b: Si[5.9 Ωcm, p]
e
a
c: Si[1.3 Ωcm, p]
a: Si[ 51 Ωcm, n]
10-4
d: Si[0.015 Ωcm, p]
10-5
b: Si[1.3 Ωcm, p]
e: Si[9.5 Ωcm, p]
c: Si[2.7 Ωcm, p]
10-5
I(A) 10-6
I(A)
rent
c
b
rent 10-6
Cur 10-7
Cur
a
10-7
b
10-8
10-8
a
c
10-9
10-9
0.1
1
10
0.01
0.1
1
10
Voltage V(V)
Voltage V(V)
(a) organic-PTCDA;
(b) organic-perylene
Fig. 5.3. I-V characteristics for Si/organic/Si heterostructures for different Si electrodes
(Stanculescu, 2008)
10-3
a: (+)Cu /PTCDA /Si[p](- )
c
10-4
b: (- )Cu /PTCDA /Si[p](+)
c: (- )Cu /perylene/Si[p](+)
10-5
d: (+)Cu /perylene/Si[p](- )
e: (+)Si[n]/perylene/Si[n](- )
f: (- )Si[n]/perylene/Si[n](+)
10-6
e
b
10-7
f
rent I(A)
c
Cur 10-8
d
10-9
a
10-10
0.01
0.1
1
10
Voltage V(V)
Fig. 5.4. I-V characteristics of Cu(Si)/perylene(PTCDA)/Si heterostructures at forward and
reverse bias (Stanculescu, 2008).
Because a Poole-Frenkel (PF) mechanism characterised by n=1/2, which involves the field
dependence of the mobility, has not been evidenced, the conduction could be space charge
limited (without electric field dependence of the mobility) and trap charge limited (Gao, 2002).
In the higher applied voltages region (> 1 V) the charge transport process appears to be
space charge limited, SCLC (n=2) when the charge injected from the inorganic electrodes (Si)
is larger than the charge existing in material in equilibrium, and trap charge limited, TCLC
(n>2) when exist discrete trapping levels associated with defects in the organic layer. A
transition region situated around 1V has been evidenced in the region where the conduction
process is trap charge limited, being associated with a mechanism of trap filling.
The general shape of the I-V characteristics is not very different for structures realized with
n type Si and n type conduction organic molecular compounds, n type Si/TPyP or n type
Si/Alq3, as can be seen in Figure 5.5. The current is less than one order of magnitude higher
in TPyP based heterostructure compared to Alq3 based heterostructure for an applied
voltage lower than 1 V.
Organic Semiconductor Based Heterostructures for Optoelectronic Devices
53
10-3
10-4
1
Si(p)/TPyP/Si(p)
2
Si(n)/TPyP/Si(n)
10-5
3
Si(n)/Alq3/Si(n)
) 10-6
A
2
10-7
rrent (
Cu
3
10-8
1
10-9
10-10
0.1
1
10
Voltage (V)
Fig. 5.5. I-V characteristics for SIS symmetrical heterostructures based on single n type
organic thin film (Rasoga, 2009):
1. Si(Cz, E/E, p: 1.34 Ωcm)/TPyP/ Si(Cz, E/E, p: 1.34 Ωcm);
2. Si(Cz, E/E, n: 0.008 Ωcm)/TPyP/ Si(Cz, E/E, n: 0.008 Ωcm);
3. Si(Cz, E/E, n: 0.008 Ωcm)/Alq3/ Si(Cz, E/E, n: 0.008 Ωcm)
A higher number of injected electrons was obtained in TPyP with ELUMO~4.1 eV (Antohe,
2008) compared to Alq3 with ELUMO=3.25 eV (Rajagopal, 1998). This behaviour is sustained
by the position of the electron affinity (LUMO) level in TPyP compared to the position of the
conduction band level in silicon. The difference between the energy of the conduction band
in n type Si and the energy of the electron affinity is higher in Alq3 (ΔE~0.75 eV) compared
to TPyP (ΔE~0.10 eV). The height of the energetic barrier favours the injection of the electron
at the contact n type Si/TPyP. As it is shown in Figure 5.5, for an applied voltage of 1 V, the
interface between n type Si wafer and n type conduction organic semiconductor (TPyP,
Alq3) favours the injection of a much higher number of charge carriers, ITPyP=1.5×10-7 A and
IAlq3=2.5×10-8 A, than the interface between p type Si wafer and the same n type conduction
organics (ITPyP is lower than 10-10 A).
