IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 4, NO. 4, JULY 2014 1119
A Novel Solar Simulator Based on a
Supercontinuum Laser for Solar Cell Device
and Materials Characterization
Tasshi Dennis, John B. Schlager, Senior Member, IEEE, and Kris A. Bertness, Senior Member, IEEE
Abstract—The design, operation, and application of a novel so-
lar simulator based on a high-power supercontinuum fiber laser
are described. The simulator features a multisun irradiance with
continuous spectral coverage from the visible to the infrared. By
use of a prism-based spectral shaper, the simulator can be matched
to any desired spectral profile, including the ASTM G-173-03 air-
mass 1.5 reference spectrum. The simulator was used to measure
the efficiency of gallium arsenide (GaAs), crystalline silicon (Si),
amorphous Si, and copper–indium–gallium–selenide (CIGS) thin-
film solar cells, showing agreement with independent measure-
ments. The pulsed temporal characteristic of the simulator was
studied and would appear to have a negligible influence on mea-
sured cell efficiency. The simulator light was focused to a spot of
approximately 8 μm in diameter and used to create micrometer-
scale spatial maps of full spectrum optical-beam-induced current.
Microscopic details such as grid lines, damage spots, and material
variations were selectively excited and resolved on GaAs and CIGS
cells. The spectral shaping capabilities were used to create output
spectra appropriate for selectively light-biasing multijunction cell
layers. The simulator was used to create variable blue-rich and
red-rich spectra that were applied to a GaInP/GaAs tandem solar
cell to illustrate the current-limiting behavior.
Index Terms—External quantum efficiency (EQE), metrology,
microscopy, multijunction, optical-beam-induced current, photo-
voltaic, responsivity, solar cell, solar simulator, spectral mismatch,
supercontinuum laser.
I. INTRODUCTION
FURTHER improvements to the efficiency of solar cells for
all materials and technologies depend critically on a bet-
ter understanding of their optical and electrical characteristics.
The study of defects caused by impurities or crystalline grain
boundaries, artifacts from deposition and growth processes, and
third-generation materials having microarrays or structured con-
duction paths could all potentially benefit from a solar simulator
offering a diffraction-limited focus. In addition, the character-
ization of multijunction materials could benefit from selective
light biasing of the different junctions made possible by a source
with an accurately shaped spectrum delivered by a single colli-
mated beam [1]. Solar simulators are typically based on lamps,
such as the xenon arc-lamp [2], or arrays of light-emitting diodes
[3], [4]. However, it is challenging to efficiently apply these light
Manuscript received November 27,2013; revised April 10, 2014 and February
20, 2014; accepted February 6, 2014. Date of publication May 26, 2014; date
of current version June 18, 2014. This papers constitutes work of the U.S.
government and is not subject to copyright.
The authors are with the National Institute of Standards and Technology,
Boulder, CO 80305 USA (e-mail: tasshi@nist.gov; john.schlager@nist.gov;
bertness@boulder.nist.gov).
Color versions of one or more of the figures in this paper are available online
Digital Object Identifier 10.1109/JPHOTOV.2014.2321659
sources to measurement systems that require focused and/or
spectrally shaped light. Fundamentally, bulb-based and point
light sources typically radiate into large solid angles, thereby
generating a low number of photons for any single spatial mode.
This low spatial coherence makes it difficult if not impossible to
efficiently use optical beam processing to achieve diffraction-
limited focusing and/or arbitrary spectral shaping.
Recently, high-power supercontinuum lasers that offer spec-
tral coverage from the visible (blue) out to the infrared have be-
come commercially available [5]. These white-light lasers rely
on optical-fiber amplifier technology to raise the peak power of
a seed laser to around 100 kW per pulse. Launching these am-
plified pulses into a photonic crystal fiber results in the broaden-
ing of the spectrum of the seed laser through nonlinear optical
mixing. The nonlinear interactions are enabled by the unique
propagation characteristics of the photonic crystal fiber, which
include a single spatial mode and a flat chromatic dispersion pro-
file across the entire low-loss window of silica from about 400
to 2200 nm. The single mode creates a tight confinement of op-
tical power, and the flat dispersion allows phase-matching over
a broad wavelength range. As a result, a broad spectrum con-
taining watts of optical power can be generated within the sin-
gle spatial mode, enabling diffraction-limited free-space beam
propagation. However, unlike a flashed arc lamp that is pulsed
with a millisecond period, the supercontinuum laser emits a
train of subnanosecond pulses with a repetition rate at mega-
hertz frequencies. A primary concern when considering these
novel sources for solar simulation is whether the devices and
materials being investigated will respond as if being illuminated
by continuous sunlight.
In this study, we report on the use of a supercontinuum laser
as a solar simulator and demonstrate the characterization of pho-
tovoltaic devices and materials. After shaping the spectrum of
the laser, we measure the efficiency of a variety of sample solar
cells and examine whether the temporal characteristics of the
light influence the measured efficiency. We report on the appli-
cation of our focused simulator to the microscopic generation of
photocurrent and present spatial maps of full-spectrum optical-
beam-induced current (FS-OBIC) from sample solar cells. The
ability to arbitrarily spectrally shape the simulator was utilized
to create light-biasing spectra for the characterization of a tan-
dem cell with top-junction current-limiting behavior.
II. SIMULATOR DESIGN
To construct our simulator, we used a commercially available
supercontinuum laser having more than 8 W of emission and a
U.S. Government work not protected by U.S. copyright.
