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Close collaboration between UNSW and BP Solar Espana continued during 2005 with an 8- week Postgraduate Industrial Training visit by Malcolm Abbott to the Tres Cantos production facility. This visit focused on developing BP Solar’s capabilities in advanced characterisation techniques, especially dark IV and local ideality factor analysis and photoconductance techniques. Malcolm gained invaluable industrial experience while spending 8 weeks living in nearby Madrid and absorbing everything Spanish. Suntech-Power Company, China Project 1: Innovative Emitter Design and Metal Contact for Screen-printed Silicon Solar Cells Research Team Prof. Stuart Wenham (UNSW – Team Leader) Anita Ho (Postdoc Fellow – UNSW) Ly Mai (PhD student – UNSW) Tjahjono (PhD student – UNSW) Roland Utama (PhD student – UNSW) Philip Hamer (undergraduate research scholarship) Dr Z. Shi (Suntech) Dr Jingjia Ji (Suntech) A Zhu (Suntech) Li Hua (Suntech) Victor Chen (Suntech) Aim The broad aim of this work is to develop the next generation of screen-printed solar cell for implementation on the Suntech-Power production line. In particular, the fundamental limitations of the conventional screen-printed solar cell that have limited its performance for the last 30 years have been identifi ed, and innovative approaches to redesigning the emitter and front metal contact have been devised and are being developed and analysed in this work. More specifi cally, the initial aim in this work is to develop and demonstrate an innovative emitter design for the screen-printed solar cell that overcomes the current and voltage limitations imposed by the standard design shown in Figure 4.7.1, while retaining compatibility with existing equipment and infrastructure currently used for the manufacture of screen-printed solar cells. The primary aim during 2006 will be to complete the pilot line evaluation of the new technology in readiness for large scale production by the end of 2006. In particular, effi ciencies in excess of 18% in large scale production will be targeted following the achievement of effi ciencies in the range of 18-18.5% in pilot production in 2005 using all the same materials, wafers, equipment and processes. Background Screen-printed solar cell technology dominates commercial photovoltaic manufacturing, with well over 50% share of international markets. Despite the dominance of this technology, the solar cell design shown in Figure 4.7.1, has signifi cance performance limitations that constrain the cell effi ciencies to well below those achievable in research laboratories around the world. In particular, the front surface screen-printed metallisation necessitates a heavily diffused emitter to achieve low contact resistance and also to achieve adequate lateral conductivity in the emitter since the metal lines need to be widely spaced compared to laboratory cells 120
to avoid excessive shading losses. Such cells therefore typically have emitters with sheet resistivities in the range of 40-50 ohms per square, which inevitably give signifi cantly degraded response to short wavelength light. To raise this sheet resistivity to above 100 ohms per square as required for near unity internal quantum effi ciencies for short wavelength light, serious resistive losses are introduced, both in the emitter and the contact resistance at the metal to n-type silicon interface. Furthermore, the conventional design of Figure 4.7.1 has quite poor surface passivation in both the metallised and non-metallised regions. Even if good ohmic contacts could be made to more lightly doped emitters, the large metal/silicon interface area would signifi cantly limit the voltages achievable due to the high levels of recombination in these regions and hence contribution to the device dark saturation current. These voltage limitations are not of major signifi cance at the moment due to the limitations imposed by the substrates. However, In the future as wafer thicknesses are reduced to improve the device economics, the cells will have the potential for improved open circuit voltages, but only provided the surfaces, including under the metal, are well passivated. 150-200 p+ n++ p-type 3mm patterned metal contact phosphorus bulk of wafer rear metal contact Figure 4.7.1: Conventional screenprinted solar cell with heavily diffused emitter and large metal/ silicon interface area for the front surface metallisation. metal Innovative Emitter Design The new emitter design is shown in Figure 4.7.2. The top surface is diffused to 100 ohms per square, while the heavily diffused grooves act as semiconductor fi ngers to carry the current to the screen printed silver fi ngers that run perpendicular to the grooves as shown in Figure 4.7.3. Also not shown in Fig 4.7.2 is the surface passivating dielectric that not only passivates the lightly diffused surface so as to give near unity internal quantum effi ciencies for short wavelength light as shown in Figure 4.7.4, but it also isolates the metal from these same regions to minimise the device dark saturation current. The metal only contacts the silicon within the heavily diffused grooves, therefore giving a low area contact while still achieving low contact resistance. Fill factors of 79-80% have been demonstrated with this structure on large area devices of approximately 150cm 2 , area verifying the effectiveness of this contacting scheme. Effi ciencies as high as 18.3% (inhouse measurements) have been achieved, 30 microns contributed to signifi cantly by the high short circuit currents gained through the excellent 100 / n-type spectral response of Figure 4.7.4 and the low metal and top surface shading losses. 45 microns Figure 4.7.2: Cross-section of the investigated innovative emitter design using semiconductor fi ngers, developed to address the fundamental limitations of screen-printed metal contacts with their inability to produce fi ne lines and make ohmic contact to lightly diffused emitters. 5 / n++ p-type silicon n++ rear surface of solar cell 121
