Conductivity-type anisotropy in molecular solids
Summary (2 min read)
Introduction
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- On the other hand, thin polycrystalline films of many molecular solids~such asa-6I!.
- The authors show that PTCDA transports electrons in the directions parallel to the molecular planes and mainly holes in the direction perpendicular to the molecular planes.
- Material system7 were employed to study transport properties normal to the molecular planes.
A. Field effect transistors
- The FETs were fabricated by vapor depositing vacuum purified PTCDA from a baffled Mo crucible in a high vacuum deposition chamber (,731026 Torr) on to prepatterned substrates as illustrated in the inset of Fig. structures were similarly made by deposition of various organic materials from Mo crucibles in a high vacuum deposition chamber.
- Field-effect transistors measure electronic transport along the PTCDA active layer/gate dielectric interface; therefore, the measured electronic transport represents transport parallel to the molecular planes of PTCDA.
- The increasing drain-source current with positive gate voltage is characteristic of ann-channel FET.
- It is important to note that the PTCDA FETs do not operate in moist air; however, under vacuum or at atmospheric pressure in an ambient of dry oxygen they exhibited n-channel FET behavior with the mobilities and electrical characteristics described above.
B. Light-emitting diodes
- Pioneering work by Forrest and co-workers on contact barrier diodes11,12 and LEDs13 have clearly shown that PTCDA can transport holes normal to the molecular planes.
- The authors previous work with TAD/Alq LEDs, as well as recent results18 show that Alq has the ability to transport holes in addition to electrons.
- This demonstrates that PTCDA transports holes in the direction normal to the molecular planes.
- In fact, the EL spectra of the previous structure with a single emitting layer of Alq and same thickness~100 nm! of PTCDA has been superimposed to compare the two EL spectra.
- Also do not indicate any appreciable electron transport.
C. Film morphology
- X-ray diffraction measurement were made to ensure that the PTCDA molecules oriented parallel to the substrate in both the FET and LED structures.
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- The spectrum from the Si/SiO2/PTCDA sample, the configuration of a FET, has one clear peak near 27.5 deg corresponding to the 102 plane.
- When the sample was tilted at an angle115 deg, the intensity of the diffraction pattern corresponding to the 012 plane reached a maximum, whereas a tilt of215 deg produced a reduction in intensity.
- The electron diffraction data corroborates evidence from the x-ray diffraction measurements that the PTCDA films orient with the molecular planes nearly parallel to the plane of the substrate.
D. Energy levels
- Let us now consider the relevant energy levels in the materials considered above, and how these could affect transport properties.
- The energy levels of the highest unoccupied and the lowest unoccupied molecular orbitals ~HOMO and LUMO! are shown in Fig.
- There is very little electron transport through the PTCDA layer as the LED data described above indicate.
- Unlike the case in organic LEDs, this does not present a serious problem because the FET channel becomes highly conducting in the on state because of the large induced charge density.
- C60 FETs with high mobility have been fabricated with Au S/D electrodes despite the 1.3 eV barrier between theC60 LUMO ~3.8 eV! and the Au work function.
III. SUMMARY
- The above data and discussion illustrate the complexity of transport in a molecular thin film.
- The transport properties depend on a number of factors including molecular ordering and orbital overlap, molecular orbital energy levels, and the nature of the ambient.
- Nevertheless, it is apparent from their study that hole transport dominates in the direction normal to the molecular planes~where thep orbital overlap is high!.
- While along the molecular planes PTCDA behaves as a quasitwo-dimensional electron transporter~in vacuum!.
- This work also illustrates the utility of device measurements in characterizing transport in such materials.
ACKNOWLEDGMENTS
- The authors wish to thank R. C. Dynes and L. J. Rothberg for helpful discussions and I. Brener and D. Weissman for the photoluminescence measurement.
- Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions.
- 11The detection of small hole currents in dominantlyn-channel FETs has been analyzed in detail by their group in A. Dodabalapuret al. Appl. Phys. Lett. 68, 1108~1996!.
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Frequently Asked Questions (15)
Q2. What is the purpose of the X-ray diffraction measurement?
X-ray diffraction measurement were made to ensure that the PTCDA molecules oriented parallel to the substrate in both the FET and LED structures.
Q3. What is the effect of moisture on the FETs?
The FETs did not function in a N2 ambient when the gas was bubbled through a water bath, indicating that moisture adversely affects electron transport in this material while oxygen does not appear to do so.
Q4. What is the ability of Alq to transport electrons?
Their previous work with TAD/Alq LEDs, as well as recent results18 show that Alq has the ability to transport holes in addition to electrons.
Q5. What is the effect of the field-induced carrier in the FET?
Experimental work as well as modeling has shown that, when such FETs are operated in the accumulation mode, most of the field-induced carriers are located within 5 nm of this interface.
Q6. What is the morphology of the PTCDA powder?
Additional characterization of thin film PTCDA morphology was performed with electron diffraction on samples in which PTCDA was deposited on amorphous carbon grids in parallel with the samples used in the x-ray measurements.
Q7. How do the authors test the transport properties of PTCDA?
By varying the thickness of the PTCDA layer, the EL spectra will offer us a means to identify the source of light and to test the transport properties of PTCDA.
Q8. How do the authors fit the EL spectra of PTCDA?
By varying the thickness of the PTCDA ~50 and 100 nm!, the authors can fit the measured EL spectra with the measured absorption and Alq/TAD EL spectra.
Q9. What is the quantum efficiency of LEDs with a PTCDA layer?
The quantum efficiency of LEDs with a PTCDA hole transporting layer sand-wiched between the Alq and the TAD is generally about an order of magnitude lower than LEDs without PTCDA.
Q10. What is the reason why no light is seen even when the devices are tested under vacuum?
The fact that no light is seen even when the devices are tested under vacuum is evidence that electron transport is not favored normal to the molecular planes.
Q11. What is the x-ray spectrum from the PTCDA sample?
The spectrum from the Si/SiO2 /PTCDA sample, the configuration of a FET, has one clear peak near 27.5 deg corresponding to the 102 plane.
Q12. What is the barrier between the Au source and the LUMO of PTCDA?
In the case of the field-effect transistor, there is a ; 1 eV barrier between the Au source/drain ~S/D! electrodes and the LUMO/HOMO of PTCDA.
Q13. What is the significance of the structure of PTCDA films?
More recent structural characterization of PTCDA films grown on Au substrates by Fenter et al. indicate that PTCDA films can be grown quasiepitaxially underthe right conditions resulting in highly ordered films.
Q14. What is the effect of the PTCDA interface on the quantum yield?
It is also possible that there are some effects at the PTCDA/Alq interface which lower the quantum yield relative to the TAD/Alq interface.
Q15. What is the energy barrier between the Au contacts and the PTCDA HOMO?
the energy barrier between the Au contacts and the PTCDA HOMO is unlikely to be the cause of the absence of observable hole transport parallel to the molecular planes.