Materials for OFET applications

In the past years, intensive effort has been spent on developing new polymeric or low molar mass semiconducting materials with mobilities approaching the 1 cm²/Vs of amorphous silicon.^{[25, 32]} It turned out that such high mobilities, which are on the edge of band transport, can only be obtained from organic compounds that show a high degree of molecular order.^{[37, 38]} Consequently, the highest charge carrier mobilities are obtained from single crystalline materials. Acenes like pentacene and rubrene have been investigated intensively during the last years (Figure 1-8). Purification and single crystal growth from these materials is the topic of many publications. With field-effect mobilities of about 15 cm²/Vs, single crystalline rubrene has set the benchmark among organic semiconductors.^[39] The second highest mobility value of about 5 cm²/Vs was obtained from pentacene single crystals.^[40]

Although these acenes show excellent OFET performances, it is very unlikely that theses molecules will finally be used in organic electronics.^{[41, 42]} In fact, the preparation of single crystals from soft organic materials is an expensive and painstaking process and not suitable for device production in a large scale. Furthermore the acenes are sensitive towards light and suffer from degradation when they are stored under ambient conditions.^{[43, 44]}

Figure 1-8. Structures of acenes for high mobility single crystalline OFETs.

Vacuum-deposition of small molecules offers a more simplified approach to prepare organic FETs exhibiting fairly high carrier mobilities. Especially with thiophene derivatives impressive improvements have been made. If sexithiophenes are evaporated, thin films with a polycrystalline order can be obtained. In the case of sexithiophene^[45] (Figure 1-9), mobilities of 2x10^-3 cm²/Vs have been recorded.^[46] If the sexithiophene core is substituted with hexyl

Rubrene

Pentacene

10 1. Introduction side chains (DH6T, Figure 1-9) field-effect mobilities up to 0.13 cm²/Vs were reported.^[47]

The increased mobility of DH6T can be explained by an improved packing of the single molecules what leads to a smaller intermolecular distance.

Figure 1-9. Structure of thiophene based materials for OFET applications. Above:

sexithiophene (6T) and α,ϖ-dihexylsexithiophene (DH6T). Below: 5,5’-bis-(9H-fluoren-2-yl)-2,2’-bithiophene (FTTF) and 5,5’-bis-(7-hexyl-9H-fluoren-2-yl)-5,5’-bis-(9H-fluoren-2-yl)-2,2’-bithiophene (DHFTTF)

Bao et al have reached field-effect mobilities of 2x10^-2cm²/Vs^[48] by vacuum depositing a polycrystalline layer of a bithiophene which is substituted with two fluorene units (FTTF, Figure 1-9).^[49] By introducing hexyl chains to the fluorene side groups in the 7-positions (DHFTTF, Figure 1-9) the mobility was increased up to 0.14 cm²/Vs due to a closer packing of the core molecules.^[41]

All the low molar mass compounds that were mentioned before can only be processed by vacuum deposition, what makes large scale device fabrication ineffective. For this reason liquid phase processing is the key to low price organic electronics. One possible approach to reach this target is the usage of polymeric compounds. Today, poly(3-hexylthiophene) (P3HT, Figure 1-10) is one of the best investigated polymers concerning its performance in organic FETs.^{[50, 51]} Thin films of regioregular P3HT exhibit a highly microcrystalline and anisotropic lamellar morphology what leads to two-dimensional conjugated layers with strong π-π interchain interactions. These thiophene layers are separated from each other by the alkyl side chains which act as a kind of insulating layer. This microstructure allows a fast in-plane charge transport.^[52] The charge carrier mobility of P3HT strongly depends on the degree of regioregularity. P3HT with a head-to-tail regioregularity of 81% shows mobilities of about

S

2x10^-4 cm²/Vs whereas 0.1-0.3 cm²/Vs can be obtained if the regioregularity is increased to 96%.^[52] Big disadvantages of these thiophene based materials are a poor photostability and the high sensitivity towards oxygen.^[53] Exposure to sunlight in the presence of air causes formation of carbonyl defects in the polymer with an associated loss of conjugation and mobility.^[54]

A step towards higher environmental stability of thiophene based materials was made by Koezuka et al who have prepared poly(thiophenevinylene) (PTV, Figure 1-10) from which mobility values of 0.22 cm²/Vs were obtained.^[55] McCulloch et al reached 0.15 cm²/Vs together with a reasonable atmospheric stability from poly(2,5-bis(3-decylthiophen-2yl)-thieno-[2,3-b]thiophene (PTT, Figure 1-10).^[56]

Figure 1-10. Chemical structures of thiophene based polymers: poly(3-hexylthiophene) (P3HT), poly(thiophenevinylene) (PTV) and poly(2,5-bis(3-decylthiophen-2yl)-thieno-[2,3-b]thiophene (PTT).

An alternative approach to obtain highly ordered thin films are large monodomains formed by liquid crystals (LC). The molecules can be aligned in the LC-phase at elevated temperatures.

The orientation is then frozen in either by quenching the LC-phase to room temperature or by photopolymerization of liquid crystalline compounds with photoreactive endgroups, which are known as reactive mesogens (RM). A well-established liquid crystalline material from which good charge carrier mobilities can be obtained is poly-(9,9’-dioctylfluoren-2-yl)-co-bithiophene (F8T2, Figure 1-11). Sirringhaus et al have shown mobilities of about 0.01 cm²/Vs from solution processed F8T2. Alignment of the F8T2 molecules was carried out

* S

12 1. Introduction in the nematic phase above 265 °C on rubbed polyimide perpendicular to the FET electrodes.

The orientation was frozen in by quenching the substrate to room temperature.^[57] In such supercooled LC-phases the orientation will fade over the time what is a drawback concerning field-effect mobilities. Broer et al were the first to solve this problem. They introduced photopolymerizable endgroups to the LC-core molecule in order to fix the orientation of the mesophase by chemically crosslinking the mesogens.^[58] With this reactive mesogen approach the group of McCulloch reached mobilities of 4x10^-4 cm²/Vs after photopolymerizing the methacrylate endgroups of a quaterthiophene derivative (quarterthiophene RM)^[59] which is shown in Figure 1-11.

Figure 1-11. Chemical structures of poly-(9,9’-dioctylfluoren-2-yl)-co-bithiophene (F8T2) and the reactive mesogen with photocrosslinkable endgroups 2-methacrylic acid 6-{5’’’-[5-(2-methylacryloyloxy)hexyloxymethyl][2,2’;5’,2’’;5’’,2’’’]-quarterthiophen-5-yl-ethoxy}hexyl ester (quarterthiophene RM).

The introduction of aromatic amines as active material in OFETs was an important step towards environmental stability. Compounds like poly-(triarylamine) (PTAA, Figure 1-12) do not suffer from atmospheric degradation and are not sensitive towards moisture. Due to vitrification they exhibit excellent film forming properties when they are processed from solution. The drawback of the amorphous state is a decrease of the hole mobility due to the isotropic behaviour of the material. By using organic dielectric layers with low polarities, charge carrier mobilities up to 10^-2 cm²/Vs could be reached in PTAA transistors.^[27]

O (CH₂)₆ S

S

S

S O

(CH₂)₆

O O O O

S

* S

C₈H₁₇ C₈H₁₇

n *

F8T2

Quarterthiophene RM

Figure 1-12. Chemical structure of a glass forming poly-(triphenylamine) derivative (PTAA).

1.3. Organic light emitting diodes (OLEDs)