Study On The Contact Effects In Organic Thin Film Transistors

Organic thin film transistors (OTFTs) are expected to become the core device of organic electronics and flat panel display technology due to the advantages of low preparation cost, large-scale production and available flexibility. In order to study the transmission mechanism of OTFTs and break the bottleneck of contact effect, this paper studied the contact effect in OTFTs by inserting the buffer layers between the source-drain electrodes and the active layer. By the methods of middle probe, charge transfer model fitting, ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), the size and distribution of contact resistances and its influence to the device performance were studied and analyzed. Finally, the interface of source-drain electrodes and active layer prepared by solution wet advocated by large-scale applications in the future were also studied.First, the contact effects were investigated by inserting ultra thin layer between source-drain electrodes and active layer, and the experimental results were compared with the results of theoretical simulation.(1) The influence of MoO3 thickness and the contact effects on the performance of OTFT devices were studied by inserting MoO3 thin layer between Al and pentacene. The experimental results were compared with the results of charge transfer theoretical fitting. The results show that the performance of device is best when the thickness of MoO3 was 10 nm. The field-effect mobility was increased from 0.089 cm2/Vs to 0.40 cm2/Vs, and the threshold voltage was decreased from -12.5 V to -9.2 V. The improvements of device performance were mainly due to reducing the device contact resistance and increasing hole injection by inserting MoO3, thereby, reducing the influence of contact effect on device performance. Then we studied the contact effect of devices by two methods.①The distribution of the device potential had a qualitative judgments using the method of middle probe, and found that the potential distribution of source and drain was uneven. When the gate voltage VGS and the electric potential difference of source increase, the proportion of the contact resistance RC in the total resistance Rtot increases. It is to say that the channel resistance Rch and contact resistance RC are decreasing as VGS increasing, and the decrease of Rch is faster than RC. The channel resistance Rch increases slowly as the voltage VDS of source and drain increasing.②The output curve of the device was fitted by charge transfer model. The contact potential is about 0.42 of the total potential, the theoretical field-effect mobility of the device is 1.1 cm2/Vs, which shows the influence of the contact resistance on the performance of device is relatively large.③The results fitted by charge transfer model was verified by calculating the contact resistance through Transfer Line Method.(2) In order to research the influence of ultra-thin layer on contact effect furtherly, we inserted fullerene C60 thin layers between Al and pentacene with different thickness. The thickness of C60 and the impact of the contact effect on device performance were studied, and the experimental results were compared with the results obtained through charge transfer theoretical fitting. The results showed that the device performances were significantly improved after optimizing the thickness. When the thickness of C60 is 3 nm, the performance of device is best:The field-effect mobility of linear region is 0.52 cm2/Vs, the threshold voltage is -6.0 V and the switching ratio is 2.2×104. This shows that inserting thin layer of C60 can improve performance of OTFTs, and the effect is better than MoO3 modification. This is due to the better matched of the valence band of C60 and the HOMO energy level of pentacene. And well surface contact can be obtained because both C60 and pentacene are organic materials. The contact potential is about still 0.31 of the total potential obtained through the charge transfer simulation fitting. This shows that the modification of C60 thin layer can reduce the contact effect and increase the field-effect mobility of devices.(3) The impact of dual ultra thin layers on contact resistance and device performance of OTFTs were investigated for the first time, and the experimental results were compared with the results of charge transfer theoretical fitting. MoO3 and C60 with different thickness were continuously inserted between the source-drain electrodes Al and pentacene. The field effect mobility varied in the range of 0.26～0.72 cm2/Vs and the threshold voltage varied in the range of -6.1～-9.6 V. It was found that the deposition order and thickness of MoO3 and C60 had significant impact on device performance. As first depositing 5 nm MoO3, then depositing 3 nm C60, the field effect mobility is the highest, to 0.72 cm2/Vs, which is higher than the device with MoO3 or C60 monolayer-modified. Then the impacts of contact effect on threshold voltage were studied. The results showed that reducing the contact effect can reduce the threshold voltage. Different from the change of several times of field-effect mobility, the contact effect on the impact of threshold voltage is relatively small. It is found that the inverse proportion of the contact potential in total potential has dropped to 0.18 fitted through charge transfer theory. This further illustrates dual (multi) thin layer modification can reduce the contact effect, and it is an important way to improve the performance of OTFTs.In order to study the impact of thin layer on the interface of pentacene and the device performance of OTFTs, MoO3 and C60 films were in-situ grown on the surface of pentacene films, respectively. Then, the interfaces of the films were tested by UPS and XPS. The energy level diagrams of pentacene/MoO3 and pentacene/C6o with actual contact were plotted according to the experimental results. As actual contact, the energy gap between the valence band of MoO3 and the HOMO energy level of pentacene is about 0.12 eV, rather than 0.4 eV obtained from the level schematic diagram. This shows that the actual capacity of increasing hole injection of MoO3 is smaller than that obtained from the energy level diagram. Diple layer is formed between Pentacene and C60 and facilitate the carriers transportion, which is also the reason of the energy match between the Pentacene HOMO and Al Fermi energy. The energy gap between the HOMO energy level of C60 and pentacene is 1.07 eV can be obtained from the energy level diagram. However, the actual contact energy level difference is about 1.25 eV. High energy difference is in favor of hole injection from high energy to low energy level, and facilitating charge transfer. This further illustrates that the intrinsic reason of the performance of device with C60 modified better than MoO3 modified.To meet the needs of future large-scale industrialization, low-cost preparation by solution wet is the trend of development in OTFTs. Therefore, we investigated the impact of interface on the performance of device with preparing source-drain electrodes and the active layer by solution wet method. It was found that, as depositing PEDOT, the PEDOT at the locations of source-drain electrodes was mixed with the following water-soluble CuPc, forming a miscible thin layer. The miscible layer can reduce the contact resistance, and is conducive to hole injection. It can be obtained that the inverse proportion of the contact potential in total potential is only 0.12 through charge transfer theory fitting. This indicates that the influence of contact effect on device performance is relatively small.