Engineering

Publication Search Results

Now showing 1 - 10 of 56
  • (2013) Zhang, Haoyang; Hawkes, Evatt; Chen, Jacqueline; Kook, Sanghoon
    Journal Article
    The autoignition of dimethyl ether (DME) with temperature inhomogeneities is investigated by one-dimensional numerical simulations with detailed chemistry at high pressure and a constant volume. The primary purpose of the study is to provide an understanding of the autoignition of DME in a simplified configuration that is relevant to homogeneous charge compression ignition (HCCI) engines. The ignition structure and the negative temperature coefficient (NTC) behaviour are characterised in a homogeneous domain and one-dimensional domains with thermal stratification, at different initial mean temperatures and length scales. The thermal stratification is shown to strongly affect the spatial structure and temporal progress of ignition. The importance of diffusion and conduction on the ignition progress is assessed. It is shown that the effects of molecular diffusion decay relative to those of chemical reaction as the length-scale increases. This is to be expected, however the present study shows that these characteristics also depend on the mean temperature due to NTC behaviour. For the range of conditions studied here, which encompass a range of stratification length scales expected in HCCI engines, the effects of molecular transport are found to be small compared with chemical reaction effects for mean temperatures within the NTC regime. This is in contrast to previous work with fuels with single-stage ignition behaviour where practically realisable temperature gradients can lead to molecular transport effects becoming important. In addition, thermal stratification is demonstrated to result in significant reductions of the pressure-rise rate (PRR), even for the present fuel with two-stage ignition and NTC behaviour. The reduction of PRR is however strongly dependent on the mean initial temperature. The stratification length-scale is also shown to have an important influence on the pressure oscillations, with large-amplitude oscillations possible for larger length scales typical of integral scales in HCCI engines.

  • (2013) Hawkes, Evatt; Chatakonda, Obulesu; Aspden, Andrew; Kerstein, Alan; Kolla, Hemanth; Chen, Jacqueline
    Journal Article
    Knowledge of the fractal properties of premixed flame surfaces can potentially be used to help develop turbulent combustion models. Here, direct numerical simulations of low Damköhler number flames are used to analyse the fractal nature of the flames. Two sets of data are considered: (i) thermochemical hydrogen–air turbulent premixed plane-jet flames with detailed chemistry and (ii) thermonuclear flames in type Ia supernovae. A three-dimensional box counting method is used to investigate fractal dimension of the flame surface, characterising the self similarity of flame fronts. In the premixed flames, the fractal dimension is found to vary in time between 2.1 and 2.7. The supernovae flames in distributed combustion regimes yield fractal dimension about 2.7. The results for the maximum fractal dimensions are higher than previously reported. They are explained theoretically by a Reynolds number similarity argument which posits that the high Reynolds number, low Damköhler number limiting value of the fractal dimension is 8/3. Also tested is Mandelbrot’s fractal additive law which relates the fractal dimension determined in two dimensions, which is typical of experimental measurements, to that in three dimensions. The comparison of the fractal dimension in both two-dimensional and three-dimensional spaces supports the additive law, even though the flames considered do not formally satisfy isotropy. Finally, the inner-cut off is extracted from the hydrogen flames and found to be consistent in order of magnitude with Kolmogorov scaling.

