Stephenson Institute for Renewable Energy
Chadwick Building
The University of Liverpool
Liverpool
L69 7ZF
Tel: 0151 795 9048
Director of SIRE and leader of an expermental group working on solar energy materials and solar cells. Director of the EPSRC Centre for Doctoral Training in New and Sustainable Photovoltaics. Also leader of the University of Liverpool Energy Research Theme. Research interests include thin film materials for PV, solar cell devices, transparent conductors and advanced materials and device characterisation. Durose joined the University of Liverpool in 2011.
As a Research Associate in the PV Materials and Devices Lab, his current research interests include the development of CZTS solar cells and fundamental investigations into the properties of earth abundant solar cell materials.
Thin film solar cells – new and advanced materials, devices, transparent conductors and nano-wire solar cells.
Prof Ken Durose leads this research group in the Stephenson Institute for Renewable Energy. We concentrate on the physics and chemistry that will result in improvements in the cost and sustainability of solar electricity.
For a brief history of the PV Materials and Devices Laboratory, click here.
The material below is intended as an introductory tutorial for some of the concepts of thin film solar cells and was written by Paul Edwards.
Faced with ever-increasing demand, the earth's sources of non-renewable energy are not expected to last long. Among the many contenders vying to replace fossil fuels, photovoltaic solar cells offer many advantages, including needing little maintenance and being relatively "environmentally-friendly"; the major drawback to date has been cost. In order for photovoltaics to be viable for large-scale energy conversion, their efficiency must be improved whilst making them cheaper.
In its simplest form, the solar cell consists of a junction formed between n-type and p-type semiconductors, either of the same material (homojunction) or different materials (heterojunction). The bandstructure of the two differently doped sides with respect to their Fermi levels can be seen in Figure 1.
Figure 1: Band structure of differently-doped semiconductors
When the two halves are brought together, the Fermi levels on either side are forced in to coincidence, causing the valence and conduction bands to bend (Figure 2).
Figure 2: Heterojunction band-bending
These bent bands represent a built-in electric field over what is referred to as the depletion region. When a photon, with an energy greater than the bandgap of the semiconductor, passes through the solar cell, it may be absorbed by the material. This absorption takes the form of a band-to-band electronic transition, so an electon/hole pair is produced. If these carriers can diffuse to the depletion region before they recombine, then they are separated by the electric field, causing one quantum of charge to flow through an external load. This is the origin of the solar cell's photocurrent, and is shown in Figure 3.
Figure 3: Principle of photovoltaic device
Advantages of CdS/CdTe
Currently, the semiconductor most widely used in solar cells is single-crystal silicon. Because of the cost involved in producing the bulk material, cells produced by this method are prohibitively expensive for all but the smallest scale or most specialised applications (such as on calculators and satellites). Higher efficiencies have been produced by using single-crystal III-V semiconductors and more elaborate constructions (e.g. multi-quantum wells), but this advantage has always been more than offset by the resultant increase in cost.
The thin-film cadmium telluride / cadmium sulphide solar cell has for several years been considered to be a promising alternative to the more widely used silicon devices. It has several features which make it especially attractive:
A concern often expressed about CdS/CdTe solar cells is the effect on health and the environment of the cadmium used. However, the thinness of the films means that the amount of active material used is relatively small; it has been estimated that even if CdTe solar cells were to provide more than 10% of the world's energy requirements, this would still only account for less than a tenth of the world's cadmium usage . To put the risk into perspective, B.P. Solar modules have been reported to have passed the appropriate U.S. Environmental Protection Agency tests, whereas fluorescent tubes (containing mercury) and computer screens (containing lead) do not.
Cell construction
The CdTe/CdS solar cell is based around the heterojunction formed between n-type CdS and p-type CdTe. The basic composition of the cell can be seen in Figure 4.
Figure 4: CdS/CdTe solar cell (not to scale)
The functions of the different layers are as follows:
Since the active layers of the device are those on top of the glass substrate, this construction is referred to as a superstrate configuration.
Deposition techniques for CdTe/CdS solar cells
The polycrystalline layers of CdS and CdTe can deposited by a number of different methods, including, amongst others, those outlined below:
2CdTe(s) = Cd(g) + Te2(g)
The source is of a large area and is positioned close to the substrate. The substrate is maintained at a high temperature (but below that of the source) such that the elemental vapours will not become deposited on the substrate but the compound form will, due to its lower equilibrium vapour pressure.
