Part 1: Slide 1

1
PV System Overview

• Solar cell is a diode.
• Photopower coverted to dc electrical power
• Shadows defects convert generating areas to
  loads.
• Dc is converted to ac by an inverter.
• Loads are reactive unpredictable.
• Storage helps match generation to load.

Shadows
2
General issues in PV

• The device
• Efficiency, cost, manufacturability
• automation, testing
• Encapsulation
• Cost, weight, strength,
• yellowing,
• Accelerated lifetime testing
• 30 year outdoor test is difficult
• Damp heat, light soak, etc.
• Inverter system design
• Microinverters, blocking diodes, reliability.

3
What are Photovoltaics (Solar Cells)?
Load
Solar cells are diodes. Light (photons) generate
free carriers (electrons and holes) which are
collected by the electric field of the diode
junction. The output current is a fraction of
this photocurrent. The output voltage is a
fraction of the diode built-in voltage.
-

n-type
p-type
Open-circuit voltage
Voltage
Maximum Power Point
Current
Short-circuit current
4
Standard Equivalent Circuit Model
Where does the power go?
Series resistance
(minimize)
Photocurrent source
Shunt resistance
Load
Diode
(maximize)
5
Review of diodes

• Electrons fill states in solids until you run out
  of them.
• The probability of finding an electron in a state
  is the Fermi distribution.
• The Fermi energy is the energy at which the
  probability of finding an electron is 0.5.

6
Review of diodes

• The Fermi energy of an electron is also the
  chemical potential.

Particles always move from high to low chemical
potential until the potentials are equalized.
7
Review of diodes

• Making a connection from an n-type semiconductor
  (doped with impurities with extra electrons) to a
  p-type material (extra holes) induces an electric
  field.
• This field is what separates charges generated by
  light.
• The depletion width is the region where carriers
  have diffused.

8
Operating Principle

• Electrons and holes generated by light are
  collected by the p-n junction. The energy of the
  carrier pair is the energy gap of the
  semiconductor.

9
Quasi-Fermi Levels
Under light the system is not in equilibrium so
we need a description of non-equilibrium chemical
potential to decide how electrons and holes will
move. For this we define a quasi-Fermi level.
Excess holes are present in the region where the
hole quasi Fermi level is below the Fermi energy.
Excess electrons are present in the region where
the electron quasi Fermi level is above the Fermi
energy.
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Part 1 Slide 9
10
Quasi-Fermi Levels
The separation of Quasi-Fermi levels is
determined by the balance of generation and
recombination. Less generation requires less
recombination for the same voltage.
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Part 1 Slide 10
11
Current in the Device
Optical generation rate
Quantum Efficiency (electrons out per photon in)
Photocurrent
Wavelength (nm)
Note photocurrent does not depend much upon
voltage over most of the voltage range
G(n) Incident photon flux a(n) Absorption
coefficient Q(n) Quantum efficiency Lp,
Ln carrier diffusion lengths q carrier charge
12
Current in the Device
Photocurrent
Diode (dark) current
J0 Reverse saturation current V Junction
voltage k Boltzmann constant T Temperature
(K) a Ideality factor 1 ideal 2 non-ideal
Voltage (V)
13
Current in the Device
J0 for an ideal diode (a1)
Diode (dark) current
J0 for a non-ideal diode (a2)
Current density (mA/cm2)
Voltage (V)
where
G(n) Incident photon flux mp, mn Hole
electron mobilities tp, tn Minority carrier
lifetimes e Semiconductor dielectric
constant Vbi Built-in voltage of junction NC,
NV Band edge densities of states Eg Semiconducto
r energy gap ni Intrinsic carrier concentration
This formula assumes recombination through defect
states in the depletion region of the junction
14
Calculating Cell Parameters
Open circuit voltage from current equation at
zero current
Solving for Voc gives
Lp, Ln Minority carrier diffusion lengths pn,
np Minority carrier concentrations
Notes This is for an ideal diode!! gop is
proportional to light flux Voc increases
logarithmically with light flux.
15
Calculating Cell Parameters
Short circuit current density is just
Jsc qgop(LpLn)
since at Jsc, V0.
Cell output power P JV
Maximum power output from a cell where dP/dV 0
dP/dV d(JV)/dV V dJ/dV J 0
or V dJ/dV -J
from
so the max power voltage is given by
at max power (ideal diode)
16
Voc
Graphically
Voltage (V)
JmaxVmax
Fill Factor (FF)
Powermax JVmax rectangle area
JscVoc
Current Density (mA/cm2)
Jsc
Rectangle Area Pabs JscVoc
AM 1.5 Incident Solar Power 100 mW/cm2
17
Operating Principle

