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High Efficiency Thin Film Solar Cells dti HIGH EFFICIENCY THIN FILM SOLAR CELLS GROWN BY MOLECULAR BEAM EPITAXY (HEFTY) CONTRACT NUMBER: S/P2/00369REP URN NUMBER: 06/668 dti The DTI drives our ambition of ‘prosperity for all ’ by working to create the best environment for business success in the UK. We help people and companies become more productive by promoting enterprise, innovation and creativity. We champion UK business at home and abroad. We invest heavily in world-class science and technology. We protect the rights of working people and consumers. And we stand up for fair and open markets in the UK, Europe and the world. 11 HIGH EFFICIENCY THIN FILM SOLAFt CELLS GROWN BY MOLECULAR BEAM EPITAXY (HEFTY) S/P2/00369/REP Contractor BP Solar Limited Prepared by N B Mason (BP Solar) KWJ Barnham, IM Ballard & J Zhang (Imperial College) The work described in this report was carried out under contract as part of the DTI Technology programme: New and Renewable Energy, which is managed by Future Energy Solutions. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of the DTI or Future Energy Solutions. First Published 2006 © Crown Copyright in EXECUTIVE SUMMARY Background The demand for Solar Electricity (photovoltaic power) is growing rapidly. Sales of PV products increased by 40% in 2004 and 25-30% growth rates are anticipated over the next 10 years. A major new industry is being created. The UK position in manufacturing photovoltaic modules is relatively weak compared to our main competitors in Japan, Germany and USA. More than 90% of the PV modules currently manufactured are based on crystalline silicon wafer technology. These wafers are typically 200 to 300 m thick and are cut from solid silicon crystals (cast multicrystalline blocks or Czochralski gown single crystals). The sawing (wafering) process is highly inefficient, typically wasting up to 200 m of silicon as kerf loss for each wafer produced. Consequently, the cost of the semiconductor grade silicon in a PV module represents up to half the total module cost. In addition, with the rapid growth of the PV industry the worldwide PV demand for semiconductor grade silicon is now comparable to the requirements of the semiconductor electronics industry leading to supply shortage and price increases until new feedstock production capacity becomes available. Thin films of high quality silicon deposited on low-cost substrates offer a potential solution to significant cost reduction and utilisation of silicon. Silicon films of only 10 m thickness can be engineered to produce high efficiency cells that require only 1 /40th of the silicon used in conventional PV cells. However, this field of thin film crystalline silicon solar cells is relatively new but could be a primary PV technology in the long term. Work in this field could give the UK an opportunity to establish a technology leadership which should attract manufacturing industry. The motivation for this project was to establish the technology for high quality crystalline silicon film deposition suitable for high efficiency cells and to evaluate techniques for fabricating these devices on low-cost glass substrates. Objectives The aim of the project was to demonstrate a world leading result in the field of thin film crystalline solar cells and to establish the UK as a major contributor to this field. The cell design was to be consistent with the large-scale manufacturing and provide a basis for the process to be exploited by industry. The main targets were: - 1. Demonstrate a 10 pm thick cell grown by deposition of silicon onto a monocrystalline substrate by molecular beam epitaxy (MBE) at temperatures below 700°C achieving a sunlight-to-electricity energy conversion efficiency of 18%. Cell area was to be at least 1 cm2. iv 2. Demonstration of thin film devices with the same technical characteristics as target 1 deposited on a glass substrate below 700°C by a non-MBE process. The target energy conversion efficiency of these cells to be 8%. Summary of the work The work programme was divided into four main tasks; (A) MBE growth, (B) Characterisation of MBE films and solar cells, (C) Development of a high efficiency solar cell for MBE thin silicon films and (D) Transfer of MBE processed cell to a low cost (glass) substrate. The first two tasks were largely the responsibility of Imperial College whilst the latter two were to be undertaken by BP Solar. It was anticipated that the growth of films for this project by MBE would not represent a significant problem. However, it was discovered though that the thickness of material to be grown, which is many times thicker than the usual 1 m samples, did in fact present problems with early samples suffering from very poor surface morphology. This substantially slowed progress as numerous samples were grown to optimise the growth conditions required for of p+- and p-doped epilayers. Initially, samples