This corresponded to using approximately 0.1 kg to 0.4 kg of CdS per deposition. Materials utilization (the amount of source material deposited in usable film) was typically 2 5 % at a substrate temperature of 230 °C. The remainder was deposited on evaporation shields and the walls of the coater. A very small amount was presumed to go into the pumping stack where it was trapped in oil. Pumps were vented to the roof without scrubbing. After eighteen months no deposits were found in the pipes connecting the pumps to the vent stack. A principal concern was the release of particles containing cadmium from deposits on the evaporation shields and chamber walls into the work place when the vacuum coater was opened for cleaning and reloading. To contain the particulates, a polyethylene enclosure was built around the vacuum chamber. Entry to the enclosure was through a baffled passageway. High volume exhaust fans were mounted on top of the enclosure to pull airborne particles up and to minimize the amount of CdS dust which could settle on the floor. As with batch-type evaporators, personnel wore protective clothing, respirators and eye protection. Additional containment was provided by using disposable jumpsuits and foot covers. Initialmonitoring of the airborne cadmium level using personal-sampling pumps (as described in Section 3) during the cleaning of the large,unit operations vacuum coater showed 0.7 m g m -3 of cadmium. This is fourteen times the T L V for airborne cadmium. Swab samples of the floor inside the enclosure yielded 1.1-3.8 m g ft-2 of cadmium. The level outside the enclosure at the doorway to the unit operations laboratory was 0.5 m g ft-2 . Employee exposure to cadmium was also monitored by blood and urine analyses which were generally found to yield levelsbelow the occupational exposure limit. However, there were instances when an individual's urine cadmium concentration tested at or above the occupational exposure limit. These results could not be reconciled with those individuals' possible exposure history, i.e. they had minimal contact with CdS or with the equipment. U p o n retesting,their new level was found to be below the limit. These results indicated the need for improved containment of CdS particulates. However, exposure of operators was minimized acceptably (as determined from biological sampling) by their use of personal-protection equipment and adherence to safety procedures at all times. Recommendations for improved particulate control include reducing the airborne level by capturing the particlesat the opening of the deposition unit (as described in Section 6) and isolation of the deposition laboratory from the rest of the facilityby installinga decontamination room for changing clothes and showering. Unit operations experimentation was concluded in December 1981 after successful demonstration of the continuous deposition of photovoltaic grade CdS but before we were able to evaluate these recommendations.
234 8. Role of the Safety Committee Many institutions have Safety Departments whose responsibility is to supply expertise and to ensure compliance with regulations. However, a strong and dedicated departmental laboratory Safety Committee composed of representatives from all interested groups is very important in organizing and implementing the above procedures to minimize exposure to hazardous materials. The Committee should include representatives of the faculty and professional researchers, technicians, students, management and safety specialists. The activities of this group should encourage a high level of safety awareness among all laboratory personnel. They should meet regularly to coordinate laboratory inspections, schedule appropriate environmental and biological testing and act as a focal point for discussion of concerns. To function effectively, the Safety Committee needs to have access to outside safety professionals and to have the support of laboratory management or department heads, who in turn must be charged with responding in a substantive way to all recommendations or concerns. The Safety Committee should familiarize itself with the toxicology of the materials in use, develop an awareness of current safety products (monitoring techniques, personal detector badges, respirators) and safety regulations imposed by their institution and state and federal governments. It should maintain records of all testing. The Committee should ensure that appropriate equipment is available, such as eye protection, chemical spill kits and specialized devices (source-loading boxes) and that proper procedures are being followed. Should a significant level of hazardous materials such as cadmium or mercury be detected by either environmental or biological monitoring, they should critically examine the situation, analyze patterns of material usage, evaluate existing controls and r e c o m m e n d changes in deposition or handling procedures.
