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Will we exceed 50% efficiency in photovoltaics?
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Figures

Image of FIG. 1.

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FIG. 1.

(Color) Unlike most other cell types, which appear to have reached close to their maximum efficiency performance in recent years, research teams are still showing significant progress with multi-junction devices. Over the past decade, the best-performing multi-junction cells have moved from around 35% efficiency to the latest record of 42.3% achieved by Spire Semiconductor. Production line cell efficiency tends to lag the champion-cell figure by about 2 years. Courtesy of NREL (Dr. Nozik).

Image of FIG. 2.

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FIG. 2.

SQ efficiency limit for an ideal solar cell versus bandgap energy for: (a) unconcentrated 6000K blackbody radiation (1595.9 Wm−2); (b) fully concentrated 6000K blackbody radiation (7349.0 × 104 Wm−2); (c) unconcentrated AM1.5-Direct13 (767.2Wm−2); (d) AM1.5 Global13 (962.5 Wm−2). Reprinted with permission from Ref. 6 (Hegedus and Luque, Wiley).

Image of FIG. 3.

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FIG. 3.

Stack of solar cells ordered from left to right in decreasing bandgap (Eg1 > Eg2 > Eg3). Reprinted with permission from Ref. 14 (Martí and Araujo, Wiley).

Image of FIG. 4.

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FIG. 4.

(color) Combinations of bandgap and lattice constant for various materials used in III-V solar cells. The potential substrate materials silicon (Si) and germanium (Ge) are also indicated. As an example, the bandgap combinations of the lattice-matched triple-junction solar cell on Ge as well as of a metamorphic triple-junction solar cell developed at FhG-ISE are indicated. The upper cells of the metamorphic solar cells have a lattice-mismatch of 1.2% compared to the Ge substrate. In order to achieve sufficient material quality, a buffer layer is integrated into the structure.

Image of FIG. 5.

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FIG. 5.

Schematic cross-section of a monolithic two-terminal series-connected three-junction solar cell. An n-on-p configuration is illustrated. Doping indicated by n++, n+ and n (or p++, p+, p) corresponds to electron (or hole) concentrations of the order of 1019–1020, 1018 and 1017, respectively. Typical materials, bandgaps, and layer thicknesses for the realization of this device structure as a GaInP/GaAs/Ge cell are indicated. Note that not all layers in an actual device (e.g., tunnel-junction cladding layers) are included in the illustration. The figure is not to scale. Reprinted with permission from Ref. 17 (Friedman, Olson, and Kurtz, Wiley).

Image of FIG. 6.

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FIG. 6.

(color) Detailed balance calculations for the efficiency of different triple-junction solar cell structures under the AM1.5d ASTM G173–03 spectrum at 500 kW/m2 and 298 K. The black haze represents bandgap combinations which admit efficiencies from 60.5% to 61.0% and hence mark the optimum. The gray haze represents efficiencies of 59.0–60.5% (see contours). Five specific triple junction solar cell structures are shown. The lattice-matched Ga0.5In0.5P/Ga0.99In0.01As/Ge (LM), two metamorphic GaInP/GaInAs/Ge (1.8, 1.29, 0.66 eV for MM1) and (1.67, 1.18, 0.66 eV for MM2), as well as two inverted metamorphic GaInP/GaInAs/GaInAs (1.83, 1.40, 1.00 eV for Inv1) and (1.83, 1.34, 0.89 eV for Inv2) devices. Reprinted with permission from Ref. 19 (Guter et al., AIP).

Image of FIG. 7.

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FIG. 7.

(a) Lattice matched and metamorphic three-junction cell grown on Ge. Dislocations are shown in the stressed layers. (b) inverted metamorphic cell grown on GaAs, detached and bonded to carrier. (c) bifacial epitaxy metamorphic cell. Reprinted with permission from Ref. 22 (Wojtczuk et al., IEEE).

Image of FIG. 8.

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FIG. 8.

Schematic band structure for a multi-QW solar cell. Reprinted with permission from Ref. 24 (Barnham and Duggan, AIP).

