PV Materials and Devices Laboratory
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.
How PV works
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.
Background to photovoltaics
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.
Principle of p-n junction solar cell
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
The CdS/CdTe solar cell
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:
- The cell is produced from polycrystalline materials and glass, which is a potentially much cheaper construction than bulk silicon.
- The chemical and physical properties of the semiconductors are such that the polysilicon thin-films can be deposited using a variety of different techniques (see below).
- CdTe has a bandgap which is very close to the theoretically-calculated optimum value for solar cells under un-concentrated AM1.5 sunlight.
- CdTe has a high absorption coefficient, so that approximately 99% of the incident light is absorbed by a layer thickness of only 1µm (compared with around 10µm for Si), cutting down the quantity of semiconductor required.
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.
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:
- Glass The solar cell is produced on a substrate of ordinary window glass, because it is transparent, strong and cheap. Typically around 2-4 mm thick, this protects the active layers from the environment, and provides all the device's mechanical strength. The outer face of the pane may have an anti-reflective coating to enhance its optical properties, but this is not the case for production mdels
- Transparent conducting oxide Usually of tin oxide or indium tin oxide (ITO), this acts as the front contact to the device. Some research devices use other TCO materials. Most usually, the TCO is a multi-layer, and may contain a ‘high resistive transparent’ layer as the junction partner to the CdS window
- Cadmium sulphide window layer The polycrystalline CdS layer is n-type doped (as CdS invariably is), and therefore provides one half of the p-n junction. Being a wide band gap material (Eg ~ 2.4 eV at 300K) it is transparent down to wavelengths of around 515 nm, and so is referred to as the window layer. Below that wavelength, some of the light will still pass through to the CdTe, due the thinness of the CdS layer (~ 100 nm).
- Cadmium telluride absorber layer The CdTe layer is, like the CdS, polycrystalline, but is p-type doped. Its energy gap (1.5 eV) is ideally suited to the solar spectrum, and it has a high absorption coefficient for energies above this value. It acts as an efficient absorber and is used as the p side of the junction. Because it is less highly doped than the CdS, the depletion region is mostly within the CdTe layer. This is therefore the active region of the solar cell, where most of both the carrier generation and collection occur. The thickness of this layer is in the range 1 - 10 µm, with most labs growing in the lower half of this range.
- Back contact. P-type CdTe is a notoriously difficult material on which to produce an Ohmic contact, and so the junction will inevitably display some Schottky diode (rectifying) characteristics. In the research lab we use gold, which due to its high conductivity, needs only be a few tens of nanometres in thickness. Prior to gold deposition, the CdTe is etched to yield a Te-rich surface, which is considered to give superior contacts – at least by some labs. Industrially, contacts are a closely guarded secret. Most labs and probably as many industries use copper telluride compositions at the back surface of the CdTe, and they go to some effort to stabilise the copper against diffusion.
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:
- Physical vapour deposition (PVD) (or evaporation) involves the vaporisation in a vacuum of a source of either the compound (CdS or CdTe) or the separate elements (Cd + S or Cd + Te). The resulting vapours recombine on the surface of the substrate (which can be heated, but is still much cooler than the source) to deposit the required polycrystalline material. The stoichiometry of the deposited layer is difficult to control accurately, as it depends strongly on the equilibrium vapour pressures of the elements, as well as the stoichiometry of the source material .
- Close-space sublimation (CSS), which has been used to produce the highest efficiency cells so far , is based on the reversible dissociation of the materials at high temperatures e.g.
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.
- RF magnetron sputtering, in which a solid target is eroded by the action of an RF plasma and the products impinge on a substrate to form a film. RF sputtering offers excellent control of thickness uniformity. This is particularly used for CdS for which the control of thickness and the integrity of the films with respect to pinholes are of special importance, the films being ~100 nm thick.
- Close-space sublimation (CSS),is similar to PVD, but in the case of CSS the source is contained in a tray having the same dimensions as the substrate. The source – substrate are closely spaced – usually just a few mm apart, and this conventionally adopted name has stuck, even though it doesn’t sound quite right in English. CSS is widely used in industry and in the research labs making the most efficient CdTe solar cells. However First Solar use a method in which CdTe is powderised and then entrained in a hot gas stream that is directed at the moving substrate.
