**Fthenakis (BNL): Environmental, Health, and Safety Issues in
Photovoltaic Technologies: A Summary (draft)**

Photovoltaic (PV) technologies have distinct environmental advantages
for generating electricity over conventional technologies. The operation
of photovoltaic systems does not produce any noise, toxic-gas emissions,
or greenhouse gases. Photovoltaic energy not only can help meet the
growing worldwide demand for electricity, but it can do so without
incurring the high economic and environmental costs of burning fossil
fuels and installing power lines. Compared to burning coal, every
gigawatt-hour of electricity generated by photovoltaics would prevent
the emission of about 10 tons of sulfur dioxide, 4 tons of nitrogen
oxides, 0.7 tons of particulates, and up to 1000 tons of carbon dioxide.

As with any energy source or product, there are environmental, health
and safety (EHS) hazards associated with the manufacture, use, and
disposal of solar cells. Although the PV industry uses far smaller
amounts of toxic- and flammable-substances than many other industries,
its use of hazardous chemicals can entail occupational and environmental
hazards. Addressing EHS concerns is the focus of numerous studies at
Brookhaven National Laboratory, under the auspices of the US Department
of Energy's National Photovoltaic Program. More than 150 articles
highlighting these studies are posted in our bibliography. Below is a
summary of EHS issues pertaining to the manufacture of crystalline-Si,
amorphous silicon, CuInSe~2~ and CdTe solar cells. We will promptly send
the full articles to the reader who wants more details.

### Crystalline Silicon Solar Cells

#### [Occupational Health Issues]{.underline}

The occupational health issues are related to chemical burns and the
inhalation of fumes from hydrofluoric acid (HF) and other solutions of
acids (e.g., HNO~3~) and alkalis (e.g., NaOH) used for cleaning wafers,
removing dopant oxide, and cleaning the reactor. Dopant gases and vapors
(e.g., POCl~3~, B~2~H~3~), also are hazardous when inhaled. POCl3 is a
liquid, but in a deposition chamber it can generate toxic P~2~O~5~ and
Cl~2~ 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 (SiH~4~) and its
byproducts from silicon nitride deposition; these hazards are discussed
in the a-Si section below.

#### [Environmental Issues and Waste Minimization]{.underline}

The environmental issues are related to the generation of liquid- and
solid-wastes during wafer slicing, cleaning, and etching, and processing
and assembling of solar cells.

The PV industry has embarked upon programs of waste minimization and
examines environmentally friendlier alternatives for solders, slurries
and solvents. Waste minimization can be effective in crystalline Si-cell
manufacturing, as demonstrated by successful programs in the laboratory
and in manufacturing plants. For example, the Photovoltaic Device
Fabrication Laboratory at Sandia National Laboratories reduced waste
generation by 75% since 1990 by substituting re-using and recycling
materials. By switching from ID saws to multiple-wire saws, Siemens
Solar Corporation reduced by about 20% the caustic waste generated by
etching that removes saw-damage after wafer slicing. Wire-sawn wafers
require less etching to remove saw damage, and simultaneously, increase
productivity as more wafers per inch of Si ingot are obtained.
Additional options for waste minimization under investigation include
recycling stainless-steel cutting wires, recovering the SiC in the
slurry, and in-house neutralization of acid and alkali solutions.

**Amorphous Silicon Solar Cells**

[Occupational Safety Issues]{.underline}

Amorphous silicon cells are currently by glow discharge deposition from
mixtures of SiH~4~/H~2~, or SiH~4~/GeH~4~/H~2~. Because of the low
material utilization of deposition process (e.g., 10%), a relatively
large volume of these gases is used. The main safety hazard of this
technology is the use of SiH~4~ 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 %. If SiH~4~ escapes in a partially
confined space at flow rates above 300 lpm, an explosion is likely.
Releases in open air usually ignite and burn smoothly; under appropriate
circumstances, burning or flaring can be a mechanism for preventing an
explosion. In addition to SiH~4~, hydrogen used in a-Si manufacturing
and in producing small quantities of crystalline-Si also is explosive.
Earlier publications discussed SiH~4~ and H~2~ safety in detail. 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 in tube trailers to avoid
frequently changing gas cylinders. 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.

Toxic doping-gases (e.g., AsH~3~, PH~3~) 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 were presented in
previous publications.

[Public Health and Environmental Issues]{.underline}

For a-Si facilities that use bulk silane, the potential consequences
from a catastrophic release need to be assessed on a facility-specific
basis. No significant environmental issues have been identified with
this technology.

**Cadmium Telluride Solar Cells**

CdTe solar cells can be manufactured by several deposition techniques,
such as close-spaced sublimation, electrodeposition, and spraying. The
CdS films needed for heterojunction formation usually are formed by
spray pyrolysis or solution growth. The efficiency of material
utilization in these processes ranges from a high of 90% for
electrodeposition, to a low of 5% to 10% for spray pyrolysis.

[Occupational Health Issues]{.underline}

The occupational health hazards from Cd- and Te-compounds in various
processing steps vary with the compounds\' toxicities, their physical
state, and the mode of exposure. No clinical data are available on human
health effects associated with exposure to CdTe. Research at the
National Institute of Environmental Health Sciences (NIEHS), compared
the toxicity of CdTe, CIS, and CGS in animal experiments simulating
ingestion and inhalation; CdTe had the highest toxicity and CGS the
lowest. In the United States, the Occupational Safety and Health
Administration (OSHA) regulate one of the parent compounds, Cd, that is
considered both toxic and a lung carcinogen OSHA considers all Cd
compounds (including CdTe) to be toxic.

