brynda1991

Thin SolidFilms, 199 (1991) 375 384
375
LANGMUIR--BLODGETT AND RELATED FILMS
ELECTRICAL AND PHOTOELECTRICAL PROPERTIES OF COPPER
TETRA[4-t-BUTYLPHTHALOCYANINE] LANGMUIR BLODGETT
FILMS
E. BRYNDA, I. KOROPECK~', L. KALVODA AND S. NE~POREK
Institute of Macromoh, cular Chemistry, Ciechoslovak Academy o[ Sciences,
( C:echoslo vakia)
(Received August 8, 1990; accepted October 12, 1990)
16206 Prague 6
The structure of Langmuir-Blodgett (LB) films prepared from copper tetra[4-tbutylphthalocyanine] (CuTTBPc) was studied using transmission electron microscopy, transmission electron diffraction and X-ray diffraction. Molecules in the LB
film were stacked in layers with phthalocyanine rings tilted by 14+ to the substrate
normal. Each molecular layer consisted of extended two-dimensional crystalline
domains oriented anisotropically in the film plane. No relation between the
anisotropic molecular orientation and in-plane electrical conductivity was observed. The in-plane electrical conductivity increased with time after voltage application reaching a stationary value 10-1 S m-1. The A1/CuTTBPc LB film/Ni
sandwich sample under illumination operated as a photovoltaic cell giving an opencircuit voltage of 0.4V. The charge photogeneration quantum efficiency via
excitation of the second singlet state of CuTTBPc was four times higher than that via
excitation of the first singlet state. Possible photogeneration models are discussed.
1. INTRODUCTION
The electrical and photoelectrical properties of organic materials have received
considerable attention over the years. In the last decade there has been an increasing
interest in studies of electronic phenomena in monomolecular and multimolecular
films prepared by the Langmuir Blodgett (LB) technique. LB films provide welldefined multilayer structures, the thickness of which can be varied on a nanometer
scale by simply adding the next molecular layer. It makes them suitable for the
measurement of molecular electronic phenomena such as excitation energy
transfer 1, tunnelling2 or hopping 3 of charges between molecular layers, charge
injection, charge dissociation and trapping at interfaces between inorganic
electrodes and adjacent molecular layers. The preparation of specifically designed
multimolecular LB structures can provide systems in which vectorial photoelectricity is reached by electron transfer between donor and acceptor layers4'5. In
more complex systems similar to photosynthetic systems a molecular antenna could
transfer the light energy to active sites of charge separation 6.
Among a large number of organic materials, phthalocyanines have potential for
0040-6090/91/$3.50
~5 Elsevier Sequoia/Printed in The Netherlands
376
E. BRYNDA. 1. KOROPE('K'~', 1,. KAI,VO1)A, S. NESPI~JREK
practical applications owing to their high chemical, thermal and optical stability.
Their electric and photoelectric properties havc been exploited in gas sensors,
copying media, photodetectors, and photovoltaic devices. LB films have been
prepared from various phthalocyanme derivatives ~ ~". Although numerous papers
and patents have dealt with the dielectric properties tt semiconductivity 1-',
photoelectric properties i ,~.~,~ energy transfer I s sensitivity to gas adsorption i,, and
third-order optical non-linearity t" of the films, tile electronic phenomena responsine for these properties have not been satisfactorily understood. LB fihns are
particularly suitable for studying photovoltaic effects in which surface states and
Schottky barriers at the phtlaalocyanine electrode interfiice play a principal
role 13'~r. The film can be prepared so thin that the region active in the charge
photogeneration includes the whole film thickness.
In this work electrical processes at the copper tetra[4-t-butylphthalocyaninc]
(CuTTBPc):air and CuTTBPc/metal electrode were studied using very thin LB
films. The morphology and molecular arrangement of the films were determined as a
necessary basis for other physical measurements. Time changes in d.c. conductivity
in the film plane were compared with optical anisotropy. Dark current w)ltagc
characteristics and photoelectricity were measured m an AI CuTTBPc Ni sandwich
cell.
