“ A new field of electronics has emerged with the discovery of conducting and semiconducting
organic materials in solid forms. To understand organic semiconductors, it may be instructive to
see them in contrast to the more common inorganic semiconductors. But before that, it is
necessary to understand organic solids.
ORGANIC ELECTRONICS II
Deepak Gupta
A special issue of Directions in February 2004
focused on Display Technology and Organic
Semiconductors. In that, an article by the same
title, Organic Electronics1, focused on thin film
transistors. Therefore, along with the report
of the proceedings of the REACH
symposium (2007) on organic electronics, the
present ar ticle emphasizes another
application, the organic light emitting diodes
(OLED).
Most interesting, and also wide-spread, use of
electronics is based on semiconductors.
When we think of semiconductors, we think
of silicon, germanium, gallium arsenide,
gallium nitride etc., which are all inorganic
crystalline materials, conventionally included
in the class of ceramic materials. On the
contrary, the term organic, in form of
engineering materials, evokes the thought of
plastics that are largely regarded as electrical
insulators. But, a new field of electronics has
emerged with the discovery of conducting and
semiconducting organic materials in solid
forms.
To understand organic semiconductors, it may
be instructive to see them in contrast to the
more common inorganic semiconductors.
But before that, it is necessary to understand
organic solids. Among the organic materials
for electronic applications, we are mostly
interested in that class which contain pelectrons. For example, consider ethylene
(C2H4) molecule, in which the sp2 hybridized
Department of Materials and Metallurgical
Engineering & Samtel Centre for
Display Technologies
carbon forms three co-planar s-bonds; the
carbon atom bonds with the other carbon
and two hydrogen atoms. The fourth orbital
(pz) is perpendicular to the sp2 hybridized
orbital plane, and leads to additional p
bonding between the two carbon atoms. The
molecular orbitals thus formed split into
bonding and anti-bonding states, which
commonly are also known as HOMO
(highest occupied molecular orbital) and
LUMO (lowest un-occupied molecular
orbital), respectively (see Figure 1 on page
112). These molecules form solids via a weak
intermolecular bonding, clubbed under the
name of “van-der waal” bonds. On account
of this weak bonding, although the HOMO
and LUMO levels may take shape of an
energy band, but the bands are rather narrow.
Furthermore, consequence of this weak
intermolecular bonding is that the electronic
properties of the organic solids are largely
determined by the molecules themselves; role
of the weak forces is only to hold the organic
molecules together in a solid.
In contrast, a crystalline inorganic
semiconductor with covalent/ionic interatomic bonding throughout the material,
because of periodicity of the lattice, allows
for a description of the electronic states in
reciprocal space, k. Correspondingly, an
inorganic semiconductor has an occupied
valence band separated by an empty
conduction band, as shown in figure 2 on
page 3. In that sense, an analogy between
111
 bond
H
C
 bond
H
C
C
C
H
H
LUMO

 (anti-bonding
C
orbital)
ε
pz
HOMO
C
 * 
1
1  2 
2
 
1
1  2 
2
pz
(bonding orbital)

C
C
Discrete or Band
Figure 1. Et hyle ne molecule with  and  bonds. Molec ular bonding
leads to bonding (occupied) and anti-bonding (empty) states, both
corresponding to - and  bonding orbitals. In solid form, the resulting
HOMO and LUMO states take a form of bands, analogous to c rystalline
semiconductors, but the band-widths ar e significantly smaller.
valence band and HOMO level and conduction
band and LUMO is possible. But, the width of
the energy bands in inorganic semiconductors
is much larger, which has a consequence on the
mechanism of the charge transport.
Comparison of organic and inorganic
semiconductors
The properties of the two types of
semiconductors are compared in Table 1 on
page 114. Apart from differences in bonding,
there are free carriers in form of electrons and
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holes in inorganic semiconductors, whereas the
organic semiconductors generally do not
support free electrons and holes. The charge in
this case is in form of a positive or negative
polaron. Fur ther more, in inorg anic
semiconductors, the positive and negative
charges form excitons (Wannier-Mott type)
with small binding energies; thus excitons are
rarely observed at room temperature in
common semiconductors. In contrast, organic
semiconductors demonstrate large binding
energies for excitons (Frenkel type) and hence
they play a major role in determining the optical
ε
Conduction Band
Eg
k
Valence band
Figure 2. Valence and conduction band in crystalline semiconductors. The valence band is completely occupied by
electrons and conduction band is empty at 0K. At higher temperatures, some electrons from the valence band
populate the conduction band, as shown in figure above.
behavior of the organic semiconductors. In
context of charge transport, the effective mass
of carriers in organic semiconductors is huge
compared to that in common inorganic
semiconductors, and hence, mobility for
charge transport is poor. This mobility of
charge in organic semiconductors is defined by
a hopping mechanism for transport, which
occurs in a band in inorganic semiconductors.
