Efficiency Enhancement of GaAs Photovoltaics Employing Antireflective Indium Tin Oxide Nanocolumns

Abstract

Global-warming issues coupled with high oil prices have become a major driving force for the use of advanced solar power technology, where a key component lies in the development of high-efficiency and low-cost photovoltaic cells. Next generation photovoltaics, hence, demand an efficiency-boosting mechanism in order to render solar energy cost competitive with conventional sources of electricity.[1] Fundamentally, the conversion efficiency of a solar cell depends on the photon absorption, carrier separation, and carrier collection. [2,3] Therefore, an effective antireflection (AR) coating, minimized recombination loss, and good Ohmic contacts are particularly important. Metal grids that inevitably block the transmission of solar energy also require optimization in order to reduce the series resistance. The trade-off between the electrode and the AR coating areas is one of the efficiency-limiting factors in a conventional solar cell.The conventional AR coating is usually composed of a quarter wavelength stack of dielectrics with different refractive indices. Broad angular and spectral AR is achievable at the price of multiple layers.[ [4][5][6][7] Over the past few years, versatile subwavelength structures (SWS) have emerged as promising candidates for AR coatings, due to the characteristics of zero-order gratings, or the so-called moth-eye effects. [8][9][10][11][12][13][14] However, the fabrication costs, which involve either electron-beam (e-beam) lithography or various etching processes, can be significant. In addition, the resulting surface-recombination loss due to dry or wet etching could further hinder the applications of SWS in commercial solar cells. Recently, multiple studies have been carried out on indium tin oxide (ITO), titanium dioxide (TiO 2 ), and silicon dioxide (SiO 2 ) nanostructures employing oblique-angle deposition methods, [15][16][17] where the refractive indices of the nanoporous materials can be engineered by adjusting the air volume ratio. Still, the materials require multiple layers to effectively suppress the Fresnel reflection.In this paper, we demonstrate a practical photovoltaic application of ITO nanocolumns serving as a conductive AR layer for GaAs solar cells. As in standard GaAs cells, the use of a nanostructured AR layer could be otherwise limited due to severe front-surface recombination. The characteristic ITO nanocolumns, prepared by glancing-angle deposition with an incident nitrogen flux, offer omnidirectional and broad-band AR properties for both s-and p-polarizations, up to an incidence angle of 708 for the 350-900 nm wavelength range. Calculations based on a rigorous coupled-wave analysis (RCWA) method indicate that the superior AR characteristics arise from the tapered column profiles, which collectively function as a graded-refractive-index layer. The conversion efficiency of the GaAs solar cell with the nanocolumn AR layer increases by 28% compared to a cell without any AR treatment. Moreover, nearly 42% enhancement is achieved for photocurrents generated at wavele...

Perspectives on oblique angle deposition of thin films: From fundamentals to devices

“…The fabrication of highly porous and/or sculptured ITO thin films has been attempted by e-beam OAD from ITO pellets, both under high vacuum conditions [316][317][318][319][320][321][322][323][324][325][326][327] and in a carrier gas flux (usually nitrogen) [320,[328][329][330]. Fig.…”

General Design Procedures for Airport-Based Solar Photovoltaic Systems

“…It would thus be expected that for a given angle reflection from a PV front glass surface without any antireflecting (AR) coating is less intense than that of water. Now, with the current progress in solar module technology and development in anti-reflection materials such as materials with an index of refraction of 1.05 [91,92], it is safe to assume that solar PV module will have reflection off their surface dropped further with future technologies [93][94][95][96]. However, even today with the refractive index off PV with AR coating dropping below 1.33 to 1.20-1.30 [97], PV poses no (or presents tolerable/safe) hazards from reflection for airport solar PV projects.…”

Arrays of Lucius microprisms for directional allocation of light and autostereoscopic three-dimensional displays

“…1a, henceforth to be referred to as the oblique angle) in a thermal or electron gun evaporator maintained at 10 − 6 Torr. The oblique deposition, which has been widely used to create smart micro-or nanostructures in various applications 2,3,[12][13][14][15][16][17] , completes the fabrication of the Lucius prism array ( Supplementary Fig. S1).…”