Nanoimprinted Tio2 solgel passivating diffraction gratings for solar cell ...

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1131

RESEARCH ARTICLE

Nanoimprinted Tio2 sol–gel passivating diffraction gratings for solar cell applications Jérémy Barbé*, Andrew Francis Thomson, Er‐Chien Wang, Keith McIntosh and Kylie Catchpole The Australian National University, Center for Sustainable Energy Systems, Canberra, Australian Capital Territory, Australia

ABSTRACT We report the fabrication and characterization of TiO2 sol–gel diffraction gratings on silicon substrates by using nanoimprint lithography. The gratings are homogeneous and free of defects and cover an area of 25 cm2. Minority carrier lifetimes of up to 900 µs for imprinted samples under illumination are reported, which Kelvin probe measurements indicate is due to light‐generated negative charge in the films. The structures reported here are very promising as light‐trapping, passivating coatings for solar cells. Copyright © 2011 John Wiley & Sons, Ltd. KEYWORDS TiO2 sol–gel; diffraction grating; solar cells; nanoimprint; passivation; transient photoconductance decay *Correspondence Jérémy Barbé, The Australian National University, Center for Sustainable Energy Systems Canberra, Australian Capital Territory, Australia. E‐mail: [email protected] Received 5 January 2011; Revised 4 March 2011

1. INTRODUCTION Reduction of surface losses is a major issue for the development of cheaper and more efficient solar cells. In particular, the reduction of optical and recombination losses at the surface plays an increasingly important role in the development of thinner and cheaper solar cells. Conventional silicon texturing, that is, the use of pyramidal structures of crystal planes at the surface of solar cells, is less effective in providing anti‐reflection and light trapping for multicrystalline silicon than for single crystal silicon due to the range of crystal orientations. In addition, it is not applicable to thin‐film cells because of the reduced thickness of material. In response to this problem, it has been shown that diffraction gratings can form a very effective structure as an alternative technology for light trapping [1–5]. Nanoimprint lithography is a suitable high resolution, large area and cheap technique to realise these patterned structures in the nanometer range [6–8]. Whitesides and co‐workers have achieved structures with feature sizes ranging down to 30 nm [9]. Hampton et al. have shown excellent patterning of sub‐500 nm inorganic oxide structures and, in particular, TiO2 sol–gel on glass or a flash layer [10]. Verschuuren and van Sprang have

Copyright © 2011 John Wiley & Sons, Ltd.

realised high‐quality three‐dimensional sub‐micron structures over square centimetres by using sol–gel imprint lithography [11]. Moreover, since the early 1980s, TiO2 has shown to be an efficient anti‐reflection coating for silicon solar cells [12–14]. Its high refractive index and low absorbance at wavelengths relevant to solar cells make it a well‐suited material for light trapping as well as for anti‐reflection. The other major surfConventional silicon texturing, that is, the use of pyraace loss mechanism is the recombination of photogenerated charge carriers. This is particularly important at the front surface where the majority of incident light is absorbed. Passivation is currently achieved in most industrial solar cells by using SiNx thin films, which also act as a single‐layer anti‐reflection coating [15]. SiNx is deposited by relatively expensive plasma‐enhanced chemical vapour deposition, and the use of TiO2 deposited by cheaper techniques could reduce costs drastically. TiO2 can be deposited by a number of techniques including spin‐coating of sol–gel films and atmospheric pressure chemical vapour deposition (APCVD). Thompson et al. have recently shown that APCVD TiO2 exhibits a photo‐induced effect, which creates negative charges in the TiO2 thin film when illuminated and strongly enhances the passivation of crystalline silicon [16–18].

