Thin Film Solar Cells (II)

Lecture 8

References:1. Physics of Solar Cells. Jenny Nelson. Imperial College Press, 2003.2. Clean Electricity from Photovoltaics, Volume 1. Mary D. Archer,

Robert Hill. Imperial College Press, 2001.3. Handbook of Photovoltaic Science and Engineering. Antonio Luque,

Steven Hegedus. Wiley, 2003.4. Nanostructured Materials for Solar Energy Conversion. Tetsuo Soga,

Elsevier Science, 2007.5. Wikipedia (http://en.wikipedia.org/wiki/Main_Page).

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Non-Si Based Thin Film Technology

In a survey of Fuji Keizai Co. Ltd, the market for CIGS solar cells will sharply grow from the 4.5 billion yen in fiscal 2006 to 472.5 billion yen in fiscal 2010. That is, CIGS Solar Cell Market to Expand 105 Times in 4 Years.

Non-Si based thin film was developed with the motivation to find a low cost and high efficiency alternative to Si based photovoltaic technology.

EC

EVEF

n-CdSp-Cu2S

2.4 eV

1.2 eV

One of the most efficient thin film PV technologies in the early years: copper sulfide/cadmium sulfide (Cu2S/CdS) solar cells:

Now the most successful non-Si based thin film PV technologies are CuInGaSe2 and CdTe based thin film solar cells. Both have been manufactured in large scale and are commercialized.

In addition, organic photovoltaics (OPV) have attracted much attention as a promising new thin film PV technology for the future.

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Laboratory Thin Films Cell EfficienciesEfficiency is the Best Metric to Gauge Progress

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In 1953, CuInSe2 was synthesized for the first time by H. Hahn.In 1974, CuInSe2 was proposed as a photovoltaic material by S. Wagner with a power conversion efficiency of 12% for a single-crystal cell.

Copper indium diselenide (CuInSe2, or CIS) is a direct band gap sc with a band gap of ~1.05 eV and an optical absorption amongst the highest known for any semiconductor.

CIGS Thin Film Solar CellsMaterials Properties

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Structures Evolution: Diamond Si → Zincblende ZnS → Chalcopyrite CuInS2

CIGS Thin Film Solar Cells

Because of the native defects, the conductivity of CIGS is p-type. The defect is mainly In vacancies and Cu atoms on In sites. It can be controlled by varying the Cu/In ratio during growth of the material.

Doping:

Other I-III-VI Semiconductors:

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CIGS Thin Film Solar CellsIn most photovoltaic structures the alloy CuInGaSe2 (CIGS) are used in place of CIS. Addition of Ga improves the photovoltaic characteristics, by raising the band gap, as well as the electronic properties of the rear contact.

CIGS is deposited by two main methods: vapor co-deposition of copper, indium, gallium and selenium; and the selenization of Cu/In films.

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CIS consists of grains of size ~1 µm, which tend to grow in a columnar structure so that grain boundaries are more likely to lie normal to the p-n junction.CIS p-n homojunction solar cells can be made, but have an efficiency of only 3-4%. Better devices can be made with heterojunction structure, using n-CdS emitter on a lightly doped p-CIS base.Highly doped CdS serves as window layer to reduce the collection losses due to surface recombination and transport electrons from junction to front surface with minimum series resistance.

Structure of CIGS Thin Film Solar Cells

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Heterojunction in CIGS Thin Film Solar CellsShort diffusion length requires the use of window layer to reduce the loss of high energy photons. In addition, CuInSe2 cannot easily be doped n-type. So a heterojunction has to be used. However, it introduces several problems:• Differences between the crystal structure or lattice constants of the two

materials introduces intra-band gap defect states at the junction, which encourage SRH recombination, may lead to Fermi level pinning.

• Differences between the chemical composition of the two materials may lead to the diffusion of species across the junction, or the formation of new chemical compounds in the junction region.

• Differences between the electron affinity and band gap of the two materials may lead to the situation where there is a spike in the CB or VB, or both at the interface.

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• Reflection losses are introduced by partial coverage of the front surface by nontransparent contacts or by reflection at material interfaces.

• Buffer absorption represents one of the major losses in todayʼs CIGS thin-film solar cells. Thinning of the CdS or replacing it with a higher band-gap material are possible alternatives.

Loss in CIGS Thin Film Solar Cells

• Recombination losses are introduced by less-than-ideal collection efficiencies of photo-generated carriers. The longer the wavelength, the deeper the generation of carriers, and the higher the likelihood of recombination. Improved recombination requires a longer electron diffusion length (Ln is ~ 2 μm), which is similar to the grain size.

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Fabrication of CIGS Thin Film Solar Cells

Air annealing has been an important process step, crucial for the efficiency of the solar cells based on CuInSe2. Cahen-Noufi model: the surface defects at grain boundaries are positively charged Se vacancies. During air annealing, these sites are passivated by O atoms. Because of the decreased charge at the grain boundary, the band bending and the recombination probability for photo-generated electrons are reduced.

Oxygenation:

Eventually, the chemical bath deposition of CdS removes the passivating oxygen and re-establishes the beneficial type inversion of the film surface.

