Research topics
David Cahen 12/’11
• Hybrid molecular/non-molecular interfaces
* Do we really understand biological e- transfer?
* Taming our work horse, Silicon
• ALTERNATIVE ENERGY
Chemistry & Physics of Light  Electrical Energy conversion
Research topics
David Cahen 012/’11
Motivations
•
Understanding & Curiosity (“Everest” research) and…
•
Explore (bio) molecule-based electronics (with M. Sheves)
•
Use chemistry, (bio)physics & materials to improve
efficiency x lifetime) /cost of photovoltaic solar energy
… do we ask relevant questions?
Solar Cell Concepts and Materials
Basic science towards improving
(efficiency x lifetime) /cost of (any) solar cell
•
•
what are the real limits to PV energy conversion ?
Metal-Insulator-Semiconductor solar cells :
re-discovering Si
•
Mesoporous, nanocrystalline solid junctions 
 high voltage solar cells (with G. Hodes)
Solar Cell Concepts and Materials
Molecules as
“door-men”
V
Effects of molecule adsorption
on solar cell performance
Adsorbed molecule
CdTe
CdS
Back contact
Poly-xtline p-CdTe
Poly-xtline n-CdS
Conductive oxide
Glass
h
HOW IS THIS POSSIBLE ?
Adsorption at the PV junction
- affects VOC ! ! !
… because … of physics of dipole layers !
Molecules
SC
idealized
cartoon
Pinholes
… because … of physics of dipole layers !
i.e., we can use even
discontinuous incomplete monolayers
idealized
cartoon
Even poorly organized monolayers can do,
but
need at least average orientation
with M. Bendikov, L. Kronik, R. Naaman
A. Kahn; N. Koch, F. Würthner
Device Outline
R = Dipole-forming Molecules
+
DONOR
l
l
l
l
+
Metal Contact
l
+
l l l l l l l l l l l l l l l l l l
+
~1 nm
+
R Monolayer:
R R R R R R Trimethoxy
R R R R R R Silane
R R R R
+
~40 nm
+
Donor : Organic Light Absorber
-
or
Voc
l
~10 nm
+
Metal Contact
-
use
ACCEPTOR
But first
….
Back to Basics
Energy Levels at Interfaces?
Metal / Semiconductor (MS)
junctions according to Schottky & Mott
Metal
Metal
EL
Walter H.
Schottky
Φm
EF
EF
Sir Nevill F. Mott
Semiconductor
Semiconductor
Schottky limit:
b,n=Φm-χ
χSC
Φm
b,n
Vbi
EL
EL
χSC
EC
EE
FC
EF
EEV
V
Fermi-Level Pinning (Bardeen Limit)
•b=Eg- 0
John Bardeen
•
Barrier dictated by Charge Neutrality Level, φ0, of
surface states
•
Δ Vacuum falls over interface  no change inside
semiconductor  S = 0
Previous Works:A. Vilan,
GaAs
& D.ZnO
A. Shanzer,
Cahen,
Nature, 404, 166 (2000)
Salomon, Berkovich, & Cahen,
Appl. Phys. Lett., 82, 1051 (2003)
ZnO
S~0.6
GaAs
S~0.1
Index of Interface Behavior, S
SiO2; S ~1
b=m-sc
GaSe; S ~0.6
Schottky-Mott
 Ionic SC
b ≈ m+const.
Si; S ~0.05
Kurtin, McGill, Mead, Phys. Rev. Lett., 22, 1433, (1970)
L.J. Brillson. Surf. Sci. Rep., 2, 123, (1982)
Bardeen
Covalent SC
Need to revise textbooks!
•HQ-alcohol
Si(100)
C1
C3
C5
C7
C10
Solar Cell Result
7mm x 9mm cell, 1 mm grid spacing
Fill Factor = 50%
PEDOT:PSS
n-Si
Power Density [mW/cm2]
Current Density [mA/cm2]
~100 mW/cm2 illumination.
Solar Cell Concepts and Materials
Molecules in nano-porous, solid state solar cells?
Extremely Thin Absorber (ETA) cells
TiO2,
ZnO
optical
absorber electron
conductor
with G. HODES
hole
conductor
CuSCN
absorber PV
CuCu
SSabsorber
PV performance:
performance:
2-x
2-x
effect
monolayer
effectofofbuffer
alkyl monolayer
0
0
Cu2-xS
F
F
-0.2 -0.2
holes
2
2
EE
J (mA/cm )
electrons
electron
s
Cu2-xS
holes
TiO2
buffer
layer
J (mA/cm )
energy
energy
-0.1 -0.1
CuSCN
-0.3
-0.3
Inx(OH)ySz + Cu2-xS
-0.4 -0.4
distance
distance
-0.5 -0.5
-0.6 -0.6
0
0
Inx(OH)ySz + C12-P(O)(OH)2 + Cu2-xS
50
HO
300 300 350
350 350 400
400 400
50 100 100 150 150 200 200 250 250 300
V (mV)
(mV)
V
V (mV)
with G. HODES
P
OH
O
Which types of electronic conductors
do we know ?
Silicon
Diamond
semiconductors
Carbon Nanotubes
Carbon
Cu
metals
β-Carotene
Pentacene
Organic
(semi)conductors
Bio-molecules?
Heme
Electronics with Bio-Molecules?
Force electrons through (bio)molecules;
What is/are transport mechanism(s)?
High quality
device structures
Transport
(yield, reproducibility)
Theory
Electronic structure
Models
Transport
mechanisms
Spectroscopy
electron, electrical
optical +++
‘Dry’ Electronic Transport across
surface-immobilized proteins
• Bacteriorhodopsin (bR)
• Azurin
OPEN QUESTIONS
What is / are the etransport mechanism(s) ?
• BSA
Can we make artificial
systems, based on these?
with Sheves & Pecht
I-V characteristics
protein layers
Electrical top contact
4
2
OTMS (C18)
0
-2
1E-6
1E-7
Current (A)
Current ( A)
Linker layer
Conductive substrate
Az (on SH; ~Br)
bR (on NH2 )
BSA (on NH2 )
-4
1E-8
1E-9
Az
bR
BSA
OTMS
1E-10
1E-11
-6
1E-12
-1.0
-8
-1.0
-0.5
0.0
-0.5
0.0
0.5
Voltage [V]
0.5
Bias Voltage (on metal) [V]
1.0
1.0
Striking temperature effects
-8
Linker
Azurin
Apo Azurin
-10
-12
3.5 nm
Ln J [@ +50mV Bias]
-6
-14
-16
-18
-20
2
4
6
8
10
12
Electrical top contact
0
1000/T [1/K ]
Linker layer
Conductive substrate
OPEN QUESTIONS
• Basic limits to solar light conversion / solar cells
 Understand static and dynamic disorder effects
 Tailor solar cells with molecules
• The inorganic/organic interface, the next frontier?
 Hot electron injection?
• Why is Electron Transport across proteins so efficient ?
 Study Peptides
 Use also CP-AFM
 Use also Electrochemistry
 Study effects of biological function (e.g., CO/O2 on myoglobin)
Further cooperation/collaboration in WIS with:
O. Tal, R. Naaman, I. Lubomirsky, S. Cohen, H. Cohen
in IsraelN. Tessler, A. Zaban, C. Sukenik, A. Nitzan, N. Ashkenasy
Abroad: USA, Japan, Germany, Italy, France, Spain