Introduction to Organic Thin Film Transistors and Design of N-Channel Organic Semiconductors

4436 Chem. Mater. 2004, 16, 4436-4451 Introduction to Organic Thin Film Transistors and Design of n-Channel Organic Semiconductors Christopher R. Newman,‡ C. Daniel Frisbie,*,‡ Demetrio A. da Silva Filho,§ Jean-Luc Bre´das,§ Paul C. Ewbank,† and Kent R. Mann† Departments of Chemistry and of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 Received April 14, 2004. Revised Manuscript Received June 23, 2004 The development of new organic semiconductors with improved performance in organic thin film transistors (OTFTs) is a major challenge for materials chemists. There is a particular need to develop air-stable n-channel (electron-conducting) organic semiconductors with performance comparable to that of p-channel (hole-conducting) materials, for organic electronics to realize the benefits of complementary circuit design, i.e., the ability to switch transistors with either positive or negative gate voltages. There have been significant advancements in the past five years. In terms of standard OTFT metrics such as the field effect mobility (µFET) and on-to-off current ratio (ION/IOFF), n-channel OTFTs have achieved performance comparable both to that of n-channel amorphous silicon TFTs and to that of the best reported p-channel (hole-conducting) OTFTs; however, issues of device stability linger. This review provides a detailed introduction to OTFTs, summarizes recent progress in the development of new n-channel organic semiconductors, and discusses the critical properties that any prospective n-channel material must have. Methods important to semiconductor design such as electronic structure calculations and synthetic structural modifications are highlighted in a case study of the development of a new n-channel material based on a terthiophene modified with electron-withdrawing groups. The review concludes with a discussion of directions for future work in this area. 1. Introduction From a materials science perspective, there remain a number of important unanswered questions concern- Over the past 10 years there has been remarkable ing the performance of organic semiconductors in OT- progress in the development of thin film transistors FTs.4 For example, transistors based on polycrystalline (TFTs) based on organic semiconductors. In terms of the films of pentacene, a fused ring oligomer, set the key figures of merit, namely, the on-to-off current ratio benchmark for OTFT performance, routinely displaying (I /I ), the field effect mobility (µ ), and the ON OFF FET values of µ in excess of 1 cm2 V-1 s-1, I /I > 108, threshold voltage (V ), the performance of the best FET ON OFF T and V near 0 V.5 It is not perfectly clear how these organic TFTs now rivals that of commercial amorphous T impressive electrical characteristics depend in detail on silicon TFTs, which are commonly employed as the pixel the crystal structure and morphology of pentacene films; switching elements in active matrix flat panel displays. TFTs based on polycrystalline films of many other An important advantage of organic semiconductors vis- organic semiconductors do not have comparable trans- a`-vis amorphous silicon is that they can be deposited port characteristics. Indeed, it is reasonable to ask why onto substrates at low temperatures, meaning they are pentacene TFTs appear so far to be unique in their compatible with flexible plastic substrates.1,2 A number combination of exceptional transport properties. Basic of industrial laboratories are working hard to develop understanding of structure-property relationships is low-cost, large-area plastic electronics employing tran- essential for answering this question and a host of sistors and diodes based on organic semiconductors. The others including the following: How high can carrier good performance of organic TFTs (OTFTs) makes it mobilities be in films of crystalline organic semiconduc- more likely that these efforts will succeed. As noted by tors? Can good mobilities be obtained in solution- Dimitrakopoulos and Malenfant in their 2002 review,3 processable films that are only partially crystalline? industrial emphasis is shifting away from organic What are the structural and electronic factors that semiconductor testing and more toward manufacturing facilitate or impede transport in organic semiconductor process development. films? Are there particular molecular structures and crystal packing motifs that are especially favorable for * To whom correspondence should be addressed at the Department of Chemical Engineering and Materials Science, University of Min- transport? What is the role of grain boundaries? These nesota. E-mail: [email protected]. unanswered questions reflect a significant