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
and Mark Wanlass
National Renewable Energy Lab, Golden, CO
~ Supported by the Charlotte Research Institute and
the American Chemical Society – Petroleum Research Fund ~
Some of the experiments and analysis by . . .
Mac Read and Caroline Vaughan (’10)
Ryan Crum and Mark Crowley (’11)
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
a
free atoms
V(r)
r
atomic crystal
Energy levels
Spacing decreasing
n=3
n=2
Periodic
Potential
Physlet*
* Physlet Quantum Physics: An Interactive
Introduction by Mario Belloni et al. (2006).
n=1
-
a
Solar Cell Operation
Conduction Band
-
E-Field
-
HEAT
ELECTRON
ABSORPTION
PHOTON
CURRENT
HOLE
-
Valence Band
+
E-Field
+
+
+
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.
Good Defects: Impurities for p/n
Junction Formation
+ + + ++
+ +
+
+
+ +
+ + P+ + +
+ + + +
+ + + + +
+
+
+
+
-
+
-
Depletion Layer
← E-Field ←
-
-
N -
Semiconductor Defects
Defect Level Physlet
~ from Physlet Quantum Physics: An Interactive Introduction
by Mario Belloni et al. (2006).
Dislocation Applet
Bad Defects: Defect-Related
Trapping and Recombination
Conduction Band
ENERGY
-
Defect Level
HEAT
HEAT
+
Valence Band
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.
Radiative Recombination and Efficiency
Conduction Band
-
light in
PHOTON
+
Valence Band
Radiative Rate ~ n x p
heat
light out
Radiative Efficiency = (light out) / (light in)
= (radiative rate) / (total recombination rate)
Photoluminescence Imaging
Experiment
Excitation-Dependent Images
2
2
Sample
(b) Iex ~ 1.2 W/cm
(a) Iex ~ 12 W/cm
Laser
Lowpass filter
100 m
1.00
0.90
0.92
0.83
0.83
0.75
0.75
0.68
100 m
0.60
0.66
2
Camera
2
(c) Iex ~ 0.12 W/cm
100 m
(d) Iex ~ 0.012 W/cm
0.75
0.30
0.69
0.28
0.63
0.25
0.56
0.23
0.50
100 m
0.20
Top View of Diffusion to
Dislocations
Simulated Images
Experiment
Simulation
2
2
2
100 m
(b) Iex ~ 1.2 W/cm
1.00
0.90
1.00
0.90
0.92
0.83
0.92
0.83
0.83
0.75
0.83
0.75
0.68
0.75
0.68
0.60
0.66
0.60
0.75
100 m
0.66
2
2
2
(c) Iex ~ 0.12 W/cm
100 m
2
(a) Iex ~ 12 W/cm
(b) Iex ~ 1.2 W/cm
(a) Iex ~ 12 W/cm
2
(c) Iex ~ 0.12 W/cm
(d) Iex ~ 0.012 W/cm
(d) Iex ~ 0.012 W/cm
0.75
0.30
0.75
0.30
0.69
0.28
0.69
0.27
0.63
0.25
0.63
0.24
0.56
0.23
0.56
0.21
0.20
0.50
0.18
0.50
100 m
Simulation Details
1st Simulation
2nd Simulation
Generation, recombination, and diffusion
with augmented defect-related
recombination in dislocation pixel:
 n (t )
t

 Generation
 
rate



Defect
Radiative





  recombinat ion  recombinat ion   Diffusion 



rate
rate



d ( n )
2
LaplacianD iffusion  D n 
dx
2
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:
Theoretical Efficiency:
Efficiency

RadiativeR ate
DefectRate
 RadiativeR ate
Recombination Improvements:
1. Defect level distribution can be
tailored to achieve the best fit
2. 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) = dn
can differ from
(# of trapped holes) = dp
Efficiency

Bn
2
An  Bn
2

Bn p
A ( n  dp  p  dn )  B  n  p
Better Simulated Images
Experiment
Simulation
1.00
0.90
0.92
0.83
0.83
0.75
0.75
0.68
100 m
2
100 m
1.00
0.90
0.92
0.83
0.83
0.75
0.75
0.68
0.66
0.60
0.60
0.66
2
2
(d) Iex ~ 0.012 W/cm
(c) Iex ~ 0.12 W/cm
2
(c) Iex ~ 0.12 W/cm
(b) Iex ~ 1.2 W/cm
(a) Iex ~ 12 W/cm
(b) Iex ~ 1.2 W/cm
100 m
2
2
2
2
(a) Iex ~ 12 W/cm
(d) Iex ~ 0.012 W/cm
0.75
0.30
0.69
0.28
0.75
0.30
0.63
0.25
0.69
0.28
0.23
0.63
0.25
0.56
0.56
0.23
0.50
0.20
0.50
0.20
100 m
10
15
10
10
-3
-1
Density of States (cm eV )
The Defect-Related Density of States
10
5
-0.4
Valence
Band
-0.2
0.0
0.2
Fractional Bandgap Energy
0.4
Conduction
Band
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
(a) Dislocation Pixel
(b) Bulk Pixels
8
2x10
6
6x10
-3
-1 -1
DOS /  (cm eV s )
8
2x10
6
4x10
8
1x10
6
2x10
7
5x10
0
0
Ec
Ev
-0.6
-0.3
0.0
0.3
0.6
Fractional Bandgap Energy
Ev
-0.6
Ec
-0.3
0.0
0.3
0.6
Fractional Bandgap Energy
Confocal Photoluminescence Microscopy
Photoluminescence Contrast
Contrast
Experiment
Mirror
Bulk _ Signal  Local _ Signal
Bulk _ Signal
Lens
Lens
Spectrometer
Notch
Filter
Laser