9. Conclusions
We have described the safety-oriented regimen which has evolved at the Institute of Energy Conversion at the University of Delaware to minimize and to measure the exposure of employees to II-VI and related compounds, particularly CdS and (CdHg)Te. Various procedures and controls have been developed to ensure that the hazardous material is contained. Environmental (air and surface) and biological (blood and urine) monitoring is required to quantify the degree of exposure, the effectiveness of the controls and also to provide a record of employee exposure history. If environmental and/or biological testing indicates that employees are. being exposed to hazardous materials such as cadmium or mercury, environmental testing is repeated to establish patterns of contamination of the laboratory workplace. After improvements, monitoring is performed to verify that the new controls are effective.
235
The experience of continuous deposition of CdS at the unit operationsscale experiment indicated a high priority need for effective containment of particulates. A Safety Committee should be established in all research groups to be responsible for the following: (i) informing employees about the usage of appropriate personal-protection equipment; (ii) providing expertise in determining safe deposition conditions and equipment design; (iii) coordinating environmental and biological testing and laboratory inspections; (iv) maintaining awareness of toxicology and applicable federal and state guidelines; (v) responding to student, employee or management concerns. By following these guidelines derived from experience in batch and unit operations scale deposition of CdS and (CdHg)Te, exposure to hazardous materials can be minimized and controlled to an acceptable level. Future industrial hygiene studies will include integrated sampling for mecury and the continuation of present cadmium containment programs. Re-testing will be conducted to determine the effectiveness of current control measures.
Acknowledgments We are grateful to M. D. Stallings and T. W. F. Russell for their strong support of the IEC Safety Program, to R. W. Birkmire and B. E. McCandless for information regarding deposition equipment and E. Koronik for manuscript preparation.
References 1 2 3 4 5 6 7 8
9 10 11
D.C. Reynolds and G. M. Leies, Electr. Eng., 73 (1954) 734. R.B. Hall and J. D. Meakin, Thin Solid Films, 63 (1979) 203. R.E. Rocheleau, B. N. Baron and T. W. F. Russell, AIChE J., 28 (1982) 656. S. P. Shea, R. W. Birkmire and J. D. Meakin, 17th IEEE Photovoltaic Specialists' Conf. IEEE, New York, 1984, p. 1270. A. Lopez-Otero and W. Huber, Surf. Sci., 86 (1979) 167. R. H. Bube, A. L. Fahrenbruch, R. Sinclair, T. C. Anthony, C. Fortmann, W. Huber, C.-T. Lee, T. Thorpe and T. Yamashita, IEEE Trans. Electron Dev., 31 (1984) 528. V. Canevari, N. Romeo and G. Sberveglieri, Mater. Chem. Phys., 9 (1983) 851. J. A. Thornton, D. G. Cornog, R. B. Hall, S. P. Shea and J. D. Meakin, Materials and New Processing Technologies for Photovoltaics, Electrochem. Soc., 83 (11) (1983) 419. Progress reports on SERI sub-contracts XW-2-01313-1 and XL-504131-1. I . N . Sax, Dangerous Properties o f Industrial Materials, Van Nostrand Reinhold, New York, 5th edn., 1979. Industrial Hygiene Characterization o f Photovoltaic Solar Cell Industry, National Institute for Occupational Safety and Health, Publication 80-112, March, 1980. F. A. Patty, Industrial Hygiene and Toxicology, 2nd rev. edn., Interscience, New York, 1963.
236 12 National Institute for Occupational Safety and Health, Registry o f Toxic Effects o f Chemical Substances, 1979. 13 National Institute for Occupational Safety and Health, Criteria for a Recommended S t a n d a r d . . . Occupational Exposure to Inorganic Mercury, PB-222 223, 1973. 14 American Conference of Governmental Industrial Hygienists, Threshold Limit Values for Chemical Substances in Workroom Air, adopted by ACGIH, 1985. 15 Manual o f Analytical Methods, Vol. 5 (NIOSH publication no. 79-141), National Institute for Occupational Safety and Health, 1979, 2nd edn. 16 T . W . F . Russell, B. N. Baron and R. E. Rocheleau, in I. H. Farag and S. S. Melsheimer (eds.), Fundamentals and Applications o f Solar Energy II, AIChE Syrup. Set., S-210 (1981). 17 T . W . F . Russell, B. N. Baron and R. E. Rocheleau, Proc. 3rd European Communities Photovoltaic Solar Energy Conf., Cannes, October 1980, Reidel, Boston, MA, 1980, pp. 348-352.
