Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

Abstract

We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase of the trap DOS very close (< 0.15 eV) to the mobility edge with a characteristic slope of 10 − 20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase of the trap DOS. Moreover, we show that the trap DOS in TFT's with small molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand. Although important conclusions can already be drawn from the existing data, more experiments are needed to complete the understanding of the trap DOS near the band edge in small molecule organic semiconductors.

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.…”

“…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.…”

“…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.…”

Remarkably low flicker noise in solution-processed organic single crystal transistors

“…Black curves in In this first straightforward approach of fitting the measured noise with McWorther's model, the density of trap states as the only fitting parameter (N = 8.3 × 10 18 eV −1 cm −3 ) was assumed to be energy-independent. In organic semiconductors, however, the trap DOS typically increases towards the transport level 37 . To further investigate the effect of an energy-dependent trap distribution on the 1/f noise, we have conducted a temperaturedependent noise analysis of an C 8 -DNBDT-NW single-crystal FET from T = 295 to 115 K (see more details in "Methods" section).…”

“…Figure 5 summarizes the energydependent trap DOS for the four different semiconductors, where symbols correspond to the trap DOS determined experimentally from noise measurements (Fig. 4) and the black lines represent the trap distribution derived from the transfer characteristics via numerical modeling 37,39,40 (see details in Supplementary Fig. 9 and Supplementary Note 6).…”