Academic literature on the topic 'Aluminium Oxide, Surface Passivation, Sputtering, Solar Cells'

Aluminum oxide films were deposited on crystalline silicon substrates by reactive RF magnetron sputtering. The influences of the deposition parameters on the surface passivation, surface damage, optical properties, and composition of the films have been investigated. It is found that proper sputtering power and uniform magnetic field reduced the surface damage from the high-energy ion bombardment to the silicon wafers during the process and consequently decreased the interface trap density, resulting in the good surface passivation; relatively high refractive index of aluminum oxide film is benefic to improve the surface passivation. The negative-charged aluminum oxide film was then successfully prepared. The surface passivation performance was further improved after postannealing by formation of an SiOxinterfacial layer. It is demonstrated that the reactive sputtering is an effective technique of fabricating aluminum oxide surface passivation film for low-cost high-efficiency crystalline silicon solar cells.

The degradation of transparent electrodes’ electrical conductivity under environmental conditions is considered as a major failure mode for solar cells’ long-term efficiency. In this paper, AZO thin films were subjected to the International Electrotechnical Commission (IEC) 61646 test to examine their environmental stability and suitability as front electrodes for solar cells. To explore the interplay between AZO deposition parameters and environmental stability, AZO films were deposited by radio frequency magnetron sputtering at different parameters and without external heating. The conductivity stability evolution upon the testwas investigated via studying the AZO electrical, structural, and morphological characteristics at different deposition conditions. A direct dependence was identified between the samples’ conductivity degradation rates and the samples’ structural and morphological characteristics including grain size, grain boundary density, surface roughness, and compactness. The samples’ resistivity increases linearly over the test period due to both electron density and mobility degradations. Improved stability was observed for thicker AZO samples (360 nm) originating from enhanced grain size, surface profile, and compactness. These samplesmaintained solar cells' applicable sheet resistance of 21.24 Ω/sq (ρ=7.64×10-4 Ω.cm) following the test. The conducted aging studies demonstrated that manipulating the AZO films growth process via optimizing the deposition parameters is an effective pathway for low-temperature deposited electrodes with enhanced environmental stability

Aluminium doped zinc oxide (AZO) is fast becoming an important thin film material for applications as transparent conducting oxide (TCO) in several thin film solar cells, smart windows and many devices using touch screen displays. This is due to its good electrical and optical characteristics as well as lower cost and good abundance. Although sputtering is the general method for industrial fabrication of this material, but film characteristics depend strongly on fabrication processes. Thus, optimal films are obtained by optimization of the deposition conditions. In this work, we investigated the effects of RF deposition power on AZO thin films. Samples of similar thicknesses were grown under similar conditions in an RF sputtering chamber at different RF powers. The samples were then characterized using FESEM, AFM, UV-Vis, XRD and Hall effect measurement tools. Results indicate that the surface morphology is slightly affected with larger grain sizes obtained at higher RF powers. Also the surface roughness, average transmittance, conductivity and deposition rate all increase with the RF power. The lowest as-deposited resistivity of 15.3x10-3 Ω/cm was obtained, at the highest RF power of 100 W. This film also have the highest values of carrier concentration, mobility and figure of merit of 4.24x1020 cm-3, 0.96 cm2/V and 0.27x10-3 Ω respectively. This work highlights the significance of RF power in the fabrication of good quality AZO thin films.

Transparent conductive oxides (TCOs) play a major role as the front electrodes of thin-film silicon (Si) solar cells, as they can provide optical scattering and hence improved photon absorption inside the devices. In this paper we report on the surface texturing of aluminium-doped zinc oxide (ZnO:Al or AZO) films for improved light trapping in thin-film Si solar cells. The AZO films are deposited onto soda-lime glass sheets via pulsed DC magnetron sputtering. Several promising AZO texturing methods are investigated using diluted hydrochloric (HCl) and hydrofluoric acid (HF), through a two-step etching process. The developed texturing procedure combines the advantages of the HCl-induced craters and the smaller and jagged—but laterally more uniform—features created by HF etching. In the two-step process, the second etching step further enhances the optical haze, while simultaneously improving the uniformity of the texture features created by the HCl etch. The resulting AZO films show large haze values of above 40%, good scattering into large angles, and a surface angle distribution that is centred at around 30°, which is known from the literature to provide efficient light trapping for thin-film Si solar cells.

