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Solar Cells --- frontiers in materials and devices PowerPoint PPT Presentation


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Solar Cells --- frontiers in materials and devices. Ning Su. Outline. Introduction Market & technology comparison Low cost solar cells thin film solar cells (TFSC) High efficiency solar cells Advanced Si solar cells Tandem cells Thermophotovoltaic

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Solar Cells ---

frontiers in materials and devices

Ning Su

EE 666 Advanced Semiconductor Devices


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Outline

  • Introduction

  • Market & technology comparison

  • Low cost solar cells

  • thin film solar cells (TFSC)

  • High efficiency solar cells

  • Advanced Si solar cells

  • Tandem cells

  • Thermophotovoltaic

  • other strategies

  • Conclusions

EE 666 Advanced Semiconductor Devices


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Introduction

  • Why PV ?

  • Average power incident upon continental United states is ~ 500 times of national energy consumption ( total, not just electricity)

  • Environmentally-friendly renewable

    energy source

  • Quiet

  • Reliable

  • Applications

  • Residential

    Cost-effective way to provide power

    to remote area

  • Space applications

    satellite, space stations

EE 666 Advanced Semiconductor Devices


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Photovoltaic Cells, Modules and Systems

  • Solar cell is the basic building blocks of solar PV

  • Cells are connected together in series and encapsulated into models

  • Modules can be used singly, or connected in parallel and series into

    an array with a larger current & voltage output

  • PV arrays integrated in systems with components for charge regulation

    and storage

Cell module array system

EE 666 Advanced Semiconductor Devices


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Market for Solar PV

  • PV market grows at fast rate especially in recent years

  • Cumulatively, about 2GW of solar cells are being used in a variety of applications

EE 666 Advanced Semiconductor Devices


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Comparison of PV Technology

World PV module production in 2003

  • main technologies available: single & multi- cystalline Si, a-Si, CuInSe2, CdTe….

  • Bulk cystalline Si remains dominant

  • Different technology comparison in efficiency & cost

EE 666 Advanced Semiconductor Devices


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Low Cost vs. High Efficiency SC

Applications:

Space

Terrestrial

Demands:

Low cost

High efficiency

High efficiency

Light weight

Radiation resistance

Technology:

Thin film

Organic SC

tandem

TPV

Materials:

Multicystalline Si

III-V

Single crystalline Si

a-Si ; CIS; CdTe

EE 666 Advanced Semiconductor Devices


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Thin Film Solar Cells

  • “thin film” refers more to solar cell technologies with mass-production possibilities

  • Rather than the film thickness.

  • requirement for suitable materials: low cost, high absorption, doping, transport,

    robust and stable

    leading materials for TFSC: CdTe, CuInSe2, (CIS) ,a-SI…

  • advantages:

    -- low material requirement

    -- variety of processing methods

    -- light weight modules

  • disadvantages:

    -- low achieved efficiency

EE 666 Advanced Semiconductor Devices


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CIS & CdTe TFSC

  • CIS, direct band gap with Eg~ 1eV, α>105 cm-1

  • high cell efficiency (19.2 %), model efficiency (13.4%)

  • comparatively long lifetime

  • Current complicated and capital intensive

    fabrication

  • CdTe, direct band gap with Eg~ 1.45eV, α>105 cm-1-- ideal suited for PV applications

  • Record cell efficiency 16.5 % (NREL)

  • Numerous promising processing techniques

EE 666 Advanced Semiconductor Devices


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Solar Cell Efficiency

  • Ideal cell efficiency

  • Effect of bandgap on efficiency

  • GaAs, InP have Eg close to the optimum,

    favored for high η cells

  • Si less favorable Eg but cheap & abundant

  • Effect of spectrum on efficiency

  • improving η by concentrating light

    100 suns or more illumination

Parabolic reflector Fresnel lens

EE 666 Advanced Semiconductor Devices


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Rear metal reflector

Minimize Losses in Real SC

  • Optical loss

  • Concentration of light

  • Trapping of light:

  • AR coatings

  • Mirrors ( metallization rear surface or growing

    active layers on top of a Bragg stack)