• The maximum voltage that can be obtained from a
  solar cell is the separation of the quasi-Fermi
  levels.
• Reducing temperature increases the voltage when
  Schockley-Read-Hall recombination controls
  performance.

This shows that surface recombination at a
heterojunction is not the dominant effect.
Data from S. Hegedus, W. Shafarman, Progress in
Photovoltaics 12 (2004), 155.
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Part 1 Slide 17
18
Series and Shunt Resistance Effects
Algebraically
Series resistance drops some voltage (reduces
output voltage)
Shunt resistance drops some current (reduces
output current)
Voltage Current are coupled
19
Series and Shunt Resistance Effects
Series Resistance
Shunt Resistance
10 ?
20 ?
20 ?
50 ?
10 ?
100 ?
2 ?
5 ?
106 ?
0 ?
Rshunt(dJ/dV)-1short circuit
Rseries(dJ/dV)-1open circuit
Both mostly change the fill factor. Shunt affects
voltage. Series affects current.
20
Current/Voltage Measurements
Light Dark

• Measuremens and their Representation
• Light dark current/voltage curves
• Series and shunt resistances.

Note photoconductivity under light
Resistance per unit area (k?/cm2)
Light Current Density
Dark Current Density (mA/cm2)
dV/dJ plots show resistances
Current Density (mA/cm2)
Voltage
Photo-current
Jsc
Resistance per unit area (k?/cm2)
Voltage (V)
Voc
Power density JV
Voltage
Data courtesy W. Shafarman, University of
Delaware, Institute for Energy Conversion
21
Solar Intensity Atmospheric Effects
Sun photosphere (6000 K black body)
Extraterestrial sunlight (AM0)
Intensity
Sunlight at sea level at 40 N Lattitude at noon
(AM1.5)
AM means air mass
Wavelength (nm)
22
Sizes important to PV

• Absorption coefficient
• Thicker is better.
• You need at least 2 absorption lengths even with
  a back surface reflector.
• Carrier diffusion length
• Thinner is better.
• Need to be able to diffuse to the contacts.
• Optimal performance
• 10 nm for organics
• 1-2 microns for CdTe, CIS, a-SiH
• 2-10 microns for GaAs
• 20-100 microns for Si, Ge

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23
Absorption of Light in the Solar Cell
Light trapping can be used to extend the path
length of the light in the absorber, allowing a
thinner layer to be used.
No light trapping absorption or reflection at
back surface
Back surface patterned to reflect scatter light
Front back surface patterned to refract
scatter light
24
Limitations to Solar Cell Performance
Analysis for a 24-efficient Si solar cell
All photon energy above Voc is lost.
Energy (eV)
Unused Photons 19
Maximum energy collected Egap
31 Loss for Energy above Egap
Intensity (mW/m2-mm)
Other losses Absorption Collection Reflection Ser
ies R Shunts
Voc lt Egap 16
Fill Factor 5
Other Losses 5
Photons used
Usable power 24
Wavelength (nm)
25
Generic Solar Cell Structures
Single Crystal Device
Multijunction Device
Thin Film Device
Examples Si, GaAs cells
Examples InGaAsP/GaAs
Examples CdTe, a-Si, CuInSe2
26
Load
Multijunction Cells
Problem Single junction loses all of the photon
energy above the gap energy.
-
-
-
-




p-type
n-type
p-type
n-type
p-type
n-type
p-type
n-type
Solution Use a series of cells of different
gaps. Each cell captures the light transmitted
from above. But cant just use a graded gap.
Allows electrons to escape to low gap
region. Need separate cells.
27
Modeling Multijunction Cells
Optimization of a two-junction solar cell
total efficiency contours

