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DOI: 10.1002/adem.200800292

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Conductance Enhancement Mechanisms of PrintableNanoparticulate Indium Tin Oxide (ITO) Layers forApplication in Organic Electronic Devices**

ION

By Michael Gross*, Nicolas Linse, Ilja Maksimenko and Peter J. Wellmann

[*] M. Gross, N. Linse, I. Maksimenko, Dr. P. J. WellmannDepartment of Materials Science 6,University of Erlangen-NurembergMartensstr. 7, D-91058 Erlangen, GermanyE-mail: michael.grosz@gmx.de

[**] Acknowledgement: This work was financially supported byDFG and Evonik Degussa GmbH (contract number GRK1161). The work of Mrs. Mertens and Mrs. Hildebrand whocarried out the XRD- and XPS-measurements is gratefullyacknowledged.

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Indium tin oxide (ITO, In2O3:SnO2) thin films exhibit a high

conductance along with brilliant transparency in the optical

range.[1–3] In spite of the existence of some other transparent

conducting oxide materials and the high material costs, ITO is

widely used as transparent electrode material in state-

of-the-art optoelectronic devices such as TFT-LCD-s,[4–5]

plasma TV-s, and touchscreens. It is also utilized in novel,

smart optoelectronic applications including organic solar

cells,[6–7] electrochromic devices,[8] and organic light emitting

diodes (OLEDs) for displays[9–11] or for lighting.[12] Usually

ITO thin films are fabricated by physical vapor deposition

(PVD) methods including RF-sputtering[13,14] and magnetron

sputtering[2,15–17] as well as pulsed laser deposition[18,19] and

several other evaporation techniques.[20–24] PVD derived ITO

layers are characterized by high conductances of partly more

than 10 000V�1 cm�1,[19,22] but also by high production costs.

Cost drivers are the inevitable vacuum process which hinders

continuous production and the requirement of substractively

wet chemical patterning steps.[4]

Wet deposition techniques are of emerging interest due to

their high cost reduction potential based on missing vacuum

demand, printing possibility comprising large area fabrication

potential, and the feasibility of direct patterning. Beside the

several deposition methods, there exist two different princi-

ples of wet chemical processing: The sol–gel-technique,

characterized by deposition of precursors with subsequent

transformation into ITO[25–30] and the deposition of a

dispersion containing ITO nanoparticles.[30–34] Sol–gel ITO films

can possess adequate conductances of >350V�1 cm�1,[25,26,35]

which can be increased to >1600V�1 cm�1 by temperature

treatment in nitrogen or forming gas.[26,30] Due to the required

low precursor to solvent ratio, only thicknesses of about

10–20 nm can be achieved for the sol–gel single layers,[26,30]

which leads to high sheet resistances. To avoid this, costly

repetitions of the deposition and transformation sequence are

necessary.

In contrast to sol–gel ITO layers, nanoparticle dispersion

derived thin films enable the production of almost every

thickness. They can also be produced on flexible polymer

substrates,[34] but exhibit comparatively low conductances.

The latter is related to a poor electrical contact between the

primary particle aggregates and high inter-grain poros-

ity.[30,32,33] Typical conductance after layer deposition is in

the order of 10�2V�1 cm�1 and rises to several 100V�1 cm�1

after annealing in air at 550 8C.[33,36] In order to overcome this

conductance bottleneck, we investigated the impact of various

conductance improving techniques on the electrical, optical,

and morphological properties of nanoparticulate printed ITO

layers.

Experimental

Nanoparticulate ITO layers for electrical measurements

were made by spin-coating a commercial ethanolic dispersion

(Evonik Degussa GmbH) with a Headway EC101 spin coater

on 20mmT 20mmT 0.14mm glass substrates. Layers for

optical transmittance measurements were prepared in the

same way but on 20mmT 20mmT 1.06mm fused quartz

glass substrates due to its better optical properties.

XPS and SEM samples were spin-coated on polished

15mmT 15mmT 1.0mm stainless steel substrates to prevent

electrostatic charging. After deposition all samples were

annealed for 30min at 550 -C in air using a W.C. Heraeus

R0K/R 10/60 furnace. The experimental procedures of

the postannealing conductance enhancing processes are

described in the respective section (Results and Discussion).

