Tunable antireflection from conformal Al-doped ZnO films on nanofaceted Si templates
© Basu et al.; licensee Springer. 2014
Received: 12 March 2014
Accepted: 12 April 2014
Published: 26 April 2014
Photon harvesting by reducing reflection loss is the basis of photovoltaic devices. Here, we show the efficacy of Al-doped ZnO (AZO) overlayer on ion beam-synthesized nanofaceted silicon for suppressing reflection loss. In particular, we demonstrate thickness-dependent tunable antireflection (AR) from conformally grown AZO layer, showing a systematic shift in the reflection minima from ultraviolet to visible to near-infrared ranges with increasing thickness. Tunable AR property is understood in light of depth-dependent refractive index of nanofaceted silicon and AZO overlayer. This improved AR property significantly increases the fill factor of such textured heterostructures, which reaches its maximum for 60-nm AZO compared to the ones based on planar silicon. This thickness matches with the one that shows the maximum reduction in surface reflectance.
81.07.-b; 42.79.Wc; 81.16.Rf; 81.15.Cd
Aluminum-doped ZnO, a transparent conducting oxide (TCO), is becoming increasingly popular as window layer and top electrode for next-generation highly efficient silicon-based heterojunction solar cells[1–4]. An essential criterion to enhance the efficiency of silicon-based solar cells is to reduce the front surface reflection. However, commercial silicon wafers show surface reflection of more than 30%. Such a high level of reflection can be minimized by growing a suitable antireflection (AR) coating, preferably in the form of a TCO. On the basis of thin film interference property, these dielectric coatings reduce the intensity of the reflected wave. However, this approach needs a large number of layers to achieve well-defined AR properties. In addition, coating materials with good AR properties and low absorption in the ultraviolet (UV) range are rare in the literature. An alternative to the lone usage of dielectric coating is therefore required which can overcome some of these difficulties.
An optimal antireflective surface should contain subwavelength features where the index matching at the substrate interface leads to improved AR performance. For instance, by using a surface texture on TCO (e.g., AZO) and/or Si substrate, one can govern the light propagation and in turn the AR property due to the formation of graded refractive index[8, 9]. In particular, for solar cell applications, a patterned AZO film on a flat silicon substrate shows a significant decrease in average reflectance up to 5%, whereas a thick AZO layer on silicon nanopillars is found to give an overall reflectance of approximately 10%. In the latter case, a higher photocurrent density was achieved (5.5 mA cm-2) as compared to AZO deposited on planar silicon (1.1 mA cm-2). It is, therefore, exigent to have more control on pattern formation and optimization of AZO thickness to achieve improved AR performance.
Majority of the patterning processes are based on conventional lithographic techniques. As a result, these are time-consuming and involve multiple processing steps. On the other hand, low-energy ion beam sputtering has shown its potential as a single-step and fast processing route to produce large-area (size tunable), self-organized nanoscale patterned surfaces compatible to the present semiconductor industry, and thus may be considered to be challenging to develop AR surfaces for photovoltaics.
In this letter, we show the efficacy of one-step ion beam-fabricated nanofaceted silicon templates for growth of conformal AZO overlayer and correlate its thickness-dependent (in the range of 30 to 90 nm) AR property. We show that growth of an optimum AZO overlayer thickness can help to achieve maximum reduction in surface reflectance. As a possible application of such heterostructures in photovoltaics, photoresponsivity of AZO deposited on pristine and faceted Si has also been investigated. The results show that by using nanofaceted silicon templates, it is possible to enhance the fill factor (FF) of the device by a factor of 2.5.
Results and discussion
It may be mentioned that effect of the experimental geometry was tested by subsequent measurement of the surface reflectance after giving a perpendicular rotation to the samples. However, no difference in the reflectance values (within the experimental error) was observed in both cases. To understand this behavior, we calculated the average aspect ratio of the faceted structures (i.e., height/lateral dimension) along x and y directions which turned out to be 0.25 and 0.24, respectively. It is well known that reflectance depends on the aspect ratio of the surface features. Thus, the observed absence of change in surface reflectance, due to different directions of incident light, can be attributed to the comparable aspect ratio of the faceted structures along x and y directions.