were grown using the MBE system in Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD) mode, this was later changed to Gas Source Molecular Beam Epitaxy (GS-MBE) mode as this produced far superior material although at a lower growth rate. During the course of this project a major re-structuring of BP Solar was undertaken that severely impacted the ability of the contractor to complete the project as originally envisaged. As part of the restructuring process, BP Solar closed-down its two thin film pilot production facilities that used glass superstates as the basis for semiconductor deposition. In addition, the European Technology Centre in Sunbury (UK) was closed and the technology functions were transferred to operations in Spain and the USA. In order to continue to support the project, BP Solar arranged for cell fabrication and cell development to be undertaken at the Fraunhofer Institute for Solar Energy (ISE) in Freiburg, Germany. This institute is a leading solar cell research organisation in Europe with all the skills necessary to convert the silicon film fabricated at Imperial College into solar cells. With this arrangement the project was able to complete Task C but unable to undertake any material work on Task D. Due to these unforeseen circumstances, it was only possible to complete the first of the two project objectives defined above and little progress was made on the second objective. As a consequence of this, project expenditure was only half of that originally anticipated. Summary of the main results The first silicon films on this project were deposited by Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD) but the best PV cell energy conversion efficiencies were very low at around 4%. When the silicon film growth method was changed to v Gas Source Molecular Beam Epitaxy (GS=MBE) the resulting cell efficiency jumped to over 10% ULP-CVD would have better material quality at high temperatures but these temperatures were incompatible with growth on glass which limited the upper temperature to 700°C. It was found that emitter structures grown epitaxially had significantly poorer performance and also required exceptionally low growth rates due to the arsenic doping. Arsenic modified the film nucleation behaviour and led to a poor surface morphology at the low growth temperature requirements. Cells with a diffused emitter achieved an efficiency of 10% whereas those with an emitter diffused into the structure during processing achieved 12.7%, which represents an efficiency improvement of 27%. The use of a textured front surface improves the efficiency by two means, first a reduction of reflectivity and second, an increase in path length. Sample thickness was increased to 15 pm to allow for a 7 pm texture depth. Samples without texture achieved 13.3% efficiency, which was improved to 15% efficiency with a front surface texture despite having less material available for absorption. The volume of Si in the textured samples is equivalent to ~10pm thick planar sample. The best cells produced by the project with optimised epitaxial layer doping and the front finger metallization had a energy conversion efficiency of 16%, independently measured at the calibration facility of the Fraunhofer ISE. The electrical characteristics of the best cells fabricated by the project are listed below. isc 29.6 mA/cm2 Voc 655 mV FF 82.8% Efficiency 16.03% Area4.02 cm2 The deposited silicon material quality and the cell design and fabrication process was been significantly improved through the project. This is demonstrated by the improvement in efficiency from 4.2% to 16.0% from the first to the last samples processed by project, representing an improvement of 380%. This was been achieved by optimisation of the growth conditions, the switch from ULP-CVD mode to GS-MBE mode and diffusing the emitter structures into the epilayers as part of the post growth processing, combined with front surface texturing. The final result of this project, a cell of 4 cm2 area fabricated on MBE deposited silicon film of 15 m thickness measured at 16% efficiency represents a significant achievement. It represents over 70% of the ideal limit as calculated for this cell thickness by Greenet al. [1]. With thin silicon films grown at a low temperature compatible deposition on glass we have demonstrated a PV cell with energy conversion efficiency within 70% of the ideal limit. The only previous cells to achieve similar results were thicker than 30 m. Clearly, the project has achieved a significant result. MBE silicon films grown at Imperial College are amongst the highest quality for PV application. vi Conclusions • Thin silicon films grown at low temperature (suitable for deposition on glass substrates) by Gas Phase Molecular Beam Epitaxy (GS-MBE) has been shown to give superior performance PV cells to Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD).
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