Sheet resistances were measured in a linear four-probe

setup using a Keithley SMU236. The environmental long-term

resistance measurements were also carried out in a linear

four-probe setup but with a Keithley 2400. Layer thicknesses

depend on the spinning parameters and are typically between

800 and 2500nm. The thickness was determined with a

DekTak IIA stylus profiler. The porosity was assessed from

refractive index[30,33] which was calculated by the method of

Manifacier et al.[37] The masses were determinated using a

Mettler AT 250 scale with an accuracy of 10S5 g. XRD

characterization was done with a Philips X’Pert with Cu Ka1X-rays and an angle of incidence of 1-. XPS measurements

erlag GmbH & Co. KGaA, Weinheim 295

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Fig. 1. Optical absorption spectra of ITO nanoparticulate films treated with variouspostannealing conductance enhancing procedures. The n values denote respectiveelectron densities in [cm�3]. They were calculated from IR-plasma frequency vP usingthe equation[3]: n¼ e1e0me,cb(vP

2þg2)/e2 with vP¼ 2pc/lP. Here me,cb¼ 0.4me[32,39]

is the effective conduction band electron mass, e1¼ 3.95[3] is the dielectric function athigh energy, g ¼ 0.2 eV £�1[3] is a relaxation frequency and vP is the plasma frequency.lP is the corresponding IR-wavelength and fits the point with maximum absorptioncoefficient slope. Mobilities were calculated from corresponding conductances and theelectron densities.

Fig. 2. Conductance versus storage time after annealing as well as after annealing withadditional postbake treatment. The waves in both curves stem from daily light oscillationin the laboratory.

were carried out by means of a Physical Electronics 5600 with

Al X-rays. The spectra were shifted to the In3d5 peak at a fixed

binding energy of 445 eV[38] and normalized to the same

number of counted binding electrons.

Results and Discussion

Spin-coating of the nanoparticle dispersion leads to highly

porous layers with porosities of more than 40%.[33] Tempera-

ture dependent conductance measurements have shown

the applicability of the fluctuation induced tunnelling

model,[32,33] which presumes clusters of well connected

primary particles. The clusters are separated by tunnelling

barriers though. After the nanoparticle dispersion

deposition, the layer conductivity is in the range of

1.8–2.5� 10�2V�1 cm�1. The dispersion is deep blue colored

indicating that ITO nanoparticles are strongly reduced and

possesses a high oxygen vacancy concentration. This is

consistent with the very high free charge carrier concentration

of 8.0� 1020 cm�3 determined from optical absorption spectra

presented in Figure 1.

Low conductance with high electron density entails a quite

low mobility of �2� 10�4 cm2V�1 s�1. This denotes marginal

contact between the primary particle aggregates and thus high

and wide tunnelling barriers.

After annealing in air for 30min, the conductance rises to

5–6V�1 cm�1 due to a mobility increase to �0.15 cm2V�1 s�1

although the charge carrier concentration drops to

2.6� 1020 cm�3 due to oxygen vacancy saturation. The

mobility increase stems from exhaustion of the dispersing

agent and from sinter neck generation between nanoparticle

aggregates.[33,36] After annealing the conductance rises with

time as presented in Figure 2. A storage of 48 h in air at

room temperature leads to a conductance increase to

15–17V�1 cm�1 whereas charge carrier concentration is

296 http://www.aem-journal.com � 2009 WILEY-VCH Verlag GmbH & C

almost constant and only the mobility rises to

�0.4 cm2V�1 s�1. The reason for this increase is still under

investigation. As a possible mechanism we currently suppose

the opening of an additional charge transport channel via the

aggregates’ surfaces which is probably induced by surface

charges from adsorbates.

Even poststorage conductance is quite low. This is why we

have studied and presented several postannealing treatments

in this work. All obtained conductances, charge carrier

densities and mobilities are summarised in Table 1.