  • (2010) Tjahjono, Budi Santoso
    Thesis
    To reach the goal of grid parity, the cost of crystalline solar cells need to be reduced considerably. This can be done by using cheaper materials, such as thinner or lower quality wafers, or by reducing manufacturing costs, both process and material costs. Increasing the efficiencies of the devices while maintaining low costs via simple fabrication schemes is an ideal combination. The benefits of selective emitter have been known and quantified for many years. However, the adaptation of selective emitter solar cell designs into mass-production has been quite slow due to the relatively complex processing steps involved in applying such a design. This thesis begins with a review of various existing selective emitter solar cell technologies. A method of creating a selective emitter is then selected to be the main focus of the thesis due to its simple yet powerful features. This method is laser-doping through a dielectric layer. A new solar cell structure that employs the laser-doping process combined with a self-aligned metallisation method is then introduced, termed Laser-Doped Selective Emitter (LDSE) solar cell. A study is then presented to help further understanding of the laser-doping through dielectric process. Investigative work combines the use of defect Yang etch, SEM and EBIC analysis, light JV curve measurements and local ideality factor derived from dark JV curve measurements. These investigations help to identify several challenges associated with the laser-doping process. Theories on the causes of these challenges are then presented along with possible solutions. One of the most critical findings is the use of a stacked layer consisting of a thin layer of thermally grown SiO2 and PECVD SiNx to prevent laser-induced defects. Next, several characterisation methods that include four point-probes, Secondary Ion Mass Spectrometry (SIMS), spreading sheet resistance, Electron Beam Induced Current (EBIC) analysis and photoluminescence imaging are utilised to measure the sheet resistances, doping profiles and lifetime of the laser-doped region. These measurement techniques are used to optimise the laser parameters and other processing parameters involved in the fabrication of LDSE solar cells. In the final chapter, a novel self-aligned metallisation method that utilises the solar cell’s ability to produce voltage and current is investigated. It is found that this photoplating method is well-suited for fabrication of LDSE cells and potentially suitable in mass-production. Finally, the integration of the optimised process flow and parameters combined with the photoplating method is demonstrated through the fabrication and characterisation of a batch of LDSE solar cells made of commercial 125mm 1 Ω.cm p-type wafers. Efficiencies of up to 19% with VOC in excess of 635 mV and JSC close to 38 mA/cm2 are reported on 125 mm commercial grade CZ p-type wafers. The use of typical production line equipments for most of the fabrication steps is also demonstrated.

  • (2010) Hameiri, Ziv
    Thesis
    This thesis examines two different ways to improve the performance of single sided laser-doped solar cells. The first is replacing the aluminum rear contact with localised contacts and a high-quality passivation layer. The second is optimising the laser processing to minimise any detrimental effects. It is demonstrated that annealing in the 600-820°C range significantly improves the passivation of different SiNx films on different silicon surfaces. Significant bulk lifetime enhancement is seen when SiNx-passivated CZ wafers are annealed. Using an optimal annealing condition, the implied Voc of CZ silicon substrates increased to a value comparable to that of FZ wafers - almost 720 mV. Laser-induced defects are investigated using a wide range of characterisation techniques. It is found that laser doping degrades the electrical performance of the device. This degradation is more pronounced when a dielectric layer is present during the laser process, possibly due to the thermal expansion mismatch between the silicon and the overlying dielectric layer. Methods to reduce defect density are discussed. The influence of laser parameters on the electrical performance of laser-doped solar cells is studied. It is demonstrated that a wide range of laser diode currents can be used to create a p-n junction by laser doping. Grooves formed through intermediate levels of ablation can be used to improve the adhesion between the silicon and metal without significantly degrading the cell performance. Electroless and photo-plating are compared; higher pseudo-FFs are achieved for photoplated laser-doped solar cells. If the photoplating technique is combined with well-optimised Ni sintering, the pseudo-FF is almost independent of the laser diode current. A new double sided laser-doped structure is developed. This structure is based on silicon nitride passivation of the rear surface and the formation of a selective emitter and local back-surface field by laser doping. One-sun implied Voc above 680 mV is achieved on commercial grade CZ p-type wafers when measured after laser doping and prior to metallisation. This is ~50 mV higher than the Voc obtained for the single-sided laser-doped cell at the same stage. This high Voc demonstrates the potential of this structure to achieve efficiencies exceeding 20%.

  • (2011) Tsao, Chao-Yang
    Thesis
    Germanium (Ge) thin films and Ge-rich silicon-germanium (SiGe) alloys have potential for lowering the manufacturing cost of photovoltaic (PV) devices especially in tandem solar cells. This thesis focuses on the fabrication and characterization of Ge thin films and Ge-rich SiGe alloys prepared by radio-frequency (RF) magnetron sputtering, an inexpensive, non-ultra high vacuum deposition technique capable of fabricating large area films. The motivation is given firstly, followed by a brief review of Ge in PV applications. Secondly, fabricating polycrystalline Ge (poly-Ge) thin films on glass by RF magnetron sputtering is investigated. In addition, in situ hydrogenation is applied in an attempt to further improve the properties of the Ge films. The influence of hydrogen on the deposition rate, surface morphology, and structural, optical, as well as electrical properties of poly-Ge films is explored. Moreover, to demonstrate the potential of the in situ hydrogenated poly-Ge (poly-Ge:H) films in PV applications, the doping of poly-Ge:H thin-films on glass is studied. P type films were deposited and in situ doped by co-sputtering Ge:H with boron (B) at various power levels in a mixture of argon and hydrogen at 500C followed by a rapid thermal anneal (RTA) process. On the other hand, n type films were deposited and ex situ doped by firstly sputter-depositing a Ge:H layer and then a SiO2/P2O5+SiO2/SiO2 sandwich structure capped with a SiNx layer, and finally followed by a thermal drive-in process with RTA. The evidence for successfully p-type and n-type doping of poly-Ge:H films is presented. Furthermore, the research scope is extended into polycrystalline Ge-rich SiGe alloys to explore the properties of the Ge alloyed with small amounts of Si by RF sputtering. Finally, a novel method for growing thin relaxed single crystalline Ge heteroepitaxial layers on Si substrates by using RF sputtering was developed. By using this method, the need of ultra-high-vacuum condition and the use of costly and extremely toxic germane gas are avoided. Therefore, thin relaxed single crystalline Ge epitaxial layers on Si substrate can be obtained at low cost, making the resultant Ge layer a potential virtual substrate for III-V material growth for tandem cell applications.

