Research Project

My research focuses on two important interfaces in organic thin-film transistors (OTFTs): the metal/semiconductor interface, where charge injection and extraction occur, and the semiconductor/dielectric interface, where charge transport occurs. Instead of using OTFTs and conventional DC measurement to study these interfaces, I use organic long-channel capacitors (OLCCs), as schematically shown in Fig. 1, and displacement current measurement (DCM).

Fig. 1. Side and top schematic views of OLCC device structure.

An OLCC can be seen as a simplified version of OTFT, with one source/drain electrode removed. The conducting channel is much longer than that found in an OTFT, which is for increasing (1) the transit time of charge moving form one end of the channel to the other, and (2) the magnitude of the displacement current.

Let's first have a look at the operating principles of OLCCs under displacement current measurement. Later on, I will discuss the unique features about OLCCs and DCM.

Operating principles of OLCCs

As shown in Fig. 1, the displacement current measurement is performed by linearly sweeping the voltage bias applied on the bottom p+ Si electrode, while measuring the displacement current running through the grounded top metal electrode. Typical IV characteristics of an OLCC, with 6 different regions, are shown in Fig. 2. The dashed line is the displacement current associated with the metal contact (including pentacene under the metal contact).

Generally, a forward sweep is defined as the sweep in which charge carriers are injected into semiconductor layer; and a reverse sweep is the one in which charge carriers are extracted from semiconductor. For p-type semiconductor such as pentacene, the direction of forward sweep is from positive voltage to negative voltage, and the reverse sweep is the opposite. What happens to the semiconductor in each region on the IV characteristics is briefly explained below and schematically shown in Fig. 3.

Region 1: No charge exists in the semiconductor. The semiconductor is insulating.

Region 2: Positive charge carriers (holes) injected from the metal electrode into the semiconductor. The semiconductor transforms from an insulating state to a conductive state.

Region 3: Holes are injected into the semiconductor at a constant rate. This is possible because the semiconductor is conductive.

Region 4: Holes are extracted from the semiconductor at a constant rate. The semiconductor is still conductive.

Region 5: Holes are extracted from the semiconductor, and the semiconductor gradually loses its conductivity.

Region 6: Most of the holes are extracted from the semiconductor. The remaining (trapped) holes are slowly emitted out of the semiconductor.

Fig. 2. Typical IV characteristics of a OLCC device under displacement current measurement.

Fig. 3. Operating principle of OLCCs.

Unique features of OLCCs.

As mentioned above, OLCCs can be seen as simplified OTFTs, but the two key interfaces (metal/semiconductor and semiconductor/dielectric) are still preserved in OLCCs. In addition, both charge injection/extraction and charge lateral transport paralleled to the dielectric surface exist during OLCC measurement, although they might not be identical to the injection/extraction and transport in OTFTs due to the absence of the external lateral electrical field.

Compared to OTFTs and conventional DC measurement, OLCCs and DCM have the following unique features:

1. OLCCs and DCM focus on the dynamic processes of conducting channel formation and annihilation, which are often overlooked in conventional OTFT DC measurements.

2. In OLCC measurement, the injection and extraction processed occurs in forward and reverse sweeps, respectively, making it possible to study injection and extraction properties individually.

3. With OLCCs and DCM, the amount of charge injected in the forward sweep, and the amount of charge extracted in the reverse sweep can be individually determined by integrating the displacement current with respect to time. The difference between the two is the amount of charge trapped in one cyclic sweep.

These unique features of OLCCs and DCM make them useful tools to study charge injection/extraction at the semiconductor/metal interfaces, and charge transport at the semiconductor/dielectric interfaces.