3B01
Pentacene Thin Film Photoconductivity :Influence of the Electrode.
(Kyoto Univ., Inst. Chem. Res.)

Richard Murdey and Naoki SatoIntroduction
We have measured the photoconductivity action spectra for pentacene films on
sapphire as a function of film thickness, using either gold or aluminum electrodes. The aim
was to examine the influence of the electrode metal on the photo-generation of mobile charge
carriers in organic semiconductor films.
Experimental
The apparatus is shown schematically in Figure 1. Pentacene (TCI) was sublimed invacuum three times and once under 30 Pa N2 gas flow before use. Single crystal sapphire[0001] substrates (Shinkosha) were first annealed at 1000 °C in air to expose atomically flat
terraces, after which metal electrodes spaced
0.1 mm apart were prepared by vacuum
deposition through a metal mask. Goldelectrodes were 100 nm thick, with a 5 nm
titanium adhesion layer. Aluminum electrodes
were 35 nm thick. The substrates weredegassed at 150 °C under UHV before use.
Leakage currents (dark or photoconductance)
were below 10 fA. Pentacene films wereprepared by stepwise thermal deposition froma resistively heated crucible to a maximum

Figure 1. Diagram of the experimental

film thickness of 100 nm. The substrate

apparatus for in situ photoconductancetemperature was kept at 30 °C, and themeasurements.

deposition rate was constant at 2.0 ± 0.3 nm
min1. After each deposition step the photocurrent was evaluated for wavelengths from 400nm – 1180 nm, using a Bunkokeiki SM-25 monochromatic light source and a Keithley 6487
picoammeter-sourcemeter. The incident photon flux was 1019 photons m–2 and the applied bias
was 105 V m1. Measurements were obtained at a repetition rate of approximately 0.1 Hz,
with dark current subtraction performed for each data point.

100 nm
50 nm
40 nm
30 nm
20 nm
15 nm
10 nm
5 nm

photoconductance (offset)

AlelectrodesAu

100 nm
57 nm
42 nm
32 nm
21 nm
16 nm
10 nm
5 nm

absorbanceelectrodes

1.0

1.5

2.0

2.5
energy / eV

3.0

3.5

4.0

Figure 2. Photoconductance of pentacene films of the indicated thickness. The opticalabsorbance of an 8 nm pentacene film is shown for comparison. The reported
photoconductances are normalized against the incident photon flux and film thickness.
Results and Discussion
The electrode metal influenced the dark conductivity of the film. The conductivity ofthe pentacene film with Au electrodes was 6×108 S m1 for films less than 20 nm thick, fallingat higher thicknesses. The conductivity with Al electrodes was below the measurement
threshold of 109 S m1 for all thicknesses measured. The photoconductivity is also dependent
on the electrode metal, as shown in Figure 2. The broad peak at around 2.7 eV is attributed tointrinsic photo-generation by excitation of electrons across the transport energy gap1. The
cluster of peaks at lower energy correlates closely with the optical absorbance of the thin film
and are assigned to a de-trapping mechanism where long lived triplet excitons excite trappedcharges into mobile states. The de-trapping currents dominate for the thin pentacene films
with Au electrodes, corresponding to the thicknesses where the dark conductivity is highest.
The de-trapping photocurrent is an order of magnitude lower when Al electrodes were used,
and the intrinsic photocurrents saturate only slightly at the lowest observed thicknesses. A0.1 eV shift of the intrinsic photoconductivity peak to higher energy is also noted.

[1] D. V. Lang, X. Chi, T. Siegrist, A. M. Sergent and A. P. Ramirez, Phys. Rev. Lett. 93, 086802 (2004).