As with all renewable technologies, taking solar power from niche market to widespread use means balancing cost and efficiency.
Conventional solar cells come in two varieties. Single-junction cells, which use simple, inexpensive materials, lack efficiency. Multi-junction cells, with distinct structures that absorb different portions of the solar spectrum, are much more efficient, but they are expensive to manufacture and require complex, thin-film materials.
Now scientists have begun exploring a third option: hot-carrier devices, which harness photon energy that would otherwise dissipate as heat. These devices offer increased solar energy conversion efficiency in a single-junction cell.
Recent advances in hot-carrier device design, together with developments in affordable synthesis methods for thin-film cells, have scientists optimistic about moving toward a solar-powered future.
Reducing energy losses
When a material absorbs photons with energies larger than the material’s bandgap, energy is lost as heat. To minimize that loss, a device can use specially designed contacts to collect the charge carriers generated by photons and produce current. The challenge is that charge carriers with large kinetic energies quickly dissipate their energy, a side effect that heats the cell material. Scientists are working to develop cells that extract the energy from these “hot carriers” before heating occurs, which offers potential to improve solar conversion efficiencies. Researchers are hopeful about surpassing the Shockley–Queisser maximum theoretical efficiency of 33.7%, which is based on the assumption of considerable thermal losses.
Designing and implementing hot-carrier devices using semiconductors is difficult, so some scientists are exploring the use of metals and dielectrics. At the SPIE Photonics West conference in San Francisco in February, physicist Jeremy Munday of the University of Maryland at College Park presented one such device that allows for the generation and collection of hot carriers. He intends the design for use on its own or with a single-junction solar cell.
Munday’s device has an optically transparent and electrically conductive layer of indium tin oxide, an insulating layer of aluminum oxide, and a back metal contact made of gold. Direct illumination of the device allows absorption to occur predominantly in the gold layer. Simulations indicate that generation and absorption for incident illumination is largely angle-independent.
Munday’s team then achieved a 10-fold increase in photocurrent by building a new device that replaces the gold contact with aluminum. Under solar illumination, the aluminum device was found to be an order of magnitude more efficient than more traditional metal-insulator-metal designs.
Room still exists for improvement. Metallic nanowires placed on top of the aluminum-based device should scatter incoming light, ensuring that all absorption occurs on the back aluminum contact. Another insulating layer ensures that all carrier transfer occurs between the indium tin oxide and the aluminum. “We try to get a lot of absorption in the metal, then extract hot carriers from that metal,” Munday says.
Although hot-carrier devices almost exclusively employ noble metals, especially gold and silver, Munday’s work shows that aluminum and other materials offer advantages. “Different metals will more efficiently generate carriers that can be extracted,” Munday says.
The approach opens the door to new hot-carrier collection devices and detectors based on transparent conducting electrodes. To create next-generation solar cells, scientists will need to study which materials best generate hot carriers and transfer their energy to useful current.
Cutting costs
Thin films that combine elements from groups III and V in the periodic table are the most efficient solar cell materials in existence. They can achieve power conversion efficiencies of more than 45%. But synthesis methods are prohibitively expensive for the films’ large-scale use.
This scanning electron microscope image reveals the structure of a thin film produced by the vapor-liquid-solid growth technique. Credit: Christopher Bailey
The substrate accounts for the majority of the cost in making a solar cell. High costs are acceptable for manufacturing solar cells for use on spacecraft and satellites, where higher panel efficiencies result in fewer solar cells in the rocket payload. But manufacturing costs and the large area of solar cells make it difficult to adapt those materials to the terrestrial solar world.
A recent analysis by the National Renewable Energy Laboratory indicated that a typical substrate such as gallium arsenide or indium phosphide costs $16/W to produce, assuming that the substrate is reused five times. Silicon solar cells set the standard at less than $1/W. “We need new methods of creating substrates,” says Christopher Bailey of Old Dominion University in Norfolk, Virginia.
In 2013 a team based at the University of California, Berkeley, demonstrated the first vapor-liquid-solid (VLS) growth of high-quality polycrystalline indium phosphide thin films. In VLS, indium films are deposited on molybdenum foil with a silicon oxide cap to maintain the necessary flat geometry. After heating the foil above the melting point of indium, phosphorous vapor is introduced and indium phosphide crystals precipitate as continuous, pristine films. The Berkeley researchers achieved crystal grain sizes 100 times as large as those obtained by conventional growth processes, which leads to excellent optoelectronic properties.
Bailey and his colleagues recently built upon the Berkeley work by exploring different material concentrations, generating indium and indium gallide layers to attempt alloys of InGaP via confined VLS on metal foils. “The idea is that you could grow a solar cell on virtually any surface,” says Bailey. “If we can remove the substrate altogether by substituting otherwise very cheap materials, we should be able to approach the cost of the traditional manufacturing-only process.”
In addition to value, VLS has the potential to produce precise lattice structures. To make an efficient multi-junction solar cell, the lattice constant—a value that defines the unit cell for a specific crystal structure—must match precisely for all materials. Deviation by less than 1% can lead to degradation in performance, preventing electrons and positively charged holes from efficiently traveling through the cell. Starting with a market-available substrate means that manufacturers are locked into whatever lattice constant the designer produces. VLS may make it possible to design substrates with any value of lattice constants, allowing for many more materials and solar cell designs to be built.
Modeling the shapes of tree branches, neurons, and blood vessels is a thorny problem, but researchers have just discovered that much of the math has already been done.