Different photovoltaic parameters obtained from various AZO overlayer thicknesses grown on silicon substrates
30-nm AZO on pristine Sia
1.24 × 10-3
30-nm AZO on nanofaceted Si
3.0 × 10-2
60-nm AZO on nanofaceted Si
5.35 × 10-2
75-nm AZO on nanofaceted Si
37.57 × 10-2
Compared to the inverted pyramid approach[23, 24], which yields reflectance values between 3% and 5% for an optimized AR coating thickness between 400 and 1,000 nm, our results show a better (by a factor of 10) performance with a smaller (30 to 75 nm) AZO film thickness. Among the available techniques reported in the literature, our novel approach of fabricating faceted nanostructures is simple and can be seamlessly integrated with the modern thin film solar cell technology for better photon harvesting with the help of proper understanding of AR property of AZO films. For a flat surface having an AR overlayer, using Fresnel's reflection formula, we measured the reflectance at different wavelengths. It is observed that with varying film thickness, the position of the reflection minima shifts, while a change in the refractive index modifies the amount of surface reflectance. Although similar trends are quite evident, the experimentally observed average surface reflectance turns out to be much lower over the spectral range under consideration.
In order to explain these results, let us first try to understand the role of the Si template which is practically an ensemble of ion beam-fabricated self-organized conical nanofacets at the top of the Si substrate. It is known that grating on any surface can be used to achieve arbitrary refractive index if the geometry of the grating structures can be tuned. For instance, if we consider a binary grating, its effective refractive index, neff, can be expressed as neff = (n1 - 1)DC + 1, where n1 is the refractive index of the grating and DC is the duty cycle and is defined as the ratio of the grating line width to the grating period. If the surrounding medium is taken as air and the grating is of the same material as the substrate, the optimized duty cycle (to meet the AR criterion) can be expressed as where n2 is the refractive index of the substrate. Such binary gratings are expected to exhibit the AR property over a very narrow spectral range. This range can be broadened by continuous tuning of the refractive index (neff) between the two surrounding media. This would essentially mean a continuous change in DC along the depth (from the apex towards the base of the facets) of the grating lines, which is possible to be achieved by having tapered/conical gratings. When the grating and the substrate materials are the same, the matching of refractive index at the substrate interfaces can exhibit highly improved AR property. This explains the enhanced AR performance observed here for the faceted Si surface formed on the Si substrate. Following the same argument, further improved AR performance is expected due to the conformal growth of an AZO overlayer on nanofaceted Si template. Indeed, the experimental findings confirm the same where increasing AZO thickness leads to a systematic red shift in the reflection minima. However, such small variations in the thickness may not be sufficient to cause any significant difference in depth-dependent change of the effective refractive index for the AZO-coated faceted Si template which corroborates well with the experimentally measured reflectance minima values.
In conclusion, we show that conformally grown AZO films on ion beam-fabricated self-organized nanofaceted Si templates can work in tandem to yield improved AR performance. It is observed that tunable AR property can be achieved by varying the thickness of AZO overlayer and there exists a critical thickness (60 nm in the present case) which exhibits the best AR performance over the given spectral range (300 to 800 nm). Reduction in surface reflectance for Si templates can be understood in light of gradient refractive index effect arising from a continuous change in the effective refractive index along the depth (from the apex towards the base of the facets) and refractive index matching at the substrate interface because of self-organized nanofaceted Si structures. Following the same argument, further enhancement in the AR performance is observed due to conformal growth of AZO overlayers on Si templates. This is accompanied by a thickness-dependent systematic red shift in the reflection minima. The fabricated AZO/Si heterostructures, both on planar and faceted silicon, show significant photoresponsivity where thickness-dependent fill factor increases by a factor up to 2.5 owing to improved light absorption in the latter case. This study indicates that conformally grown AZO overlayer on nanofaceted silicon may be a promising candidate as AR coatings by optimizing the process parameters.
The authors would like to thank D. P. Datta from Institute of Physics, Bhubaneswar for his help during preparation of the revised manuscript and Pravakar Mallick from National Institute of Science Education and Research for his help during the SEM measurements.
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