Lucio Claudio Andreani

Photonic light-trapping versus Lambertian limits in thin film silicon solar cells with 1D and 2D periodic patterns.

We theoretically investigate the light-trapping properties of one- and two-dimensional periodic patterns etched on the front surface of c-Si and a-Si thin film solar cells with a silver back reflector and an anti-reflection coating. For each active material and configuration, absorbance A and short-circuit current density Jsc are calculated by means of rigorous coupled wave analysis (RCWA), for different active materials thicknesses in the range of interest of thin film solar cells and in a wide range of geometrical parameters. The results are then compared with Lambertian limits to light-trapping for the case of zero absorption and for the general case of finite absorption in the active material. With a proper optimization, patterns can give substantial absorption enhancement, especially for 2D patterns and for thinner cells. The effects of the photonic patterns on light harvesting are investigated from the optical spectra of the optimized configurations. We focus on the main physical effects of patterning, namely a reduction of reflection losses (better impedance matching conditions), diffraction of light in air or inside the cell, and coupling of incident radiation into quasi-guided optical modes of the structure, which is characteristic of photonic light-trapping.

Light trapping regimes in thin-film silicon solar cells with a photonic pattern

By patterning thin-film silicon solar cells with a periodic etching in addition to an antireflection coating, we increase the short-circuit current up to 36.5%. The pattern and the coating are investigated to recognise different coupling regimes.

Engineering Gaussian disorder at rough interfaces for light trapping in thin-film solar cells

A theoretical study of randomly rough interfaces to obtain light trapping in thin-film silicon solar cells is presented. Roughness is modeled as a surface with Gaussian disorder, described using the root mean square of height and the lateral correlation length as statistical parameters. The model is shown to describe commonly used rough substrates. Rigorous calculations, with short-circuit current density as a figure of merit, lead to an optimization of disorder parameters and to a significant absorption enhancement. The understanding and optimization of disorder is believed to be of general interest for various realizations of thin-film solar cells.

Towards high efficiency thin-film crystalline silicon solar cells: The roles of light trapping and non-radiative recombinations

Thin-film solar cells based on silicon have emerged as an alternative to standard thick wafers technology, but they are less efficient, because of incomplete absorption of sunlight, and non-radiative recombinations. In this paper, we focus on the case of crystalline silicon (c-Si) devices, and we present a full analytic electro-optical model for p-n junction solar cells with Lambertian light trapping. This model is validated against numerical solutions of the drift-diffusion equations. We use this model to investigate the interplay between light trapping, and bulk and surface recombination. Special attention is paid to surface recombination processes, which become more important in thinner devices. These effects are further amplified due to the textures required for light trapping, which lead to increased surface area. We show that c-Si solar cells with thickness of a few microns can overcome 20% efficiency and outperform bulk ones when light trapping is implemented. The optimal device thickness in presen...

Light trapping in thin-film solar cells with randomly rough and hybrid textures

We study light-trapping in thin-film silicon solar cells with rough interfaces. We consider solar cells made of different materials (c-Si and μc-Si) to investigate the role of size and nature (direct/indirect) of the energy band gap in light trapping. By means of rigorous calculations we demonstrate that the Lambertian Limit of absorption can be obtained in a structure with an optimized rough interface. We gain insight into the light trapping mechanisms by analysing the optical properties of rough interfaces in terms of Angular Intensity Distribution (AID) and haze. Finally, we show the benefits of merging ordered and disordered photonic structures for light trapping by studying a hybrid interface, which is a combination of a rough interface and a diffraction grating. This approach gives a significant absorption enhancement for a roughness with a modest size of spatial features, assuring good electrical properties of the interface. All the structures presented in this work are compatible with present-day technologies, giving recent progress in fabrication of thin monocrystalline silicon films and nanoimprint lithography.

How to assess light trapping structures versus a Lambertian Scatterer for solar cells

We propose a new figure of merit to assess the performance of light trapping nanostructures for solar cells, which we call the light trapping efficiency (LTE). The LTE has a target value of unity to represent the performance of an ideal Lambertian scatterer, although this is not an absolute limit but rather a benchmark value. Since the LTE aims to assess the nanostructure itself, it is, in principle, independent of the material, fabrication method or technology used. We use the LTE to compare numerous proposals in the literature and to identify the most promising light trapping strategies. We find that different types of photonic structures allow approaching the Lambertian limit, which shows that the light trapping problem can be approached from multiple directions. The LTE of theoretical structures significantly exceeds that of experimental structures, which highlights the need for theoretical descriptions to be more comprehensive and to take all relevant electro-optic effects into account.

