3. NOVEL STAR-SHAPED TRIPHENYLAMINE BASED MOLECULAR GLASSES
3.3. Preparation and measurement of organic FETs
The transistor performance of the new star-shaped compounds was investigated with bottom gate OFET substrates from Philips. The devices consist of a heavily doped n++ silicon wafer as gate contact. On top of the gate electrode an insulating layer of silicon dioxide is grown thermally. Afterwards gold is evaporated and photolithographically patterned to form the source and drain contacts (Figure 3-2).[28] In order to obtain the best possible FET performance of the new materials, several surface treatments of the polar SiO2 gate insulator were carried out. As these prelimininary tests are very important for the transistor results presented in paper 1, the surface treatment procedures are described in more detail at this point.
It is a well known fact that the device characteristics can be influenced by covering the gate insulator with a self assembled monolayer (SAM) of organosilane compounds.[87] For this concern we decided to use hexamethyldisilazane (HMDS) as it the most suitable silanizing agent in connection with with aromatic amine based semiconducting materials.[35]
Figure 3-2. Schematic of the used bottom gate OFET device architecture (left) and microscopic image of a Philips bottom gate OFET substrate containing several transistors with different channel lengths (right).
First of all the bottom gate FET substrates were cleaned with fuming HNO3 for 30 min at room temperature. After rinsing the substrates thoroughly with distilled water the silane coupling agent was vapor deposited at 80 °C. In order to check the influence of the evaporation time, the FET devices were kept in the HMDS vapor for 3 and 24 h, respectively.
The experimental setup for vapor deposition of the HMDS is shown in Figure 3-3.
Figure 3-3. Experimental setup for HMDS vapor deposition onto the OFET substrates.
Afterwards the substrates were flushed with isopropanol before the organic semiconductor (13) was deposited onto the FET substrates. This was either done by spin-coating from 2-wt%
toluene solution or by evaporation from the gas phase. The spin-coated films were dried under argon atmosphere at 130 °C for 30 min. An average film thickness of 40 nm was determined.
For the sake of comparison, additional FET devices without HMDS treatment were prepared.
All devices were annealed for 15 min at 90 °C under vacuum before the transistor characteristics were measured.
The preliminary measurements showed that the field-effect mobilities from the HMDS treated substrates are one order of magnitude higher than from the untreated FET devices. The turn-on voltage could be reduced from -5 V (untreated substrate) to -2 V by silanizatiturn-on with HMDS (24 h evaporation). Shorter HMDS treatment results in an insignificant drop of the turn-on voltage (-4 V). It was found that hysteresis effects can be eliminated almost completely by surface modification with the organosilane. Contact resistance could be tremendously reduced and on/off-ratios of the devices were increased by two orders of magnitude up to 105. Concerning hysteresis, contact resistance and on/off-ratio, no significant changes could be observed with of different HMDS deposition times. Furthermore it was found that the deposition of the amine glass 13 by spin-coating or vacuum evaporation has no influence on the FET performance.
Cooling Water Specimen Holder
(Teflon)
HMDS @ 80°C Transistor Substrate
28 3. Novel star-shaped triphenylamine based molecular glasses and their use in OFETs
Figure 3-4. Transfer characteristics of OFET devices with untreated (left) and HMDS silanized (24 h) gate insulator surfaces (right). In both cases the triphenylamine based star-shaped compound 13 was used as semiconducting material. The untreated OFET substrate shows significant hysteresis effects, a 3 V higher turn-on voltage and a lower on/off-ratio.
The transfer characteristics in Figure 3-4 clearly show the improvements that were achieved concerning hysteresis effects, turn-on voltages and on/off-ratios by treating the gate insulator surface with HMDS. Comparison of the output characteristics in Figure 3-5 clearly shows that the effect of contact resistance was reduced remarkably by introducing this substrate treatment. This means that the charge carrier injection from the gold electrodes into the organic semiconductor was improved significantly. Concerning the additional reduction of the turn-on voltage by evaporating HMDS for 24 h, we decided to adopt this substrate preparation procedure for all future OFET measurements.
From this series of molecular glasses the highest charge carrier mobilities were obtained from compound 15 with 3 x 10-4 cm2/Vs. The other star-shaped materials 13, 14 and 16b exhibit mobilities in the range of 10-4 cm2/Vs. Furthermore on/off-ratios of about 105 and remarkably low turn-on voltages between -5 and -1 V could be achieved with the new compounds.
The most promising result is the high environmental stability of the OFETs under ambient conditions. Storage of the devices for more than four months in air and daylight had no influence on the device performance. Merely the field-effect mobility dropped slightly. Figure 3-6 shows the transistor characteristics of the pristine device and after storing it for four months, using 13 as semiconductor.
-20 -15 -10 -5 0
Figure 3-5. Output characteristics of OFET devices with untreated (left) and HMDS silanized (24 h evaporation) gate insulator surfaces (right). The untreated OFET substrate shows a non-linear increase of the drain current between 0 and -15 V. This is clear evidence for contact resistance between the organic semiconductor and the gold electrodes. This effect could be reduced significantly by the surface modification with HMDS.
Figure 3-6. Transfer characteristics of 13. The drain potentials were –20 V and –2 V in the upper and lower traces, respectively (solid lines). Left: pristine device. Right: device performance after storage under ambient conditions and daylight for 4 months.
30 3. Novel star-shaped triphenylamine based molecular glasses and their use in OFETs
In conclusion five out of six star-shaped triphenylamines which were prepared by the Suzuki cross-coupling form stable molecular glasses. Thin amorphous films can be obtained from these compounds when they are spin-coated or evaporated. It was also shown that the HOMO levels can be tailored by exchanging the sidearm substituents. The star-shaped molecules with fluorene side groups (16a-c) exhibit HOMO levels of -5.2 eV which can be increased by 0.2 eV if carbazole units are introduced to the triphenylamine core (13-15).
Furthermore a suitable surface treatment procedure was developed for the bottom gate OFET substrates. Hole carrier mobilities up to 3 x 10-4 cm2/Vs and on/off-ratios in the range 105 were achieved form the novel star-shaped materials. The excellent longterm stability of the FET devices under ambient conditions is the most promising result of this work.