http://www.weizmann.ac.il/AERI/
Presentations are at http://www.weizmann.ac.il/AERI/presentations.html
BASIC CONCEPTS OF
Photovoltaics
(1st part based on
tutorial of Isaac Balberg
Racah Inst. of Physics
Hebrew University of Jerusalem)
Basic Physics and
Materials Science of Solar Cells
J. M. Pearce, Email: profpearce@gmail.com
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
‫ בראשית‬.... In the beginning
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Energy levels of the hydrogen atom
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The hydrogen molecule
H+H
H2
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The hydrogen molecule
Level splitting
H+H
H2
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
From atomic levels to bands
E : distance between atoms where system’s energy is minimized
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Energy Levels
Chemistry is controlled by the states around the filled/empty transition,
i.e., around the …… Fermi Level
Energy
E=0
Empty
States
LUMO
Filled
States
HOMO
An Atom
Vacuum
Level
Fermi
Level
A Small
Molecule
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
A Large
Molecule
Bulk
Material
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Band Structure
Infinitesimal
energy difference (ΔE)
between filled and
empty states
Small, but non-zero
ΔE between
filled and
empty states
Valence
Band
Large
ΔE between
filled and
empty states
Band Gap
Core
Bands
Metal
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
Semiconductor
Insulator
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Fermi Level
• focus on the electrons near the filled/empty boundary.
• each material’s energy state distribution is unique; different EF.
Minimum
energy to
remove
electron
from
sample
E=0 (vacuum level)
EF (Fermi level)
EF (Fermi level)
Metal 1
Metal 2
• the closer an electron is to the vacuum level, the weaker it is bound to the solid
• or, the more energetic is the electron
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Two Conductors in Contact
–+
–+
–+
–+
–+
electron flow
leads to charge separation
Contact potential difference
Fermi level equal throughout sample
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Metal in an Electrolyte Solution
Redox potential =
Electrochemical potential
of the electron =
Fermi level
Fermi levels
are aligned
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
+–
+–
+–
Charge is transferred to
equilibrate Fermi level
with redox potential,
producing a charge
separation and a contact
potential difference.
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Two Electrolyte Solutions
A charge separation
arises to align the
“Fermi” level
(= redox potential) and
produces a potential
at the interface.
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
“Fermi” level
+–
+–
+–
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
An Ion in Solution
• ion’s electronic structure: HOMO, LUMO, HOMO-LUMO gap.
Lowest Unoccupied
Molecular Orbital
HOMO-LUMO Gap
“Fermi” level
Highest Occupied
Molecular Orbital
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Electrochemical Thermodynamics
Every substance has a unique propensity to contribute to a system’s
energy. We call this property Chemical Potential.
m
When the substance is a charged particle (such as an electron or an ion)
we must include the response of the particle to an electrical field in
addition to its Chemical Potential.
We call this Electrochemical Potential.
m = m + zFf
These are perhaps the most fundamental measures of thermodynamics.
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Chemical Potential
Chemical potential (or electrochemical potential if it is charged) is the
measure of how all the thermodynamic properties vary when we
change the amount of the material present in the system.
Formally we can write
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Gibbs Free Energy
The free energy function is the key to assessing the way in which a
chemical system will spontaneously evolve.
dG = -SdT + V dP + å mi dni + g dA + f dl
constant T
constant P
don’t change
shape
don’t stretch it
dG = å mi dni
Dan Thomas, Univ. Guelph, Canada
http://www.chembio.uoguelph.ca/educmat/chem7234/
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semiconductor doping
“Doping” – deliberate introduction of •
impurities into a high-purity, low-defect
semiconductor crystal
Ofer Sinai, 11-2013
17
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semiconductor doping
“Doping” – deliberate introduction of •
impurities into a high-purity, low-defect
semiconductor crystal
Impurity content is low  •
host chemical/crystalline
properties preserved
Nevertheless, impurities •
completely dominate the
electrical behavior
Ofer Sinai, 11-2013
18
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semiconductor doping
Intrinsic semiconductor  very low conductivity
At room T,
Si intrinsic carrier concentration ≈ 1010 cm-3
(Cu: ~1023 cm-3)
Ofer Sinai, 11-2013
19
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semiconductor doping
Impurities introduce free charge carriers
P
B
Donor impurities
Acceptor impurities
Negative charge carriers
Positive charge carriers (holes)
n-type semiconductor
p-type semiconductor
Ofer Sinai, 11-2013
20
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semiconductor doping
Impurities introduce free charge carriers
P
Donor impurities
Negative charge carriers
n-type semiconductor
Ofer Sinai, 11-2013
21
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semiconductor Materials
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Electrons and holes in semiconductors
Doping of semiconductors
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Electrons and holes in semiconductors
Formation of p-type semiconductor
1 eenergy
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Electrons and holes in semiconductors
Formation of n-type semiconductor
1 eenergy
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Impurities determine conduction
~1010 cm-3
~5∙1022 cm-3
Si intrinsic carriers:
Si atom density:
E.g., a ppm impurity can increase the amount
of carriers a million-fold!