  • (2011) Sugianto, Adeline
    Thesis
    To enable photovoltaics (PV) to become a leading source of energy in a low-carbon future, the cost of solar electricity needs to be reduced to a level that can compete with current electricity prices. From a technology perspective, this reduction can be achieved by increasing the solar cell’s conversion efficiency at similar or reduced manufacturing costs. This thesis describes the development of a high efficiency Si solar cell technology that has the potential to lower the cost per Watt of solar cells, through the use of laser-doped selective emitter and localised rear contacts. This thesis begins with the development of laser-doped selective emitter (LDSE) technology to overcome the limitations of the front surface design associated with the industrial screen-printed solar cells. In this technology, laser doping is employed to simplify the formation of selective emitters that were conventionally fabricated by photolithographic patterning and long, high-temperature phosphorus diffusion. Defect studies and device loss analysis are performed to facilitate the selection of the most suitable laser parameters for the laser doping application. By using a continuous wave 532 nm laser, a sheet resistance as low as 2 Ω/ is achieved in the selective emitters with minimal laser-induced defects while maintaining a short process time of several seconds per wafer. Optimisation of the laser doping parameters leads to a cell efficiency of 19% being demonstrated on large area, commercial-grade CZ p-type wafers. This thesis also explores the application of an identical LDSE cell structure on commercial-grade CZ n-type wafers. In this case, the selectively-doped n+ regions at the front surface act as a front surface field (FSF) while the screen-printed Al-alloyed region at the rear surface forms the emitter of the n-type solar cells. A study of the Al-alloyed emitter formation with respect to firing conditions is presented with the aid of SEM imaging. A final Voc approaching 650 mV and pFF of 83% demonstrate the successful formation of p+ emitter with minimum junction shunting. During the development of the n-type LDSE technology, an “apparent” shunting behaviour is observed in the cell areas that are not well-plated. Investigative work involving Photoluminescence imaging and PC1D modeling is detailed to further understand the cause of the “apparent” shunting and its impact on the cell performance. Through this investigation, solutions are devised and successfully implemented to overcome the “apparent shunting”. Further optimisation on the phosphorus FSF diffusion enables the achievement of a FF approaching 80% and a final cell efficiency of 18.7%. The final stage of this work presents the development of a novel next-generation LDSE cell structure that focuses on improving the rear surface design while retaining the excellent front surface design of the LDSE cell. This cell structure employs a commercially-manufacturable thermal-SiO2/PECVDSiNx stack layer for rear surface passivation; and relies on the use of boron laser doping in conjunction with sputtered Al for localised rear contact formation. Test structures with an implied Voc over 700 mV (prior to laser doping) indicates high surface-passivation quality given by the SiO2/SiNx stack following thermal anneal. Demonstration of sheet resistance as low as 5 Ω / on the boron laser-doped p+ regions highlights the potential for low-temperature Al sintering to form an ohmic contact. Different geometries for the rear contact pattern are designed and implemented to establish optimum current collection. By improving the rear surface design, independently-confirmed cell efficiencies as high as 20% are achieved on large area, standard commercial-grade p-type wafers, with the potential for further efficiency increase exceeding 21%. These are believed to be record performance cell for this type of wafer. These results suggest that this new generation of LDSE cells have the potential of becoming an attractive cell technology in the PV industry.