The lower injection at the contact n type Si/Alq3 compared to the contact n type Si/TPyP
could be explained by the lower efficiency of the coupling between the n type Si substrate
and the π electrons system of Alq3 compared to TpyP (Rasoga, 2009).
The number of charges injected at the contact p type Si and p conduction organic, Si/ZnPc,
is higher (6×10-4 A at an applied voltage of 1 V) because ZnPc (as PTCDA as well) is a planar
aromatic molecule characterised by a preferred orientation of the molecular planes and
small distances between the intermolecular planes (Wu, 1997).
5.1.3 Si/organic multilayer/Si heterostructures
The order of preparation of organic/organic structure in multilayer heterostructures is not
critical because the system is not submitted to high thermal variations during the vacuum
evaporation process. The behaviour of the interface organic/organic has an important
influence on the shape of the I-V characteristics by a charge accumulation process. Figure 5.6
shows the I-V characteristics for the structures with double organic layer, n type organic
semiconductor (TPyP or Alq3) on p type organic semiconductor (PTCDA or perylene). The
54
Optoelectronic Devices and Properties
highest value of the current, was obtained for an applied voltage of 1 V at reverse polarization
(n type Si electrode positively biased and p type Si electrode negatively biased) in the
heterostructures Si/TPyP/PTCDA/Si (I=8×10-6 A) and Si/Alq3/perylene/Si (I=5×10-6 A).
The energetic barriers for the charge carriers at the p-n junction results from the value of the
ionisation energy and electron affinity for each organic compound:
ΔETPyP/PTCDA~0 eV; ΔEAlq3/PTCDA=0.85 eV; ΔETPyP/perylene=1.7 eV; ΔEAlq3/perylene=0.85 eV. (holes)
ΔETPyP/PTCDA~0 eV; ΔEAlq3/PTCDA=1.35 eV; ΔETPyP/perylene=1.5 eV; ΔEAlq3/perylene=0.65 eV (electrons)
The position of the HOMO (IE) level in TPyP can be estimated from the position of the
LUMO level (EA=4.1 eV) and the optical band gap (Eg=2.7 eV). The optical band gap has
been evaluated from the optical transmission spectra of TPyP thin films deposited on quartz
substrate.
10-3 1
Si/Alq3/perylene/Si
2
Si/Alq3/PTCDA/Si
10-4 3
Si/perylene/Alq3/PTCDA/Si
4
Si/TPyP/perylene/Si
5
Si/perylene/TPyP/PTCDA/Si
3
2
10-5 6
Si/TPyP/PTCDA/Si
)A
6
10-6
ent I(
urr
1
C 10-7
4
10-8
5
10-9
0.1
1
10
Voltage V(V)
Fig. 5.6. I-V characteristics for SIS heterostructures based on double n-p organic thin films
(Rasoga, 2009):
1. Si(Cz, E/E, n: 0.008 Ωcm)/Alq3/ perylene/Si(Cz, E/E, p: 6.78 Ωcm);
2. Si(Cz, E/E, n: 0.008 Ωcm)/Alq3/ PTCDA/Si(Cz, E/E, p: 1.34 Ωcm);
3. Si(Cz, E/E, p: 1.34 Ωcm)/perylene/ Alq3/PTCDA/Si(Cz, E/E, p: 6.78 Ωcm);
4. Si(Cz, E/E, n: 0.008 Ωcm)/TPyP/ perylene/Si(Cz, E/E, p: 1.34 Ωcm);
5. Si(Cz, E/E, p: 1.34 Ωcm)/perylene/ TPyP/PTCDA/Si(Cz, E/E, p: 1.34 Ωcm);
6. Si(Cz, E/E, n: 0.008 Ωcm)/TPyP/ PTCDA/Si(Cz, E/E, p: 1.34 Ωcm)
In the structures with three organic layer perylene/Alq3/PTCDA, presented in Figure 5.6,
the currents are slightly different at the reverse bias (p type, ρ=1.34 Ωcm Si electrode
positively biased and p type ρ=6.78 Ωcm Si electrode negatively biased) compared to the
structure with only two organic layers, Alq3/PTCDA at reverse bias (p type, ρ=1.3 Ωcm Si
electrode, negatively biased and n type, ρ=0.008 Ωcm, positively biased).
