inorganic materials for solar cell applications · inorganic materials for solar cell applications....

57

3 Inorganic Materials forSolar Cell Applications

Yasutake Toyoshima

3.1 INTRODUCTION

It has been more than half a century since the first solar cell was invented,1 which was made of single-crystal Si. Up to now, Si is still the major semiconductor material that has been used in solar cell for photovoltaics (PV). In this chapter, a wide-range over-view about the inorganic materials that have been used for solar cells will be presented.

Before going into the variety of materials, the fundamental aspects of PV are described. First, the sunlight spectra are shown in Figure 3.1. Due to the atmospheric absorption (mainly by ozone molecules in the UV region and water and carbon diox-ide molecules in the infrared region) and scattering, these spectra have somewhat complicated features on the earth ground. AM is abbreviation of air mass and the number means the thickness of the atmosphere (unity corresponds to vertical thick-ness), while the letter “D” or “G” means that each spectrum is directly from the sun or from the entire sky including blue light scattered by the atmosphere. In Figure 3.1, photon number–based spectrum (on the basis of the Si bandgap) is also presented since a PV cell current is proportional to the total number of absorbed photons, not to the total absorbed energy. In Figure 3.2, photoabsorption characteristics of various semiconductor materials are summarized. Recently reported corrections2 for CuInSe2 (CIS) and CdTe profiles are compiled in Figure 3.2 along with the previous ones that are shown in broken curves. In addition, a commonly used dye molecule called N719 is also plotted in this figure by simply assuming the molecular volume

CONTENTS

3.1 Introduction .................................................................................................... 573.2 Crystal Silicon Solar Cells ..............................................................................603.3 Thin-Film Silicon Solar Cells ......................................................................... 673.4 CIGS Solar Cells ............................................................................................. 713.5 CdTe Solar Cells ............................................................................................. 743.6 III–V Compound Semiconductor Tandem Solar Cells ................................... 753.7 Emerging Materials for Solar Cells ................................................................ 79

3.7.1 Metal Silicides .................................................................................... 793.7.2 Perovskites .......................................................................................... 79

3.8 Inorganic Materials Employed for Dye-Sensitized and Organic Solar Cells .....80References ................................................................................................................ 81

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58 Advanced Materials for Clean Energy

Wavelength (µm)

Radi

atio

n in

tens

ity (k

W/m

2 µm)

0.00.0

0.5

1.0

1.5

2.0

2.5

0.5 1.0 1.5 2.0 2.5 3.0

6000 K black body (relative)AM 0AM 1.5G (global)AM 1.5D (direct)Photon number base

FIGURE 3.1 Standard solar spectra.

Photon energy (eV)

Pene

trat

ion

dept

h (µ

m)

Abs

orpt

ion

coef

ficie

nt (c

m–1

)

Wavelength (µm)

0.5

0.50.60.81.01.5

1

10

100

1,000

10,000

100,000

1,000,000

1 1.5 2 2.510,000

1,000

100

10

1

0.1

0.01

3

GaP

GaAs

CdTe a–Si:H

CuGaSe2

CIS

Ge

CdSAm 1.5Gphotonnumber

based

Si N719

FIGURE 3.2 Photoabsorption characteristics of semiconductors.

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59Inorganic Materials for Solar Cell Applications

to be 1 nm3. In Figure 3.3, the maximum current corresponding to the bandgap cut-off is plotted. This is obtained simply by integrating the number of higher-energy photons and is basically the same as the one reported by Henry,3 except for two cor-rections of total intensity from 844 to 1000 W/m2 and the revision of standard solar spectra.4 This figure is useful for a brief estimation of current collecting efficiency for a semiconductor of a given bandgap. When it comes to the voltage, the situation is rather complicated. Generally speaking, half (or 2/3 at best) of the bandgap is usually obtained for open-circuit voltage. Refer to the Shockley–Queisser limit for estimat-ing the upper limit of energy conversion for a single-junction solar cell.5 Two major origins to limit the energy conversion efficiency are shown in Figure 3.4. One is

Cut-off wavelength (nm)

Inte

grat

ed cu

rren

t (m

A/c

m2 )

4000

10

20

30

40

50

600 800 1000 1200

FIGURE 3.3 Relationship between the integrated current and cutoff wavelength.

Wavelength (µm)

Absorbed but unused

Not absorbed

Inte

nsity

0.0 0.5 1.0 1.5 2.0 2.5 3.0

FIGURE 3.4 Two kinds of major losses.

