The increasing cost of petroleum extraction, awareness of anthropogenic climate change, and electrification of transportation1 have combined to help accelerate the demand for renewable power, of which direct solar photovoltaic (PV) generation is an increasingly significant fraction. Albeit not known for especially sunny skies, Germany has been the standard-bearer for solar power adoption, with multiple peak production records greater than 22 GW and up to 50% of national instantaneous demand met by PV2,3. While demonstrations such as these show the possibilities with today’s technology, wider adoption of solar power will require further development in conversion efficiency and cost-reduction.
Numerous materials and device technologies exist that are able to perform a photovoltaic action: a thin layer of material that can develop and sustain a (nearly) constant electric potential under illumination with a current that is proportional to the intensity of this illumination. The solar cell’s power conversion efficiency — defined by the ratio of output electric power to input light power — offers a guide to the suitability of these materials for real power systems. While potentially inexpensive organic polymer photovoltaics (OPV) and small-molecule die-sensitized solar cells (DSSC) have seen promising records of conversion efficiency in a lab setting at 10.7%4 and 11.9%5, respectively, they have seen negligible commercial success so far. Bulk crystalline (c-Si) and multicrystalline (mc-Si) solar cells, with record efficiencies of 25.0%6 and 20.4%7, respectively, have captured 86% of the PV market, while thin-film inorganic semiconductor technologies split the remaining 14%8. The most significant metric for solar power, the ratio of cost to peak power generation or “$/Watt,” affords substantial flexibility to the developers of PV technology, with some (OPV/DSSC) promising to aggressively attack the numerator with low-priced materials and fabrication methods, while others (high efficiency inorganic) focus on the denominator of this ratio. In theory, any solar technology reaching a low $/Watt ratio could be successful in the marketplace.
The economic challenge for low-efficiency/low-cost solar technologies is meeting a low “Balance-of-System” (BOS) and module cost target, whereby all of the system costs (packaging, wiring, installation, assembly, etc) are considered. This sum generally accounts for more than the solar cell cost in building a complete power system9. As many of these external requirements for PV systems scale linearly with device area, this suggests that there exists a minimum efficiency for solar cells that is economically feasible.
Regardless of PV technology, it is clear that minimizing efficiency losses is critical to achieving low-cost power systems. The ultimate thermodynamic conversion efficiency limit for a material with a single optical transition energy or “bandgap”, known as “detailed balance,” was first outlined by Shockley and Quisser (1961)10, where they found the upper-limit for silicon (1.1 eV bandgap) to be near 30%. The detailed balance (or “SQ”) limit defines the performance ceiling for PV technologies as we currently understand them, but designers can make practical efficiency gains by viewing total efficiency as the product of optical efficiency, quantum efficiency, and electrical efficiency, and addressing each in turn11. Optical losses occur because of reflection, incomplete absorption, and non-ideal absorption edge energies, while quantum efficiency losses result from material, and electrical efficiency losses occur from excess series resistance or insufficient shunt resistances.
In striving to achieve high conversion efficiency, researchers have found radically different solutions to many of these challenges depending upon charge carrier lifetime, optical absorptivity, and available absorption bands. Modern silicon solar cell designs take advantage of silicon’s key asset as a result of extremely low defect concentration: extremely long carrier lifetimes and diffusion lengths12. These long lifetimes allow relatively thick layers with optical treatments to be used, which reduce surface reflection and enhance total internal reflection, counter-acting silicon’s generally weak optical absorption. The high efficiency passivated emitter, rear locally-diffused (PERL)13 and the interdigitated backside-buried-contact (IBBC)14 cells incorporate these and other refinements, with SunPower Corporation currently manufacturing IBBC cells at volume within 1% of the lab efficiency record set by PERL cells15. Direct-transition compound semiconductors, such as gallium arsenide (GaAs), cadmium telluride (CdTe), and others, in contrast, have shorter carrier lifetimes, but more than compensate for this lack with much stronger optical absorptivity and flexibility in absorption bands. These materials have absorption coefficients up to a factor of one hundred higher than that of silicon, meaning correspondingly thinner layers may be used, helping to offset their increased cost16. A start-up company, Alta Devices, along with academic and National Renewable Energy Labs collaborators, has recently exploited GaAs’s well-tuned match to the solar spectrum and demonstrated cells making the closest approach to the single-junction SQ-limit yet at 28.8%17. Alta achieved this impressive result by first creating an optimized and extremely low-defect GaAs solar cell structure, then exploiting a layer-lift-off technique to transport the thin (~ 10 microns) active layers to a highly reflective metal back mirror and applying optical treatments to the sun-incident side of the device. By maximizing the optical coupling efficiency of the cell-sun system and avoiding parasitic optical absorption, the Alta group works near the theoretical limit for their material18. To climb beyond the single-junction SQ-limit, designers must combine different absorption band thin-film compound semiconductors in monolithic multijunction (optically and electrically in series), PV cells to more efficiently use the full solar spectrum. Researchers at Sharp have demonstrated the highest efficiency at standard solar intensity (1000 W/m2) using such a device with three semiconductors at 37%19, while a slightly different design by start-up company Solar Junction has yielded a record conversion efficiency of 44% under solar concentration20. From advanced multijunction record breakers, to optimization of the market-favorite — bulk silicon — the numerous methods available for achieving high conversion efficiency offer significant advantages for market flexibility and research avenues.
