Aluminium-induced Texturing Research Articles

Studies on the efficacy of alkaline and acidic etching in aluminium induced texturing of glass for solar cell application✰

Enhancing photon absorption by light trapping is an important way to improve efficiency of photovoltaic devices. In this work, we discuss one such method called aluminium induced texturization (AIT) which can help textured glass act as a scattering element in solar cells and introduce a sustainable fabrication method by understanding the different parameters that can control the texturization. The optical properties of AIT glass were studied using UV–VIS-NIR spectrophotometer which shows the haze of these textured glass is directly related to the surface roughness which can be tuned by thickness of Aluminium deposited. We have done a comparative study of acidic and alkaline etching techniques optimized for lower thermal budget and lesser operational hazard by eliminating concentrated (49%) hydrofluoric acid in the process. Investigation of the surface topography using atomic force microscopy and scanning electron microscopy corroborates the change in haze that comes from the change in rms roughness due to differences in surface morphology from different etch conditions. The AIT glass we developed has a high transmittance (~93%), peak haze value between (70%-25%) and rms roughness between 40 and 190 nm which can be tuned by changing the aluminium thickness and etch conditions. A proof of concept of forming superpositioned texture in combination with transparent conducting oxides to use the AIT glass as an active component of the device has also been elucidated.

Glass surface etching with Aluminium-induced texture process for thin film solar cell applications

Various approaches have been explored to enhance the efficiency of thin film solar cells by adopting new materials, novel device structures and implementing various light-trapping techniques. Generally, the light trapping is achieved by texturization of thin film transparent conducting oxide (TCO) layer by chemical etching. In the present study, we have etched the surface of the glass substrate by Aluminium-Induced Texturization (AIT) process. A set of experiments is carried out with different thicknesses of the Aluminium and annealed for different durations. The effect of AIT on the glass surface roughness is studied using AFM analysis. The optical transmission and haze of the etched glass substrates are measured using UV–Vis–NIR spectrophotometer. Aluminium-doped Zinc oxide (AZO) layer is deposited on the etched glass and the effect of glass etching on the properties of AZO thin film in terms of surface roughness, and optical properties are studied.

Aluminium induced texturing of glass substrates with improved light management for thin film solar cells

Aluminium induced texturing (AIT) method has been used to texture glass substrates to enhance photon absorption in microcrystalline thin film Si solar cells. In this process, a thin Al film is deposited on a glass substrate and a non-uniform redox reaction between the glass and the Al film occurs when they are annealed at high temperature. After etching the reaction products, the resultant glass surface presents a uniform and rough morphology. In this work, three different textures (σrms~85, ~95, ~125nm) have been achieved by tuning the dc sputtering power and over them and over smooth glass, pin microcrystalline silicon solar cells have been fabricated. The cells deposited over the textured substrates showed an efficiency improvement in comparison to the cells deposited over the smooth glass. The best result was given for the glass texture σrms~125nm that led to an average efficiency 2.1% higher than that given by the cell deposited on smooth glass.

Optimization of surface morphology and scattering properties of TCO/AIT textured glass front electrode for thin film solar cells

Aluminium induced texture (AIT) method has been used for obtaining highly textured glass substrate suitable for silicon based thin film solar cell technology. Wet etch step parameters of AIT process have been varied and effect of different etchants and different etching times on morphological and optical properties has been analyzed. The resulting morphology features (shape, size distribution, inclination angle) have been optimized in order to obtain the best scattering properties. ZnO:Ga (GZO) films have been deposited by sputtering technique on AIT-processed glass. Two different ZnO surface morphologies have been obtained, strongly depending on the underlying glass substrate morphology induced by different etching times. Very rough and porous texture (σrms∼150nm) was obtained on glass etched 2min showing cauliflower-like structure, whereas a softer texture (σrms∼78nm) was obtained on glass etched 7min giving wider and smoother U-shaped craters. The effect of different glass textures on optical confinement has been tested in amorphous silicon based p-i-n devices. Devices fabricated on GZO/high textured glass showed a quantum efficiency enhancement due to both an effective light trapping phenomenon and an effective anti-reflective optical behaviour. Short etching time produce smaller cavities (<1μm) with deep U-shape characterized by high roughness, high inclination angle and low autocorrelation length. This surface morphology promoted a large light scattering phenomenon, as evidenced by haze value and by angular resolved scattering (ARS) behaviour, into a large range of diffraction angles, giving high probability of effective light trapping inside a PV device.

Light Scattering and Current Enhancement for Microcrystalline Silicon Thin-Film Solar Cells on Aluminium-Induced Texture Glass Superstrates with Double Texture

Microcrystalline silicon (μc-Si:H) thin-film solar cells are processed on glass superstrates having both micro- and nanoscale surface textures. The microscale texture is realised at the glass surface, using the aluminium-induced texturing (AIT) method, which is an industrially feasible process enabling a wide range of surface feature sizes (i.e., 700 nm–3 μm) of the textured glass. The nanoscale texture is made by conventional acid etching of the sputter-deposited transparent conductive oxide (TCO). The influence of the resulting “double texture” on the optical scattering is investigated by means of atomic force microscopy (AFM) (studying the surface topology), haze measurements (studying scattering into air), and short-circuit current enhancement measurements (studying scattering into silicon). A predicted enhanced optical scattering efficiency is experimentally proven by a short-circuit current enhancementΔIscof up to 1.6 mA/cm2(7.7% relative increase) compared to solar cells fabricated on a standard superstrate, that is, planar glass covered with nanotextured TCO. Enhancing the autocorrelation length (or feature size) of the AIT superstrates might have the large potential to improve theμc-Si:H thin-film solar cell efficiency, by reducing the shunting probability of the device while maintaining a high optical scattering performance.