  • textured surface

  • Photon recycling

    reabsorption of photons emitted by radiative recombination inside the cell

Double path length in metallized cell

  • Electrical loss

  • Surface passivation

  • Resistive loss

    ……

EE 666 Advanced Semiconductor Devices


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Advanced Si Solar cells

Crystalline Si efficiency

PERL cell

  • large improvement in the last 15 years

    1) textured surface & AR coating

    2) Improved surface passivation

  • PERL cell ( 24% in 1994 )

  • Buried contact cell commercialized by BP Solarex

    advantage: fine grid– reduced shading–Jsc

    reduced contact recombination – Voc

    series resistance – concentrator sc

Burried contact sc

  • Martin A. Green etc.,” Very high efficiency silicon solar cells-science and technology,” IEEE Trans. Electron

  • Devices,vol. ED-46,pp1940-47,1999.

EE 666 Advanced Semiconductor Devices


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Tandem Cells – beyond efficiency limit

  • Concept

  • Intrinsic efficiency limit using single semiconductor material is 31%

  • Stack different band gap junctions in series larger band gap topmost

  • efficiency of 86.8% calculated for an infinite stack of independently operated cells *

* A. Marti, G. L. Araujo, Sol. Energy Mater. Sol. Cells 43 (1996) 203.

EE 666 Advanced Semiconductor Devices


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Tandem Cells -- Practical approaches

  • Advantages : high efficiency

  • Cover wider range of solar spectrum

  • reduce thermerlisation loss

    (absorbed photon with energy just little higher than Eg)

  • Practical approaches

  • individual cells grown separately and mechanically stacked

  • monolithically grown with a tunnel-junction interconnect

EE 666 Advanced Semiconductor Devices


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GaInP/GaAs/Ge Dual- and triple-junction SC

  • Dual-junction (DJ)

  • GaInP/GaAs cells on Ge (average AM0 η 21.4 %) *

  • small-area lab cells large-scale manufacturing

    approach megawatt level **

  • Triple-junction (TJ)

  • efficiency of 27.0% under AM0 illumination at 28 0C *

* N. H. Karam etc. Solar Energy Materials & Siolar cells 66 (2001) 453-466.

**N. H. Karam etc. Trans. Electron Dev. 46 (10) 1999 pp.2116.

EE 666 Advanced Semiconductor Devices


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Multiple Junction Cells

  • Four-junction cells under development

  • addition of 1-eV GaInNAs subcells

    under GaAs to form 4 junctions

  • InGaN – potential material

    for MJ cells

  • Direct energy gap of InGaN cover

    most of the solar spectrum*

  • MJ solar cells based on this single

    ternary could be very efficient

* LBNL/Conell work: J. Wu et al. APL 80, 3967 (2002).

EE 666 Advanced Semiconductor Devices


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Thermophotovoltaic (TPV)

  • TPV solar energy conversion

Photovoltaic conversion with the addition of an intermediate thermal

absorber/emitter is known as thermophotovoltaic (TPV) energy conversion.

Solar radiation is used to heat absorber/emitter to temperature of 1200-2500 K

emitter radiates photons PV cell converts the energy of radiation

into electrical power.

  • Advantage

By matching the spectrum of the emitter to the PV cells, efficiency improved.

EE 666 Advanced Semiconductor Devices


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TPV Configuration

  • Components of a TPV system

All TPV systems include: 1) heat source 2) radiator 3) PV converter

4) means of recovering unusable photons

Selective emitter matched to PV cells

EE 666 Advanced Semiconductor Devices


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Other Strategies – for high efficiency

  • Intermediate band solar cells

  • A.Luque and A. Marti,”Increasing the effiency of ideal solar cells by photon

  • Induced transitions at intermediate levels”, Phys. Rev. Lett. 78, 5014 (1997)

  • Low-dimentional strucutrues, QWs, QDs

  • Impact ionization solar cells

  • P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).

  • Hot carrier solar cells

  • P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).

    ……

EE 666 Advanced Semiconductor Devices


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Conclusions

  • Remarkable progress made in synthesis, processing and characterization

    leads to major improvement in PV efficiency and reduction in cost

  • Silicon continues to dominate the PV industry

  • Thin film and organic solar cells offer promising options for substantially

    reducing the cost, competitive for terrestrial applications

  • Very high efficiency achieved in multiple junction III-V semiconductors

    presently commercialized for space applications

  • New device concept for high efficiency facing challenges and prospects

EE 666 Advanced Semiconductor Devices


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