Solar-cell efficiencies have exceeded 40% in recent years. The keys to achieving these high efficiencies include: 1) use of multiple materials that span the solar spectrum, 2) growth of these materials with near-perfect quality by using epitaxial growth on single-crystal substrates, and 3) use of concentration. Growth of near-perfect semiconductor materials is possible when the lattice constants of the materials are matched or nearly matched to that of a single-crystal substrate. Multiple material combinations have now demonstrated efficiencies exceeding 40%, motivating incorporation of these cells into concentrator systems for electricity generation. The use of concentration confers several key advantages.
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As the world looks for sustainable sources of energy, conversion of sunlight to electricity is a key promising approach. The distribution of the solar resource throughout the world allows it to be used by all, but also requires that sunlight be captured from large areas. Higher solar-cell efficiencies imply that more electricity can be generated from a given area, potentially reducing the requirements for real estate, support structure, glass, and other materials/costs that scale with the area of the solar system. Over the years, research has led to higher and higher efficiencies. Champion efficiencies for a variety of types of solar cells are summarized in Fig. 1 .
The efficiency of a solar cell depends on the incident spectrum, the temperature of the cell, and the irradiance or concentration of the light; the efficiencies reported in Fig. 1 were measured at standard reference conditions. A striking conclusion from Fig. 1 is that one technology has achieved substantially higher efficiencies than all of the rest. In Fig. 1, the purple triangles denote efficiencies achieved for GaAs-based solar cells, including single-junction, two-junction, and three-junction cells. Not only have these cells achieved the highest efficiencies, their improvement within the last ten years is greater than for any other technology. Furthermore, most researchers studying these cells predict that efficiencies may reach 45% or even 50% with further work.
The detailed-balance approach to calculating the efficiency limits for solar cells provides an elegant method for determining the highest possible efficiency for one or more materials operating under a fixed set of conditions [1–8]. The theoretical efficiency limit for solar cells operating under ~1000 suns irradiance is ~61% for a 3-junction cell and ~65% for a 4-junction cell . Today’s champion cells typically achieve 75%–80% of their detailed-balance theoretical limits . Recent efficiency records and the associated structures are summarized in Table 1 . The highest efficiencies are achieved with non-optimal band-gap combinations because of the relative maturity of these structures compared with the new designs.
2. Use of multiple materials to reach high efficiency
As shown in Fig. 1, many semiconductor materials can serve as photovoltaic materials. High-band-gap materials can provide the highest photovoltages, but absorb only a small portion of the solar spectrum, resulting in low photocurrents. Because the power delivered is the product of the current and the voltage, the optimum efficiency is predicted for a material with a band gap that provides a good compromise of photocurrent and photovoltage. To improve on the efficiency of a single-junction solar cell, a multijunction solar cell is fabricated with each material absorbing a different portion of the solar spectrum, as shown in Fig. 2 .
While the group-IV semiconductors, Si and Ge, are the most commonly used semiconductor materials, they provide limited options for creating multijunction structures. By contrast, the III-V compound semiconductors have band gaps that span most of the solar spectrum, from 0.3 to ~2.2 eV for direct-gap materials. Figure 3 shows band gaps of a number of semiconductors plotted as a function of their lattice constants.
For series-connected multijunction solar cells, the band gaps should be chosen so the photocurrents generated in each subcell are matched. If an upper layer has a band gap that is lower than ideal, the layer may be grown thinner so as to transmit some light to the next lower junction. In this way, multijunction cells can be assembled from many material combinations. The current champion cells all use GaInP as the top cell, with a band gap of ~1.8–1.9 eV and GaAs or GaInAs (with 1%–8% indium) as the middle cell [9,10]. Today’s commercially available cells are grown on germanium substrates in such a way that a junction is formed in the germanium, providing the third junction. Because the band gap of germanium is lower than ideal, excess photocurrent is generated in the germanium.