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longer-wavelength light that cannot be absorbed, and the other is shorter-wavelength light that has excess energy above the bandgap that cannot be utilized for conver-sion although absorbed. To partly improve this situation, a multi-bandgap PV cell, commonly called as tandem cell, is employed. The basic concept of a tandem cell is schematically shown in Figure 3.5. It should be noted that a semiconductor will not absorb the light whose energy is below the bandgap. Practical implementation of this tandem concept into PV cells is discussed in Section 3.6.

3.2 CRYSTAL SILICON SOLAR CELLS

Currently, crystal silicon–based PV cells are still in major use in the field of solar cell application. First, the purification process is shown in Figure 3.6, since purity is the most important factor in semiconductor materials. These processes are commonly called Siemens method as a whole. The key feature of this method is purification by multiple distillations up to 300 times (or more) of trichlorosilane (SiHCl3) gas, whose boiling point is close to ambient temperature. Ultimate purity of 11 nine (99.999999999%) can be achieved by this method, which is good (and necessary) for semiconductor use. For solar cells, however, such an ultimate purity is sometimes said to be unnecessary to achieve practical conversion efficiency, from the viewpoint of production cost. Those moderate-purity materials of five to seven nine (99.999-99.99999%) level are called solar grade. The problematic feature in this ultimate purification is the low conversion efficiency in the solidi-fication of SiHCl3 gas by electric current heating in a bell jar–shaped reactor. As shown in Figure 3.6, the maximum (theoretical) conversion is 25% at best. In addi-tion, the heat loss is quite significant since higher pressure is preferred in this solidification reaction. Attempts to improve this reactor are still ongoing.6 These solidified polysilicon rods are broken into small pieces and then melted and made to a single-crystal rod by the Czochralski (CZ) method as shown in Figure 3.7. For solar cells, polycrystalline Si made by cast methods is also used.7 Since the cuboid made by casting would be usually larger than a meter, it is suitable for mass productions. These crystal Si ingots are sliced into wafers by a multiwire saw as shown in Figure 3.8.

White sunlight

Green Red Absorbed

Blue

Wide gap Medium gap Narrow gap

FIGURE 3.5 How tandem structure works efficiently to absorb white light.

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A typical sequence for solar cell production from p-type polycrystalline wafer is shown in Figure 3.9. The silicon nitride (SiNx) layer at the light incident side pro-duced by plasma-enhanced chemical vapor deposition (PECVD) will play multiple roles, namely, (a) an antireflecting layer for light incidence, (b) interface passivation, (c) carrier collecting effect by positive fixed charges in the layer, and (d) hydrogen reservoir to passivate point defects in polycrystalline Si, mainly at the grain bound-aries. Passivation is carried out not only by placing an insulating layer but also by minority (positive) carrier repulsion due to the fixed charge in this layer. Since this material is insulating, some trick is necessary for front electrode contact. This can be achieved by so-called glass frit incorporation into the Ag paste for front grid electrodes. This glass frit is mainly made of lead borosilicate8 and/or other metal oxide mixtures9 that melt at relatively low temperature. During the firing process, this glass frit melt penetrates through the nitride layer, forming the Ag wire contact to the underlying Si emitter. The counterdoped layer at the light incident side is usually called emitter for convention. At the same time of this firing, rear-side Al electrode metallization is also performed, which also includes glass frit mainly made of silica particles to reduce the mechanical stress (which causes wafer bowing) due to a large thermal expansion coefficient in Al. This Al electrode works as a backside reflector (BSR) for transmitted (unabsorbed) light back into the active layer. In addi-tion, a small amount of Al diffused into the rear end of the Si layer will work as a p-type dopant, forming an enforced p-type-doped part to enhance positive carrier collection. This effect is called backside field (BSF). It is proposed that Al oxide pro-duced by atomic layer deposition can also be employed to enhance positive carrier collection, due to the negatively charged defects in this layer.10 A heavily doped layer

Single-crystal rod

Si meltCzochralski method~1500°C

HCl~300°C

Si + 4HCl → SiCl4 + 2H2

4SiHCl3 → Si + 3SiCl4 + 2H2 (η = 25% at best)SiHCl3 + H2 → Si + 3HCl (ideal η = 100%)

Si + 3HCl → SiHCl3 + H2

Metal grade Si Solar grade Si

VLD (Vapor to

liquid depositi

on)

Extreme purity(99.999999999%)

SiHCl3Multiple distillation

(>300 times)

Deoxidation

Boiling point (°C)SiCl4SiHCl3SiH2Cl2SiH3ClSiH4

PH3PCl3B2H6BCl3

57.631.8

8.2–30.4

–111.5

–87.776.