Between doping rates of 1013 – 1020 cm-3,
doping determines
Carrier concentration
Carrier polarity
Ofer Sinai, 11-2013
26
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
What is the effect of doping?
The Fermi level, EF, is a key parameter
Intrinsic  EF is near the center of the forbidden gap
E
E
Conduction band (CB)
Egap
EFermi
Valence band (VB)
1
f E   E  E f  kT
e
1
Fermi-Dirac distribution
Ofer Sinai, 11-2013
27
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
What is the effect of doping?
Donor impurities add occupied levels near the CB edge
Added free electrons  Fermi level is raised
E
E
Conduction band (CB)
EFermi
Valence band (VB)
Ofer Sinai, 11-2013
28
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
What is the effect of doping?
Acceptor impurities add unoccupied levels near VB edge
Added free holes  Fermi level is lowered
E
E
Conduction band (CB)
EFermi
Valence band (VB)
Ofer Sinai, 11-2013
29
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
Basic component in electronics
E
≈
Local vacuum level
Local vacuum level
Conduction band
Conduction band
EFermi
EFermi
Valence band
n-type side
Ofer Sinai, 11-2013
Valence band
p-type side
30
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
Basic component in electronics
E
Local vacuum level
Local vacuum level
Conduction band
Conduction band
Valence band
Valence band
≈
n-type side
Ofer Sinai, 11-2013
p-type side
31
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
Charge carriers diffuse in both directions
+ –
E
Local vacuum level
Local vacuum level
Conduction band
Conduction band
Valence band
Valence band
≈
n-type side
Ofer Sinai, 11-2013
p-type side
32
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
A space-charge region (SCR) is formed
E
Local vacuum level
≈
Conduction band
EFermi
Valence band
n-type side
p-type side
33
Ofer Sinai, 11-2013
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
The junction is rectifying:
n-type side
Ofer Sinai, 11-2013
p-type side
34
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
Forward bias:
–
+
n-type side
Ofer Sinai, 11-2013
p-type side
35
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction
Reverse bias:
–
+
n-type side
Ofer Sinai, 11-2013
p-type side
36
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The concept of the Fermi level
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Formation of p-n junction
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The energy diagram of the p-n junction
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n (homo)junction
I = 0
actually means
Idiffusion = -Idrift (= -I0)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction under forward bias
As V  , the current  ;
 diffusion current dominates in the
circuit
# of carriers that can
diffuse over the potential
barrier increases by
Boltzmann factor
exp(qV/kT) :
Idiffusion = I0 [exp(qV/kT)]
(I0 ~ same as in equilibrium)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The concept of the quasi-Fermi level
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Forward & reverse bias
in p-n junction
@ large |Vreverse|
 “breakdown”
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction under reverse bias
Reverse current
saturates @ I0
For small |V|,
I0 is maintained;
Large |V|  “breakdown”.
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The photoelectric effect
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Photon-induced electron-hole pair
generation in a semiconductor


Photon enters, is
absorbed, and excites
electron from VB to CB
 hole left in VB,
 electron-hole pair.
incident
photon
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Photoexcitation in the p-n junction
In illuminated junction,
IL is drift current, due
to e--h+ hole generation,
which adds to I0, the
diffusion current.
