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.

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