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Nanoimprinted Tio2 sol–gel passivating diffraction gratings

In this paper, we report the fabrication of TiO2 diffraction gratings on silicon substrates by using sol–gel imprint lithography and characterise their surface morphology and electrical properties. We show that defect‐free TiO2 sol–gel diffractions gratings can be achieved over areas of 25 cm2, which is the size of the nanoimprinting stamp used. These layers can provide excellent surface passivation, as indicated by minority carrier lifetime measurements of up to 900 µs under illumination. The surface passivation is due to the presence of light‐generated negative charge in the layers.

2. EXPERIMENTAL: TITANIA SOL–GEL PREPARATION Titania (TiO2) sol–gel was prepared following traditional sol–gel chemistry techniques [10]. The precursor titanium (IV) butoxide, Ti(0CH2CH2CH2CH3)4, was first mixed with acetylacetone in order to moderate its hydrolysis rate by chelation [19]. After 15 min of stirring (120 rpm), the solution was diluted with the solvent 2‐propanol, and then glacial acetic acid was added dropwise to the stirring solution as a catalyst. The solution was stirred for 1 h and filtered before use. The molar ratio of chemicals was optimised in order to obtain the desired viscosity and thickness. In particular, we noticed that a high amount of solvent was more appropriate for nanoimprints. The optimised mass ratio of titanium IV butoxide, Hacac, 2‐propanol and glacial acetic acid that we used was 10:6.8:30:1. The filtered solution was then poured on a silicon substrate and spin‐coated following the recipe: resist dosing for 10 s at 100 rpm, resist flying for 10 s at 1500 rpm then 2 s at 1000 rpm and pre‐drying for 10 s at 300 rpm. To avoid a fast drying of the sol–gel while spinning because of the high amount of isopropanol, we used a cover adapted to the spin‐coater to prevent solvent removal before printing. For the nanoimprinting, we used the soft stamp ‘substrate conformal imprint lithography’ method of Verschurren and van Sprang [11]. The polydimethylsiloxan stamp was patterned with a square array of circular pillars with a diameter of 343 nm, a height of 200 nm and a pitch of 513 nm. The master stamp was provided by Philips and made by e‐beam lithography. After the spin‐coating, the cover was removed and the polydimethylsiloxan stamp quickly placed face‐down on the coated wafer. No high pressure was applied; stamp cavities were only filled by capillarity‐ fill processes, but it may be necessary to help this process by gently pressing with fingers. The TiO2 sol–gel was printed and dried for 2 h by solvent removal through the stamp. The stamp can then be peeled off without damaging the nanoimprinted surface. In this work, transient photoconductive decay was used to measure the effective minority lifetime at the surface of the semiconductor material. This technique described by Kane and Swanson [20] in 1985 is a contactless method, which involves the measurement of sheet photoconductivity over time [21,22]. Kelvin Probe measurements were conducted

to characterise both the magnitude and the polarity of the photoinduced charges at the surface of TiO2 sol–gel on silicon. This technique used a vibrating tip to measure the work function difference with the conducting sample [23].