–92.812.5

SiO2

Thermal decomposition/hydrogen reduction~1000°C

Polysilicon

Bell jar method

Siemens method

Slicing/polishing~12N

~7N

>11N

2N

Floating zone(low oxygen content) Si wafer

FIGURE 3.6 Purification process of silicon.

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also has an effect on interfacial passivation. Since carrier recombination requires coexistence of both positive and negative carriers, it can be reduced by minimizing the minority carrier density in heavily doped layers. In Figures 3.10 and 3.11, typical appearances of Si cell and such cell-based PV module are shown, respectively, where PVF, PET, PEN, and EVA stand for polyvinyl fluoride, polyethylene terephthalate, polyethylene naphthalate, and ethylene-vinyl acetate, respectively.

Carbon support

Si melt

Ar atmosphere

Silica crucibleSeed crystal Neck

ChuckCarbon heater

Rotating

FIGURE 3.7 CZ method.

FIGURE 3.8 Multiwire saw.

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Front-side n-type doping, often by POCl (thermal drive-in) into p-type wafer

Front-side texture by wet etching (or by other methods)

SiNx layer formation by PECVD

Pasting light-reflecting electrode (typically Al) at rear sideScreen painting of finger electrode pattern with Ag-based paste

Ag paste firing to form front contact, melting through the SiN layer (front)Al sintering and slight diffusion to backside field effect formation (rear)< These two processes are accomplished at the same time >

Base bar (Al strap) soldering at front side Interconnecting tab

(surface passivation, anti reflection, defect termination by residual H,and enhancement of negative carrier collection by fixed positive charge)

FIGURE 3.9 Typical procedure for PV cell formation (multicrystal Si).

Base bar Finger electrode

Monocrystal cell(round corners,

from columnar ingot)

Multicrystal cell(right angle corners,

from cast-made cuboid)

Interconnector(Tab)

FIGURE 3.10 Typical appearance of Si-based PV cell.

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Hereafter, several types of high-efficiency cells are presented. One of them shows the highest efficiency* among the Si-based cells, called passivated emitter, rear locally diffused (PERL) cell11 (Figure 3.12). Among all, this cell has a quite unique structure of inverted pyramids for antireflection at the front side, although the cell area is considerably small.

The next one is the heterojunction with intrinsic thin layer (HIT) cell12 (Figure 3.13), which has already been applied for commercial production of PV modules. The key feature in this PV cell is p–n junction formation not by thermal drive-in of impu-rity but by doped amorphous layer formation, which will result in less heat stress in the single-crystal Si wafers. In addition, hydrogen contained in the intrinsic inter-mediate layer plays efficient passivating roles, resulting in a quite high open-circuit voltage. However, since the light absorbed in the amorphous layer cannot contribute to the photocurrent generation, the short-circuit current is somewhat lower (below 40 mA/cm2). It should be noted that in this HIT structure, a transparent electrode such as transparent conductive oxide (TCO) that can be produced without heating up is necessary since the top amorphous layer is low conducting and will crystallize with heating up. Room temperature sputtered indium tin oxide (ITO) layers will match this requirement. Care should be taken in producing such TCO layer so as not to dis-turb the light absorption close to the absorption threshold as described in Figure 3.14.

* The most efficient Si-based cell is now the HIT type.51

Junction box

Frame (Al)

Si cell

Cover glass

Backsheet(PVF, PET, PEN) Filler (EVA)

+

–

FIGURE 3.11 Typical structure of Si-based PV module.

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Rearside electrode

Insulating oxide

FZ Si (p type)

Surface texture (inverted pyramids)

Thermal oxide for surface passivationFinger electrode

p+

n+ n

FIGURE 3.12 PERL cell.

TCO layer

TCO layer p type

n type

UndopedCZ Si wafer (n type)a-Si:H

Finger electrodes

FIGURE 3.13 HIT cell (surface textures are omitted for simplicity).

Amorphous Si

Absorption bysemiconductor

500

Care should be taken to avoid overlapping around this region

1000 1500

Light reflectiondue to carriers in TCO

Wavelength (nm)

Phot

oabs

orpt

ion

and/

or re

flect

ion

Crystal Si Carrierdensityin TCO

Low

High

FIGURE 3.14 Consideration on interference between c-Si absorption and TCO transmission.

