Nasim Sahraei

Analytical solution for haze values of aluminium-induced texture (AIT) glass superstrates for a-Si:H solar cells

Light scattering at randomly textured interfaces is essential to improve the absorption of thin-film silicon solar cells. Aluminium-induced texture (AIT) glass provides suitable scattering for amorphous silicon (a-Si:H) solar cells. The scattering properties of textured surfaces are usually characterised by two properties: the angularly resolved intensity distribution and the haze. However, we find that the commonly used haze equations cannot accurately describe the experimentally observed spectral dependence of the haze of AIT glass. This is particularly the case for surface morphologies with a large rms roughness and small lateral feature sizes. In this paper we present an improved method for haze calculation, based on the power spectral density (PSD) function of the randomly textured surface. To better reproduce the measured haze characteristics, we suggest two improvements: i) inclusion of the average lateral feature size of the textured surface into the haze calculation, and ii) considering the opening angle of the haze measurement. We show that with these two improvements an accurate prediction of the haze of AIT glass is possible. Furthermore, we use the new equation to define optimum morphology parameters for AIT glass to be used for a-Si:H solar cell applications. The autocorrelation length is identified as the critical parameter. For the investigated a-Si:H solar cells, the optimum autocorrelation length is shown to be 320 nm.

Optical Absorption Enhancement in Amorphous Silicon Films and Solar Cell Precursors Using the Aluminum-Induced Glass Texturing Method

One of the key issues of thin-film silicon solar cells is their limited optical absorptance due to the thin absorber layer and the low absorption coefficient for near-infrared wavelengths. Texturing of one or more interfaces in the layered structure of these cells is an important technique to scatter light and enhance the optical pathlength. This in turn enhances the optical absorption of the solar radiation in the absorber layer and improves the solar cell efficiency. In this paper we investigate the effects of textured glass superstrate surfaces on the optical absorptance of intrinsic a-Si:H films and a-Si:H p-i-n thin-film solar cell precursors deposited onto them. The silicon-facing surface of the glass sheets was textured with the aluminium-induced glass texturing method (AIT method). Absorption in both intrinsic silicon films and solar cell precursor structures is found to increase strongly due to the textured glass superstrate. The increased absorption due to the AIT glass opens up the possibility to reduce the absorber layer thickness of a-Si:H solar cells.

Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector.

Optimum feature size of randomly textured glass substrates for maximum scattering inside thin-film silicon solar cells

Optimization of light scattering by designing proper randomly textured surfaces is one of the important issues when designing thin-film silicon solar cell structures. The wavelength region that needs to be scattered depends on the absorber material and the thickness of the solar cell. The optimum morphology of the textured substrate can be defined regarding the wavelength range intended for scattering. Good scattering is experimentally achieved by optimizing the fabrication process of the randomly textured substrate. However, optimum morphological parameters have not been analytically formulated. In this work we develop the morphological criteria for optimum light scattering in a-Si:H solar cells using Aluminum Induced Texture (AIT) glass superstrates. Transmission haze is widely used as an evaluating factor for scattering properties. Haze can be easily measured for the substrate/air interface. However, the relevant scattering properties are those in the absorber material. These properties cannot be measured directly, but can be predicted by an appropriate model. The simple model for haze calculation based on scalar scattering theory cannot correctly estimate the haze value because it only considers the root mean square (RMS) roughness of the textured surface, which does not contain information about lateral feature size. In addition, the opening angel of the haze measurement is not considered in the equation. In this work, we demonstrate that the power spectral density (PSD) function of the randomly textured surface can provide the missing information in the haze equation. A general formulation for calculating the lateral feature size based on the PSD function is presented. We use this calculated haze value based on PSD to find the optimum lateral feature size for scattering a specific wavelength into the desired material. The optimum lateral feature size for scattering 620-nm light, which is weakly absorbed in a-Si:H, is shown to be 100 nm.

Thin-film a-Si:H solar cells processed on aluminum-induced texture (AIT) glass superstrates: prediction of light absorption enhancement

Light scattering superstrates are important for thin-film a-Si:H solar cells. In this work, aluminum-induced texture (AIT) glass, covered with nonetched Al-doped ZnO (AZO), is investigated as an alternative to the commonly used planar glass with texture-etched AZO superstrate. Four different AIT glasses with different surface roughnesses and different lateral feature sizes are investigated for their effects on light trapping in a-Si:H solar cells. For comparison, two reference superstrates are investigated as well: planar glass covered with nonetched AZO and planar glass covered with texture-etched AZO. Single-junction a-Si:H solar cells are deposited onto each superstrate, and the scattering properties (haze and angular resolved scattering) as well as the solar cell characteristics (current-voltage and external quantum efficiency) are measured and compared. The results indicate that AIT glass superstrates with nonetched AZO provide similar, or even superior, light trapping than the standard reference superstrate, which is demonstrated by a higher short-circuit current Jsc and a higher external quantum efficiency. Using the trapped light fraction δ, a quantity based on the integrated light scattering at the AZO/a-Si:H interface, we show that Jsc linearly increases with δ in the scattering regime of the samples, regardless of the type of superstrate used.

Optical Modeling of Alkaline Saw-Damage-Etched Rear Surfaces of Monocrystalline Silicon Solar Cells

In this study, we propose a geometric optical model to represent alkaline saw-damage-etched (SDE) surfaces of monocrystalline silicon wafers. An experimental study is carried out to characterize the optical properties of alkaline SDE surfaces on monocrystalline silicon wafers. Based on the surface characteristics measured by goniometry and height profiling, a geometric optical model is developed to describe the SDE surface with two parameters: characteristic angle and planar fraction. Using the path-tracing method, spectral reflectance simulations are carried out for four different types of samples. With the measured characteristic angle of 22° and planar fraction of 0.25 or 0.36, we find that this representation of SDE surface can predict the reflection and transmission with a root-mean-square error (RMSE) of the equivalent current density from 0.19 to 0.57 mA/cm2. The developed model is also applied to the optical loss analysis of aluminum local back surface field (Al-LBSF) solar cells with an SDE rear surface. We find that SDE rear surfaces provide better light trapping than planar surfaces. As a consequence, Al-LBSF solar cells with pyramids on the front and an SDE rear are predicted to produce 0.6 mA/cm 2 more photocurrent than similar cells with a planar rear surface.

On the use of a charged tunnel layer as a hole collector to improve the efficiency of amorphous silicon thin-film solar cells

A new concept, using a negatively charged tunnel layer as a hole collector, is proposed and theoretically investigated for application in amorphous silicon thin-film solar cells. The concept features a glass/transparent conductive oxide/ultra-thin negatively charged tunnel layer/intrinsic a-Si:H/n-doped a-Si:H/metal structure. The key feature of this so called t+-i-n structure is the introduction of a negatively charged tunnel layer (attracting holes from the intrinsic absorber layer), which substitutes the highly recombination active p-doped a-Si:H layer in a conventional p-i-n configuration. Atomic layer deposited aluminum oxide (ALD AlOx) is suggested as a potential candidate for such a tunnel layer. Using typical ALD AlOx parameters, a 27% relative efficiency increase (i.e., from 9.7% to 12.3%) is predicted theoretically for a single-junction a-Si:H solar cell on a textured superstrate. This prediction is based on parameters that reproduce the experimentally obtained external quantum efficiency and cu...