2. EXPERIMENTAl, I)ETAII.S
CuTTBPc was synthesized from 4-t-butylphtfialonitrile ~ and purified on a
Flurosil (Fluka AGt chromatographic colunm. Purity was checked by thin layer
chromatography, and the structure was confirmed by mass spectroscopy. Substratcs
for the LB film deposition were microscope glass fiydrophobized by trimetfiylchlorosilane, silica, and aluminium or silver films obtained by vacuum deposition on
glass. Monomolecuhu lilms wcrc spread from 4.5 nagml 1 CuTTBPc in xylene
solution on water bidistilled in a quartz apparatus {resistivity, 1 Mf~ cm: pH 6.4)at
17 C. A limiting area of 0.6 nm-' per CuTTBPc molecule was observed when the
monolayer was compressed at a rate of 1.5 mm min ~. Before deposition tile
monolayer usually relaxed at 25 mN m 1 for 15 min, decreasing its area by about
1.8"i>. The quality of the monolayer on water was checked visually. Blue spots in the
monolayer were recognized in some cases. Only optically homogeneous monolayers
were deposited. The transfer on solid substrates was by vertical dipping at a rate
3.3 mm min 1 at a surface presstire of 25 mN in 1. Tens of CuTTBPc monolayers
could be transferred on the substrates by Y-type deposition. The deposition ratio for
both withdrawing and lowering the substrate through the monolayer on water was
usually close to unity for about 25 layers. It decreased during further deposition. The
optical measurements were performed with a 8451A Hewlett Packard spectrophotometer. Transmission electron microscopy {TEM) and transmission electron
diffraction (TED) were carried out with a J EM 7A transmission electron microscope
operated at 50 or 80 kV. The monotayer sample for TEM and TED was transferred
directly from the water surface onto a copper microscope grid by horizontal lifting of
thc latter. The multilayer film was deposited on a glass slide provided with
evaporated silver strips. By etching on silver with nitric acid, the fihn was released
Co TETRA[4-t-BUTYLPHTHALOCYANINE] LB FILMS
377
from the slide surface and transferred onto the water surface. A part of the film,
which had originally covered the glass surface between silver strips, was cut off and
transferred onto microscope grids.
The X-ray diffraction was measured for LB multilayer films deposited on
h y d r o p h o b i z e d single-crystal silicon wafers. Wide angle Bragg scattering patterns
were recorded with a sample located in a symmetrical reflection position under
irradiation by a filtered Cr K s beam. The X-ray diffraction on C u T T B P c powder
was measured under analogous conditions. The in-plane electrical conductivity was
measured using surface gold electrodes evaporated in v a c u u m on top of the
C u T T B P c LB film deposited on hydrophobized glass. The gap between two
electrodes was 0.12mm. Photoelectric properties were measured in glass/Al/
C u T T B P c , / N i sandwich cell, of area 4 m m 2 ( C u T T B P c , denotes an LB film
consisting of n monolayers). The LB film was deposited on an aluminium electrode
and the nickel electrode was deposited in vacuum on top of the film. The electrode
thickness was 13 nm. The current was measured with a Keithley 616 electrometer
using a Keithley 240 A power supply. Samples were illuminated with a m o n o chromatic light source built from a 1000 W xenon lamp and m o n o c h r o m a t o r of
dispersion 10nm m m 1. The light intensity was measured with a E G & G 580
radiometer and adjusted at each wavelength to a constant p h o t o n flux incident on
the C u T T B P c film using correction for electrode transmission.
3. RESULTSAND DISCUSSION
Figure 1 shows the optical absorption spectra of C u T T B P c . The spectrum of
the LB film with a m a x i m u m at 620 nm was identical with that of polycrystalline
samples obtained by evaporation of solution on a glass slide. The phthalocyanine
m o n o m e r absorption peak at 676 nm was d o m i n a n t in solutions used for the LB film
0./+
.:" !\
0.06
;
:..I
;
i
EP
/
II
I
..i I
"t.
!
0.04
0.2
0.02
• ".. ..............
,.'"
I
I
1
I
I
300
400
500
600
700
wovetengfh [nm]
Fig. 1. Optical absorption spectra of CuTTBPc: - - , 2 monolayers on silica (one on each side of the
silica); . . . . , 1.06x 10 4 wt.i!,,xylene solution: .... , polycrystalline film prepared by evaporation of the
solution on silica.
