3.1 Occupational Health Issues

In the manufacture of wafer-based crystalline silicon solar cells, occupational health issues are related to potential chemical burns and the inhalation of fumes from hydrofluoric acid (HF), nitric acid (e.g., HNO₃) and alkalis (e.g., NaOH) used for wafer cleaning, removing dopant oxides, and reactor cleaning. Dopant gases and vapors (e.g., POCl₃, B₂H₃), also are hazardous if inhaled. POCl₃ is a liquid, but in a deposition chamber it can generate toxic P₂O₅ and Cl₂ gaseous effluents. Inhalation hazards are controlled with properly designed ventilation systems in the process stations. Other occupational hazards are related to the flammability of silane (SiH₄) and its byproducts used in silicon nitride deposition; these hazards are discussed in the a-Si section below.

3.2 Public Health and Environmental Issues

No public health issues were identified with this technology. The environmental issues are related to the generation of liquid and solid wastes during wafer slicing, cleaning, and etching, and during processing and assembling of solar cells. The x-Si PV industry has embarked upon programs of waste minimization and examines environmentally friendlier alternatives for solders, slurries and solvents. Successful efforts were reported in laboratory and manufacturing scales in reducing the caustic waste generated by etching. Other efforts for waste minimization include recycling stainless-steel cutting wires, recovering the SiC in the slurry, and in-house neutralization of acid and alkali solutions. Finally, the content of Pb in solder in many of today’s modules creates concerns about the disposal of modules at the end of their useful life. One x-Si manufacturer has developed and is using Pb-free solders, and has offered the technology know-how to others who are considering such a change [1].

4.1 Occupational Safety Issues

The main safety hazard of this technology is the use of SiH₄ gas, which is extremely pyrophoric. The lower limit for its spontaneous ignition in air ranges from 2% to 3%, depending on the carrier gas. If mixing is incomplete, a pyrophoric concentration may exist locally, even if the concentration of SiH₄ in the carrier gas is less than 2%. At silane concentrations equal to or greater than 4.5%, the mixtures were found to be metastable and ignited after a certain delay. In an accident, this event could be extremely destructive as protection provided by venting would be ineffective. Silane safety is discussed in detail elsewhere [2–5]. In addition to SiH₄, hydrogen used in a-Si manufacturing, also is flammable and explosive. Most PV manufacturers use sophisticated gas-handling systems with sufficient safety features to minimize the risks of fire and explosion. Some facilities store silane and hydrogen in bulk from tube trailers to avoid frequently changing gas cylinders. A bulk ISO (International Standards Organization) module typically contains eight cylindrical tubes that are mounted onto a trailer suitable for over the road and ocean transport. These modules carry up to 3000 kg of silane. Another option is a single, 450 I cylinder, mounted on a skid, which contains up to 150 kg of silane (mini-bulk). These storage systems are equipped with isolation and flow restricting valves.

Bulk storage decreases the probability of an accident, since trailer changes are infrequent, well-scheduled special events that are treated in a precise well-controlled manner, under the attention of the plant’s management, safety officials, the gas supplier, and local fire-department officials. On the other hand, if an accident occurs, the consequences can be much greater than one involving gas cylinders. Currently, silane is used mainly in glow discharge deposition at very low utilization rates (e.g., 10%). To the extent that the material utilization rate increases in the future, the potential worst consequences of an accident will be reduced.

Toxic doping gases (e.g., AsH₃, PH₃, GeH₄) are used in quantities too small to pose any significant hazards to public health or the environment. However, leakage of these gases can cause significant occupational risks, and management must show continuous vigilance to safeguard personnel. Applicable prevention options are discussed elsewhere [5]; many of these are already implemented by the US industry.

