PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696
Published online 29 October 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2709
ACCELERATED PUBLICATION
Pushing the limits of concentrated photovoltaic solar
cell tunnel junctions in novel high-efficiency GaAs
phototransducers based on a vertical epitaxial
heterostructure architecture
Denis Masson, Francine Proulx and Simon Fafard*
Azastra Opto Inc, M50-IPF206, 1200 Montreal Rd., Ottawa, ON K1A 0R6, Canada
ABSTRACT
A monolithic compound semiconductor phototransducer optimized for narrow-band light sources was designed for
achieving conversion efficiencies exceeding 50%. The III-V heterostructure was grown by metal-organic chemical vapor
deposition, based on the vertical stacking of 5 partially absorbing GaAs n/p junctions connected in series with tunnel
junctions. The thicknesses of the p-type base layers of the diodes were engineered for optimal absorption and current
matching for an optical input with wavelengths centered near 830 nm. Devices with active areas of ~3.4 mm2 were
fabricated and tested with different emitter gridline spacings. The open circuit voltage (Voc) of the electrical output is
five times or more than that of a single GaAs n/p junction under similar illumination. The device architecture allows
for improved Voc generation in the individual base segments because of efficient carrier extraction while simultaneously
maintaining a complete absorption of the input photons with no needs for complicated fabrication processes or reflecting
layers. With illumination powers in the range of a few 100 mW, the measured fill factor (FF) varied between 88 and 89%,
and the Voc reached over 5.75 V. The data also demonstrated that a proper combination of highly doped emitter and window layers without gridlines is adequate for sustaining such FF values for optical input powers of several hundred milliwatts. As the optical input power is further increased and approaches 2 W (intensities ~58 W/cm2), the multiple tunnel
junctions sequentially exceed their peak current densities in the case for which typical (n++)GaInP/ (p++)AlGaAs concentrated photovoltaic tunnel junctions are used. Lower bandgap tunnel junctions designed with improved peak current
densities result in phototransducer devices having high FF and conversion efficiencies for up to 5 W optical input powers
(intensities ~144 W/cm2). Measurements at different temperatures revealed a Voc reduction of 6 mV/°C at ~59 W/cm2.
Copyright © 2015 John Wiley & Sons, Ltd.
KEYWORDS
phototransducers; III-V n/p junctions; heterostructures; GaAs, photon recycling; monolithic; MOCVD; tunnel junctions; concentrated photovoltaic
*Correspondence
Fafard, Simon, Azastra Opto Inc, M50‐IPF206, 1200 Montreal Rd., Ottawa, ON K1A 0R6, Canada.
E‐mail: simon.fafard@azastra.com
Also at Institut Interdisciplinaire d’Innovation Technologique (3IT), Université de Sherbrooke, Sherbrooke, Québec, Canada.
Received 29 December 2014; Revised 29 July 2015; Accepted 1 October 2015
1. INTRODUCTION
High performance tunnel junctions have been developed for
concentrated photovoltaic (CPV) solar cell applications. High
peak tunneling currents and optical transparency are key
requirements for such tunnel junctions. CPV solar cells having conversion efficiencies exceeding 40% for concentrations
up to and in excess of 1000 suns (~100 W/cm2) are now
Copyright © 2015 John Wiley & Sons, Ltd.
routinely obtained with success by using highly doped (n++)
GaInP/(p++)AlGaAs tunnel junctions [1,2]. Typically, for
multijunction solar cells operating above 1000 suns, the peak
tunneling current density of the tunnel junctions must be significantly greater than 14A/cm2. These high bandgap tunnel
junctions are therefore interesting technological building
blocks for other photonic devices operating with similar
photocarrier current densities. In addition, for the case of
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D. Masson, F. Proulx and S. Fafard
Tunnel junctions in novel vertical epitaxial heterostructure
single junction photovoltaic devices, record efficiency solar
cells have been demonstrated with GaAs n/p junctions
[3–6]. Also, in the field of CPV, such GaAs n/p junctions
have been shown to be well suited for operating at high
photon fluxes [7]. Furthermore, it has been demonstrated
that thin single p-n junctions can be advantageous for
obtaining higher open circuit voltages (Voc) [8–12]. However, a thin junction may not absorb all the input photons,
and the reduction in short-circuit current (Isc) typically
nullifies the gain in Voc unless complicated architectures
or manufacturing processes are engineered.
