ED Communications

MAERIALS
Evidence for n-TypeConduction in a Perylene
Tetracarboxylic Diimide Derivative**
By Gilles Horowitz,* Faygal Kouki, Peter Spearman, Denis
Fichou, Claude Nogues, Xavier Pan and Francis Garnier

The use of organic semiconductors in practical electronic

devices first emerged when photovoltaic cells were fabri-
cated with organic dyes.[’-41More recently, non-intention-
ally doped conjugated polymers and oligomers have been
used as the active component in field-effect transistors
(FETS)[~-’’] and light-emitting diodes (LEDs).[’~-’~]
Because of their high band gap and low electronic affinity,
most of the “classical” organic semiconductors (anthracene,
phthalocyanines and most of the conjugated polymers) are
p-type, or hole-transporting.
Pioneering work by Tang has shown that the performance Fig. 1. Energy diagram of an organic diode at equilibrium (top) and under
forward bias (bottom). The anode and cathode are high and low work function
of both photo~oltaic[’~~ and light-emitting diodes[’*]can be metals, respectively. a): Hole transporting (p-type) organic semiconductor. b):
substantially enhanced by combining a p-type layer with an “Intermediate” case. c): Electron transporting (n-type) semiconductor. In all
n-type, or electron-transporting (ET), organic compound. A cases, a forward current Rows when the cathode is negatively biased. Small
band curvatures that are likely to occur near the interfaces have been omitted
prototype of such an organic n-type semiconductor is c.50,in for clarity.
which ET properties are thought to come from its high
electron affinity. Several other ET organic materials have Both devices provide clear evidence for electron-transport in
been reported re~ently.[’~-~’] For example, an organic this compound. The role of the electron injecting electrode is
semiconductor can be made n-type by adding an electron also discussed.
donating group such as cyanide[’g1or imide[’s~201 to the Most organic semiconductors have very low conductivity.
structure. Accordingly, organic diodes are often depicted as insulator-
The n-type-electron transporting-character of these metal interfaces, in which a uniform electric field extends
materials is most often identified on the basis of the across the organic layer thickness, rather than as conven-
improved photovoltaic or light-emitting properties of multi- tional Schottky diodes. Figure 1 shows the energy diagram
layer structures made with them, and direct evidence for n- (at equilibrium and under forward bias) of hole transporting
type conduction is not always given. We note however that (p-type, Fig. la) and electron transporting (n-type, Fig. lc)
organic FETs based on C60[22-241and tetracyanoquino- semiconductors sandwiched between high (anode) and low
dimethane (TCNQ)[251 have been reported, in which work function (cathode) metals. Figure l b corresponds to
evidence for n-type conduction arises because a positive an intermediate case, where the barrier heights at the anode
gate bias is required to induce an increase in the source- and cathode would be equal, which would lead to a
drain current under positive drain voltage (FETs made with symmetrical current voltage (I- V) characteristic. We note
p-type semiconductors operate with negative gate and drain that in the case of a non-symmetric I- V curve, the forward
voltages). current (bottom part of Fig. 1) would flow when the cathode
In the present work, we have fabricated photovoltaic is negatively biased, independently of the type of the
diodes and field-effect transistors with N,N’-diphenyl- semiconductor. In other words, a non-symmetric I- V
3,4,9,1O-perylenetetracarboxylic-diimide (DPP, Scheme 1). curve cannot by itself indicate the sign of the carriers
(electrons or holes).
Figure 2 shows the I-V characteristic (in the dark) of a
SnOZ/DPP/Al diode. The curve is non-symmetric, and the
forward current flows when the aluminum electrode is
negatively polarized. According to the above discussion, the
Scheme 1. low rectification ratio may be accounted for by a small
difference between the barrier heights at the anode and
cathode.
When the diode is illuminated, a current flows at short-
[*I Dr. G . Horowitz, Dr. F. Kouki, Dr. P. Spearman, Dr. D. Fichou, Dr. C.
Nogues, Dr. X. Pan, Dr. F. Garnier circuit. Figure 3 shows the photocurrent action spectrum
Laboratoire des MatQriauxMoleculaires, CNRS recorded when illuminating the diode through the Sn02 and
2 rue Henry-Dunant, F-94320 Thiais (France)
A1 electrodes. The absorption spectrum of the DPP layer is
[**I Xerox Corporation, Canada, is acknowledged for a generous gift of DPP.
Work supported by the European Union, through the “Human Capital shown for comparison. We note that zph is symbatic (in
and Mobility” Project SELMAT. phase) with the absorption spectrum when the diode is
242 0 VCH VerlagsgesellschaftmbH. 0-69469 Weinheim, 1996 093S-9648/96/0303-U242$ lO.O0+.25/U Ad”. Mater. 1996,8, No. 3
Communications
ADVANCED
MATERIALS
may be lost through recombination or trapping before
reaching the active electrode. If the thickness t of the film is
higher than the exciton diffusion length L, strongly absorbed
light will generate the majority of excitons close to the
surface. These excitons have less chance to reach the active
electrode (that is, the other side of the film), which results in
antibatic photocurrent and absorption spectra. If t 5 L, the
shape of the action spectrum will be independent of the
direction of illumination. In our case, t =200 nm. As L
ranges between 5 and 10 nm,[261we can expect that the
photoresponse will depend on the illuminated side. The fact
that here the active electrode is SnO2, whereas it is aluminum
-4 -3 -2 -1 0 1 2 3 4 in most p-type organic semiconductors,is at least a hint that
Voltage (V) DPP is electron transporting.
Field-effect measurements were carried out on structures
Fig. 2. Current-voltage characteristic in the dark of a 200 nm thick Sn02/
DPP/Al diode. In forward bias, the Al electrode is negatively charged. The similar to those described previously.[271 They are fabricated
device area is 0.2 an2. on glass substrates, and comprise an aluminum gate
electrode insulated by a poly(methylmethacry1ate)
(PMMA) layer, and gold or aluminum source and drain
electrodes. Figure 4 shows the variation of Id as a function
of Vd for various source-gate voltages in DPP based FETs.
2 Top curves correspond to a structure with gold source and
drain, and bottom curves to aluminum source and drain.
1.5 The curves are typical, and present a linear regime at low v d
and a saturation regime at high vd.The positive sign of both
1 the drain and gate voltages indicates a transport by
electrons. The field-effect mobility pFETof the carriers can
0.5 be obtained either in the linear regime or in the saturation
regime through Equations 1 and 2, respectively.[281 Here, W
0 ' ' z . I '.' ' I ' ' ' "' ' . . --I .L.,
. :-'. 0
400 450 500 550 600 650 700
Wavelength (nm)
Fig. 3.Photocurrent action spectra of the diode from Fig. 1, measured in short
cicuit (no bias applied). Full curve: illumination through SnOz; dotted curve:
illumination through Al. The absorption of the DPP film is shown by the
dashed curve.

