achieve dramatic improvements in the economics of solar energy. This strategy is illustrated in
Figure 5.
BASIC SCIENCE CHALLENGES, OPPORTUNITIES, AND RESEARCH NEEDS IN
SOLAR ELECTRICITY
Inorganic Photovoltaics
As shown in Figure 2, single-crystal Si solar cells, defined as Generation I cells, have module
efficiencies of about 10–12% — leading to module costs of about $3.50/Wp. In order to lower
the areal cost, Generation II approaches to PV systems involve the use of semiconductor thin
films (amorphous Si, polycrystalline CdTe and CuInSe 2 , dye cells, and organic PV cells).
However, the system cost will be limited by BOS costs, which means that the Generation II
systems cannot have arbitrarily low efficiency even if the module costs are negligible. With low
areal costs for both modules and BOS of $75/m^2 each, 10% efficiency for Generation II modules
(17% cell efficiency) would yield PV costs of $1.50/Wp ($0.075/kWh). Thus, unless the module
and BOS costs can be reduced well beyond the aggressive target of $75/m^2 each, achieving the
goal of $0.40/Wp ($0.02/kWh) will require efficiency and system costs per unit area that lie in
the Generation III area of Figure 2.
A critical feature of Generation III PV systems is that their module efficiencies are above 32%
— the thermodynamic limit calculated by Shockley and Queisser (1961). In the Shockley-
Queisser analysis, a major assumption is that electrons and holes created by absorption of
photons with energies above the band gap lose their excess energy (the difference between the
photon energy and the semiconductor band gap) as heat through excitation of the lattice
vibrations (this is called phonon emission). However, thermodynamic calculations that do not
make this assumption show that the thermodynamic efficiency limit can be above 65% if the
energetic electrons and holes created by the high-energy photons do not convert the excess
energy to heat but produce higher photovoltages or photocurrents (Green 2004; Ross and Nozik
1982; Marti and Luque 2004). Reaching the goals of ultra-high efficiencies and low cost will
require basic research to achieve the revolutionary advances indicated in Figure 5.
Another way to achieve efficiencies above the Shockley-Queisser limit is to use a series of
semiconductor p-n junctions arranged in a tandem configuration (these cells are called multi-
junction solar cells). Of the approaches proposed to achieve higher efficiency, only multi-
junction solar cells have been able to actually exceed the performance of conventional single-
junction solar cells. Figure 6 shows the current world-record efficiencies (as a function of the
number of junctions) compared with the efficiency limit that is predicted by the thermodynamic
analyses. At one sun, the single-junction efficiency is about 75% of the theoretical limit; the
multi-junction and concentrator cells show even more opportunity for improvement. The
thermodynamic limit for solar-energy conversion is significantly higher still: 66% at one sun and
86% at full solar concentration (46,200 suns). The achievement of even higher efficiencies and
lower costs for multi-junction solar cells will require basic research to bring about the revolution
in existing technology and change the slope of the learning curve, as presented in Figure 5. The