Illumination yields a drift photocurrent, IL
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The p-n junction under illumination
I = Idark - IL
Voc = (kT/q) . ln(IL/I0+1)
@Voc, Idiff is so large that it cancels (I0+IL)
 Voc is determined by the light-induced drift current
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
What a Semiconductor is good at:
p-n junction = photocurrent ‘slide’
Internal field
creates
(minority) carrier drift
Long-lived
excess
electrons
and holes
Dad
Light
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Solar Cell Summary
Conventional p-n junction
1 e- energy
Absorb light •
Absorbed light creates carriers •
Carrier collection, by diffusion, drift •
space
after textbooks & R. Collins, CSM
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Solar Cell Summary
Conventional p-n junction + I-V characteristic
Absorb light •
Absorbed light creates carriers •
Carrier collection, by diffusion, drift •
1 e- energy
Voc = 0.602 V
Jsc = 26.7 mA/cm2
FF= 73.3%
 = 11.8%
space
after textbooks & R. Collins, CSM
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
A schematic of a Solar Cell
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Inside a p/n junction Solar Cell
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Chapin, Fuller, Pearson
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The Photovoltaic (PV) effect:
Generalized picture
contact
e-
High
energy
state
contact
one electron energy
Absorber
Low
energy
state
space
p+
Metastable high and low •
energy states
Absorber transfers charges •
into high and low energy state
Driving force brings charges to •
contacts
Selective contacts •
(1) cf. e.g., Green, M.A., Photovoltaic principles. Physica E, 14 (2002) 11-17
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Current Types of PV Cells
Primarily based on solid-state electronic material systems
Elemental Semiconductors •
(non)
concentrator;
single-& multijunction
Single or multi-crystal –
Polycrystalline films –
Amorphous thin film –
Inorganic Compound •
Semiconductors
Single crystal –
Polycrystalline thin film –
homo- &
hetero-junction;
photoelectrochem;Organic, Excitonic •
MIS-inversion (molecules, polymer)
Polycrystalline thin film –
Interpenetrating network –
Nanocrystalline; –
dye-sensitized
thin films
………………………………………………………
Si
,Ge
(Ga,In)(As,P)
Cu(In,Ga)Se2
CdTe
P3HT/PCBM;
porphyrins ++
perovskites*
dye+TiO2
(ZnO)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Single Junction PV Lab Cells: Present Status
(1-4 cm2 ; most tandems are much smaller)
• ~ [75%] 25% single crystal Si;
• ~ [80%] ~20 % single jctn. PX thin films (CIGS, Si)
• ~ [88%] ~12% dye sensitized solar cell (DSSC)
• ~ [89%] ~11% organic molecule (~[90%] 10% polymer)
• ~
(~
[62%]
[56%]
38% “big Mac”
tandem triple junction
44% “bigger Mac” tandem, +concentration)
Definition of efficiency:
Electrical PowerOUT 100%

Solar Radiative PowerIN
Data from Solar Cell Eff #41, Progr in PV 2013
and other sources
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Solar Cell (r)evolutions
1st
generation
Si
Single- crystalline
cm
CdTe, CIGS
3d generation
TiO2 Organic (polymer/
small molecule)
poly-crystalline
mm
nano crystalline
~ 20 nm
2nd generation
amorphous
(a-Si:H;
polymers)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Crystalline Si Cell Manufacturing Process
Wafer
Saw
Crystal Growth
Vacuum furnace
Wafer clean & Prep
Wet bench (clean& etch)
P-doping
Inline furnace
Module
Back contact
PVD or screen print
SiN ARC deposition
LPCVD or PVD
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Different process steps through production of c-Si
solar cells
+ their relative part of the gross energy requirement
(GER)
Stoppato Energy 33, 224 (2008)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Power Losses in Solar Cells
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The Solar Spectrum
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
External Quantum Efficiency (EQE ~ IPCE)
η() ≡ Ip()/qN()
N() = # of impinging
photons
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
External Quantum Efficiency (EQE ~ IPCE)
η() ≡ Ip()/qN()
N() = # of impinging
photons
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
External quantum efficiency of
different types of cells
Wavelength (nm)
In organic based solar cells EQE does not have sharp edge.
This limits current efficiency.
Solar Cell Eff #35, Progr. in PV, 2010
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Internal Quantum (or collection)
Efficiency (IQE)
Y() ≡ η() / T()
≡ Ip() / qN()T()
N()T() = absorbed photons
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Optical Problems for Quantum
conversion of solar energy
In Solar Cells Most Solar Energy is •
“Wasted” as Heat
why is this so?
In any system with concentration, •
most of the diffuse radiation is “lost”
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
in Solar Cells Most Solar Energy is “Lost” as Heat !
Photovoltaic Conversion is a
Quantum (threshold) Conversion Process
Solar
Energy
Spectrum
ultra-violet
(UV)
visible
Infra-Red
(IR)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Single p-n junction solar cell
e-
Energy
e-
hn
hn
p-type
n-type
useable photovoltage (qV)
h+
space
O. Niitsoo
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
 Shockley-Queisser* (SQ) Limit
photosynthesis
SQ Limit
30
GaAs
25
Prince, JAP 26 (1955) 534
Loferski, JAP 27 (1956) 777
Shockley & Queisser JAP (1961)
Efficiency (%)
c-Si
InP
20
CIGS
CdTe
15
DSC
a-Si
10
OPV
5
0.5
1.0
1.5
2.0
2.5
Band Gap (eV)
detailed balance, photons-in = electrons-out + photonsout;
on earth, @ RT, for single absorber / junction;
*
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Losses in PV cell
Etendu; Photon entropy –TD
80
~0.3eV @RT, lack of concentration
Current (mA/cm2)
70
Carnot factor –TD
60
Eg
Emission loss- (current)
50
40
30
< Eg
not absorbed
Electrical power out
Current – Voltage
Characteristics
20
>Eg
thermalized
10
0
0
1
2
4
3
Energy (eV)
After Hirst & Ekins-Daukes
Prog.Photovolt:Res:Appl. (2010)
Nayak, ……, Cahen., Energy Environ. Sci., 2012
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
What else can we do?