4.2 Public Health and Environmental Issues

Silane used in bulk quantities in a-Si facilities may pose hazards to the surrounding community if adequate separation zones do not exist. In the USA, the Compressed Gas Association (CGA) Guidelines specify minimum distances to places of public assembly that range from 80 ft to 450 ft depending on the quantity and pressure of silane in containers in use [6]. The corresponding minimum distances to the plant property lines are 50–300 ft. Prescribed separation distances are considered sufficient to protect the public under worst-condition accidents.

No environmental issues have been identified with this technology.

5.1 Occupational Health Issues

In CdTe manufacturing, the main concerns are associated with the toxicity of the feedstock materials (e.g., CdTe, CdS, CdCl₂). The occupational health hazards presented by Cd and Te compounds in various processing steps vary as a function of the compound specific toxicity, its physical state, and the mode of exposure. No clinical data are available on human health effects associated with exposure to CdTe. Limited animal data comparing the acute toxicity of CdTe, CIS, and CGS, showed that from the three compounds, CdTe has the highest toxicity and CGS the lowest [7]. No comparisons with the parent Cd and Te compounds have been made. Cadmium, one of CdTe precursors, is a highly hazardous material. The acute health effects from inhalation of Cd include pneumonitis, pulmonary edema, and death. However, CdTe is insoluble to water, and as such, it may be less toxic than Cd. This issue needs further investigation.

In production facilities, workers may be exposed to Cd compounds through the air they breathe, as well as by ingestion from hand-to-mouth contact. Inhalation is probably the most important pathway, because of the larger potential for exposure, and higher absorption efficiency of Cd compounds through the lung than through the gastrointestinal tract. The physical state in which the Cd compound is used and/or released to the environment is another determinant of risk. Processes in which Cd compounds are used or produced in the form of fine fumes or particles present larger hazards to health. Similarly, those involving volatile or soluble Cd compounds (e.g., CdCl₂) also must be more closely scrutinized. Hazards to workers may arise from feedstock preparation, fume and vapor leaks, etching of excess materials from panels, maintenance operations (e.g., scraping and cleaning), and during waste handling. Caution must be exercised when working with this material, and several layers of control must be implemented to prevent exposure of the employees. In general, the hierarchy of controls includes engineering controls, personal protective equipment, and work practices. Area and personal monitoring would provide information on the type and extent of employees’ exposure, assist in identifying potential sources of exposure, and gather data on the effectiveness of the controls. The US industry is vigilant in preventing health risks, and has established proactive programs in industrial hygiene and environmental control. Workers’ exposure to cadmium in PV manufacturing facilities is controlled by rigorous industrial hygiene practices and is continuously monitored by medical tests, thus preventing health risks [8].

5.2 Public Health and Environmental Issues

No public health issues have been identified with this technology. Environmental issues are related to the disposal of manufacturing waste and end-of-life modules; these are discussed in Section 9.

6.1 Occupational Health and Safety

The main processes for forming copper indium diselenide solar cells are co-evaporation of Cu, In, and Se and selenization of Cu and In layers in H₂Se atmosphere. The toxicity of Cu, In, and Se is considered mild. Little information exists on the toxicity of CIS. Animal studies have shown that CIS has mild to moderate respiratory track toxicity; in comparing CIS, CGS, and CdTe, CIS was found to be less toxic than CdTe and somewhat more toxic than CGS.

The selenium TLV-TWA of 0.2 mg/m³ as selenium were set to prevent systemic toxicity, and to minimize the potential of ocular and upper respiratory tract irritation. Interestingly, selenium is an essential element in the human diet and daily intakes of 500–860 μg of selenium are tolerated for long periods [9].

Although elemental selenium has only a mild toxicity associated with it, hydrogen selenide is highly toxic. It has an Immediately Dangerous to Life and Health (IDLH) concentration of only 1 ppm. Hydrogen selenide resembles arsine physiologically; however, its vapor pressure is lower than that of arsine, and it is oxidized to the less toxic selenium on the mucous membranes of the respiratory system. Hydrogen selenide has a TLV-TWA of 0.05 ppm to prevent irritation, and prevent the onset of chronic hydrogen selenide-related disease. To prevent hazards from H₂Se, the deposition system should be enclosed under negative pressure and be exhausted through an emergency control scrubber. The same applies to the gas cabinets containing H₂Se cylinders in use.