In production facilities, workers could be accidentally exposed to Cd
compounds through inhalation or ingestion from hand-to-mouth contact.
Processes in which Cd compounds are used or produced as fine particles
or fumes present larger risks. Spray pyrolysis with its very low
utilization efficiency generates a large quantity of by-products and
fine particles. Some of these materials will be deposited on the
reactor's wall, requiring periodic scraping or chemical removal and
cleaning. The large fraction remaining will be contained in the exhaust
gas. In this type of process, hazards to workers may arise from
preparing the feedstock, from fume and vapor leaks, and such maintenance
operations. Since feedstocks in less fine form are inherently safer

Hazard management options include the use of CdTe and CdS in coarse
particle or pellet sizes, process automation, isolation and local
ventilation, air monitoring, personal protective equipment, and prudent
work practices. These were presented in earlier articles. CdTe PV
manufacturing facilities implement industrial-hygiene programs that
include biomonitoring. In today's CdTe facilities, no detectable levels
of cadmium have been reported from monitoring people during
manufacturing and maintenance operations.

[Public Health and Environmental Issues]{.underline}

The health of the public may be affected from chronic exposure to Cd
compounds released to the environment as a by-product of different
manufacturing steps, or as a waste from the uncontrolled disposal of
spent photovoltaic modules. The magnitude of potential releases from
manufacturing depends on the methods and controls implemented in each
facility. For example, a 10 MWp PV fabrication plant, using
electrodeposition with an efficiency of 90% in forming a 2 μm CdTe
layer, would generate only 11 kg/yr of Cd and 12 kg/yr of Te, mostly as
ions in solution. Therefore, electrodeposition poses minimal
public-health hazards during routine operations. Only if accidentally
the electrodeposition batch becomes contaminated, or a spill occurs, may
additional material need treatment.

A less efficient deposition process (e.g., spray pyrolysis with an
estimated efficiency of 10%), would generate about 950 kg of CdCl~2~,
CdO, HCl, H~2~S and thiourea byproducts. Some of this material will be
deposited on the reactor\'s walls, but most will be in a gaseous form in
the exhaust stream. Under these assumptions, a 10 MW~p~ plant may
generate emission flow rates of about 0.45 kg/hr during an 8 hr/day, for
250 days-per-year. These can be removed by 99+% efficiency by using HEPA
filters or other pollution control equipment.

**Copper Indium Selenide Solar Cells**

Copper Indium Selenide (CuInSe~2~, CIS) thin-films can be formed by
several processes, including co-evaporation (i.e., physical vapor
deposition) of Cu, In, and Se, and selenization of Cu and In layers in a
H~2~Se atmosphere. In the second method, Cu and In are deposited by
electron-beam evaporation and sputtering techniques. Other options
involve gallium sources and the formation of copper gallium selenide
layers (CGS). Heterojunctions typically are formed with CdS film. The
health and safety issues related to CdS were discussed in the context of
CdS/CdTe solar cells, above.

[Occupational Health and Safety Issues]{.underline}

There is limited information on the toxicity of CIS and CGS. As
discussed earlier, limited animal testing comparing CdTe, CIS, and CGS
showed that CGS has the lowest toxicity, and that CIS is somewhat more
toxic than CGS but less so than CdTe. Measurements of airborne
concentrations of copper, indium, and cadmium from mechanical scribing
and deposition operations on CIS/CdS modules demonstrated that they were
well below their threshold levels. .

The main safety issue in producing CIS cells is related to the highly
toxic gas hydrogen selenide used as feedstock in some processes.
Accident-prevention options were described in earlier articles.

[Hazard Management and Environmental Issues]{.underline}

The options for substitution, isolation, work practices, and personnel
monitoring outlined for CdTe are applicable to CIS manufacturing too. In
addition, using hydrogen selenide in some CIS fabrication processes
requires engineering and administrative controls to safeguard workers
and the public from exposure to this highly toxic gas; these may include
limited inventories, and flow-restricting valves. Routine and accidental
emissions of hydrogen selenide from process tools can be controlled with
wet or dry scrubbing. Such systems were discussed in earlier articles.

**CONCLUSION**

The manufacture of photovoltaic modules uses some toxic- and
explosive-gases, corrosive liquids, and suspected carcinogens in solid
form. Routine conditions in manufacturing facilities should not pose any
threats to health and the environment. Quality control in these
facilities demands especially clean conditions, with air concentrations
of contaminants in occupational space much below the threshold exposure
limits. However, as general guidance, all facilities working with
hazardous materials should continue to control exposures by monitoring,
employing engineering controls and personal protective equipment, and
instituting good work practices.

Hazardous materials could adversely affect occupational health and, in
some instances, public health during accidents. Such hazards arise
primarily from the toxicity and explosiveness of specific gases.
Accidental releases of hazardous gases and vapors can be prevented
through choosing safer technologies, processes, and materials, better
use of materials, and by employee training and safety procedures. As the
PV industry vigilantly and systematically approaches these issues and
mitigation strategies, the risk to the industry, the workers, and the
public will be minimized. This effort is especially important in view of
the anticipated multi-hundred MW production facilities to meet the
increased demand for PV.