Hot-carrier effects in type II heterostructures
Louise C. Hirst1, Michael K. Yakes1, Christopher G. Bailey2, Joseph G. Tischler1, Vincent R. Whiteside3,
Ian R. Sellers3, Matthew P. Lumb1,4, David V. Forbes5, Robert J. Walters1
1
U. S. Naval Research Laboratory, 4555 Overlook Ave. SW., Washington DC, 20375
2
Old Dominion University, Norfolk, VA 23529
3
The University of Oklahoma, 440 W. Brooks St., Norman, OK 73019
4
The George Washington University, 2121 I Street NW, Washington DC 20037
5
NanoPower Research Labs, Rochester Institute of Technology, Rochester, NY
Abstract — Hot-carrier solar cells have high theoretical
limiting efficiency however, absorber materials with slow carrier
thermalization remain a development barrier for these devices.
In previous studies, charge separation in core-shell colloidal
quantum dots has been shown to result in slow carrier relaxation.
Charge separation also occurs in III-V heterostructures with
type-II band alignments. We characterize hot-carrier effects in
InAlAs/InP and InAlAs/InGaAsP quantum well structures, with
type-II and quasi-type-II band alignments respectively. InGaAsP
is identified as a promising hot-carrier absorber candidate, with
thermalization coefficient 1.77±0.12 W.K-1.cm-2, corresponding to
limiting solar conversion efficiency >42%, under 2000X.
Index Terms — Hot-carrier solar cell, type II band alignment,
InP, InGaAsP.
I. INTRODUCTION
A hot carrier solar cell (HCSC) is a device in which
energetic photo-excited carriers are extracted before they have
time to dissipate their heat energy and equilibrate with the
surrounding lattice. In a traditional single-junction thermalequilibrium solar cell, thermalization and transmission losses
account for >50% of incident solar energy [1]. These
dominant intrinsic loss mechanisms are targeted in HCSCs,
allowing for substantial enhancement in limiting efficiency [2,
3]. There are two key challenges associated with HCSC
development: energy selective contacts and absorbers in
which the rate of carrier thermalization is restricted.
Nanostructured materials, such as resonant tunneling
quantum dots [4] and quantum wells [5], make promising
candidates for energy selective extraction. Primitive HCSC
functionality has been demonstrated in devices which
integrate these contacts with HC absorbers [6-8].
Nanostructured materials have also been proposed as hotcarrier absorber candidates. Slow carrier cooling in GaAs
quantum well structures, relative to bulk material, at high
carrier density, has been experimentally demonstrated [9].
Recent studies [10, 11] have characterized this HC effect in
terms of thermalization coefficient (Q) [12]. A value of Q =
2.5 W.K-1cm-2 was characterized in an InAlAs/InGaAs QW
structure grown lattice matched to InP. This corresponds to a
limiting efficiency enhancement >5%, over a single-junction
thermal equilibrium equivalent device [13].
The core-shell (CdSe-ZnSe) colloidal quantum dot (QD)
has also been proposed as a nanostructured hot-carrier
Fig.1 Simulations of single QW structures (well width 20
nm): InAlAs/InP and InAlAs/InGaAsP.
absorber. Intraband relaxation rates >1 ns have been
characterized by pump probe spectroscopy [14]. This slow
rate of cooling has been attributed to the band-alignment
between core and shell layers leading to charge separation
which prevents rapid excitonic cooling. The rapid transfer of
hot-electrons to an electron-acceptor has since been
demonstrated [15, 16] however, integrating the colloidal QD
absorber with an energy selective contact remains a significant
challenge.
As with colloidal QDs, III-V heterostructures can also be
fabricated with band-alignments which induce charge
separation. In this study, hot-carrier effects in InAlAs/InP and
InAlAs/InGaAsP single quantum well structures are
characterized by photoluminescence. These structures have
type II and quasi-type II band alignments, which cause
electrons and holes to delocalize. Structures with type II band
alignments are not usually considered desirable as
photovoltaic absorbers because the alignment results in a
voltage drop relative to the absorption edge, however in a
HCSC such a voltage drop would be compensated for by
above bandgap carrier extraction. These structures also exhibit
extended radiative lifetime and hence higher steady-state
carrier densities, as well as reduced Auger scattering, both of
which could result in slowed electron cooling.
II. SAMPLES AND GROWTH
In this study, photoluminescence from two single quantum
well structures with different well materials were compared:
InP and InGaAsP. The structures were simulated using NRL
Q = 1.77 ± 0.12 W.K-1.cm-2
Fig.3 Pabs / exp(−ELO /k.Teh ) (W · cm−2 ) as a function of
ΔT, for the InGaAsP QW sample, the gradient of which
provides thermalization coefficient.