Image of FIG. 9.

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FIG. 9.

(Color) Efficiency-concentration data of champion certified cells in the world.

Image of FIG. 10.

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FIG. 10.

(top) basic structure of an intermediate band solar cell; (middle) simplified bandgap diagram in equilibrium; (bottom) simplified bandgap diagram under illumination and forward bias. Reproduced with permission from Ref. 34 (Canovas et al., Elsevier).

Image of FIG. 11.

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FIG. 11.

Layer structure of a quantum dot intermediate band solar cell grown by molecular beam epitaxy. Reproduced with permission from Ref. 34 (Canovas et al., Elsevier).

Image of FIG. 12.

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FIG. 12.

Schematic of a Fresnel lens showing the teeth. Reproduced with permission from Ref. 57 (Sala and Anton, Wiley).

Image of FIG. 13.

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FIG. 13.

Detail of the dome shaped Fresnel lenses used by Daido Steel (Japan).

Image of FIG. 14.

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FIG. 14.

(Color) Angular transmission curves for the different SOE studied. Circular marks over the lines indicate the deviation angle where optical efficiency becomes 90% and 80% of the maximum. Lens to cell geometrical concentration 1000X. Reproduced with permission from Ref. 59 (Victoria et al., OSA)

Image of FIG. 15.

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FIG. 15.

(Color) View of the Fresnel-Koehler POE and SOE. It is formed of four POE in a single Fresnel lens each one imaged onto the cell by the four SOE elements. The four SOE elements are cast in the same block. Below: Irradiance distribution on the cell for the LPI’s FK concentrator with Cg = 625x, f/1, no AR coating on the SOE, when the sun is on axis and the solar spectrum is restricted to: (a) the top subcell range (360–690 nm), and (b) the middle-subcell range (690–900 nm). Reproduced with permission from Ref. 60 (Cveckovic et al., WIP and LPI).

Image of FIG. 16.

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FIG. 16.

(Color) The Solfocus concentrator (USA) used initially hexagonal glass reflectors in a Cassegrain structure (two mirrors) and a glass truncated pyramid coupled to multijunction cells: scheme of the principle of the concentrator Reproduced with permission from Ref. 57 (Sala and Anton, Wiley).

Image of FIG. 17.

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FIG. 17.

Typical receiver for two-stage optics with a Fresnel lens in the primary. The substrate provides insulation and good thermal transmission. It houses a bypass and a dome type secondary. The hollow part of the dome is filled with silicone rubber for good optical matching. Reproduced with permission from Ref. 57 (Sala and Anton, Wiley).

Image of FIG. 18.

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FIG. 18.

(Color)The beam is split by means of a band-pass filter and directed onto two cells by means of free form optics. The filter is under medium level concentration on filter for reducing area (low cost) and good performance (moderate angular dispersion). Additionally, a mirrored cavity is recycling light reflected by the MJ cell. The optics performs the Koehler integration for good irradiance uniformity on top of cells. (a) Perspective. (b) Cross-section. (Courtesy of Prof Miñano, UPM).

Image of FIG. 19.

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FIG. 19.

(Color) Inspira 36 m2 tracker for HCPV modules.

Image of FIG. 20.

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FIG. 20.

(Color) Solfocus subfield installed at ISFOC, Puertollano, Spain.

Image of FIG. 21.

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FIG. 21.

(Color) Cumulated production during 2009 in a plant of ISFOC per kW (kWnormal in the text). The nameplate kW is measured under OTC according to the definition in the picture. This production is compared with the production per kWp of neighboring flat module plants with and without tracking. The KWp’s are defined under STC, as usual. Irradiation (kWh/m2) data are also provided (Direct normal irradiation, DNI and global normal irradiation GNI). Courtesy of F. Rubio, ISFOC.

Image of FIG. 22.

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FIG. 22.

(Color) AC efficiency of a 6 kW array with its own inverter during a day at ISFOC, Puertollano, Spain. In the central hours of the day the output is limited by the inverter to avoid injecting above the permitted power. Courtesy of F. Rubio, ISFOC.

