A New Real-Time Quantum 

Efficiency Measurement System

David L. Young1, Brian Egaas2, 

Scott Pinegar1, and Paul Stradins1

1 National Renewable Energy Laboratory, Golden, Colorado 80401 (USA) 

2 Colorado School of Mines, Golden, Colorado 80401 (USA) 

NREL/PR-520-43323

Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, California
Information-Rich Quantum Efficiency Graphs

c-Si/a-Si, heterojunction

• Spectral response (current loss)

• Spatial response

“Blue” – front, “Red” - back

surfaces, bulk

thin-film layers (thickness, composition, Bandgap)

• Diffusion length (modeling) (Kieliba, JAP 2006)

• Recombination centers (QE(T)) (Wagner,APL 2003)

• Junction physics, impurity diffusion (QE(V)) (Batzner TSF 2003),(a-Si:H cells)

CIGS thin-film cell
Information-Rich Quantum Efficiency Graphs

c-Si/a-Si, heterojunction

• Spectral response (current loss)

• Spatial response

“Blue” – front, “Red” - back

surfaces, bulk

thin-film layers (thickness, composition, Bandgap)

• Diffusion length (modeling) (Kieliba, JAP 2006)

• Recombination centers (QE(T)) (Wagner,APL 2003)

• Junction physics, impurity diffusion (QE(V)) (Batzner TSF 2003),(a-Si:H cells)

CIGS thin-film cell
Information-Rich Quantum Efficiency Graphs

c-Si/a-Si, heterojunction

• Spectral response (current loss)

• Spatial response

“Blue” – front, “Red” - back

surfaces, bulk

thin-film layers (thickness, composition, Bandgap)

• Diffusion length (modeling) (Kieliba, JAP 2006)

• Recombination centers (QE(T)) (Wagner,APL 2003)

• Junction physics, impurity diffusion (QE(V)) (Batzner TSF 2003),(a-Si:H cells)

CdS

ZnO:Al

ZnO

Bandgap

CIGS thin-film cell
Monochromator

Serial measurement ~ 5-20 mins

Electronically-controlled

light source

Parallel measurement ~ 0.1 sec

1000x decrease in time

New method

So, why don’t we use QE graphs more in research and industry?

Traditional QE method Time! (money, lack of graduate students)

chopper Lock-in Amp.
New method: Real –Time Quantum Efficiency measurement system (RTQE)

Sine wave generators

(frequency, amplitude)

f1 f2 f3 f4

...

Full-spectrum

LED array

Light gathering device

Focusing optics

Electronically-Controlled LED light source

LED

(#, color)

λpeak

(nm)

Drivefrequency 

(Hz)

1, Red 700 1000

2, Yellow 600 1153

3, Green 550 1262

4, Blue 470 875

... - -

Parallel processing of information 

from an array of spectral channels 

encoded in modulated frequency bands
New method: Real –Time Quantum Efficiency measurement system (RTQE)

Electronically-Controlled LED light source

Humphreys, MRS Bulletin 33 April (2008)

LED materials, wavelengths
New method: Real –Time Quantum Efficiency measurement system (RTQE)

58-color LED array (2004)

Electronically-Controlled LED light source
f

f, Sine wave drive frequency

λ, LED emission wavelength

Current-to-voltage

preamp

I

V

LED

(λpeak)

f1 f2 f3 f4

Current-to-voltage

preamp

voltage vs time

power spectrum

FFT

f λ1, QE1=K1( I (fλ1))

, QE2=K2( I (fλ2))

f λ3, QE3=K3( I (fλ3))

f λ4, QE4=K4( I (fλ4))

FFT

f λ, QE=K( I (fλ))

f

I

power spectrum

Frequency (Hz)

LED1

LED2

LED3 LED4

analog-to-digital 

converter DAQ card

voltage vs time

Time

f

software

Principle of operation

f λ2 f λ2
Proof of concept: 10-LED Real-Time QE system prototype

10 LEDs were chosen to span the 

Spectral response range of C-Si.

10-LED Array

Optical Fibers

Focusing optics

Test cell
f1 f2 f3 f10 f4 f9 f8 f7 f6 f5

10-LED array prototype Raw Data

Power Spectrum

Peak Filter/Peak Integration

QE

Standard QE (20 mins)

LED QE (real-time)
Technical Considerations:

1) LED emission: Spectral width, Asymmetric spectra, Spectral overlap

Calibration accounts for LED spectra using Singular Value Decomposition mathematics

2) LED drive signal: sinusoidal LED emission, non-multiple drive frequencies.

high data acquisition < drive frequency < 1/response time

3) Data acquisition rate: 2x highest LED drive frequency (avoid aliasing)

Singular Value Decomposition (SVD)

“least-squares-fit” for matrices

Current (LED) = Illumination (LED,λ) * QE(λ)

=

I1

I2

I3

.

.

.

λ11,λ12,λ13,...

λ21,λ22,λ23,...

λ31,λ31,λ33,...

..........

QE1

QE2

QE3

.

.

.

*

(QE (λ)) = (V)(S)(UT)(current(LED)) LED #

LED spectrum
• fast

• inexpensive

• all solid-state 

• robust

• Replace traditional lab-based QE systems

Expanded Applications

• In-line diagnostics

• Spatial QE mapping

• Multi-junction QE measurements

The Real-Time QE system is:

Parallel processing of information 

from an array of spectral channels 

encoded in modulated frequency bands
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
QE

Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

JV QE

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

JV QE

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

JV QE

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
QE

Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

JV QE

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

JV QE

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
Benefits: 

• In-line diagnostics for process control

• Device physics feedback

• Spectral-matching cell binning to maximize module KW-hr output

JV QE

Bin B (good blue response)

Bin E (good red response)

In-line QE for solar cell manufacturing

Bin A

Bin C

Bin D

Bin F

Bin G
X (mm)

Y (mm)

Spatial spectral-response Mapping 

(cells, modules)
X (mm)

Y (mm)

Spatial spectral-response Mapping 

(cells, modules)

Cell and module uniformity

Defects

Process control
Electronic control of light spectrum

Multi-junction solar cell QE measurements

• RTQE measurement on subcell under test

• Light bias other subcells to allow transport
Real Time Quantum Efficiency Technique

• Electronically controlled full-spectrum LED light source

• Parallel data processing

• Simple, robust, “inexpensive”, solid-state, FAST (~1000 vs 1 QE Measurement during this talk)

Expanded Applications

• Industrial In-line diagnostics, spectral-matching cell sorting

• Spatial spectral response mapping

• Electronic filtering – tandem solar cell QE measurements

• Technique applicable to other spectroscopy techniques

Further information: 

Technical: david_young@nrel.gov 

Technology Licensing: david_christensen@nrel.gov 
Pure sine wave drive voltage:

FFT

FFT

fast data acquisition rate <Drive frequencies < 1/minority life time, acquisition/2

No multiple frequencies
LED spectral width