225
LABORATORY SAFETY PROCEDURES FOR PROCESSING I I - V I AND RELATED COMPOUNDS F O R THIN FILM PHOTOVOLTAICS*
STEVENS. HEGEDUS, J. D. MEAKIN and B. N. BARON
Institute of Energy Conversion, University of Delaware, Newark, DE 19716 (U.S.A.) J. A. MILLER
Safety Division, Department of Public Safety, University of Delaware, Newark, DE 19716 (U.S.A.) (Accepted January 31, 1986)
Summary The Institute of Energy Conversion (IEC) at the University of Delaware has been fabricating and evaluating thin film solar cells from I I - V I and related compounds for over a decade. Materials of interest have included CdS, (CdZn)S, CdTe, (CdHg)Te, CuInSe~, ZnS, ZnSe and ZnO. In this paper, we describe specific safety-driven procedures which have evolved for material handling, operating and cleaning of equipment, as well as environmental and biological monitoring. The emphasis of the paper is based on our experience with small-scale batch and unit operations-scale depositions of CdS and (CdHg)Te by evaporation in an academic research and development laboratory. The toxicology and techniques for monitoring environmental levels of cadmium and mercury are described.
1. Solar cells based on I I - V I and related compounds A distinguishing feature of photovoltaic research on thin film s of comp o u n d semiconductors is that each research group must produce its own semiconductor films and very frequently must also synthesize its own starting material. This is in marked contrast to work on any of the commercially available single~rystal wafers where the research can focus on junction formation and device analysis. As a consequence, much of the research directed towards cell development is actually devoted to film preparation. The different deposition techniques used serve to distinguish the various research efforts. In this paper we will review the procedures used at the Institute of Energy Conversion (IEC) to ensure the safe production and *Paper presented at the SERI Photovoltaics Safety Conference, January 16-17, 1986, Lakewood, CO, U.S.A. 0379-6787/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
226 utilization of II-VI and related compounds. Most experience has been gained with physical vapor deposition (PVD) of thin film c o m p o u n d semiconductor cells with a focus on the production at the laboratory scale of experimental devices. However, some unit-operations pilot-scale work will be discussed. Much of the early work in thin film photovoltaics was carried out on the CdS/Cu2S and CdS/Cu2Te heterojunctions [1]. Within the last few years there has been a rapid growth in the use of the chalcopyrite I-III-VI 2 compounds, principally CuInSe2, with the elements sulfur, selenium and tellurium from Group VI and aluminum, gallium and indium from group III. Formation of the heterojunction solar cell is most commonly made with CdS or (CdZn)S. Less extensively investigated are the IIa-V 2 compounds such as ZnaP 2. The III-V compounds are under intensive investigation but will not be included in this paper as they are dealt with elsewhere in this conference. The commonly used I I - V I compounds evaporate congruently and PVD is probably the most widely used film-growth process. Relatively unsophisticated equipment can be used to grow films by PVD. The techniques used at IEC for laboratory scale PVD of CdS have been described in refs. 2 - 4 . Growth techniques related to PVD which also use the c o m p o u n d as a starting material include hot-wall [5] and close-spaced vapor transport [6] and flash evaporation [7]. Because of the enclosed nature of the growth equipment, the safety concerns for all these techniques center on the load and unload cycles, the periodic cleaning procedures and the exhaust venting which are described in the following sections. Conventional sputtering or reactive sputtering share many of the features of PVD. The reactive materials are well enclosed and safety issues center on the load/unload steps. However, the reactive sputtering for CdS and CuInSe 2 involve the highly toxic gases H2S and H2Se and appropriate handling and venting procedures have been described by Thornton [8]. A specific safety issue recently arose with the addition of mercury to CdTe films during the growth process to form (CdHg)Te. In recognition of the volatile nature of elemental mercury, specific handling procedures will be described which were developed to ensure total containment at all stages of source loading and substrate loading and unloading.
