This document summarizes spectroscopic characterization of zinc phosphide (Zn3P2) for photovoltaic applications. Zn3P2 is an abundant material with promising properties for solar cells. The author grew Zn3P2 samples using molecular beam epitaxy on graphene and indium phosphide substrates. Photoluminescence and Raman spectroscopy were used to characterize the samples. Passivating the Zn3P2 surface with aluminum oxide improved photoluminescence intensity by reducing non-radiative recombination. Temperature-dependent measurements provided insight into direct and indirect bandgap transitions in Zn3P2. The goal is to understand recombination processes in Zn3P2 to enable efficient solar cells using this material.

1. Spectroscopic Characteriza/on of Zinc Phosphide for
Photovoltaic Applica/ons
1. Laboratory of Semiconductor Materials, Ins9tute of Materials, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
2. Xiong Qihua Group, Division of Physics and Applied Physics, School of Physical and Mathema9cal Sciences, Nanyang Technological University, Singapore.
Nicolas Humblot 1,2
Supervision: Elias Stutz1
, Anna Fontcuberta i Morral1
, Xiong Qihua2
Mo/va/on
The terrestrial abundance of materials used in photovoltaic (PV) devices is of great interest as a more sustainable way of genera9ng power. Zinc phosphide (Zn3P2) is an earth abundant
material with a direct bandgap that is promising for solar cells applica9ons. Understanding the recombina9on processes and finding ways to reduce non-radia9ve pathways are crucial for
the design of efficient Zn3P2-based solar cells.
Thesis goal
The long term goal is to produce stable and efficient solar cells with Zn3P2 as an ac9ve material. The aim of this thesis is to characterize Zn3P2—grown at EPFL using molecular beam epitaxy
on graphene (Van der Waals) and indium phosphide (InP) substrates—by means of photoluminescence (PL) and Raman spectroscopy*. The effects of passiva9ng the surface with an
aluminum oxide layer is another ongoing inves9ga9on.
Mo/va/on & thesis goal
The recombina9on processes in Zn3P2 are
dominated by surface recombina9ons. One
Zn3P2 grown on graphene sample was coated
with 5 nm of aluminum oxide (Al2O3) by atomic
layer deposi9on (ALD). No etching of the na9ve
oxide was performed. The ALD passiva9on
increases the average PL intensity by a factor of
2.3 (Fig. 6), likely reducing one source of non-
radia9ve recombina9on.
Time-resolved PL of coated and uncoated Zn3P2
samples will help to iden9fy the bulk and
surface recombina9on contribu9ons.
Effects of a passiva/on layer
• Op9mize the passiva9on of surfaces with adequate oxide and remove of the na9ve one.
• Master the growth of high quality Zn3P2 crystals (e.g. NWs) and study their defects by
means of PL and unusual Raman peaks.
• Create heterojunc9ons using zinc nitride (Zn3N2) as n-type layer and Zn3P2 as p-type.
Outlook
The PL spectra described here were acquired with 532 or 633 nm illumina9ons.
• Room Temperature (RT)
PL emissions from two Zn3P2 samples grown
on graphene show two dis9nct spectra (Fig. 1)
at RT. The sample with a smaller growth 9me
(red curve) emits at 1.53 eV with a lower
energy shoulder around 1.43 eV. The posi9on
of the emission is unchanged with increasing
laser power and the power law k=log(PL
intensity)/log(Laser intensity)>1 suggests an
excitonic origin[1] of the signal. The longer
growth 9me sample (blue curve) broadly emits
around 1.45 eV, but increasing the laser power
redshijs the emission, sugges9ng several electronic transi9ons to happen. The crystal
quality may be lower for this sample (perhaps containing more grain boundaries).
• From 300K to 200K
With decreasing temperature, on the lower growth 9me
sample, the 1.53 eV transi9on quenches while the lower
energy one increases (grey arrows Fig. 2). A similar
behavior was observed on a previous study[2]. The origins
of the peaks we observe are likely to be comparable to the
direct (1.50 eV) and indirect (1.38 eV) interband
transi9ons that the authors reported for Zn3P2.
The 1.41 eV—well defined—peak at 200K can be related
to an indirect transi9on in Zn3P2. This PL peak does not
shij with increasing laser power and its high energy tail
suggests band-to-band recombina9ons. The power law k =
log(PL intensity)/log(Laser intensity) > 1 hints an exciton
line[1].