Aperture
Contrast Map
Lens
1.00
0.75
Sample
Translation
Stage
0.50
0.25
5 m
0
20 microns
Confocal Maps
Before
2
2
(b) Iex ~ 78 KW/cm
(a) Iex ~ 650 KW/cm
0.050
0.25
0.038
0.19
0.025
0.13
0.012
5 m
0.06
5 m
0
0
2
2
(c) Iex ~ 6.2 KW/cm
(d) Iex ~ 0.9 KW/cm
0.45
0.008
0.34
0.006
0.23
0.004
0.11
5 m
0
5 m
0.002
0
Confocal Maps
Before
After: Ugly Defects!
2
2
2
2
(b) Iex ~ 78 KW/cm
(a) Iex ~ 650 KW/cm
(b) Iex ~ 78 KW/cm
(a) Iex ~ 650 KW/cm
5 m
5 m
0.25
0.038
0.19
0.03
0.15
0.025
0.13
0.02
0.10
0.06
0.01
0.05
0
0
0
0.012
5 m
5 m
0
2
2
2
(c) Iex ~ 6.2 KW/cm
5 m
0.20
0.050
0.04
2
(c) Iex ~ 6.2 KW/cm
(d) Iex ~ 0.9 KW/cm
5 m
(d) Iex ~ 0.9 KW/cm
5 m
0.25
0.004
0.006
0.19
0.003
0.23
0.004
0.13
0.002
0.11
0.002
0.06
0.001
0
0
0
0.45
0.008
0.34
0
5 m
Confocal Maps of an Ugly Defect
High Magnification
(b) Iex ~ 6.2 KW/cm
(a) Iex ~ 25 KW/cm
(b) Iex ~ 78 KW/cm
20 m
5 m
5 m
2
2
2
2
(a) Iex ~ 650 KW/cm
20 m
0.04
0.20
0.8
0.45
0.03
0.15
0.6
0.34
0.02
0.10
0.4
0.23
0.01
0.05
0.2
0.11
0
0
0
0
2
2
2
(c) Iex ~ 6.2 KW/cm
5 m
Low Magnification
5 m
2
(c) Iex ~ 2 KW/cm
(d) Iex ~ 0.9 KW/cm
(d) Iex ~ 0.9 KW/cm
20 m
20 m
0.25
0.004
0.100
0.008
0.19
0.003
0.075
0.006
0.13
0.002
0.050
0.004
0.06
0.001
0.025
0.002
0
0
0
0
Radial Contrast Profile
650
78
25
6
2
0.9
Contrast
1
2
KW/cm
2
KW/cm
2
KW/cm
2
KW/cm
2
KW/cm
2
KW/cm
0.1
0.01
0
10
20
30
40
Distance (microns)
50
Radial Contrast Profile
650
78
25
6
2
0.9
0.9 KW/cm
2
2 KW/cm
1
2
Contrast
Contrast
1
2
KW/cm
2
KW/cm
2
KW/cm
2
KW/cm
2
KW/cm
2
KW/cm
0.1
0.1
0.01
0
10
20
30
40
Distance (microns)
50
0
20
40
60
Distance (microns)
80
Effective Diffusion Length
Diffusion Length (microns)
after
before
10
1
3
10
4
5
10
10
2
Excitation (KW/cm )
6
10
Effective Diffusion Length
Diffusion Length (microns)
after
before
Electrons?
10
Holes?
1
3
10
4
5
10
10
2
Excitation (KW/cm )
6
10
Top View of Confocal Measurement with
Diffusion to a Dislocation
Low-Excitation
+P
P
+P
-
L
P
+
P
P
D
P
Excitation &
Detection
-
P
+
P
P
P
P
Mid-Excitation
-
P
P
P
D Dislocation
+P
+P
+P
P
P
Pt. Defect
-
+P -
P
- +P
D -
L
-
+P
-
P
P
Electron
P
+
Hole
Top View of Confocal Measurement with
Diffusion to a Dislocation
High-Excitation
Mid-Excitation
+
P
+P
+P
+
P
+P
P
-
+P -
P
- +P
D -
L
-
+P
Excitation &
Detection
- + - ++ -+ P
+
- - + L - -+
+ - +
P
+ --+P -
P
P
D
P
P
P
P
P
P
D Dislocation
P
P
P
P
P
Pt. Defect
-
Electron
P
+
Hole
Side View of a Solar Cell Under
High Illumination
PHOTONS
DISLOCATIONS
ELECTRICITY!
+
p/n Junction
E-Field
-
+
-
Side View of a Solar Cell Under
Low Illumination
DISLOCATIONS
PHOTONS
OK
LOST!
+
p/n Junction
E-Field
-
+
-
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