Cd(OH)2 = Cd2+ + 2(OH)-
and sulphide ions from an alkaline aqueous solution of thiourea:
(NH2)2CS + OH- = CH2N2 + H2O + HS-
HS- + OH- = H2O + S2-
The aim of our research is to make a contribution to low cost and sustainable PV through improvements to the physics and chemistry of the materials and devices. It is a key feature of our lab that we make devices routinely, and that our ideas are translated to lab scale prototypes. Ongoing research themes are listed below, while details of our funded projects and intellectual property listed in line.
Ken Durose is listed by EPSRC as PI for the UK SUPERGEN PV-21 team project on thin film PV materials and devices.
The lab has achieved the following recent results – all are in-house measurements:
16.5% CdTe/CdS CSS-grown CdTe - on soda lime glass (5*5 mm2 contacts).
13.6% CdTe/CdS CSS-grown CdTe - on soda lime glass (10*10 mm2 contacts).
12.4% CdTe/CdS all-sputtered cell with our own TCO - on soda lime glass (5*5 mm2 contacts).
In addition to routine J-V and EQE characterisation the lab undertakes C-V profiling and frequency response analysis of thin film solar cells and materials.
We grow ZnO, SnO2, ITO and dielectrics for use in solar cells. A number of studies to control doping in the sputtering environment have also been made. In addition, both transmission and ellipsometry measurements have been used to extract dispersion parameters of key layers in solar cells, and these have been used in modelling the design of layer stacks to optimise the optical absorption in absorber layers of solar cells that we grow in the lab.
By sputtering and close space sublimation. We have also undertaken fundamental studies of the mechanisms by which thin films nucleate and grow, and used these to inform strategies to create more efficient devices.
We are using the V-L-S method to grow CdTe nano-wire arrays with a view to incorporating them into core-shell PV devices. Good control of the nano-wires has been achieved with gold catalysed V-L-S growth in the CSS reactor, and coating of them with CdS has been most successful using the CBD method.
Extended defects in semiconductors may have a profound effect on the performance of minority carrier devices, including solar cells. The study of grain boundaries, twins and other defects in bulk and thin film solar materials is an on-going theme of our research. We have particular knowledge of EBIC and electron microscope methods.
Contact Ken Durose for further information.
The PV Materials and Devices Laboratory of the Stephenson Institute is housed in the Chadwick building at theUniversityofLiverpool.
Three chambers of mostly RF magnetron sputtering for thin film deposition of solar energy materials and solar cells. The systems were supplied by AJA International in theUSA.
Chambers 1 and 2 are connected and allow a full CdTe/CdS/TCO/glass solar cell structure to be fabricated in the same system. We have made cells with 5 x 5 mm2 contacts with up to 12.5% efficiency (in-house measurement) using this kit.
We have four chambers for CSS deposition in two kits, they are dedicated to solar energy materials including complete PV devices and nanowire structures. The kits were built specially for us by Electro-Gas Systems Ltd,UK.
Our kits have the following features:
Oriel solar simulator with approximate AM1.5 spectrum and 100W/m2 illumination.
External Quantum Efficiency measurement system – Bentham.
Solartron 1260 frequency response analyser with dielectric interface. Used for
Janis closed cycle He cryostat with computer controlled Keithley electrical measurement system. J-V-T measurement gives information about the current transport mechanism.
Shimadzu Solid Spec 3600 UV –Vis– IR spectrophotometer
Capable of measuring 30 x 30 cm2 plates with mapping capability
CODE software – used for:
a) modelling physics of dielectric dispersion and fitting to transmission spectra – this allows the determination of the dispersion of refractive index and extinction coefficient.
b) modelling optical transmission through stacks of thin films and in solar cell design
Ambios XP – 200 stylus profilometer. Used for film thickness measurement and roughness evaluation.
Mr Takahiro Nakao - Tohoku University, Sendai, Japan
Our first visitor, with us during the summer of 2011, Takahiro worked on sputtering thin films of TiO2:Nb
Dr Douglas Halliday – Durham University
Senior Lecturer in Physics. Photoluminescence investigations of thin film solar energy materials and solar cells, especially CdTe. PL spectroscopy and mapping.
Dr Budhika Mendis – Durham University
Lecturer in Durham G J Russell Electron Microscope Facility and member of the Dept of Physics. Specialist in TEM and electron energy loss spectroscopy. Interests include crystal defects, recombination at grain boundaries and extraction of quantitative information from semiconductor materials at the nano-scale.