gap in basic † Department of Chemistry, University of Minnesota. knowledge of structure-property relationships on all ‡ Department of Chemical Engineering and Materials Science, University of Minnesota. length scales in organic semiconductors. Perhaps the § Georgia Institute of Technology. most significant point is that this knowledge gap 10.1021/cm049391x CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004 Reviews Chem. Mater., Vol. 16, No. 23, 2004 4437 impedes rational efforts to design organic semiconductors with further enhancements in electrical performance. A particular design need is in the area of n-channel (also commonly referred to as “n-type”) organic semiconductors. As described in the next section, TFTs based on n-channel semiconductors conduct electrons and achieve their high conductivity “on” state with positive gate voltages, whereas most OTFTs (e.g., pentacene TFTs) are p-channel devices (i.e., they conduct holes) and turn on with negative gate voltages. The motivation for seeking good n-channel OTFTs is that they enable complementary circuit design. Electrical engineers are Figure 1. Schematic of top (a) and bottom (b) contact organic proficient at developing low-power complementary cir- TFTs. (c) Relevant voltages and geometry for a TFT. cuits that utilize both positive and negative gate voltages to turn transistors on and off. The lower power and drain electrodes. In the top-contact case, Figure 1a, requirement of complementary circuits is an attractive the organic film is deposited first, followed by the metal feature for many potential applications for plastic 6 electrodes. In the bottom-contact case, Figure 1b, this electronics, such as radio frequency identification (RFID ) deposition sequence is reversed. Usually, the gate/ tags, in which the organic circuit must be energized by insulator assembly consists of a metal or doped semi- a local radio frequency (rf) field. Consequently, the conductor gate electrode coated with an insulating oxide development of good n-channel OTFTs with perfor- (typically 200-400 nm thick), though polymeric insula- mance comparable to that of pentacene TFTs is a major tors are also used and might ultimately be preferable goal for organic electronics. for flexible electronics. The organic semiconductor film Historically there have been more examples of good ( 30-50 nm thick) can be deposited from the vapor p-channel materials than n-channel materials for OTFTs. p∼hase or coated from solution. If desired, surface treat- In the past five years there has been increased attention ments on the insulator may be used prior to deposition to n-channel materials, but early efforts can be traced of the semiconductor layer. These treatments have been back to the mid-1990s. Very recently, electron mobilities 2 -1 -1 observed to have profound effects on the resulting thin near 1 cm V s have been achieved in several organic film structure and electrical characteristics.7,8 The metal semiconductors, which is on par with hole mobilities in source and drain electrodes are often vapor-deposited pentacene films. However, there is currently no n- through a shadow mask, but conductive inks that can channel analogue for pentacene, i.e., an n-channel be printed are also employed. Source-drain channel organic semiconductor that is clearly superior to all lengths, L, shown in Figure 1c, typically range from 10 others in terms of overall performance. This situation to 100 µm, and channel widths, W, are usually between may change soon given the intense research in this field, 100 µm and 1 mm. and progress on the vexing issue of air stability to be discussed later. The voltage applied between the source and drain is referred to as the source-drain voltage, V . For a given In this paper we review recent progress in the design D V , the amount of current that flows through the of good n-channel semiconductors for OTFTs and specify D semiconductor film from source to drain is a strong the key requirements that these materials must satisfy function of the voltage, V , applied to the gate electrode. to be practical. Our review complements the previously G The semiconductor film and the gate electrode are mentioned review by Dimitrakopoulos and Malenfant, capacitively coupled such that application of a bias on who summarized p-channel and n-channel OTFT de- the gate induces charge in the semiconductor film, as velopment efforts up to the summer of 2001. Specifically, shown schematically in Figure 1c. Much of this charge we include developments that have occurred in the past is mobile and moves in response to the applied source- two and a half years (e.g., more new n-channel materi- drain voltage V . Ideally, when no gate

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