Postbake Treatment

Postbake treatment means a short temperature step in air

on a hot plate at moderate temperatures. Figure 3 shows the

obtained conductances after postbake treatments at various

temperatures. We found a maximum conductance of about

16V�1 cm�1 at a postbake temperature of 250 8C caused by

slightly increased charge carrier density from 2.6 to

3.0� 1020 cm�3 (see Fig. 1) and a rising mobility from �0.15

to �0.3 cm2V�1 s�1. Optimum postbake time was ascertained

to be 5–10min.

On the one hand Figure 2 clearly points out that the

conductance enhancement by postbake is long-term stable. On

the other hand, postbake and storage effects are additive,

which indicates different origins for both. Postbake and

subsequent storage leads to a conductance of more than

35V�1 cm�1 within 48 h. As a reason for the postbake effect

we assume a decomposition of neutral electron trapping

and scattering complex defects which are generated during

high temperature annealing. Probable defect candidates

are the tin–oxygen complexes (Sn2.Oi

00)�, (Sn2O4)� and

[(Sn2.Oi

00)(Sn2O4)�] that were first reported by Frank

et al.[40] and verified by Yamada et al.[41] These defects distort

the lattice, therefore act as scattering centers and electron traps

and additionally decrease Sn-doping efficiency. The repeat-

ability of the postbake effect by a second annealing and

postbake cycle supports this assumption of defect generation

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Table 1. Achieved conductances, charge carrier densities and mobilities for reference samples and after all applied postannealing treatments and some combinations of them

Treatment Conductance [V�1 cm�1] Charge carrier density [cm�3] Mobility [cm2V�1 s�1]

As deposited 0.02 8.0� 1020 1.4� 10�4

Air annealing 6 2.6� 1020 0.15

Storage (48 h) 16 2.5� 1020 0.4

Postbake 16 3.0� 1020 0.3

Postbakeþ storage 31 n.m.[a] n.m.[a]

Infiltration 22 3.0� 1020 0.5

Infiltr.þpostbake 73 3.4� 1020 1.3

Forming gas 65 3.6� 1020 1.1

Infiltr.þ forming gas 71 n.m.[a] n.m.[a]

Vacuum annealing 121 5.6� 1020 1.4

Infiltrationþ vacuum 132 n.m.[a] n.m.[a]

[a] n.m., not measured.

and decomposition. The defect decomposition is a diffusion

controlled process. Hence, due to kinetics, we observed

diminished conductance enhancement at lower postbake

temperatures. From the oxygen diffusion coefficient in ITO of

D(T)¼ 0.1exp(�18 000[K]T�1) cm2 s�1[42] an oxygen diffusion

length of 3.7 nm at 250 8C within 5min postbake time can be

calculated. This firstly seems marginal but it is enough for a

decomposition of tin–oxygen complex defects and diffusion of

tin and oxygen atoms to regular lattice places. At temperatures

higher than 250 8C, defect generation is apparently thermo-

dynamically favored.

Infiltration Technique

The idea behind the self-developed infiltration technique is

to improve the interaggregate contact by generation of

additional ITO material in the pores and mainly at the

interconnections of the nanoparticle aggregates. For this aim,

porous annealed layers were infiltrated with a standard ITO

sol–gel precursor solution, which was dried and afterwards

transformed into ITO. This technique combines the print-

ability of nanoparticle layers with every desired thickness and

the high conductance of sol–gel material. The ITO precursor

solution from indium chloride and tin chloride was prepared

Fig. 3. Conductance versus postbake temperature.

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by exactly following the recipe of Daoudi et al.[28] Infiltration

into porous nanoparticle layer was carried out in both vacuum

and air by vertically putting the sample in a puddle of

precursor solution (capillary infiltration) as well as by dipping

it completely into the precursor (dipping infiltration). In case

of capillary infiltration the precursor solution visibly ascends

in the pores of the sample. It takes about 10min until the

infiltration frontier reaches the upper sample edge, either

infiltrated in vacuum or under ambient conditions. Using the

Washburn equation x¼ {r2t[paþ (2g cosu)/r]/4h}½ for infiltra-

tion into capillaries we calculated a mean infiltration capillary

radius of �30 nm in both cases. Therby an infiltration time

t¼ 10min, an infiltration depth x¼ 17mm, an external applied

pressure of pa¼ 0 mbar as well as the measured parameters

precursor’s surface tension g ¼ 2.76� 10�2Nm�1, precursor’s

viscosity h¼ 8.4� 10�4 Pas and contact angle u¼ 08were used.