2. A NOVEL APPLICATION FOR
HIGH PEAK CURRENT TUNNEL
JUNCTIONS
Optical fiber technology is steadily replacing copper wires
for data transmission. In principle, optical fibers can also be
used for power transmission where light instead of electrons
are used. This is a field often referred to as power-over-fiber
[13,14]. While the transmission loss of light in an optical fiber can be quite small, the losses associated with the light
sources, generally lasers or LEDs, and the phototransducers
are still too large to compete with traditional metal wires
for efficient power transmission. However, unlike metals,
an optical fiber made of glass is immune to lightning, high
voltage, radio frequency noise, magnetic fields, and corrosion. Glass will not react with most chemicals and will not
spark. All of these make power-over-fiber an attractive candidate for special applications in utilities dealing with high
voltages, transmission towers, medical, and telecom equipment manufacturers as well as for chemical plants, refineries,
and aviation applications dealing with corrosive and explosive materials [15–21].
It is often beneficial for such optical power over fiber applications, or other applications of photonic power delivery
systems, to obtain an electrical output voltage higher than
the input light photovoltage (hν/e). For example, the
phototransducer can be used to power sensors, communication devices, actuators, gauges, or other optoelectronic devices requiring hundreds of milliwatts or watts of power
at voltages typically between 2 and 12 V direct current, depending on the application. It is therefore desirable to have
a photovoltaic (PV) device or a phototransducer capable of
producing an output voltage that is usually more than twice
the input light photovoltage, for example, typically >5 V
for many electronic applications. Consequently, single p-n
junction PV devices have the limitation that their output
voltage is lower than desirable, and multi-junction PV devices have the limitation that they cannot operate effectively with a narrowband optical source because of
current-matching constraints. However, this paper will
demonstrate that the high peak tunneling current tunnel
junction, as a technological building bloc developed in the
field of CPV, can greatly benefit novel phototransducers
designs.
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2.1. Phototransducers based on a novel
vertical epitaxial heterostructure
architecture utilizing concentrated
photovoltaic solar cell tunnel junctions
In order to increase the output voltage of the phototransducers
to higher levels, parquet geometries of single or tandem junction devices have previously been devised using planar configurations with multiple series connections [22–27]. The
segments are typically arranged in pie-shaped configurations,
or in groups fabricated in circular patterns. The number of
segments connected in series is selected to obtain the desired
device output voltage. The maximum power conversion efficiency occurs when photo-current matching is obtained between the series-connected segments. Therefore, this
approach requires a segment geometry and alignment that
are well matched to the intensity distribution of the impinging
light [28]. Furthermore, wafer processing in this approach requires to first separate and then re-connect the various segments, therefore reducing or shadowing the active surface
area, which decreases the overall conversion efficiency. The
performance of the planar geometry can also be limited by
the high sheet conductivity necessary for the lateral current
extraction, not only from the front contact but also from the
back contact.
Instead here, we engineered a vertical heterostructure design capable of higher efficiencies and also better tolerances
with respect to the alignment and the non-uniformity of the
input light. In this work, we study such phototransducers
intended for an optical input with wavelengths centered near
830 nm. The heterostructure has been designed [29] to produce output voltages significantly higher than the corresponding photovoltage of the input light. The vertical
heterostructure allows for higher overall power conversion
efficiencies and operation with reduced restrictions on the
impinging beam shape within the phototransducer active surface. The fabrication of such phototransducer is also significantly simplified by using a III-V heterostructure obtained in
a single epitaxy run and by including tunnel junctions (TJs)
similar to the TJ developed in multijunction CPV solar cells.
The TJs are used as connecting elements, joining together the
multiple base segments stacked in the propagating direction
of the incoming beam of light.
2.2. Novel phototransducer structure
The heterostructure used for the phototransducer devices is
shown in Figure 1. The layers were grown on 150 mm diameter (100) p-type GaAs substrates by MOCVD with an
Aixtron 2600 multi-wafer reactor using standard GaAs
growth conditions [29]. The light enters from the top.