illuminated through the Sn02 electrode, whereas it is

antibatic (out of phase) when illuminated through the A1
electrode. Reversed behavior (namely, the photocurrent and
absorption are symbatic when the diode is illuminated
through the A1 electrode) has been reported for a number of
organic photovoltaic that involved p-type organic
semiconductors. This phenomenon was first rationalized by
Ghosh and Feng.[261
The generation of a photocurrent in an organic semi-
conductor occurs through the formation of excitons,
followed by their dissociation into charges. The dissociation
is enhanced either by an electric field, or by charge accepting
states at electrode interfaces. The latter is generally more
efficient in organic semiconductors, and can account for a
photovoltaic effect in the aforementioned case where no 0 10 20 30 40 50
Schottky barrier is formed. If the light is directed towards Drain voltage (V)
the exciton dissociating electrode, excitons generated close Fig. 4. Drain current Id vs. source-drain voltage Vdmrves for various source-
to the surface readily dissociate and the photocurrent action gate voltages V, for DPP field-effect transistors. Top curves: gold source and
drain; bottom curves: aluminum source and drain. In both cases the insulator is
spectrum is "in phase" with the absorption spectrum. On the PMMA, with a capacitance C,= 5 nF/anZ, and the channel length and width
other hand, excitons generated at the other side of the diode are L = 50 pm and W=O.5 cm, respectively.