378
1. IIRYNI)A, 1. K(IROP't{(K~'. I . KAI.V(H)A, S. NI{9,PURI{K
p r e p a r a t i o n . 1 h e abscncc of m o l e c u l a r aggregates in the solutions was confirmed by
a well-resolved line structure of the c o p p e r electron p a r a m a g n e t i c resonance signal
at 77 K. At high ( ' u T T B P c c o n c e n t r a t i o n s in solutions or in a p o o l solvent, such as
c h l o r o f o r m acctonc, a b r o a d a b s o r p t i o n peak a p p e a r c d at 620 nm. similar to that
m e a s u r c d in p o w d c r samples and LB litms. The peak indicated thc occurrencc o[
cofacial m o l e c u l a r a g g r e g a t i o n ~'~. There was no observable dill'erencc between the
p o w d e r and L B lilm a b s o r p t i o n spcclra. Both spectra exhibited the main a b s o r p t i o n
peak at 620 nm with a small a b s o r p t i o n peak at a b o u t 680 nm. The latter indicated
the prescncc of p h t h a l o c y a n i t l e molecules in d i s o r d e r e d positions in which they did
not cofaciall> interact with their ncighbours. The spectral shape did not d e p e n d on
the n u m b e r o f m o n o l a y e r s m LB lilms. The optical density was p r o p o r t i o n a l to the
n u m b e r of m o l c c u l a r la3ers in the LB film. being 1.28 x t0 e per m o n o l a y e r for
a b o u t 25 layers. It decreased in further layers. In p o l a r i z e d light t: L E w a s 1.7.
where Ez and /=, were the a b s o r b a n c e s of light p o l a r i z e d with the electric vector
p e r p e n d i c u l a r and parallel respccti~el> to thc dircction of d i p p i n g of the substrate.
This suggested that lhe p h t h a l o c y a n i n e m o l e c u l a r plancs ~ e i e prcfercntiall5
oriented perpendicularly to the d i p p i n g direction. In lhe expcriment this direction
coincided with that o t ' c o m p r e s s i o n of the m o n o l a y c r on v, atcr surface.
Thc X-ray dilTraction ~ a s m e a s u r c d on ( ' u T T B P c polycrystallinc powder.
S h o r t - r a n g e o r d e r distances of 0.335 nm. 0.538 nm, 1.698 nm and e l e m e n t a r y cell
paramcters a
2.7Snm,/~
0.53nm. c
1.91 nm, and fi
117.5 were obtained.
Thc d a t a ~ c r c consislcnt \xith a model suggestcd b~ K o v a c s et al. 2" from ~ hich the
d i m e n s i o n s of the C u - l " l B P c molecule wcrc c x a l u a t e d 1Fig. 2{all and the m o l e c u l a r
a r r a n g e m e n t in p o w d e r was suggested I Fig. 2(bll.
/" ~ \k
iX
I#?- " \
.>\%
\/~, . . . . . . . . .
.,(
/;
~
/'" --"
[
(ill
k_ - - - . . . .
J
]['i:''~';r,
f~",~ 1,:~-.,.
(hi
I ig. 2. ta) ( u ITBPc mtqcculc dimcn,i~,n> ;ll/d ({)limqccuJal tll'l'itl/gClllClll In it ply'~,dcl ~amplc suggested
[rom X-ra\ dil]'laction.
The I I . I M m i c r o g r a p h of a ( ' u T T B P c m o n o m o l c c u l a r iilm transfcrrcd fl'om
v~atcr Oll a m i c r o s c o p i c grid is shown in Fig. 3{a). Extended d a r k d o m a i n s are visible
on a n a n o m e t r e scale. The T E l ) pattern m Fig. 3{b) reflects a long-range o r d e r
379
C u TETRA[4-t-BUTYLPHTHALOCYANINE] LB FILMS
molecular arrangement in the domains. Using T E D interplanar spacings di
determined from the seven observed reflections and the model molecular dimensions 2°, a possible arrangement of molecules in the domains was suggested
(Fig. 4(a)). A two-dimensional texture in the film plane was apparent. The
orientation of domains was anisotropic with a ratio of about 2:1, which was
consistent with the optical anisotropy discussed above. The similar anisotropy of
molecular arrangement obtained by vertical dipping of the quarz plate in the optical
experiments or by horizontal lifting of the microscopic grid suggested that the
domains had been oriented by compression of the monolayer on the water surface
before the transfer on the substrates. The data support a hypothesis 2° according to
which stacks of C u T T B P c molecules are formed on the water surface. There are
strong intermolecular forces among molecules in a stack, but the interaction
between the stacks is weak. When the surface pressure is increased during
compression of the films on water, the stacks rotate as a whole. The extended
domains in Fig. 3(a), 2 - 4 nm wide, could be formed by one or two molecular stacks.