• Assumes all photons reach the top absorber
• Absolute maximum 28.2 efficiency

Model by Timothy J. Coutts
28
Multijunction cells

• Typical multijunctions
• GaInP/GaInAs/Ge triple junction on a Ge substrate
• 1.75 eV gap GaInP
• 1.17 eV gap GaInAs
• Spectrolab record efficiency devices typical of
  these.
• Metamorphic (strain relieved) 38.8 efficient
• Lattice matched 39 efficient
• Alternate approaches
• Can grow an inverted structure on a GaAs
  substrate and transfer (c.f. figure right).
• Eliminates thick Ge substrate (Ge is rare in the
  crust and indirect gap so it requires a thick
  layer)

Figure example from Geisz, J.F. et.al. Appl.
Phys. Lett. 91, 023502 (2007)
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29
Concentrator PV

• Advantages
• gt12 hours of power during summer
• Maximum power all day
• Less cell area required
• High efficiency devices practical

http//www.amonix.com/Amonix_Installation_Examples
.html

• Disadvantages
• Mechanically complex, more maintenance required
• Less visually appealing for urban installations
• Not for cloudy locations
• Valuable for bulk power, poor for individuals

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30
Concentrator PV

• Insolation level
• Even under sunny conditions atmospheric
  scattering reduces light level to 85 of global
  intensity (e.g. AM 1.5).
• Optics transmission limited to 85
• Net light flux 0.850.85 72 of global flux.
• Non-focusing/non-tracking optics allows 2 to12x
  concentration but cost is prohibitive.
• Theoretical limit to concentration 40,000
• Practical limit 500-1000 typically
• Receiver temperature limit
• Pointing tollerance reduced to practical level
• Flux homogeneity on the collector (use secondary
  optics to improve)

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31
Record laboratory thin film cell efficiencies
32
Thin Film Photovoltaics Achieved
Volume production competition will reduce
margins. Improved Si module efficiencies will
reduce Si price by another 50. Other thin film
concentrator technologies may enter the
competition.
Conventional crystalline silicon technology
Projection to be competitive with any current
energy technology is forecastable based on
existing proven concepts, increasing competition
and mass production volumes.
33
Modules vs. cells
Rain, hail, ice, snow
Design of the module is critical to the success
of the total system. Module efficiency typically
3/4 of the cell efficiency. Module provides
protection to the device.
Local failures, shadows
Heat, humidity, corrosive gases
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34
Interconnect Schemes

• Soldered (standard for Si cells)
• Mostly automated
• Local heating of surface
• Interconnects flexible, helps mitigate thermal
  expansion stresses.

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Part 1 Slide 34
35
Interconnect schemes

• Monolithic interconnects
• Converts single layer structure to many
  junctions.
• Used in all thin film devices.
• Issue width of cell determines current. More
  cells less current/more voltage (good) but more
  cells more interconnect area (bad)

P3
P2
P1
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36
Encapsulation

• Cover glass
• 2-4 mm thickness for hail protection
• Provides mechanical strength to the module
• Typically soda-lime glass is sufficient
• Must have low Fe content to improve transparency
• Add cerium to block UV to reduce polymer
  degradation

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37
Encapsulation

• Back encapsulation
• Insulating (at least inner surface)
• Gas barrier
• Heat dissipation thermal expansion
• Typically a multilayer polymer sheet (glass for
  greater strength)

• Polymer sealant
• Outgassing gas permeability
• Cost of polymer
• Usually fast curing ethylene vinyl acetate (EVA)
  sheets.

• Metal frame (optional)
• Mechanical strength
• Mounting hardware
• Issue dirt accumulation

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38
Blocking diodes -- protection from low voltage
strings
Two good cells One good cell, one weak cell
Cell arrays including a weak cell, the array
performs at the level of the worst cell.
Monolithically-interconnected Cell Array
Diodes can be used to prevent current from strong
arrays flowing through weak array segments.
However, they reduce output voltage.
Front contact
Back contact
For a full discussion, see H.S. Rauschenbach,
Solar Cell Array Design Handbook (Van Nostrand
Reinhold, New York, 1980)
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39
Bypass diodes -- protection from low current
strings

• Bypass diodes are used in modules to route
  current around shadowed or defective strings
  (series connected strings must maintain constant
  current throughout).
• Used typically every 15-20 cells.