The top n++GaAs layer serves only to form an ohmic contact with the top metal pattern and is otherwise etched
away. A 1 μm thick GaInP window layer, with an
n ~1E18cm3 doping, is included over the top pn-GaAs
junction. The composition for the window layer is
Ga0.51In0.49P, lattice-matched to GaAs. With a bandgap
of ~1.8 eV, Ga0.51In0.49P is transparent to the input light.
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
DOI: 10.1002/pip
D. Masson, F. Proulx and S. Fafard
Tunnel junctions in novel vertical epitaxial heterostructure
must be designed to absorb 1/5th of the incoming light.
The remaining light is transmitted to the second junction
designed to absorb another 1/5th. Because the intensity of
the transmitted light profile follows Beer’s law, it is
straightforward to calculate the optimal thicknesses of the
absorbing layers, based on the absorption coefficient of
the emitter and base layers. For a device with an absorption
coefficient α(λ), at a wavelength λ, this condition can be
written as follows:
ti
tiþ1
∫0 I i eαðλÞx dx ¼ ∫0
I iþ1 eαðλÞx dx
I iþ1 ¼ I i eαðλÞti
Figure 1. Schematic of the epitaxial layers used to obtain a harmonic photovoltage up-converting phototransducer based on a vertical epitaxial heterostructure architecture. Tunnel junctions (TJs)
are used as connecting elements to interconnect multiple GaAs
base segments (pn-GaAs). The thickness of each n/p-GaAs junction
increases progressively from top to bottom to obtain photo-current
matching of all the base-segments. The layers are approximately
drawn to scale, and the total thickness of all the base segments is
~3.5 μm allowing the absorption of substantially all the input light.
The absorbing region of the phototransducer is made of 5
pn-GaAs junctions, that is, five p-type base segments separated by four TJs. Results for two types of TJs are presented here. A number of phototransducers have been
fabricated based on such a heterostructure grown using a
typical (n++)GaInP/(p++)AlGaAs CPV tunnel junctions.
Another set of phototransducers are based on a lower
bandgap p++AlGaAs / n++AlGaAs TJs. The latter is
intended to increase the peak tunneling current density
for allowing operation of the phototransducers at higher
optical input powers. In all cases, the TJs used for a given
heterostructure are all identical. The III-V alloys used for
the TJs all have large enough bandgaps to be transparent
to the optical input. The five thin n/p-GaAs base segments
are therefore the only absorbing layers of the device.
2.3. Current matching the base segments of
the vertical heterostructure phototransducer
To optimize the power conversion, all n/p-GaAs base segments must generate the same currents, otherwise, the
overall current will be limited by the weakest junction. In
the structure studied, shown in Figure 1, the top junction
where ti is the thickness of the ith layer and Ii is the intensity of the light entering the ith layer. Also, to optimize the
conversion efficiency, most of the input light must be
absorbed. For that purpose, single junction GaAs n/p junctions are typically ~3.5 μm thick in practice. The thickness
is essentially very similar to the single junction GaAs solar
cell devices. Therefore, the thickness of the five GaAs base
segments of the phototransducer can here be determined
such that each base segment absorbs 0.2(1 e α(λ)3.5μm)
~19.6% of the incident optical input. For example, the
measured value for α(830 nm) is ~1.2E4cm1 [30]. For that
absorption coefficient value, the required thicknesses for
the GaAs absorbing segments are found to be 183, 235,
328, 548, and 2207 nm, from top to bottom, respectively.
Clearly, the thicknesses of the individual n/p junctions
are much smaller than standard single junction solar cells.
Similarly, it is also much thinner than the thickness of
multijunction solar cells. It is therefore particularly interesting to study the electro-optical properties of such n/p
junctions in heterostructures having progressively thinner
bases.
Also, because the amount of light absorbed in each base
segment is not expected to depend neither on the beam
non-uniformities nor on the beam alignment, the currents
should remain matched despite such possible variations.
The tolerance to the latter variations is expected to be a
fundamental advantage over devices based on planar configurations, as further discussed in Section 3.4 latter.
