Solar Energy Perspectives - IEA

Chapter 6: Solar photovoltaics
or additional electricity generation. Concentrating photovoltaics and thermal (CPVT) designs
are being explored in various forms.
Figure 6.4 Output of tracking and fixed PV systems
Dual-axis
tracking array
Energy production
Fixed array
Time of day
Key point
Two-axis tracking increases and evens out the production of PV over the day.
Organic cells
Emerging new technologies include advanced thin films and organic solar cells. Organic
solar cells are either full organic cells (OPV) or hybrid dye-sensitised solar cells (DSSC). They
have lower efficiencies and shorter life-times, but can be made using roll-to-roll and usual
printing technologies, which could lead to very low manufacturing costs. They have a place
in niche markets such as consumer devices, but it is yet unproven whether they will
contribute to larger electric systems.
Novel devices: quantum dots and wells, thermoelectric
cells
Research is underway on novel devices that may offer the possibility of breaking efficiency
records – quantum dots and wells, and thermo-electric cells.
Most current PV cells are limited in efficiency to a theoretical maximum of about 30%
for crystalline silicon because the photovoltaic effect takes place in only one “band” of
solar radiation, corresponding to only one energy level of photons. Photons with lower
energy levels fall short. Photons with greater energy levels work, but part of their excess
energy is wasted for electricity and only heats the cell. It is possible to improve on this
efficiency by stacking materials with different band widths together in multi-junction
(“tandem” or “sandwich”) cells. The same “trick” is used in most efficient thin films, but
adding low-efficient layers at best allows reaching the efficiency level of c-Si cells.
However, their complex manufacturing process and high costs reserve them for CPV
devices.
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