High Mobilities in Organic Molecular Crystals
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INTRODUCTION Optoelectronic devices based on organic semiconductors are very promising for future applications [1-3]. However, several basic questions concerning the microscopic picture of the charge transport and intrinsic performance limits of field-effect transistor (FET) devices still remain unanswered. In this study we investigate the intrinsic charge transport properties in various organic semiconductors, such as polyacenes or oligothiophenes, by measurements of the mobility in single crystals as function of temperature and applied electric field.
EXPERIMENTAL Single crystals of various organic semiconductors (tetracene, pentacene, rubrene, quaterthiophene, sexithiophene, anthradithiophene,…) were grown from the vapor phase. Details of the technique have been described earlier [4]. The charge carrier mobility in these materials was determined by space charge limited current (SCLC) spectroscopy [5] and FET measurements [6]. Furthermore, the density and mobility of photoexcited charge carriers were measured as function of temperature by photo Hall effect measurements.
RESULTS AND DISCUSSION Polyacenes The hole mobility in pentacene, tetracene, and rubrene single crystals increases with decreasing temperature following a power law (µ∝T -n) indicating band-like charge transport in delocalized states. All three investigated polyacenes exhibit many similarities. Therefore, we will discuss only pentacene as one example. Mobility values as high as 400 cm2/Vs are measured in pentacene for an applied electric field of 4×103 V/cm (see Figure 1) by the SCLC technique. The mobility anisotropy at room
temperature is on the order of three. The mobility values obtained by the different measurements reveal a good agreement between surface (FET) and bulk (SCLC) mobility. The differences at low temperatures are caused by the different purity of the samples.
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Figure 1. Mobility in pentacene single crystals determined by SCLC, FET, and Photo-Hall measurements as function of temperature. The dashed line corresponds to the low field mobility µo (see Equation (1)).
Figure 2. Hole velocity in pentacene single crystals as function of applied electric field. The dashed line corresponds to a fit to Equation (1). At high fields the hole velocity saturates.
Moreover, the mobility exhibits a strong electric field dependence (see Figure 2), which can be ascribed to acoustic phonon scattering. Using the deformation potential approximation, the carrier velocity vh as function of the electric field E is given by [7] 3π v h = µ o E 2 1 + 1 + 8
1 2 2
µoE cl
− 12
(1)
where cl is the longitudinal sound velocity and µo the low-field mobility. The value of µo, which cannot be obtained directly from the SCLC measurements, is shown in Figure 1 as function of temperature. At h
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