3. RESULTS AND DISCUSSION 3.1. TiO2 as a diffraction grating Figure 1 shows four scanning electron microscope images of sol–gel TiO2 features on silicon. The features are arrays of holes that are approximately 50 nm in depth and 460 nm in width. The thickness of the TiO2 thin‐film is about 100 nm. Samples are free of defects, the thickness is homogeneous, and no cracks can be observed. The imprinted area is bigger than 5 × 5 cm square and is only limited by the size of the stamp that we used. The holes in titania are 9.5% wider than the original stamp grating, which is due to shrinkage of the titania during the evaporation of the solvent. To study the light‐trapping efficiency of such structures, we used the software GSOLVER 4.20c (Grating Solver Development Co., Allen, TX, USA) to simulate the above grating. In parallel, the reflectance of this sample was measured with a Cary 5000 (Varian Inc., Palo Alto, California) spectrophotometer in the range 400–1100 nm. The simulation and measurement data are shown in Figure 2. Between 500 and 1000 nm, simulation and experimental results fit well. The shift in the infrared is due to the fact that we simulate an infinite structure so that reflectance from the back surface is not taken into account. The weighted average reflectance calculated from the AM1.5G solar spectrum and spectrophotometric measurements is 14.2% for the imprinted sample against 36% for bare silicon. The grating is not optimised for anti‐reflection and light trapping, but is clearly promising in this regard. Future works will focus on the geometry of such structures to enhance light trapping. 3.2. TiO2 as a passivation layer In the second part of this work, we have studied the effectiveness of TiO2 sol–gel as a passivation layer for crystalline silicon. In the following experiments, we used 5 Ω cm, 700‐µm‐thick, , one‐side‐polished, n‐type, flat zone silicon wafers. These wafers were first etched and RCA cleaned. One side was oxidised at 1050 °C for 30 min, whereas the other side was coated with non‐imprinted TiO2 sol–gel as described earlier. It is necessary to passivate one side with SiO2 in order to reduce the effect of the surface on the minority carrier lifetime. The high quality of the wafers means that bulk recombination will have a negligible effect. At the end of the process, samples were baked for 30 min at 275 °C. The effective minority lifetime of the wafers’ surface was measured using transient photoconductive decay. Samples were measured both with and without a light bias with an intensity of 0.5 suns to observe the effect of Prog. Photovolt: Res. Appl. (2011) © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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Figure 1. Scanning electron microscope image of TiO2 sol–gel on silicon patterned with a hole grating.

Figure 2. Simulation and reflectance measurements for TiO2 sol–gel on silicon patterned with a hole grating.

illumination on the minority carrier lifetime. It was found that TiO2 layer passivates silicon much better under illumination than in the dark. In Figure 3, the effective carrier lifetime at the surface coated with non‐imprinted TiO2 sol–gel is plotted over time as the bias light is switched on and off. The lifetime is plotted for an excess carrier density of 1015 cm−3. From this graph, we see that relatively high lifetimes can be reached in crystalline silicon coated with TiO2 sol–gel. Moreover, we observe a reversible light‐induced effect which significantly increases the carrier lifetime: Prog. Photovolt: Res. Appl. (2011) © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

The sample has an initial lifetime of 140 µs in the dark. After the light bias is switched on, the lifetime increases rapidly in the first few seconds and then increases exponentially with a time constant of 70 s, saturating at 700 µs after 5 min. Then, after the bias light is switched off, the lifetime decays to its initial value, also with a time constant of 70 s. These results for sol–gel TiO2 are consistent with those for APCVD TiO2 [16–18]. Annealing is necessary to enhance the TiO2 passivation effect. Figure 4 shows the effect of different annealing temperatures between 250 °C and 400 °C on the carrier lifetime for an excess carrier density of 1015 cm−3. In all cases, the samples were baked for 30 min under nitrogen flow and the lifetime was measured soon after. We observe that the highest lifetime up to 900 µs under illumination, is reached for a 250 °C annealing. We also notice that the light‐induced effect is reduced with increasing temperatures and almost disappears after 300 °C. If we measure the sample after 3 days and more, the lifetime is somewhat deteriorated. However, after 6 weeks, the lifetime under illumination is still high, with a decrease from 900 to 400 µs for imprinted and annealed samples. As a first step towards our goal of producing light‐ trapping, passivating coatings, we have studied the passivation efficiency of imprinted samples. The same grating as shown in Figure 1 was used. Table I shows lifetime measurements for imprinted TiO2 before and after annealing. First, we observe that when imprinted samples are not annealed, light no longer induces any effect. However, after baking at 200 °C for 30 min, light‐induced surface passivation returns. These observations are not true for

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Table I. Effect of annealing (200 °C for 30 min) on the effective minority carrier lifetime for nanoimprinted samples.

τeff before annealing (µs) τeff after annealing (µs)

Dark

Light

122 276

129 902

Figure 3. Effect of bias illumination on the effective minority carrier lifetime for 5 Ω cm, n‐type crystalline silicon wafers coated with non‐imprinted TiO2 sol–gel. The effective lifetime is determined for an excess carrier density of 1015 cm−3.