Dual gratings for enhanced light trapping in thin-film solar cells by a layer-transfer technique

Thin film solar cells benefit significantly from the enhanced light trapping offered by photonic nanostructures. The thin film is typically patterned on one side only due to technological constraints. The ability to independently pattern both sides of the thin film increases the degrees of freedom available to the designer, as different functions can be combined, such as the reduction of surface reflection and the excitation of quasiguided modes for enhanced light absorption. Here, we demonstrate a technique based on simple layer transfer that allows us to independently pattern both sides of the thin film leading to enhanced light trapping. We used a 400 nm thin film of amorphous hydrogenated silicon and two simple 2D gratings for this proof-of-principle demonstration. Since the technique imposes no restrictions on the design parameters, any type of structure can be made.

Cascade luminescent solar concentrators

We propose a luminescent solar concentrator (LSC) characterized by a strong enhancement of the concentration factor in which the area covered by photovoltaic cells is independent of the area over which sunlight is collected. We name this device cascade-LSC (c-LSC), as sunlight is both geometrically and spectrally concentrated by cascading absorption and emission into different LSCs. We demonstrate a prototype and measure the generated photocurrent. The results are in good agreement with those predicted by our numerical model based on Monte Carlo simulations.

Light trapping and electrical transport in thin-film solar cells with randomly rough textures

Using rigorous electro-optical calculations, we predict a significant efficiency enhancement in thin-film crystalline silicon (c-Si) solar cells with rough interfaces. We show that an optimized rough texture allows one to reach the Lambertian limit of absorption in a wide absorber thickness range from 1 to 100 μm. The improvement of efficiency due to the roughness is particularly substantial for thin cells, for which light trapping is crucial. We consider Auger, Shockley-Read-Hall (SRH), and surface recombination, quantifying the importance of specific loss mechanisms. When the cell performance is limited by intrinsic Auger recombination, the efficiency of 24.4% corresponding to the wafer-based PERL cell can be achieved even if the absorber thickness is reduced from 260 to 10 μm. For cells with material imperfections, defect-based SRH recombination contributes to the opposite trends of short-circuit current and open-circuit voltage as a function of the absorber thickness. By investigating a wide range of ...

The importance of light trapping in thin-film solar cells

For solar energy to provide a significant contribution to the energy needs of our society, the development of more efficient and less expensive photovoltaic cells is critical. Producing energy on a global scale—the so-called terawatt challenge—makes it essential to use a minimum amount of rare or costly photoactive elements. Photovoltaic cells based on thin semiconductor films are well placed to meet these criteria and have the potential to replace the dominant photovoltaic technology based on expensive bulk silicon wafers. To do this, their efficiency needs to be increased beyond current values, and the required thickness of the semiconductor film should be minimized as much as possible. However, ultra-thin semiconductor layers are less effective at absorbing sunlight, unless smart light-trapping techniques can be designed and implemented. Such techniques aim at increasing the optical path of light in the semiconductor, which enhances the absorption efficiency for the same material thickness. The ultimate limit to light trapping in thick semiconductor layers was determined by Yablonovitch and Cody1, 2 for the case of weak absorption, and was later generalized by Green3 to the case of arbitrary absorption. It is based on the concept of a Lambertian scatterer, i.e., a rough interface that randomizes the direction of propagation of incoming light when it enters the sample. This limit to light trapping is called the Lambertian limit. Such treatments assume a ray-optics approach and are rigorously valid only when the film thickness is much larger than the wavelength of visible light, which amounts to a few hundreds of nanometers. When the film thickness is less than this, a wave-optics approach becomes necessary and the light-trapping limit to absorption is not known. While it is generally agreed that wavelength-scale or nanophotonic structures should be employed, the optimal geometry for such structures is yet to be determined. A point of debate is whether ordered structures, Figure 1. Short-circuit current density (Jsc) under air mass 1.5 solar spectrum as a function of thickness for materials with (indirect bandgap) crystalline silicon (c-Si), and (direct bandgap) amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS). Solid lines refer to the single-pass case, while dashed lines indicate the Lambertian light-trapping limit.