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All these cell structures have front-side electrodes of base bars and fingers that will hinder the light incidence to some extent. Increase in the photocurrent, resulting in elevated conversion efficiency, is expected upon removal of these front-side elec-trodes. Such a concept for PV cell is called back contact,13 as shown in Figure 3.15. To avoid shadowing would be one reason for high efficiency. However, removing the

Surface passivation/AR/texture

Mono Si

p layer n layer

Floating emitterto minimize

recombination

+ –

FIGURE 3.15 Back contact cell.

Metal electrodes

Contact holes

n+ region

Trench

Oxide layerThermal oxide formation

Lithography to open trench

n+ diffusion through trench

Additional oxide formation

Opening of contact holesby lithography

Rear electrode formationand separation

+–

p layerp-layer formation

n-type waferStarting from n-type wafer

FIGURE 3.16 Rear contact fabrication for back contact cell.

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metal contacts from the front side, where photocarrier generation is most abundant, should be the most important feature in achieving high efficiency in this type of PV cell since the metal/semiconductor interface usually works to enhance carrier recombination. In this sense, front-side passivation by heavy doping, usually called floating emitter, is another key issue to achieve a higher efficiency. The somewhat complicated production procedure of a back contact cell is schematically presented in Figure 3.16.14

There used to be thin-wafer production techniques called ribbon methods in which very thin Si plates are directly produced.15 These methods were meaningful to avoid kerf loss at wafer slicing especially when the Si materials are quite expensive. However, it has a certain disadvantage in mass production. That is, producing each single wafer one by one takes a lot of time. So the solidification process should be done quite quickly, resulting in a not so good enough crystal quality. In Figure 3.17, the last-survived ribbon method, called string ribbon, is displayed, which had ended commercial production a few years ago.

3.3 THIN-FILM SILICON SOLAR CELLS

In 1975, an epoch-making report was published, which showed that the semicon-ducting type of n and p in a silicon-based amorphous material can be controlled by impurity doping.16 Soon after this report, the amorphous material was employed for

String(carbon wire)

Si melt

FIGURE 3.17 String ribbon method.

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solar cell application.17 In spite of large expectations to the potential of this material, light-induced degradation effect (called Staebler–Wronski effect after the reporter) is found,18 which seems fatal for PV use. After some time, these features are regarded to be originating from hydrogen incorporation in this material. So this material is formally called hydrogenated amorphous silicon, or a-Si:H, although a simple way of amorphous silicon or a-Si is most commonly used alternatively.

Preparation of a-Si is done in the PECVD reactor from monosilane (SiH4) gas. A schematic representation of a PECVD reactor is shown in Figure 3.18. The thin-film growth is usually performed at the anode side to minimize ion impingement–induced damages. The cathode electrode is self-biased negatively by the presence of a blocking capacitor in the impedance matching circuit attached between the cathode electrode and the power supply unit that is generating a radiofrequency of 13.56 MHz. A drawback in this growth technique is a quite low utilization of start-ing gas material (<5%), which is necessary to keep the quality of film reasonable and uniform over the entire deposited area.

Although both the electrons and holes can transport in this material, their mobil-ity is quite different. Electrons can move quicker than holes by about 1000 times. This feature puts a stiff restriction on the solar cell structure, as schematically explained in Figure 3.19. Carrier generation is most abundant at the light incident side. Meanwhile, one of the carriers only has to move a short distance to the front-side electrode, and the other has to move all the way to the rear electrode. If the latter carrier moves slowly, it may not reach the electrode. This is why a-Si solar cells must have a p-type layer at the light incident side. If only this is reverse, tandem-type cell of quite high efficiency can be achieved with the CIS bottom layer. In addition, poor conductivity in the lateral direction of the top p layer makes it necessary to cover the entire top surface by TCO, commonly made of F-doped tin oxide (FTO). It should also be noted that since doped n and p layers include too much defects, photogen-erated carriers can only survive in the undoped layer. As a result, the p–i–n structure is a must for a-Si-based solar cells.

Anode

Cathode

Earth shield

Blocking capacitor

Matching box

RF generator

Substrate

Electric potential

Plasma potential

Self-bias

+0–

FIGURE 3.18 PECVD reactor. RF—radio frequency.