Figure 5. Kelvin probe voltage versus time measured with a Kelvin probe for patterned TiO2 on silicon after an annealing at 200 °C.

Figure 4. Effective minority carrier lifetime of TiO2‐passivated silicon wafers as a function of annealing temperature with and without light bias for an excess carrier density of 1015 cm−3. The samples were measured soon after the annealing process.

non‐imprinted samples for which light induces passivation both before and after annealing. Kelvin probe measurements were then performed to assess the effect of illumination on the charge associated with the TiO2. Figure 5 plots the measured voltage VKP as a function of time when the illumination is switched on and off for a sample annealed at 200 °C. Because of difficulties in the calibration, the absolute VKP is unknown, and we have offset VKP such that VKP ~ 0 mV when the sample is in steady state under illumination. Although this offset does not affect the change in VKP due to the changing light conditions, the relationship between VKP and illumination is not trivial because it depends on the location of the charge in the TiO2, the interface‐trapped charge at the TiO2–Si interface, the degree of band‐bending in the silicon and the location of the electron and hole Fermi levels

in the Si; furthermore, all of these parameters interrelate and have a complicated dependence on illumination [24]. Nevertheless, it is possible to construe some information about charge in the TiO2 from Figure 5. Firstly, after the light is switched off in region 1 of the graph, there is an immediate decrease in VKP of about 400 mV. It arises because of (i) the change in the quasi‐ Fermi levels that occurs under illumination and, possibly, (ii) an increase in the band‐bending caused by a negative charge in the film (as band‐bending is significantly greater at equilibrium than under illumination). Thus, from this effect alone, one cannot determine whether there is a charge in the TiO2 (or at the TiO2–Si interface), but one can conclude that if there is a charge, it is not large and positive, which would have caused an increase in VKP when the light is switched off. A second salient feature of Figure 5 is the slow increase in VKP of ~300 mV after the light is switched off in region 2. The increase in VKP is approximately exponential with a time constant of a similar magnitude (30 s) to that of the decay in surface recombination (70 s). Because the light remains off, there can be no change in the Fermi level; instead, the increase in VKP must come from a ‘downward’ movement of the energy bands relative to the Fermi level, as occurs if the charge in the TiO2 becomes less negative (more positive). We can conclude, therefore, that the TiO2 charge must be negative and decreasing with time, or else there would not be a corresponding reduction in lifetime (increase in surface recombination) in the Prog. Photovolt: Res. Appl. (2011) © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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non‐diffused samples of Section 3.2 and, more particularly, in the boron‐diffused samples of [16–18]. Finally, it is evident in Figure 5 that when the light is switched back on, VKP increases by just 150 mV in region 3 rather than the decrease of 400 mV in region 1; this is consistent with the increase in VKP due to the change in Fermi levels being countered by an immediate decrease in VKP due to the introduction of negative charge. An immediate increase in charge passivation is apparent from the photoconductance data. Unlike the photoconductance signal, however, only a barely discernible reduction in VKP occurs over time when illuminated. In summary, it can be concluded from the Kelvin probe measurements that the illumination introduces a negative charge into the TiO2 (or at the TiO2 interface) that dissipates over minutes when the illumination is extinguished.

4. CONCLUSION In conclusion, we have produced titania sol–gel diffraction gratings on silicon substrates by using nanoimprint lithography. The patterned layers show promise as an anti‐reflection and light trapping structure for solar cells and can provide very good passivation of crystalline silicon. A light‐induced effect creates negative charges in the TiO2 layer, which strongly enhances the carrier lifetime at the surface of silicon, with the sample lifetime being high as 900 µs for both non‐imprinted and imprinted samples.

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Prog. Photovolt: Res. Appl. (2011) © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip