13
secondsthat
isthe
phthalocyanine
just
started
to
exciteit
will
generate
thermal
CT
excitonsthen
reduce
the
energy
and
shorten
the
distance
between
electron-holes
At
the
same
timeas
mentioned
abovefor
typical
molecular
size
and
dielectric
constantthe
interfacial
CT
exciton
binding
energy
of
several
hundred
millivolts
of
electrons
was
estimated
to
be
an
order
of
magnitude
higher
than
the
thermal
energy
at
the
condition
of
room
temperature
Another
important
application
was
the
use
of
the
FE
with
the
lowest
energy
and
CT
excitons
in
the
quasi-1D
structure
for
NN1049011-dimethylperylene-349
10-dicarboximide(MePTCDI)and
PTCDA
crystals
The
authors
proposed
a
Hamiltonian
that
included
some
vibrating
FE
and
a
nearest
neighbor
CT
exciton
that
could
describe
the
polarization
directionpeak
intensityand
energy
position
of
a
1D
crystal
for
a
molecule
that
contained
only
one
molecule
The
intrinsic
oscillator
strength
of
crystal
and
CT
excitons
was
very
small
Electro-absorption
measurement
was
the
most
ideal
direct
observation
tool
It
proved
that
CT
excitons
had
large
internal
transition
dipole
moments
in
quasi-1D
crystals
having
strong
orbital
overlapwhich
seriously
affected
the
polarization
direction
about
the
mixed
excitons
Howeverthe
CT
transition
dipole
reached
a
polarization
ratio
for
a
three-dimensional(3D)crystal
containing
two
molecules
and
having
a
weak
coupling
between
1D
stacks
This
mechanism
directly
demonstrated
the
mixing
about
Frenkel
and
CT
excitons
in
MempCDIwhich
was
not
directly
related
to
electroabsorption
measurements[17]
Other
applications
of
CT
excitons
included
polymerfullerene
mixturesbulk
heterojunctions
of
polyfluorene
copolymers
and
fullerene
derivativesvan
der
Waals
interfacespolymer
semiconductor
heterojunctionsorganic
solar
cells
narrow
band
gap
polymer-based
volume
heterojunctionsand
organic
solids
in
first
principle[18-22]
Among
themthe
molecular
donoracceptor
and
the
van
der
Waals
interface
of
the
graphene-based
2D
semiconductor
were
the
key
to
the
photo-electrical
mutual
conversionincluding
photodetectorslight-emitting
diodes
and
solar
cells
A
distinguishing
characteristic
of
the
two
van
der
Waals
46 Nanoscale
Photonics
and
Spectroscopy
interfaces
was
the
poorly
shielded
Coulomb
potentialresulting
in
bound
electron-hole
pairsieCT
excitons
For
organic
solar
cellsthe
dissociation
efficiency
about
the
CT
state
of
the
weakly
bonded
interface
was
very
significant
for
organic
heterojunction
solar
cells
And
the
authors
examined
a
variety
of
donor
polymers
and
acceptors
through
photoluminescence(PL)
quenching
to
observe
that
the
dissociation
is
not
different
from
CT
excitons
and
FE
Studies
had
shown
that
the
field-dependent
photocurrent
about
pure
polymers
was
related
to
the
quenching
effect
Howeverthe
correlation
between
CT
exciton
quenching
and
photocurrent
was
not
significant
It
was
worth
noting
that
for
pure
polymersPL
and
electroluminescence
were
the
samebut
red
shift
of
blend
electroluminescence
could
still
be
observed
The
above
indicated
that
the
energy
in
the
blending
was
low
and
was
not
visible
in
PL
Thusthe
luminescent
state
of
the
PL-detected
blend
was
produced
by
photocurrent[23]
Surface
excitons[24-26]
In
previous
studiesthe
theory
about
surface
excitons
in
molecular
crystals
showed
that
the
presence
of
surface
excitons
and
their
states
on
or
below
the
exciton
state
were
based
on
the
nearest
neighboring
environmental
displacement
term
and
the
exciton
transfer
term
determines
In
additionthe
surface
exciton1049011s
localization
energy
was
not
sensitive
to
the
thickness
of
crystal
Surface
excitons
were
widely
used
For
exampleionic
crystals
and
rare
gases
Here
was
a
brief
introduction
to
the
application
of
surface
excitons
in
ZnO
crystalswhich
were
n-type
semiconductors
with
upwardly
band
bending
on
the
surfaceand
generated
by
negative
acceptor-like
surface
states
When
the
light
was
excitedthe
surface
electric
field
separated
the
photocarrier
from
the
surfaceand
the
photocarrier
was
accompanied
by
movement
to
the
bulk
of
the
electrons
and
holes
Note
that
the
surface
excitons
studied
here
had
very
specific
time
behaviors
For
bound
excitonstheir
decay
time
was
very
short
56CHAPTER
3 Photonics
and
Plasmonics
The
study
of
the
radiation
and
non-radiative
decay
mechanisms
about
surface
excitons
needed
to
require
more
in-depth
research
Thereforethe
authors
suggested
that
the
surface-bound
excitons
decay
might
be
related
to
low
temperatures
Surface
excitons
could
still
be
observed
in
rare
gases
The
first
evidence
showed
that
surface
excitons
were
observed
only
under
ultrahigh
vacuum(HHV)conditionsand
their
strength
was
quickly
reduced
even
in
very
small
rangessuch
as
10-10
Torr
More
than
twice
as
much
as
half
an
hour
The
residual
gas
adsorbed
by
the
single
layer
causes
the
surface
excitons
to
disappear
When
covering
different
rare
gas
filmsthe
surface
excitons
will
disappearwhile
the
surface
excitons
of
the
overlay
will
emerge
For
studying
the
dependence
of
surface
excitons
on
film
thicknesssince
the
transmittance
of
each
film
was
constantthe
transmittance
of
the
multilayer
film
decreased
with
increasing
thickness
The
authors
speculated
that
surface
excitons
caused
the
sample
to
be
limited
to
1
layer
to
2
layers
In
addition
studies
had
shown
that
the
position
of
excitonssplittingand
the
strength
of
the
oscillator
needed
to
be
considered
Compared
to
bulk
excitationsurface
excitation
was
caused
by
changes
in
the
spatial
environment
close
to
the
surface
Another
important
application
was
volume
and
surface
excitons
in
solid
neon
Rare
gas
solids(RGSs)were
prototype
materials
for
insulators
because
RGS
had
a
relatively
simple
electronic
structure
at
the
ground
state
The
valence
band
was
produced
by
the
relatively
weak
van
der
Waals
force
in
the
crystal
and
the
outermost
closed
p-shell
electron[24]
Atomic
and
molecular
excitons
In
previous
studiesmolecular
exciton
models
were
used
to
deal
with
excited-
state
resonance
interactions
in
weakly
coupled
electronic
systemswhich
was
seen
as
an
important
tool
for
studying
the
photochemistry
and
spectroscopy
of
complex
molecules
For
composite
moleculesloosely
bound
light
absorbing
66 Nanoscale
Photonics
and
Spectroscopy