Adv. Mater. 1996,8, No. 3 Q VCH Veriagsgeseihchafr mbH, 0-69469 Weinheim. 1996 0935-9648/96/0303-0243$10,00+.25/0 243
 ED Communications

MATERIALS
and L are the channel width and length, respectively, Ci is Experimental
the capacitance (per unit area) of the insulating layer and V , Zone refined N,N'-diphenyl-3,4,9,1O-perylenetetracarboxylic-diimide (DPP,
the threshold voltage. With gold source and drain we get a Scheme 1) was kindly donated by the Xerox Corporation and used without
mobility pFET= 1.5 x 10-5cmZV-'s-'. further purification. The FET structure has been largely described previously
[27]. It consists of a glass substrate on which the gate electrode (100 nm wide
aluminum stripe) is first evaporated. The poly(methylmethacry1ate) (PMMA)
insulator is subsequently spin-coated on top of the gate electrode. Source and
drain contacts are made by evaporating the appropriate metal through a
shadow mask giving a channel width W of 5 mm and a channel length L of
50 pm. The organic layer was vacuum evaporated on top of the structure by
heating the compound in a molybdenum boat at a temperature of 350-4OO0C,
under a base pressure of 3 to 5 x 10-6Torr.
Photovoltaic diodes were made by evaporating DPP onto SnOz coated glass,
The figure obtained with aluminum electrodes is three followed by vacuum deposition of the top contact (aluminum). The thickness
times lower. We note however that the performance of the of the DPP film was estimated from its UV-vis absorbance. Current-voltage
characteristics were recorded with a computer-controlled Hewlett-Packard HP
structures rapidly degrades with time, which we attribute to 4140B picoammeter/dc voltage source.Optical absorption measurements were
the fact that all measurements were carried out in air. The performed with a Varian Cary 2415 UV-vis-NIR spectrophotometer.
Photocurrent action spectra were obtained by illuminating the photovoltaic
field-effect completely vanished after two or three days for diodes with a tungsten-halogen lamp through a Jobin-Yvon H25 mono-
the gold electrode device, and even faster with the aluminum chromator. Spectra were recorded under chopped light with a PAR 128A lock-
electrodes, which would indicate that the degradation takes in amplifier. The incident light intensity was calibrated using a UDT silicon
radiometer. Spectra are corrected for the absorption of the front (SnOz or Al)
place at the metal-semiconductor interface. A similar electrode.
instability has been reported on Cm transistors,[241and is
Received: September 13, 1995
probably a general characteristic of n-type organic semi- Final version: November 13, 1995
conductors. As the delay between fabrication and measure-
ment was identical for the gold and aluminum devices, the
lower current (and field-effect mobility) measured with A1 [l] B. R. Weinberger, S. C. Gau, Z. Kiss, Appl. Phys. Lett. 1981, 38, 555.
electrodes may be accounted for by its faster degradation, [Z] S . Glenis, G. Horowitz, G. Tourillon , F. Gamier, Thin SolidFilms 1984,
which is consistent with the higher reactivity of Al. 111, 93.
[3] R. N. Marks, J. J. M. Halls, D. D. C. Bradley, R. H. Friend , A. B.
A very interesting feature of DPP transistors is that Holmes, J Phys. Candens. Mutter 1994,6, 1379.
electron injection is equally obtained with gold and [4] H. Antoniadis, B. R. Hsieh, M. A. Abkowitz, S. A. Jenekhe, M. Stolka,
aluminum. We have checked that unlike the DPP FET, a Synth. Met. 1994, 62, 265.
[5] A. Tsumura, H. Koezuka , Y. Ando, Synth. Met. 1988,25, 1 1 .