Future breakthroughs in materials development, device modeling, and manufacturing methods promise to continually boost top efficiency records while reducing cell costs. Perhaps one of the most promising future areas in high solar energy production is concentrated photovoltaics (CPV)21. The ability to offset the cost of expensive solar cells with inexpensive focusing optics (fresnel lens or dish reflector) and solar tracking systems provides for a cost-effective and higher energy-output system compared to a fixed angle flat-plate system of equivalent peak efficiency. Additionally, higher-efficiency cells could act as “drop-in” upgrades to existing CPV systems, offering dramatic increases in power output. Improvements in multijunction (MJ) cell technology, including low resistance tunnel junctions,22 optical coupling between subcells23, and the addition of higher energy subcells for 4-junction and 5-junction MJ cells will help push efficiency records above 50%24. Movements to integrate high-efficiency III-V semiconductors onto inexpensive platforms such as large-wafer silicon may prove transformative to reducing cost of these high efficiency cells. Progress in integrating these materials for MJ-on-Si through reduction of interface defects (such as anti-phase domains) of gallium phosphide (GaP) on Si25,26,27 and graded buffers for high energy cells on GaP28 indicate practical implementations are on the horizon.
The challenges for any of these new technologies in moving from laboratory result to mass-market product are, of course, myriad. A mechanically, thermally, and electrically robust device at reduced cost and high manufacturing yield is required. Additionally, competition is never sitting still: the exponential rise in conventional silicon PV manufacturing from China and Taiwan from 2004 (< 1 gigawatt-peak (GWp), <1% of world production) to 2011 (13 GWp, 56% of world production) has resulted in a precipitous drop in bulk silicon prices and average $/W for installed PV29. The shifting sands of government subsidy and market conditions mean start-up company business plans require flexibility, rapid execution, and the ability to flawlessly scale production. Given the right combination of technical novelty and business acumen, however, and PV manufacturers are sure to develop, while solar power continues to provide an ever-increasing fraction of our diverse energy future.
 “Global EV Outlook 2013.” International Energy Agency. April 2013. http://www.iea.org/publications/freepublications/publication/name,37024,en.html. Accessed: April 26, 2013.
 Kirshbaum, E.. “Germany sets new solar power record.” Reuters. 26 May 2012. http://www.reuters.com/article/2012/05/26/us-climate-germany-solar-idUSBRE84P0FI20120526. Accessed: 26 April 2013.
 Burger, B. “Electricity production from solar and wind in Germany in 2013,” Fraunhofer, ISE. 22 April, 2013. http://www.ise.fraunhofer.de/en/news/news-2013/electricity-production-from-solar-and-wind-in-germany-in-2013. Accessed: 26 April, 2013.
 Service, Robert F. “Outlook brightens for plastic solar cells.” Science, 332(6027): 293, 2011.
 Komiya R., Fukui A., Murofushi N., Koide N., Yamanaka R., Katayama H. “Improvement of the conversion efficiency of a monolithic type dye-sensitized solar cell module.” Technical Digest, 21st International Photovoltaic Science and Engineering Conference, Fukuoka, 2 C-5O-08, 2011.
 Zhao J., Wang A., Green M. A., Ferrazza F. “Novel 19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells,” Applied Physics Letters 73: 1991–1993, 1998.
 Schultz O, Glunz S. W., Willeke G. P., “Multicrystalline silicon solar cells exceeding 20% efficiency,” Progress in Photovoltaics: Research and Applications, 12: 553–558, 2004.
 “Photovoltaics Report.” Fraunhofer Institute for Solar Energy Systems, ISE. Freiburg, 11 December, 2012. http://www.ise.fraunhofer.de/de/downloads/pdf-files/aktuelles/photovoltaics-report.pdf, Accessed: 27 April, 2013.