The options for substitution, isolation, work practices, and personnel monitoring discussed for CdTe are applicable to CIS manufacturing as well. In addition, the presence of hydrogen selenide in some CIS fabrication processes requires engineering and administrative controls to safeguard workers and the public against exposure to this highly toxic gas.

6.2 Public Health and Environmental Issues

Potential public health issues are related to the use of hydrogen selenide in facilities that use hydrogen selenide as a major feedstock material. Associated hazards can be minimized by using safer alternatives, limiting inventories, using flow restricting valves and other safety options discussed in detail elsewhere [10]. Emissions of hydrogen selenide from process tools are controlled with either wet or dry scrubbing. Also, scrubbers that can control accidental releases of this gas are in place in some facilities. Environmental issues are related to the disposal of manufacturing waste and end-of-life modules; these are discussed in Section 9.

7.1 Occupational Health and Safety

MOCVD is today’s most common process for fabricating III–V PV cells; it employs the highly toxic hydride gases, arsine and phosphine, as feedstocks. Similarly to silane and hydrogen selenide handling, the safe use of these hydrides requires several layers of engineering and administrative controls to safeguard workers and the public against accidental exposure. Such requirements pose financial demands and risks that could create difficulties in scaling up the technology to multimegawatt levels. One part of the problem is that today’s use of the hydrides in MOCVD is highly ineffective. Only about 2–10% are deposited on the PV panels, as a 10–50 times excess of V to III (As to Ga) is required. Metal-organic compounds are used more effectively, with their material utilization ranging from 20 to 50%. In scaling up to 10-MW/yr production using MOCVD, the current designs of flat-plate III–V modules will require approximately 23 metric tons of AsH₃, 0.7 tons of PH₃, 7 tons of metal organics, and 1500 tons of hydrogen. (These estimates are based on generic data applicable to the largest current MOCVD reactors [e.g., EMCORE Enterprise E400 and Aixtron AIX3000], and carry some unquantified uncertainty. Production for 24 hours a day by one of these reactors could provide about 100 kWp/year, so 100 such reactors will be needed for the 10 MW production-basis we are considering herein. Therefore, larger reactors will be needed at this scale, thereby introducing more uncertainty in our estimates.) These quantities can be effectively delivered only with tube trailers, each carrying 2–3 tons of gases. The potential consequences of a worst-case failure in one of these tube trailers could be catastrophic. On a positive note, however, it is more likely that terrestrial systems will be concentrators, not flat plates, because the former would be less expensive to manufacture. PV cells have been shown to be capable of operation under concentrated sunlight, between 100 and 2000 times the one-sun level (see Parts IId and IIId). A possible practical strength is 500×concentrators; the material requirements for producing such concentrators are 600 times less than those needed for flat plates.

The best way to minimize both the risks associated with certain chemicals and the costs of managing risk is to assess all alternatives during the first steps of developing the technology and designing the facility. These hydrides may be replaced in the future by the use of tertiary butyl arsine (TBAs) and tertiary butyl phosphine (TBP) [11]; it appears that there are no intrinsic technical barriers to growing PV-quality GaAs with TBAs and GaAsP or GaInP₂ with TBP. Until substitutes are tested and implemented, however, it might be prudent to use arsine and phosphine from reduced-pressure containers, which are commercially available. Research efforts are being made in Europe [12] to replace hydrogen by inert nitrogen. Apparently, there is no inherent reason to prohibit such a substitution. However, since molecular hydrogen decomposes to some extent, and atoms participate in the gas-phase chemistry, the PV research community is challenged with learning how to optimize III–V growth conditions with nitrogen.