The supplementary perylene layer decreases with more than one order of magnitude the
value of the current at an applied voltage of 1 V in the heterostructure Si/TPyP/PTCDA/Si.
Organic Semiconductor Based Heterostructures for Optoelectronic Devices
55
This is a result of the high energetic barrier at the contact perylene/TPyP (ΔE=1.7 eV). In the
structure Si/Alq3/PTCDA/Si, a supplementary perylene layer has no influence on the
current for applied voltages lower than 1 V. The effect of the energetic barrier height at the
contact perylene/Alq3 (ΔE=0.85 eV) becomes important in the heterostructure
Si/perylene/Alq3/PTCDA/Si at applied voltages higher than 1 V (Rasoga, 2009).
5.2 Effect of the dopant
The doping process, which refers to the introduction of a strong electron donor or acceptor
in the host organic matrix, could have a strong impact on the material properties, increasing
the electrical conductivity of these host compounds and involving or not charge transfer
between the organic host matrix and the guest dopant (organic or inorganic). For example,
by the intercalation of benzil matrix with monovalent alkali (sodium) or non-alkali (silver)
metallic atoms can be induced delocalised charge carriers, electrons and holes, respectively.
Similarly results are expected for organic dopants containing substituent groups on the
aromatic nucleus with strong electron donors (such as OH) or acceptors (such as NO2 and
CO), properties.
The organic molecular films show some particularities of the transport mechanism of the
charge carriers, small amount of impurity or dopant strongly affecting or even masking or
hiding the intrinsic properties by a trapping mechanism.
It has been evidenced that the relatively low intrinsic conductivity of optical wide-gap
organic semiconductor, benzil [Eg=2.84 eV (Stanculescu, 2004; Stanculescu, 2006 a] and m-
DNB [Eg=2.92 eV (Stanculescu, 2004; Stanculescu, 2006 a)] has been improved by doping: in
benzil doped with m-DNB and, in m-DNB doped with 8-hydroxyquinoline (oxine) or 1,3
dihydroxybenzene (resorcinol). In these situations the resistivity of the organic layer
decreases to 106-107 Ωcm. New states have been generated within the band gap by doping,
which can be involved in the conduction process.
Benzil can be easily doped with electrons donors such as alkali metals (sodium) because it is
characterized by a high electron affinity. At low concentration, c=1 wt %, it was evidenced
an increased conductivity compared to the pure compound.
Another factor with an important effect on the electrical properties of the heterostructures is
the crystalline quality of the intermediate organic layer. An increase in the resistivity is
obtained for higher concentration of sodium, c=6 wt %, or by p type doping with silver. A
high concentration of dopants can induce a higher concentration of defects. Microstructured
patterns of the organic crystalline layers are created either by dopants or by the grain
boundaries whose generation is controlled by the thermal regime of the solidification
process. The effect of dopant on the resistivity of the organic layer is presented in Table 1.
The heterostructures prepared with doped intermediate benzil layer show a non-linear
behaviour of the I-V characteristics, as illustrated in Figure 5.7.
We have emphasised a special behaviour for the organic layer doped with organic molecule.
In benzil doped with m-DNB (3 wt %) we have obtained an important decrease in resistivity
and a higher conductibility. m-DNB doped with resorcinol shows a different I-V
characteristic. The exponential dependence can be correlated with a field intensity
dependence of the mobility of the charge carriers.
An even stronger effect of the dopant over the resistivity of the organic layer is evidenced in
m-DNB. The resistivity of the m-DNB layer can be reduced with three orders using, as
dopants, oxine (c=1 wt %) or resorcinol (10 wt %) as mentioned in Table 1.
