Exploring defect level position and occupation in GaAsP via recombination

analysisKyungTaek Lim and Tim Gfroerer, Davidson College, Davidson, NC

Mark Wanlass, National Renewable Energy Lab, Golden, COAbstract

Motivation: Solar Cell Operation

When a photon is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. Then an internal electric field sweeps the electrons and holes away, creating current through the solar cell and providing electricity.

Conduction Band

Valence Band

PHOTON

ELECTRON

E-Field

HOLE

-

E-Field

+ +

++

-

ABSORPTION

Field

-E--

-

I n our experiment, laser light is incident upon the semiconductor sample, producing luminescence. We collect this emitted light and f ocus it onto a photodiode f or effi ciency measurements, or into the spectrometer f or spectral analysis.

Experimental Setup

When electron-hole pairs are created in a semiconductor, recombination can occur either radiatively or nonradiatively. We measure the radiative efficiency (the fraction of recombination that is radiative) as a function of laser excitation intensity and temperature to characterize recombination mechanisms and predict how defects will impact solar cell performance. In a 1.6eV-bandgap GaAsP heterostructure, we observe complex changes in the photoluminescence spectrum and radiative efficiency with excitation and temperature. The changes suggest that two defect bands may be contributing to recombination in this alloy. Further evidence for the presence of these bands is obtained by modeling the defect level occupation and recombination statistics. The analysis suggests that thermal activation out of shallow defect levels can improve the performance of solar cells incorporating GaAsP.

1.60 1.65 1.70 1.75 1.80

102

103

104

105

Inte

nsit

y (

Ab

s. U

nit

)

Energy (eV)

78K 120K 165K

Laser Power:

2.83 W/cm2

B-B

D-R

Luminescence Spectra at Low Excitation

Radiative recombination reveals a Defect-Related (D-R) transition approximately 40 meV below the Band-to-Band (B-B) emission. The D-R transition is primarily radiative at 78K, but becomes primarily nonradiative at higher temperatures.

D-R

D-R

B-B

B-B

1.60 1.65 1.70 1.75 1.80104

105

106

107

108

Inte

nsit

y (

Ab

s. In

ten

sit

y)

Energy (eV)

78K 120K 165K

Laser Power:

74 W/cm2

B-BD-R

Luminescence Spectra at High Excitation

The radiative effi ciency increases dramatically with laser power at 165K, while the D-R band is still prominent in the 78K spectrum. This behavior suggests that another, lower-density defect level is contributing to the recombination under low excitation at 165K.

B-B

B-B

D-R

D-R

Theoretical FitThe Model:

1. ∆p∆n is the product of hole and electron densities in the valence and conduction bands, respectively.

2. Defect-related recombination is assumed to be purely non-radiative so we do not include the 78K results in the fi t.

3. Temperature-dependent def ect level occupation allows f or thermal activation out of shallow traps.

4. The least error between theory and experiment is obtained with the defect-related density of states (DOS) shown in the inset, which matches our interpretation of the data.

1030 1032 1034 1036

1022

1023

1024

1025

EV

DO

S

EnergyE

C

120K 165K 207K 250K 297KR

ecom

bina

tion

Rat

e (c

m-3

s-1)

pn (cm-6)

Def ect-Related Trapping and Recombination

At low temperature, electrons can recombine with holes by hopping through defect levels and releasing heat. This loss of energy as heat reduces the effi ciency of a solar cell. At high temperature, trapped electrons can escape to conduction band and then be swept away to create electricity or drop down to valence band by emitting photons.

Defect Level

At High Temperature

Conduction Band

Valence Band

ENER

GY

--

-

-

-

++ + + +

Band to Band (Radiative )

Recombination

kT

PHOTON

Conduction Band

Valence Band

ENER

GY

-

+

HEAT

--

+ +

-- -- --

-

-

-

+ +

Defect-Related (Non-Radiative) Recombination

At Low Temperature

HEAT

Trapping Escape

Effi ciency Results: Band Gap Energy = 1.75 eV

•Excitation-dependent radiative effi ciency measurements are consistent with the luminescence spectra

•Radiative effi ciency drops dramatically with temperature between 78K and 120K via 2 mechanisms:

•First, radiative recombination via the shallow D-R band becomes nonradiative

•Second, transitions through another, less-dense defect level begin to contribute to nonradiative recombination

•Radiative effi ciency increases again above 120K as carriers are thermally activated out of the shallow D-R band.

1022 1023 1024 1025

0.0

0.2

0.4

0.6

0.8

1.0

T 78 T120 T165 T207 T250 T290

Rad

iativ

e Ef

ficie

ncy

Recombination Rate (cm-3s-1)

The Low-Density Def ect Band

The hypothetical low-density defect level which reduces the radiative effi ciency at low excitation and high temperature is not an eff ective recombination center at 78K. Perhaps there is an energetic barrier that surrounds these defects, inhibiting occupation at 78K.

Conduction Band

Valence Band

- - - -

Photons

At 77.7K

Conduction Band

Valence Band

-

- -

-

Defect Related Recombination

Heat

Trapping

At 120K

B- B RadiativeRecombination

The Higher-Density Shallow Band

The D-R recombination at 78K is radiative – the D-R luminescence is as strong as the B-B emission and the radiative effi ciency is relatively high. At 120K and above, radiative D-R recombination quenches, producing heat. However, thermal activation out of the shallow level enhances B-B recombination at 165K and above.

At 77.7K

Conduction Band

Valence Band

- - - - - -

Defect Related Recombination

Photons

kT

At 120K

Conduction Band

Valence Band

- - - - - -

Defect Related Recombination Heat

kT

- -kT

---

B- B RadiativeRecombination

Conduction Band

Valence Band

At 165K

Conclusions, Significance, and Acknowledgement

• We observe complex changes in the radiative efficiency of GaAsP with excitation and temperature.• The changes suggest that 2 defect bands may be contributing to recombination.• The presence of 2 bands, with expected characteristics, is confirmed by modeling the defect level occupation and recombination statistics.• Thermal activation out of shallow defect levels can reduce the negative impact of these states on GaAsP-based solar cell performance.• We thank Jeff Carapella for growing the test structure and we acknowledge the Donors of the American Chemical Society – Petroleum Research Fund for supporting this work.