- Chemical vapour deposition (CVD) can also be used to deposit both semiconductors. It involves pyrolysis of precursors on the substrate on the substrate to form the compound. One variation of this method, Metal-Organic CVD (MOCVD), uses metallo-organic precursors: this method is widely used in III-V technology as it allows control of solid solution compositions and doping profiles. At the time of writing just one research lab uses MOCVD for CdTe solar cells.
- Chemical bath deposition is sometimes used for depositing CdS films, and involves producing the required ions in a solution by chemical means, which combine and precipitate out onto the substrate if the required equilibrium conditions are met. For example, cadmium ions can be produced by the hydrolysis of Cd(OH)2:
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-
- Electrodeposition may also be used to deposit many semiconductor materials at low temperature from solution. CdTe has been made in this way, but attempts to commercialise it ultimately failed.
Research in PV materials and devices
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.
High efficiency thin film device fabrication
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).
Advanced electrical characterisation
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.
Transparent conducting oxides/optical modelling
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.
Thin film crystal growth
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.
Nanostructure growth and core-shell PV 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.
- SUPERGEN PV-21 is a UKnational programme on thin film and inorganic PV materials and devices. The overall aim is to make a significant contribution to low cost PV. Team members include the universities of Bath, Cranfield, Edinburgh, Glyndwr, Imperial College, Liverpool, London South Bank, Northumbriaand Southampton. The PV-21 web site is pv21.org
- SOBONA is a network action under the FP7 IRSES Marie Curie PEOPLE scheme. It focuses on Solar Cells By Nanowire Arrays.
Contact Ken Durose for further information.
- Grain size control in CdTe films – a method to increase the grain size and hence increase the efficiency of PV devices. Filed – commercial relationship developed.
- Increased doping level in CdTe for PV devices – processing methodology that increases the effect of chloride doping and has a profound effect on the efficiency of PV devices. Filed.
- Method of increasing process yield in CdTe PV – to be announced.
Facilities and methods
The PV Materials and Devices Laboratory of the Stephenson Institute is housed in the Chadwick building at theUniversityofLiverpool.
Sputter deposition of thin films
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.
- Chamber 1 Transparent conductors and dielectrics. Used for ZnO, SnO2, SiO2, TiO2. and doping oping method exploration.
- Chamber 2. Solar absorber and window layers. Used for CdTe and CdS i.e. heterojuncions for solar cell devices.
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.
- Chamber 3. Contacts and metallisation. Used for Mo, Te, dry etching, Sb2Te3 and As2Te3.
Close space sublimation (CSS) deposition
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:
- 5 x 5 cm2 substrates and source trays
- Substrate heating up to about 600°C
- Choice of gas ambient for N2 or H2 or O2 and mixtures
- Safety interlocks
- Gas pressure control in the range 2 mBarr to atmospheric pressure
- No metallic components in the growth chamber to limit contamination
Solar cell performance measurement (AM1.5)
Oriel solar simulator with approximate AM1.5 spectrum and 100W/m2 illumination.
External Quantum Efficiency measurement system – Bentham.
Frequency response analysis
Solartron 1260 frequency response analyser with dielectric interface. Used for
- C-V measurements
- Equivalent circuit analysis of devices and materials
- Spectroscopy of levels in the band gap of semiconductors in devices – thermal admittance spectroscopy
Temperature dependent current – voltage measurement (J-V-T)
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.
Collaborators and visitors
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.
Jobs and studentships
Our research team welcomes the best talent regardless of your background.
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/.
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 firstname.lastname@example.org 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.
FAQs about Solar Energy in the UK
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.
What does Photovoltaic mean?
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.
Is there enough sunlight in the UK to make it worthwhile investing in solar power here?
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.
What subsidy is available for domestic installations in the UK?
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’.
How can I find out more about installing a domestic PV system?
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.
How much power can theUKexpect to get from solar?
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.
Why do solar electricity research in the UK?
Solar electricity (PV) is set to become a major industry worldwide. We need to be part of that in a European and worldwide context.
What companies are active in PV in the UK?
There are many – here’s a selection:
- Sharp – module assembly
- Pilkington – glass supply to PV industry world wide
- Dyesol – solar cell maker
- G21i – solar cell maker
- Crystallox – major supplier of multicrystalline silicon worldwide
- Sial – supplier of chemicals for the PV industry (III-V)
Contact for PV research
Prof Ken Durose
Stephenson Institute for Renewable Energy
University of Liverpool
+44 (0) 151 795 9048
AM 1.5 spectrum
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.
A brief history of the PV Materials and Devices research group
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.