Candidate:
Tyler
Davidson-Hall
Title:
The
Influence
of
Charge
Transport
Layer
Interfaces
on
the
Performance
of
Electroluminescent
Colloidal
Quantum
Dot
Light-Emitting
Devices
Date:
July
20,
2020
Time:
1:00
PM
Place:
REMOTE
PARTICIPATION
Supervisor(s):
Aziz,
Hany
Abstract:
Electroluminescent
quantum
dot
light-emitting
devices
(QDLEDs)
are
a
relatively
new
technology
for
displays
that
have
all
of
the
benefits
of
emissive
displays
commonly
associated
with
organic
light-emitting
devices
(OLEDs)
such
as
low
power
draw,
deeper
black
levels,
and
fast
refresh
rates.
However,
QD
emitters
confer
additional
valuable
characteristics
for
displays
due
to
their
high
photoluminescence
quantum
yield
(PLQY)
approaching
100%,
narrow
emission
full-width
at
half
maximum
(FWHM
<
20
nm),
the
tunability
of
their
peak
luminescence
wavelengths
across
the
entire
visible
spectrum,
and
their
solution-processability
which
makes
them
compatible
to
low-cost
and
flexible
fabrication
techniques.
Unfortunately,
like
their
organic
counterpart,
the
long-term
stability
of
QDLEDs
has
been
limited
for
many
years.
While
high-
efficiency
red
and
green
QDLEDs
have
recently
been
demonstrated
with
operational
lifetimes
of
over
1,000,000
hours,
this
may
not
be
suitable
for
some
applications
given
the
luminance
of
the
devices
for
which
these
lifetimes
are
rated.
Furthermore,
like
OLEDs,
blue
QDLEDs
are
far
less
stable
than
red
and
green
with
the
record
lifetime
being
orders
of
magnitude
less.
Therefore,
it
is
important
to
understand
the
mechanisms
governing
QDLED
stability
in
order
to
improve
device
lifetimes.
In
general,
the
main
degradation
mechanism
affecting
QDLEDs
has
been
attributed
to
multi-charge
carrier
interactions
referred
to
as
Auger
recombination
which
uniquely
affects
QDs
to
a
greater
degree
due
to
their
nanoscale
structure
but
also
the
inherent
charge
imbalance
within
QDLEDs.
However,
there
is
little
focus
on
the
degradation
of
devices
outside
of
this
process.
Given
the
similarity
in
structure
between
QDLEDs
and
OLEDs,
this
work
aims
to
elucidate
upon
additional
degradation
mechanisms
that
may
influence
the
long-term
stability
of
these
devices
by
considering
the
degradation
mechanisms
that
affect
organic
semiconductors.
This
thesis
investigates
each
interface
present
within
the
QDLED
structure
to
identify
its
influence
on
QDLED
performance,
with
the
device
stability
as
the
main
focus.
The
main
interfaces
of
interest
occur
between
the
electron
transport
layer
(ETL)
and
the
QD
emission
layer
(EML)
as
well
as
between
the
QD
EML
and
the
hole
transport
layer
(HTL).
However,
the
interface
between
the
HTL
and
the
hole
injection
layer
(HIL)
will
also
be
shown
to
influence
the
operational
stability
of
QDLEDs
despite
not
interacting
with
the
QD
layer
directly.
This
work
utilizes
the
hybrid
inverted
QDLED
structure
which
consists
of
an
inorganic
ETL
and
an
organic
HTL
as
this
structure
has
proven
to
be
far
superior
to
devices
with
all-organic
or
all-inorganic
transport
layers.
Therefore,
it
is
the
HTL
that
will
be
of
particular
interest
in
this
thesis.
Red
and
green
QDs
were
utilized
as
the
EML
over
the
course
of
this
work,
but
the
primary
focus
will
be
on
the
red
QDLEDs
as
they
resulted
in
the
most
efficient
and
long-lives
devices
allowing
for
a
better
environment
in
which
the
influence
of
the
interfaces
on
QDLED
performance
could
be
evaluated.