3. PHOTOTRANSDUCER
PERFORMANCE
3.1. Fabrication of the phototransducers
The phototransducer devices were fabricated using a CPV
solar cell process. A contact mask lithography process is
used to define the features of the top metal contacts. The
fabrication steps included the formation of narrow silver
gridlines (~5 μm wide). After annealing the top metal to
form ohmic contacts, the remaining top n++GaAs layer
was etched away using a selective wet etch process designed to stop on the GaInP window layer. A blanket metal
layer deposited on the backside of the substrate forms the
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
DOI: 10.1002/pip
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Tunnel junctions in novel vertical epitaxial heterostructure
p-electrode ohmic contact. The devices were electrically
isolated using first, a shallow sawing process, followed
by a cleaning step, and the deposition of dielectric passivation layers; and then, the final sawing cuts were made to
singulate the monolithic phototransducer devices. The
dielectric passivation layers also served as an antireflection coating designed for incident light at wavelengths in the vicinity of 830 nm.
From each 150 mm epiwafer, the fabrication process
produced several hundreds of individual phototransducer
devices, which were each tested to study their currentvoltage (I-V) characteristics. The geometries of the top
metal contact are shown in Figure 2. Designs D and E have
a gridline spacing of 325 and 425 μm, respectively, while
design F has no gridlines. All the devices had an open active area with a diameter of 2.1 mm (active area of
0.034 cm2).
3.2. Performance under 830 nm illumination
The phototransducers were tested in a modified commercial solar cell wafer tester, running at a concentration of
1000 suns but equipped with a 50 nm bandpass filter centered at a wavelength of 830 nm. The filter was inserted
in the light path to eliminate the other wavelengths from
the probe beam.
Figure 3 shows the resulting average values measured
in forward bias from the population of phototransducers
probed under the above illumination. The Isc, the Voc,
and the fill factor (FF) are plotted separately for the three
different gridline geometries shown in Figure 2. The
Figure 3(a) summarizes the resulting I-V characteristics
corresponding to the average values measured in Figure 3
(b),(c), and (d). The I-V curve of Figure 3(a) was obtained
from modeling 5 photodiodes connected in series by fitting
their ideality factors (n-factors) and the diode saturation
current (Is) to reproduce the measured combination of
Voc and FF for the observed Isc [31]. An ideality factor between 1.1 and 1.2 reproduces the observed FF and Voc
values. The Isc is normally linearly proportional to the
amount of light reaching the active regions of the devices.
Figure 2. Drawing of the top contact gridline geometries for
three different designs. The gridlines are shown by thin gray
lines and the surrounding gray areas is covered by metal. The
gridline spacing is 325 and 425 μm for designs D and E, respectively, while design F has no gridlines. The device active area is
2
0.034 cm . The geometrical gridline shadowing are 1.8%, 1.3%,
and 0% for D, E, and F, respectively.
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It can be increased by reducing the density and the width of
the current-collecting gridlines, which shadow the input
light. However, fewer and narrower gridlines will increase
the series resistance of the device, and a compromise typically needs to be established for CPV solar cells, depending on the light intensity under operating conditions. The
Voc is also expected to increase with the amount of incident light following a logarithmic dependence. The latter
can be approximated by Voc = (nkbT/e)ln(Il/Is), from the
ideal diode equation where n is the ideality factor; kbT/e
is the product of the Boltzmann constant (kb) and the temperature (T) over the electron charge (e), and Il is the
photo-current. The Voc of a single homo-junction PV device depends mainly on the bandgap, Eg, of the semiconductor material used. The Voc is typically close to (Eg–
0.4 eV)/e. The phototransducer output voltages measured
here, correspond to Voc values clearly higher than those
expected from a single GaAs device. For a PV device with
a bandgap of 1.42 eV, ~1.0 V is anticipated from incident
photons having an energy of ~1.49 eV. The photovoltage
up-conversion demonstrates the added Voc’s of the five individual n/p segments connected in series. Furthermore,
the output voltage is significantly greater than ~5 times
the ~1.0 V expected for typical GaAs n/p diode with a
3.5 μm thick base. The voltage increase is therefore a combination of the higher voltage expected for the higher photon fluxes [7] and also for GaAs n/p diodes having a
thinner base, yielding higher Voc’s [8,9].
