A summary of Photovoltaic (PV) technology in Wales; part of the Low Carbon Revolution.

Dr Tracy Sweet Cardiff University, 2010.

Page 1 27/09/2010

Executive Summary

The UK is on the verge of an energy transformation. The 2008 Climate Change Act [1], created a legislative framework for the effective management and delivery of policies to tackle climate change, has set ambitious reductions in carbon dioxide emission. At least 34% from 1990 baseline levels is required by 2020, with 80% reduction required by 2050 [2]. The Low Carbon Revolution outlines a move to resilient low carbon energy production via secure, indigenous renewable sources, on both a centralized and localized basis within Wales [3]. It is clear that no single technology provides the complete answer, but maintaining a portfolio of technologies increases society’s chances of identifying the best options. Photovoltaic technology can make a significant contribution [4]. It has been estimated that the annual solar energy resource in Wales supplies between 2.6 to 2.8TWh /km2 of Solar Energy. Estimates from the Low Carbon Research Institute (LCRI) state that achievable solar electricity generation rates could provide up to 20% of Wales’ total electricity needs by 2050 [5]. The benefits of photovoltaic technology are that the running costs are extremely low in comparison with other renewable sources, there are no moving parts, no fuel requirements and minimal maintenance is required. However, systems are currently relatively expensive to purchase (£10,000 for 3kW system) [6]. Traditionally within the UK there has been comparatively low-level of adoption of PV. In contrast, the worldwide photovoltaic industry has reported growth rates of 30-40% per year since 1996 [7] largely due to government-subsidized, grid-connected, residential rooftop programmes. Most manufacturing capacity growth has been met by investment in traditional silicon wafer-based technology. The UK government incentive (Low Carbon Buildings Programme installation grant) was withdrawn (February 2010) in light of implementation of the feed-in-tariff (April 2010). Consumers now receive payment for micro-generation (up to 41.3p/kWh for <4kW retrofit or 36.1p/kWh for <4kW on a new build). This is free from income tax and guaranteed on installations until 2012) [8]. Using Germany as a case study, government investment has made a region of East Germany grow into a sector leader in PV technology. It is therefore anticipated that the photovoltaic industry within the UK will become more standardized and efficient as demand increases, making the economics of adoption much more favourable. Within Wales, further investment in fundamental research, development and characterization of photovoltaic materials will contribute enormously to future industrial and governmental targets. Interdisciplinary research efforts are extremely important within the field, from fundamental physics through to material growth technology and development, testing and device fabrication (scaling up from modules to arrays) through to the economic and social science of public acceptance. Given the current levels of environmental, political and economic pressures of energy supply, with sufficient strategic investment of time and resources, photovoltaic technology has significant potential to contribute strongly to the Low Carbon Revolution in Wales.

Page 2 27/09/2010

Table 1 – list of contents

Section Description & hyperlink PageTitle Report title 1Executive summary Executive summary 2Contents Table 1 – list of contents 31 Introduction Figure 1 – Sun star UV image from space 42 Fundamentals Equation 1 - Planck’s equation 4 Figure 2 Typical solar spectrum 5 PV test conditions 5 Table 2 – Standard PV test conditions 6 Equation 2 – Fill Factor equation 7 Equation 3 – Conversion Efficiency equation 73 Current technologies First generation technology Monocrystalline Silicon 7 Polycrystalline Silicon 7 Second generation technology Thin Film Amorphous Silicon 8 Thin Film Compound Semiconductors 8 Third generation technology Compound Semiconductor - Concentrating PV (CPV) 10 Table 3 – Key semiconductor materials properties 12 Fourth generation technologies Nanotechnology – DSC technology 12 Organic / Polymer Solar Cells 134 Technology comparison Figure 3 – NREL best research-cell efficiencies timeline 2010 14 Figure 4 – Data-sort on best photovoltaic efficiencies measured 155 Photovoltaic Industry Table 4 – PV industry, HQ or supply-chain connection to Wales 176 Conclusions Conclusions 187 Acknowledgements List of acknowledgements 188 References References 19

Page 3 27/09/2010

1 Introduction

Solar radiation is an inexhaustible, clean, renewable source of energy necessary for life on Earth. The sun converts, via nuclear fusion, Hydrogen to Helium at an estimated rate of 4 million tonnes per second. Approximately 70% of the Sun’s energy reaches Earth as light and heat.