The use of a 0.9-eV material for the third junction gives a more optimal match of the three photocurrents. So far, this structure has been implemented most optimally by inverted growth: GaInP and Ga(In)As junctions are grown in the inverted configuration, followed by a graded layer that increases the lattice constant to that desired for the final GaInAs junction . The inverted growth causes the complication of needing to transfer the cell to a different substrate and remove the original substrate. However, the inverted approach has the dual advantages of the possibility of reusing the substrate and providing a pathway to four or more junctions with whatever band-gap combination is desired .
3. Importance of crystal quality and passivation in reaching high efficiency
The highest efficiencies have been achieved for material sets with near-optimal band gap combinations, but even more important in achieving high efficiency is the use of near-perfect materials. Non-radiative recombination of photocarriers can reduce the photocurrents and can have an even more important effect on the photovoltage. Crystallographic defects associated with energy levels within the band gap are known to catalyze recombination of photocarriers. The kinetics of the non-radiative recombination were described by Shockley, Read, and Hall [12,13] to elucidate that the most problematic recombination occurs when the energy of the defect lies near the middle of the gap and when the Fermi level is close enough to the middle of the gap that the level is often in a partially filled state. Non-radiative recombination is usually decreased when crystallographic defects are avoided.
The equilibrium concentrations of point defects in III-V materials are generally very low. When carefully grown, III-V materials have III/V concentration ratios very close to unity, limiting the number of point defects. When grown epitaxially on single-crystal substrates, dislocation densities <105/cm2 can be expected. Thus, in the case of high crystallographic quality, III-V materials show the highest concentration of defects on the surfaces of the crystal. In silicon solar cells, the surfaces are typically passivated using an oxide or nitride material. These insulating layers are an essential part of the best silicon cells, but they complicate the cell geometries because conductive paths must exist to carry the photocurrent out of the cells. A key difference between silicon and III-V cells is that single-crystal, conductive, passivating layers are available for use in the III-V cells. Figure 4 shows a schematic of a solar cell, indicating how the use of passivating layers on the front and back help to confine the minority carriers while providing a conduction path for the majority carriers to leave the device.
4. Benefits of concentration for increasing efficiency and reducing use of semiconductor material
The photocurrent of a solar cell typically increases linearly as the intensity of the light is increased; the photovoltage increases logarithmically with light intensity. Thus, the efficiency typically increases logarithmically with light intensity until the current increases to the point that series-resistance losses dominate, as shown in Fig. 5 .
Concentrated sunlight usually increases the efficiency of a solar cell; it also reduces the required solar-cell area. This reduced need for semiconductor material can reduce cost, increase the efficiency, and reduce business risk. Most of the optical designs currently used with multijunction solar cells concentrate the light by a factor of 500–1000. To a first approximation, the cost of a specific type of solar cell scales with area, so a concentration ratio of 500 can reduce the solar-cell cost by a similar factor―making the cost of the concentrator cells only a small part of the overall system cost. If the cell cost can be reduced this dramatically, it can be advantageous to use a cell with a higher efficiency (even if this comes at a higher cost), making the investment in the optics all the more valuable. Higher efficiency can translate into lower cost at the system level because requirements for area-related costs are reduced. Potential savings include a smaller mounting structure, smaller system to install, and less glass or plastic to cover the area.
In today’s uncertain economic climate, reducing the need for semiconductor material is attractive to investors because of the smaller (and therefore less risky) capital investment. Farsighted decisions about investment in new silicon purification facilities have been a primary driver in the growth/health of Si PV companies in recent years, and, thus, investors are attracted to concepts that use less semiconductor material. Additionally, to provide terawatts of power, the materials used must be in adequate supply, which is more likely when the semiconductor material needs are reduced.
Although many estimates have shown that high-efficiency concentrator systems have the potential to deliver lower cost solar electricity than other approaches, the requirements to simultaneously achieve high performance and reliability also tend to add cost. Today’s concentrator photovoltaic industry has shown that the performance, reliability and cost goals can be met individually, but the challenge remains to demonstrate that the performance, reliability and cost goals can be met simultaneously.
We appreciatively acknowledge S. Moon, A. Hicks, and B. Kurtz for their help with editing and figures. This review represents a short summary of years of work by dozens of people. This work was supported by the U.S. Department of Energy under Contract No. DOEAC36-08GO28308 with the National Renewable Energy Laboratory.
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