This is in agreement with SEM measurements of the ITO

layers. Taking into account this radius we achieve an

infiltration time of about 5ms for the dipping infiltration

assuming an average layer thickness of 1.5mm for infiltration

in vacuum or in air. Hence we only shortly immersed the

samples in the case of dipping infiltration.

Afterwards the samples were dried for 30min at 150 8C on

a metallic hot plate. This temperature step leads to an

evaporation of most of the solvent which is considered as start

of the sol–gel formation. However, transformation into ITO

takes place at temperatures above 350 8C during the

subsequent annealing in air at 550 8C for 30min.

As amain result we found a conductance increase from 6 to

22V�1 cm�1 by applying the infiltration and transformation

process as described above. By combining infiltration and

following postbake process we achieved a conductance of

73V�1 cm�1. IR optical absorption denotes a slight increase in

charge carrier density from 2.6 to 3.0� 1020 cm�3 in case of

pure infiltration process and to 3.4� 1020 cm�3 for the

combined process (see Fig. 1). Mobility rises from 0.15 to

0.5 as the casemay be 1.3 cm2V�1 s�1. Wewere not able to find

a correlation between infiltration manner, infiltration atmo-

sphere and conductance improvement. This is important

considering a possible future application of this technique. On

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Table 2. Calculated porosity versus number of infiltration and transformation cycles

Infiltration steps 0 1 2 3 4 5

Porosity [%] 42.2 43.2 43.7 43.7 44.0 43.8

the one hand dipping infiltration saves much time compared

to capillary infiltration and is viable in a continuous printing

process. On the other hand a vacuum process should be

avoided if possible, because the omitability of vacuum

processes is a main argument of ITO nanoparticle dispersion

derived layers against sputtered or evaporated films.

While investigating the structural and morphological

properties of infiltrated layers we found a quite unexpected

result: absolutely no porosity decrease was observed after the

infiltration step, not even in the case of multiple infiltration

and transformation cycles. We rather found a small porosity

increase, as presented in Table 2. We however expected a

decrease since we have proved the generation of ITO material

during the sol–gel process: By spin-coating the precursor

solution on uncoated glass substrates and subsequent

transformation, thin conductive ITO layers were produced,

that were detected by resistance measurement as well as XRD

as presented in Figure 4.

To verify the result of nondecreasing porosity we

accomplished mass measurements of samples before and

after infiltration as well as after the subsequent transformation

step. The mass of typically used glass substrates exceeds the

ITO layer mass by the factor of �150. To avoid impreciseness

impact by the substrates we used several pellet samples

pressed from ITO powder. The required infiltration time was

calculated to be 1.4 s taking into account an infiltration depth

of 1mm. Nevertheless the samples were dipped into the

precursor solution for 15 s to guarantee complete infiltration.

Subsequent drying and heat transformation was carried out as

in the case of ITO layers.

Fig. 4. XRD-spectra of pure sol–gel layers annealed at various temperatures. The peaksclearly indicate generated ITO for transformation temperatures of 400 8C and above.

298 http://www.aem-journal.com � 2009 WILEY-VCH Verlag GmbH & C

We found a significant mass increase after infiltration by

23.5% being evidence that infiltration of the precursor solution

into the porous particle network actually takes place. After the

transformation we even observed a slight mass decrease of

0.2% compared to starting mass. This is consistent with the

results of nondecreasing porosity.

Both results lead us to the opinion that the precursor

solution slightly etches the ITO aggregates before the new

sol–gel material is generated in the transformation process.

Most likely, the origin for this etching is hydrochloric acid

(HCl) which is a product of hydrolysis of indium chloride.[1,25]

We found that mass of ITO pellets decreases if dipped into 1M

HCl.

To investigate the mechanism of the conductance enhance-

ment by the infiltration process, we carried out X-ray

photoelectron spectroscopy (XPS) measurements. In

Figure 5 the XPS survey spectra are presented.

