Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance

Abstract

We studied the influence of oxygen on the electronic trap states in a pentacene thin film. This was done by carrying out gated four-terminal measurements on thin-film transistors as a function of temperature and without ever exposing the samples to ambient air. Photo-oxidation of pentacene is shown to lead to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of 10 18 cm −3 . As the gate voltage is ramped up, these trap states are occupied at first and cause a reduction in the number of free carriers at a given gate voltage. Moreover, the exposure to oxygen reduces the mobility of the charge carriers above the mobility edge. We correlate the change of these transport parameters with the change of the essential device parameters, i.e., subthreshold performance and effective field-effect mobility. This study supports the assumption of a mobility edge for charge transport and contributes to a detailed understanding of an important degradation mechanism of organic field-effect transistors. Deep traps in an organic field-effect transistor reduce the effective field-effect mobility by reducing the number of free carriers and their mobility above the mobility edge.

Trap density of states in n-channel organic transistors: variable temperature characteristics and band transport

“…charge transport has been proposed and widely accepted, 13,14 and there have been many attempts to determine the trap density of states (trap DOS). However, the trap DOS has been analyzed mainly for p-channel materials, [13][14][15][16][17][18][19][20] and comparatively limited works have been done for n-channel materials. 15,[21][22][23] In particular, the trap DOS has not been systemically studied from the viewpoint of the basic transistor operation.…”

“…Lang et al have extracted the trap DOS from the Arrhenius plot of the transconductance for single-crystal pentacene transistors, 32,33 and similar analysis has been applied to other thin-film organic semiconductors. 15,16 In addition, a variety of methods to calculate the trap DOS have been investigated, and an exponential distribution of the DOS has been established in pentacene thin-film transistors, [17][18][19] and in single crystal transistors. 20 The DOS distribution has been also extracted from space-charge-limited-current spectroscopy and Kelvin probe microscopy.…”

“…Intrinsic carrier transport, in particular, band transport, has been explored in organic single crystals, [9][10][11][12] and trap-limited charge transport has been examined in organic thin films, which have higher density of trap states due to the structural defect, grain boundaries, and chemical impurities. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The multiple trapping and release model for a Author to whom correspondence should be addressed. Electric mail: cho.j.ad@m.titech.ac.jp.…”

Role of band states and trap states in the electrical properties of organic semiconductors: Hopping versus mobility edge model

“…35 However, there is now increasing evidence from a number of investigations that in many organic systems the deep gap states exhibit an exponential distribution. [1][2][3][4][5][36][37][38] These studies also suggest that the shallow trap states, i.e., the states located closer to the ME (or the valence band or conduction band edge) deviate from an exponential shape. For instance, very recent results obtained by means of scanning Kelvin probe microscopy on a self-assembled monolayer fieldeffect transistor (SAMFET) based on quinquethienyl molecules, reveal that the DOS consists of an exponential distribution of deep trap states with an additional group of localized shallow states that can be modeled via a Gaussian function.…”

“…It was recently shown that a substantial density of localized (trap) states (~10 15 -10 18 cm -3 ) is present even in the case of high-mobility organic single crystals such as rubrene and pentacene. [1][2][3][4][5] Therefore, understanding how disorder affects the electrical properties of the system is important in the quest for new materials and devices with improved performance.…”