(a)
(b)
Fig. 3. (a) TEM micrograph (50 kV) and (b) TED pattern (80 kV) of one monomolecular layer of
CuTTB Pc.
• • •
f
.
f[
.
U./
((i . ~ ( i
k A>JL-"J~J
~ ,
~"..//t"-~
{a)
dv ' ~ ' ~ /
[- d2 L
I
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.
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1,65
nm
I
I
-' ." . . .'substrate
(b}
.
.
.
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]
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[
-.
'
Fig. 4. Schematic drawings of the molecular arrangement of C u T T B P c molecules in an LB film: (a) inplane structure of crystalline regions in one molecular layer determined from T E D ffor the observed
interplanar distances d~, i ~ 1 7, see Table l}: (b) multilayer structure of an LB film determined from
X-ray diffraction.
380
t:. BRYNI)A, 1. KOROPECKY, 1,. KALVODA, S. NESPISREK
The T E D patterns obtained from LB films consisting of 6, 12, 22, and 24
molecular layers were similar to that obtained from the monomolecular fihn without
any evidence of three-dimensional crystalline structures. Each of them could be
interpreted as a superposition of diffractions from the individual monolayers lying
randomly on one another. No systematic change in interplanar distances was
observed which would increase the number of layers in the fihns (Table l).
Apparently, there was no signiticant epitaxial deposition or three-dimensional
crystallization in the fihn. The distance of 1.65 nm between molecular layers was
determined by measuring X-ray diffraction from an LB film consisting of 22
CuTTBPc layers. A comparison of the interlayer distance with the molecular
dimensions of 1.7 nm suggested that the phthalocyanine rings m the layers were
tilted by about 1 4 to the substrate surface normal (Fig. 4(b)). The arrangement
corresponded to an area per molecule of 0.6 nm e, as observed in the monomolecular
film on water.
TABLE 1
INTERPLANAR
DISTANCES
di
AND
"IHIilR
I!XPIRIMIN'IAI.
SIANI)ARD
TRANSMISSION ELECTRON DIFFRACTION PATTERNS ~)t C u T T B P c
DEVIAFII)NS
()BIAINEI)
FROM
L A N G M U I R BLOI)GETT FILMS ('ONSISTING
()E DIFFERENT N U M B E R S OF LAYERS
R~flecti(m
re;taker
hllerplanar di,s'tam'e.~ d i (11111)a;ld uxperhnenlal slamtard det'ialiolls ( in
parentheses ) ( × lO 3 nmJ /br lhu jbllowing.systems
Monolaver
1
2
3
4
5
6
7
1.581130)
0.482 (7)
0.33311 )
0.265 I4)
0.202121
0.16511)
0.12711)
6 ]ayer,s
]2 h{w'rs
_~ ]avers
0.450 (3)
0.32911 )
0.458 t7)
0.334 t3)
1.503 ~50)
0.518 {16)
0.328 t2)
0.201 tl)
0.163{1)
0.12811)
0.204{11
0.163{1)
0.130(11
0.19611)
0.16411)
0.124 1)
24 kt.w'r,s
0.447 {4)
0.332 ( 1)
0.269 ~2)
0.202{11
0.163{I)
0.12911)
The electrical conductivity measured in the CuTTBPc LB film plane was time
dependent (Fig. 5). The current through the sample increased after the bias voltage
was applied. At a voltage of 100 V the stationary current value was reached in about
150 rain (curve 1 in Fig. 5). After the polarity of the bias voltage was reversed, the
current decreased quickly to 20,,, of the stationary value and then increased slowly
again up to a stationary value (curve 2 in Fig. 5). The initial and stationary
conductivity values for an electrical field of 5 x l0 s V m 1 were 2 x 10 2 S m ~ and
10 - 1 S m 1 respectively. When the voltage was disconnected, the sample relaxed in
two days at 22 ~C to the original state characterized by the initial conductivity, and
the same current-time plot could be measured if the voltage was applied again. The
in-plane conductivity was measured on one sample by electrodes oriented
perpendicularly and parallel to the dipping direction. No difference in the
conductivity value or the time dependence was observed with respect to the
direction. Apparently, there was no relation between the preferential molecular
orientation reflected by the optical anisotropy and electron diffraction, on the one
Cu TETRA[4-t-BUTYLPHTHALOCYANINE]
LB FILMS
381
hand, and the electrical properties, on the other. The film resistivity seemed to be
affected more by amorphous regions among the crystalline domains than by the
conductance within the anisotropically oriented domains. The optical anisotropy
measured simultaneously with the electric current at the applied voltage did not
change during the increase in conductivity.