ABSTRACTHigher silicon solar efficiencies are possible if metal contact is made to the cell though openings in a well-passivated surface. Patterning for rear point-contact schemes has typically been achieved using deterministic patterning methods involving either the use of photolithography, laser or inkjet patterning. However, with these approaches it is difficult to achieve cost-effective, high-throughput and robust processing if very small and closely-spaced openings are required. In this paper we review recent progress in the use of self-patterning anodised aluminium oxide layers to both passivate and enable point metal contacts to the rear surface of silicon solar cells. We describe a wet chemical method for anodising aluminium layers thermally-evaporated on the rear surfaces of silicon solar cells, and demonstrate that the layers can result in excellent passivation of the underlying silicon and also enable metal contact to the solar cell. Additionally, we describe how patterning of either the anodic aluminium oxide layer or the source aluminium layer can result in patterns of metallic and dielectric regions on a surface, and how currently-available solar cell electroplating tools can be adapted to achieve anodisation of solar cells at commercial processing throughput rates.

The global energy market is continuously changing due to changes in demand and fuel availability. Amongst the technologies considered as capable of fulfilling these future energy requirements, Photovoltaics (PV) are one of the most promising. Currently the majority of the PV market is fulfilled by crystalline Silicon (c-Si) solar cell technology, the so called 1st generation PV. Although c-Si technology is well established there is still a lot to be done to fully exploit its potential. The cost of the devices, and their efficiencies, must be improved to allow PV to become the energy source of the future. The surface of the c-Si device is one of the most important parts of the solar cell as the surface defines the electrical and the optical properties of the device. The surface is responsible for light reflection and charge carrier recombination. The standard surface finish is a thin film layer of silicon nitride deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). In this thesis an alternative technique of coating preparation is presented. The HiTUS sputtering tool, utilising a remote plasma source, was used to deposit the surface coating. The remote plasma source is unique for solar cells application. Sputtering is a versatile process allowing growth of different films by simply changing the target and/or the deposition atmosphere. Apart from silicon nitride, alternative materials to it were also investigated including: aluminium nitride (this was the first use of the material in solar cells) silicon carbide, and silicon carbonitride. All the materials were successfully used to prepare solar cells apart from the silicon carbide, which was not used due to too high a refractive index. Screen printed solar cells with a silicon nitride coating deposited in HiTUS were prepared with an efficiency of 15.14%. The coating was deposited without the use of silane, a hazardous precursor used in the PECVD process, and without substrate heating. The elimination of both offers potential processing advantages. By applying substrate heating it was found possible to improve the surface passivation and thus improve the spectral response of the solar cell for short wavelengths. These results show that HiTUS can deposit good quality ARC for silicon solar cells. It offers optical improvement of the ARC s properties, compared to an industrial standard, by using the DL-ARC high/low refractive index coating. This coating, unlike the silicon nitride silica stack, is applicable to encapsulated cells. The surface passivation levels obtained allowed a good blue current response.

