Light-trapping Layer Research Articles

Macroporous Silicon Structures for Light Harvesting

Macroporous silicon light trapping layers on the surface of p-type substrates were manufactured by using electrochemical etching. Optical measurements have shown the decreased optical reflection as compared to bulk silicon. The p-n junction was formed by phosphorus diffusion from phosphosilicate glasses. Scanning Kelvin Force Microscopy technique was used to determine the location of p-n junction in samples with porous silicon layers.DOI: http://dx.doi.org/10.5755/j01.ms.21.1.5725

Design of Anodic Aluminum Oxide Rear Surface Plasmonic Heterostructures for Light Trapping in Thin Silicon Solar Cells

A metal-dielectric heterostructure that provides the combined capability of light trapping and surface passivation is reported. The light-trapping layer employs a porous aluminum anodic oxide (AAO) with metal nanoparticles formed in the pores on the rear surface of a thin crystalline silicon solar cell. Numerical finite-difference time domain (FDTD) simulations were performed to determine the pore diameter and spacing that would result in optimal light trapping for this metal-dielectric heterostructure. For a 2.5-μm-thick crystalline silicon device, the optimal pore diameter and spacing were determined to be ~250 and ~450 nm, respectively. These conditions resulted in an enhancement of the simulated photocurrent by ~12.6% compared with a device in which the heterostructure was replaced with a homogenous aluminum oxide layer. Simulations also confirmed that the thickness of an underlying dielectric layer should be minimized to 10-20 nm, with the AAO barrier layer being maintained as thin as possible. Finally, it was shown that replacement of silver by aluminum in the pores resulted in a reduction in the photocurrent of 6.3% and would necessitate much larger pore spacing that is difficult to achieve experimentally and would result in thicker AAO barrier layers, which are undesirable.

Silicon Nanostructures For Efficient Light Absorption In Photovoltaic Devices

Porous silicon light trapping layers (por-Si LTL) were manufactured using electrochemical etching. Optical measurements have shown the improved por-Si LTL optical properties versus bulk silicon. The p-n junction was formed by boron diffusion from borosilicate glasses. Scanning Kelvin Force Microscopy (SKFM) and Tunnelling Atomic Force Microscopy (TUNA) techniques were used to determine the location of p-n junction in samples with porous silicon light trapping layers. DOI: http://dx.doi.org/10.5755/j01.ms.20.2.6326

Light management for photovoltaics using high-index nanostructures.

High-performance photovoltaic cells use semiconductors to convert sunlight into clean electrical power, and transparent dielectrics or conductive oxides as antireflection coatings. A common feature of these materials is their high refractive index. Whereas high-index materials in a planar form tend to produce a strong, undesired reflection of sunlight, high-index nanostructures afford new ways to manipulate light at a subwavelength scale. For example, nanoscale wires, particles and voids support strong optical resonances that can enhance and effectively control light absorption and scattering processes. As such, they provide ideal building blocks for novel, broadband antireflection coatings, light-trapping layers and super-absorbing films. This Review discusses some of the recent developments in the design and implementation of such photonic elements in thin-film photovoltaic cells.

Enhancing Electrochemical Hydrogen Generation by Platinum-Modification of p-Type Silicon Wires Array under Visible Light

An electrochemical study involving the pulse electrodeposition of platinum onto p-type silicon nanowires array in non-fluoride solution for photo enhanced electrochemical hydrogen generation is presented. Pt was deposited onto the surface of silicon nanowires array (light trapping layer) as a photocatalyst with a radial multi-layers nanocluster structure, of which the photocurrent density of hydrogen generated was increased by about 300% compared to the bare silicon nanowires array under simulated solar radiance, while the value of onset potential also positively shifted to 0.40 V. Furthermore, the impedance of Pt/silicon nanowires array was smaller than that of bare silicon nanowires array as indicated by EIS analysis. We tentatively ascribed the mechanism of enhancement to the effect of the Schottky junction at the Pt/silicon nanowires array interface.

Optimization of non-periodic plasmonic light-trapping layers for thin-film solar cells

Non-periodic arrangements of nanoscale light scatterers allow for the realization of extremely effective broadband light-trapping layers for solar cells. However, their optimization is challenging given the massive number of degrees of freedom. Brute-force, full-field electromagnetic simulations are computationally too time intensive to identify high-performance solutions in a vast design space. Here we illustrate how a semi-analytical model can be used to quickly identify promising non-periodic spatial arrangements of nanoscale scatterers. This model only requires basic knowledge of the scattering behaviour of a chosen nanostructure and the waveguiding properties of the semiconductor layer in a cell. Due to its simplicity, it provides new intuition into the ideal amount of disorder in high-performance light-trapping layers. Using simulations and experiments, we demonstrate that arrays of nanometallic stripes featuring a limited amount of disorder, for example, following a quasi-periodic or Fibonacci sequence, can substantially enhance solar absorption over perfectly periodic and random arrays.