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Still, the tandem structure is meaningful to reduce photodegradation (Figure 3.20). Since photodegradation is caused by carrier recombination, a steeper band profile produced by the thinner i-layer, which separates the carriers more quickly, will be useful to reduce photodegradation, although light absorption is also decreased. Texture formations of TCO at the incident side have a complementary effect on light absorption. A tandem structure can also complement this decrease. However, the bottom cell must be thicker than the top cell in order to match the photocurrent, a prerequisite of tandem structures. Instead of simply increasing the thickness of the bottom cell, several attempts to reduce the bandgap of the bottom cell is performed. One way to do this is by adding the Ge (by adding the GeH4 gas into the SiH4 gas) to make the bandgap of the bottom cell narrower. An example is shown in Figure 3.21, which is called a series connection through apertures formed in film (SCAF) module.19 The other way to avoid the rather expensive GeH4 use is by employing the microcrystal Si (μc-Si) material, as shown in Figure 3.22, which is called a hybrid cell.20 The μc-Si materials can be made basically by employing

FIGURE 3.19 Selection of p–i–n or n–i–p upon carrier mobility.

Single junction Tandem (double) junction

pp nn np

Recombination occursin moderate slope

Current matchingis a must

in tandem cell

FIGURE 3.20 How tandem structure works to reduce photodegradation.

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Connection hole Collection hole Connection holeTCO Active layer Polymer-base film

Connectedaround thecollection

hole

Connectedthrough theconnection

hole

RearFront

Single cell(tandem)

Top cell

ITO p a-Si n np

a-Si

Ge

Elec

trod

e

Elec

trod

e

Poly

mer

-ba

se fi

lm

Bottom cell

1 µm 50 µm

FIGURE 3.21 Details of substrate-type SCAF module structure.

Backside electrode

Bottom cell(μc-Si)

TCO

Glass superstrate

Top cell(a-Si)

Transparentintermediate

layer

FIGURE 3.22 Amorphous and microcrystalline hybrid cell.

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the hydrogen dilution conditions in the PECVD reactors. In the latter case, an inter-mediate layer of low refractive index is inserted between the top and bottom cells so as to reflect the top-cell transmitted light back. By employing this interlayer, the thickness of the top cell can be further reduced, preferred for the reduction of photodegradation. Such an interlayer should be desired to be resistive to reduction (deoxidation) by hydrogen atoms that are necessary for microcrystal formation. It is known that ZnO-based TCO is more resistive to such reduction than FTO. The doping element Al or Ga is mostly used to increase the conductivity of ZnO, called AZO or GZO, respectively.

In Figure 3.23, the preparation sequence for the so-called monolithic structure of a thin-film-type PV module is presented.21 Among these processes, laser scribing for cell separation would be most time consuming in mass production, in addition to the microcrystal growth for the bottom cell of the hybrid type. Since the glass plate initially employed as the substrate for the film growth will be at the front side of light incidence in practical use, this module structure is called superstrate type. The other one is called substrate type, as usual.

3.4 CIGS SOLAR CELLS

Because copper indium diselenide (CuInSe2 or CIS) shows strong photoabsorption due to its direct transition gap nature, it is expected to be a candidate for highly efficient thin-film solar cells, although the bandgap is almost as short as Si. So it is quite important to increase the bandgap by substitutional doping of Ga in the lattice position of In. Such materials are called CIGS. Based on the crystal structure, these

TCO

Soda lime glass

Active layer(semiconductor)

Back sideelectrode(Ag or Al)

Scribing the electrode/active layers

Formation of back electrode

Scribing the active layer

Formation of active layer(p/i/n semiconductor layers)

Scribing the TCO layer

TCO formation on glass superstrate

FIGURE 3.23 Procedure of monolithic structure formation for thin-film PV module.

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materials (and also this type of PV cells and modules) are sometime called chalcopy-rites. Currently, only the CIGS-based solar cells have reached efficiency over 20%22 in the thin-film category. Quite recently, 20.8% efficiency is reported.23 A typical cell structure is shown in Figure 3.24. In addition, Figure 3.25 shows the desired band profile in the CIGS layer. Since CIGS is natively p-type, this profile should be ben-eficial for the minority carrier (electron) transport. At the front-side interface of the CIGS layer, however, a slight increase of bandgap would be advantageous to reduce

SpikeMo back electrode

Soda

lim

e gla

ss su

bstr

ate

CIGS1.2 eV

CdSbuffer2.4 eVZnO

3.2 eVITO

AR (MgF2)

+–

FIGURE 3.24 Basic structure of CIGS cell.