The sunlight which hits Earth is polychromatic comprising of; Ultra-violet ( ≤350nm) Blue ( = 351 to 499 nm) Green ( = 500 to 649 nm) Red ( = 650 to 999 nm) Short-wave infra-red ( = 1000-2000nm)

Figure 1 - Sun star, UV image from space

The term Photovoltaics was derived from combining the Greek word for light, Photos, with Volt, the name of the unit of electromotive force. Photovoltaic is a term which literally defines electricity generation from light (solar photon energy). Essentially, solar energy is absorbed by a combination of semiconductor materials which have a p-n junction (diode) enabling electrons to flow in one direction only. This flow of electrons creates direct current (DC) electricity which can either be stored in batteries or converted into alternating current (AC) electricity by utilizing an inverter for input to the national grid. Tremendous advances in the technology have been made by research into the nature and property of light and the mechanisms of photon energy conversion from its humble discovery in 1839 by Alexandre Edmond Becquerel. Space exploration programs of the 1960s pushed the boundaries of photovoltaic technology and provided the basis for the photovoltaic technology in operation today. In recent years growth of the photovoltaic industry has been largely facilitated by the rapid expansion of the semiconductor and electronics markets providing both experience and excellent material quality availability. Three areas of photovoltaic research & development currently active are;

1) To manufacture solar cells cheaper & more efficient to compete with other energy sources.

2) Develop new technologies based on new solar cell architectural designs. 3) Develop new materials to serve as light absorbers and charge carriers.

2 Fundamentals

Planck’s equation describes the relationship between energy and wavelength;

Energy = hc/ Equation 1

h= Planck constant (6.62606896×10−34 Js); c=Speed of light in a vacuum (2.9979 x 108ms-1) Wavelength (m)

Page 4 27/09/2010

As Planck’s equation contains two constant terms (h and c), it is clear that energy has a reciprocal relationship with wavelength – as wavelength increases, energy decreases. Total solar radiation can approach a power density of 7kWhm-2 per day during the summer and ~2kWhm-2 per day during the winter in Southern Europe. This means that useful amounts of solar energy are received for the entire year. In Northern Europe solar radiation can reach 5kWhm-2 per day during summer. This drops to around 10% (0.5kWhm-2 per day) during winter; 50% of the solar radiation is diffuse - created by the sun’s rays hitting the atmosphere & scattering the light. The reduction in solar irradiation during winter months therefore limits the useful output of photovoltaic technology in Wales for approximately one-third of the year. This fact emphasizes the importance of conversion efficiency for PV devices – the maximum amount of energy has to be harvested from solar irradiation for the entire year for both practical & economic purposes. At the Earth’s surface the various gases in the atmosphere (N2, O2, H2O, CO2, O3. etc) attenuate the solar radiation selectively at different wavelengths. Solar spectra are highly variable - according to both the time of year & climatic conditions. Figure 2 illustrates a typical solar spectral power distribution according to wavelength. This graph also shows the areas of the solar spectrum converted by crystalline silicon into electricity (green area). It is clear that as wavelength increases from ultra-violet (UV) to infra red (IR) so the available energy from the spectrum of sunlight decreases.

Figure 2 - Solar spectrum [9]

Paired electrons constitute chemical bonds between atoms. Any electron within a material has either a minimum (low) energy when bound, or it gains enough energy to become a “free” flowing electron. This minimum energy to become a free electron is the "band gap" of the semiconductor material; only higher spectral energy than the material band gap can be utilized for PV energy production. The band gap of Silicon (1.1eV) limits the absorption of solar energy to wavelengths of <1100nm. Photons incident on the surface of a semiconductor will be either reflected from the surface, absorbed in the material or, failing either of the above two processes, will be transmitted through the material. Reflection and transmission are loss mechanisms as photons which are not absorbed do not generate power.

Page 5 27/09/2010

To obtain the maximum conversion efficiency of light into electric power by a PV device, the band gap of the material has to be well-matched to the spectrum of light incident upon it. Semiconductor materials with band gaps of between 1.0-1.5eV meet this requirement. Approximately 50% of available solar energy is not converted into electrical energy by conventional crystalline silicon. IR radiation (heat) in the spectrum remains unutilized. A combination of techniques, PV + Thermal Energy Storage (TES) via Phase Change Materials (PCM) is currently being extensively investigated in many Chinese Universities (Xi’an Jiaotong, Sun Yat-Sen). The Research focuses on PCM (inorganic salts) to enable cooling to the rear side of PV devices during operation to assist in keeping device operating temperatures as close to optimum as possible, maximizing conversion efficiency.

PV test conditions

The performance of PV cells and modules are measured under standard test conditions tabulated below.

Parameter Condition Temperature of the cell 25oC Solar radiation on the cell - power density 1000 Wm-2 Spectral power distribution Air Mass* 1.5 (AM 1.5)

* Defined as 1/Cosθ (ratio of the path length of the sun’s rays through the atmosphere when the sun is at a given angle (θ) to the zenith, to the path length)

Table 2 – Standard PV test conditions

In practice, the power rating in peak watts (Wp) of a cell or module is determined by measuring the maximum power it will supply when exposed to radiation from lamps designed to produce the AM1.5 spectral distribution at 1000Wm-2 total power density.

