Solar energy conversion George W. Crabtree and Nathan S. Lewis If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to con- vert photons into electricity, fuel, and heat. The need for better conversion technologies is a driving force behind many recent developments in biology, materials, and especially nanoscience. George Crabtree is a senior scientist at Argonne National Laboratory in Argonne, Illinois, and director of its materials science division. Nathan Lewis is a professor of chemistry at the California Institute of Technology in Pasadena, California, and director of the molecu- lar materials research center at Caltech’s Beckman Institute. The Sun provides Earth with a staggering amount of Sun continuously delivers to Earth, 1.2 × 105 terawatts, energy—enough to power the great oceanic and atmospheric dwarfs every other energy source, renewable or nonrenew- currents, the cycle of evaporation and condensation that able. It dramatically exceeds the rate at which human civi- brings fresh water inland and drives river flow, and the ty- lization produces and uses energy, currently about 13 TW. phoons, hurricanes, and tornadoes that so easily destroy the The impressive supply of solar energy is complemented natural and built landscape. The San Francisco earthquake of by its versatility, as illustrated in figure 1. Sunlight can be con- 1906, with magnitude 7.8, released an estimated 1017 joules of verted into electricity by exciting electrons in a solar cell. It energy, the amount the Sun delivers to Earth in one second. can yield chemical fuel via natural photosynthesis in green Earth’s ultimate recoverable resource of oil, estimated at plants or artificial photosynthesis in human-engineered sys- 3 trillion barrels, contains 1.7 × 1022 joules of energy, which tems. Concentrated or unconcentrated sunlight can produce the Sun supplies to Earth in 1.5 days. The amount of energy heat for direct use or further conversion to electricity.1 humans use annually, about 4.6 × 1020 joules, is delivered to Despite the abundance and versatility of solar energy, we Earth by the Sun in one hour. The enormous power that the use very little of it to directly power human activities. Solar HO2 HO2 O2 Electrons O2 CO2 H H NN CO2 NN Holes H,CH24, Sugar CH3 OH O H NC 400–3000 °C Natural 50–200 °C Heat engines, photosynthesis Artificial Space, water electricity generation, (biomass) photosynthesis heating industrial processes Solar electric Solar fuel Solar thermal Figure 1. Solar photons convert naturally into three forms of energy—electricity, chemical fuel, and heat—that link seam- lessly with existing energy chains. Despite the enormous energy flux supplied by the Sun, the three conversion routes supply only a tiny fraction of our current and future energy needs. Solar electricity, at between 5 and 10 times the cost of electricity from fossil fuels, supplies just 0.015% of the world’s electricity demand. Solar fuel, in the form of biomass, accounts for approximately 11% of world fuel use, but the majority of that is harvested unsustainably. Solar heat provides 0.3% of the energy used for heating space and water. It is anticipated that by the year 2030 the world demand for electricity will double and the demands for fuel and heat will increase by 60%. The utilization gap between solar energy’s potential and our use of it can be overcome by raising the efficiency of the conversion processes, which are all well below their theoretical limits. © 2007 American Institute of Physics, S-0031-9228-0703-010-4 March 2007 Physics Today 37 electricity accounts for a minuscule 0.015% of world electric- ity production, and solar heat for 0.3% of global heating of Photovoltaic conversion efficiencies space and water. Biomass produced by natural photosynthe- Laboratory Thermodynamic sis is by far the largest use of solar energy; its combustion or best* limit gasification accounts for about 11% of human energy needs. Single junction 31% However, more than two-thirds of that is gathered unsus- tainably—that is, with no replacement plan—and burned in Silicon (crystalline) 25% small, inefficient stoves where combustion is incomplete and Silicon (nanocrystalline) 10% the resulting pollutants are uncontrolled. Between 80% and 85% of our energy comes from fossil Gallium arsenide 25% fuels, a product of ancient biomass stored beneath Earth’s surface for up to 200 million years. Fossil-fuel resources are Dye sensitized 10% of finite extent and are distributed unevenly beneath Earth’s Organic 3% surface. When fossil fuels are turned into useful energy though combustion, they produce greenhouse gases and Multijunction 32% 66% other harmful environmental pollutants. In contrast, solar Concentrated sunlight photons are effectively inexhaustible and unrestricted by (single junction) 28% 41% geopolitical boundaries. Their direct use for energy produc- tion does not threaten health or climate. The solar resource’s Carrier multiplication 42% magnitude, wide availability, versatility, and benign effect on *As verified by the National