Solar Energy Perspectives - IEA

Chapter 6: Solar photovoltaics
of the manufacturing processes; to increase the efficiency and lifetime of the cells; and to
reduce other system costs. Of particular concern is the use of silver, the price of which
doubled in the past year, and now represents about 5% of module prices. PV already
represents about 10% of the global demand of this precious metal.
Thin films
Thin films are made from semi-conductors deposited in thin layers on a low-cost backing.
There are four main thin-film categories:
• amorphous (a-Si) with efficiencies from 4% to 8%;
• multi-junction thin silicon films (a-Si/ μc-Si), made of an a-Si cell with additional layers
of a-Si and micro-crystalline silicon (μc-Si) with efficiencies up to 10%;
• cadmium-telluride (CdTe) with efficiency of 11%; and
• copper-indium-(di)selenide (CIS) and copper-indium-gallium-(di)selenide (CIGS), with
efficiencies from 7% to 12%.
Thin film manufacturing has been highly automated, and some use roll-to-roll printing
machines, driving costs down. Thin films now offer life-time almost similar to those of the
crystalline silicon wafer. Lower efficiencies of thin films versus crystalline silicon modules
means that a greater surface area is needed to produce the same electrical output; however
the ratio of kWh over kW of peak capacity (kWp) depends only on the solar resource. One
advantage of non silicon-based thin films, important in hot climates, is that their efficiency
does not decrease with temperature levels, or decreases much less than that of silicon
PV cells.
The use of cadmium, a poison and environmental hazard, in thin films often raises concerns.
However, cadmium residue from the manufacturing process is recovered, and studies show
that CdTe in glass-glass modules would not be released during fires. Recycling of CdTe thin
films is operational and allows recovery of 95% of cadmium and tellurium. CdTe films use
cadmium 2500 times more efficiently than Nickel-Cadmium (Ni-Cd) batteries in delivering
electricity. In sum, the life-cycle cadmium emissions from the use of CdTe PV modules are
about 0.2 μg/kWh to 0.9 μg/kWh and mostly result from the electricity used in the process
when it is produced by burning coal, which itself entails Cd life-cycle emissions of about
3.1 μg/kWh (Fthenakis, 2004).
Thin film “modules” can be made flexible and offer a great diversity of sizes, shapes and
colours – especially CIGS thin films. This helps in developing specific applications for
integration into the envelopes of buildings, going from building-adapted PV (BAPV) to
building-integrated PV (BIPV).
Hybrid PV-thermal panels
To maximise the energy efficiency per surface area of receiving panels, manufacturers now
offer hybrid systems, which collect electricity from the PV effect and heat simultaneously,
thereby adding the efficiency of PV to that of heat collectors, reaching a cogeneration
efficiency of 80% or more. While this combination was initially developed with air collectors,
it is now available with water collectors as well (Photo 6.1). Non-covered PV-thermal (PV-T)
115
© OECD/IEA, 2011