 “$1/W Photovoltaic Systems: White Paper to Explore a Grand Challenge for Electricity from Solar,” U.S. Department of Energy: Advanced Research Projects Agency-Energy, 2010. http://www1.eere.energy.gov/solar/sunshot/pdfs/dpw_white_paper.pdf , Accessed: 25 April, 2013.
 Shockley W., Queisser H. J., “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” Journal of Applied Physics, 32, 3:510-519, 1961.
 Nelson, J. “Physics of Solar Cells,” Imperial College Press, London, 2003.
 Roulston D. J., Arora N. D., Chamberlain S. G. “Modeling and measurement of minority-carrier lifetime versus doping in diffused layers of n+-p silicon diodes,” IEEE Transactions on Electron Devices, 29,2:284-291, 1982.
 see ref. 6.
 Guo J.-H., Cotter J. E., “Optimizing the diffused regions of interdigitated backside buried contact solar cells,” Progress in Photovoltaics: Research and Applications, 15:211-223, 2007.
 Cousins P. J., Smith D. D., Luan H.-C., Manning J., Dennis T. D., Waldhauer, A., Wilson, K. E., Harley, G., Mulligan, W. P. “Generation 3: Improved performance at lower cost,” 35th IEEE Photovoltaics Specialists Conference (PVSC), 275-278, 2010.
 Handbook of Optical Constants of Solids, edited by Edward D. Palik, (1985), Academic Press, New York.
 Kayes, B. M., Nie H., Rose T., Spruytte S. G., Reinhardt F., Kizilyalli, I. C., Higashi, G. S. “27.6 % Conversion efficiency, a new record for single-junction solar cells under 1-sun illumination,” 37th IEEE Photovoltaics Specialists Conference (PVSC), 2011.
 Miller O. D., Yablonovitch E., Kurtz S. R., “Strong Internal and External Luminescence as Solar Cells Approach the Shockley-Quisser Limit,” IEEE Journal of Photovoltaics, 2, 3:303-311, 2012.
 Yoshida A., Agui T., Katsuya N., Murasawa K., Juso H., Sasaki K., Takamoto T. “Development of InGaP/GaAs/InGaAs inverted triple junction solar cells for concentrator application.” 21st International Photovoltaic Science and Engineering Conference (PVSEC-21), 2011.
 Wiemer, M., Sabnis V., Yuen H., “43.5 % efficient lattice-matched solar cells,” Proceedings of SPIE 8108, High and Low Concentrator Systems for Solar Electric Applications VI, 810804, 2011.
 Swanson R. M., “The Promise of Concentrators,” Progress in Photovoltaics: Research and Applications, 8, 93-111, 2000.
 Zide J. M. O., Kleiman-Shwarsctein A., Strandwitz N. C., Zimmerman J. D., Steenblock-Smith T., Gossard A. C., Forman A., Ivanovskaya A., Stucky G. D. “Increased efficiency in multijunction solar cells through the incorporation of semimetallic ErAs nanoparticles into the tunnel junction,” Applied Physics Letters 88, 162103, 2006.
 Steiner, M. A., Geisz, J. F. “Non-linear luminescent coupling in series-connected multijunction solar cells,” Applied Physics Letters 100, 251106 , 2012.
 King, R R., Bhusari, D., Larrabee, D., Liu, X.-Q., Rehder, E., Edmonson, K., Cotal, H., Jones, R. K., Ermer, J. H., Fetzer, C. M., Law, D. C., and Karam, N. H. “Solar cell generations over 40% efficiency,” Progress in Photovoltaics: Research and Applications. 20:801–815, 2012.
 Kunert B., Nemeth I., Reinhard S., Volz K., Stolz W. “Si(001) surface preparation for the antiphase domain free heteroepitaxial growth of GaP on Si substrate,” Thin Solid Films 517:140-143, 2008.
 Geisz J. F., Olson J. M., Romero M. J., Jiang C. S., and Norman A. G., “Lattice-mismatched GaAsP Solar Cells Grown on Silicon by OMVPE,” 2006 IEEE 4th World Conference on Photovoltaic Energy Conference, 772–775, 2006.
 T. J. Grassman, M. R. Brenner, M. Gonzalez, A. M. Carlin, R. R. Unocic, R. R. Dehoff, M. J. Mills, S. A. Ringel, “Characterization of Metamorphic GaAsP / Si Materials and Devices for Photovoltaic Applications,” 57, 10: 3361–3369, 2010.
 Tomasulo S., Simon J., Simmonds P. J., Biagiotti J., Lee M. L., “Molecular beam epitaxy of metamorphic InGaP solar cells on mixed anion GaAsP/GaAs graded buffers,” Journal of Vacuum Science and Technology - B, 29, 3:03C118-1-6, 2011.
 see ref. 9.