Cardiff University hosts UKAS accredited Solar Energy Testing Service (SETS) which performs environmental solar radiation testing of electrical, electronic and energy devices. The efficiency of commercially available modules is lower than that of individual cells in the laboratory. The reasons for this are; It is difficult to mass-produce the product to the same standards as a one-off cell. Laboratory cells are not usually glazed or encapsulated. There are inactive areas in a module, between the cells and due to the inactive frame. Small resistive losses in the wiring between cells & in the diodes used as short circuit

protection. Losses due to mismatching between cells of different electrical characteristics

connected in series. The overall device efficiency is the product of individual efficiencies. These parameters can not all undergo direct measurement; the important characterization parameters are;

Integral photocurrent density (iph) Open-circuit voltage (Voc)

Page 6 27/09/2010

Short-circuit current (Isc) Fill factor (FF) - the ratio of the maximum power point (Pm) divided by the open circuit voltage and the short circuit current as shown in equation 2.

FF = Pm /( Voc x Isc) Equation 2

The intensity of the incident light (Is = 1000 W/m2) Complete current-voltage (I-V) curve Maximum power point (Pmp) Cell temperature (oC)

The overall efficiency (ηglobal) of the photovoltaic cell is calculated as shown in equation 3;

ηglobal = (iph x Voc x FF) / Is Equation 3

Many UK-based companies send material overseas for testing and device feedback. The test conditions applied overseas may not be rigorous, accurate or repeatable and add significant delays to development programs.

3 Current Technologies

Silicon.

Monocrystalline Silicon was the dominant photovoltaic technology until 2006. The element is manufactured in extremely pure form with a single, continuous cubic crystal lattice structure having virtually no defects or impurities. The Czochralski process [10], developed for the electronics industry, is used to grow the crystals. However, all steps in the manufacturing process are energy intensive (heating to purify the raw material, sand SiO2, and treatment of noxious by-products). The final Silicon product is highly expensive and has a considerable Carbon footprint. To form a PV device, impurity (p- and n-type dopant) atoms are introduced to the silicon to form a p-n junction. This junction acts as a diode allowing electric charge to flow in one direction only. Light generated charge carriers, electrons and holes, flow through the junction in opposing directions, enabling useful electricity generation. Antireflection coatings are applied to the cell, front contacts are formed which are interconnected. The cell is tested, encapsulated and fitted into modules, and finally assembled into solar arrays. Polycrystalline Silicon has been manufactured in recent years. It has lower purity but is less costly and ~5% less efficient than solar-grade Silicon. Polycrystalline Silicon consists of randomly packed “grains” of mono-crystalline Si. Controlled casting of molten polycrystalline Si into cube-shaped ingots is then cut into thin square wafers and fabricated into PV cells. Electrons & holes can recombine at the boundaries between the grains of polycrystalline silicon. Therefore, grain size and orientation (i.e. light penetration) has a considerable impact on cell efficiency.

Page 7 27/09/2010

Thin Film Amorphous Silicon can be used in very thin film form to manufacture solar cells. Amorphous Silicon does not have the long-range ordering and symmetry of crystalline Silicon atomic lattices; the band gap is wider but is less clearly defined. The advantages of amorphous Silicon are summarized below;

It is cheaper to produce than mono-crystalline Silicon. It is a better absorber of light (thinner, cheaper). It is manufactured at lower temperature. Continuous production technologies can be utilized for its production. A range of deposition media (rigid & flexible) are possible Widespread commercial adoption for low efficiency low cost applications Suitable for multi-junction device development

The disadvantages of amorphous Silicon are;

It is less efficient with 9.5% maximum conversion efficiency (see Figure 4) Photo-degradation occurs due to the Stabler-Wronski [11] effect. The defect

density increases with light exposure causing an increase in the recombination current. This reduces the conversion efficiency by ~2% within a few months.

Currently 85% of the global photovoltaic market is Silicon-based and this is predicted to dominate the market for the next 10-20 years [12]; Developments in Silicon photovoltaic technology include;

1) A drive to increase commercial conversion efficiency from 15% to 20%. 2) Reducing Silicon material costs by using thinner wafers (from ~200m to

~100m); higher efficiencies are produced due to shorter electron diffusion distances.

3) Increasing the mechanical stability of the Silicon or develop new handling methods.

4) Recycling reclaimed wafers [13]. 5) Investigating alternatives to Boron (p-type dopant); its use is historical,

conferring high radiation stability in space. 6) Reduction in Oxygen incorporation during the original Silicon ingot growth.

Under illumination Boron-Oxygen complexes reduce device performance (causing 3% drop in efficiency over the first few hours).

7) Improvement of Silicon material quality to extend the lifetime of excited electrons.

8) Increasing the absorption of solar energy into the near-infra red.

