Calculating the trap density of states in organic field-effect transistors from experiment: A comparison of different methods

Abstract

The spectral density of localized states in the band gap of pentacene (trap DOS) was determined with a pentacene-based thin-film transistor from measurements of the temperature dependence and gate-voltage dependence of the contact-corrected field-effect conductivity. Several analytical methods to calculate the trap DOS from the measured data were used to clarify, if the different methods lead to comparable results. We also used computer simulations to further test the results from the analytical methods. Most methods predict a trap DOS close to the valence band edge that can be very well approximated by a single exponential function with a slope in the range of 50 − 60 meV and a trap density at the valence band edge of ≈ 2 × 10 21 eV −1 cm −3 . Interestingly, the trap DOS is always slightly steeper than exponential. An important finding is that the choice of the method to calculate the trap DOS from the measured data can have a considerable effect on the final result. We identify two specific simplifying assumptions that lead to significant errors in the trap DOS. The temperature-dependence of the band mobility should generally not be neglected. Moreover, the assumption of a constant effective accumulation layer thickness leads to a significant underestimation of the slope of the trap DOS.

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.…”

Quasi Two-Dimensional Dye-Sensitized In2O3 Phototransistors for Ultrahigh Responsivity and Photosensitivity Photodetector Applications

“…Its advantage over most other methods of DOS extraction is that it does not require temperature dependent measurements while producing comparable results to other methods. 31 The DOS analysis relies on two main assumptions: firstly that transport through the material is limited by multiple trapping and thermal release, 32,33 and secondly that there is a conduction band edge, above which the conduction band DOS is assumed to be constant with fixed electron mobility and below which exists only traps that produce a localized bandgap DOS. These bandgap states control the subthreshold characteristics of a TFT and the measured field effect mobility via the relative fraction of charge carriers distributed in immobile and mobile states.…”

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

“…Note that in the ME model, we construct the band-tail states using a superposition of a half-Gaussian and an exponential distribution. We attribute the overall fitting distribution widths of 2 to 5 k B T, indicated in Table 1, to structural defects and chemical impurities 1 , previously reported for small organic molecules 3,53,54 . A recent study 55 , and compared it to the evolution observed in the UPS measurements.…”