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Buffer Layer Deposition:Surface passivation and junction formation is most easily achieved by the CBD deposition of a thin CdS film from a chemical solution containing Cd ions and thiourea. The benefit of the CdS layer is manifold:• CBD deposition of CdS provides complete coverage of CIGS surface,

which provides protection against damage and chemical reactions resulting from the subsequent ZnO deposition process.

• The chemical bath removes the natural oxide from the film surface and thus re-establishes positively charged surface states and, as a consequence, the natural type inversion at the CdS/CIGS interface.

• The Cd ions, reacting first with the absorber surface, remove elemental Se, possibly by the formation of CdSe. The Cd ions also diffuse to a certain extent into the Cu-poor surface layer of the absorber material supporting the type inversion of the buffer/absorber interface.

• The VOC limitations imposed by interface recombination can be overcome by a low surface recombination velocity in addition to the type inversion of the absorber surface.

Fabrication of CIGS Thin Film Solar Cells

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CIGS Thin Film Solar Cells

One inherent advantage of thin-film technology for photovoltaics is the possibility of using monolithic integration for series connection of individual cells within a module.The interconnect scheme has to ensure that the front ZnO of one cell is connected to the back Mo contact of the next one. The first Mo patterning by laser scribing. The second patterning is performed after absorber and buffer deposition, and the final one after window deposition. Scribing of the semiconductor layer is performed by mechanical scribing or laser scribing.

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CIGS Thin Film Solar Cells

Major companies of CIGS cells: ZSW, Germany; Energy Photovoltaics, USA; Global Solar (flexible cells), USA; Angstrom Solar Centre, Sweden; Siemens, USA and Germany; Showa, Japan, etc.

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Laboratory Thin Films Cell EfficienciesEfficiency is the Best Metric to Gauge Progress

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CdTe Thin Film Solar Cells

Cadmium Telluride (CdTe) is the semiconductor from the II-VI group of materials. It has a direct band gap of 1.44 eV, close to the optimum for photo-conversion, and a very high optical absorption.It is one of the only two II-VI compounds (the other is ZnTe, Eg=2.26 eV) which can be doped p type as well as n type.

Materials Properties

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In most deposited CdTe films, hexagonally packed alternating Cd and Te layers tend to lie in the plane of the substrate (the 111 axis being perpendicular to the substrate), leading to columnar growth of crystallites.CdTe grows natively p-doped in thin-film form, no additional doping has to be introduced.

The crystal of CdTe adopts the wurtzite crystal structure, like GaAs, but has poorer transport properties, due to a high density of native defects at the grain boundaries giving rise to defect states deep in the band gap.

CdTe Thin Film Solar CellsMaterials Properties

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Structure of CdTe Thin Film Solar CellsThe n-CdS/p-CdTe heterojunction solar cell must be illuminated through the CdS window, so that the light is absorbed in the CdTe close to the junction.In the preferred fabrication procedure, the n-CdS film is deposited onto a transparent TCO film, typically SnO2. Next the CdTe is deposited onto the CdS, and finally a low-resistance contact is made to the CdTe followed by a back electrode.CdS is heavily n-doped, CdTe is lightly p-doped. So all the electric field drops within the CdTe layer, extending to a depth of ~1 μm, a value comparable with the optical absorption length. Light-generated electrons in the CdTe experience a drift field and move toward the junction into the CdS.

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Fabrication of CdTe Thin Film Solar Cells

The CdS film should be as thin as is feasible with pinhole-free and continuous. To maximize the photovoltage, the CdS should be as highly doped as possible. Physical vapor deposition and chemical spraying are the main options. An alternative option is chemical bath deposition.

CdS Window Layer Deposition

CdTe Absorption Layer DepositionThe deposition process used for CdTe should utilize the advantageous materials properties such as the native p-doping, good crystallinity and highminority carrier mobility. Numerous deposition technologies have been developed: Sublimation-condensation (Close-spaced sublimation), Chemical spraying, Galvanic deposition, Screen printing, Chemical vapor deposition, Atomic layer epitaxy (ALE), and sputtering, etc.Cell ActivationIt has become common practice to activate the cells by using the influence of CdCl2 at elevated temperatures. The deposited CdTe films are wetted with solutions of CdCl2 in methanol, dried and annealed or CI- ions are supplied during the growth process. It may improve the crystallinity of the material.

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Fabrication of CdTe Thin Film Solar Cells

There are two general principles for making Ohmic contacts to p-CdTe:1. Use a metal of higher work function. As the electron affinity of CdTe is 4.3

eV, low-cost metals of work function greater than 4.5 eV are not available,2. Create a highly doped back-surface layer in the semiconductor. However,

p-doping in CdTe suffers from a strong acceptors self-compensation, and dopants generally diffuse preferentially along grain boundaries.

Back Contact

In practice, some methods may combine the above two approaches by first contacting CdTe with a more easily doped semiconductor (generating a p-p+ junction), and then with metals of high work function. For example, HgTe, ZnTe:Cu, Te and Cu2Te, etc.