Growth and Properties of ZnO:Al on Textured Glass for Thin Film Solar Cells

Aluminium induced texturing (AIT) method has been used to texture glass substrates in order to enhance the photon absorption in thin film solar cells. The resultant glass roughness has been analyzed by varying the AIT process parameters and it has been found that the deposition method of Al is a decisive factor in tuning the texture. Two types of textures, a soft (texture E) and a rough texture (texture S), were achieved from the thermally evaporated and sputtered Al layers through AIT process. Aluminium-doped zinc oxide (AZO) layers of different thickness were deposited over both textures and over smooth glass. Haze values above 30% were obtained for texture S + AZO and above 10% for texture E + AZO. The resultant morphologies were free from sharp edges or deep valleys and the transparency and the resistivity values were also good enough to be used as front contact for thin film solar cells. In order to demonstrate the light absorption enhancement in a solar cell device, 200 nm of a-Si:H followed by 300 nm of Ag were grown over the textured and smooth substrates with AZO, and an optical absorption enhancement of 35% for texture E and 53% for texture S was obtained in comparison to the smooth substrate.

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.

Investigation of the Optical Absorption of a-Si:H Solar Cells on Micro- and Nano-Textured Surfaces

Light trapping is essential for thin-film silicon solar cells due to their thin absorber layers. Textured surfaces are widely used to scatter light, thereby increasing the pathlength of light in these solar cells. In a-Si:H solar cells different interfaces can be textured, e.g. the glass/TCO (transparent conductive oxide) interface, the TCO/a-Si:H interface, or the rear TCO/Al (rear reflector) interface. Nano-textured TCOs are widely used for scattering light at the TCO/a-Si:H interface. Additionally, it was shown that micro-texturing of the glass/TCO interface is effective for light scattering, especially in the long-wavelength range. Aluminium induced texturing (AIT) is a method to texture glass surfaces. Previous studies have shown that AIT glass increases the quantum efficiency of micromorph thin-film solar cells in the long-wavelength range. In this contribution, the Advanced Semiconductor Analysis (ASA) software is used to investigate the effect of textured interfaces on the absorption enhancement of single- and double-junction a- Si:H thin-film solar cells. Surface morphologies of AIT glass (micro-textured) and a commercially available TCO- coated glass (whereby the TCO is nano-textured) are examined. The results suggest that nano-texturing increases the parasitic absorption in the TCO and the p-doped silicon layer, thus decreasing the blue response of the solar cell. However, absorption in the intrinsic layer is significantly enhanced in the long-wavelength region. Particularly in the double-junction a-Si:H solar cells on AIT micro-textured superstrates, the current is enhanced in both the top and the bottom cell.

Analysis of Optical and Morphological Properties of Aluminium Induced Texture Glass Superstrates

Texturing the glass surface is a promising method for improving the light trapping properties of superstrate thin-film silicon solar cells, as it enables thinner absorber layers and, possibly, higher cell efficiencies. In this paper we present the optical and morphological properties of borosilicate glass superstrates textured with the aluminium induced texture (AIT) method. High haze values are achieved without any reduction in the total optical transmission of the glass sheets after the AIT process. Scanning electron microscope and atomic force microscope (AFM) measurements reveal a laterally uniform surface morphology of the AIT texture. We demonstrate that the surface roughness and thus the transmission haze can be controlled by adjusting the AIT process parameters. From the AFM images, we extract histograms of the local height and angle distributions of the texture. Samples with a wide angle distribution are shown to produce the highest optical haze. The results of this analysis provide a better understanding of the correlation between the AIT process parameters and the resulting surface morphology. This analysis is further extended to an amorphous silicon pin solar cell deposited onto the textured glass substrate.

Analysis of Optical and Morphological Properties of Aluminium Induced Texture Glass Superstrates

Analysis of Optical and Morphological Properties of Aluminium Induced Texture Glass Superstrates

Crystalline silicon growth in the aluminium-induced glass texturing process

Aluminium-induced texturing (AIT) is a method to texture glass surfaces by utilising the reaction between aluminium (Al) and glass at high temperature (above 500°C) and a subsequent wet-chemical treatment that removes the reaction products. In this work, we studied the solid state reaction between a sputtered Al layer and a borosilicate glass sheet during AIT annealing. Raman spectroscopy showed that crystalline silicon (c-Si) is formed during the AIT process. An optical microscope was used to visualise the evolution of the c-Si growth. Plan-view scanning electron microscopy (SEM) investigations performed on samples after completed AIT reaction showed that separate c-Si clusters formed at the glass surface. Atomic force microscopy revealed that the c-Si clusters grew upwards and were on top of the glass surface. Cross-sectional SEM examination showed that the c-Si layer is not uniform and that crater-shaped nodules are embedded into the glass. The widths and depths of the nodules are in the micrometre range. Energy-dispersive X-ray spectroscopy showed that the nodules consist mainly of aluminium oxide (Al2O3). X-ray diffraction analysis showed that the c-Si grains are preferentially (111) oriented. The activation energy of the reaction between Al and borosilicate glass is 3.0±0.2eV based on in-situ XRD analysis of the c-Si growth. Finally, a phenomenological model of the AIT process is proposed and we suggest that the topology of the glass texture strongly depends on the size, depth and lateral separation of the Al2O3 nodules embedded in the glass.