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

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 density of states (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 in the trap DOS very close $\left(<0.15\text{eV}\right)$ 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 in 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.

Figure 1

Schematic representation of the mobility edge separating localized states (traps) from extended states. At the mobility edge, the mobility as a function of energy abruptly rises and only the charge carriers that are thermally activated to states above the mobility edge contribute to the charge transport.

Figure 2

(Color online) Spectral density of localized states in the band gap (trap DOS) of pentacene as calculated with several methods from the same set of transistor characteristics. The transistor characteristics were measured with a pentacene-based TFT employing a polycrystalline pentacene film and a ${\text{SiO}}_2$ gate dielectric. The energy is relative to the valence band edge (VB). The choice of the method to calculate the trap DOS has a considerable effect on the final result. Adapted from Ref. 7.

Figure 3

(Color online) Trap DOS from thin-film transistors (TFT’s) made with small-molecule organic semiconductors. Several different semiconductors, gate dielectrics and methods to calculate the trap DOS were used. Some details of the TFT fabrication are listed in Table along with the method that was used to calculate the trap DOS and the reference of the data. Small-molecule semiconductors tend to be crystalline and can be obtained in high purity. Typical materials are oligomers such as pentacene or sexithiophene but this class of materials also includes, e.g., rubrene or ${\text{C}}_{60}$. The molecules interact by weak van der Waals-type forces and have loosely bound $\pi$ electrons which are the source of charge conduction.

Figure 4

(Color online) Trap DOS from single crystal field-effect transistors (SC-FET’s). Different small-molecule semiconductors and gate dielectrics as well as different calculation methods were used. Details of the data are summarized in Table . Remarkable is the very low trap density from the rubrene-based SC-FET with Cytop™ fluoropolymer gate dielectric (data no. 12).

Figure 5

(Color online) Trap DOS in the bulk of single crystals. The trap densities were calculated from SCLC measurements and the small-molecule semiconductor is, e.g., pentacene, rubrene or sexithiophene. Details of the underlying SCLC measurements and samples are summarized in Table .

Figure 6

(Color online) Representative trap DOS data in small-molecule organic semiconductors from thin-film transistors (TFT’s, black lines), single crystal field-effect transistors (SC-FET’s, red lines in online version), and in the bulk of single crystals (blue lines in online version). The data were selected from Fig. 3, 4, 5 as typical examples. The trap densities from SC-FET’s can be much lower than the trap densities in the best TFT’s. This strongly suggests that traps due to structural defects tend to dominate in thin films. The trap densities in the bulk of single crystals are typically lower than the trap densities from SC-FET’s. Importantly, if a highly hydrophobic Cytop™ fluoropolymer gate dielectric is used, bulk trap densities can be reached in organic field-effect transistors made with small-molecule semiconductors. Thus, water adsorbed on the gate dielectric appears to be the dominant cause of traps if the semiconductor has a low density of traps due to structural defects (e.g., single crystals). A steep increase in the trap DOS very close to the valence band edge $\left(<0.15\text{eV}\right)$ can sometimes be observed (data no. 6, 12, 16, 24, and 25). These states are attributed to the thermal fluctuations of the intermolecular transfer integral.

Figure 7

(Color online) Oxygen-induced traps in rubrene and pentacene. The exposure of rubrene single crystals (SC) to oxygen in combination with light leads to a sharp peak of trap states centered at about 0.28 eV (two different samples, blue and red lines, data from Ref. 12). Exposing pentacene thin films (TFT’s) to oxygen in combination with light leads to a much broader peak of trap states also centered at 0.28 eV (full black line, data from Ref. 63). For the pentacene thin film, the width of the peak is 0.16 eV and the volume density of traps as calculated from integrating the peak is $4\times {10}^{18}{\text{cm}}^{-3}$. The large width of the peak is thought to result from the increased local disorder in a thin film as compared to the bulk of single crystals.

Figure 8

(Color online) Comparison of typical trap densities in small-molecule organic semiconductors/transistors with typical trap densities in hydrogenated amorphous silicon (a-Si:H). The dashed-dotted green line (data a) is a typical distribution of hole traps in a-Si:H (energy relative to VB). The full green line (data b) marks typical electron trap densities in a-Si:H (energy relative to CB). Details of the data are given in Table . The hole trap DOS in a-Si:H is surprisingly similar to the hole trap DOS in small-molecule-based TFT’s.

Figure 9

(Color online) Comparison of typical trap densities in small-molecule semiconductors with typical trap densities in polycrystalline silicon (poly-Si). The dash-dotted blue line (data c) represents hole traps in poly-Si (energy relative to VB) and the full blue line (data d) marks electron traps in poly-Si (energy relative to CB). Details of the data are given in Table .