As expected the carbon C1s- as well as the chloride Cl2s-

and Cl2p-peaks are significantly increased after infiltration

(before transformation) due to the infiltrated precursor

chloride salts and the organic solvent. After the transforma-

tion these peaks disappear again. Beside this all Sn-peaks are

clearly increased compared to the spectrum before infiltration.

This reveals the difference of the deposited sol–gel material

compared to the nanoparticle ITO in the content of tin. These

additional Sn donor atoms are presumably the reason for the

increased charge carrier density after the infiltration and

transformation process. The rise in mobility though pre-

sumably stems from a preferably accretion of the new sol–gel

material in indentations with negative radius inter alia small

pores and the inter-aggregate contacts due to capillary forces.

This should increase the contact area and diminish the

interaggregate tunnelling barrier. The infiltration method

leads quasi to a relocation of ITOmaterial from the aggregates’

surfaces to the contacts by etching oldmaterial and generating

new one. This is depicted in Figure 6.

Fig. 5. XPS survey spectra of nanoparticulate ITO layers before infiltration process,after infiltration with precursor solution and finally after the precursor to ITOtransformation step.

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Fig. 6. The infiltration method: a porous network of aggregated primary particles a) will be infiltrated and etchedby the ITO precursor solution b). After precursor-to-ITO transformation Sn-rich material is generated preferablyat indentations as the inter-aggregate touch points due to capillary forces c).

Temperature Treatment Under Reducing Conditions

It is well-known that conductance of ITO is improved by

temperature treatment in environments with low oxygen

partial pressures. This applies to sol–gel layers[26,30,43] as well

as nanoparticle dispersion derived thin films.[30,44,45] In the

prevailing opinion the reason is an increase in free charge

carrier density due to partial pressure gradient driven oxygen

diffusion from regular lattice places out of the material

generating oxygen vacancies and two free electrons per

vacancy:[3,46,47]

OxO! 1=2 O2 gð Þþv::

Oþ2e0

We investigated the impact of forming gas (95% N2þ 5%

H2) treatment for 30min as well as vacuum anealing at 550 8Cfor 30min on already air annealed ITO layers. In Figure 7 the

conductance results of samples treated in forming gas at

various temperatures are presented.

In contrary to the results of Guenther et al.[45] themaximum

conductance of 65V�1 cm�1 is achieved at a treatment

temperature of 200 8C. The conductance enhancement comes

alongwith a small rise in free charge carrier density from 2.6 to

3.6� 1020 cm�3, but a substantial improvement of the mobility

from 0.15 to 1.1 cm2V�1 s�1 (see Fig. 1 and Table 1). This result

seems unlikely since a more significant electron density

increase would rather be expected. Presumably this is caused

by the special structure of nanoparticulate films: The

macroscopic conductance is not limited by bulk properties,

but by the barriers between the nanoparticle aggregates.[32,33]

We assume the aggregates possessing oxidized surface layers

Fig. 7. Conductance of already air annealed nanoparticulate ITO layers post-treated informing gas at various temperatures.

ADVANCED ENGINEERING MATERIALS 2009, 11, No. 4 � 2009 WILEY-VCH Verlag GmbH & Co. KGaA

which therefore are electron depleted and act

as wide tunnelling barriers. Consequently,

the reduction of these oxidized surface layers

may only lead to a small rise in electron

density but can significantly decrease the

width of the tunnelling barriers between the

aggregates. This would lead to a highly

increased mobility. The combination of infil-

tration technique and forming gas treatment

leads to a conductance of 71V�1 cm�1.

Furthermore we observed a darkening and loss of transpar-

ency of the layers at forming gas treatment above 300 8C.Probably this stems from an overreduction of In2O3 to brown

or black In2O.[48,49] Guenther et al. even demonstrated

reduction to the metallic indium–tin phase.[45]

Annealing in vacuum for 30min at a temperature of 550 8Cleads to a conductance of 121V�1 cm�1 accompanied by an

electron density increase to 5.6� 1020 cm�3 and a rise in

mobility to 1.4 cm2V�1 s�1 (see Fig. 1 and Table 1). Previously

infiltration treated layers exhibit a conductance of 132V�1 cm�1,

the maximum we achieved. The origin of conductance

enhancement is the same as in the case of forming gas

treatment and the same arguments apply to the strong

mobility increase. Beside this we found a relation between

pressure and reduction. At pressures of less than �10�4mbar

overreduction to In2O takes place and the layers become black.

