Field effect transistors and materials and methods for their manufacture

1. A field effect transistor in which a continuous semiconductor layer forms a field effect channel between a source electrode and a drain electrode, wherein the continuous semiconductor layer comprises:

a) an organic semiconductor which is a conjugated aromatic molecule containing at least four aromatic rings; and,

b)an organic binder which has an inherent conductivity of less than 10 −6 Scm −1 and a permittivity at 1,000 Hz of less than 3.0;

with the proviso that the organic semiconductor does not comprise 2,5-di-n-octyloxy-1,4-bis((4′,4″-bisstyryl)styrylbenzene)(Ooct-OPV5).

2. A field effect transistor according to claim 1 wherein the organic semiconductor has inherent conductivity of less than 10 −6 Scm −1 .

3. A field effect transistor according to claim 2 wherein the organic binder is a homopolymer of styrene and its molecular weight is less than 20,000 daltons.

4. A field effect transistor according to claim 1 wherein the organic binder has a permittivity of less than 2.8.

5. A process of producing a field effect transistor according to claim 1 which comprises:

a) coating a substrate with a liquid layer which comprises the organic semiconductor, the binder and a solvent or which comprises the organic semiconductor and a material capable of reacting to form the binder; and

b) converting the liquid layer to a solid layer comprising the semiconductor and the binder by evaporating the solvent or by reacting the material to form the binder as the case may be.

6. A field effect transistor in which a continuous semiconductor layer forms a field effect channel between a source electrode and a drain electrode, wherein the continuous semiconductor layer comprises:

a) an organic semiconductor; and,

b) an organic binder which has an inherent conductivity of less than 10 31 6 Scm −1 and a permittivity at 1,000 Hz of less than 3.0;

with the proviso that the organic semiconductor does not comprise 2,5-di-n-octyloxy-4-bis((4′,4″-bisstyryl)styrylbenzene)(Ooct-OPV5) or a liquid crystalline phenyl naphthalene compound.

7. A field effect transistor according to claim 1 or claim 6 wherein said continuous semiconductor layer comprises an organic semiconductor having an inherent conductivity of less than 10 −6 Scm −1 and a field effect mobility of more than 10 −5 cm 2 V −1 s −1 and the organic binder has an inherent conductivity of less than 10 −6 Scm −1 and a permittivity at 1,000 Hz of from 2.0 to 2.8 with the proviso that if the binder is a homopolymer of polystyrene its molecular weight is less than 20,000 daltons and greater than 1,000 daltons.

8. A field effect transistor in which a continuous semiconductor layer forms a field effect channel between a source electrode and a drain electrode, wherein the continuous semiconductor layer comprises:

a) an organic semiconductor; and,

b) an organic binder which has an inherent conductivity of less than 10 31 6 Scm −1 and a permittivity at 1,000 Hz of less than 3.0;

with the proviso that the organic semiconductor does not comprise 2,5-di-n-octyloxy-4-bis((4,′,4″-bisstyryl)styrylbenzene)(Ooct-OPV5), said semiconductor being selected from the group consisting of:

(a) compounds having repeat units of Formula 1:

 where each Y 1 is independently selected from P, S, As, N and Se; Ar 1 and Ar 2 are aromatic groups and Ar 3 is present only if Y 1 is N, P, or As in which case it too is an aromatic group;

(b) conjugated oligomeric and polymeric heterocyclic compounds comprising a repeat unit of an optionally substituted 5-membered ring and terminal groups A 1 and A 2 as shown in Formula 2:

 in which X may be Se, Te, O, S, or —N(R)— where R represents H, optionally substituted alkyl or optionally substituted aryl; R 1 , R 2 , A 1 and A 2 may be independently H, alkyl, alkoxy, thioalkyl, acyl, aryl or substituted aryl, a fluorine atom, a cyano group, a nitro group or an optionally substituted secondary or tertiary alkylamine or arylamine —N(R 3 )(R 4 ), where R 3 and R 4 are independently H, optionally substituted alkyl, optionally substituted aryl, alkoxy or polyalkoxy groups;

(c) oligomers containing a conjugated linking group of Formula 3:

 in which X may be Se, Te, O, S, or —N(R)—, R is as defined above; R 1 , R 2 , A 1 and A 2 as defined above for Formula 2, Linking group L represents —C(T 1 )=C(T 2 )-, —C═C, —N(R′)—, —N═N—, (R′)═N—, —N═C(R′)— with T 1 and T 2 defined as above;

(d) polymers having repeat units of Formula 4:

 in which X, R 1 and R 2 are defined as above;

(e) polymers having repeat units of Formula 7:

 in which X is as defined above and the bridging group A is optionally fluorinated C 1-6 alkyl;

(f) polymers having repeat units of Formula 8:

 in which X, R 1 and R 2 are defined as above;

(g) polymers having the repeat units of Formula 9:

 in which X is as defined above; R 5 and R 6 may be independently H, alkyl, aryl or substituted aryl;

(h) polymers having the repeat units of Formula 10:

 wherein R 5 and R 6 are as defined in Formula 9.

9. A field effect transistor comprising:

an insulator layer,

a gate electrode,

a semiconductor layer,

a source electrode,

a drain electrode,

wherein the semiconductor layer is as defined in any one of the preceding claims.

10. A field effect transistor according to claim 9 , wherein the gate electrode and the semiconductor layer contact the insulator layer, and the source electrode and the drain electrode contact the semiconductor layer.