Better utilization of sunlight: Photon management:
Multi-bandgap, multi-junction photovoltaics
Four-junction device with bandgaps
1.8 eV/1.4 eV/1.0 eV/0.7 eV
Theoretical efficiency > 52%
5
6
7
8
9
1
2
Bandgap (eV)
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Photon Management 
multi-junction device structures
Thermodynamic Efficiency Limits
non-concentrated Sunlight (AM 1.5)
# of
Junctions
Efficiency
Optimum EG (eV)
1
30%
1.3
2
42%
1.9 - 1.0
3
49%
2.3 - 1.4 - 0.8
4
53%
2.6 - 1.8 - 1.2 - 0.8
infinite
68%
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Optical Problems for Quantum
conversion of solar energy
In Solar Cells Most Solar Energy is •
“Wasted” as Heat
in any system with concentration, most of •
the diffuse radiation is “lost”
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Improve performance using
concentrated sunlight
but … diffuse (scattered)
radiation lost upon concentration
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
The Solar Spectrum
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Optical Frequency Shifting
BASIC RESEARCH NEEDS FOR SOLAR ENERGY UTILIZATION
Report on the Basic Energy Sciences Workshop on Solar Energy Utilization
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Up-conversion for a single
junction
2 photons of energy 0.5 Eg< hν< Eg
are converted to 1 photon of hν> Eg
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Down-conversion for a single
junction
1 photon of energy hn > 2Eg is
converted into 2 photons of hn > Eg
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
IN PRINCIPLE … efficiency 
possible in quantized systems
Why ? Quantization changes relative rates of
carrier relaxation channels:
Slows carrier cooling - Phonon bottleneck•
Can break selection rules •
Increases carrier confinement •
Allows non-equilibrium carrier populations•
Hot carrier collection
Hot carrier
distributions
Energy selective contacts
Carriers must equilibrate
and be collected before
relaxing to band edges
from / after R. Collins, CSM
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Normalized efficiency
Up- and down-conversion together,
based on AM1.5
a-Si:H
GaA
a-Si:H:F
Cu2
s
Si S
Cd
S
Ge
Eg
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Types of junction for solar cells
1. Homojunctions
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Types of junction for solar cells
2. Heterojunctions
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Thin Film PV
Three major approaches
amorphous Si &
Micromorph Tandem
Solar Cell
Cu(InGa)Se2
CdTe
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Polycrystalline Thin Film PV
2nd Generation
© Materials Research Society
From Ginley & Cahen, Ch, 18 (Ginley, Collins, Cahen)
2012
Fundamentals of Materials for Energy and Environmental Sustainability< Cambridge Un. David
Press,Cahen,
2011 Weizmann Inst., 11 – 2013 Caesarea Program
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Organic Photovoltaics:
Basics
Thin Organic Film PV
Cathode
A layer of Donor and a layer of
Acceptor between electrodes
Anode
Exciton dissociation at Donor/Acceptor interface.
glass
Limited efficiency due to:
Small dissociation sites (only D/A interface).
High recombination possibility
if D or/and A layer is thicker than ~10 nm.
Low light absorption, because films are thin.
90
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
p/n vs. excitonic solar cells
ORGANIC
INORGANIC
high dielectric constant •
Exciton
minority carrier device•
low dielectric constant •
excitonic device•
m*e4
includes •
EB  
2
2 2
(4 0 ) 2  jiggling & wiggling
 dielectric constant
from B. Kippelen, Georgia Tech
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Electron-hole pair:
Organic vs. Inorganic PV cells
Exciton
binding
energy >> kT
Exciton binding
energy < kT
→ dissociation
by space charge
region E-field
MOLECULAR PICTURE
Inorganic semiconductor
from A. Kahn, Princeton U
→ requires
donor/
acceptor,
(D/A) type
structure
Organic semiconductor
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
(hybrid) PV Cells: decreasing STATIC disorder
Bulk heterojunction cell
Dye-sensitized / ETA
Cathode
-
D
D
A
Anode
Substrate
Light
OM Perovskite
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program
Semi-transparent “Plastic” OPV:
Light for greenhouse plants
and
Power the fans, Pump the water
courtesy David Ginley, NREL
David Cahen, Weizmann Inst., 11 – 2013 Caesarea Program