2. Toxicology The development and production of solar cells from I I - V I materials involves various metals and their compounds. The toxicological properties of these compounds are not fully understood; however, detailed literature is available on the characteristics of exposure to the elemental forms such as cadmium and mercury [9, 10]. There can be significant biochemical differ-
227 ences between the element and its compounds, e.g. cadmium is water soluble and therefore easily absorbed by h u m a n tissue whereas CdS is insoluble. Potential exposure of workers to cadmium and its compounds can occur in m a n y operations in the manufacturing of CdS solar cells. The toxicity rating for cadmium compounds on the scale of 0 to 3 is 3, indicating that a short exposure to small quantities may cause death or permanent injury [9]. Routes of entry of concern are by ingestion and inhalation with the lungs, liver and kidneys confirmed as target organs [9, 11]. The very high oral toxicity of this metal is offset by the violent emetic action caused by its ingestion [9]. Inhalation of cadmium dust or fumes can cause dryness of the throat, coughing, headache, chest constriction, dyspnea and vomiting [9, 10]. These symptoms are usually delayed, so that even fatal concentrations can be inhaled without sufficient warning [9]. The toxicity of cadmium is partly because of its affinity for combining with sulfhydryl-containing enzymes [ 11 ]. Cadmium compounds including CdS are listed as carcinogens of the connective tissue, lungs and liver in test animals [9, 10, 12]. Accordingly, great care must be taken to minimize exposure to even very low levels of CdS. Mercury and its salts have been confirmed to be general cellular poisons and protein precipitants owing to the release of Hg 2+ ions [11]. The insidious chronic form of mercurialism has been well documented throughout history. The classical symptoms include gingivitis, stomatitis, excessive salivation and metallic taste accompanied by behavioral changes and tremors [13]. Exposure to high concentrations by the respiratory route can cause pneumonitis, bronchitis, chest pains, dyspnea and coughing [11, 13]. Classical manifestations such as tremors may also be present. Oral, respiratory and dermal exposure are all important routes of entry [ 11 ]. The distribution of mercury in tissues and organs exhibits significant interspecies variation with accumulations in the kidney, liver, myocardium, intestines, upper respiratory tract, oral mucosa, testis, skin, bone marrow and the placenta [13]. The kidney accumulates the highest concentrations in most cases [13]. Because of the proven toxicity of cadmium and mercury and the nature of the various procedures used in solar cell technologies, it is necessary to monitor the exposure of workers using classical industrial hygiene methods.
3. Monitoring the workplace 3.1. Cadmium The evaluation of the potential exposure of workers to cadmium consisted of the following: (i) a review of materials and methods; (ii) inspections o f facilities; (iii) air and surface monitoring; (iv) biological screening. Identification of the individuals at risk on the basis of their daily operations was the first step to be taken. These workers and their areas of
228 operation were then inspected for blatant problems such as improper techniques and housekeeping. As an example, the source material CdS is a bright yellow powder. Housekeeping problems can be easily identified by visual observation of deposits. Air and surface monitoring for cadmium were conducted in three separate ways. Area air samples were used most frequently in locations of high potential exposure. An area sample is defined by positioning the sampling train in a fixed location. A personal sample collected by the sampling train being worn by the worker. Surface or swipe samples were also taken in locations of high concentration such as the CdS loading box, the evaporator and doorways to the laboratory. 3.2. Materials and m e t h o d s