• From 150K to 10K
The 1.4 eV peak at 150K
— p r e s e n t i n b o t h
samples—either redshijs
in one sample (red arrow
Fig. 3) as in a previous
study[3] or shows a
c h a n g e i n t h e
temperature coefficient
(red/blue arrows Fig. 4)
upon cooling. In this
laner case, a combina9on
of several peaks with
posi9ve and nega9ve
temperature coefficients
can be responsible for the change[3]. A low energy (1.25 eV) defect peak rises below 150K
and suggests a lower crystal quality of this longer growth 9me sample.
• 10K
At low temperature, a phonon replica is
suspected to contribute to the low energy tail of
the asymmetric emission peak (Fig. 5). The
fiong with two gaussian func9ons leads to a 45
meV energy of the phonon, close to the 43 meV
reported in the literature[2,3]. Power series
suggests that the origin of the emission is no
longer excitonic but rather donor-acceptor
transi9ons. The blueshij (blue arrow) could be
an indicator of slow recombina9on processes
through the indirect bandgap of Zn3P2 and be
used to study the material quality.
Photoluminescence from Zn3P2 grown on graphene
900875850825800775
Wavelength [nm]
1.41.451.51.551.6
Energy [eV]
0
2
4
6
8
10
PLintensity[arb.u.]
Uncoated
ALD coated
900875850825800775750
Wavelength [nm]
1.351.41.451.51.551.61.65
Energy [eV]
PLintensity[arb.u.]
Sample A
Sample C
1 μm
1 μm
(1)
950900850800
Wavelength [nm]
1.31.41.51.6
Energy [eV]
PLintensity[arb.u.]
200K
220K
240K
260K
280K
300K
1000950900850
Wavelength [nm]
1.21.251.31.351.41.451.5
Energy [eV]
0
2
4
6
8
10
PLintensity[arb.u.]
12 W
53 W
129 W
194 W
391 W
1.17 mW
(5)
1000950900850
Wavelength [nm]
x3
x2
1.21.31.41.5
Energy [eV]
PLintensity[arb.u.]
6.8K
21K
34K
39.9K
50.5K
60K
80K
110K
150K
1000950900850
Wavelength [nm]
1.21.31.41.5
Energy [eV]
PLintensity[arb.u.]
12.1K
22K
31K
40.8K
50.3K
60K
70K
80K
100K
120K
150K
(4)1 μm 1 μm
Raman spectrum of Zn3P2 was acquired at
10K, 391 μW at 633 nm (Fig. 8). The arrows
iden9fy the peaks ascribed to different
irreducible modes (A1g, B1g, B2g or Eg)[3]. The
good correla9on of the 11 observed peaks
with previous studies supports the presence
of the desired phase (α-Zn3P2) in our
material. In addi9on to the zone-centered
Raman ac9ve phonons, the two ∗ annotated
peaks are unusual addi9onal A1g modes that may be related to phosphorus inters99als
ac9ng as acceptors in Zn3P2 (intrinsically p-type). Using different laser excita9ons for the
study if these peaks could be a useful tool for understanding the material quality.
Raman spectroscopy of Zn3P2 grown on graphene
*
*
200 250 300 350 400 450
Raman shift [cm-1
]
0
2
4
6
8
10
Ramanintensity[arb.u.]
(8)
1 μm
1 μm
1 μm
(6)
Zn3P2 thin films grown on InP cover the
whole substrate. The PL from Zn3P2 thin
films and nanowires grown on InP is
dominated by emission from the substrate.
InP has three characteris9c peaks that we
all observe (inset Fig. 7). We addi9onally
observed a shoulder (dashed line) on the
defect InP peak (full line). It was first
suggested that Zn3P2 contribute to this
shoulder. However, this hypothesis was
discredited by the observa9on of the
phonon replica of the defect InP peak at 1.34 eV (inset Fig. 7). This InP phonon replica
shows the same shoulder and behavior with increasing power. The shoulder is thus due
to a transi9on happening in InP (most likely a donor-acceptor pair transi9on).
905.0901.9899.1892.0
Wavelength [nm]
1.371.37481.37891.39
Energy [eV]
0
2
4
6
8
10
PLintensity[arb.u.]
5.55 W
20.8 W
40.8 W
145 W
928899876
1.341.381.42
PLint.[a.u.]
Photoluminescence from Zn3P2 grown on InP
(7)
1. Schmidt, T. et al., Physical Review B 45, 8989 (1992).
2. Kimball, G. et al., APL 95, 112103 (2009).
3. Briones, F. et al., APL 39, 805–807 (1981).
4. Pangilinan, G. et al., Physical Review B 44, 2582
References
* Both spectroscopic techniques are widely known and not described in this poster
(3)