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40
Silicon Solar Modules

• Crystalline silicon
• Single crystals
• Cast polycrystals
• Well understood
• Cost analysis easy
• Source material is expensive
• Material sensitive to impurities defects

most manufactured technology
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41
Silicon Solar Modules

• Crystalline silicon
• The major limitation to Si technology is the
  availability of electronic-grade Si.
• Thinner devices would be less suscetible to
  recombination centers caused by impurities
• Absorption coefficient is small so it is hard to
  make the devices thin.
• Single crystals or very large-grained
  polycrystals are required.
• Concepts for use of lower-purity Si now being
  studied.

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42
Silicon Solar Modules

• Steps to make a Si module
• Growth of the Si bulk crystal (ingot)
• Cutting of the wafers from the grown ingot.
• Diffusion of dopants to form the junction.
• Interconnection
• Packaging.

175 W BP Solar Monocrystalline module
Figure courtesy R. Birkmire, Univ. of Delaware
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Part 1 Slide 42
43
Czochralsky Si

• Bulk single crystal boules grown by this method.

Growth rate 0.5-3 mm/min Typical PV boule
diameter 100-150 mm length 40-150 cm
Photographs courtesy MEMC Electronic Materials
Inc, St. Peters MO.
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44
Czochralsky Si

• Issues
• Heat extraction
• Convection currents
• High oxygen content
• Low throughput
• High energy use
• Requires skilled operator

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Part 1 Slide 44
45
Czochralsky Si
Undercooling is required. The amount is
determined by impurity content.
Temperature (C)
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46
Crystalline Si by Casting
Cast multicrystalline Si is formed by cooling
slowly either in a large mold or by the Bridgeman
method where the crucible is dropped slowly
through an inductive heater.
Quartz crucible Si3N4 coating Heaters to control
cooling rate
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47
Cast Multicrystalline Silicon
48
Cast Multicrystalline Silicon
Encapsulated multicrystalline device performance
15.2

• High surface recombination losses
• Emitter recombination vs. ohmic loss trade off
• Hard to texture polycrystalline surfaces.
• Contacts cause shadowing losses

49
Cast Monocrystalline Silicon
Seed crystal
50
Cast Monocrystalline Silicon
Cast monocrystal has a (100) surface orientation
so it can be textured for light trapping using a
chemical etch.
Performance varies with height in the brick and
near the edge of the casting. Long lifetime
allows devices designed with back surface contact
for high efficiency.
51
Cast Monocrystalline Silicon
After doping/passivation
52
Cast Monocrystalline Silicon
Using a seeded casting yields a single crystal
material with longer lifetimes and higher
diffusion lengths.
Surface area fraction
Fraction of product
Cell efficiency bin ()
53
Cast Monocrystalline Silicon
Lifetime controlled by dislocation density.
Dislocation density 850 270 mm-2
Inclusions (carbides, nitrides) cause cascades.
54
Impurities affect lifetime
Casting crucible can result in impurity inclusion
in the material.
Fe is a major issue.
55
Crystalline Si by Casting

• Bulk Polycrystals grown by this method

Wire Saw
Slow cooling
Cast Polycrystal

• Features
• Large grains possible
• Batch process is fast
• Low technology, cost
• Relatively fast
• Lower quality crystals