This also was reported by Thiel and Luckmann.[50]

Low Sheet Resistance Layers

By applying the conductance enhancingmethods described

above we produced several low sheet resistance ITO layers. If

conductance enhancement already is utilized, sheet resistance

can only be decreased by an increase in the layer thickness. Of

course, this diminishes the optical transmittance. For example

a 5V/& layer exhibits a total transmittance (through layer

and glass substrate) of 60%, whereas a 17V/& layer possesses

80% transparency. Without using any vacuum process

15V/& and a transmittance of more than 70%were achieved.

Hence, it is important to find a compromise between a low

sheet resistance required for the functioning of a device and a

high transparency which is important for its efficiency. For the

assembly of polymer LEDs we typically used ITO layers with

a sheet resistance of �25V/&.

Organic Light Emitting Diode as Optoelectronic Test Device

The aim of OLED device fabrication was not to assemble an

optimized device or to compete against published high

efficiencies, the goal was to demonstrate the applicability of

conductance enhanced nanoparticle ITO layers in modern

optoelectronics. Until now similar experiments have been

carried out only by Cirpan and Karasz.[51] One main problem

we facedwas the formation of micrometer-sized agglomerates

in the ITO dispersion, presumably resulting from a drying at

the dispersion bottle walls, in pipette tips or in printing

nozzles. Since, the size of these agglomerates substantially

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Fig. 8. Photographies of two OLEDs with 1.3 nm LiF/100 nm Al-cathode at differentworking voltages.

exceeds the thickness of the active polymer layers, they

inevitably result in electrical shorts in the device. By filtration

of the dispersion with microfiber glass membrane filters we

were able to reduce number and size of the agglomerates to

values which allow a fabrication of working OLED devices.

The roughness of nanoparticulate ITO layers accounts to Rrms

� 4 nm and Rpv� 25 nm. This exceeds roghness of sta-

te-of-the-art sputtered material by a factor of four

(Rrms� 1 nm, Rpv� 7 nm). This is related to the special

structure of nanoparticulate ITO layers with a primary

particle size of 15–30 nm.

Patterning of the ITO electrodes was achieved by

appropriate etching in concentrated HBr. OLEDs with a

diameter of 2mm were produced on nanoparticulate ITO

films by spin-coating a 80 nm layer of Baytron poly-3,4-

ethylenedioxythiophene/polystyrenesulfonate (Pedot/PSS)

followed by a 100 nm layer of Merck’s polyphenylene

vinylene (PPV) ‘‘Super Yellow.’’ As cathode a double layer

of 0.7–1.4 nm LiF and 100nm Al was evaporated from

resistance heated crucibles.

As the main result we announce the functioning of these

devices. In Figure 8 pictures of two fabricated OLED devices

operating at 12 and 15V are shown. LiF backing contact

thickness was 1.3 nm. The I–V characteristics of these both

devices are presented in Figure 9. The onset voltage is quite

high indicating that HOMO and LUMO levels of adjacent

Fig. 9. I–V characteristics of two OLED devices fabricated on printed conductanceenhanced nanoparticle dispersion derived ITO layers (photographies depicted in Fig. 8).

300 http://www.aem-journal.com � 2009 WILEY-VCH Verlag GmbH & C

layers in the stack are poorly adjusted to each other. To

improve the alignment of the energy levels further investiga-

tions would be necessary. However, this lies beyond the scope

of our research activities.

Conclusions

Beside the well-known conductance improvement by

annealing at high temperatures in air we investigated four

other postannealing conductance improving mechanisms of

nanoparticle dispersion derived transparent ITO layers.