  • (2012) Eggleston, Bonneville
    Thesis
    A large area cw diode laser is utilised in this thesis to develop three new processes to improve the efficiency and reduce the cost of silicon thin-film solar cells made on glass substrates. The first is a defect annealing process which replaces a belt furnace anneal for the removal of crystallographic defects and activation of dopants in solid-phase crystallised silicon. The highest substrate temperature required during the process sequence is reduced from 960C to 620C, which opens the door to the use of cheaper glass substrates. A peak 1-sun voltage of 492 mV is achieved on planar samples, which is an improvement of 30 mV over the belt furnace process. The films are shown to be partly recrystallised during the process, significantly improving the material quality of the melted section while retaining the dopant profile within the un-melted section. Completed devices are shown to match the performance of the optimised belt furnace process. The second is a complete laser crystallisation process applied to thick a-Si films. Here the film is molten across the entire thickness, and a regime is found whereby long linear grains are formed by continuous growth with the previous crystallised region acting as a seed for the newly crystallising material. Grains up to millimetres in length and hundreds of microns in width have been grown with virtually zero detectable intra-grain defects. A rear diffused emitter is used to create a p-n junction, which produces 1-sun voltages as high as 539 mV. IQE measurements show that the glass-side of the films is very well passivated and that the diffusion length is much longer than the device thickness. Cells up to 7.01% efficiency have been demonstrated without light trapping or optimisation of many of the key processes. The third new process is large area solid-state diffusion using a diode laser to drive in spin-on dopants. This process has been used to form a junction and demonstrated some of the highest voltages on the diode laser crystallised silicon and to form a back surface field for solid-phase crystallised cells. A thermal/doping model is presented which predicts the depth of diffusion and its accuracy is confirmed by dopant profile measurements.

  • (2012) Bilbao, Jose I
    Thesis
    This Thesis presents the work of the author on PVT-water modules and systems as retrofits for existing households. Two computer models were developed in order to analyse and characterise PVT systems: one for steady state conditions, used for annual yield simulations, and a full transient model, used for the detailed analysis of PVT modules performance and behaviour. Also, a PVT module was designed based on commercially available parts and materials. The design was built as part of the experimental setup that was used to fine tune and verify the accuracy of the computer models developed. Results show that both models offer high accuracy when compared to experimental data and a better agreement with the data than commercial software packages. Simulations for domestic PVT-water systems were carried out for different climates and load profiles, characterised by the demand found for the average households in two countries: a developed economy (Australia), and an emerging economy (Chile). The results show that not only the performance and type of PVT module must be considered in the design of the system, but also the demand profiles and the other system components. This differs from the PV market where a “one design fits all” approach is possible. It is determined that a systemic approach should be used when characterizing PVT-modules because of the very nature of the technology, and that different solutions should be applied for a same system under different conditions. When all these effects are taken into account, it is concluded that PVT-water systems are a viable solution for the growing energy demand in the residential sector.

