Research On The Improvement Of Contact Between Source/Drain Electrode And Organic Active Layer For Organic Thin Film Transistor

Field effect transistor (FET) is the fundamental device in the modern domain of IC industry. And organic thin film transistor (OTFT) as a kind of FET which is based on organic semiconductor display a lot of advantages on large-area, low-cost electronics. Radio frequency identification tags (RFIDs), smart cards, electronic paper, flat panel display are the examples which employ OTFT technology. The huge economic potential leads to the great attention to research on OTFT.In this thesis, we mainly focus on enhancing the performance of OTFT through improvement of contact between Source/Drain electrode and organic active layer.First of all, different structures of OTFT and its principle of operation, working process are shown. Then we introduce a method which is based on theory of inorganic MOSFET to abstract characteristic parameters for OTFT. Finally, we fabricated a bottom-gate and top-contact OTFT based on pentacene and abstract its characteristic parameters. The field-effect mobility in saturation region is 0.097 cm²/Vs, threshold voltage is -11V, current on/off ratio is 103.In the following, we introduce several factors which have vital influences on the Source/Drain contact. For example, high work function of metal and suitable buffer materials should be employed to the contact. Especially, we focus on the dipolar layer formed in the interface between the metal and organic active layer. According to the former analysis, for the first time we apply m-MTDATA, a kind of hole-injection material having relatively thick membrane structure and fine surface topography, doped in the pentacene as a buffer between metal electrode and organic active layer. The performance of the improved device is enhanced obviously. The field-effect mobility in region of linearity is 0.51 cm²/Vs (Vd=-4V), compared with undoped-device with 0.16cm²/V·s. And for improved device the threshold voltage is -2V, current on/off ratio is 104 whereas for the undoped-device, the two parameters are -11 and 103 respectively. We attribute the improvement to the main reason that the metal atoms are prevented from diffusing into organic active layer by utilizing m-MTDATA having relatively thick membrane structure and fine surface topography doped into the active layer. Therefore impact from dipolar layer is weakened, the barrier of injecting hole decreases. At last the performance is enhanced.Meanwhile we put forward employing a kind of combination electrode, MoO3-Au, to fabricate OTFT. The performances are improved as well. The highest field-effect mobility in region of linearity for the improved device reaches to 0.7 cm²/Vs, nearly four times as the one 0.17 cm² V^-1·S for original device. And current on/off ratio is advanced with a factor of ten, from 103 to 10⁴. In our opinions, there are two reasons to explain the improvement. On the one hand, MoO3 as a kind of compact inorganic material, can effectively block Au atoms from diffusing into organic active layer, Therefore impact from dipolar layer is weakened, the barrier of injecting hole decreases. On the other hand, the suitable energy band matching can be obtained using MoO3 between Au and the organic active layer.Finally, for further comprehension on principle of operation for OTFT, we deuce an electricity theoretical model for output characteristic of saturation region for P type OTFT using the model of Gilles Horowitz et for N type OTFT and inorganic a-Si:H TFT as references. According to the model we deduced, the explanation why the mobility of saturation at high gate voltage is much higher than the one at low gate voltage. The instruction is as follows. At low gate voltage, most of injected holes are captured by the trap state from interface, defect and so. The free carries density is too small to have obviously conduction. The mobility at the low gate voltage from the model isθ₀μ₀,θ₀<<1,μ₀ is microscopic mobility for organic active layer. Low mobility explains the low conduction. While OTFT is working at high gate voltage, the density of free holes is much lager than the density of holes captured by trap states. From out model, the mobility now is nearlyμ₀.