Elimination of Efficiency Degradation Mechanisms in Silicon Photovoltaic Solar Cells

Lead Research Organisation: University of Manchester
Department Name: Electrical and Electronic Engineering

Abstract

Photovoltaic Solar Cells seem certain to make a significant contribution to the world's energy needs in the 21st Century. At the present time such devices only convert a small part of the 1kW per square meter of radiation from the sun which falls on the earth's surface into electricity. Typical commercial silicon devices that use un-concentrated sunlight are less than 15% efficient although laboratory devices with efficiencies as high as 24% have been made. Many very interesting ideas have been put forward for higher efficiency 3rd generation cells but most of these are a very long way from commercial realization on a large economic scale. At the present time 90% of production is based on wafered silicon (Czochralski single crystal or cast poly-crystalline). The predicted production for 2010 using existing silicon technology will have a peak output of 26GW. Unfortunately in their initial few hours of operation most silicon solar cells suffer from degradation which stabilizes after a reduction in efficiency of 10% relative ie a 20% cell becomes an 18% cell. In the context of total photovoltaic power lost on a world scale this is very significant. It is believed that the reduction in efficiency is due to a defect reaction in the silicon in which oxygen dimers diffuse to the boron dopant to form a powerful recombination centre. The problem is not restricted to the commercial devices of the next decade but is also highly relevant to some of the most promising third generation (high efficiency) cells which use silicon as part of the active structure. This research proposal aims to eliminate the degradation process by removing the reaction path from the silicon prior to the formation of the recombination centre. An essential pre-requisite to this is to achieve a detailed understanding of the defect centres and their formation.We have previously studied the oxygen dimers which are currently thought to be the precursors of the recombination centre. These can be detected using optical absorption measurements, ideally at low temperatures (~10K). Preliminary work indicates that it should be possible to develop treatments of the silicon material which reduce the concentration of the dimers to a negligible level in the finished cell and, because the dimers do not form at normal operating temperatures, so eliminate the formation of the defect. It would be quite feasible using this approach to maintain the concentration of interstitial oxygen which provides mechanical strength to the silicon with consequent yield and cost benefits. In this work the recombination centre will be studied using minority carrier Laplace Deep Level Spectroscopy and its structure determined by the application of uniaxial stress. The reaction of the oxygen dimer will be studied as the recombination centre forms, in real time, using optical absorption techniques. The work will be done in collaboration with MEMC who are one of the leading manufactures of solar silicon, the Institut fr Solarenergieforschung Hameln/Emmerthal (ISFH) in Germany who have undertaken much experimental work recently on solar cell degradation and the University of Aveiro in Portugal who will collaborate on theoretical calculations to support the Manchester work.

Planned Impact

If the project is successful there will be an important industrial and social benefit in terms of more efficient power generation and consequent greater uptake of renewable energy. As a result of this work the absolute gain in efficiency of wafered silicon cells is anticipated to be 2%. This is very significant in the photovoltaic field (typically 10% improvement in relative terms). This type of cell constitutes more than 90% of the photovoltaic market at the present time and is expected to dominate sales for at least the next ten years despite other materials taking some of the market. This is an inevitable result of massive investment in silicon technologies for solar and recent improvement in cost effectiveness (thinner slices, cheaper poly silicon and new cell designs) If the effect of this is factored into the estimated output of the silicon solar cells susceptible to this form of degradation manufactured over the next 15 years it is equivalent to an additional 12GWp generating capacity without building any additional manufacturing capacity or generating any additional greenhouse gases. The equivalent value in today's costs of this is over twenty billion US dollars. In addition, if successful, the project outcomes are almost certain to be applicable to silicon based third generation cells which in the long term are likely to replace existing technologies. Commercial beneficiaries will be the UK and European companies to which the development is licensed who, in consequence, will have an improved competitive advantage. These are both cell manufacturers and solar materials manufactures. The general area of oxygen reactions in silicon is also one of very considerable interest to the microelectronics community as well as those specializing in photo-voltaics. The properties and structure of the recombination centres, the diffusion of the oxygen dimer are issues which are highly relevant to many semiconductor devices. Specific beneficiaries beyond solar are: a) Integrated circuits and b) Silicon power devices. Two of the world's major research establishments in Solar have provided letters of support for this proposal. The Fraunhofer-Institut fr Solare Energiesysteme ISE in Freiburg and the National Renewable Energy Laboratory, Golden, Colorado.

Publications

10 25 50
 
Description One of the most important early life degradation mechanisms in silicon solar cells has been correlated with the presence of boron and oxygen. It has been proposed prior to our work that a recombination centre formed by the reaction of boron and two oxygen atoms is responsible for the degradation. The mechanism of formation was proposed as being due to the migration of the oxygen dimer via the Bourgoin-Corbett mechanism. This can only occur as a result of a change in charge state of the dimer ena
Exploitation Route The findings are of considerable importance to manufacturers of silicon solar cells and solar cell material in the sense that hopes of a quick fix to the boron oxygen degradation problem now seem ill founded. We know of one manufacturer who is developing a technique for controlling the doping gradient in gallium doped ingots by using continuous addition of feedstock during ingot growth and our work has given a boost to those companies who re trying to develop a solar cell process using an n-type
Sectors Electronics,Energy,Environment

 
Description Elimination of Efficiency Degradation Mechanism in Silicon Photovoltaic Cells
Amount € 268,000 (EUR)
Funding ID EP/H019987/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 02/2010 
End 07/2012
 
Description SupersiliconPV: extending the limits of materials performance (Supergen award)
Amount £350,000 (GBP)
Funding ID EP/M024911/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 09/2015 
End 11/2018
 
Description NREL 
Organisation U.S. Department of Energy
Department National Renewable Energy Laboratory (NREL)
Country United States 
Sector Public 
PI Contribution We have measured advanced multi crystalline Si PV material as part of an information exchange process with NREL.
Collaborator Contribution NREL have supplied specialist advanced material to Manchester and have hosted several visits by us.
Impact Publications, invitations to conferences etc.
Start Year 2010
 
Description North Carolina 
Organisation University of North Carolina at Chapel Hill
Country United States 
Sector Academic/University 
PI Contribution We have provided specialist measurements to a consortium led by North Carolina which include Industrial Labs.
Collaborator Contribution North Carolina and associates have carried out specialist electron microscopy in support on Manchester based research.
Impact Numerous publications and invited presentations at major PV meetings.
Start Year 2012
 
Description Si PV Fraunhofer 
Organisation Fraunhofer Society
Department Fraunhofer Institute for Solar Energy Research
Country Germany 
Sector Charity/Non Profit 
PI Contribution We have interacted and advised on transient capacitance spectroscopy
Collaborator Contribution The Fraunhofer have advised us on specialised currentspectroscopy.
Impact The key outcomes of this collaboration are improved experimentation and diversification of characterisation methods.
Start Year 2009