Nanophotonic light trapping with patterned transparent conductive oxides

Transparent conductive oxides (TCOs) play a crucial role in solar cells by efficiently transmitting sunlight and extracting photo-generated charge. Here, we show how nanophotonics concepts can be used to transform TCO films into effective photon management layers for solar cells. This is accomplished by patterning the TCO layer present on virtually every thin-film solar cell into an array of subwavelength beams that support optical (Mie) resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. We first demonstrate these concepts with a model system consisting of a patterned TCO layer on a thin silicon (Si) film and outline a design methodology for high-performance, TCO-based light trapping coatings. We then show that the short circuit current density from a 300 nm thick amorphous silicon (a-Si) cell with an optimized TCO anti-reflection coating can be enhanced from 19.9 mA/cm2 to 21.1 mA/cm2, out of a possible 26.0 mA/cm2, by using an optimized nanobeam array. The key differences and advantages over plasmonic light trapping layers will be discussed.

Advanced light trapping materials: Double layer ZnO:B based transparent conductive oxide

We present double layer structures consisting of ZnO:B/ZnO:B (BZO) and In 2O 3:Mo (IMO)/BZO films. The structure offers the unique opportunity of separating the conductivity of transparent conductive oxides from their light scattering behavior and allows their optimization for use in thin film solar cells. The layers serve as carrier transport and light trapping layers, respectively. BZO films were prepared by mid-frequency magnetic sputtering from a ZnO:B 2O 3 ceramic target. In order to enhance the conductivity of the BZO films, hydrogen was introduced into the sputtering atmosphere. Introducing hydrogen increased the mobility of the BZO-based double layer films to near 30 cm 2/V•s. Efficient scattering was achieved by etching the film in dilute hydrochloric acid. IMO films were also tested as the transport layer. An unconventional surface morphology was obtained by etching the IMO/BZO double layer film. Using this cascading multilayer structure IMO/BZO film as the front contact in a-Si:H solar cell, 20.4% and 7.4% enhancements in short circuit current density were obtained compared to smooth IMO films and textured single layer BZO films.

Quantifying the effectiveness of SiO2/Au light trapping nanoshells for thin film poly-Si solar cells

In order to enhance light absorption of thin film poly-crystalline silicon (TF poly-Si) solar cells over a broad spectral range, and quantify the effectiveness of nanoshell light trapping structure over the full solar spectrum in theory, the effective photon trapping flux (EPTF) and effective photon trapping efficiency (EPTE) were firstly proposed by considering both the external quantum efficiency of TF poly-Si solar cell and scattering properties of light trapping structures. The EPTF, EPTE and scattering spectrum exhibit different behaviors depending on the geometric size and density of nanoshells that form the light trapping layer. With an optimum size and density of SiO2/Au nanoshell light trapping layer, the EPTE could reach up to 40% due to the enhancement of light trapping over a broad spectral range, especially from 500 to 800 nm.

Red-shifting the surface plasmon resonance of silver nanoparticles for light trapping in solar cells

AbstractSurface plasmons in metallic nanoparticle arrays have been shown to increase the absorption of an underlying silicon substrate. This has wide ranging applications, not least in the photovoltaic industry. Incident light excites localised surface plasmons in the silver nanoparticles and is coupled into the silicon in trapped modes. The radiative behaviour of the nanoparticle film is changed by the proximity of a high refractive index surface, causing radiation to be directed into the silicon and providing a light-trapping layer. We investigate a simple and effective method of tuning the surface plasmon resonance frequency, and hence the spectral region at which the absorption enhancement is seen, by varying the underlying dielectric. The particle geometry and distribution are modified by the surface conditions provided by the dielectric layer, and both this and the change in refractive index alter the resonance position. Three common dielectrics used in the photovoltaic industry were investigated as surfaces on which to form arrays of self-assembled silver nanoparticles atmospheric pressure chemical vapour deposited titanium dioxide (APCVD TiO2), low pressure chemical vapour deposited silicon nitride (LPCVD Si3N4) and thermally grown silicon dioxide (SiO2). We show, by optical and electrical measurements, that the red-shifted resonances produced by nanoparticle films on APCVD TiO2, and LPCVD Si3N4 with relatively high refractive indices, correspond to an increase in optical absorption and external quantum efficiency in thin, crystalline solar cells at longer wavelengths.

Microporous Silicon as a Light Trapping Layer for Photodiodes

Porous silicon films with micropore structures were grown on the surface of silicon wafers by an anodization process. The microporous silicon film was found to be an excellent light trapping layer. It was demonstrated that a quantum efficiency of higher than 90% could be obtained with incident angles of 40, 80, and 58°, and for s-, p-, and randomly polarized light, respectively. © 2000 The Electrochemical Society. S1099-0062(00)03-069-8. All rights reserved.

Photoresponse study of diamond thin film/C60-dopedpoly(3-dodecylthiophene) heterostructure

We have studied the photoresponse of diamond thin film/C60-doped poly-(3-dodecylthiophene) heterostructure. The fine grain diamond thin film in the heterostructure was used as a protective photoactive window and also as an anti-reflecting and light-trapping layer due to its chaotically-oriented and well-faceted nano-sized polycrystalline structure. We have obtained a quantum efficiency of 1.8% in the UV region.