oo
D
2AO 8
/
o
2
% 1 0 .8
/
\-
\
2
j~
//
/
/
/
/
/
/
/
/
/
J
/
i
30 min
I
time
Fig. 5. Time dependence of the surface conductivity of the seven-layer C u T T B P c film: curve 1 ( - - ) ,
after the ( + 100 V) voltage application; curve 2 (. . . . ), after the reversal of polarity ( 100 V).
The dark current-voltage characteristics of a sandwich sample glass/A1/
C u T T B P % / N i are shown in Fig. 6, curve 1. The thickness of the CuTTBPc8 LB film
consisting of 8 monolayers was 13.2 nm. The high resistivity of the sample was
probably due to the resistivity of the Al/O3 layer at the aluminium electrode surface.
At low voltages the current was ohmic, being proportional to the voltage. The
transition voltage Ux between the ohmic and the quadratic (I oc U 2) space-chargelimited current regions was determined at 0.55 V. At a voltage above 3 V the current
followed a superquadratic dependence l oc U", where m > 2 and is voltage
dependent.
The current-voltage characteristic under illumination at 350 nm were similar
to the dark characteristics (Fig. 6, curve 2). At voltages below 0.6 V the short-circuit
photocurrent prevailed. The sample operated as a photovoltaic device giving an
open circuit voltage Uoc = 0.4 V (the aluminium electrode was negative), a shortcircuit photocurrent l s c = 2 X 1 0 - 1 1 A cm 2 at a photon flux JPh = 2 . 4 x 1 0 1 2
photons s -1 mm -z, and a fill factor f = 0.17. Isc was linearly proportional to the
photon flux jph within the tested region 2.5 z 101°-2.5 x 1012 photons s- 1 mm-2. A
dependence IscoCjph ~, e < 1, should have been observed if the photocurrent had
been limited by the charge recombination or surface traps zl. Thus the low value of
Isc was caused mainly by the resistivity of the oxide layers on the metal electrodes.
382
IL BRYNI)A, 1. KOROPE('K{', 1,. KAI.V()DA. S. NESPUREK
g
>
'/
~ i0~
o
oo
o / 2 •I~U" "
I0 '1
° •
~ ~:
go D
1/I~U
ld'/
10 "
101
I0'
101
voltage, U [V]
Fig. 6. Current w)ltage characteristics of the dark current I d and photocurrenl lph in an AI C u T T B P c s
Ni cell: cur~,e 1, I a for positive 10, plotted as ld) and negative ( ×, plotted as 1,0 bias of the a l u m i n i u m
electrode: curves 2 and 3, l,j + lph (O) and lph ([J) respectivel3 under illumination of aluminiuin electrode
with a photon i]t.IX incident on the (~uTTBPc lilm (2.4 ~, 10 i2 p h o t o n s s i thin 2.
35 nm, positixc
aluininium electrodc].
The spectral response of lsc was symbatic with the absorption spectrum of
C u T T B P c film {Fig. 71. Apparently, the excitation of C u T T B P c molecules was the
first step in the charge photogeneration. The direction and value of photocurrent for
the constant photon flux in the LB film were the same when the sample was
illuminated through aluminium or nickel electrodes. The aluminium electrode was
always negative. The bulk charge photogeneration observed earlier in vacuumdeposited phthalocyanine f i l m s 22 could not explain the high photovoltage measured in the LB film because the film thickness was too small. Thus, probably
because of the Schottky barrier, the charge injection from the aluminium electrode
into the C u T T B P c was responsible for the photovoltaics in the LB film sandwich.