Air samples were collected on 0.8 pm mixed cellulose ester filters, 37 mm in diameter, closed faced, using personal-sampling pumps calibrated to a flow rate of 2 1 min-1. Swipe samples were collected on distilled water moistened Fisher brand quantitative filters (catalog 9-790-4C) 9 cm in diameter. One floor tile (1 f t : ) was wiped once in one direction in a back and forth motion followed by wiping a second time perpendicular to the first wipe. Both air and swipe samples were analyzed for cadmium by atomic absorption spectroscopy using a National Institute for Occupational Safety and Health (NIOSH) physical and chemical analysis method (P and CAM) 173 [14] by an independent American Industrial Hygiene Association (AIHA) certified laboratory. Some representative sampling results for airborne cadmium are shown in Table 1 and results for surface
TABLE 1 Airborne cadmium concentrations measured from a small bell-jar type PVD CdS system Sample description
Cadmium level (mg m-3)
Personal sample during cleaning of PVD system Conventional vacuum exhaust during cleaning Area sample in room during cleaning of PVD system with conventional vacuum cleaner Area sample in room with PVD system open, no vacuum cleaner TLV for airborne Cd (8 h exposure) [ 15]
0.025 0.280 0.019 0.0002 0.050
229 TABLE 2 Surface-deposited cadmium concentrations measured from a small bell-jar type PVD CdS system Location
C a d m i u m level (rag ft -2)
Floor tile in front of CdS loading box Floor tile in front of PVD system Floor tile at door to CdS room
0.140 0.480 0.280
3.3. Mercury Owing to the very recent introduction of mercury to form (CdHg)Te in our laboratory, the study of potential exposures to mercury has not been as extensive as the sampling and analysis for cadmium b u t essentially the same approach is being followed. A detailed review of the proposed experimentation with (CdHg)Te was conducted initially to identify potential risks. Qualitative air samples for mercury were collected on mercury vapor detector tubes using a universal sampling pump. Samples were taken in the room and from the exhaust of the evaporation pump during a (CdHg)Te deposition. In all cases, results were below the detection limits of the tubes (0.05 mg m -z) which coincides with the TLV.
3.4. Biological screening There are several reasons for including monitoring of blood and urine for metals when individuals are occupationally at risk. These are as follows: (i) even with extensive air sampling, exposures to high concentrations of short duration m a y be missed; (ii) for some elements, the b o d y burden of the metal m a y be cumulative; (iii) some individuals could also be subject to non-occupational exposure. Cadmium blood and urine levels have been determined for workers having potential exposures. All samples were collected b y Delaware Medical Laboratories, Newark, Delaware with analyses conducted by Smith Kline Clinical Laboratories, Inc., King of Prussia, Pennsylvania. The results of the tests are recorded in Table 3. These results suggest a trend to lower cadmium uptake with time; however, because of the small number of workers sampled, test-to-test variations and results less than the lower detection limit, this cannot be definitively concluded. Only one set of data has been accumulated on blood and urine mercury levels. The average blood mercury level was 0.4 #g d1-1 (normal range 0 10 #g 1-1) and the average urine mercury value was less than 3/~g 1-1 (normal range 0 - 3 0 #g 1-1). The University of Delaware has recently initiated a medical screening program for employees at high risk from hazardous chemicals such as pesti-
230 TABLE 3 Average c a d m i u m c o n c e n t r a t i o n in b l o o d a n d u r i n e for IEC e m p l o y e e s b y y e a r
Year
1981 1982 1983 N o r m a l range D e t e c t i o n limit
Number o f workers
12 13 5
Average blood cadmium
Average urine cadmium
(/.tg 1-1 )
(/.tg
1.8 2.0 none detected 0-10 5.0
7.0 0.44 none detected 7-22 5.0
1-1 )
cides and heavy metals. This program includes a medical history, annual physical examination and appropriate biological tests. The IEC has been included in this program. From our experience with biological testing for cadmium, we must caution that blood and urine concentrations reported b y the analytical laboratories have n o t been completely reliable and repeatable.
