Threshold Voltage Shifts in Organic Thin‐Film Transistors Due to Self‐Assembled Monolayers at the Dielectric Surface

Abstract

Recently, it has been shown by several groups that the electrical characteristics of organic thin‐film transistors (OTFTs) can be significantly influenced by depositing self‐assembled monolayers (SAMs) at the organic semiconductor/dielectric interface. In this work, the effect of such SAMs on the transfer characteristics and especially on the threshold voltage of OTFTs is investigated by means of two‐dimensional drift‐diffusion simulations. The impact of the SAM is modeled either by a permanent space charge layer that can result from chemical reactions with the active material, or by a dipole layer representing an array of ordered dipolar molecules. It is demonstrated that, in both model cases, the presence of the SAM significantly changes the transfer characteristics. In particular, it gives rise to a modified, effective gate voltage Veff that results in a rigid shift of the threshold voltage, ΔVth, relative to a SAM‐free OTFT. The achievable amount of threshold voltage shift, however, strongly depends on the actual role of the SAM. While for the investigated device dimensions, an organic SAM acting as a dipole layer can realistically shift the threshold voltage only by a few volts, the changes in the threshold voltage can be more than an order of magnitude larger when the SAM leads to charges at the interface. Based on the analysis of the different cases, a route to experimentally discriminate between SAM‐induced space charges and interface dipoles is proposed. The developed model allows for qualitative description of the behavior of organic transistors containing reactive interfacial layers; when incorporating rechargeable carrier trap states and a carrier density‐dependent mobility, even a quantitative agreement between theory and recent experiments can be achieved.

Threshold voltage control in organic thin film transistors with dielectric layer modified by a genetically engineered polypeptide

“…These dipoles, generated at the semiconductor-dielectric interface shift V T positively or negatively based on their direction. The dipoles generated by NH 3 + Cl − ion pair shift V T positively while those generated by COO − K + ion pair shift V T negatively. The ions do not appear to bind to the SiO 2 surface when no peptide is present as there is no change in device performance if the bare SiO 2 / Si substrate is soaked in HCl or KOH solution prior to pentacene and gold evaporation.…”

“…Addition of base yielded negative shifts in V T of 5-15 V, larger with increasing basicity, with respect to the initial shift. These shifts can be explained by the presence of ion pairs on the QBP layer such as NH 3 + Cl − or COO − K + dependent on the acidic or basic assembling conditions. Based on the calculations using PROPKA server, 18 the isoelectric point ͑pI͒ of QBP is 6.0, and the pK a s of the C-terminus ͑Ser-12͒, N-terminus ͑Pro-1͒, and side chain group ͑the phenol side chain on tyrosine ͑Tyr-7͒ are 2.9, 6.9, and 13.4, respectively.…”

“…Controlling V T can yield higher circuit performance and lower power consumption in electronics. 3,4 It can be shifted by modifying the dielectric with self assembled monolayers ͑SAMs͒ to shift V T by tens of volts. 5,6 A less studied route in the pursuit of improved device performance involves integrating functional biological materials into OTFTs.…”

Analysing organic transistors based on interface approximation

“…18 In addition, it has been suggested that in thin-film transistors, where the bulk potential is not fixed in contrast to the conventional metal-oxide-semiconductor field-effect transistors (MOS-FET), the band bending is considerably reduced due to the global energy-level shift. 19 If the potential is flat, charge exists only at the interface, and the interface model applies perfectly.…”

Continuous tuning of the threshold voltage of organic thin-film transistors by a chemically reactive interfacial layer

“…As can be shown by drift-diffusion-based device modelling [19,20], the threshold voltage shift can be associated with a density of fixed charges at the interface (in our case the -SO − 3 groups) via…”