Firstly, we observed a self-occurring conductance increase

by storage in air, which presumably stems from a change in

aggregates’ surfaces electronic states by postbake adsorption

of species. Secondly, we found a long-term stable enhance-

ment by postbare treatment in air at moderate temperatures

probably due to diffusion controlled decomposition of neutral

complex tin–oxygen defects. The main focus of this paper was

on our self-developed infiltrationmethod, whereas the porous

layers are infiltrated by an ITO precursor solution with a

subsequent transformation into ITO. We detected a particle

etching by the solution and a preferably accretion of sol–gel

derived material at indentations inter alia the interaggregate

contacts which significantly affects the mobility. Furthermore

we investigated the well-known tempering in reducing

conditions and obtained a strong mobility increase due to

chemical reduction of the aggregates’ surfaces, diminishing

the inter aggregate tunnelling barriers. Finally, we assembled

several functioning polymer OLEDs based on these layers to

demonstrate their applicability in modern optoelectronic

devices.

Received: September 5, 2008

Final Version: November 26, 2008

[1] R. Groth, Phys. Status Solidi 1966, 14, 69.

[2] S. Ray, R. Banerjee, N. Basu, A. K. Batabyal, A. K. Barua,

J. Appl. Phys. 1983, 54, 3497.

[3] I. Hamberg, C. G. Granqvist, J. Appl. Phys. 1986, 60, R123.

[4] M. Katayama, Thin Solid Films 1999, 341, 140.

[5] M. Gaillet, L. Yan, E. Teboul, Thin solid films 2007, 516,

170.

[6] C. J. Brabec, Sol. Energy. Mater. Sol. Cells 2004, 83, 273.

[7] C.Waldauf,M. C. Scharber, P. Schilinsky, J. A. Hauch, C.

J. Brabec, J. Appl. Phys. 2006, 99, 104503.

[8] C. G. Granqvist, Nat. Mater. 2006, 5, 89.

[9] T. Dobbertin, E. Becker, T. Benstem, G. Ginev, T.

Heithecker, H.-H. Johannes, D. Metzdorf, H. Neuner,

R. Parashkov, W. Kowalsky, Thin solid films 2003, 442,

132.

[10] D. Hertel, C. D. Muller, K. Meerholz, Chem Unserer Zeit

2005, 39, 336.

[11] W. Kowalsky, P. Gorrn, J. Meyer, M. Kroger, H.-H.

Johgannes, T. Riedl, Proc. SPIE 2007, 64860F.

o. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2009, 11, No. 4

COM

MUNIC

ATIO

N

M. Gross et al./Conductance Enhancement Mechanisms of Printable Nanoparticulate

[12] C. Garditz, A. Winnacker, F. Schindler, R. Paetzold,

Appl. Phys. Lett. 2007, 90, 103506.

[13] J. C. C. Fan, J. B. Goodenough, J. Appl. Phys. 1977, 48,

3524.

[14] J. Calderer, Vacuum 1987, 37, 441.

[15] M. Buchanan, J. B. Webb, D. F. Williams,Appl. Phys. Lett.

1980, 37, 213.

[16] M. Libra, L. Bardos, Vacuum 1988, 38, 455.

[17] P. Frach, D. Gloß, K. Goedicke, M. Fahland, W. M.

Gnehr, Thin Solid Films 2003, 445, 251.

[18] H. Kim, C. M. Gilmore, A. Pique, J. S. Horwitz, H.

Mattoussi, H. Murata, Z. H. Kafafi, D. B. Chrisey, J.

Appl. Phys. 1999, 86, 6451.

[19] A. Suzuki, T. Matsushita, T. Aoki, Y. Yoneyama, M.

Okuda, Jpn. J. Appl. Phys. 2001, 40, L401.

[20] M. Mizuhashi, Thin Solid Films 1980, 70, 91.

[21] A. Hjortsberg, I. Hamberg, C. G. Granqvist, Thin Solid

Films 1982, 90, 323.

[22] I. A. Rauff, Mater. Lett. 1993, 18, 123.

[23] J. George, C. S. Menon, Surf. Coat. Technol. 2000, 132, 45.

[24] J. W. Bae, J. S. Kim, G. Y. Yeom, Nucl. Instrum. Methods

Phys. Res, B 2001, 178, 311.

[25] K. Nishio, T. Sei, T. Tsuchiya, J. Mater. Sci. 1996, 31, 1761.