Thin Film Compound Semiconductors

Compound Semiconductors are an alternative to amorphous Silicon for thin film technology. Some of the best known technologies are based on the following materials;

Copper Indium Diselenide (CuInSe2 - CIS), 26% efficiency Copper Indium Gallium Diselenide (CIGS), 19.4% efficiency Cadmium Telluride (CdTe), 16.7% efficiency

Page 8 27/09/2010

Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light (CPV). CPV systems focus a large amount of sunlight onto a small area of PV material to generate electricity. Unlike conventional flat panel systems, CPV systems are often less expensive to produce because concentration enables a smaller area of PV material to be used. The band gap of Cadmium Telluride (CdTe) is 1.44eV at 300K which is close to optimum for absorption of sunlight and was historically manufactured via an inexpensive electroplating-type process. The Centre for Solar Energy Research (CSER), Glyndwr University, employ Metal Organic Chemical Vapour Deposition (MOCVD) for thin-film CdTe growth. Photovoltaic conversion efficiency of 13.3% has been achieved. First Solar has produced >1GW of thin film CdTe PV material to date [14], and is a collaborative partner with CSER. One significant drawback of CdTe is that it is toxic upon ingestion, dust inhalation or improper handling (i.e. without appropriate safety precautions). This issue led to companies such as BP solar withdrawing from CdTe production. Brookhaven National Laboratory (BNL) and the U.S. Department of Energy (DOE) nominated Cadmium Telluride (CdTe) for inclusion in the National Toxicology Program (NTP) in 2003. However, once properly encapsulated, CdTe is rendered harmless and the U.S. Department of Energy announced (August 2010) loan guarantees of $400 million to Abound Solar Manufacturing to develop manufacturing facilities for thin-film CdTe solar panels. The technique of CdTe deposition on glass has been developed by Colorado State University and NREL. The amount of semiconductor material used is reported as 100 times less than that required for silicon panels, with a largely automated process. Total CdTe released into the environment during panel production (mining, recycling, production & installation) are reported within permitted limits [15]. PV-21 is the UK Supergen consortium which focuses on the science that will deliver 'making a major contribution to achieving competitive PV solar energy'. The Consortium runs the UK network for PV materials and device research, PV-NET, a forum for the UK academic and industrial research communities [16]. In its initial period of activity, the Consortium put in place lab-scale facilities for making three main types of solar cells based on thin film absorbers at CSER;

1) Copper indium diselenide (CIS) 2) Cadmium telluride (CdTe) 3) Ultra thin silicon.

Future work will focus on both thickness reduction and on finding alternative sustainable low cost materials (absorbers and transparent conductors). To increase conversion efficiency, grain boundaries, nanostructure thin films (to harvest more light) and doping are being investigated alongside surface sensitization of thin film silicon cells. Energy transfer from fluorescent dyes will also be investigated to potentially reduce the required film thickness to as low 0.2 m.

Page 9 27/09/2010

Compound semiconductor - Concentrating Photovoltaic

Concentrating Photovoltaic (CPV) technology uses concentrating optics such as Fresnel lenses (having a large aperture and short focal length) to focus light onto small PV cells. Tracking systems, although costly, can also be employed to follow the trajectory of the sun from sun rise to sun set, maximizing the photovoltaic conversion efficiency. Low concentration systems (2-100 suns) use Silicon cells; higher concentration systems (>400 suns) use higher efficiency multi-junction cells. CPV operate most efficiently in regions of direct sunlight. Because of size and land-cost issues, many power plants utilizing CPV systems are placed far away from the electrical grid. As a result, they are often located far from transmission lines adding to both time and cost for deployment. Multi-junction solar devices provide the highest solar conversion efficiency. Different junctions in the structure are designed with complementary band gap energies. Therefore by increasing the number of heterojunctions (different materials) within the device more of the sunlight spectral energy can be absorbed. Gallium Arsenide (GaAs) is a crystalline compound semiconductor with a cubic crystal lattice structure similar to that of Silicon. The known technological advantages are that;

GaAs has a high light absorption coefficient. GaAs has a wider band gap than Si (1.42 vs 1.12eV); close to the theoretical

optimum for absorbing solar energy. GaAs cells are more efficient than those of mono-crystalline Silicon in converting

photons into electricity. Operation can occur at relatively high temperatures without appreciable reduction

in efficiency. Very small areas of semiconductor crystal are required in CPV, reducing the

Carbon footprint of the technology dramatically. However, the disadvantages are;

GaAs cells are substantially more expensive than those made from Silicon. Production process is less mature. Precursor materials are not abundant & are expensive GaAs is a known carcinogenic material

Quantum dots (QD), discovered by Brus at Bell Labs [17], are a feature of many CPV structural designs. QDs are an effective method of engineering the band gap of the semiconductor crystals deposited and the high levels of crystal size and shape control technology required are now possible with techniques such as MOCVD. Due to the thin layers deposited, excitons, electrically neutral combination of an electron (e-) and hole (h+) - are confined in all directions. Whereas typical semiconductor optoelectronic devices operate at fixed wavelengths defined by their constituent chemistry, quantum dots can be tuned to absorb or emit light of different wavelengths simply by varying their size [18]. As previously discussed, infra-red energy must be absorbed from the solar spectrum – otherwise around half of the sun’s energy remains unutilized. Various companies are developing CPV technology within Wales (e.g. IQE Silicon & IQE Europe, details are listed in Table 4 ). Highly efficient, reproducible triple junction (3J) solar cell technology enables absorption of radiation from different areas of the solar spectrum. This results in higher efficiency than a single-junction structure.