However,
the
results
should
be
applicable
to
a
wide
range
of
QD
EMLs
of
different
colours
and
compositions.
The
role
of
the
ETL
/
EML
interface
in
QDLED
performance
was
investigated
by
introducing
the
wide
band
gap
work
function
modification
layer
between
the
two
layers.
While
a
1.6×
improvement
in
maximum
EQE
was
observed,
this
coincided
with
a
worsening
of
the
charge
imbalance.
Rather
than
a
result
of
improved
charge
balance,
this
efficiency
enhancement
was
attributed
to
a
reduction
in
QD
exciton
quenching
by
the
ZnO
due
to
the
passivation
of
surface
states
at
this
interface.
While
this
resulted
in
a
negligible
effect
on
the
EL
stability
of
these
QDLEDs
at
the
time,
further
experiments
investigating
the
effect
of
excitons
at
the
EML
/
HTL
interface
would
identify
these
excitons
as
an
issue
for
long-term
QDLED
stability.
Ultimately,
the
main
influence
of
the
ETL
/
EML
interface
on
QDLED
performance
lies
in
the
exciton
quenching
effect
of
ZnO
surface
states
which
limits
the
QD
film
PLQY.
The
influence
of
the
ETL
and
EML
layers
on
morphological
stability
was
investigated
by
a
systematic
study
on
the
processing
parameters
of
ZnO
and
the
QDs
for
spherical
and
rod-shape
QDs.
The
main
factor
determining
the
degree
of
morphological
degradation
in
the
emissive
area
homogeneity
was
the
roughness
of
each
layer.
This
is
alsoÂ
aÂ
limitationÂ
inÂ
OLEDÂ
morphologicalÂ
stability,Â
asÂ
theÂ
organicÂ
semiconductorsÂ
areÂ
proneÂ
to
crystallization
when
deposited
on
rough
surfaces.
Therefore,
although
rough
layers
are
the
catalyst
of
this
morphological
instability,
it
is
the
organic
HTL
that
is
the
limiting
factor
in
long-term
morphological
stability.
The
role
of
degradation
of
the
HTL
on
overall
QDLED
stability
was
further
investigated
through
PL
measurements
from
QDLEDs
over
time
while
irradiated
with
UV
light.
By
measuring
changes
in
the
QD
PL
over
time
under
constant
UV
illumination
in
devices
with
a
variety
of
organic
semiconductors,
it
became
clear
that
the
HTL
has
a
significant
influence
over
the
loss
in
QD
PLQY
during
aging.
While
the
QDs
individually
are
relatively
stable
during
the
measurement
period,
the
PLQY
loss
in
the
QDLEDs
correlated
to
the
individual
PLQY
loss
of
the
organic
layers.
It
was
therefore
found
that
the
exciton-induced
degradation
of
the
organic
molecules
at
the
QD
/
HTL
interface
that
is
particularly
deleterious
to
QDLED
stability.
A
multi-layer
HTL
structure
was
investigated
in
order
to
improve
charge
balance
and
reduce
hole
accumulation
at
the
QD
/
HTL
interface.
With
the
optimized
structure,
a
25×
enhancement
in
QDLED
lifetime
was
observed.
This
HTL
structure
allows
for
the
use
of
alternate
HILs
which
do
not
damage
the
underlying
organic
layers
during
device
fabrication,
restricts
the
accumulation
of
holes
at
the
QD
/
HTL
interface
and
reduces
charging
of
the
QD
layer,
and
limits
degradation
of
the
HTL
at
the
QD
/
HTL
interface.
Ultimately,
this
work
has
identified
a
degradation
mechanism
affecting
long-term
EL
stability
of
QDLEDs
been
identified
and
characterized
and
presents
a
device
structure
which
utilizes
a
cascading
HTL
with
small
gaps
between
each
component’s
HOMO
energy
level
with
an
organic
HIL
which
minimizes
the
effects
of
this
mechanism
on
QDLED
EL
stability.
Monday, July 20, 2020 1:00 pm
-
1:00 pm
EDT (GMT -04:00)