Hole injection from aluminium could explain the negative polarity of the aluminium
electrode. A small Uoc ~ 4 mV observed in glass/Au/CuTTBPc.'Ni samples under
the same conditions as described above suggested that nickel operated like a noninjecting electrode. The photovoltaic effect due to the Schottky barrier at the
phthalocyanine-Al contact both in vacuum-deposited films 23'za and in LB
films 13.14 was reported earlier. On the contrary, the low rectifying effect observed in
the glass/Ni/CuTTBPcs/AI samples (Fig. 6) did not confirm the presence of the
Schottky barrier. The photogeneration quantum efficiency was determined as 10 4
charges per absorbed photon and 2.5 ×10 s charges per absorbed photon for
illumination at wavelengths ). shorter than 4 0 0 n m and longer than 500nm
respectively (Fig. 7, curve 3). It can be supposed that different processes were
included in the charge photogeneration via excitation of the first excited singlet state
Cu
TETRA[4-t-BUTYLPHTHALOCYANINE]
LB
383
FILMS
(2 > 500 nml or of the second excited singlet state (). < 400 nm). The first process can
p r o b a b l y be described by a m e c h a n i s m suggested for the charge p h o t o g e n e r a t i o n at
the p h t h a l o c y a n i n e - m e t a l c o n t a c t 25. The excitation energy migrates as a first
singlet exciton into the d e p l e t i o n region of S c h o t t k y barriers at the interface. T h e r e
an e l e c t r o n - h o l e pair is formed by exciton dissociation in the inner electric field. A
decrease in the q u a n t u m efficiency at 680 nm could be c o n n e c t e d with the lower
p r o b a b i l i t y of excitation energy m i g r a t i o n from molecules in d i s o r d e r e d positions
c h a r a c t e r i z e d by the optical a b s o r p t i o n b a n d at 676 nm.
- I0× I[34 #c ~
c
o
=2
"~ o_
g o=
-- L
E
c
i_
0/
-7.5× lU ~0
g~
05
E
D
¢3
L
-~10
/I./.\ \
..
•
5
0.10" -5.0x 10 o-
" .
-5
/
0 0 5 -2.5x10
I
400
500
600
700
wovetenCh,X [nm]
Fig. 7. Action spectrum of the short-circuit photocurrent lsc and the photoinjection efficiency ~l of an
AI/CuTTBPcs/Ni cell (photon flux incident on the CuTTBPc film was 2.4x 1012 photons s ~mm 2):
curves 1( x ) and 2 (O), lsc under illumination through aluminium (plotted as IscJ,nickel (plotted as - Isc)
electrodes respectively; curve 3 (
), quantum efficiencyof charge generation; curve 4 (-----), optical
absorption spectrum of the CuTTBPc films.
A direct charge g e n e r a t i o n from the second singlet state could explain the
higher efficiency of charge generation at short wavelengths. In a different case the
excitation energy would be dissipated to the first singlet state by the fast energy
c o n v e r s i o n and the charge generation efficiency w o u l d be c o n t r o l l e d by the singlet
exciton dissociation. The latter process is d o m i n a n t in e v a p o r a t e d p h t h a l o c y a n i n e
films, in which the q u a n t u m efficiency of charge g e n e r a t i o n via the second single
state is the same as that via the first singlet state 23'24. The p h o t o c u r r e n t values
o b t a i n e d by i l l u m i n a t i o n of the a l u m i n i u m electrode side did not differ from those
t~. BRYNI)A, 1. KOROPECKY, 1_. KAI,VO1)A, S. NESPI~JREK
384
obtained by illumination of the opposite sample side (Fig. 7). Owing to the high
absorption coefficient of CuTTBPc the light intensity at the illuminated surface of
the film was about 20",, higher than that at the opposite surface. Thus the interfacc
region active in photogeneration probably included the whole film thickness
(13 nm). This is consistent with 45 nm estimated by Hua et al. ~ for depletion width
plus the exciton diffusion length in silicon tetra[t-butylphthalocyanine] LB films at
the alum/n/urn electrode. A spectral dependence of the charge generation efficiency
s i m i l a r to that on glass..'Ni/CuTTBPc AI (Fig. 7) was observed also on g l a s s g l a s s
A g / ' C u T T B Pc,,,A1 s a m p l e s . The higher efficiency at short wavelengths d e c r e a s e d w i t h
aging of the samples.