Page 10 27/09/2010

The technology generally used to grow these compound semiconductor crystals is MOCVD [19]. The maximum theoretical conversion efficiency of a single-junction PV cell is about 30% if light trapping techniques are applied to ensure maximum photon absorption. Theoretically 3J solar cells have been estimated to provide conversion efficiencies of ~66% (for an infinite number of junctions). Efficiencies of 41-43% are currently reported, so there remains a significant margin for improvement. Further developments include;

Increasing the number of junction within the structure (5J structures are now emerging) to absorb more solar energy (in the 1.1eV region).

Developing better reproducibility of the growth process. This entails increased accuracy of temperature monitoring & control.

Modifying the structural design (layer thicknesses, doping levels and composition)

Innovative strategies include patented technologies growing Si(0.03)Ge on GaAs [20]. The effect of stretching the SiGe crystal lattice produces enhanced electron mobility within the device leading to increased performance. GaAs has the following advantages over Ge substrates;

It is 50% of the price of Ge (Umicore or AXT) It has higher mechanical strength It is removed by back-side etch & replaced with a heat-sink It is readily available (electronic industry) & of excellent crystal quality

Modelling multi-junction structures using simulation software (e.g. Silvaco) provides extremely useful developmental design information. The most commonly utilized elemental and binary semiconductor materials together with their key material properties are compiled in Table 3. The Chemical Abstracts Service (CAS) registry unique numbers are assigned in the table for rapid reference.

Page 11 27/09/2010

IUPAC name Cadmium Telluride

Gallium Arsenide

Silicon Germanium Indium Phosphide

CAS number 1306-25-8 1303-00-0 7440-21-3 7440-56-4 22398-80-7Properties Molecular formula

CdTe GaAs Si Ge InP

Molar mass g mol−1

240.01 144.645 28.09 72.64 145.792

Density g/cm3 5.85 5.316 2.3290 5.323 4.81 Melting point °C 1092 1238 1414 938.25 1062 Band gap (300K) eV

1.44 1.424 1.12 0.67 1.344

Refractive index (nD)

2.67 3.0 - 5.0 4.01 4.009 3.1-3.6

Structure

Crystal structure Zinc blende Zinc blende Diamond

cubic Diamond cubic Zinc blende

space group F-43m T2d-F-43m Fd3m Fd3m T2

d-F-43m

Hazards

EU classification Harmful (Xn) Dangerous to the environment (N)

Toxic (T) Dangerous to the environment (N)

None None Toxic (T), hydrolysis phosphine

Table 3 – Key semiconductor materials properties

Nanotechnology – Dye-sensitized Solar cell technology.

Dye sensitized solar cell (DSC) technology is an emerging economic alternative to conventional p-n junction photovoltaic devices [21]. In DSC the functions of absorption and charge separation are differentiated. Light is absorbed by an inorganic complex – the sensitizer - which is anchored to the surface of a wide band-gap semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. Ideal characteristics of the sensitizer are that it has;

A broad absorption band across the visible spectrum Attachment groups (carboxylate/phosphonate) to graft it to the semiconductor It should inject electrons into the solid with a high quantum yield upon excitation Energy level of the excited state should be well matched to the lower band of the

oxide conduction band to minimize energy losses Redox potential should be high enough so that it can be regenerated via electron

donation from the redox electrolyte Stable enough to sustain about 20 years exposure to natural light

Overall solar (AM 1.5) to current conversion efficiencies (IPCE) of over 10% have been reached and there are good prospects to produce these cells at a lower price than conventional devices due to lower manufacturing cost.

Page 12 27/09/2010

A key advantage of DSC over competing technologies is that its performance is remarkably insensitive to temperature change. Raising the temperature from 20 to 60oC has practically no effect on power conversion efficiency. In contrast Silicon cells exhibit a significant decline over this temperature range of ~20%. Future developments include developing a sensitizer with optical features similar to GaAs. Broad optical absorption through the visible and near-IR region can be achieved using a combination of two co-adsorbed dyes that are complementary in their spectral features (eg porphyrins and phthalocyanines). Synthesizing structures with a higher degree of order than that found in random TiO2 particle networks would also increase device efficiency. Prof David Worsley (Director – Centre in Steel Technology at Swansea University) is researching DSC technology in conjunction with Corus Colors. G24i (R&D facilities based in Wentloog, Cardiff) have publicized commercial scale DSC technology for a number of novel applications.