Efficient and inexpensive solar cells are necessary for photovoltaics to be widely adopted for mainstream electricity generation. For this to occur, the recombination losses of charge carriers (i.e. electrons or holes) must be minimised using a surface passivation technique suitable for manufacturing. In the literature, it has been shown that the aluminium oxide films are negatively charged dielectrics that provide excellent surface passivation of silicon solar cells. Meanwhile, sputtering has been shown to be an inexpensive thin film deposition method that is suitable for manufacturing. This thesis work aims to combine the excellent passivation properties of aluminium oxide with the manufacturing advantages offered by sputtering. We show - for the first time - that sputtering is capable of depositing negatively charged aluminium oxide films that provide very good surface passivation of crystalline silicon. Effective surface recombination velocities of 24.6 cm/s and 9 cm/s are achieved on 0.8 Ohm.cm p-type crystalline silicon and 1 Ohm.cm n-type crystalline silicon respectively, with charges in the range of -1E11 to -1E13 per square centimetre. We specify the sputtering requirements and processing conditions required for achieving these results, showing the effect of the various deposition and annealing parameters. After investigating the physical characteristics of the sputtered aluminium oxide films using thin film measurement techniques such as Rutherford Backscattering Spectrometry and Secondary Ion Mass Spectroscopy, we conclude that the current levels of surface passivation attained using aluminium oxide films appear to be closely related to the interfacial layer and the presence of hydrogen. In some cases the level of surface passivation is most likely limited by the incorporation of unwanted impurities. We determine the composition and bonding of aluminium oxide films, discussing their significance to the various hypotheses concerning the origin of the negative charge. Finally, we demonstrate that sputtered aluminium oxide can be applied to solar cells by fabricating passivated emitter and rear cells with efficiencies as high as 20%. The results of this thesis provide the foundation for the sputtered aluminium oxide technology and its application to industrial solar cells.

High-efficiency crystalline silicon solar cells must suppress recombination at their p-type surfaces. Thin-film, amorphous aluminium oxide (Al₂O₃) has been shown to provide very effective passivation of such surfaces, assisted by its negative fixed charge. However, many details of Al₂O₃ passivation remain poorly understood. Furthermore, conventional means of depositing passivating Al₂O₃ are too slow or too expensive to be suitable for high-volume commercial production. This thesis addresses these issues in three ways: 1) by contributing to a deeper understanding of semiconductor-dielectric interfaces and semiconductor surface recombination mechanisms in general, 2) by investigating the properties of Al₂O₃ as a passivating dielectric for silicon surfaces, and 3) by demonstrating the viability of APCVD as a high-throughput, industrially compatible deposition method for Al₂O₃, enabling its application to commercial solar cells. Using Al₂O₃ as a test case, it is shown how a novel analysis of the extended conductance method can be used to i) distinguish the separate contributions to the interface state distribution at a semiconductor-dielectric interface, and ii) determine their capture cross-sections for both minority and majority carriers. Furthermore, the direct link between these measured interface state properties and the recombination rate at the semiconductor surface is experimentally demonstrated by showing that the former can be used to accurately predict the latter. Investigations of the surface passivation properties of Al₂O₃ reveal a remarkably consistent picture. It is shown that the properties of the Si‒Al₂O₃ interface states are essentially independent of the Al₂O₃ deposition conditions and technique. The interface properties are found to be independent of the surface dopant concentration at boron- and phosphorus-doped surfaces, while recombination is shown to be only weakly dependent on surface orientation and morphology as a result of the remarkable orientation-independence of the Si‒Al₂O₃ interface state properties. Meanwhile, the chemical origin of the charge at the Si‒Al₂O₃ interface is elucidated by correlating FTIR and electrical measurements. APCVD is clearly shown - for the first time - to be capable of depositing Al₂O₃ films with exceptional surface passivation properties, comparable to the best results achieved using other deposition techniques. In the best case, interface state densities as low as 5 x 10¹⁰ eV¯¹ cm¯² at midgap, and negative fixed charge concentrations of 3.3 x 10¹²cm¯² are measured, resulting in a saturation current density of 7fAcm¯² on undiffused p-type surfaces. The APCVD films are shown to be thermally stable under standard solar cell processing conditions and are demonstrated in large-area solar cells with peak efficiencies of 21.3 %. These results demonstrate the viability of APCVD Al₂O₃ as a surface passivation layer for industrial silicon solar cells.