Defects in solar cell materials: the good, the bad, and the ugly Tim Gfroerer Davidson College, Davidson, NC with Yong Zhang University of NC @ Charlotte.
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Defects in solar cell materials: the good, the bad,
and the ugly
Tim GfroererDavidson College, Davidson, NC
with Yong ZhangUniversity of NC @ Charlotte
and Mark WanlassNational Renewable Energy Lab, Golden,
CO
~ Supported by the Charlotte Research Institute andthe American Chemical Society – Petroleum Research Fund
~
Some of the experiments and analysis by . . .
Ryan Crum and Mark Crowley (’11)
Mac Read and Caroline Vaughan (’10)
Outline
• Semiconductors, solar cells, and defects• Recombination, radiative efficiency, and
dependence on defect level distributions• Photoluminescence imaging and
modeling• Confocal photoluminescence microscopy
and the role of diffusion
Semiconductors
PeriodicPotentialPhyslet*
r
V(r) Energy levels
Spacing decreasing
n=3
n=2
n=1
a
a
--
f ree atoms atomic crystal
* Physlet Quantum Physics: An Interactive Introduction by Mario Belloni et al. (2006).
Solar Cell OperationConduction Band
Valence Band
PHOTONEN
ER
GY
ELECTRON
E-Field
E-Field
HOLE
E-Field
E-Field
+ +
++
---
-
-
CURRENTABSORPTION
When a photon is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. Some heat is lost, reducing efficiency. Then an internal electric field sweeps the electrons and holes away, creating electricity.
HEAT
Good Defects: Impurities for p/n Junction Formation
NP+
++
+++++++
+
--
- -
-
+++++++
+++++
Depletion Layer
E-Field
+-
+
+
-
-
+
+
+
-
-
-
+
+
+
-
-
-
+
Semiconductor Defects
Dislocation Applet
Defect Level Physlet~ from Physlet Quantum Physics: An Interactive Introduction
by Mario Belloni et al. (2006).
Bad Defects: Defect-Related Trapping and Recombination
Conduction Band
Valence Band
EN
ER
GY Defect Level
-
+
HEAT
Electrons can recombine with holes by hopping through defect levels and releasing more heat. This loss mechanism also reduces the efficiency of a solar cell.
HEAT
Radiative Recombination and Efficiency
Conduction Band
Valence Band
PHOTON
EN
ER
GY
-
+
Radiative Efficiency = (light out) / (light in)= (radiative rate) / (total recombination rate)
heatlight in
light out
Radiative Rate ~ n x p
Photoluminescence Imaging
(c) Iex
~ 0.12 W/cm2(d) I
ex ~ 0.012 W/cm2
(b) Iex
~ 1.2 W/cm2
100 m0.20
0.23
0.25
0.28
0.30
100 m 0.50
0.56
0.63
0.69
0.75
0.60
0.68
0.75
0.83
0.90
100 m100 m
0.66
0.75
0.83
0.92
1.00
(a) Iex
~ 12 W/cm2
Laser
Camera
Lowpass filter
Sample
Experiment Excitation-Dependent Images
Top View of Diffusion to Dislocations
Simulated Images
(d) Iex
~ 0.012 W/cm2(c) Iex
~ 0.12 W/cm2
(b) Iex
~ 1.2 W/cm2
0.18
0.21
0.24
0.27
0.30
0.50
0.56
0.63
0.69
0.75
0.60
0.68
0.75
0.83
0.90
0.66
0.75
0.83
0.92
1.00
(a) Iex
~ 12 W/cm2
(c) Iex
~ 0.12 W/cm2(d) I
ex ~ 0.012 W/cm2
(b) Iex
~ 1.2 W/cm2
100 m0.20