In ribbon growth the casting produces a ribbon
without much cutting.
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56
Crystalline Si Ribbon methods
Edge-defined Film-fed Growth (EFG) process Large
grain multicrystals close to 110. Throughput 1
MW/furnace/year _at_ 15 efficiency, 90 yield
Dislocation density 105 - 106 cm-1 Cells
15-16 best Manufactured as ASE Schott Solar
modules
For details see United States Patent 6562132
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Part 1 Slide 56
57
Crystalline Si Ribbon methods
String-ribbon growth (STR) method Large grain
multicrystals close to 110. Throughput 160
kW/furnace/year _at_ 15 efficiency, 90 yield
Dislocation density 105 - 106 cm-1 Cells 15-16
best
Developed and used by Evergreen solar
Image courtesy Evergreen solar www.evergreen.com
used with permission
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Part 1 Slide 57
58
Crystalline Si Ribbon methods
Dendtritic web growth (WEB) method Single
crystals with (111) faces. Throughput 50
kW/furnace/year _at_ 15 efficiency, 90 yield
Dislocation density 104 - 105 cm-1 Cells 17.3
best
Developed for manufacturing at EBARA Solar.
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Part 1 Slide 58
59
Crystalline Si Ribbon methods
Ribbon growth on substrate (RGS) method Large
grain multicrystals (through thickness), random
orientation. Throughput 250 MW/furnace/year _at_
15 efficiency, 90 yield
Dislocation density 105 - 107 cm-1 Cells 12.9
best
Under development at the Energy Research Center
of the Netherlands and the University of Konstanz
C.f. Burgers, A. et.al., 2006 IEEE 4th World
Conference on Photovoltaic Energy Conversion
(IEEE, 2006)
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60
Crystalline Si Defect Passivation
Phosphorous and Aluminum gettering Growth of
phosphosilicate glass on the wafer surface leads
to Si interstitial injection. Causes kick out of
impurities that are trapped at surfaces. Formatio
n of Al contacts with a Si-Al eutectic the
eutectic dissolves excess impurities which
diffuse by concentration gradient to the Al-Si
eutectic.
Carrier lifetimes 5 cm x 5 cm oxygen rich RGS
wafer as grown P gettering
Al gettering
Hahn, G. and Schönecker, J. Phys. Condens.
Matter. V. 16 (20040 R1615-48.
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Part 1 Slide 60
61
Crystalline Si Defect Passivation
Hydrogen passivation Annealing under hydrogen
allows hydrogen to bond to dangling bonds.
Reduces the effect of defect states on devices.
Carrier lifetimes 5 cm x 5 cm oxygen rich SR
wafer as grown P gettering
Hydrogenation
Hahn, G. and Schönecker, J. Phys. Condens.
Matter. V. 16 (20040 R1615-48.
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Part 1 Slide 61
62
Breakthrough combination
Sanyo HIT gt16-efficient modules use amorphous
Si to provide contacts that are minority-carrier
mirrors and do not drive strong recombination.
Price currently comparable to existing
12-efficient technology. This technology may
overwhelm all other Si technologies eventually.
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63
Thin Film Device Process

• Single junction device process (example from
  ADVANCIS CIGS device)
• Deposit back contact
• Deposit absorber layer
• Scribes to produce integrated devices.
• Top contact deposition
• Test and encapsulate.
• Other thin film devices made by similar processes.

Substrate device process Figure courtesy R.
Birkmire, University of Delaware
64
Thin Film Device Comparison
Cu(In,Ga)Se2 alloy (glass/Mo/Cu(In,Ga)Se2/CdS/TCO)
CdTe alloy (glass/TCO/CdS/CdTe/contact)
Si-based alloys (PECVD, stabilized)
Table courtesy R. Birkmire, University of Delaware
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Part 1 Slide 64
65
One-sun Devices

• Amorphous Silicon
• Lower cost
• Lower efficiency
• Multijunction devices (moderately complex)
• Source material (SiH4) is expensive.

many have tried, few are left
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66
Typical Amorphous Si Triple Junction

• Typical a-SiH triple junction characteristics
• p-type at the top to reduce distance holes must
  move (generation is strongest at the top of the
  device).
• Tunnel junctions are n/p contacts that
  recombine minority carriers.
• The device can be translucent if the back
  material is transparent.

n ITO conductor
p-type contact
i, a-SiH 1.83 eV gap
i, a-SiGeH 1.63 eV gap
i, a-SiGeH 1.45 eV gap
n-type contact
n ZnO conductor
Metal reflector
Stainless Steel substrate
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67
Typical a-SiH Triple Junction

• Example of the response of a triple-junction
  a-SiH solar cell.
• Note interference effects for the smooth surface.

• Record small cell performances good.
• Modules fair

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68
a-SiH Solar Cell Structure

• Doped end regions
• Doping is easier if the material is crystalline
• Intrinsic center
• Energy gap is critical to performance.
• Alloys for band-gap engineering.
• Need low density of defect states.

