Marius Peters

Black silicon: fabrication methods, properties and solar energy applications

Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

Nanoimprinted diffraction gratings for crystalline silicon solar cells: implementation, characterization and simulation

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electro-optical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential.

The effect of photonic structures on the light guiding efficiency of fluorescent concentrators

It is possible to increase the efficiency of fluorescent concentrator systems with photonic structures. This is achieved by reducing the losses caused by the loss cone of total internal reflection. Examples of fluorescent concentrators we are currently working with are given and different photonic structures designed for the application on these fluorescent concentrators are presented. We discuss the optical characteristics of the photonic structures and their effects on the light guiding efficiency of the fluorescent concentrators. An analytical model is established to analyze and quantify the effects of these filters on the light guiding efficiency theoretically. This model is used to analyze the given photonic structures in detail. We show that with a real photonic structure the loss cone losses can be reduced by more than 75%.

Matrix formalism for light propagation and absorption in thick textured optical sheets

In this paper, we introduce a simulation formalism for determining the Optical Properties of Textured Optical Sheets (OPTOS). Our matrix-based method allows for the computationally-efficient calculation of non-coherent light propagation and absorption in thick textured sheets, especially solar cells, featuring different textures on front and rear side that may operate in different optical regimes. Within the simulated system, the angular power distribution is represented by a vector. This light distribution is modified by interaction with the surfaces of the textured sheets, which are described by redistribution matrices. These matrices can be calculated for each individual surface texture with the most appropriate technique. Depending on the feature size of the texture, for example, either ray- or wave-optical methods can be used. The comparison of the simulated absorption in a sheet of silicon for a variety of surface textures, both with the results from other simulation techniques and experimentally measured data, shows very good agreement. To demonstrate the versatility of this newly-developed approach, the absorption in silicon sheets with a large-scale structure (V-grooves) at the front side and a small-scale structure (diffraction grating) at the rear side is calculated. Moreover, with minimal computational effort, a thickness parameter variation is performed.

Hexagonal sphere gratings for enhanced light trapping in crystalline silicon solar cells

Enhanced absorption of near infrared light in silicon solar cells is important for achieving high conversion efficiencies while reducing the solar cells thickness. Hexagonal gratings on the rear side of solar cells can achieve such absorption enhancement. Our wave optical simulations show photocurrent density gains of up to 3 mA/cm2 for solar cells with a thickness of 40 µm and a planar front side. Hexagonal sphere gratings have been fabricated and optical measurements confirm the predicted absorption enhancement. The measured absorption enhancement corresponds to a photocurrent density gain of 1.04 mA/cm2 for planar wafers with a thickness of 250 µm and 1.49 mA/cm2 for 100 µm.

Advanced upconverter systems with spectral and geometric concentration for high upconversion efficiencies

In this paper we present an advanced upconverter system concept to reduce the sub-bandgap losses of silicon solar cells. We address the issue of the narrow absorption range of common upconverter materials. This problem can be overcome by the combination of the upconverter with a broadly absorbing fluorescent material, which emits in the absorption range of the upconverter. However, possible fluorescent materials also absorb in the emission range of the upconverter. We therefore propose an advanced system setup, which avoids unwanted absorption by separating upconverter and fluorescent material with a selectively reflective photonic structure. The incorporation of the fluorescent material in a fluorescent concentrator allows for additional geometric concentration increasing the efficiency of the upconversion process. We estimate that a total system efficiency of up to 25% could be possible with an Er based upconverting system and a silicon solar cell.

Advanced modeling of the effective minority carrier lifetime of passivated crystalline silicon wafers

A strong injection level dependence of the effective minority carrier lifetime (τeff) is typically measured at low injection levels for undiffused crystalline silicon (c-Si) wafers symmetrically passivated by a highly charged dielectric film. However, this phenomenon is not yet well understood. In this work, we concentrate on two of those possible physical mechanisms to reproduce measured τeff data of c-Si wafers symmetrically passivated by atomic layer deposited Al2O3. The first assumes the existence of a defective region close to the c-Si surface. The second assumes asymmetric electron and hole lifetimes in the bulk. Both explanations result in an adequate reproduction of the injection dependent τeff found for both n- and p-type c-Si wafers. However, modeling also predicts a distinctly different injection dependence of τeff for the two suggested mechanisms if the polarity of the effective surface charge is inverted. We test this prediction by experimentally inverting the polarity of the effective surfac...

Evaluating the electrical properties of silicon wafer solar cells using hyperspectral imaging of luminescence

A line-imaging spectrometer is used to collect the spectrum of electroluminescence at each point of a multicrystalline silicon wafer solar cell. Characterization of the diffusion lengths of minority charge carriers is developed using a specific feature of the luminescence spectral signature. It is shown that various material and device parameters affecting the luminescence spectral signature may be determined independently. Diffusion length images derived from the proposed hyperspectral method are assessed against diffusion lengths obtained by light beam induced current measurements. Using hyperspectral imaging, diffusion lengths of minority charge carriers in a silicon wafer solar cell can be determined.

Electro--optical simulation of diffraction in solar cells.

A simulation method is presented and evaluated for simulating two- and three dimensional wave optical effects in crystalline silicon solar cells. Due to a thickness in the 100 µm range, optical properties of these solar cells typically are simulated, primarily through the use of ray-tracing. Recently, diffractive elements such as gratings or photonic crystals have been investigated for their application in crystalline silicon solar cells, making it necessary to consider two- and three dimensional wave optical effects. The presented approach couples a rigorous wave optical simulation to a semiconductor device simulation. In a first step, characteristic parameters, simulated for a reference setup using the electro-optical method and the standard procedure are compared. Occurring differences provide a measure to quantify the errors of the electro-optical method. These errors are below 0.4% relative. In a second step the electro-optical method is used to simulate a crystalline silicon solar cell with a back side diffractive grating. It is found that the grating enhances to short circuit current density jSC of the solar cell by more than 1 mA/cm².

Enhanced light trapping in thin-film solar cells by a directionally selective filter

A directionally selective multilayer filter is applied to a hydrogenated amorphous silicon solar cell to improve the light trapping. The filter prevents non-absorbed long-wavelength photons from leaving the cell under oblique angles leading to an enhancement of the total optical path length for weakly absorbed light within the device by a factor of kappa(r) = 3.5. Parasitic absorption in the contact layers limits the effective path length improvement for the photovoltaic quantum efficiency to a factor of kappa(EQE) = 1.5. The total short-circuit current density increases by DeltaJ(sc) = 0.2 mAcm(-2) due to the directional selectivity of the Bragg-like filter.