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An Introduction to Semiconductor
Electrochemistry
Laurie Peter
University of Bath
ELCOREL Workshop, Oud Poelgeest Castle, Oegstgeest
energy
DOS
energy
D(E)
Egap
VB
CB
Egap
energy
CB
VB
Egap
density of states parabolic approximation simplified band diagram
Basic Ideas: Band Diagrams
( ) 1
1 exp
= −+
FD
F
B
f EE E
k T
Fermi-Dirac Function Determines Concentrations of
Electrons and Holes in Electronic Energy Levels
EF – the Fermi level
( ) ( )
( ) ( )( )
exp
1 exp
C
V
C FC C
BE
E
V FV V
B
E En D E f E dE N
k T
E Ep D E f E dE N
k T
∞
−∞
−= = −
− = − =
∫
∫
To obtain the total concentrations of electrons and holes
we need to integrate the product of the occupation probability fFD(E) and the
density of electronic states in the conduction or valence bands, DC and DV
exp gapC V
B
Enp N N
k T
= −
In thermal equilibrium
CB
EF
VB
Integrate the green curve to obtain the concentration of electrons
CB
VB
EF
intrinsic n-type p-type
Intrinsic and Doped Semiconductors
D →→→→ D+ + eCB A →→→→ A- + hVB
Evac – vacuum energy level.
Ec conduction band edge energy.
Ev – valence band edge energy.
Egap – energy gap.
A – electron affinity.
I – ionization energy.
Φ - work function.
Φ EF Fermi energy.
Energy diagram for an n-type semiconductor
__
, , ,
_
0 0
_ _
ln ln
j
ii
T P n
i i i
n n B p p BC V
n pF F
G
n
z F
n pk T k T
N N
E E
ϕ
µ
µ µ ϕ
µ µ µ µ
µ µ
∂ = ∂
= +
= + = +
= = −
The Fermi level, Free Energy and the
Electrochemical Potential of Electrons and Holes
Electrochemical potential or Partial Molar
Gibbs Free Energy of Charged Species
potential dependent termchemical potential
Nc and NV – effective density of states
thermodynamic standard states
e-vac
-qϕϕϕϕ
EC
EF
EV
µµµµn,sc
µµµµn,m
semiconductor metal
EF
EC
EV
-qϕϕϕϕ
semiconductor metal
Equilibration of Fermi Levels in Solid State Junction
Formation of a Schottky Barrier
Cr3+/Cr2+
Fe3+/Fe2+
H+/
H2
-4.5
evac0
0
-1.0
+1.0
≈≈ ≈≈ ≈V vs. SHE
-3.5
-5.5
eV
Vacuum Energy Scale and Electrode Potential Scale
0 0, 4.5≈ − −F redox redoxE eV qU
Outer Sphere Redox Systems – Marcus Theory
Redox
Fermi level, ln
o oR
R O R OF redox BO
NE k T
Nµ µ µ µ− − − − = − = − +
( )( )
( )20
1/2
1exp
44 BB
E EW E
k Tk T λπλ
− = −
Equilibration of Fermi Levels in Electrolyte Junctions
metal/redox semiconductor/redox
CB
VB
Charge, Field and Potential Distribution
in the Case where a Depletion Layer or
Space Charge Region (SCR) is formed
The charge distribution corresponds to
a space charge capacitance
not to scale!
���� ��∆����
���
�/�
equilibrium flat band accumulationreverse bias
EF
EF,redoxEF
EF EF
EF,redox
EF,redox
EF,redox
Band Bending as a Function of Applied Potential
In the dark the electrode behaves as a diode
n-type electrode
depletion region
accumulation region
Mott Schottky Plots of the Space Charge Capacitance
1/C2
U
VBCB
p-type
n-typeEF
EF
p-type n-type
20
1 2 Bsc
sc d SC
k T
C qN qφ
ε ε
= ∆ −
( )0 0.059fb fbU U pH pH= −
Band Edge Positions at pH = pHpzc
( ) ( )2
0
21
6W
W
rqNr r r
rφ
εε = − +
Nanostructured ElectrodesSpace Charge in Spherical Particles
Band bending for a spherical
anatase particle (Nd = 1017 cm-3,
r = 25 nm, ε = 30) as a function
of rW .
rW defines the edge of the space
charge region.
The maximum band bending of
ca. 6 meV is reached when rW is
reduced to zero, i.e. the space
charge region extends to the
centre of the particle.
Band bending for complete depletion
in spherical anatase particles with
different doping densities (r = 25 nm,
ε = 30, doping density as shown).
In the case of the lower doping, band
bending is limited to only a few mV.
For the higher doping, saturation
occurs when the potential drop
across the depletion layer reaches ca.
120 mV. The effects of band banding
cannot be neglected in this case.
Influence of Doping Density on Space Charge in Spherical Particles
( )2 2 2
0
1ln
2 2φ
εε ∆ = − − +
WSC W
rqNR r R
R
Space Charge in Nanorod Electrodes
Fe2O3 nanorod arrays
rW
R
���,��� ���
����
��
E
gss(E)
EF
filled
emptyEC
EV
Non-Ideal Systems: Surface States
Surface States and Fermi Level Pinning
Rss CSs
CSC
CH
RF
Equivalent Circuit Including Surface State Capacitance
What happens when we illuminate a semiconductor electrode?
We create electron-hole pairs
Thermal equilibrium no longer applies
neq → neq +∆n
peq → peq +∆p
∆n = ∆p
e-
h+
CB
VB
Quasi-Fermi Levels and Fermi Level Splitting
EC
EV
EFnEF
pEFhνννν
µ µ µ= + = −eh n p n F p FE E
exp exp V p FC n FC V
B B
E EE En n N p p N
k T k T
+ −+ ∆ = − + ∆ =
majority carriers - small change minority carriers - large change
stored free energy
Quasi Fermi Level in Illuminated Semiconductor-Electrolyte Junction
O
R
nEF
pEF
e-
h+
R + h+ →→→→ O
photocurrent due to reaction of minority carriers (holes)
Generation and Collection of Minority Carriers
( )0 min
exp1
1photo SCj W
EQEqI L
αα
−= = −
+
Gärtner Equation
min min min min minBk T
L Dq
τ µ τ= =
Minority carrier diffusion length
Anodic Photocurrent for n-type Electrode
Photocurrent increases
as space charge region
gets wider
Simple theory predicts photocurrent onset at flat band potential
recombination
U
jphoto
jrec
jG
jphoto
no recombination
Delayed Photocurrent Onset Due to Surface Recombination of Electrons and Holes
E - Efb
End of the first lecture….