Fron

t jun

ctio

n sid

e

e

Mo

met

al b

ack

elec

trod

e

FIGURE 3.25 Desired band profile for the CIGS layer.

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carrier recombination. This is also the origin of placing a buffer layer of a wider gap with a spike at the conduction band side.

As these cells are substrate type, a Mo back electrode is sputtered on the soda lime glass substrate at first, as shown in Figure 3.26. It is somewhat mysterious that Na diffusion from the soda lime glass into the CIGS layer through this Mo layer is a must for a high-efficiency cell. Next, the way to fabricate the CIGS layer is categorized into two cases, of small cells and of large modules. For small cell fabrication, a multisource coevaporation technique is employed, and selenization (and sulfuration) is done at the same time, since Se (and possibly S) is evaporated simultaneously. For large modules, on the other hand, metal components of desired compositions are deposited mainly by sputtering, and selenization (and sulfuration) is done afterwards in the ambient of H2Se and H2S at elevated temperature. So a precise control of compositions to achieve a desired bandgap profile, such as shown in Figure 3.25, can only be accomplished easily by coevaporation, leading to much higher efficiencies in small cells. This, in return, would be a big issue for large-area module productions. The buffer layer production is usually performed by chemical bath deposition (CBD), which uses a chemical solution to form the layer of desired composition. Although the best performance cell is achieved using a Cd-based buf-fer layer, attempts to realize a Cd-free structure of high efficiency is a matter of great interest from an environmental viewpoint.

Sputtered Mo (500–900 nm)

Three-step method CIGS (2.5–3.0 µm)

Chemical bath deposited CdS buffer layer (40–50 nm)

Sputtered undoped ZnO (50–100 nm)

Sputtered Al-doped ZnO (150–200 nm)

MgF2-based AR coating (105 nm)

Ni/Al-grid

Substrate: soda-lime glass (3 mm)

FIGURE 3.26 High-efficiency CIGS cell.

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3.5 CdTe SOLAR CELLS

It has been quite some time now since CdTe is said to be the best candidate for solar cells due to its best matched bandgap of about 1.5 eV. However, the toxic Cd is problematic to use for solar cells, especially in Japan. The improvement of conver-sion efficiency had not been so prominent, except for the recent few years. It is quite rapidly increasing now, reaching 19.0%.24 In addition, its production cost is probably most inexpensive among all the commercialized PV modules. Unfortunately, since the high-efficiency cell structure is not disclosed, a somewhat old one25 is shown in Figure 3.27. The light-absorbing CdTe layer seems rather too thick in this case, so it would be much suitable to make it thinner for commercial production. The CdTe layer is produced by close-spaced sublimation (CSS), which is quite rapid (takes only a few minutes for a 10 μm absorber growth). In addition, high vacuum is not neces-sary for CSS. Instead, oxygen-based gas ambient is usually employed. These features are quite advantageous for low-cost production. The window layer of CdS is, like the CIGS case, produced by the CBD method. However, this CBD layer is grown prior to the CdTe layer because this cell structure is the superstrate type.

Borosilicate glass superstrate

CdTe (~10 µm)

Frontcontact

(In)CBD-CdS (0.07–0.1 µm)

ZnSnOx (0.1–0.2 µm)

Cd2SnO4 (0.15–0.3 µm)

Back contact (C:HgTe:CuxTe)

FIGURE 3.27 Basic structure of CdTe cell.

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3.6 III–V COMPOUND SEMICONDUCTOR TANDEM SOLAR CELLS

The III–V compound semiconductors generally have superior properties in carrier transport over Si and are a better match to PV applications. A comprehensive dia-gram26 of the relationship between the lattice constant and the bandgap is shown in Figure 3.28. Among them, GaAs is the best match to the solar spectrum due to its adequate bandgap of 1.4 eV for single-junction use. Actually, the best conversion efficiency of GaAs single-junction cell is higher than that of Si.27 Because of its direct transition nature, the thickness necessary (and thus adequate) for a solar cell is much thinner than the thickness commonly used for wafers. So a lift-off technology is employed to fabricate the thin-film-type GaAs cells,28 which is also beneficial for space use. In addition, the open-circuit voltage is also increased by thinning, as is commonly expected.

