Carrier mobility in organic field-effect transistors

Abstract

A study of carrier transport in top-gate and bottom-contact TIPS-pentacene organic field-effect transistors (OFETs) based on mobility is presented. Among three mobilities extracted by different methods, the low-field mobility obtained by the Y function exhibits the best reliability and ease for use, whereas the widely applied field-effect mobility is not reliable, particularly in short-channel transistors and at low temperatures. A detailed study of contact transport reveals its strong impact on short-channel transistors, suggesting that a more intrinsic transport analysis is better implemented in relatively longer-channel devices. The observed temperature dependences of mobility are well explained by a transport model with Gaussian-like diffusivity band tails, different from diffusion in localized states band tails. This model explicitly interprets the non-zero constant mobility at low temperatures and clearly demonstrates the effects of disorder and hopping transport on temperature and carrier density dependences of mobility in organic transistors.

Transport models in disordered organic semiconductors and their application to the simulation of thin‐film transistors

“…proposed an approach to describe the transport properties in OSCs using the generalized Einstein relation . To describe the variation of the transport properties with accessible state energies E those authors proposed associating each elementary energy layer [ E , E + d E ] with an ‘energetic conductivity’ given by σ ′ ( E ) = q 2 DOS( E ) D ( E ). The parameter D ( E ) is the diffusion coefficient of individual states. The conductivity of the OSC is given by the Kubo–Greenwood integral …”

Theoretical analysis of carrier mobility in organic field-effect transistors

“…Because ͑E͒ = q 2 N͑E͒D͑E͒, 7,8 where D͑E͒ is the diffusivity for each energy, a generic distribution of D͑E͒ is assumed having a Gaussian-like shape with a relatively smaller standard deviation than for the DOS, i.e., ␣⌬E͑␣ Յ 1͒. 9 Thus, the diffusivity decreases from band center to band tails as the localized states distributed at the band edges degrade the overall transport from bandlike to a more hoppinglike process, hence smaller ␣ represents more hopping contribution in the band tails. It should be mentioned that Kubo-Greenwood integral directly offers the total conductivity in the band, irrespective of which kind of transport exactly contributes to the microscopic conductivity.…”

Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers

“…At high temperatures, the larger number of microscopic conductivities spanning over a wide energy region concurrently contributes to the mobility, often manifesting as thermal activation. In addition, due to the presence of energetic disorder and the already-high Fermi level for hole transport (discussed below), the apparent E a would not be very different before and after incorporating the interlayers44. Regarding the E a for electron transport, more dramatic improvement was observed by inserting interlayers: Not only was the mobility magnitude improved over the entire temperature region, but also the activation energy was significantly reduced from 55 to 27 meV.…”

“…To comprehend how much the charge injection is improved more quantitatively and to understand why the mobility magnitude changes so dramatically by adding the interlayers, we implemented mobility calculations based on a solid mobility model that has been reported previously4447. As there was only one difference in the contact interlayer, we kept the transport profile constant and only varied the charge concentration.…”