PHOTONIC ENGINEERING OF ABSORPTION AND EMISSION IN PHOTOVOLTAICS
Munday, Jeremy N
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As modern society advances, the demand for clean and renewable energy resources becomes more and more important. The sun is by far the most abundant source of renewable energy and is indirectly responsible for many other energy resources on earth (e.g. sunlight enables photosynthesis, biofuels, wind, and even carbon-based fuels). A solar cell directly converts the energy of solar illumination into electricity through the photovoltaic effect and is expected to play a crucial role in the future total power generation globally. Our work has focused on photonic approaches to improving the conversion efficiency of solar cells. Toward this goal, we present results describing the use of quantum dot emission to redirect light within a solar cell, as well as the modification of absorption and emission of light from a solar cell using nanostructures and thin films to increase the efficiency to approach (or possibly surpass) the currently understood efficiency limits for traditional devices. The Shockley-Queisser (SQ) limit describes the maximum solar power conversion efficiency achievable for a p-n junction composed of a particular material and is the standard by which new photovoltaic technologies are compared. This limit is based on the principle of detailed balance, which equates the photon flux into a device to the particle flux (photons or electrons) out of that device. Based on this theory, we describe how the efficiency of a photovoltaic cell is altered in the presence of new anti-reflection coatings, nanotexturing (e.g. plasmonic nanoparticle, nanowire), and more advanced photonic structures (e.g. photonic crystals) that are capable of modifying the absorption and emission of photons. Nanostructured solar cells represent a novel class of photovoltaic devices. By careful selection of materials, as well as particle shapes and positions, the device performance can be improved by increasing the optical path length for scattered light, improving the modal distribution of the light within the absorber, and increasing light concentration (or angle restriction). For example, nanowires can yield microscale concentration effects to improve device performance; however, it has been unclear whether or not they can exceed the Shockley-Queisser limit. We show that single-junction nanostructured solar cells have a theoretical maximum efficiency of ∼ 42% under AM 1.5 solar illumination. While this exceeds the efficiency of a non-concentrating planar device, it does not exceed the Shockley-Queisser limit for a planar device with optical concentration. For practical devices, we include the effect of diffuse illumination and find that with the modest optical concentration available from nanostructures (× 1,000), an efficiency of 35.5% is achievable even with 25% diffusive solar radiation. Finally, we discuss how photon emission modification offers an approach for low bandgap materials to achieve higher efficiencies. By incorporating specifically designed photonic structures that restrict the absorption and emission of above bandgap photons, the bandgap of materials can be effectively tuned. Similarly, restriction of the emission angle leads to increased optical concentration. For realistic devices, we consider how both of these effects are affected by non-ideal materials and photonic structures. We find that the photonic crystal bandgap required to achieve maximum efficiency depends critically on the reflectivity of the photonic crystal. We experimentally demonstrated that the semiconductor bandgap of a material need not be an intrinsic property of that material but can be changed through photonic structuring of the surrounding layers. GaAs has a natural bandgap of 1.43 eV; however, we show that optical reflectors can be used to induce photon-recycling effects, which result in a bandgap shift of 0.13 eV. When a p-n junction is created within the GaAs, we find that its electrical properties are also shifted resulting in a 1.71 mV improvement in the open-circuit voltage of the device under 0.6 suns equivalent illumination. These results show that both the optical and electrical properties of a semiconductor can be modified purely by photonic manipulation, which enables a fundamentally new method for designing semiconductor structures and devices. We anticipate that our result will enable a range of optoelectronic devices.