3.3. Temperature coefficients
The performance of the GaAs phototransducers based on a
vertical epitaxial heterostructure architecture was measured
at different temperatures for various input powers. Figure 4
(a) shows the results obtained in the range of 10 and 50°°C
for the three optical input powers of 1.0 W, 1.5 W, and
2.0 W obtained from the output of a 830 nm laser diode.
The Voc reduction with temperature, as determined from
the slopes in Figure 4(a), varies slightly with the incident
optical power, between d(Voc)/dT = 7.3 mV/°C for 1 W
input to d(Voc)/dT = 6.4 mV/°C for 2 W optical input
power. The aforementioned values can be compared with
typical solar cell values. For GaAs, the bandgap variation
d(Eg)/dT is 0.45 meV/°C [32], and d(Voc)/
dT ~ 2.2 mV/°C has been measured for GaAs solar cells
around one sun concentration, that is, for ~0.1 W/cm2
[33–35]. CPV triple-junction cells have roughly three
times the temperature dependence with d(Voc)/dT between
6 and 7 mV/°C at one sun. The temperature dependence of CPV cells is reduced with concentration, decreasing to a d(Voc)/dT of ~4 mV/°C at 1000 suns (~100 W/
cm2) [33,36]. The temperature behavior observed in Figure 4(a) is therefore quantitatively in agreement with
multijunction CPV solar cell data and can be directly compared with the results from triple junction (3 J) devices by
simply rescaling the measured results by the number of
p/n junctions incorporated in the heterostructure. Therefore, the observed d(Voc)/dT ~-7.3 mV/°C for the
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
DOI: 10.1002/pip
D. Masson, F. Proulx and S. Fafard
Tunnel junctions in novel vertical epitaxial heterostructure
Figure 3. (a) I-V response under illumination with diode parameters matching the measured data for Isc, Voc, and FF shown in (b), (c),
and (d), respectively. The data represent the average values at 25 °C from hundreds of devices for the grid designs, D, E, and F
2
2
depicted in Figure 2. The active device area is 0.034 cm . The illumination intensity is ~8 W/cm . An average current density of
2
0.94A/cm is obtained for grid design E.
phototransducer with five base segments would correspond, after rescaling by a 3/5 factor, to a d(Voc)/dT of
4.4 mV/°C for a triple junction CPV solar cell. The latter
value is typically observed around a few hundred suns for
3 J CPV cells. It should be noted that the optical density on
the phototransducer at 1 W on an area of 3.4 mm2 corresponds to approximately 29 W/cm2. The phototransducer
therefore exhibits temperature behavior very similar to a
multijunction CPV solar cell.
The temperature behavior of the spectral response has
also been investigated. For example, in Figure 4(b), the
quantum efficiency (QE) is shown at different temperatures. The measured spectra demonstrate a near optimum
QE of ~100% / 5 at the peak near 830 nm at around room
temperature. It matches closely the wavelength at which
the heterostructure design was optimized. When the device
temperature is increased to 60 °C, the peak is shifted by
~0.2 nm/°C because of the bandgap shift. The QE amplitude however clearly remains near the same value at its
peak. It demonstrates that the current balancing in the various base segments is not affected significantly. Furthermore, the spectral response of the phototransducer has
been found to be relatively broad because, at least in part,
of photon reabsorption effects in the optically coupled thin
structures. A detailed study of the spectral response of the
phototransducers has indeed been providing interesting
insight in the luminescent coupling effects present in these
devices and is the subject of another publication [37].
3.4. Impact of enhanced sheet conductivity
The FF values of Figure 3 are obtained from the maximum
power point on the IV curves [38] and are all between ~88
and ~89%. Series and/or shunt resistances can affect the FF
values. The series resistance can be reduced by adding
gridlines but more gridlines diminish the Isc. The effect
is seen in Figure 3(b), (c), and (d) by comparing the segregated values of Isc, Voc, and FF obtained for the three different grid designs. Isc shows, as expected, a slight
increase from design D with the highest density of
gridlines to the grid-less design F. A detailed statistical
analysis of the measured Isc values confirms that the measured Isc values follow the expected geometrical
shadowing values, namely, 1.78 +/ 0.06%, 1.26 +/
0.03%, and 0% for grid patterns D, E, and F, respectively.
