Investigation of Gold and Poly(3-Alkylthiophene) interface in top and bottom contact structures

• Crystallinity variations in polymers occurring at Gold on P3AT (top contact) and P3AT on Gold (bottom contact) interface were quantitatively studied.
• Techniques used were J–V, UV–vis, PL, Raman and FESEM.
• Significantly decreased P3AT film crystallinity near the Gold on P3AT interface was observed.
• Observations were attributed to the creation of disrupted π-conjugation inside the polymer matrix due to Gold diffusion effect.
• These results can be significant for understanding the nature of charge transport in the vicinity of contacts in low voltage organic optoelectronic devices.

In this report we present the variations in the degree of crystalline order in π-conjugated polymer based thin films occurring at the interface between Gold (Au) and Poly(3-Alkylthiophene)s (P3ATs). For the purpose of study two possible electrode configurations were considered, i.e. the top contact (Au on top of Polymer) and the bottom contact (Polymer on top of Au). The current-voltage characteristics of Au(top)/P3AT/Au(bottom) devices revealed a near ohmic hole injection from bottom electrode but a non-ohmic hole injection from top electrode. Furthermore, the presence of barrier at the Au(top)/P3AT interface was estimated from the first order derivative of lnJ and lnV characteristics of the device under the low electric field biasing. It was observed that the process of coating metal (Au) itself led to significantly decreased polymer crystallinity and that the effect was more profound in top contact configuration compared to bottom contact ones. These observations were further verified through some non-contact mode characterization techniques such as absorption, photoluminescence, Raman spectroscopy and scanning electron microscopy. The difference in the hole injection current from the top and bottom contact electrodes in a Au/P3AT/Au device has been attributed to the creation of distorted π-conjugation due to Au diffusion effect in polymer films in top contact configuration.

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