Optoelectronic devices using tailored- and low-refractive index nanoporous thin films

Abstract

A multiple-discrete-layer tailored- and low-refractive index antireflection (AR) coating is designed and experimentally demonstrated on an inverted metamorphic (IMM) triple-junction solar cell. Such AR coating has broadband and omnidirectional AR characteristics achieved by a global optimization based on multiple-discrete-layer AR coating design. This high performance AR coating shows an averaged omnidirectional short-circuit current enhancement of 34% on an IMM solar cell.

At large layer thickness, nanoporous thin film alumina shows interesting optical anisotropy. A 550 nm nanoporous alumina (n ≈ 1.07, 410 nm) is deposited on a sapphire substrate and illuminated by a pulsed laser at 266 nm. At normal incidence, the scattering distribution is found to be highly asymmetric, with a backscattering peak at θ = 62° with respect to the layer surface normal. A novel model employing a finite difference time domain simulation method is proposed for the optimization of such films.

Nanoporous thin film materials are suitable for applications in deep-ultraviolet (UV) frequency range. A deep-ultraviolet (280 nm) light-emitting diode deposited with an optimized two-layer nanoporous alumina AR coating shows over 8% efficiency enhancement, in excellent agreement with the calculation using the effective refractive index model. At a total thickness of 172 nm for the two-layer AR coating and at a wavelength of 266 nm, the non-ideal extinction of the nanoporous alumina thin film is negligible.

By tuning the thickness period of high / low refractive index of a multilayer ITO color filter, interference-based blue, green, and red color filters up to four pairs (eight layers) are experimentally demonstrated. A maximum transmittance of 95.2% and a minimum transmittance of 26.2% are realized for an eight-layer red color filter.

Tailored- and low-refractive index nanoporous thin film materials possess a wide range of tunable refractive indices by varying the degree of nano-porosity. This tailorability of refractive indices enables the realization and improvement of advanced optical functional coatings. In this dissertation, tailored- and low-refractive index nanoporous thin films are designed, optimized and demonstrated for high energy-efficiencies of solar cells, liquid crystal displays, and deep-ultraviolet light emitting diodes.

Matching the refractive index of indium-tin-oxide (ITO) electrode to its ambient in liquid crystal display significantly reduces Fresnel reflection loss. The transmittance through glass substrates with nanoporous ITO electrodes is shown to have an averaged transmittance enhancement of 13% compared to glass substrates with dense ITO electrodes.