Solar Cells ---

1
Solar Cells --- frontiers in
materials and devices
Ning Su
2
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

3
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

4
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
5
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

6
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

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Low Cost vs. High Efficiency SC
Space
Applications
Terrestrial
Low cost
High efficiency
Light weight
High efficiency
Demands
Radiation resistance
Thin film
Organic SC
tandem
TPV
Technology
Materials
Multicystalline Si
III-V
Single crystalline Si
a-Si CIS CdTe
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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

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CIS CdTe TFSC

• CIS, direct band gap with Eg 1eV, agt105 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, agt105
  cm-1-- ideal suited for PV applications
• Record cell efficiency 16.5 (NREL)
• Numerous promising processing techniques

10
Solar Cell Efficiency

• Ideal cell efficiency

• Effect of bandgap on efficiency
• GaAs, InP have Eg close to the optimum,
• favored for high ? cells
• Si less favorable Eg but cheap abundant

• Effect of spectrum on efficiency
• improving ? by concentrating light
• 100 suns or more illumination

Parabolic reflector Fresnel lens
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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

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Advanced Si Solar cells
PERL cell
Crystalline Si efficiency

• 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 shadingJsc
• 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.

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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.
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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

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

• Dual-junction (DJ)

• GaInP/GaAs cells on Ge (average AM0 ? 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.
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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).
17
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.
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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
19
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).

20
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