p-type sexithiophene (6T) FET with an aluminum source [6] J. H. Burroughes, C. A. Jones , R. H. Friend, Nature 1988, 335, 137.
and drain does not operate. This indicates that, as expected [7] A. Assadi, C. Svensson, M. Willander , 0. Inganas, Appl. Phys. Lett.
1988,53, 195.
from its low work function, A1 can inject electrons, but not [8] G. Horowitz, D. Fichou, X.Z. Peng, Z. G. X u , F. Gamier, Solidstate
holes. On the other hand, since both 6TLS1and DPP FETs Commun. 1989, 72, 381.
with gold source and drain do operate, gold can inject holes [9] J. Paloheimo, P. Kuivalainen, H. Stubb, E. Vuorimaa, P. Yli-Lahti, Appl.
Phys. Lett. lm,56, 1157.
and electrons. The latter is confirmed in that p-n organic [lo] H. Akimichi, K. Waragai, S. Hotta, H. Kano, H. Sakati, Appl. Phys. Lett.
photovoltaic cells have been recently reported to operate 1991, 58, 1500.
with a gold contact to the n-type layer,[201and that all n-type [ll] Z. Xie, M. S. A. Abdou, X. Lu, M. J. Deen, S. Holdcroft, Can. J. Phys.
1992, 70, 1171.
organic FETs reported so far were equipped with gold [12] J. H. Burroughes, D. C. C. Bradley, A. R. Brown, R. N. Marks, K.
source and drain.[2z-251 To rationalize these observations, we McKay, R. H. Friend, P. N. Bums, R. B. Holmes, Nature 1990,341,539.
[13] D. Braun, A. J. Heeger, Appl. Phys. Lett. 1991,58, 1982.
note that the nature of a metal contact to a conventional [14] G. Grem, G. Leditzky, B. Ullrich , G. Leising, Adv. Muter. 1992, 4, 36.
inorganic semiconductor is most often governed by the [I51 J. Gmeiner, S. Karg, M.Meier, W. Riess, P. Strohriegl , M. Schwoerer,
reactivity between both materials rather than their respec- Actu Polym. 1993, 44, 201.
[16] H. Suzuki, H. Meyer, J. Simmerer, J. Yang ,D. Haarer, Adv. Muter. 1993,
tive work function,IZ9]and that the reactivity of gold with 5, 743.
organic materials is known to be much less than that of 1171 C. W. Tang, Appl. Phys. Lett. 1%,48, 183.
aluminum. [18] C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913.
[I91 N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend, A. B.
In summary, we have fabricated Schottky diodes and Holmes, Nature 1993,365, 628.
field-effect transistors with evaporated thin films of a [ZO] M. Hiramoto, H. Fujiwara , M. Yokoyama, Appl. Phys. Lett. 1991, 58,
1062; J . Appl. Phys. 1992, 72, 3781.
perylenetetracarboxylic diimide derivative. The photore- [Zl] M. Strukelj, F. Papadirnitrakopoulos, T. M. Miller, L. J. Rothberg,
sponse of the former shows that the active electrode is the Science 1995, 267, 1969.
high work function cathode, whereas the current-voltage [22] J. Paloheimo, H. Isotalo, J. Kastner, H. Kuzmany, Synth. Met. 1993, 56,
3185.
characteristics of the latter indicate that charge carriers are [23] K. Hoshimono, S. Fujimori, S. Fujita , S. Fujita, Jpn. J. Appl. Phys. Pt 2
electrons. Both features are consistent with an n-type 1993,32, L1070.
semiconductor. It was also shown that gold as well as [24] R. C. Haddon, A. S. Perel, R. C. Morris, T. T. M. Palstra, A. F. Hebard,
R. M. Fleming, Appl. Phys. Lett. 1995,67, 121.
aluminum can inject electrons in DPP, which we account for [25] A. R. Brown, D. M. Deleeuw, E. J. Lous , E. E. Havinga, Synth. Met.
by the low chemical reactivity of gold with organic 1994, 66,257.
materials. Work is currently underway to extend this study [26] A. K. Ghosh, T. Feng, J . Appl. Phys. la%, 49, 5982.
1271 G. Horowitz, F. Deloffre, F. Gamier, R. Hajlaoui, M. Hmyene, A.
to other metal electrodes. Yassar, Synth. Met. 1993,54,435.

