Powłoki antyrefleksyjne otrzymywane metodami zol-żel oraz ALD do zastosowań w krzemowych ogniwach słonecznych

The application of one or more antireflection coatings on the front surface of the solar cells reduces the reflection of the incident light, which increases the device efficiency. For solar cells it is important to have a minimum reflection over all the visible spectrum range [1]. The studies of solar cells can be divided into two approaches. One is to reduce production costs, the second is to increasing efficiency of solar cells.

The design of low-cost antireflection coatings is possible by using the sol gel method. It is a process for preparing materials at low temperatures from solutions. We can produce different forms of materials include gels, gel-derived glasses, ceramics in form of nano- and micro-powders, bulk, fibres, thin films and coatings as well as more recent materials such as hybrid organicinorganic materials and composites. Controlling of the physical and chemical parameters of production process gives materials with precisely adjusted parameters such as mechanical strength, transparency, size and distribution of the pores. Generally the sol gel method consists in a few steps as shown in Fig. 1 [2–4].

There are many kinds methods of deposition antireflection coatings from solutions. For example, by spinning, dipping or spraying. Spin coating is a fast and easy method to prepare thin and homogeneous organic and inorganic films. Spin coating is a procedure used to deposit uniform thin films onto flat substrates. An excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. A typical spin process consists of a dispense step in which the fluid is deposited onto the substrate surface, a high speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film (Fig. 2) [6–9].

Fig. 1. Schematic representation of the sol-gel method [5]

Fig. 2. Schematic representation of the spin coating method

Fig. 3. Schematic representation of one cycle of the atomic layer deposition (ALD) process

The design of antireflection coatings with very low reflection is possible by using the atomic layer deposition method (ALD). It is a process which is based on the sequential self-limiting surface reactions from generally two gaseous precursors coating [10]. By depositing one layer per cycle, ALD offers extreme precision in ultra-thin film growth since the number of cycles determines the number of atomic layers and therefore the precise thickness of deposited film.

Schematic representation of one cycle of the atomic layer deposition (ALD) process is shown in Fig. 3.

In the study of antireflection coatings it is important to optimize their surface topography, thickness, and therefore their optical properties. For this purpose very useful are microscopic and spectroscopic studies.

Experiment 

TiO₂ thin films were prepared by the sol–gel spin coating technique. The Titanium isopropoxide was used as precursor material.

Absolute ethyl alcohol was used as a solvent and hydrochloric acid as a catalyst. The mixture was vigorously stirred using a magnetic stirrer for 2 h. Then the solution was spin coated with the various spin speed (2000, 3000 and 4000 rpm) on the polished monocrystalline silicon substrates and heated at 600°C for 2h. The whole process was in accordance with the scheme shown in Fig. 4.

The Al₂O₃ thin films was prepared by the atomic layer deposition technique. Trimethylaluminum (TMA) was used as precursor material. ALD process was carried out in such a way as to obtain a layer thickness of 60, 80 and 100 nm. The thin films was deposited on the monocrystalline silicon substrates with the texture.

The obtained layers were studied using atomic force microscope (AFM) and spectrometer UV/VIS.

Fig. 4. The schematic representation of the process

Results

The atomic force microscope was used to investigate the surface topography of obtained thin films. The representative pictures of the surface of sol gel thin films TiO₂ and ALD thin films Al₂O₃ are shown in Fig. 5 and 6.

As shown in the Fig. 5 the thin film deposited at 4000 rpm spin rate is uniform without any major irregularities. Similarly, the Fig. 6 shows that the texture pyramids were covered evenly with Al₂O₃ layer.

The UV/VIS spectrometer was used to investigate optical reflection of the obtained thin films (Fig. 7–9). Reflectance spectra were taken on sol-gel prepared TiO₂ thin films before and after heat treatment. The minimum values of the reflection were obtained in the wavelength range 500…700 nm and ranged between 1…7%. With the increase of spin speed the range is shifted to lower wavelengths. The heat treatment at 600°C has worsened the optical properties of the deposited thin films.

The minimum values of the reflection (for the ALD Al₂O₃) were obtained in the wavelength range 360…1000 nm and ranged between 0.2…7%.

It is expected that the shape of the reflectance spectrum of TiO₂ films is modified by interference effects. Such effects are missing in case of ALD deposited Al₂O₃ thin films.

Fig. 5. The AFM topography image of the surface of thin film TiO2 coated with a spin speed 4000 rpm on the monocrystalline silicon 

Fig. 6. The AFM topography image of the surface of ALD thin film Al2O3 deposited on the monocrystalline silicon with the texture

Fig. 7. Reflection as a function of wavelength, for the TiO2 thin films obtained with three different spin speed (2000, 3000 and 4000 rpm)

Fig. 8. Reflection as a function of wavelength, for the TiO2 thin films obtained with three different spin speed (2000, 3000 and 4000 rpm) after heat treatment

Fig. 9. Reflection as a function of wavelength for the Al2O3 thin films with three different thickness (60, 80 and 100 nm) obtained by ALD method

Summary

The both methods (sol gel and ALD) are provide a uniform coating of substrates. The sol-gel method allows the coating of substrates with low surface roughness and the ALD method allow the coating of the substrates of different shapes. Definitely better results of reflection were obtained for the ALD thin films. The reflection was about range 0.2…7%. for the wavelength from 360 to 1000 nm. Reflectance spectra of TiO2 thin films are modified by interference effects. 

Marek Szindler is a holder of scholarship from project POK L.04.01.01-00-003/09-00 entitled „Opening and development of engineering and PhD studies in the field of nanotechnology and materials science” (INFONANO), co-founded by the European Union from financial resources of European Social Fund and headed by Prof. L.A. Dobrzański.

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