Fig. 2 Power dependent photoluminescence spectra
(295K) for InP and InGaAsP samples, with corresponding
absolute carrier distribution temperature (Teh) as a function
of incident power density. Fit ranges: InP QW (1.18-1.24
eV) and InGaAsP QW (1.10-1.14 eV).
multi-bands software [17], which solves the time-independent
Schrödinger equation for a finite square well using material
parameters from Vurgaftman et al. [18] (Fig. 1). The
InAlAs/InP structure has a type II band-alignment, while the
InAlAs/InGaAsP structure has a fully flat valence band
(quasi-type II). Samples were grown on Fe-semi insulating
InP substrates by MOCVD (background doping, n-type
~3x1015 cm-3), with a thick InGaAs buffer, to prevent photoexcitation in the substrate, and a thin (10 nm) InP cap.
II. PHOTOLUMINESCENCE SPECTROSCOPY
Photoluminescence (PL) spectroscopy was performed using
a Princeton Instruments Acton 2500 and cooled Ge detector.
Fig. 2 shows 295K PL spectra with steady-state excitation
from a focused 532 nm laser (FWHM ~10 µm).
Peaks in the PL spectra, from the InGaAsP QW, are consistent
with the simulated confined energy levels for this structure
(within 20 meV of the absolute values and with matching
energy separation between energy levels). The PL spectra
from the InP QW however, is not consistent with simulation.
The observed lowest energy transition is 95 meV lower than
the simulated value and the second confined energy level
(present in simulation) is not observed. Relative to the
InGaAsP sample, PL emission from the InP QW has a less
pronounced exciton peak and significantly reduced emission
intensity. These trends are consistent with recombination at a
type II interface in the InP sample however, it is clear that
material composition at this interface is more complex than is
represented by the simulations. This is a known issue with
MOCVD growth of InAlAs/InP QWs [19]. At the front QW
interface (InAlAs/InP), As still present in the line can be
incorporated into the InP, producing a thin graded InAsP
layer. This traps electrons at the interface preventing diffusion
through the relatively thick (20 nm) QW.
Broadening of the high energy tail of the PL spectra, with
increasing power density, is characteristic of an increase in
carrier distribution temperature (Teh). This was quantified by
fitting a Plank-like distribution (Eqn. 1) to the high energy tail
of the PL spectrum (Ipl(E)). In each case, a calibration factor
was derived from a low power PL measurement to extract
energy dependent emissivity (ε(E)) and any other energy
dependent experimental features, such as detector sensitivity
and optics transmission, from the experimental data.
 E
I pl ( E )   ( E )  exp 
 kTeh



(1)
The InGaAsP QW sample exhibited significant carrier
heating over the given incident power density range
(ΔT>100K for incident power density <1000W.cm-2).
Conversely, no carrier heating was observed in the InP QW
sample. The absence of HC effects in this sample can be
attributed to the unintentional presence of a thin InAsP layer
at the InAlAs/InP interface. Previous studies have shown
enhanced HC effects in thick QWs [13], similarly colloidal
QD studies showed long carrier relaxation times only
occurred in samples with a thick shell layer (hole trap).
Further studies of InAlAs/InP structures, fabricated with
different growth conditions, would be required to determine
the efficacy of this material system as an HC absorber.
III. SIMULATED HCSC EFFICIENCY
In order to make quantitative comparison with previous
studies, the HC effect in the InGaAsP QW sample was
evaluated in terms of thermalization coefficient. PL
Fig. 4 Contour map of simulated HCSC efficiency (%), as
a function of Q and Eg, under 2000X. Color map shows
corresponding
carrier
distribution
temperature.
Characterized
structures
are
marked:
AlGaAsSb/InGaAsSb QW [12], GaAsP/InGaAs [13],
InAlAs/InGaAs [15] and InAlAs/InGaAsP [this study].
spectroscopy was performed using a 1064 nm excitation laser.
This wavelength is not absorbed in the InAlAs barrier region
of the sample and thus the density of photo-excited carriers
can be accurately determined.
The rate at which heat energy is thermalized to the lattice
(Pth) is related to the LO phonon energy (ELO), the temperature
gradient between the carrier population and the lattice (ΔT)
and the thermalization coefficient (Q) via Eqn. 2 [12].
 E
Pth  Q  T  exp  LO
 kTeh



(2)
The rate of absorbed power (Pabs) is high, relative to the
rate of emitted power and the approximation Pabs = Pth can
be made. Q is thus determined from the gradient of Pabs/exp(Elo.kTeh) versus ΔT as 1.77±0.12 W.K-1.cm-2 (Fig. 3). Fig. 4
shows a contour plot of simulated HCSC efficiency under
2000X solar concentration, as a function of Q and bandgap.
Efficiency is calculated using a detailed balance, energy and
particle conservation model, with a partially thermalizing
absorber and idealized energy selective contacts. Previously
characterized structures are compared with the values of the
InGaAsP QW sample in this study. The low Q value, along
with Eg ~1eV, makes the InGaAsP QW structure particularly
favorable for HCSC device development, with projected
limiting efficiency >42%.
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