ITO contact
p-type
Intrinsic
n-type
ZnO contact
Glass
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69
PECVD of a-Si

• This is the production process for a-Si solar
  cells.
• Silane decomposition
• SiH4 -gt a-SiHx (2-x/2) H2
• Process is modified by H2 addition to the gas.
• Gas chemistry is controlled
• by the pressure regime and
• plasma conditions.
• Reactor geometries vary.
• Process parameters interact.

Rf or dc power source
plasma
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70
PECVD of a-Si

• Advantages
• Relatively simple, scalable process
• Conformal coating (good with textured contacts)
• Problem
• Control of crystallinity, density, and electronic
  properties is difficult.
• Solution Sputter deposition
• Sputter Si target in ArH2 discharge
• Allows control of crystallinity and energy gap
• Other properties do not degrade seriously.

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71
Sputtered a-SiH Phase Diagram
Crystallinity is controllable by adjusting H
pressure
G. Feng. M. Kaytyar, Y.H. Yang, J.R. Abelson, and
N. Maley, Proc. Spring MRS, 1992
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Part 1 Slide 71
72
a-SiH Energy Gap Control

• Hydrogen takes states from the band edge (weakest
  bonds) in Si and increases bond energy.
• This increases energy (mobility) gap.

C.R. Wronski et.al., 11th European PVSEC,
Montreaux, Switzerland, Oct., 1992
1.9
1.8
Mobility (Tauc) Gap (eV)
1.7
1.6
1.5
0
10
20
30
Density of States
Conduction band
Hydrogen Content (at. )
Valence band
Increasing PH in the sputtering system directly
controls H.
2
Energy
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73
Thin Film Silicon

• Amorphous silicon
• Issues
• Instability under light exposure (degrades by
  20)
• Inefficient use of SiH4 source material.
• Low overall efficiency
• Already doing triple junctions, not much space
  for improvement.

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74
Thin Film Devices

• Cadmium Telluride
• Basic processes easy
• Methods well understood
• Basic devices well understood
• Important developments at First Solar reported.

easier than CuInSe2 better than a-Si thin
film technology
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75
CdTe Solar Cell Structure

• Graphite Contact
• Stability of this contact is a major issue
• CdTe
• Needs to be treated with CdCl for best
  performance.
• CdS
• Formed by chemical bath deposition.

These are superstrate devices (sunlight enters
from the glass side)
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76
CdTe by CSS

• Closed-space sublimation is the primary method
  for deposition of CdTe.

Gas inlet He O2 (1-50 Torr)
Sealed Volume
Diffusion
CdTe
Substrate
of
CdTe
Source
Pump
600-800 C
600 C
Deposition and re-evaporation rates from the
substrate and source are controlled by relative
temperatures of source and substrate.
O2 increases acceptor density in the CdTe (more
p-type)
For example of process, see C.S. Ferekides et.al.
High efficiency CSS CdTe solar cells. Thin Solid
Films, vol.361-362, 21 Feb. 2000, pp.520-6.
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Part 1 Slide 76
77
CdTe Treatment with CdCl

• CdTe grain structure is significantly improved by
  CdCl2 treatment.

CdCl2 acts as a flux, allowing an increase in
grain size and grain quality. The CdCl2 is not
incorporated into the film. Some oxygen may be
added to the gas phase as well to increase p-type
doping.
CdCl2
400C
CdTe
O2
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78
CdS by Chemical Bath Deposition

• CdTe/CdS and CuInSe2/CdS heterojunctions formed
  by dip coating.
• Typical dip recipe

Sample Holder
Temperature probe

• Solutions
• 0.015 M Cd salt e.g. CdSO4
• 1.5 M Thiourea SC(NH2)2
• 30 Ammonium Hydroxide HN4OH
• Deionized water
• 60-80C
• Reaction occurs spontaneously over several
  minutes.
• Nanocrystalline Cd deposited on all surfaces.

Solution
Samples
Water bath
Magnetic Stirrer
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79
CdTe Device Contacts

• Back contact

• Top contact (glass side)

• Best devices use SnO2F
• Surface defects (especially O vacancies) are
  critical to the contacts properties.
• These affect the Fermi energy at the interface.

• Best devices use Cu-doped graphite.
• Cu reacts at the interface to form Cu2Te.
• This forms the contact.
• Instabilities are a problem.