Applications of Organic Electronics
Comparison with inorganic crystalline
semiconductor reveals that mobility of carriers
in organic semiconductors, typically of
positively charged species, is limited to 1
2
cm /Vs, two or three order of magnitude lower
than that in conventional semiconductors.
Hence, applications are also limited to those
devices where requirements on mobility of
charges are not too high. Such applications can
broadly be classified into those where only the
charge transport is important, such as organic
1
thin film transistors (performance is
equivalent to that by demonstrated by
amorphous silicon), or where optical behavior
is more important, such as solar cells, solid state
lighting source or organic light emitting
displays. In the latter set of applications, the
charge transport is not as important as the
balance of electron and hole like conduction in
a device and absorption/emission of photons.
As an example, Figure 3 on page 115 shows two
displays. The first is the state of art 11 inches
OLED television announced to be introduced
113
C ry stallin e/ In o rgan ic
solid
M olecu la r/ O rgan ic so lid s
Io nic, C o valent, M etallic
(2-4ev)
Io nic o r co valent w ithin mo lecu le
(intramo lecu lar), but the is so lid
held to gether by V an-der W aal
fo rce (0.01 eV ). T herefo re, so lid ’s
behavio r to a large extent go verned
by ind iv idual mo lecu les (except
increased vibratio nal mo des )
E lectro ns, ho les, io ns
P o laro ns, excito n (tho ugh neutral)
B on d in g
C h arge
C arrier
E ffective m ass
m e o r less
(10 2 - 10 3 ) m e
T ran sp o rt
B and
H o pping
M ob ility
10 2 -10 4 cm 2 /V s
10 -6 -1 cm 2 /V s
E xciton
W annier- M o tt
Frenkel, C harge transfer
L u m in escen ce B and to band
reco m binatio n (at
practical temperature )
E xcito n reco m binatio n
Table 1: Comparison of molecular/organic with crystalline/inorganic solids
in the market by the year-end. The other
picture shows the real potential of organic
electronics where the OLED display is built on
a flexible substrate. Implicitly, the electronics
therefore will be thin, light-weight and truly
portable. In addition it could also be form
fitting; for more details refer to [1].
At the Samtel Center for Display Technologies,
the research gamut on organic electronics
includes OLED, OLED displays, thin film
transistors, solar cells and RFID (radio
frequency identification) tags. Of these, as an
example, we elaborate further on the OLED
devices.
114
Basics of OLED technology
A pixel of an OLED display, as shown in Fig. 4
on page 116, consists of multiple organic layers
sandwiched between two electrodes marked as
cathode and a transparent anode, typically
made up of indium tin oxide (ITO). Each
organic layer has a specific function, with a total
thickness all organic layers on the order of 100
nm. When electrons and holes are injected into
this device from their respective electrode, the
structure in Fig. 4 is so engineered that the two
types of charges meet up in the emissive layer.
Here they form an exciton, which upon
relaxing emits light corresponding to the
(a)
(b)
Figure 3. (a) Sony 11 -inch OLED TV (b) a flexible display ( www.spectrum.ieee.org/publicfeature /aug00/orgsf1.html)
nature of the emissive layer. Furthermore, the
emissive layer can be modified by doping to
change the color of emission; that is color
tuning is possible.
1989 included dopants in a host, allowing for
energy transfer from the latter to the dopant
molecules. This, then, allowed modulation of
color in the same device.
OLED History
The emission from these devices was
fluorescent. That meant, even the ideal
electroluminescent quantum (internal)
efficiency could not be more than 25%,
especially in OLEDs fabricated with small
organic molecules. In this regard, another
development in late 1990 is noteworthy.
Forrest's group succeeded in harvesting the
triplet excitons, taking the quantum efficiencies
to nearly 100% with Ir-complexes based
5
phosphorescent devices .