Image of FIG. 23.

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FIG. 23.

Evolution of yearly energy yield in Almeria (Spain) for several tracking strategies, considering a loss of power proportional to the shading and assuming a constant soiling loss of 3%. Reprinted with permission from Ref. 75 (Narvarte and Lorenzo, Wiley).

Image of FIG. 24.

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FIG. 24.

(Color) (top) 60 MW Olmedilla (Spain) fixed Si plant covering 180 Ha. (bottom) 46 MW Moura (Portugal) two-axes tracking flat Si module plant covering 250 Ha.

Image of FIG. 25.

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FIG. 25.

(Color) Material costs for a CPV module with the following assumptions: Concentration 1000 X, module efficiency 20% 850 W/m2 and 25°C. Wafers refers to the epi-wafers with the MJ cells integrated.

Tables

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Table I.

Bandgaps and detailed balance efficiency limits for 6000/300K Sun/ambient temperature at full concentration for unconstrained cells (not series connected). In the rightmost column, achieved efficiencies are presented or, when not available (marked with an asterisk), calculated as 70% of the detailed balance limit.

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Table II.

Bandgaps and the detailed balance efficiency for 6000/300K Sun/ambient temperature at full concentration for unconstrained and series connected IB and single junction cell stacks with a single tunnel junction. In the rightmost column, 70% of the detailed balance limit is presented.

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Table III.

Ultimate yearly ground efficiency for two localities of good (Madrid, Spain) and excellent (Albuquerque, NM, USA) irradiation conditions and four PV technologies. Irradiation data from reference75. The irradiances on module are DNI for HCPV, GNI for Flat module tracking and global irradiance on optimally tilted modules for fixed plants.

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Table IV.

Specific ground coverage (MW/Ha) and estimated GCR for several large scale plants of different technologies.

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/content/aip/journal/jap/110/3/10.1063/1.3600702
2011-08-08
2014-04-23

Abstract

Solar energy is the most abundant and reliable source of energy we have to provide for the multi-terawatt challenge we are facing. Although huge, this resource is relatively dispersed. High conversion efficiency is probably necessary for cost effectiveness. Solar cell efficiencies above 40% have been achieved with multijunction (MJ) solar cells. These achievements are here described. Possible paths for improvement are hinted at including third generation photovoltaics concepts. It is concluded that it is very likely that the target of 50% will eventually be achieved. This high efficiency requires operating under concentrated sunlight, partly because concentration helps increase the efficiency but mainly because the cost of the sophisticated cells needed can only be paid by extracting as much electric power form each cell as possible. The optical challenges associated with the concentrator optics and the tools for overcoming them, in particular non-imaging optics, are briefly discussed and the results and trends are described. It is probable that optical efficiency over 90% will be possible in the future. This would lead to a module efficiency of 45%. The manufacturing of a concentrator has to be addressed at three levels of integration: module, array, and photovoltaic(PV) subfield. The PV plant as a whole is very similar than a flat module PV plant with two-axes tracking. At the module level, the development of tools for easy manufacturing and quality control is an important topic. Furthermore, they can accommodate in different position cells with different spectral sensitivities so complementing the effort in manufacturing MJ cells. At the array level, a proper definition of the nameplate watts, since the diffuse light is not used, is under discussion. The cost of installation of arrays in the field can be very much reduced by self aligning tracking control strategies. At the subfield level, aspects such as the self shadowing of arrays causes the CPV subfields to be sparsely packed leading to a ground efficiency, in the range of 10%, that in some cases will be below that of fixed modules of much lower cell efficiency. All this taken into account, High Concentration PV (HCPV) has the opportunity to become the cheapest of the PV technologies and beat the prevalent electricity generation technologies. Of course the way will be paved with challenges, and success is not guaranteed.

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Scitation: Will we exceed 50% efficiency in photovoltaics?
http://aip.metastore.ingenta.com/content/aip/journal/jap/110/3/10.1063/1.3600702
10.1063/1.3600702
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