In Figure 3(c), Voc values of ~5.75 V are obtained at ~8 W/
cm2, even for the devices with no gridlines. Remarkably,
the FF results are most interesting because they show excellent values, even when no gridlines are present for design F. This confirms that the sheet conductivity,
obtained from the combined window and upper emitter
layers in our heterostructure design, is adequate to carry
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
DOI: 10.1002/pip
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Tunnel junctions in novel vertical epitaxial heterostructure
enhanced window thickness successfully implemented in
the phototransducer architecture.
The improved sheet conductivity leads to a less stringent requirements for the gridline spacing as demonstrated
earlier. Furthermore, it also leads to improved performances under non-uniform illumination. For example,
Figure 5 shows the measured I-V response under nonuniform illumination at 25 °C. Here, the output of a
multi-mode optical fiber was used to illuminate a portion
of the 0.034 cm2 active area. The data for a number of devices are shown in Figure 5(a) when the central portion of
the device is illuminated, here covering about half of the
active area. The resulting parameters are indicated in the
insert of the figure. Despite the non-uniform illumination,
FF values higher than 80% are observed. It allows maintaining high conversion efficiencies of around 60% for all
the different devices measured under these operating
conditions. Figure 5(b) shows the results obtained when
illuminating varied fractions of the active area. The distance between the fiber output and the device surface
was increased to produce an illuminated spot about the
size of the active area, thus creating a more uniform illumination. The I-V curves of Figure 5(b) are zoomed in the
Pmax region to compare both cases in details. It is apparent that the beam uniformity does not significantly impact
Figure 4. Temperature dependence measured on monolithic
GaAs phototransducer with five thin based segments together
having a total thickness of 3.5 μm as depicted in Figure 1. The
Voc is shown in (a) for optical input powers of 1.0 W, 1.5 W, and
2.0 W, at a wavelength of 830 nm. A linear regression is fitted to
the data to extract d(Voc)/dT for each input powers. The caption
displays the results. The QE is shown in (b) displaying near optimum QE of ~100% / 5 similarly for 10 and 60 °C.
the lateral current without introducing significant resistive
losses for illumination intensities of at least 10 W/cm2.
The measured Isc values for Figure 3 are about 32 mA, corresponding to an average current density Jsc ~0.94A/cm2 at
an optical input intensity of ~8 W/cm2. It is relevant to note
that at similar optical intensity for typical 3 J CPV solar
cells, the FF would be much lower for the gridline pitch
shown in Figure 2. For example, to verify the corresponding FF behavior in typical CPV cells under such conditions, single junction InGaP solar cells were grown and
fabricated with a typical CPV InGaP emitter and AlInP
window layer configuration and dopings [36]. These solar
cells were then tested under equivalent conditions and
using the same cell geometries as indicated in Figure 2.
The average FF data was then: FF ~ 71% for cell pattern
D, FF ~ 60% for cell pattern E, and FF ~ 47% for cell
pattern F. It thus further demonstrates that the design of
the window layer of our phototransducer offers an
improved sheet conductivity and therefore allows using
a reduced gridline pitch compared with a typical CPV
cell. By design, the improved sheet conductivity obtained
here is due to the different emitter/window layer configuration as mentioned earlier, and predominantly due to the
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Figure 5. Measured I-V response under non-uniform illumination at 25 °C. The output of a multi-mode optical fiber was used
to illuminate the central portion of the device covering about half
2
of the 0.034 cm active area. In (a), the data for a number of devices are shown and the resulting parameters are indicated in
the insert and feature high FF in all cases. I-V curves zoomed
in the Pmax region are compared in (b) when illuminating varied
fractions of the active area.
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
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D. Masson, F. Proulx and S. Fafard
the shape of the I-V curves under these conditions of nonuniform illumination.
The high values of FF, observed in Figure 3(d) or in
Figure 5, with the vertical heterostructure phototransducer
also demonstrated that the CPV solar cell TJ and BSF layer
design that we used here functioned properly. Indeed, for
optical input intensities up to at least 10 W/cm2, the
heterostructure is not introducing excessive internal series
resistance losses or unwanted barriers at the interfaces having bandgap discontinuities. But the phototransducer device allows straightforward testing and operating of the
heterostructure at higher optical input intensities by increasing the power of the laser source incident on the
phototransducer. The following section will therefore
study the device behavior at higher optical powers.