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80
Thin Film Devices

• Cadmium Telluride
• Issues
• Cathode contact to the back of the device is
  unstable
• Cd causes cancer.
• Single junction devices allow great improvement
  potential.

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81
Thin Film Devices

• Copper-Indium Diselenide
• Hard little understood
• Works great when done right
• Basic methods complex
• Yield is difficult to obtain

Shell Solar CIS modules
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82
Cu(In1-xGax)Se2 Solar Cells

• Non-CIGS Layers
• ZnO Top Contact
• Sputtered or by MOCVD
• One layer intrinsic
• One layer n
• Intrinsic CdS
• Grown from solution
• Mo Back Contact
• Rf or dc magnetron sputtered
• High stress typical

n ZnO contact
i-ZnO
i-CdS
Cu(In1-xGax)Se2 p-type
Mo contact
Glass
1/1/2010
Part 1 Slide 82
83
Deposition of Cu(In1-xGax)Se2

• Current Processes
• Evaporation
• High rate
• Easy control
• Difficult to scale up
• High temperature process
• Solid Phase Reaction
• Easy to scale up
• High residual stresses
• Sequential room temperature process followed by a
  high temperature reaction

Se
1/1/2010
Part 1 Slide 83
84
Issues in CIGS Deposition

• Control of point defects in CIGS
• Ordered point defects modify energy gap
• Point defects control type and carrier
  concentration
• Back contact
• Selenization (solid phase reaction) causes stress
  that produces adhesion failures
• Mo produces a 0.3 eV barrier Schottky contact to
  CIGS
• Supply of Na is critical to device optimization.

1/1/2010
Part 1 Slide 84
85
Point Defects in CIGS

• Cu-deficient Cu(In,Ga)Se2 dissolves point
  defects.
• p-type
• Egap and p depend upon Ga content.
• b-phase ordered defect compound
• n-type
• Egap1.2 eV without Ga

d sphalerite a chalcopyrite b P chalcopyrite
d
a d
a
a Cu2Se (HT)
Temperature (C)
b
a b
a Cu2Se (LT)
15
20
25
30
Phase diagram from T. Haalboom et.al. Inst. Phys.
Conf. Ser. No. 152, Proc. 11th Int. Conf. on
Ternary Multinary Compounds (ICTF-11) IOP
Publishing, Bristol, 1998, p. 249.
Atomic Cu
1/1/2010
Part 1 Slide 85
86
Thin Film Devices

• Cu(In,Ga)Se2
• Issues
• What limits the device performance is unknown.
• Limited supply of In and Ga
• Hard to make by simple methods
• Single junction devices allow great improvement
  potential.

Shell Solar CIS modules
1/1/2010
Part 1 Slide 86
87
(Ga1-xInx)N

• Inorganic alloy that covers the entire solar
  spectrum for multijunction devices.
• Defects and their impact are little known.

• Issues
• Epitaxial devices processing is expensive
  difficult
• Ga In are rare

1/1/2010
Part 1 Slide 87
88
Novel Concepts Nano

• Carrier extraction how do you get carriers out
  of a nanoparticle?
• Coulomb interaction amplification in the dot --
  enhances multiexcitons but enhances carrier loss
  and increases exciton energy.
• Indirect gap nano reduces recombination but
  loses energy.
• Exciton extraction requires two identical
  contacts with equal carrier transmission.
• Surface recombination Almost impossible to
  eliminate.
• Exciton barriers slow extraction.

89
Novel Concepts Organics

• Exciton binding energy Hundreds of meV binding
  energies are energy losses that are intrinsic.
• Diffusion length so small that it requires bulk
  heterojunctions. Large junction areas produce
  large dark currents.
• Molecular distortions increase HOMO/LUMO gap and
  create trap states.
• Low mobility produces Coulomb barriers at
  contacts.
• Molecular stability carriers are intrinsic
  reaction sites.

90
Novel Concepts Photo Hydrogen

• Some proposals are to make hydrogen directly from
  sunlight in a photoelectrochemical process.
• Problem compromises both electrochemical cell
  and the photovoltaic device.
• Better design these components separately
  because voltages and currents can be adapted with
  high efficiency in a circuit.