Organic / Polymer Solar Cells (OPV / PSC)

Organic / Polymer Solar Cells, an emerging technology, have recently attracted wide scale interest in both the academic and, increasingly, the commercial sector. It is promising in terms of electronic properties, cost, and versatility of functionalization, flexibility and ease of processing. OPV/PSCs provide low-cost fabrication of large-area devices. They have low specific weight; they have mechanical flexibility, and have tunable chemical properties of the active layer organic materials. They can be manufactured in attractive colors, have better performance in low and indirect light and also have the potential for the manufacture of transparent solar cells. Relatively low conversion efficiencies (~5-7%) are currently produced and this needs to be increased for this technology to compete with alternative solar cell technologies. Many challenges remain with this technology which includes;

The development of reliable manufacturing processes Improvement in lifetimes in these low-cost flexible cells Development of predictive models relating structure and photovoltaic device

performance Nanomorphology, which is crucial in determining device efficiency. Achieving efficient charge photo-generation, which remains a key challenge Conjugated polymers typically have high extinction coefficients but optical band

gaps are often not well matched to the solar spectrum which limits the fraction of solar radiation absorbed.

This field is currently an active emerging area for research (Peter Holliman, Bangor; Simon Pope, Nancy Dervisi & Ian Fallis, Cardiff). Research goals include increasing the photon conversion efficiency to ~10% or <1$/Wp for general commercialization of the technology.

Page 13 27/09/2010

4 Technology comparison

A historical summary of champion cell efficiencies are given below [22];

Figure 3 – NREL best research-cell efficiencies, timeline 1975-2010.

It must be noted that testing is generally conducted on cells (1cm2). Commercial efficiencies are significantly lower. The last reported world efficiency record for mono-crystalline Silicon (25%) was reported in 1999. Multi-crystalline & amorphous Silicon technologies have made marginal improvements over the past 10 years. Unsurprisingly, the largest increases in conversion efficiency are observed for both multi-junction concentrators (purple) and emerging PV technologies (red), dye-sensitized solar cell, organic & inorganic technology. Multijunction (3J) structures provide the highest solar conversion efficiency at 41.6%; Organic PV provides the lowest conversion efficiency at 7.9%. There is scope for significant improvement in these emerging technologies. Impact from fundamental particle physics research (e.g. CERN’s Large Hadron Collider [23]) should further contribute to fundamental understanding of sub-atomic processes involved in photovoltaic technologies. The choice of PV technology to provide the required device with acceptable performance criteria is application dependent. For example, building integrated PV (BIPV) are generally large area solutions which require roof or wall mounting & long device lifetimes

Page 14 27/09/2010

(25 years +) due to high installation costs. Alternatively, solar powered calculators are low-price commodities; therefore low-cost and low performance thin film technologies are adequate within this market place. The major types of PV technology vs % conversion efficiency are presented in Figure 4.

Classification of PV vs Efficiency (%)

0

5

10

15

20

25

30

35

GaInP

/GaA

s/Ge

GaInP

/GaA

s

GaAs (

tf)

GaAs/C

IS (t

f)

Si (c)

InP (c

)

Si (m

c)

CIGS (C

ell)

GaAs (

mc)

Si (tft

)

CIGS (s

ub-m

)

CdTe

(cell

)

a-Si/m

c-Si

Si (tfs

)

DSSC

Si (nc

)

Si (a)

DSSC (sub

-m)

Org (2

-cell

tand

em)

Org P

oly

Org (s

ub-m

)

Eff

icie

ncy

(%

)

Figure 4 – Data-sort on best photovoltaic efficiency measurements [19]

The data-sort in figure 4 illustrates that multi-junction compound semiconductor (CS) technologies are the most efficient at converting sunlight into electricity. This is due to the choice of materials closely matching the spectral distribution of the sun, and the fact that the materials are mostly direct-gap. These materials can be grown with near perfect crystal lattice quality. Silicon technologies then DSSC technologies follow, with OPV giving lowest results. However, this ordering of efficiency also trends in general terms with cost and complexity of production. Compound semiconductors are expensive and technically challenging to manufacture, followed by Silicon technologies with lower substrate costs, DSSC and finally relatively low-cost OPV. The efficiency values presented are significantly less than theoretical limits would suggest. The reasons for this are various & in some cases specific to the materials selected for the solar devices. However, general issues for reduced PV conversion efficiency are;

1) Reflection from the surface of the solar device back to atmosphere. 2) Inevitable heating of the device during operation. 3) Competing processes; electron-hole recombination rather than charge separation 4) Impurities – disruption of the crystal lattice 5) Poor electrical contact or encapsulation.

Page 15 27/09/2010

As with consideration of any energy source, the cost per kWh of energy running from photovoltaic cells consists of capital costs (PV modules) together with running costs. The Balance of System (BoS) costs are usually large, fixed costs and include; Interconnection of modules to form arrays The array support structure Land and foundations (if the array is not roof mounted) The costs of cabling, Charge regulators & switching Inverters (to convert DC to AC electricity with about 15% loss of energy) Metering, plus the cost of either storage batteries or connection to the grid.