0.23
0.25
0.28
0.30
100 m 0.50
0.56
0.63
0.69
0.75
0.60
0.68
0.75
0.83
0.90
100 m100 m
0.66
0.75
0.83
0.92
1.00
(a) Iex
~ 12 W/cm2
Experiment Simulation
Simulation Details2nd Simulation1st Simulation
Generation, recombination, and diffusion
with augmented defect-relatedrecombination in dislocation pixel:
Recombination Assumptions:1. Defect levels clustered near the
middle of the gap –no thermal excitation out of traps
2. (# of electrons) = (# of holes) = n
Theoretical Efficiency: 2
2
BnAn
Bn
ateRadiativeRDefectRate
ateRadiativeREfficiency
Recombination Improvements:1. Defect level distribution can be
tailored to achieve the best fit2. Theory accounts for thermal
excitation out of traps
3. (# of e-s in conduction band) = n can differ from (# of holes in valence band) = p
4. (# of trapped e-s) = dncan differ from(# of trapped holes) = dp
Theoretical Efficiency:pnBdnpdpnA
pnBEfficiency
)(
Diffusion
rate
ionrecombinat
Radiative
rate
ionrecombinat
Defect
rate
Generation
t
tn
)(
2
2 )(
dx
ndDiffusionLaplacianD n
Better Simulated Images
(d) Iex
~ 0.012 W/cm2
(b) Iex
~ 1.2 W/cm2
(c) Iex
~ 0.12 W/cm2
0.20
0.23
0.25
0.28
0.30
0.50
0.56
0.63
0.69
0.75
0.60
0.68
0.75
0.83
0.90
0.66
0.75
0.83
0.92
1.00
(a) Iex
~ 12 W/cm2
(c) Iex
~ 0.12 W/cm2(d) I
ex ~ 0.012 W/cm2
(b) Iex
~ 1.2 W/cm2
100 m0.20
0.23
0.25
0.28
0.30
100 m 0.50
0.56
0.63
0.69
0.75
0.60
0.68
0.75
0.83
0.90
100 m100 m
0.66
0.75
0.83
0.92
1.00
(a) Iex
~ 12 W/cm2
Experiment Simulation
-0.4 -0.2 0.0 0.2 0.4
105
1010
1015
De
nsi
ty o
f S
tate
s (c
m-3e
V-1)
Fractional Bandgap Energy
The Defect-Related Density of States
Valence Band
Conduction Band
EN
ER
GY
The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above.
Defect-Related Density of States Used for Better
Simulation
-0.6 -0.3 0.0 0.3 0.6
0
2x106
4x106
6x106
Fractional Bandgap Energy
(b) Bulk Pixels
Ev E
c
-0.6 -0.3 0.0 0.3 0.6
0
5x107
1x108
2x108
2x108
DO
S / (
cm-3eV
-1s-1
)
Fractional Bandgap Energy
(a) Dislocation Pixel
Ev
Ec
Confocal Photoluminescence Microscopy
Laser
Spectrometer
NotchFilter
Mirror
Sample
Lens
Translation
Stage
Lens
5 m 0
0.25
0.50
0.75
1.00
Experiment
Contrast Map
20 microns
SignalBulk
SignalLocalSignalBulkContrast
_
__
Photoluminescence Contrast
Aperture
Lens
Confocal Maps
(c) Iex
~ 6.2 KW/cm2(d) I
ex ~ 0.9 KW/cm2
(b) Iex
~ 78 KW/cm2
5 m
0
0.002
0.004
0.006
0.008
5 m0
0.11
0.23
0.34
0.45
0
0.06
0.13
0.19
0.25
5 m5 m
0
0.012
0.025
0.038
0.050
(a) Iex
~ 650 KW/cm2
Before
Confocal Maps
(c) Iex
~ 6.2 KW/cm2(d) I
ex ~ 0.9 KW/cm2
(b) Iex
~ 78 KW/cm2
5 m
0
0.001
0.002
0.003
0.0045 m
0
0.06
0.13
0.19
0.25
5 m
0
0.05
0.10
0.15
0.205 m
0
0.01
0.02
0.03
0.04
(a) Iex
~ 650 KW/cm2
(c) Iex
~ 6.2 KW/cm2(d) I
ex ~ 0.9 KW/cm2
(b) Iex
~ 78 KW/cm2
5 m
0
0.002
0.004
0.006
0.008
5 m0
0.11
0.23
0.34
0.45
0
0.06
0.13
0.19
0.25
5 m5 m
0
0.012
0.025
0.038
0.050
(a) Iex
~ 650 KW/cm2
Before After: Ugly Defects!