[26] Y. Takahashi, S. Okada, R. B. H. Tahar, K. Nakano, T.

Ban, Y. Ohya, J. Non-Cryst. Solids 1997, 218, 129.

[27] M. J. Alam, D. C. Cameron, Thin Solid Films 2000, 377,

455.

[28] K. Daoudi, B. Canut, M. G. Blanchin, C. S. Sandu, V. S.

Teodorescu, J. A. Roger, Thin Solid Films 2003, 445, 20.

[29] T. F. Stoica, M. Gartner, M. Losurdo, V. Teodorescu, M.

Blanchin, T. Stoica, M. Zaharescu, Thin Solid Films 2004,

455, 509.

[30] N. Al-Dahoudi, M. A. Aegerter, Thin Solid Films 2006,

502, 193.

[31] C. Goebbert, R. Nonninger, M. A. Aegerter, H. Schmidt,

Thin Solid Films 1999, 351, 79.

[32] J. Ederth, P. Heszler, A. Hultaker, G. A. Niklasson, C. G.

Granqvist, Thin Solid Films 2003, 445, 199.

ADVANCED ENGINEERING MATERIALS 2009, 11, No. 4 � 2009 WILEY-VCH Verl

[33] M. Gross, A. Winnacker, P. J. Wellmann, Thin Solid Films

2007, 515, 8567.

[34] T. Koniger, H. Munstedt, J. Soc. Inf. Disp. 2008, 16, 559.

[35] K. Daoudi, B. Canut, M. G. Blanchin, C. S. Sandu, V. S.

Teodorescu, J. A. Roger, Mater. Sci. Eng, C 2002, 21,

313.

[36] J. Ederth, A. Hultaker, P. Heszler, G. A. Niklasson, C. G.

Granqvist, A. van Doorn, C. van Haag, M. J. Jongerius,

D. Burgard, Smart Mater. Struct. 2002, 11, 675.

[37] J. C. Manifacier, J. Gasiot, J. P. Fillard, J. Phys. E Sci.

Instrum. 1976, 9, 1002.

[38] (Eds.: D. Briggs, M. P. Seah), Practical Surface Analysis by

Auger and X-ray Photoelectron Spectroscopy, Vol. 1, Wiley

& Sons, Chichester 1990, Appendix 7b.

[39] I. Hamberg, C. G. Granqvist, K.-F. Berggren, B. E. Ser-

nelius, L. Engstrom, Phys. Rev. B 1984, 30, 3240.

[40] G. Frank, H. Kostlin, Appl. Phys. E 1982, 27, 197.

[41] N. Yamada, Y. Shigesato, I. Yasui, H. Li, Y. Ujihira, K.

Nomura, Hyperfine Interact. 1998, 112, 213.

[42] J. Berger, I. Riess, D. S. Tannhauser, Solid State Ionics

1985, 15, 225.

[43] J. Liu, E. Radlein, G. H. Frischat, Phys. Chem. Glasses

1999, 40, 282.

[44] A. Hultaker, Transparent conductive tin doped indium

oxide, Dissertation, ACTA University, Uppsala 2002.

[45] G. Guenther, G. Schierning, R. Theissmann, R. Kruk, R.

Schmechel, C. Baehtz, A. Prodi-Schwab, J. Appl. Phys.

2008, 104, 034501.

[46] H. K. Muller, Phys. Status Solidi 1968, 27, 723.

[47] D. C. Paine, H. Y. Yeom, B. Yaglioglu, in: Flexible Flat

Panel Displays (Ed.: G. P. Crawford), Wiley-Verlag, Chi-

chester 2005, p. 79.

[48] C. Winkler, J. Prakt. Chem. 1867, 102, 273.

[49] (Ed.: Deutsche Chemische Gesellschaft), Gmelins Hand-

buch der anorganischen Chemie - Indium, 8th Edn., Chemie

GmbH, Berlin 1936.

[50] A. Thiel, H. Luckmann, Z. Anorg. Chem. 1928, 172, 353.

[51] A. Cirpan, F. E. Karasz, J. Appl. Polym. Sci. 2006, 99, 3125.

ag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 301