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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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slide1
Solar Cells ---

frontiers in materials and devices

Ning Su

EE 666 Advanced Semiconductor Devices

slide2
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

slide3
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

slide4
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

slide5
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

slide6
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

slide7
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

slide8
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

slide9
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

slide10
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

slide11
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

slide12
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

slide13
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

slide14
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

slide15
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

slide16
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

slide17
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

slide18
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

slide19
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

slide20
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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