Richard King: Spectrolab, Inc.
Raising the Efficiency Ceiling in Multijunction Solar Cells
February 16, 2011 | 4:00pm | ESB 1001
Photovoltaics research represents one of the greatest opportunities to impact the climate change and energy security problems that we face today. Over 1.5×1022 J (15,000 EJ) of solar energy reach Earth everyday, compared to a daily energy consumption of approximately 1.3 EJ by human activity. The potential for new 4-, 5-, and 6-junction solar cell architectures, capable of greater than 70% efficiency in theory, to reach practical efficiencies over 50% is highly leveraging for the economics of concentrator photovoltaic (CPV) systems.
The theoretical performance of such next-generation cells, and experimental results for 3- and 4- junction CPV cells, are examined in this talk to evaluate their impact for real-world solar electricity generation. Semiconductor device physics equations are formulated in terms of the band gap-voltage offset Woc ≡ (Eg / q) – Voc , to give a clearer physical understanding and more general analysis of the multiple subcell band gaps in multijunction cells. Band gap-voltage offset is shown experimentally to be largely independent of band gap Eg for a wide range of metamorphic and lattice-matched semiconductors from 0.67 to 2.1 eV. Its theoretical Eg dependence is calculated from that of the radiative recombination coefficient, and at a more fundamental level using the Shockley-Queisser detailed balance model, bearing out experimental observations. First-principle efficiency limits are analyzed for some of the main candidates for high-efficiency multijunction terrestrial concentrator cells. Energy production of 4-, 5-, and 6- junction CPV cells, calculated for changing air mass and spectrum over the course of the day, is found to be significantly greater than for conventional 3-junction cells. The spectral sensitivity of these next-generation cell designs is fairly low, and is outweighed by their higher efficiency. Both upright metamorphic and lattice-matched GaInP/GaInAs/Ge 3-junction cells have reached an independently confirmed efficiency of 41.6%. Light I-V and quantum efficiency measurements of these high-efficiency concentrator solar cells and of next-generation 4-junction CPV cells are presented.
Dr. King is currently Principal Scientist responsible for Photovoltaic Cell R&D at Spectrolab, Inc. His research on photovoltaics over the last 25 years has explored high-efficiency solar cells in a number of semiconductor materials systems, from silicon, to the GaInP, GaInAs, and germanium subcells in III-V multijunction cells. Dr. King's solar cell research led the emergence of III-V multijunction concentrator cells as the photovoltaic technology with the highest and most rapidly rising efficiency, helping to enable the recent growth of the concentrator photovoltaics industry, which now primarily uses this type of solar cell. In his Ph.D. research at Stanford University, Dr. King worked to develop high-efficiency one-sun back-contact silicon solar cells, and his characterization studies of minority-carrier recombination at the doped Si/SiO2interface are still in use today for high-efficiency silicon solar cell design. Dr. King was inducted into the Space Technology Hall of Fame in 2004, and has 12 patents and over 100 publications on photovoltaics and semiconductor device physics.