Amorphouslike Density of Gap States in Single-Crystal Pentacene

Abstract

We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field-effect transistors fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.

Thermoelectric model to characterize carrier transport in organic semiconductors

“…24 The carrier mobility can be calculated using the generalized Einstein relation (valid to high carrier concentrations): 25,26 μ…”

Optical, Photoluminescent, and Photoconductive Properties of Novel High-Performance Organic Semiconductors

“…Therefore, the differences in the trapping properties of TIPS-pentacene, Pc, Rub and Tc crystals most likely account only for a factor up to ~2 in the differences in µη values obtained in these crystals at RT, while further differences are due to factors (i) and (ii). For example, the lower transient photoconductivity observed in Rub compared to Pc single crystals at RT at 400 nm could be mostly due to the lower photogeneration efficiency η (factor (ii)) in Rub, in qualitative agreement with Podzorov et al, 2004 andLang et al, 2004. The decay dynamics of the transient photoconductivity yield information about the nature of charge transport, trapping and recombination (Etemad et al, 1981;Moses et al, 1987;Yu et al, 1990).…”

“…Indeed, it is a complicated task to reveal and utilize the intrinsic properties of organic materials, since they are often masked by the influence of impurities, the presence of which is sensitive to the methods of material purification and device fabrication. As a result, measurements performed in the same material using different experimental techniques often provide conflicting results (Nelson et al, 1998;Hegmann et al, 2002;Podzorov et al, 2003;Jurchescu et al, 2004;Lang et al, 2004;Thorsmølle et al, 2004;Ostroverkhova et al, 2005a;Ostroverkhova et al, 2006a;Koeberg et al, 2007;Laarhoven et al, 2008;Najafov et al, 2008;Marciniak et al, 2009). Experimental methods that probe charge carrier dynamics on picosecond (ps) time-scales after a 100-femtosecond (fs) pulsed photoexcitation have had most success in revealing intrinsic properties of organic semiconductors (Hegmann et al, 2002;Thorsmølle et al, 2004;Hegmann et al, 2005;Ostroverkhova et al, 2005a;Ostroverkhova et al, 2005b).…”

Analysis of interface states of the pentacene organic thin-film phototransistor by conductance technique

“…Several techniques, such as photoelectron emission microscopy (PES) [36], potential imaging mapping [37], and scanning Kelvin probe microscopy (SKPM) [38,39] have been used to probe local defects originating from structural disorders, crystalline domain size, shape, and height of the pentacene layer itself. Metastable defects under bias voltage using space-charge limited current spectroscopy [40], deep charge traps using electric force microscopy [41] and shallow traps by using scanning tunneling microscopy (STM) [33] have been observed to clarify the charge transport mechanism at the interface between pentacene and insulating dielectric layers on various substrates [42]. The field-effect mobility and performance of an OTFT device depend also on deposition rate and substrate temperature that affect the morphology and molecular ordering of semi-crystalline organic semiconductors like pentacene [43,44].…”