Although the conversion efficiency of a GaAs single-junction cell is already supe-rior than the Si cells, the key issue using the III–V compound semiconductors for solar cells is their versatility in composition and thus the band gap, as schematically shown in Figure 3.29. This feature may be quite desirable in the multijunction (tan-dem) cell design. Note that a semiconductor cannot absorb the light whose energy is below the bandgap. Utilizing this nature, a triple-junction cell made of high-, medium-, and low-energy absorbers is expected to perform a quite high efficiency, as previously shown in Figure 3.5. A well-designed combination of bandgap for these three absorbers is essential for high performance. As shown in Figure 3.30, there are several ways for such choice. Because there is a large valley at around 1.4 μm in the

Lattice constant (nm)

To GaN

Ge

InAs

GaSb

GaAsInP

AlSb

AlAs

AlP

GaP

InSb

Band

gap

(eV)

0.540.0

0.5

1.0

1.5

2.0

2.5

0.56 0.58 0.60 0.62 0.64 0.66

FIGURE 3.28 Relationship between the lattice constant and the bandgap for III–V compound semiconductors.

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sunlight spectra due to the strong absorption of water molecules in the atmosphere, two ways are available for an ideal combination of these three bandgaps.

One of them utilizes the longer-wavelength region beyond this valley by using the low bandgap material. This choice seems most practical from the viewpoint of fabrication, which employs the Ge wafer for the bottom junction. By chance, it is lucky that the lattice constant of Ge and GaAs is quite similar and thus rather easy to perform heteroepitaxial growth between these two semiconductors. In order to further continue the lattice-matched heteroepitaxy, the top layer composition is selected to be In1/2Ga1/2P, whose lattice constant is also close to the underlying two

Wavelength (µm)

AM1.5D (photon number-based)

Lattice matched 0.67

Current matched 0.66Ge substrate

High currentHigh voltage

Ideal0.70

Inverted metamorphic

GeGaAs (InxGa1-xAs:N)In½Ga½P

0.931.341.86

1.74 1.17

1.181.67

1.83 1.42

1.83 1.34 0.89

0.5 1.0 1.5 2.0 2.5

FIGURE 3.30 Bandgap design for triple-junction cells: ideal and practical.

Bandgap (eV)

Ge

0.67 1.42 1.8

In In

In½Ga½PGaAs5.675 5.653

N

Latti

ce co

nsta

nt (Å

)

AlGa

~5.655

FIGURE 3.29 Schematics on how compositional change affects the bandgap and lattice constant.

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layers (Figure 3.29). However, this choice, denoted as lattice matched/Ge substrate in Figure 3.30, has a problematic nature. That is, the number of photons absorbed in the middle layer is far smaller than the other two layers. So the current matching, one of the most important prerequisites in the multijunction solar cell, will not be satis-fied. To overcome this problem, the bandgap of the middle layer should be reduced to increase the current generation therein. For this purpose, a metamorphic technol-ogy using the buffer layer is necessary since the bandgap–lowered middle layer is no longer lattice matched to the base Ge layer. Figure 3.31 schematically describes how the buffer layer works in the lattice-mismatched growth. Although the thickness necessary for this buffer layer is considerable, it works quite well for high-efficiency tandem cells.29

For an ideal bandgap combination, another way, denoted as high voltage in Figure 3.30, also requires this buffer technology to achieve the so-called inverted metamor-phic structure.30 In this choice, only the sunlight whose wavelength is shorter than the valley is designed to be absorbed. Although this will cause a decrease in the generated current, the bandgap of the bottom junction layer, and thus the total volt-age, will be increased in return. So the Ge substrate is not adequate to fabricate this structure. Instead, a GaAs wafer is employed for the starting substrate. At first, the lattice-matched InGaP layer, or the top layer, is grown on the substrate. This is why it is called inverted. Next, a little bit modified GaAs layer is grown as the middle junction. Then, the lattice-mismatched bottom layer, whose bandgap is larger than that of Ge, is grown with the aid of the previously mentioned buffering technology.

Overlayer

Substrate

Defect (dislocation)

FIGURE 3.31 Schematics on how the buffer layer works in lattice-mismatched heteroepitaxy.

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Again in this case, the buffer layer is considerably thick as shown in Figure 3.32. Finally, the starting base of the GaAs substrate is removed (etched out), and the triple-junction cell is transferred onto a holding baseplate, such as a Si wafer or glass plate. Generally speaking, a higher voltage with lower current will be welcomed from the viewpoint of large-scale integration, since Joule loss is reduced by lowering the cur-rent. However, this inverted metamorphic process totally sacrifices the valuable GaAs wafer, and it needs another breakthrough to be utilized for practical productions.