56
Optoelectronic Devices and Properties
An exponential dependence between I and V has been evidenced in Figure 5.8 for m-DNB
layer highly doped with resorcinol (10 %), in the low voltage range, around 1 V, suggesting
a Poole-Frank conduction mechanism involving the field dependence of the charge carrier
mobility (Silveira, 2005) :
C V
I ~ V ⋅ e
(5.1)
Sample Organic
material Film Si
Si surface Resistivity
thickness conductivity processing
of the
(μm)
type
organic
film
ρ (Ωcm)
P01 m-DNB as synthesized
21
p
E/E
2.1×109
P03 m-DNB purified by melting zone
16
p
E/E
6.6×109
P07 m-DNB+oxine
75
n
E/E
0.49×106
P08 m-DNB+resorcinol
20
n
E/E
5.3×106
P11 benzil as synthesized
52
p
E/E
0.76×109
P12 benzil purified by melting zone
17
p
E/E
5.0×109
P15 benzil+m-DNB
30
p
E/E
2.3×107
P17 benzil+1 wt % Na
44
p
E/E
0.79x109
P18 benzil +6 wt % Na
43
p
E/E
1.9×1010
P19 benzil+2.4 wt % Ag
72
p
E/E
4.5×1010
Table 1. The properties of prepared SIS heterostructures (Stanculescu, 2006 b)
P15
10-3
P17
P18
10-4
P19
10-5
(A)
nt I 10-6
urreC 10-7
10-8
10-9
1
10
Voltage V (V)
Fig. 5.7. I-V plots (Stanculescu, 2006 b) for:
a) benzil doped with 1 wt% Na, Si(p), (P17);
b) benzil doped with 6 wt% Na, Si(p), (P18);
c) benzil doped with 2.4 wt% Ag, Si(p), (P19);
d) benzil doped with 3 wt% m-DNB, Si(p), (P15)
Organic Semiconductor Based Heterostructures for Optoelectronic Devices
57
-4
P08
-6
-8
( I / V )
ln
-10
ln(I/V) = -11.89438+2.03244*V1/2
-12
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
V1/2
Fig. 5.8. The plot ln( I V ) − V for a layer of m-DNB highly doped (10 wt%) with resorcinol
grown between two Si wafers, Si (n), E/E, ρ=0.90-1.9 Ωcm, (P08 in Table 1) (Stanculescu, 2006 b)
6. Effect of metallic contact on the electrical properties of organic
semiconductor thin films
The charge carrier injection at this type of interface is more complicated than at the interface
metal/inorganic semiconductor because in this case, the contact properties can be
influenced by the defects induced on the organic surface by the metallic contact and by the
space charge effect that characterises the organic semiconductors, effect that can obscure or
alter the real contact properties (Stanculescu, 2007 a).
The injection efficiency at the interface metal/organic semiconductor depends on many
factors such as the energetic barriers, which must to be overcome by the charge carriers, the
morphology and composition of the interface. Diffusion processes or reaction tacking place
at the interface could also influence the behaviour of the metal/organic contact (Hirose,
1996). When the physical contact between metal and organic solid is realized, interface states
are induced by metallization or interdiffusion (Hirose, 1996).
6.1 MIS type structures: metal/organic semiconductor/inorganic semiconductor
Different types of structures have been investigated from the point of view of the electrical
conduction properties. In the structures based on perylene and PTCDA the organic layer has
been obtained by vacuum deposition, while in the structure based on wide band gap
organic semiconductor, with Tmelting < 100 °C, Cu/m-DNB/Si, by rapid directional
solidification between two substrates one of Si and the other of an organic insulator
substrate covered with Cu (Stanculescu, 2007 a).
The order of the steps for the deposition of the layers that compose the heterostructure
(metal on organic or organic on metal) can be very important for the behaviour of the
heterostructure (Hill, 1998). The chemisorption of the organic molecules in determined sites
controlled by the extended π-electrons system irrespective of the functional groups appear
when the organic layer is deposited on a metallic layer. When a metal is deposited on an
organic layer an important modification of the interface may take place as a result of the
58
Optoelectronic Devices and Properties
chemical reaction between the metal and the organic molecules and/or of diffusion of a
large number of metallic atoms into the organic layer. These processes generate a high
density of states in the organic semiconductor band gap (Hill, 1998).
The diffusion into the organic layer depends on the type of metal and plays an important
role in the behaviour of the metal/organic contact. The main difference between different
metals is generated by the degree of diffusion of the metal atoms in the organic film. The
diffusion process depends on the reactivity of the metal atom with the organic compound.
The reactivity of the organic molecule is strongly dependent on the molecular structure. The
groups that substitute the hydrogen to the aromatic nucleus, and sometimes the ends
groups like anhydride in the case of PTCDA, control the interaction of the metal with the
organic molecule.
