Photocapacitance measurements on GaP alloys for high efficiency solar cellsDan Hampton and Tim Gfroerer, Davidson College, Davidson, NC

Mark Wanlass, National Renewable Energy Lab, Golden, CO

AbstractMotivation: Multi-junction solar cells

If higher energy photons are absorbed in higher bandgap alloys, the heat loss caused by excess photon energy relative to the gap is reduced.

400 800 1200 1600 2000 2400

0.00

0.25

0.50

0.75

1.00

1.25

1.502.0 eV GaInP

1.75 eV GaAsP

So

lar

Sp

ectr

al Ir

rad

ian

ce

(W

m-2n

m-1)

Wavelength (nm)

GaAs bandgap

Visible

Photocapacitance: Optically Stimulated Escape

Deep level transient spectroscopy (DLTS) employs transient capacitance measurements on diodes during and after the application of a bias pulse to monitor the capture/emission of carriers into/out of defect-related traps. The photocapacitance experiment is a modified version of DLTS that uses light (rather than heat) to excite the charge carriers out of traps.

Photocapacitance Experimental Setup

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.

Conduction Band

Valence BandIn

crea

sing

Ene

rgy

For

Hol

es

+Hole

Cap

ture

Therm

al Escape

+

Tra

p D

epth

While stacking materials of different bandgaps will increase theefficiency of solar cells, it will also create defects within the device because of lattice-mismatching.

Defects provide energy levels that restrict the movement of charge carriers. This inhibits the production of electricity. Once a carrier becomes trapped, it must be excited out of the defect state either thermally or optically.

AcknowledgementsWe thank Jeff Carapella for growing and processing the test structures. We also thank the Donors of the American Chemical Society – Petroleum Research Fund and the Davidson Research Initiative for supporting this work.

Defect Levels

Large bandgap materials are an essential component of ultra-high efficiency solar cells. While multi-junction devices harness more of the solar spectrum for electricity production, challenges in the construction of a lattice-mismatched system lead to defects that trap charge carriers and inhibit overall efficiency. Characterization of these defects may enable device engineers to minimize their effects. This work uses photocapacitance measurements to obtain optical escape energies from defect levels in high-bandgap GaInP and GaAsP alloys. We combine these optical results with previous thermal measurements to construct a working model that incorporates a lattice configuration-dependent energy structure. The model explains why the optical escape energy is significantly larger than the thermal capture and escape energies, and in the GaAsP device, it helps explain why the number of escaping carriers depends on the energy of the incident light.

N+ P+

+

Depletion Layer

-

-

-+

-

--

+

+

+

+

-

-

- -

-

--

---

--

-

--

-

--

-

---

-

- -

+

++

+

Depletion without bias

77 K

The computer operates the temperature controller and retrieves data from the digital oscilloscope. The pulse generator applies the reverse and pulse biases to the sample while the capacitance meter reads the resulting change in capacitance as a function of time. During this process, the sample is illuminated with a particular wavelength of light from the monochromator.

GaInP Results

0.7 0.8 0.9 1.0 1.1 1.2 1.3

10-12

10-11

Cro

ss S

ecti

on

(cm

2 )

Energy (eV)

EThreshold

= 0.695 (eV)

Cross Sections GaInP T = 77K

50 100 150 200 250 300

0.320.340.360.380.400.420.440.460.480.500.520.540.560.580.60

Optical Escape

Thermal Escape

Cap

acit

an

ce (

nF

)

Temperature (K)

Capacitance vs Temperature

Thermal Capture

PHOTON

+

Optical E

scape

PHONON

Optical Escape

Working Model for Observed Threshold Energies

0 20 40 60 80 100

450

500

550

600

650

700

750

800

850

Ca

pa

cita

nce

(p

F)

Time (s)

975nm 1350nm 1150nm 1050nm 1800nm

Example Raw Data

GaAsP Results

The optical escape threshold energy of 0.695 eV is significantly higher than the thermal escape energy of 0.322 eV for GaInP found in previous work. These results support the model described below.

In the configuration-dependent model, the energy of the system changes as a function of the local configuration of the lattice. This means that lattice vibrations, or phonons, can push the defect level closer to the valence band, but photons, which carry very little momentum , have no effect on the defect level. As a result, the optical escape energy is much larger than the thermal escape and capture energies.

PHOTON

Valence Band

Defect Levels

Thermal Escape (0.394 eV)

Thermal Capture (0.370 eV)

Enlarg

ed V

iew

Optical Cross Sections

Hig

h E

ner

gy

Lig

ht

Low

En

erg

y Li

gh

t

As the device cools, charge carriers become frozen into the defects (thermal capture), which extends the depletion region and causes the capacitance to decrease. The charge carriers escape when the device is exposed to light. On the warming cycle the thermal capture occurs at a lower temperature than the thermal escape (the thermal capture energy is smaller than the thermal escape energy).

When the energy of the light is reduced, the optical escape rate decreases as expected, but the transient amplitude is also reduced. The reduced amplitude indicates that carriers in some traps are not optically activated by the lower energy light. This result implies the defects in our GaAsP device have a range of energies as shown in the diagram above. For a given defect energy, the optical cross section, which control the optical escape rate, depends on the energy of the incident light. The diagram also shows how low energy light only activates traps in the upper portion of the energy range.