Substrate and TCO FilmThe substrate onto which thin-film solar cells are deposited will to a large extent determine the cost of the final module. Generally a TCO film is considered good if its conductivity is <10 ohms square-1 and its optical transmission is >80% in the region of solar sensitivity of the device.

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Industrial Status of CdTe Solar CellsThe following are the individual steps for module production from glass to module: (1) cleaning of substrate glass; (2) TCO-deposition; (3) scribing step; (4) CdS deposition; (5) CdTe deposition; (6) activation; (7) scribing step 2; (8) back contact deposition; (9) scribing step 3; (10) edge insulation and contact attachment; (11) lamination; (12) safety check; (13) power test and inspection. For mass production, these processes must be integrated into a production line.

A typical module connected in series is made by applying three sets of separation cuts to the growing film stack during production by means of laser ablation or mechanical machining.

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Commercial Module Target (15%)

Industrial Status of CdTe Solar Cells

First Solar Inc. is the largest manufacturer of thin film solar modules, having expanded manufacturing capacity to 735 MW in 2008. And the total capacity is expected to be more than 1 GW by the end of 2009. First Solar has achieved the lowest manufacturing cost per watt in the industry, $0.98/watt for the fourth quarter of 2008, breaking the $1 per watt price barrier.

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Organic Photovoltaic Devices: A Brief History

1906 – Pochettino discovered photoconductivity in solid anthracene 1950 – Chlorophyll and related organic dyes were studied. Kearns and Calvin worked with magnesium phthalocyanine 1980 – First polymer based solar cells 1986 – Breakthrough: first cell with donor and acceptor by Tang 1991 – First dye/dye based cell by Hiramoto 1993 – First polymer/C60 based cell by Sariciftci 1995 – First polymer/polymer based cell by Yu and Hall 2000 – Oligomer-C60 dyads/triads as active materials by Peters and Van Hall2001 – Cell with double-cable polymers by Ramos

Organic photovoltaics (OPV) are comprised of electron donor and electron acceptor materials rather than semiconductor p-n junctions.

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Materials for Organic Photovoltaic DevicesPolymer Photovoltaics

Donor

Acceptor

Conjugated polymers possess delocalized π electrons.

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Materials for Organic Photovoltaic DevicesPhotovoltaics Made of Small Molecules

phthalocyanine

These organic semiconducting materials have tunable band gap and other electronic properties. And there is no lack of the resources of elements like some of the inorganic semiconductors.

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Bulk Heterojunction in Organic Photovoltaics

Bulk heterojunctions constitute intimate blends of organic donor and acceptor materials that allow for efficient charge separation throughout the photoactive layer and provide independent pathways to transport the charge carriers to the contacts.

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1. Exciton creation and diffusion

LUMO PPV

HOMO PPV

-

CT-state

LUMO PCBM

HOMO PCBM

-

+

Charge transfer 2. Charge transfer at D-A interface

Device Physics of Organic Solar Cells

Because of the difference in ionization potential and electron affinity between donor and acceptor, photon induced charge transfer process takes place very rapidly (fs), which forms the metastable bound e-h pairs (CT-state).

LUMO PPV

HOMO PPV

-

+

Exciton diffusion

LUMO PCBM

HOMO PCBM

hνUpon illumination, photon absorption within the PPV layer first leads to the creation of a bound e-h pair–the exciton. Excitons may diffuse during their lifetime within the material, where they were created. The exciton diffusion length is limited to ~5-20 nm.

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LUMO PPV

HOMO PPV

Dissociation to free carriers

LUMO PCBM

HOMO PCBM

-

+

- -

++

3. Dissociation of e-h pairs at D-A interface:

LUMO PPV

HOMO PPV

Charge transport and collection

LUMO PCBM

HOMO PCBM

-

+

--

+ +

-Charge transport and collection

+

4. Transport of free charge carrier to the electrodes:

Device Physics of Organic Solar Cells

Once the charge carriers have been separated, the donor material serves to transport the holes whereas the electrons travel within the acceptor material, which highly dependent on the internal phase structure of the D–A blend.

Because of the larger dielectric constant, weakly interacting molecules, and localized wave functions, exciton binding energy is normally stronger in organics. The e-h pairs are dissociated at the interface across by the energy level offset.

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Recombination in Organic Solar CellsLUMO PPV

HOMO PPV

-

+

Exciton recombination

LUMO PCBM

HOMO PCBM

+ ++++

- - - ---

Recombination to CT-state

Decay to ground state

Firstly, percolating paths are required to ensure that the charge carriers will not experience the fate of recombination due to trapping in dead ends on an isolated material’s domain.In addition, if mobility is not sufficiently high, the carriers will not reach the contacts, and recombine instead at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are highly imbalanced, such that one species is much more mobile than the other.

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Fabrication of Organic Solar CellsRoll-to-roll printing of organic solar cells by Konarka

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Siemens AG

Konarka Organic Thin-Film Solar Cell Modules

Industrial Status of Organic Solar Cells

Before the stability issue could be solved and higher efficiency could be achieved in solar modules, there is still long way to go for OPV to be used in niche market.

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Summary

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