"I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that." We have come a long way since these words of wisdom from Thomas Edison way back in 1931! Solar energy or photovoltaic (PV), a key component in the renewable energy mix, continues to grow, with a record 57 GWp installed in 2015. The compound annual growth rate of PV installations was 41 % in the 2000 to 2015 period (Fraunhofer ISE). Wafer-based crystalline silicon is still the dominant technology, accounting for more than 90% of global PV production. Still playing catch up are the many thin film technologies led by cadmium telluride (CdTe). The early predictions of thin film technologies surge did not materialize, resulting in the failure of many startups. But the technologies are alive and well, amidst continuing efforts to lower cost and improve conversion efficiency. Advanced materials still hold the keys to success in the race to boost PV efficiency levels.
"More energy from sunlight strikes the Earth in one hour than all the energy consumed by humans in one year." Solar cells are engineered with light absorbing advanced materials to convert incoming solar radiation into direct electrical current but, unfortunately, not all the light is converted. Some of the energy is lost as heat while a portion is simply not absorbed. Mismatch in bandgap energy levels is a major contributor to the low conversion. This restricts the theoretical efficiency, without concentration, of a single junction solar cell to only ca. 33%, the so-called Shockley-Queisser limit. But all is not lost. There are several design tactics and advanced materials driven innovations to bypass the hurdle, allowing efficiencies to possibly climb to about 74%! Some of these include multi-junction cell designs, light concentration, such as concentrator photovoltaic (CPV) or concentrating solar power (CSP), tandem cell arrangements, hot carriers, thermophotovoltaics, circulators, and quantum antennas.
Higher efficiency results in higher power output that lowers overall cost, typically measured in $/watt ($/W) and levelized cost of electricity (LCOE). Other benefits include higher power density and module size reduction. In 1954, Bell demonstrated the first practical solar cell with about 6% conversion efficiency. Today, silicon-based cells are getting close to 4X this number. But comparing efficiency results from different sources can be tricky. 3rd party validation by credible independent laboratories is the appropriate way to support claims. The National Renewable Energy Laboratory (NREL) in the USA, Fraunhofer Institute for Solar Energy (ISE) in Germany, and the National Institute of Advanced Industrial Science and Technology, (AIST) in Japan fulfill the referee role. NREL maintains a great web-based reporting on the best research cell efficiencies.
First generation solar technology based on silicon is still first, in terms of global installations. The combination of high efficiency, reduced materials, durability and lower production costs easily put silicon as the most widespread solar technology. This wafer-based silicon comes in different forms of single crystal, polycrystalline, mono- and multi-crystalline. In July 2016, Sun Power claimed the record for an NREL-validated rooftop silicon solar module efficiency of 24.1% while the best non-concentrating single crystal cell efficiency is at 25%. Amonix had earlier reported a concentrating (92X) silicon cell at 27.6%. At the same time, material usage has been declining, reaching about 6 g/Wp in 2015 from 10 g/Wp years ago.
Thin film technologies ushered in the second generation of solar power generation. The three major types, CdTe, copper indium gallium di-selenide (CIGS) and amorphous silicon, enable low cost from reduced materials usage and simpler manufacturing processes. Earlier this year, First Solar broke its own record with a 22.1% efficient CdTe cell, as they explore pathways to 25% efficiency. CIGS technology has struggled despite reaching 22.3% conversion cell efficiency while amorphous silicon cells remain at the bottom at 13.5%.
Third generation PV technologies exhibit lower efficiency levels. These include nanocrystalline, polymer, and dye-sensitized technologies. These cells exhibit lower efficiencies but offer the promise of low cost and ease of fabrication. However, mass deployment is yet to happen. In February this year, Heliatek reported 13.2% efficiency with their organic PV multijunction architecture, on the road to15%. 15% efficiency has been claimed for solid-state dye sensitized solar cells, though NREL validated cells show a lower level efficiency. Quantum dot technology is also in the game, with the University of Toronto group recently reporting power conversion efficiencies exceeding 10%. Perovskite-based cells are new on the block. Already capturing headlines from research labs and startups, these perovskite-based cells have reached 22.2%! The cells can be made very thin and tuned to adjust color and impart transparency. Hong Kong Polytechnic University is claiming the world's highest power conversion efficiency with a perovskite-silicon tandem cell at 25.5%.
Multijunction cells have achieved the highest conversion efficiencies. While offering premium lightweighting advantages for space applications, high costs and complexity have limited much terrestrial use. These cells typically use compound semiconductor III-V materials and can be engineered in various configurations, such as, lattice-matched, metamorphic, and intermediate band. Earlier this year, a CPV mini-module from Fraunhofer measured 43.4% conversion, a world record. Spectrolab showed off another record-breaking 5-junction cell with 38.8% efficiency. The high efficiencies can be pushed even higher with concentration. A 4-junction cell with concentration at 508 suns reached 46% efficiency.
High efficiency modules are already being touted as the "future of solar". I do expect that we will see increasing efficiencies across the various technologies. At the same time, costs will continue to decrease. GTM Research projects that utility-scale solar PV installed cost will fall below $1/W by 2020. Coincidentally, this is line with the Department of Energy (DOE) SunShot Initiative launched in 2011. While incremental improvements will emerge from novel designs, major breakthroughs will remain dependent on advanced materials innovation.