Angelo Bozzola

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.

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...

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.

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 ...

Silicon solar cells reaching the efficiency limits: from simple to complex modelling

Numerical modelling is pivotal in the development of high efficiency solar cells. In this contribution we present different approaches to model the solar cell performance: the diode equation, a generalization of the well-known Hovel model, and a complete device modelling. In all three approaches we implement a Lambertian light trapping, which is often considered as a benchmark for the optical design of solar cells. We quantify the range of parameters for which all three approaches give the same results, and highlight the advantages and limitations of different models. Using these methods we calculate the efficiency limits of single-junction crystalline silicon solar cells in a wide range of cell thickness. We find that silicon solar cells close to the efficiency limits operate in the high-injection (rather than in the low-injection) regime. In such a regime, surface recombination can have an unexpectedly large effect on cells with the absorber thickness lower than a few tens of microns. Finally, we calculate the limiting efficiency of tandem silicon–perovskite solar cells, and we determine the optimal thickness of the bottom silicon cell for different band gaps of the perovskite material.

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.

Light trapping in thin film solar cells with sub-wavelength photonic crystal patterns

In this paper we investigate light-trapping in thin film crystalline silicon (c-Si) solar cells with shallow photonic crystal (PhC) front patterns. Coupling of light to the quasi-guided optical modes of the structures becomes a dominant feature when the thickness approaches the wavelength scale. Simple 1D and 2D patterns are investigated using a scattering matrix approach, and best results are compared with the Lambertian limit to light trapping. We show that light trapping can be enhanced by adding a controlled amount of disorder inside the PhC unit cell. We present some preliminary results about a Gaussian size-disorder in 1D structures, and we conclude that an advanced light-trapping design should rely on both order and disorder to maximize the absorption of sunlight.

Light trapping in thin film solar cells: towards the Lambertian limit

In this work we theoretically investigate the light trapping properties of one- and two-dimensional periodic patterns etched in crystalline silicon solar cells with anti-reflection coating and back-reflector, in a wide range of active material thicknesses. The resulting short-circuit current (taken as the figure of merit for efficiency) and the optical spectra are compared with those of an unpatterned cell, and with the ultimate limits to light trapping in the case of a Lambertian (isotropic) scatterer. Photonic patterns are found to give a substantial absorption enhancement, especially for twodimensional patterns and for thinner cells, thanks to physical mechanisms like reduction of reflection losses, diffraction of light into the cell, and coupling into the resonant optical modes of the structure.

Light absorption and carrier collection in thin-film crystalline silicon solar cells with light trapping

In this paper, we present a theoretical study of the effects of light trapping and carrier recombination in thin-film crystalline silicon (c-Si) solar cells. We develop a new electro-optical model which is based on the analytic solution of drift-diffusion equations. We explore the effects of different thickness and material qualities on the energy conversion efficiency of the device. The results clearly point out that c-Si absorbers with a thickness between 10 and 80 microns are very attractive for future high efficiency applications. We find that in this range of thickness, thin-film devices can be more efficient than those based on bulk wafers. The requirements in terms of bulk and surface quality that ensure this result are quantified by our model. This analytic framework can be applied as a valid tool in understanding experimental and numerical results for c-Si solar cells with rough interfaces or other isotropic optical structures for light trapping.

Tailoring randomly rough textures for light trapping in thin-film solar cells

In this contribution, we use a rigorous electro-optical model to study randomly rough crystalline silicon solar cells with the absorber thickness ranging from 1 to 100 μm. We demonstrate a significant efficiency enhancement, particularly strong for thin cells. We estimate the “region of interest” for thin-film photovoltaics, namely the thickness range for which the energy conversion efficiency reaches maximum. This optimal thickness results from the opposite trends of current and voltage as a function of the absorber thickness. Finally, we focus on surface recombination. In our design, the cell efficiency is limited by recombination at the rear (silicon absorber/back reflector) interface, and therefore engineering the front surface to a large extent does not reduce the efficiency. The presented model of roughness adds a significant functionality to previous approaches, for it allows performing rigorous calculations at a much reduced computational cost.