Renewable Energy Laboratory. the environment and climate make it an appealing energy Organic cell efficiencies of up to 5% have been reported in source. the literature. Raising efficiency The enormous gap between the potential of solar energy and junction interface drives electrons in one direction and holes our use of it is due to cost and conversion capacity. Fossil fuels in the other, which creates at the external electrodes a po- meet our energy demands much more cheaply than solar al- tential difference equal to the bandgap, as sketched in the ternatives, in part because fossil-fuel deposits are concen- left panel of figure 1. The concept and configuration are sim- trated sources of energy, whereas the Sun distributes photons ilar to those of a semiconductor diode, except that electrons fairly uniformly over Earth at a more modest energy density. and holes are introduced into the junction by photon excita- The use of biomass as fuel is limited by the production ca- tion and are removed at the electrodes. pacity of the available land and water. The cost and capacity With their 1961 analysis of thermodynamic efficiency, limitations on solar energy use are most effectively addressed William Shockley and Hans Queisser established a milestone by a single research objective: cost effectively raising conver- reference point for the performance of solar cells.2 The analy- sion efficiency. sis is based on four assumptions: a single p–n junction, one The best commercial solar cells based on single-crystal electron–hole pair excited per incoming photon, thermal re- silicon are about 18% efficient. Laboratory solar cells based laxation of the electron–hole pair energy in excess of the on cheaper dye sensitization of oxide semiconductors are bandgap, and illumination with unconcentrated sunlight. typically less than 10% efficient, and those based on even Achieving the efficiency limit of 31% that they established for cheaper organic materials are 2–5% efficient. Green plants those conditions remains a research goal. The best single- convert sunlight into biomass with a typical yearly averaged crystal Si cells have achieved 25% efficiency in the laboratory efficiency of less than 0.3%. The cheapest solar electricity and about 18% in commercial practice. Cheaper solar cells comes not from photovoltaics but from conventional induc- can be made from other materials,3 but they operate at sig- tion generators powered by steam engines driven by solar nificantly lower efficiency, as shown in the table above. Thin- heat, with efficiencies of 20% on average and 30% for the best film cells offer advantages beyond cost, including pliability, systems. Those efficiencies are far below their theoretical lim- as in figure 2, and potential integration with preexisting its. Increasing efficiency reduces cost and increases capacity, buildings and infrastructure. Achieving high efficiency from which raises solar energy to a new level of competitiveness. inexpensive materials with so-called third-generation cells, Dramatic cost-effective increases in the efficiency of indicated in figure 3, is the grand research challenge for mak- solar energy conversion are enabled by our growing ability ing solar electricity dramatically more affordable. to understand and control the fundamental nanoscale phe- The Shockley–Queisser limit can be exceeded by violat- nomena that govern the conversion of photons into other ing one or more of its premises. Concentrating sunlight al- forms of energy. Such phenomena have, until recently, been lows for a greater contribution from multi-photon processes; beyond the reach of our best structural and spectroscopic that contribution increases the theoretical efficiency limit to probes. The rise of nanoscience is yielding new fabrication 41% for a single-junction cell with thermal relaxation. A cell techniques based on self-assembly, incisive new probes of with a single p–n junction captures only a fraction of the solar structure and dynamics at ever-smaller length and time spectrum: photons with energies less than the bandgap are scales, and the new theoretical capability to simulate assem- not captured, and photons with energies greater than the blies of thousands of atoms. Those advances promise the ca- bandgap have their excess energy lost to thermal relaxation. pability to understand and control the underlying structures Stacked cells with different bandgaps capture a greater frac- and dynamics of photon conversion processes. tion of the solar spectrum; the efficiency limit is 43% for two junctions illuminated with unconcentrated sunlight, 49% for Electricity three junctions, and 66% for infinitely many junctions. Solar cells capture photons by exciting electrons across the The most dramatic and surprising potential increase in bandgap of a semiconductor, which creates electron–hole efficiency comes from carrier multiplication,4 a quantum-dot pairs that are then charge separated, typically by p–n junc- phenomenon that results in multiple electron–hole pairs tions introduced by doping.