For perylene or perylene derivative (such as PTCDA) the transport of the charge carriers at
the contact metal/organic is influenced by the properties of the single silicon electrode and
by the polarization of the heterostructure. A low height injection barrier is sustained by a
value of the metallic cathode work function (Φ) close to the value of the electron affinity of
the organic. This assures a high level of electrons injection from metal to the organic layer.
Some studies have mentioned In with the work function, ΦIn =4.2 eV (Hirose, 1996) and Al
with ΦAl=4.25 eV (Hirose, 1996), as the most adequate metals for contacting PTCDA (EA=4
eV or 4.6 eV depending on the reference).
For example, the distance between the molecular planes parallel to the substrate in PTCDA
films is large compared to the metal atomic and ionic radius and assures a natural path for
the motion of the metallic atom/ion in the organic crystalline layer of PTCDA. The motion is
also facilitated by the local deformation of the organic crystalline lattice characterised by
weak van der Waals bonds (Hirose, 1996). The motion of the metal atoms can also be
influenced by the presence of defects, such as grain boundaries, which have influence on the
structural quality of the film.
The chemical reaction between In or Al on one hand and PTCDA on the other hand involves
predominantly the anhydride group of the organic molecule generating the oxidation of the
adatoms and the reduction of C atoms from the carbonyl group, as a consequence of the
high affinity of these metals for oxygen. The heat of formation of the reactive metals oxides
is with orders of magnitude higher (ΔH298=-1676 KJ/mol for Al2O3 and ΔH298=-926 KJ/mol
for In2O3) than the heat necessary for Ag2O formation (ΔH298=-31 KJ/mol) and the heat
necessary for the formation of ordinary metal oxides. This means that the generation of In,
and Al oxides take place in the detriment of metal-metal bonding, because the more
negative is the heat of formation the more stable is the compound. This is due to the fact that
more energy is lost to the surroundings when the compound is formed and the compound
has a lower energy being more stable (Stanculescu, 2007 b).
For PTCDA, the attachment of the electronegative anhydride group to the perylene core
causes the increase of the ionisation energy of perylene, IE=5.1 eV (Hirose, 1996). PTCDA
has higher ionisation energy, IE=6.2 eV (Hirose, 1996) or IE= 6.8 eV (Gao, 2001) and electron
affinity EA=4 eV or EA=4.6 eV respectively and its contact with metals are characterised by
the transfer of the negative charge carriers (electrons) from metal (In, Al, Ag) to organic
molecule (Hill, 1998).
The charged metallic ions created at the interface are driven through the materials by the
electric field and can react with the organic molecule, depending on its chemical structure
and reactivity. Considering Cu and Al metallic electrodes, the low atomic radius of Cu (1.57
Organic Semiconductor Based Heterostructures for Optoelectronic Devices
59
Å) and Al (1.82 Å) compared to the distance of separation between the molecular planes (for
compounds characterised by a nearly planar molecular configuration such as PTCDA and
ZnPc) assures a structural way for the motion of metal atoms (ions) in this type of
compounds.
This high diffusivity of metal atoms/ions can be attributed to a relatively low value of the
first ionisation energy that favours the transfer of an electron to the host organic matrix, this
ionisation leading to a Coulomb type repulsion between positively charged In ions situated
in interstitials sites (organic crystalline materials are characterised by large dimension of the
interstitials zones), acting as driving forces for diffusion. The diffusion rate is also
substantial for the other selected metals (Al) but slowly decreases in the succession In>Al
because the first ionisation energy for these metals increases in the following succession:
5.786 eV (In) < 5.985 eV (Al) (Stanculescu, 2007 b).
The reactivity of Al with perylene is not significant (there are not substituent groups to the
aromatic nucleus), Al atoms diffuses through the perylene film characterised by a separation
of 3.46 Å between the parallel plans of molecules (Hirose, 1996). At an applied voltage of
0.1-0.2 V has been remarked a region of transition though a region characterised by a power
dependence between I and V. This power dependence with an exponent n=2 characterises
the presence of the space-charge limited currents (SCLC) and with n>2 characterises the
presence of the trap-charge limited currents (TCLC).
As can be seen from Figure 6.1, the I-V characteristics of the Si/perylene/Al heterostructure
are not linear at low applied voltages (< 1 V) , but tend to an ohmic behaviour through a