3.5. Pushing the limits of concentrated
photovoltaic solar cell tunnel junctions
Given the common availability of high power laser diodes
nowadays, it is straightforward to push the input optical intensities and the output current densities into a regime that
might exceed the typical operating conditions of CPV solar
cells. For example, Figure 6 shows the measured I-V
curves of the phototransducer heterostructures, which were
studied in Figure 3 at lower excitation intensities and
which are using 4 (n++)GaInP/(p++)AlGaAs CPV tunnel
junctions that are connecting five base segments. At an optical input power of 5 W (average optical input intensity of
144 W/cm2), clearly the TJs are exceeding their peak currents. As the photo-generated current starts exceeding the
TJs peak tunneling current, the negative differential resistance region of the I-V curves of the respective TJs impedes the current flow. Then the I-V curve of the
phototransducer follows the negative differential resistance
behavior of the individual TJ’s as the voltage is swept
across the heterostructure, as clearly observed in Figure 6.
Figure 6. Measured current–voltage (I-V) curves of the
phototransducer heterostructures using 4 (n++)GaInP/ (p++)
AlGaAs CPV tunnel junctions at high optical input intensities.
Here, at an optical input power of 5 W (average optical input in2
tensity of ~144 W/cm ), the TJs, which are connecting the different base segments, are exceeding their peak tunneling
currents. The peak intensity can be significantly higher than
the average intensity.
Tunnel junctions in novel vertical epitaxial heterostructure
The peak tunneling current of the TJs can be made
higher by using higher doping values and by using lower
bandgap semiconductor alloys [1,2]. Therefore, for operation at higher optical input intensities, we have grown
and fabricated phototransducer heterostructures using improved TJs. These TJs were especially designed with lower
bandgap (n++)AlGaAs/ (p++)AlGaAs layers. Figure 7
shows the measured FF values as a function of the optical
input power for the corresponding two TJ designs. As
shown in Figure 6, the phototransducer heterostructures
that are using (n++)GaInP/ (p++)AlGaAs CPV TJs to connect the different base segments are exceeding their peak
tunneling currents. This is clearly observed, for example,
at an optical input power above ~1 W (average optical intensities greater than ~30 W/cm2). This is distinguished
here as a reduction in FF. For these measurements and
for the typical phototransducer operation, the optical input
is emerging from a multimode fiber. It is delivering the
continuous wave output of a high power laser diode. Under
such conditions, the peak intensity can be significantly
higher than the average optical intensity. For example,
the (n++)GaInP/ (p++)AlGaAs CPV TJ design used here
has been demonstrated before and typically provides stable
CPV operation for concentrations up to several thousand
suns, that is, above 100 W/cm2 [36]. Nevertheless, as can
be observed in Figure 7, the (n++)GaInP/ (p++)AlGaAs
CPV TJs are exceeding their peak tunneling currents at a
much lower optical input power compared with the
phototransducer heterostructures comprising the improved
(n++)AlGaAs/ (p++)AlGaAs TJs. The latter TJs are providing stable operation up to 5 W input powers corresponding to an average intensity of close to 150 W/cm2.
Figure 7. Measured FF values as a function of the optical input
2
power for two types of TJ designs. The device area is 0.034 cm
and a 5 W optical input power corresponds to an average intensity
2
of ~144 W/cm . The peak intensity can be significantly higher than
the average intensity. The phototransducer heterostructures
using 4 (n++)GaInP/ (p++)AlGaAs CPV TJs to connect the 5 different base segments are exceeding their peak tunneling currents at
a much lower optical input power compared with the
phototransducer heterostructures using the four improved (n++)
AlGaAs/ (p++)AlGaAs TJs. The latter TJs were designed specifically for the phototransducer heterostructure using a lower
bandgap to obtain improved peak tunneling currents.
