ab“RN

R”R” = (CH2)6Br

SNn(PFBT-Pr)

R” = (CH2)6N+Me3Br –(PFBT-Pr)
F –4

R” = (CH2)6N+Me3BAr(PFBT-BArF4)

Figure 1. (a) Chemical structures of CPEs used in this study and (b) energy levels of the conducting substrate, metal-coated probe, and CPEs.

carrier mobilities can be extracted.
In this application note we describe
the application of C-AFM for chargetransport measurements of CPE thinfilms used in PLEDs (see figure 2). Thismeasurement is particularly difficult
to perform with conventional electrodeevaporated, hole-only diodes because of
the thickness restriction in the CPEs.
Several advantages of using C-AFM forcharge transport measurements are1) no separate set of devices is needed
because the measurements can bedone on the same PLED; 2) correlation
between surface morphology and chargetransport can be made; 3) thin and softpolymer films can be studied (these are

typically difficult to study by conventional
diode techniques); 4) distribution of holemobility can be computed; and 5) C-AFM
provides a charge transport measuring
technique that by-passes the effects
and restrictions caused by thermal
evaporation of a top electrode in holeand electron-only diodes. Often, a highwork function metal such as Pt or Au is
used and damage to the organic thin film
can occur during metal evaporation.
SAMPLE PREPARATION AND
MEASUREMENTS

Thin films of approximately
20 nanometers were spin-coated onto
indium tin oxide (ITO) substrates. Silver
paste was used to provide electricalacontact between the ITO substrate/
sample and the metal puck forbias application.
C-AFM measurements were performed
with a Veeco MultiMode® equipped witha C-AFM module. A Pt-coated Si tip witha 0.2 N/m spring constant and ~25 nm
tip radius was used. The Pt-coated tip
in contact with the CPEs complete thesandwiched hole-only device. Pt andITO with work functions of 5.6 eV and
4.8 eV ensure a large electron injection
barrier when either is negatively biased
and allows hole injection when Pt is
positively biased (see figure 1b). A
contact force of 40 pN was used for all
measurements showing no apparentb

Metal-coated tip

C-AFM probeAVprobe

Conjugated polyelectrolyteITOGlassCathodeEmissive Layer
PEDOT
ITO
Glass

Figure 2. Schematic drawing of C-AFM setup to study (a) thin polymer film or (b) organic light-emitting diode.

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Nanoscale Charge Transport in Light Emitting Diode Materials Using Conducting Atomic Force Microscopy

abc

Figure 3. AFM topography images of a) PFBT-Br, b) PFBT-BArF4, and c) PFBT-Pr.

variation upon repeated applications. Atip/sample contact area of 84 nm2 was
calculated using the Hertz model, where
the contact radius, a, is described by:a = (3FR/4Y*)1/3 and 1/Y* = (1 – ν21)/Y1 +(1 – ν22)/Y2, where F the load, R the tipradius, Y the Young’s modulus, and ν thePoisson’s ratio. With F = 40 nN,
R = 25 nm, Y1 = 168 GPa and ν1 = 0.38 for
Pt, Y2 = 3 GPa and ν2 = 0.35 for polymer,
and a is calculated to be around 6.03 nm.
SURFACE TOPOGRAPHY BY AFM

The surface topography AFM images of
the CPE films along with the film of theneutral precursors are shown in figure 3.
The AFM images of the CPEs are similar
while that of the neutral precursor ismuch rougher. Similarities between
the CPE films are likely due to identicalsolution processing; CPEs solutions
were spin-coated from methanol while
the neutral precursor from toluene.5aNANOSCALE CHARGE TRANSPORTBY C-AFM

I-V measurements at various locations
were performed in each sample andaveraged manually (see figure 4). As
expected, significantly lower currents
are observed in PFBT-Pr and PFBTBArF4 films at positive tip bias thannegative due to larger hole injection
barriers from ITO.
For the PFBT-Br film, a large current isobserved at positive tip bias, indicating
reduced hole injection barriers from ITO
with this CPE. This was not anticipatedbut is believed to be due to the formationof an interface dipole at the ITO/CPE
interface caused by the more exposedionic charge of the Br- counter anion.5
Carrier mobilities were extractedusing the space charge limited current
model commonly adopted in organic

diode charge transport measurements(see figure 4b).6 LogJ-logV plots of thefilms exhibit two regions; space chargetransport with shallow traps region(J ~ V2) and a trap limiting regime(J ~ Vm, m > 2). Hole mobilities of
3.4 x 10-4 cm2/Vs, 1.1 x 10-5 cm2/Vs and6.5 x 10-6 cm2/Vs were determine for
PFBT-Br, PFBT-BArF4 and PFBT-Pr,
respectively. One order decrease in holemobility with exchange of the counter_anion from Br- to BArF4 was observed
supporting our hypothesis that the_dimensionally larger BArF4 anion act asa spacer that separates polymer chainsand hence affects the optical and chargetransport properties.5CONCLUSION

C-AFM can be employed for chargetransport measurements of sensitiveand soft organic films with little damage.
This is in contrast to the irreversiblebFigure 4. Average I-V curves of a) the three polymer films and b) a representative logJ-logV plot showing general behavior observed in all samples.

Nanoscale Charge Transport in Light Emitting Diode Materials Using Conducting Atomic Force Microscopy

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