Enhancing the efficiency of photovoltaics with photonics
The widespread adoption of solar energy is inhibited primarily by its high cost,1 which is linked to the efficiency of converting energy from sunlight to electricity.2 A photovoltaic (PV) device's efficiency is limited by the inability of a single bandgap (energy gap) absorber to convert the broad solar spectrum into electric power.3 Photons with energy less than the bandgap are not absorbed at all, while those with energy much higher than the bandgap lose this excess energy through thermalization. As a result, more than 40% of the incident power is lost right at the start.3
Increasingly, manufacturers of PV devices are proposing ultrathin absorbers in an attempt to reduce materials costs, and because such cells are flexible enough to enable unique applications.4,5 Unfortunately, in such devices only a very small fraction of the incident light is absorbed. However, recent advances in photonics are providing solutions to these challenges.
Historically, multijunction solar cells have mitigated the problem of limited absorption by placing absorbers of increasing bandgaps on top of one another.6–8 However, such devices require expensive and complex fabrication processes, and are not readily extendible to more than three bandgaps. Many approaches have been proposed to improve absorption in thin-film PV cells,9, 10 most of them aiming to increase the current density of the device. In contrast, we are developing manufacturable nano- and microphotonics that enhance the overall power-conversion efficiency, potentially enabling cost-effective devices with efficiencies of 50% or greater.
We extended a direct-binary-search algorithm to design broadband microphotonic elements that can efficiently separate sunlight into spectral bands and concentrate these onto optimized PV absorbers: see Figure 1(a).11, 12 We call these optical elements ‘polychromats.’ Compared with previous approaches, using prisms, dichroic filters, or holograms,13–16 the polychromat should achieve optical efficiencies of greater than 90% across the solar spectrum. Furthermore, it can be manufactured inexpensively using embossing, and can be incorporated into the glass covers of solar panels. Using this approach, we experimentally demonstrated an increase of 42% in peak power density using a pair of copper indium gallium (di)selenide (CIGS) solar cells, and saw an increase of around 22% when using a combination of silicon (Si) and gallium arsenide (GaAs) cells under simulated sunlight.12 An important advantage of the polychromat approach is that we can independently control the number of spectral bands and the geometric concentration of each.17 Therefore, it might be possible to combine expensive and inexpensive materials (at different concentrations) in order to maximize the output power.
In our experiments, we fabricated the polychromat using grayscale lithography on a glass substrate: see Figure 1(b). We used an optoelectronic model to design the polychromat so as to maximize the total peak power density of all the cells. We typically use 1D polychromats to avoid tracking the daily motion of the sun, although 2D devices are also feasible. Figure 1(b) shows the experimental configuration comprising the polychromat and the two solar cells (gallium indium phosphide, or GaInP, and GaAs). We further demonstrated that appropriate design can reduce the distance between the polychromat and the cells to ∼2mm, enabling a compact device. Figure 1(c) shows the image formed by the polychromat, where a high-energy band was interspersed with a low-energy band. A periodic array of polychromats illuminates a periodic array of solar cells.
At the lower end of the efficiency scale are ultra-thin PV devices, which have the potential to be extremely low cost. In some cases, thinner absorbers enable high charge transport and carrier-collection efficiencies,18, 19 but the performance of such devices is fundamentally limited by their poor light absorption. In order to mitigate this problem, scientists have used random or simple geometries of micro- and nanostructures, which scatter light into the absorber layer.9, 10,20–23 The resulting increased optical path lengths lead to higher absorption. As pointed out by Yablonovitch, in thick absorbers light absorption may be enhanced compared to an unpatterned absorber by a factor of up to 4n2, where n is the refractive index of the absorber.24 This is the so-called ergodic limit.24–26
Recent theoretical work suggests the possibility of overcoming the ergodic limit and capturing more of the incident light in absorbers with subwavelength thicknesses.25–27 This may be achieved by locating the absorber between two cladding layers, whose refractive index is higher than that of the absorber. This configuration enables photonic modes that are primarily trapped within the plane of the absorber. However, such modes do not easily couple to incident light from free space, especially over a broad spectrum. We developed optimization algorithms that can design nanostructured scatterers at the interfaces between the cladding and absorber layers, as shown in Figure 1(d), to enhance the coupling of broadband sunlight into resonant guided modes within the absorber, and thereby to substantially increase the output power density of such PV devices.21,28–31 Our design algorithms are easily adapted to account for oblique incidence angles (such as when the sun moves across the sky), and simulations indicate that a properly optimized device can produce over seven times more energy in the course of a year compared to an equivalent unpatterned device.29, 31 At present, fabricating such devices remains a challenge. However, recent advances in aligned nanoimprint lithography show promise for addressing these issues.32–35
Photonics can play an important role in increasing the efficiency of PV devices, and may thereby reduce their cost and enable their widespread adoption. Conditioning sunlight using micro- and nanophotonics offers unprecedented opportunities for innovation and interdisciplinary research at the intersection of computation, nanofabrication, and photonics. One interesting thrust of our future research is the combination of spectrum splitting and light trapping using nanophotonics. Furthermore, advances in high-quality thin-film GaAs devices foretell the potential for much greater efficiencies in the future.36,37
I would like to acknowledge Daniel Friedman, who provided the GaInP and GaAs cells for this work. I would also like to thank Jose Dominguez-Caballero, Daniel Friedman, Howard Lee, S. V. Sreenivasan, Henry Smith, Pooran Joshi, Keith Emery, and my students Peng Wang, Ganghun Kim, and Bing Shen for useful discussion and contributions. The research was partly funded by a Department of Energy Bridge grant EE0005959 and the Utah Science and Technology Research initiative. Assistance for device fabrication at the Utah nanofabrication facility is gratefully acknowledged.
Rajesh Menon's research into the intersection of optics and nanotechnology has inspired more than 50 publications, 30 patents, and two companies. In 2011 he won the National Science Foundation CAREER Award and in 2009 the International Commission for Optics Prize. He holds SM and PhD degrees from the Massachusetts Institute of Technology.