From organic single crystals to solution processed thin-films: Charge transport and trapping with varying degree of order

Abstract

We have fabricated single crystal, thermally evaporated, and spin-coated thin-film transistors (TFTs) from the same organic semiconductor N,N′-1H,1H-perfluorobutyl dicyanoperylene carboxydiimide (PDIF-CN2) using various combinations of deposition methods and gate dielectrics to investigate how the charge transport properties vary with the degree of crystalline order. Never before has a semiconductor been studied in such a wide variety of processing methods, allowing cross-comparison of the microscopic factors influencing the charge transport, and in particular the trap density of states (DOS). Excellent transistor performance was achieved for PDIF-CN2 single crystals in combination with Cytop as a dielectric layer resulting in a mobility of up to 6 cm2/Vs, an on/off-ratio exceeding 108, and a subthreshold swing of 0.45 V/dec. Furthermore, gate-bias-stress effects are not present in these transistors and we observed low stress effects in the evaporated TFTs with Cytop as the gate dielectric. These findings are reflected in the trap DOS. The single crystal field-effect transistor with Cytop has a low trap DOS, whereas in evaporated TFTs, the trap DOS is higher by 2–3 orders of magnitude. Surprisingly, the trap DOS of the spin-coated TFT is similar to that in evaporated TFTs, except for additional discrete trap states centered around 0.24 eV below the conduction band.

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

“…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.…”

“…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.…”

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

“…(34) and (36) can also be generalized to the case of degenerate electron systems (see Ref. [70] and Sec.…”

“…Such intrinsic tails have been ascribed to the presence of thermal molecular motions. [35,36] Optical conductivity.-Measuring the optical conductivity of charge carriers in organic semiconductors is experimentally challenging, as these must be injected either in a field-effect transistor (FET) geometry (in which case the absorption measurement is complicated by reflections at the interfaces) or by optical pumping (which is in principle free from interface effects but drives the system out of equilibrium). Despite the experimental difficulties, there have been several reports of the optical conductivity of excess carriers in rubrene.…”

Analysing organic transistors based on interface approximation

“…[10][11][12][13][14][15][16][17] Kalb et al have analysed organic transistors based on these methods and proved that various methods afford approximately consistent exponential trap distribution. [24][25][26][27][28] Although Lang's analysis has been extensively used due to its simpleness for (4)). The free carriers Q f are activated from E F to the interface E c according to Eq.…”