244 0 VCH VerlagsgesellschaftmbH, 0-69469 Weinheim. 1996 0935-964819610303-0244$ 10.00+.25/0 Adv. Muter. 1996, 8, No. 3
Communications
ADVANCED
MATERIALS
1281 S. M. Sze,Physics of Semiconductor Devices, Wiley, New York 1981. PDMS I
[29] E. H. Rodherick , R. H. Williams, Metal-Semiconductor Contacts, master
Oxford University Press, New York 1988.

1 Place on a support

Two- and Three-Dimensional Crystallization of

Polymeric Microspheres by Micromolding in
Capillaries**
By Enoch Kim, Younan Xia, and George M . Whitesides*
i Place a drop of latex
at one end

Two-dimensional (2-D) arrays of colloidal particles are

interesting for potential applications in optical devices, data
storage, and microelectronics and as models for protein Fill channels by capillary flow;
crystallization.['-51 Fabrication of these arrays usually i allow evaporation of solvent
involves deposition of a thin layer of a suspension of
monodisperse colloids on a flat surface, followed by
evaporation of the solvent.[61 The mechanism of the
assembly of the particles involves nucleation initiated by
capillary forces and growth driven by a laminar flow and
evaporation of the The most commonly used
1 RemovePDMS

system for colloidal assembly has been the fabrication of 2-

D arrays of latex microspheres (usually polystyrene); these
particles can be synthesized with precise control over sizes
from lOOnm to 1OOpm.
N N

This paper describes the fabrication of crystalline 2-D and Fig. 1. Schematics of MIMIC in the crystallization of polystyrene micro-
quasi 3-D arrays of microspheres, patterned in the plane of spheres.
the support, using a technique we refer to as MIMIC
(micromolding in capillaries)['] that was developed for the
fabrication of polymeric microstructures of organic materi- air to escape. Opening both ends was essential for this
als. Using MIMIC, we have fabricated 2-D and 3-D arrays process to be successful.
of polystyrene microspheres in enclosed, continuous chan- Prior to MIMIC, the support (glass or Si/SiO2) was
nels formed by conformal contact between a support and an washed with a solution of peroxysulfuric acid
elastomeric m a ~ t e r [ ' ~ - whose
' ~ ] surface was patterned with (H202:H2S04= 1:3 by volume), rinsed with water, and
relief regions. This simple procedure has generated pat- dried with nitrogen. When the mold was placed on the
terned, 2-D and 3-D arrays of microspheres on different support, the compliant nature of the elastomer allowed
substrates, and this patterning is what distinguishes this conformal contact between the mold and the support, and a
process from previous work. network of channels formed. When a drop of a latex
Figure 1 shows a schematic outline of the process used to solution (Polybeads, Polyscience) containing polystyrene
crystallize microspheres using MIMIC. An elastomeric microspheres was placed at one end, the fluid filled the
mold was fabricated from poly(dimethylsi1oxane) (PDMS, network of the channels by capillary action. A typical rate
Sylgard 184, Dow Coming) using procedures for making for filling capillaries with cross-sectional area of 3-6 pm2
stamps used in microcontact printing (pCP),['0-'31 by over glass at room temperature was approximately 1-2 cm/
casting PDMS against a master that contained a pattern min. We usually positioned the elastomeric mold and
complementary to that to be reproduced. A typical length of support in a such way that the capillary filling was not
the mold was approximately 0.5 to 1.5 cm, and the mold influenced by the force of gravity.['41
covered approximately 0.5 to 4 cm2.Both ends of the Once the channels were filled completely, the liquid was
channels in the mold were cut to allow the fluid to enter and allowed to evaporate at room temperature, and the micro-
spheres crystallized onto the support within the confinement
of the channels: a pattern complementary to that in the
I*] Prof. G. M. Whitesides, E. Kim, Y. Xia
elastomeric mold was generated in a structure of crystallized
Department of Chemistry, Harvard University
12 Oxford Street, Cambridge, MA 02138 (USA) microspheres. Once crystallized completely, the crystals of
[**I This research has been supported in part by the Advanced Research microspheres were stable and rigid enough that when the
Projects Agency and the Office of Naval Research and in part by the elastomeric mold was peeled away from the support, they
National Science Foundation (PHY 9312572). It used MRSEC Shared
Facilities supported by the NSF under Award Number DMR-9400396. retained their shape and structural integrity; the surfactants

Adv. Mater. 1996.8, No. 3 0 VCH Verl~gsgesellschaftmbH, 0-69469 Weinheim, 1996 0935-9648/96/0303-0245$10.00+.25/0 245