The ratio R of charge generation efficiency via the second excited singlet to the
charge generation efficiency via the firsl excited singleS observed on the
CuTTBPc~ 5. AI sample was similar to that on the C u T T B P c 8 , AI s a m p l c d i s c u s s e d
above. A smaller ratio R w a s o b s e r v e d f o r t h e C t I T T B P % o ' A I
s a m p l e . Unfortunately, the thick sample was not comparable with the thin s a m p l e s b e c a u s e o f t h e
decrease m m o l e c u l a r p a c k i n g in t i l m s c o n s i s t i n g o f m o r e t h a n 25 monolayers.
REI:EREN('ES
11. Kuhn. D. M,3bius and tt. Bfichur. in A. Wemhcrgcr and B. Ros:-,itcr (c<.ls.). Phrygia~ .!,lcH~mL~ o/
Chem/~trv. V,.d.I. F'art 11 I B. Wiley, Nc,,~, ~ elk. 1972. p. 57-:.
E. E. P,.',lymerop,.mlc, s. J. 4pp/. Phw., 4,"; (197,7) 2404.
J. K. Sevcrll. R. V. Sudi~ala and E. (;. Wilson. /7ms Solid/qlm~, 16# ( 19~4Si 171 17",
M. f:ttjihira alll_] t4. Yanl;.ida. 77#n Solid l'Ths>. /£)0(19S~;) 12D 132.
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7 S. Baker. (J. (L Roberts and M. ('. Pets,.. Pro¢. lss.~z, t:h'¢tr. EtLC'., 130 119X3~ 2611.
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9 W. Davida. W. K a l i n a a n d S . W . ( ' r a n c . T h m S o / i d b i / m s . / 3 4 ( 1 9 ~ 5 ) 109 l lg.
10 .I.-H. Kim. T. M. ('c)lton, R. A. l_lphatis and ('. (" [.cznoi]'. Thi#l .So/id/'7/m.~. 157(198N) 141 147.
I1 J. D. S h u n and S. [!. Rickcrl, .L Jlol. E/co/ross., 5 ( I 9~49) 129 134.
12 Y. L. [ lua, G. G. Robcrls, M. M. Abroad, M. (". PetI'.. M. I hinack and M. Rein, t'hi/os..41J<,,. 13, 5_7
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13 M . Yt)lae}alla:.l.M. Sugi. M. Saito. K. Ikcgami, S. Kurod~l alld S. li×inla. ,]p#t. ,I..tppI /'isis.. 25
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14 ¥. |.. U tlcl. M. (~'. Pen 3 , (i. (7. Roberts. M. M. ml'~nlach, M. Hanack and M. Rein. 771iH Solid/-7tm~,
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16 H. Wohlijen. W. R. Bargcr, ,,\. W. Snow and N. I...larvis. I E E E Trall.v. E/eezron/)cri,'c.s. 72 (It)gS)
I 170 I [ 74
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Boston, MA, 1989. p p . l ~ 5 It)3.
S J. H. Moscl +and A. L. Thonlas. I~>hl/mlocvas##u , (om/mum,(v, Reinhold. London, It)6:l.
9 A. W. Snow and N..I. ,hn". is. J. Am. ("'hem..So{ .. /#6 l191441 47116
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21 R. Signerski. J. Kalinowski, 1. Koropcck 5' and S. Nc~purck, Tll#s Solid bThn~. / 2 / t I gl'441.
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23 K..1. Hall, J. S. Bonham and L. E. L3 ons, .-tu~l..]. ('hc###...7/I197g) 1661.
24 S. Neg,p{Irck, R. tl. [farl. ,I. S. Bonllam and L. E. Lyons. 4usz. J. C/u,m., .7<'~{ 1995i 10ill
25 R. I). Louify..I. 11. Sharp, ('. K. l tsiao and R. lie, ,I. ,-Ippl. P/tm.. 5 2 ( I 9g l) 521S.