Mr Leon Bowen – Durham University
Experimental Officer in electron microscopy at the Durham G J Russell Electron Microscope Facility atDurhamUniversity. Expertise includes high resolution SEM, EDX and quantitative analysis, EBIC evaluation of solar cells, FIB cross sectioning and TEM sample prep by FIB.
Dr Mohammed Alturkestani - Umm Al-Qura University, Taif branch,Saudi Arabia
Lecturer in Physics. Research interests include advanced electrical characterisation including J-V-T, parameter extraction from solar cell devices, rapid screening device methodologies, role and control of key interfaces in thin film solar cells. Barrier height measurement of electrical contacts to thin film solar cells.
Drs Murat and Habibe Bayhan – Mugla University Turkey
Murat Bayhan: Advanced electrical characterisation of thin film solar cells, especially by J-V-T measurement. Extraction of optical constants from PV materials from modelling of transmission measurements.
Habibe Bayhan: Impedance analysis and frequency response analysis of thin film solar cells.
Our research team welcomes the best talent regardless of your background.
Jobs
By European law, all jobs must be advertised in an open competition. We always advertise on the website www.jobs.ac.uk and the University HR site http://www.liv.ac.uk/working/jobvacancies/currentvacancies/research/.
Research Visitors
We are able to welcome a small number of research visitors, e.g. visiting academics for sabbatical and research leave. However, we do not have funds to support such visits. Please do contact Ken Durose to discuss any proposals.
PhD places – overseas students (outside the EU)
We welcome applications for PhD places from overseas students, but please understand that we do not have any funds to support your travel, living expenses or tuition fees. Also, you will need to achieve the required standard of English before travelling to the UK in order to do research at the highest level. Your interview will be in English. You will need to achieve a high standard in your first degree to be eligible. To get the best from a PhD opportunity you must have a genuine commitment to advanced research, and the ability to work both alone and in as part of a team. Applicants with funding should make first enquiries to ken.durose@liverpool.ac.uk or apply direct to http://www.liv.ac.uk/study/postgraduate/applying/index.htm.
PhD places – UK and European students
We host the EPSRC Centre for Doctoral Training in New and Sustainable PV and we regularly have PhD positions available for October entry. Please see www.cdt-pv.org for details. The eligibility criteria for the funding is that there is full funding (tuition fees and living expenses) for UK students and EU students who have spent the previous three years in the UK. For other EU students, we can pay the tuition fees, but not the living expenses. If you wish to apply for a PhD then you may apply directly through http://www.liv.ac.uk/study/postgraduate/applying/index.htm. For further information on projects that are available please see www.cdt-pv.org.
Internships and summer studentships
We do not regularly offer internships and have no internship scheme. If any vacancies become available, they will be advertised here on this website. If there are none advertised then there are none available.
Undergraduate research projects at Liverpool
Opportunities for UoL Physics students to join us for their level 4 projects will be advertised through Departmental channels.
At the Stephenson Institute we focus on research for future renewable energy, but we are often asked some more general questions about solar energy – here’s the answers to the questions that we hear most.
There are two common kinds of solar panels that you can buy for domestic use: 1. Water heaters which are used to boost your domestic power and 2. Solar electricity generating panels – these are photovoltaic or PV.
Londongets jut over 40% of the energy from the sun as the sunniest places on Earth, such as Ethiopiaand the Australian desert. Its true that ‘the more light the better’, but it doesn’t have to be direct sunshine – if its cloudy then PV still works and any light is good light. It has been said that if you can see the solar panels, then they are working. Sunlight energy falling on the Earth is known as insolation and is measured in kW-hrs/m2/day averaged over a whole year, night and day. So with an insolation level of 2.5 kW-hrs/m2/day, the kind of level you might expect in theUK,each square meter receives 2.5 x 365 = 900 kW.hr in a year – for free.
|
Location |
Insolation kW.hrs/m2/day |
|
Australia |
5.89 |
|
England(London) |
2.61 |
|
Scotland(Edinburgh) |
2.26 |
|
Finland(Helsinki) |
2.41 |
The UK Feed In Tarriff works like this: You install a PV system and it generates power in your home. You use that power, but you also get paid a premium for having generated the power in a pioneering and eco-friendly way. The government has obliged power companies to pay you at a rate of 42p per unit, when you would normally buy it from them at 13.5p per unit (2011). That way you can expect a return on your investment at the level of 2 - 3 times over the lifetime of the solar cells. TheUKhopes to benefit from this by stimulating a PV industry (supply chain for the manufacture of solar cells, inverters, installation companies etc). There is also a wider benefit from the early adoption of ‘green power’.