  • (2013) Li, Hua
    Thesis
    To reach the goal of grid parity, technology improvements to enhance the conversion efficiency of solar cells at similar or reduced manufacturing cost must be found. The research presented in this thesis describes industrially applicable high efficient Passivated Emitter and Rear Locally Diffused and Contacted (PERL) cell technology with a hydrogenated amorphous silicon and silicon nitride (a-Si:H/SiNx:H) layer stack for rear surface passivation. This thesis starts with studying the dependence of the film physical characteristics, chemical bond configurations, surface passivation quality and the thermal stability of the intrinsic a-Si:H on the deposition conditions. The deposition conditions include deposition time, deposition temperature and microwave power. This was done for the purpose of growing uniform films with good film quality and ideal surface passivation. The surface passivation properties of the intrinsic a-Si:H on a textured surface with relatively small pyramid size was also studied. As a result, an effective surface recombination velocity (Seff) below 8 cm/s was achieved on 1 Ω.cm boron doped p-type FZ wafers. The second part of this work focuses on developing a thermally stable a-Si:H/SiNx:H stack with good surface passivating property. The dependence of the surface passivating property and the thermal stability of the a-Si:H/SiNx:H stack on the deposition conditions of both layers was investigated for the purpose of optimizing these two aspects. With the optimized deposition conditions, excellent τeff values of 1.7 ms was achieved on 1 Ω.cm p-type FZ wafers with an upper bound for the corresponding Seff of 5.8 cm/s. The surface passivating quality of the optimized a-Si:H/SiNx:H stack remains stable for over 90 min at 400°C which is sufficiently stable for the formation of metal contacting schemes that require relatively low metal sintering temperatures and the implementation of hydrogenation passivation processes. Optimized iVoc of 717 mV was achieved on cell precursors prior to laser-doping processes. As high as 54% reflectance at 1200 nm indicates that the a-Si:/SiNx:H stack acts as a good rear reflector when combined with an overlying thin layer of Al. The third part of this work focuses on developing a rear localised metal contact in conjunction with the a-Si:H/SiNx:H passivated rear surface, based on laser-doping combined with a low temperature annealing step for an overlying Al layer. A 355 nm Q-Switched laser was selected as a suitable tool to form high quality local back surface field (LBSF) through the a-Si:H/SiNx:H stack. The quality of the LBSF, including the sheet resistance (R□), doping profile and the recombination properties of the laser-doped LBSF were investigated in a wide range of laser parameters to optimize the process. As a result, an LBSF with R□ of about 5 Ω/□, a surface doping concentration above 1020cm-3 and the lowest effective SRV in the LBSF region of 6400 cm/s was obtained with the lowest laser scanning speed and a relatively low laser power. The laser-induced defects in the LBSF region can be hydrogenated by the H atoms released from the passivating layers during a thermal annealing process, resulting in a remarkably reduced effective SRV to about 4600 cm/s. To avoid potential damage induced by Al during the thermal annealing process for ohmic contact formation, the influence of the capping SiNx:H layer thickness and the characteristics of the underlying a-Si:H layer on the performance of cell test structures was studied. Contact resistance (Rc) values as low as 2 mΩ.cm2 were achieved after annealing for 20 min at temperatures ranging from 350°C to 450°C. The final stage of this work presents the development of the rear a-Si:H/SiNx:H passivated PERL cells with the developed a-Si:H/SiNx:H rear surface passivating stack and the rear local contacting scheme. Best cell efficiency of 20.24% with a Jsc of 40.2 mA/cm2 and Voc of 671 mV was achieved on 1.8-2.4 Ω.cm p-type commercial CZ wafers. To optimize the cell performance, the influence of the rear contact pattern on the electrical performance, including point contact patterns and line contact patterns, was studied. It was found that, by using closer spaced point contact pitch, the Rs can be effectively reduced, which therefore improves the FF. However this leads to increased dark saturation current and hence reduced Voc and Jsc. As a compromise, the highest efficiency of 20.3% was achieved on the cell with an optimal point contact pitch of 0.4 mm. The industrial feasibility of the developed technique is discussed.

  • (2012) Kim, Taekyun
    Thesis
    This thesis is concerned with the development of metallisation and interconnection schemes for e-beam evaporated polycrystalline silicon thin-film solar cells on glass substrates. Of particular interest is a method for series connection of individual cells to primarily minimise light leakage to the glass substrate and thus to reduce associated current loss. In this thesis, two new metallisation schemes have been developed and investigated. The first scheme is called the “aligned bifacial interconnected metallisation”. The diode is divided into individual cells by laser scribing an isolation groove, which is then filled with photoresist to act as an isolating dielectric. Aluminium is deposited over the whole device and patterned to form interconnected bifacial mini-modules. The open-circuit voltage of the mini-module was the sum of individual cells' voltages, indicating a successful interconnection. The mini-module short-circuit was slightly higher than the individual cell current due to lower light leakage in the glass substrate when a diffuse back surface reflector is used for light trapping. Current gain of about 2.6 % of a mini-module was achieved through the interconnection of individual cells. The second scheme investigated is the "self-aligned bifacial interconnected metallisation". In this scheme the emitter grooves are etched down to the glass substrates rather than to the emitter layer as in aligned bifacial scheme. Emitter electrodes are formed by filling the grooves with evaporated aluminium, which does not need alignment. The resulting emitter electrodes were wider, which was expected to reduce series resistance. However, the series resistance of both the individual cells and interconnected mini-modules were found to be higher due to reduced area of contact between the emitter electrodes and the emitter sidewalls. Devices metallised using this self-aligned scheme also recorded low shunt resistances due to Schottky contacts between emitter electrodes and the cell absorber. E-beam evaporated thin-film cells were successfully interconnected using the aligned bifacial scheme. Additional short-circuit current of the mini-module was reported, due to less current loss from light leakage through the glass substrate. Cells metallised with the self-aligned scheme suffered from series and shunt resistance.