4. Cleaning Cleaning the film-growth equipment is potentially the most hazardous operation. The operator is directly exposed to large quantities and surface areas of deposited material on insides of bell jars, substrate holders and other fixtures inside the system. Mechanical scraping or brushing is preferred for removing the bulk of the deposit. Full personal protection, which includes gloves, goggles, respirator and lab coat, should be worn. If a final chemical etch and rinse is needed, it should be performed under a wellventilated hood. Toxic gases such as H2S, H2Se and H2Te result from the etching in acid of CdS, CuInSe 2 and CdTe respectively. These gases are initially readily detected b y their strong foul odor. However, owing to olfactory fatigue, an exposure to significant levels may occur when the operator is unable to detect the gases. H2Se is particularly toxic with an 8 h, time weighted average (TWA) value of only 0.05 ppm compared to 10 ppm for H2S [15]. Little is k n o w n about the toxicity of H:Te. It is c o m m o n practice to use vacuum cleaners for localized removal of deposited material in the form of flakes and powder. Only equipment fitted with a high efficiency particulate air (HEPA) filter should be used. These filters have 99.7% particle retention efficiency at 0.3 #m particle size. As shown in Table 1, sampling the exhaust of a conventional vacuum cleaner which was being used to remove post-deposition material from a CdS evaporator indicated that a very high level of CdS was being dispersed into the room. Vacuum cleaners must not be used to clean up spills or small
231 droplets of mercury since the heat and air flow atomize and disperse the material promoting vaporization. Mercury spill kits should be available and operators should be familiar with their use. Materials contaminated in the cleaning process, especially with mercury, must be disposed of properly. Solvents should be bottled and labeled whereas solids (rags, tissue paper, etc.) should be placed in sealable bags. These bottles and bags should be set aside for collection by an authorized toxic waste disposal organization.
5. Personal-protection equipment Many educational and governmental institutions as well as private corporations have policies governing the use of protective eyewear and respirators, which should be viewed as minimum requirements. A variety of eye, breathing and skin protection devices should be available in the laboratory, including full-face shields for splash protection. Respirators must be fit-tested for each individual instead of assuming that they fit properly. Some individual facial features, such as a narrow bone structure, may make it impossible to fit everyone with one brand of respirator. The correct filter cartridge should be used to match the hazardous situation, i.e. organic mists and vapors, etc. (It should be noted that there are currently no cartridges available which are rated as being effective for filtering mercury vapor.) Training sessions should be held so that employees understand the differences between various eye and breathing-protection equipment to enable them to select the appropriate unit for each operation. For example, they should know when to use a properly fitted respirator with filter cartridges instead of a disposable dust mask; when a face shield is needed in addition to safety glasses. This education is a vital role of the Safety Committee, which will be discussed later. Properly fitted respirators with the appropriate filters, eye-protection, gloves and lab coats should be worn during the loading of source material, post
6. Safety-related reactor design and operation features Procedures and equipment must be designed to confine the source materials and deposited materials to either the source loading area or the film-deposition equipment. Source loading should take place in glove-boxes or other sealable containers. Special handling systems, such as spillproof bottles, are needed with mercury. The mercury evaporation source should be tightly sealed during transportation from the loading area to the evaporator. Extreme care is
232
required to prevent spills in re-filling the source bottle in the ventilated glove-box. Standard safety features should be incorporated in all deposition systems to minimize potential hazards owing to high temperatures, explosion or implosion, radiation, etc. For example, glass bell-jar vacuum systems should have fine mesh or solid metal screens to contain glass in the event of an implosion. Roughing pumps should be in a well-ventilated area, preferably vented directly out of the building. When appropriate, the exhaust should be passed through a scrubber. After a deposition, the reactor should be flushed with an inert gas and cycled back down to a mild vacuum ( ~ 1 0 0 pm Hg) to reduce the concentration of any hazardous reactants. After going through this cycle at least once, the system should be opened slightly (i.e. the bell-jar should be raised a few centimeters) and vacuum applied around the opening to further remove any airborne material while venting the vacuum to an external exhaust system. Any exhaust system which may carry toxic aerosols or vapors should be tested with a smoke bomb or other m e t h o d to ensure adequate airflow. Other work stations which share c o m m o n ductwork should be monitored for back~treaming. Air-flow patterns on the roof should be checked under different wind conditions to ensure there is no cross,contamination of building air intake from the exhaust stacks. Extra precautions have been taken to ensure safe handling of mercury. The mercury level in the exhaust from an evaporator used to deposit (CdHg)Te was monitored with calibrated detector tubes. The mercury source was at 90 °C. The system had a cold-trap directly under the base plate. No mercury was detected in the exhaust under sampling conditions which had a lower limit of detectability of 0.05 mg m -3 indicating that all of the unreacted mercury was being condensed on the cold trap.