5 Photovoltaic Industry

The photovoltaic sector in Wales is incredibly fragmented and it is a difficult area for potential Consumers to negotiate. Insufficient historical demand in the UK has meant that the supply chain has not yet consolidated into an easily accessible service. Inverter supply has been haphazard from mainland Europe, with up to 20-week lead times. Installation has to be undertaken by Microgeneration Certification Scheme (MCS) accredited installers for feed-in-tariff eligibility. Installations are often sub-contracted to large organizations (e.g. the Mark group). In order for the PV market to significantly establish itself within Wales a holistic & simplified system approach needs to be taken. In Global terms, Germany, Japan and the USA are the three world-leading countries which collectively represent nearly 90% of the total worldwide PV installed capacity. Taking Germany’s “Solar Valley” (Mitteldeutschland) as a case study, Government subsidy has made this region of East Germany grow into World leader. Solar Valley accounts for 43% of total German PV revenue and 14.5% of global solar cell production. During 2009 the German solar industry had revenues of £4.8 billion [24]. Currently, €0.39 per kW feed-in-tariff are paid to German Households with up to 30kW installed capacity (higher prices are subsidised by a surcharge each month on all German electric bills where grid rate averages around €0.22). By the end of 2009, 600,000 solar panel systems were installed thus providing ~2% of Germany’s annual electricity consumption. There is a comparatively large, and increasing amount of activity in the emerging photovoltaic (& renewable energy) sector within Wales. This is largely facilitated by local Welsh semiconductor industries, together with the proximity of Cardiff, Swansea, Bangor and Glyndwr Universities – all of which are active in the research & development of different facets of PV technology. Partial financial support by the Welsh Assembly Government for some of the photovoltaic development projects undertaken by organizations listed in Table 4 is gratefully acknowledged. The major PV industrial activity in Wales (primary & supply chain businesses) is compiled in Table 4.

Page 16 27/09/2010

Company Business Website

Bright Light Solar Renewable Energy http://www.brightlightsolar.com/

CAG Energy Solar Energy http://www.cagenergy.co.uk/

Carbon Zero UK Low Carbon consultation http://carbonzeroco.com/

Centre for Alternative Technologies

Demonstration of renewable energy & training/education

http://www.cat.org.uk/

Corus Steel/Energy mounting systems http://www.corusenergy.com/en/

Corus Colors Pre-finished steel http://www.corusgroup.com/en/company/divisions/strip_products_division/corus_colors/

Cyrium Technologies CPV development & supply http://www.cyriumtechnologies.com/

Dragon’s Breath Home microgeneration http://www.solardragons.co.uk/

Dulas Ltd Renewable Energy http://renewable-resources.com/

Eaga plc Sustainable living http://www.eaga.com/about/

Eco Energy Systems Ltd Energy & engineering http://www.ecoenergy.ltd.uk/

Eco House Energy Solutions http://www.ecohousesolar.co.uk/

Elltec Solar Renewable Energy Engineers http://www.elltecsolar.co.uk/

Eon Sustainable Energy http://www.eonenergy.com/In-Business/Sustainable-Energy/

Filsol Solar Energy solutions http://www.filsolsolar.com/

G24i DSC R&D / manufacture http://www.g24i.com/

GB-Sol Solar panel assembly supply http://www.gb-sol.co.uk/

Gensol PV & renewable energy http://www.gensol.biz/

Green Warmth Renewable Energy Solutions http://www.greenwarmth.co.uk/

IQE Silicon & Europe Ltd CPV wafer manufacture http://www.iqep.com/

Lectogic Electrical services http://www.lectogic.co.uk/

Llani Solar Renewable Heating Specialists http://www.llanisolar.co.uk/

Mark Group Energy saving – supply & install http://www.markgroup.co.uk/

Micaul Renewable Energy Systems http://micaul.com/

Natural Power Wales Renewables http://www.r-e-a.net/members/natural_power_wales

Parity Solar Global PV manufacture & supply http://www.paritysolar.com/

Pure Wafer Silicon reclaim / PV manufacture http://www.purewafer.com/contact.html

PV Systems Power from the sun http://www.pvsystems.com/

Quantasol CPV development & supply http://www.quantasol.com/

Romasol Sustainable Technologies http://www.romasol.co.uk/

SEI Ltd Renewable Energy Solutions http://www.sei-energy.co.uk/

Sharp Solar panel manufacturers http://www.sharpdirect.co.uk/page/solar

Southern Solar Solar Power http://www.southernsolar.co.uk/

The Energy Academy, Tredegar, British Gas / WAG

Train 1300 people/year. Renewable energy technologies.

http://fs4b.wales.gov.uk/bdotg/action/detail?site=230&itemId=5001566977&type=ONEOFFPAGE

Thermal Earth Renewable energy http://www.thermal-earth.uk.com/

Therm-a-sol Solar energy http://www.thermasoluk.co.uk/

Tomorrow’s Energy Ltd Renewable Energy Solutions http://www.tomorrowsenergy.co.uk/