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ARC Centre of Excellence for Advanc
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Page 1. DIRECTORS' REPORT 1 2. HIGH
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the cell. Cells based on “hot”
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New Semiconductor Finger Technology
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Stanford University GCEP Grant The
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ARC Centre Review The Centre was re
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Adjunct Fellows Kylie Catchpole, BS
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Student Administration Manager Tric
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Figure 4.1.2: Example of “second-
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The fourth “spin-off” Centre st
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4 3 2 1 G During 2005 the developme
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system, microwave carrier lifetime
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Semiconductor Nanofabrication Facil
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4.3 FIRST GENERATION: WAFER-BASED P
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According to the calculated data in
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It is seen that the measured IQE in
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The re-PERT cells on n-type CZ subs
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In silicon wafers that have natural
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Based on our fi ndings, we consider
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Conductive Paste Firing Tests Basic
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Figure 4.3.2.9: Illuminated IV curv
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Silicon Nitride Passivation of Bare
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Wafer Handling PL imaging was used
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(a) (b) (c) Figure 4.3.2.18: SiN sa
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4.3.2.4 Process Engineering - Conta
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4.4 SECOND GENERATION: THIN-FILMS U
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Another highlight of the Group in 2
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whereas ALICE cells can be made by
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To illustrate the signifi cance of
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4.4.8 Optimisation of Evaporated So
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Secondary ion mass spectroscopy (SI
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Current Density (mA/cm 2 ) 16 14 12
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The fact that the SPE diodes are n=
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The current response of the cell as
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Page 76 and 77: The parallel project with Toyota on
Page 78 and 79: 4.5.2.1 Alternative matrices for Si
Page 80 and 81: 4.5.3.1.1 TEM of SiQDs in nitride H
Page 82 and 83: Figure 4.5.9: Two Theta Plots of th
Page 84 and 85: 4.5.3.3 Electrical characterisation
Page 86 and 87: Generating a description of the sam
Page 88 and 89: Many lanthanide energy levels are b
Page 90 and 91: 4.5.6.1 Selective Energy Contacts R
Page 92 and 93: ias to about 2.2V (Figure 4.5.23c).
Page 94 and 95: The phonon dispersions show that if
Page 96 and 97: In addition the thesis project inve
Page 98 and 99: 4.5.8 Concluding remarks for the Th
Page 101 and 102: 4.6 SILICON PHOTONICS RESEARCH GROU
Page 103 and 104: Some of these SOI LEDs were designe
Page 105 and 106: (a) (b) Figure 4.6.7: (a) Top view
Page 107 and 108: The photonic crystal processing of
Page 109 and 110: 4.6.5 Improved SOI LEDs Using Surfa
Page 111 and 112: Another interesting aspect of the P
Page 113 and 114: 4.6.6 Photoluminescence-Based Chara
Page 115 and 116: Surprisingly high sensitivity of PL
Page 117 and 118: Fig.4.6.6.6: Analysis of experiment
Page 119 and 120: In qualitative PL imaging, only the
Page 121 and 122: 4.7 COLLABORATIVE RESEARCH Universi
Page 123: UNSW Centre for Energy and Environm
Page 127 and 128: diffusing the groove walls. The per
Page 129 and 130: Thus this represents a QD areal map
Page 131 and 132: Summary The Centre for Advanced Sil
Page 133 and 134: strand is to provide students with
Page 135 and 136: 5.2.3 Screen-printed Solar Cells Sc
Page 137 and 138: 5.4 Scholarships The undergraduate
Page 139 and 140: 5.5.1 New Flexible First Year In 20
Page 141 and 142: The main purpose of this tool is to
Page 143 and 144: 5.8.3 UNSW Information Day Local un
Page 145 and 146: For the fi rst time in 2005, UNSW M
Page 147 and 148: 5.9.6 Murdoch University Western Au
Page 149 and 150: The Management structure of the Cen
Page 151 and 152: 7.1 PROGRESS AGAINST CENTRE DESIGNA
Page 153 and 154: In parallel, the AIT glass texturin
Page 155 and 156: A.10 Demonstration of photon down-c
Page 157 and 158: A.17 Demonstration of operational m
Page 159 and 160: Governance Milestones H.1 Establish
Page 161 and 162: Number of postgraduate completions
Page 163 and 164: Quality of the Centre Strategic Pla
Page 165 and 166: Centre 2005 Income The total income
Page 167 and 168: Centre 2005 Expenditure of ARC Gran
Page 169 and 170: BOOKS Stuart R. Wenham, Martin A. G
Page 171 and 172: R.A. Bardos, T. Trupke, M. Schubert
Page 173 and 174: D. Song, D. Inns, A. Straub, M.L. T
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F.W. Chen, J.E. Cotter, A. Cuevas,
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M.A. Green, “Photovoltaics - Elec
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A. Shalav, B. Richards, G. Conibeer
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P.I. Widenborg, S.V. Chan, D. Inns,
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