7. Unit operations experimentation A key unit operation in the manufacture of CdS-based thin film photovoltaic modules is deposition of CdS. To achieve low manufacturing costs it is desirable to continuously deposit onto a moving substrate [16]. However, the design of commercial scale equipment for continuous deposition of CdS requires information that cannot be readily obtained from laboratory scale experiments. Unit operations experimentation is an integral part of process research and development in the chemical industries. It provides the quantitative information and experience that is needed for efficient translation of scale from the laboratory to commercial production. Unit operations experiments on the continuous deposition of CdS were performed from June 1980 to December 1981 at the University of Delaware using a reel-to-reel vacuum coater which is described elsewhere [17]. Depositions were carried out with substrate throughputs ranging
233
from 180 c m 2 h -I to 600 c m 2 h -I at film growth rates from 0.5/~m min -I to 2/~m rnin-i. Runs yielding up to 3000 c m 2 of 25/~m thick CdS have been carried out at throughputs of approximately 400 c m 2 h -I. This corresponded to using approximately 0.1 kg to 0.4 kg of CdS per deposition. Materials utilization (the amount of source material deposited in usable film) was typically 2 5 % at a substrate temperature of 230 °C. The remainder was deposited on evaporation shields and the walls of the coater. A very small amount was presumed to go into the pumping stack where it was trapped in oil. Pumps were vented to the roof without scrubbing. After eighteen months no deposits were found in the pipes connecting the pumps to the vent stack. A principal concern was the release of particles containing cadmium from deposits on the evaporation shields and chamber walls into the work place when the vacuum coater was opened for cleaning and reloading. To contain the particulates, a polyethylene enclosure was built around the vacuum chamber. Entry to the enclosure was through a baffled passageway. High volume exhaust fans were mounted on top of the enclosure to pull airborne particles up and to minimize the amount of CdS dust which could settle on the floor. As with batch-type evaporators, personnel wore protective clothing, respirators and eye protection. Additional containment was provided by using disposable jumpsuits and foot covers. Initialmonitoring of the airborne cadmium level using personal-sampling pumps (as described in Section 3) during the cleaning of the large,unit operations vacuum coater showed 0.7 m g m -3 of cadmium. This is fourteen times the T L V for airborne cadmium. Swab samples of the floor inside the enclosure yielded 1.1-3.8 m g ft-2 of cadmium. The level outside the enclosure at the doorway to the unit operations laboratory was 0.5 m g ft-2 . Employee exposure to cadmium was also monitored by blood and urine analyses which were generally found to yield levelsbelow the occupational exposure limit. However, there were instances when an individual's urine cadmium concentration tested at or above the occupational exposure limit. These results could not be reconciled with those individuals' possible exposure history, i.e. they had minimal contact with CdS or with the equipment. U p o n retesting,their new level was found to be below the limit. These results indicated the need for improved containment of CdS particulates. However, exposure of operators was minimized acceptably (as determined from biological sampling) by their use of personal-protection equipment and adherence to safety procedures at all times. Recommendations for improved particulate control include reducing the airborne level by capturing the particlesat the opening of the deposition unit (as described in Section 6) and isolation of the deposition laboratory from the rest of the facilityby installinga decontamination room for changing clothes and showering. Unit operations experimentation was concluded in December 1981 after successful demonstration of the continuous deposition of photovoltaic grade CdS but before we were able to evaluate these recommendations.