True Energy Renewable Energy http://www.trueenergy.co.uk/

Warm Wales Community Interest Company http://www.warmwales.org.uk/

WDS environmental Environmental solutions http://www.wdsenvironmental.co.uk/

Table 4 - PV industry, HQ or supply chain connection to Wales

Page 17 27/09/2010

6 Conclusions

The UK government incentive implementation of the feed-in-tariff for micro-generation of renewable energy (April 2010; up to £0.41 per kWh) has stimulated growth within the UK photovoltaic sector. However, comparing Wales to Germany (with similar solar irradiance levels), government incentives can not be guaranteed to support PV industries past the mid-term milestone (to 2020). Therefore economic viability of the technology is essential. As outlined in this report, all photovoltaic technologies can be improved in terms of theoretical photon conversion efficiencies. For widespread photovoltaic technology adoption, and its contribution to significant reduction of greenhouse gas emissions, further research and development is required to increase conversion efficiencies and reduce costs. Close collaboration between academia and industry is of great importance to the rapid ongoing development of photovoltaic technology, utilising the attributes of the Energy Generation & Supply Knowledge Transfer Network [25]. Silicon technology is set to dominate the market in Wales for the next 10-20 years, and will make a positive contribution to the Low Carbon Revolution in Wales. Recent £35 million investments by Sharp Solar UK into its Wrexham plant will double production of both crystalline and thin-film solar panels [26]. However, material band gap limitations of silicon will eventually lead to alternative semiconductor technologies competing on cost and efficiency basis. The introduction of emerging technologies and applications are set to become increasingly important over coming decade. Wales is well-positioned both academically and commercially to maximise both the challenges and opportunities inherent in photovoltaic technology.

7 Acknowledgements

Prof Nick Jenkins and Dr Janaka Ekanayake (Institute of Energy, School of Engineering, Cardiff University); HEFCW; Dr Anthony Giles (School of Engineering, Cardiff University); Prof Alex Ivanov (School of Physics and Astronomy, Cardiff University); Dr Ian Fallis & Dr Simon Pope (School of Chemistry, Cardiff University); Mr Russel Pyecoft (WAG); Mr Simon Parker (KTC, Cardiff University); Dr Hywel Edwards (RACD, Cardiff University); Dr Andrew Johnson, Dr Roy Blunt & Mr Robert Harper (IQE plc, Cardiff); Dr Vicki Stevenson & Prof Peter Pearson (LCRI, Cardiff University); Dr Carl Griffiths (FusionIP); Dr Vincent Barrioz & Prof Stuart Irvine (CSER); Dr Simon Westwater (IMEC); Prof David Worsley (Swansea University).

Page 18 27/09/2010

Page 19 27/09/2010

8 References

1 http://www.decc.gov.uk/en/content/cms/legislation/cc_act_08/cc_act_08.aspx 2 DECC Climate Change Act 2008, http://www.decc.gov.uk/en/content/cms/legislation/cc_act_08/cc_act_08.aspx 3 http://wales.gov.uk/topics/environmentcountryside/energy/renewable/policy/lowcarbonrevolution/?lang=en 4 Decarbonising the GB power sector: evaluating investment pathways, generation patterns and emissions through to 2030. A report to the Committee on Climate Change, September 2009. 5 Anne Stafford/Stuart Irvine, UK Photovoltaic Solar Energy Road Map, CSER/Photonics KTN January 2009. 6 Quotation from Southern Solar Ltd (member of the Solar Trade Association) May 2010 7 Martin A. Green, Solar Energy 74 (2003) 181–192. Crystalline and thin-film silicon solar cells: state of the art and future potential. University of NSW, Sydney, Australia. 8 http://www.gb-sol.co.uk/news-fit/ 9 IQE Marketing / Presentation literature 10 http://www.cematsil.com/Prof.htm 11 http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=74554&tag=1 12 Chemistry World – feature article “Solar Storms”, Ned Stafford, June 2010 13 http://www.purewafersolar.com/ 14 http://www.firstsolar.com/en/index.php 15 Rebecca Trager, Alternative Energy; The US solar push, Chemistry World, August 2010 p.7. 16 http://www.pv21.org/ 17 L.E. Brus (2007). Chemistry and Physics of Semiconductor Nanocrystals. 18 Edward H. Sargent, “Connecting the Quantum Dots” February 2010, IEEE Spectrum Int p.47-50. 19 http://www.aixtron.com/index.php?id=1&L=1 20 Andrew Johnson & Robert Harper, Lattice-matched SiGe on GaAs for triple-junction CPV solar cells. Semiconductor Today, Vol 5 issue 5, June/July 2010. 21 Michael Grӓtzel, Review of Dye-sensitised solar cells, Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) 145-153 22 NREL – National Renewable Energy Laboratory, National Centre for Photovoltaics, 2010 23 http://www.lhc.ac.uk/ 24 Chemistry World – feature article “Solar Storms”, Ned Stafford, June 2010. 25 https://ktn.innovateuk.org/web/energyktn 26 http://wales.gov.uk/newsroom/businessandeconomy/2010/100729sharpinvestment/?skip=1&lang=en (29/07/10)