We are working with P3HT OFETs. We have used degenerate n-Si as substrate, a thin layer of PMMA (10 mg/ml in chlorobenzene, 1000 rpm for 30s ) over it, and a thin film of P3HT (3 mg/ml in xylene, 2000 rpm for 2 min) over PMMA. We have deposited Al for source and drain using shadow masking over P3HT layer. For gate contact we are directly depositing Al over earlier protected n-Si by shadow masking, but we are not getting desired FET I-V and C-V characteristics. Is there anything wrong in the process? Are we missing some important steps?
Use polar solvents for gate insulators (since P3HT is solebule in nonpolar solvents). For example polyvinyl acetate (PVA) is soluble in water only. Other possibility is to use some polymers (such as polydiaceylenes) which you can polymerize by UV and they are not soluble anymore.
Few suggestions:
(1) Try to use heavily doped substrate with thermal oxide layer. You will use silicon as a gate electrode and SiO2 layer will serves as a gate insulator (not problem with dissolving during the P3HT spin-coating). Keep in mind that xylene can re-dissolve PMMA so you will obtained film with PMMA-P3HT mixture.
(2) Use gold as source and drain electrodes. P3HT has p-type conductivity and gold is favourable for the holes injection into P3HT. Aluminium creates blocking contact.
Following these conditions I always obtain common OFET with mobility abou 5x10^(-2) cm^2/V.s. The mobility can be even higher if it is done in glove-box and P3HT is annealed to improve the crystallinity.
Thank You Martin, for valuable suggestions....
But xylene is only a weak solvent for PMMA, and if we want to make all-organic FET (organic insulator and organic semiconductor), then which solvent will be better for P3HT as almost all solvents for P3HT dissolve/partially dissolve PMMA?
If we do not use inorganic oxide layer, then other than lesser mobility, which other issues will arise?
Idea of gold contacts is better indeed.
Use polar solvents for gate insulators (since P3HT is solebule in nonpolar solvents). For example polyvinyl acetate (PVA) is soluble in water only. Other possibility is to use some polymers (such as polydiaceylenes) which you can polymerize by UV and they are not soluble anymore.
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