234 8. Role of the Safety Committee Many institutions have Safety Departments whose responsibility is to supply expertise and to ensure compliance with regulations. However, a strong and dedicated departmental laboratory Safety Committee composed of representatives from all interested groups is very important in organizing and implementing the above procedures to minimize exposure to hazardous materials. The Committee should include representatives of the faculty and professional researchers, technicians, students, management and safety specialists. The activities of this group should encourage a high level of safety awareness among all laboratory personnel. They should meet regularly to coordinate laboratory inspections, schedule appropriate environmental and biological testing and act as a focal point for discussion of concerns. To function effectively, the Safety Committee needs to have access to outside safety professionals and to have the support of laboratory management or department heads, who in turn must be charged with responding in a substantive way to all recommendations or concerns. The Safety Committee should familiarize itself with the toxicology of the materials in use, develop an awareness of current safety products (monitoring techniques, personal detector badges, respirators) and safety regulations imposed by their institution and state and federal governments. It should maintain records of all testing. The Committee should ensure that appropriate equipment is available, such as eye protection, chemical spill kits and specialized devices (source-loading boxes) and that proper procedures are being followed. Should a significant level of hazardous materials such as cadmium or mercury be detected by either environmental or biological monitoring, they should critically examine the situation, analyze patterns of material usage, evaluate existing controls and r e c o m m e n d changes in deposition or handling procedures.
9. Conclusions
We have described the safety-oriented regimen which has evolved at the Institute of Energy Conversion at the University of Delaware to minimize and to measure the exposure of employees to II-VI and related compounds, particularly CdS and (CdHg)Te. Various procedures and controls have been developed to ensure that the hazardous material is contained. Environmental (air and surface) and biological (blood and urine) monitoring is required to quantify the degree of exposure, the effectiveness of the controls and also to provide a record of employee exposure history. If environmental and/or biological testing indicates that employees are. being exposed to hazardous materials such as cadmium or mercury, environmental testing is repeated to establish patterns of contamination of the laboratory workplace. After improvements, monitoring is performed to verify that the new controls are effective.
235
The experience of continuous deposition of CdS at the unit operationsscale experiment indicated a high priority need for effective containment of particulates. A Safety Committee should be established in all research groups to be responsible for the following: (i) informing employees about the usage of appropriate personal-protection equipment; (ii) providing expertise in determining safe deposition conditions and equipment design; (iii) coordinating environmental and biological testing and laboratory inspections; (iv) maintaining awareness of toxicology and applicable federal and state guidelines; (v) responding to student, employee or management concerns. By following these guidelines derived from experience in batch and unit operations scale deposition of CdS and (CdHg)Te, exposure to hazardous materials can be minimized and controlled to an acceptable level. Future industrial hygiene studies will include integrated sampling for mecury and the continuation of present cadmium containment programs. Re-testing will be conducted to determine the effectiveness of current control measures.
Acknowledgments We are grateful to M. D. Stallings and T. W. F. Russell for their strong support of the IEC Safety Program, to R. W. Birkmire and B. E. McCandless for information regarding deposition equipment and E. Koronik for manuscript preparation.
References 1 2 3 4 5 6 7 8
9 10 11
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236 12 National Institute for Occupational Safety and Health, Registry o f Toxic Effects o f Chemical Substances, 1979. 13 National Institute for Occupational Safety and Health, Criteria for a Recommended S t a n d a r d . . . Occupational Exposure to Inorganic Mercury, PB-222 223, 1973. 14 American Conference of Governmental Industrial Hygienists, Threshold Limit Values for Chemical Substances in Workroom Air, adopted by ACGIH, 1985. 15 Manual o f Analytical Methods, Vol. 5 (NIOSH publication no. 79-141), National Institute for Occupational Safety and Health, 1979, 2nd edn. 16 T . W . F . Russell, B. N. Baron and R. E. Rocheleau, in I. H. Farag and S. S. Melsheimer (eds.), Fundamentals and Applications o f Solar Energy II, AIChE Syrup. Set., S-210 (1981). 17 T . W . F . Russell, B. N. Baron and R. E. Rocheleau, Proc. 3rd European Communities Photovoltaic Solar Energy Conf., Cannes, October 1980, Reidel, Boston, MA, 1980, pp. 348-352.