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
DOI: 10.1002/pip
1693
D. Masson, F. Proulx and S. Fafard
Tunnel junctions in novel vertical epitaxial heterostructure
3.6. Novel high-efficiency GaAs
phototransducers based on a vertical
epitaxial heterostructure architecture
The performance of the phototransducers based on a vertical
epitaxial heterostructure architecture comprising the lower
bandgap (n++)AlGaAs/ (p++)AlGaAs TJs has been measured from low to very high optical input intensities. For example, Figure 8 shows the measured Voc and FF as a
function of the output power for various optical input powers. The data confirm that the novel high-efficiency GaAs
phototransducers based on a vertical epitaxial heterostructure
architecture demonstrated conversion efficiencies in excess
of 50% for up to 5 W optical input yielding average current
densities of up to 17A/cm2. The FF decreases slightly at
higher input powers, but as can be observed in Figure 8(b),
excellent performance can be obtained over the entire range
covered here. Designs with larger active areas and/or incorporating tunnel junctions with even higher peak currents
could readily be implemented for applications that might
benefit from more output powers.
Using the same parameters as in Figure 3(a), the
5-photodiode model was further used to investigate the
expected dependence of the phototransducer parameters
on the optical input intensity. The expected Voc and conversion efficiencies are consistent with the measured data.
Figure 8. Measured Voc in (a) and FF in (b) as a function of the
output power for the various optical inputs indicated in the caption. The data demonstrate that the novel high-efficiency GaAs
phototransducers based on a vertical epitaxial heterostructure
architecture are capable of conversion efficiencies in excess of
50% with up to 2.5 W electrical output powers using here five
base segments. The lower bandgap (n++)AlGaAs/ (p++)AlGaAs
2
TJs allow an input power of up to 5 W on a 3.4 mm device, cor2
responding to an average optical intensity of ~144 W/cm . The
input is a continuous wave (CW) laser diode, at a wavelength
of 830 nm unless specified.
1694
The Voc and the efficiencies are increasing with optical input intensities and can reach an efficiency of about 70% at a
Voc of around 6 V at high powers. However, the measured
data display a slight Voc reduction at the highest input optical powers because of an increase in the operating temperature of the device as discussed in Figure 4 because no
active cooling was used to obtain the results of Figure 8.
As a benchmark, the measured I-V data of a control single
n/p GaAs PV diode having a 3.5 μm p-type base was also
modeled. A higher n-factor of 1.45 was obtained with the
latter. The lower n-factor with the thinner n/p junctions is
consistent with a more efficient extraction of the minority carriers in the heterostructure exploiting the thin base segments.
4. CONCLUSIONS
We have designed, fabricated, and tested a novel type of
phototransducer based on a vertical epitaxial heterostructure
architecture. It comprises thin n/p GaAs diodes connected
vertically in series using tunnel junctions grown in a single
epitaxial run. The current-matching condition necessary for
optimal power conversion was obtained by tailoring the
fraction of light absorbed in the various base segments. In
this structure, the thinnest n/p junction is the top one, with
each subsequent n/p junction increasing in thickness. This
insures that each n/p junction absorbs the same fraction of
photons and generates the same photo-current. The resulting
devices operated as designed by showing a photovoltage
up-conversion proportional to the number of junctions
used. The output photovoltage is significantly higher
than the equivalent input voltage of the absorbed photons
(hν/e). The vertically stacked epitaxial layers in these
phototransducer devices can remove the non-uniformity
and misalignment problems, which may occur in alternative
planar geometries. The resulting phototransducer can be
made very efficient by selecting a bandgap close to the band
edge of the absorber to minimize the thermalization losses.
The thin GaAs n/p diodes demonstrated good ideality factors and high Voc’s. The novel phototransducers have been
used to obtain an electrical output of up to 2.5 W with a Voc
of about 5.8 V with a conversion efficiency of 50% or better.
The temperature dependence of the device has been characterized. It will be possible to obtain higher output powers
and even higher conversion efficiencies by further optimizing the tunnel junctions and/or by using larger devices.
ACKNOWLEDGEMENTS
We are grateful to Azastra Inc for their financial support.
We also wish to thank M. Wilkins, Dr. C. E. Valdivia, Prof.
K. Hinzer, Prof. V. Aimez, and Dr. A. Jaouad for valuable
discussions and experimentations connected to this work.
We would also like to thank the Industrial Research
Assistance Program from the National Research Council
of Canada for their financial assistance supporting part of
this work.
Prog. Photovolt: Res. Appl. 2015; 23:1687–1696 © 2015 John Wiley & Sons, Ltd.
DOI: 10.1002/pip
D. Masson, F. Proulx and S. Fafard
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