Confocal Maps of an Ugly Defect
(c) Iex
~ 2 KW/cm2(d) I
ex ~ 0.9 KW/cm2
(b) Iex
~ 6.2 KW/cm2
20 m
0
0.002
0.004
0.006
0.008
20 m
0
0.025
0.050
0.075
0.100
20 m
0
0.11
0.23
0.34
0.45
20 m
0
0.2
0.4
0.6
0.8
(a) Iex
~ 25 KW/cm2
(c) Iex
~ 6.2 KW/cm2(d) I
ex ~ 0.9 KW/cm2
(b) Iex
~ 78 KW/cm2
5 m
0
0.001
0.002
0.003
0.0045 m
0
0.06
0.13
0.19
0.25
5 m
0
0.05
0.10
0.15
0.205 m
0
0.01
0.02
0.03
0.04
(a) Iex
~ 650 KW/cm2
High Magnification Low Magnification
Radial Contrast Profile
0 10 20 30 40 50
0.01
0.1
1
650 KW/cm2
78 KW/cm2
25 KW/cm2
6 KW/cm2
2 KW/cm2
0.9 KW/cm2
Con
tras
t
Distance (microns)
Radial Contrast Profile
0 10 20 30 40 50
0.01
0.1
1
650 KW/cm2
78 KW/cm2
25 KW/cm2
6 KW/cm2
2 KW/cm2
0.9 KW/cm2
Con
tras
t
Distance (microns)
0 20 40 60 80
0.1
1 0.9 KW/cm2
2 KW/cm2
Con
tras
t
Distance (microns)
Effective Diffusion Length
103 104 105 1061
10
after before
Dif
fusi
on L
engt
h (m
icro
ns)
Excitation (KW/cm2)
Effective Diffusion Length
103 104 105 1061
10
after before
Dif
fusi
on L
engt
h (m
icro
ns)
Excitation (KW/cm2)
Electrons?
Holes?
D Dislocation Electron HoleExcitation &Detection
L
D
Mid-Excitation
P
P
P
P
P
PP
P
P
P
+
P Pt. Defect - +
P
P
P
L
D
Low-Excitation
P
P
P
P
P
PP
P
P
P
+
P
P
P
++
++
+
+ ++
-
-
--
-
--
-
--
Top View of Confocal Measurement with Diffusion to a Dislocation
D Dislocation Electron HoleExcitation &Detection
L
DP
P
P
P
P
PP
P
P
P
+
P Pt. Defect - +
P
P
P+
++
+
+ ++
--
-
--
-
--
Top View of Confocal Measurement with Diffusion to a Dislocation
Mid-Excitation
L
DP
P
P
P
P
PP
P
P
P
+
P
P
P
+
+
++
+
+ -
--
-
--
--
High-Excitation
--- -
++
++
Side View of a Solar Cell Under High Illumination
DISLOCATIONS
PHOTONS
+
- +p/n JunctionE-Field
ELECTRICITY!
-
Side View of a Solar Cell Under Low Illumination
DISLOCATIONS
PHOTONS
+
- +
LOST!
-
p/n JunctionE-Field
OK
Conclusions
• Defects reduce solar cell efficiency by providing new recombination pathways (loss)
• Photoluminescence is a powerful tool for examining the properties of defects
• Depletion of electrons and holes near dislocations depends strongly on illumination
• The physics of confocal microscopy differs dramatically from the physics of imaging
• Ultimately, understanding diffusion near defects will facilitate better solar cell design
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