In the context of displays, more important was
the time constant of triplet emission, in
microsecond range. An advantage of an
OLED display over the liquid crystal display
(LCD) is that the former can display fast
moving images much better owing to their
faster response speed, typically microsecond in
O L E D s a n d m i l l i s e c o n d i n L C D s.
Phosphorescence is usually associated
The organic light emitting diode (OLED)
based display bear their origin to often quoted
invention of Tang and Van Slyke2 in 1987,
wherein a heterostructure device, much like its
inorganic counterpart, yielded measurable light
emission at voltages less than 10 V. Prior to this
invention, only a faint glow came out of an
OLED device at voltages on the order of 100
V. In most organic materials, the hole mobility
is at least 10-100 times greater than that of an
electron. Thus, the bilayer OLED reported in
1987, moved the recombination zone away
from the cathode, leading to a better efficiency
of the device. But, the OLED, reported in [2]
emitted in green; the displays require three
primary colors, usually red, blue and green.
Another report from the same group3 in
115
Cathode
Electron Injection Layer
Electron Transport Layer
Emissive Layer
Hole Transport Layer
Hole Injection Layer
ITO
Glass
116
Hole Blocking Layer (optional)
Figure 4. An OLED device with ITO anode
with slow emission, but a fast emission in the
Ir-complex based devices preserved the
advantage of the OLED devices.
commercialization of OLED displays, refer to
t h e w e b - s i t e : h t t p : / / w w w. o l e d info.com/history.
The OLED devices are made from two types
of organic materials - the first are called small
molecules and the other polymers. The early
development of OLED occurred with small
molecules, whose thin films are obtained by
thermal evaporation, and other processes
which are well established in micro-electronics.
In early to mid 1990's, Friend and his coworkers demonstrated OLED devices based
on polymers, with very different processing
methods. Since, most polymers are difficult to
thermally evaporate, but can be made soluble in
many volatile solvents, it became possible to
spin-coat the polymer thin films required in an
OLED. This option of solution processing, by
spin coating or ink-jet printing, opens up
potentially a much cheaper route for
fabricating OLEDs. However, lack in pace of
development of required equipment, and easy
availability of equipment/process for small
molecules, early commercialization of OLED
displays has been through small molecule
organic materials. Nonetheless, a few small
sized monochrome displays based on polymers
did become available commercially. For details
of major industrial landmarks that have led to
The Display Prototype made at IITK
These major developments described here
allowed the possibility of OLED displays. In
general, the displays are of two types passive
matrix and active matrix. We have made a full
color passive matrix displays with a resolution
of 96 (3)x64 in a 1” diagonal and 4:3 format.
The images generated on these displays are
depicted in Fig.5 below. These pictures are
taken from the module which is shown in
Figure 6 on page 117.
Fig. 5. Images captured from a passive matrix OLED
display. Picture on the top left is a B/W image
because of the efforts of a team consisting of
J.Narain, K. N. N. Unni, G. S. Samal, S.
Bhagwat, A. Awasthi, J. Bhatia, Boby C. Villari,
A. Rajouria, S. Bhattacharya and S. Bharat.
References
Fig. 6. Display module, full color 96 (3) x 64, 1”,
passive matrix, consisting of the OLED
display, COF and driver.
Acknowledgement: The development of the
passive matrix display has become possible
[1].
Deepak, “Organic electronics,”
Directions, Vol. 6, No. 1, pp. 19-22
[2]. C. W. Tang and S. A. VanSlyke, “Organic
electroluminescent diodes,” Appl. Phys. Lett.,
vol. 51, 1987, pp. 913-915.
[3]. C. W. Tang and S. A. VanSlyke,
“Electroluminescence of doped organic thin
films,” J. Appl. Phys.., vol. 65, 1989, pp. 36103616.
[ 4 ] . B a l d o e t a l . ,” H i g h l y e f f i c i e n t
phosphorescent emission from organic
electroluminescent devices,” Nature, vol. 51,
1998, pp. 151-154.
[5]. M. A. Baldo, “The electronic and optical
properties of amorphous organic
semiconductors,” Ph. D. dissertation,
Princeton University, 2001.
[6]. J. H. Burroughes et al.,”Light emitting
diodes based on conjugated polymers,” Nature,
vol. 347, 1990, pp. 539-541.
Prof. Deepak Gupta is a Ph.D from the University of California,
Berkeley, California, USA. His research interests are materials
development: electronic and optical, organic electronics, displays and
modelling and simulation of materials and processes.
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