Laser Enhancement of Pristine PEDOT:PSS Conductivity and Applications in Organic Electronics

Poly(3,4-ethylenedioxythiophene) : polystyrene sulfonate (PEDOT:PSS) is a well-known material used in a wide range of applications, including as hole transport layer in optoelectronics and as a transparent electrode in many devices. Most notably in bioelectronics, PEDOT:PSS is one of the most common materials in OECTs, favored for its mechanical properties, ease of fabrication, and ionic/electronic transducing abilities. Indeed, the conductivity and the ability to control it are the biggest draws of PEDOT:PSS. However, in the commonly used aqueous suspension forms, such as Cleveios PH1000™, pure PEDOT:PSS has a low conductivity, well below 1 S/cm. To overcome this, additional compounds are added during- or post-deposition, increasing conductivity by several orders of magnitude. However, these treatments detrimentally affect the fabrication process, particularly wettability of the solution and homogeneity of the subsequent film.<br/>Here we demonstrate the use of 1064nm laser irradiation to precisely control the conductivity of PEDOT:PSS, without additives, showing control over 3 orders of magnitudes and achieving conductivities over 300 S/cm, on par with those achieved through the use of standard additives. In line with the small amount of existing work, we show that a minimum laser fluence is required to initiate the conductivity enhancement, above which a maximum conductivity is quickly achieved, followed by a gradual reduction as the laser power is increased further. This electrical control is correlated with a range of physical and chemical parameters, including changes in surface roughness, film thickness, work function, ionic conductivity, and local bonding environment. In this context, a discussion of likely mechanisms is presented.<br/>Finally, this direct-write conductivity control is coupled with laser ablation to masklessly fabricate planar OECTs with performance similar to equivalent devices using standard additives.