Here at the Stephenson Institute we are scientists working on the next generation of lost cost sustainable solar cells – you can’t buy them in the shops yet! We’re experts on PV science in the lab, so we’d recommend that you contact a PV installation company for advice about domestic installations. We can’t recommend a particular installer, but we would advise you to get several quotes and some independent advice that you feel that you can trust. Generally you’ll need a south facing roof, a house in good structural condition, and no plans to move home in the immediate future – these will all help.
Some people predict that mostly in theUK, solar electricity will be generated from small scale rooftop installations. As much as 10 – 20% of national demand might be satisfied in this way.
Solar electricity (PV) is set to become a major industry worldwide. We need to be part of that in a European and worldwide context.
There are many – here’s a selection:
Prof Ken Durose
Stephenson Institute for Renewable Energy
University of Liverpool
Chadwick Building
Peach St
Liverpool
L69 7ZF
United Kingdom
+44 (0) 151 795 9048
The Air Mass (AM) value describes the spectrum (but not necessarily the intensity) of sunlight at a particular latitude. It is defined as the distance through the atmosphere that the light from the sun travels in order to reach the solar cell. This is expressed relative to conditions at the equator, where the sun is almost directly overhead, and where the light is therefore described as AM1. Thus in space, with no atmosphere, the spectrum is referred to as AM0.
The path length, in units of Air Mass, changes with the zenith angle
For most terrestrial applications, the generally accepted solar cell testing standard is that of AM1.5 conditions. In addition, it is usual to also specify the intensity of the light, integrated over the spectrum, as being 100mW.m-2. When using solar concentrators, for example, the intensity might be increased by a factor of 1000, but the shape of the spectrum would remain AM1.5.
The group has its origins at the University of Durham from where Ken Durose moved to Liverpool in March 2011.
The Department of Applied Physics and Electronics was founded in 1960 at Durham by Prof D A Wright and Dr John Woods who moved there from GEC. Its ethos was to educate students in electronics and also in the physics necessary to both understand electronic devices and to deploy electronics usefully in science, engineering and society. Although small, the department quickly established a reputation for the quality of both its teaching and research. Woods’ personal research interests included semiconductors, and in the early 1960s the landscape for electronic materials was considerably less certain than at present. Germanium was arguably the best understood material, and the III-VIs (GaAs etc) and II-Vis were considered to have untapped potential. Wood specialised on the understanding of the electrical properties of II-Vis, in the early days the work being focussed on CdS and trapping. However, he quickly realised that the development of the physics of these materials was limited by the availability of single crystals. He therefore initiated an influential crystal growth theme inDurham, that had far-reaching consequences. For those II-VIs with high melting points, vapour- rather than melt-growth was seen as the way forward, and the lab instigated a two-zone furnace method with stoichiometric control that was unique toDurham. It was used for CdS, ZnS, ZnSe, CdTe and the mixed crystals CdZnTe. CdZnS, ZnSSe and others, and these materials provided fertile ground for a 30 year stream of research ideas and PhDs.
In the 1970s the group became home to Dr Graham Russell, who ran the university’s materials electron microscope facilities and developed a special expertise in semiconductor microscopy. During his career he promoted an energetic and enthusiastic approach to research in both materials and microscopy for which he was recognised by the naming of the G J Russell Electron Microscope Facility in 2010.
1980 saw the arrival of two newcomers to Durham, Dr Andy Brinkman, a new academic member of staff, and Ken Durose, then a student of Applied Physics and Chemistry. Andy was a multi-talented research leader with a background in electronic engineering and thin film crystal growth. He went on to develop research themes in deep levels, thin film epitaxy, electroceramics, bulk crystal growth, solar cells and environmental physics. It is he who was responsible for the CdTe bulk crystal growth research that gave rise to the spin out company Durham Scientific Crystals, now Kromek, a company specialising in detector elements and systems for security and medical applications.
Having trained with Graham Russell, Ken Durose returned to the Department to take an academic position after three years at British Telecom Research Labs and following the untimely death of Graham in 1989. In the following years Ken developed research themes in semiconductor electron microscopy, especially in special modes for the SEM, in thin film growth and solar cells. The latter became a major theme, one which migrated to the Stephenson Institute en bloc in March 2011.
Call the department
+44 (0)151 795 8137