The behavior of nitrogen addition to GaAs, as shown in Figure 3.30, is quite exceptional since both the bandgap and lattice constant are decreasing. This is due to a quite large band bowing occurring between GaAs and GaN, wherein the interme-diate bandgap profile does not follow the linear interpolation but shifts downwards.31 A similar behavior, although the shift is smaller, can be seen in other binary combi-nations shown in Figure 3.29.

Since these triple-junction cells are quite expensive, installation with the opti-cal concentrator system is a must in a terrestrial use. It also needs a quite precise mechanical tracking system to follow the moving sun, called a heliostat. Although such systems are already installed in the world, their outdoor performance is not as good as the previously mentioned cell efficiency.32 In addition, one should

Lattice sizeHandling base (Si, Glass, etc.)

Bottom (InGaAs:1.0 eV)

Buffer (GaInP:transparent)

Middle (GaAs:1.4 eV)

Top (GaInP:1.8 eV)

GaAs substrate (removed)

FIGURE 3.32 Schematics of inverted metamorphic cell.

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note that a tracking system has a low ground cover ratio, to avoid shadowing, and thus has a lower performance per unit land area compared to nonconcentrating systems.33

3.7 EMERGING MATERIALS FOR SOLAR CELLS

3.7.1 Metal SilicideS

There has been some attention to use metal silicides for solar cells. Typically, β-FeSi2

used to gather much attention because of its strong and wide-range photoabsorp-tion.34 However, its bandgap, which is narrower than Si, seems too small for a single-junction solar cell, resulting in a not so high efficiency.35

Instead, wide-gap silicides, such as BaSi2, are of interest recently. The band gap of BaSi2, which is not larger than that of Si, can be increased by the addition of Sr,36 although the bandgap of pure SrSi2 is quite small (about 43 meV37). Attempts to fab-ricate high-efficiency solar cells are underway.38

3.7.2 PerovSkiteS

Quite recently, there have been intensive works on this new category of solar cell materials, which is called perovskites, due to their crystal structure (see Figure 3.33). The key material is CH3NH3PbI3. This inorganic–organic hybrid compound is first employed39 as a sensitizer in a dye-sensitized solar cell (DSC), focusing on its unique optical properties.40 That is, this iodide,41 along with its bromide counterpart,42 shows strong excitonic absorption near the band edge. The bandgap of about 1.5 eV43 is also suitable for a solar cell material candidate. Recent reports have prevailed that the TiO2-based DSC structure is no longer necessary for high-efficiency cells exceeding 15%.44,45 Further increase in conversion efficiency is greatly expected utilizing these new materials. If the excitonic behavior is ruling the conduction mechanism, the so-called bulk heterostructure46 might be useful, as for the organic semiconductor–based solar cells. Since these compounds are soluble in water, long-term stability against humidity will be a big issue for solar module applications.

I (anion)

Pb (cation)

CH3NH3 (cation)

FIGURE 3.33 Crystal structure of CH3NH3PbI3.

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3.8 INORGANIC MATERIALS EMPLOYED FOR DYE-SENSITIZED AND ORGANIC SOLAR CELLS

A dye-sensitized cell (Figure 3.34) has three major components, in addition to the common thin-film solar cells: porous TiO2 layer for electron transport, sensitizer dye for optical absorber, and iodine-based organic redox solution.47 The use of ZnO instead of TiO2 seems to be not working suitably in this DSC application probably because of the hole transporting behavior. TiO2 is said to have a hole blocking nature, while ZnO can easily transport holes judging from its bipolar nature (both n- and p-type conduction are possible).48 The Ru-based complex dyes, such as N3, N719, and N749, are most commonly used for sensitizers. The redox reaction of 3I− ↔ I− +I2 + 2e− is carrying the electrons from the back electrode (typically Pt) to the photo-excited and electron-removed dye molecules.

For solar cells made of organic semiconductors, TiO2 is also used for the electron transporting layer. It is quite interesting that MoO3 and Cs2CO3 are used as the dop-ing material for organic semiconductors to make them p and n types, respectively.49 Note that in this field of organic solar cells, a hole conductor is called donor and an electron conductor is called acceptor. Other inorganic compounds, such as LiF, are used to improve the transport at the electrode interface.50

Rear glass

Pt back electrode

Dye

TCO

Front glass

TiO2

I–/I3– redox solution

FIGURE 3.34 Schematic structure of DSC.

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