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'Appl. Phys. A 36, 163-170 (1985) Applied .o,,.. Physics A Surfar 9 Springer-Verlag 1985 Ultrapure, High Mobility OrganicPhotoconductors* W. Warta, R. Stehle, and N. Karl Physikalisches Institut, Teilinstitut 3, Universit/it, Pfaffenwaldring 57, D-7000 Stuttgart 80, Fed. Rep. Germany Received 13 September1984/Accepted2 October 1984 Abstract. This contribution demonstrates that high charge carrier mobility (< 400 cm2/Vs) is an inherent property of ultrapure organic molecular crystals at low temperatures. Small concentrations of traps, however, can completely obscure these microscopic transport properties on macroscopic scales. We describe extensive purification procedures with naphthalene and perylene, which led to the observation of high mobilities. At the same time we demonstrate that charge carrier transport measurements are a sensitive tool for the analytical characterization of high purity organic molecular crystals. PACS: 72.80L, 72.20H, 81.10 This report is concerned with ultrapurification, and characterization by electrical transport measurements, of two typical representatives of organic photoconductors, the aromatic hydrocarbons naphthalene, C~0H8 (I), and perylene (peri-dinaphthalene), C2oH12 (II). (I) naphthalene (i1) perylene Both form molecular crystals of monoclinic symmetry, space group P21/a, with Z = 2 and Z = 4 molecules in the unit cell, respectively. The "dimeric" perylene structure, projected along the molecular planes, is shown in Fig. 1 F1]. Increasing interest in this class of organic molecular crystals arose, on the one hand, from their potential applicability as low-cost photoconductors: Due to the extended re-electron systems of the constituent molecules 1, these crystals exhibit a separation between the * This work has been presented in part at the VIIth Intern. Conf. on Crystal Growth, Stuttgart (1983) 1 and in contrast to the purely or-bonded molecules such as, e.g., polyethylene ill/ Fig. 1. The e-perylene crystal structure projected along the intersection line of the molecular planes of the two symmetryrelated molecular pairs [1] valence and the conduction band of only a few eV; consequently charge carriers can be excited by light in the visible or near UV range. On the other hand, an understanding of the basic physical processes which can occur in these molecular crystals is interesting in itself. For example, the question how their charge carrier transport properties can be described theoretically is one of the major unresolved problems in modern solid-state physics. 164 W. Warta et al. The investigations presented here are part of a larger program designed to obtain reliable experimental data over wide temperature ranges on charge-cartier transport in organic photoconductors. Naphthalene and perylene were selected, among others, to serve as model substances. Naphthalene is one of the most simple aromatic molecules; very extensive investigations are available of both its molecular and its crystal properties. Its melting point, however, (m.p. = 80.5 ~ is too low for any potential practical application. In contrast, perylene, which melts at 278 ~ is an example of a fairly stable, larger aromatic molecule. Both materials can be zone-refined and obtained as fairly perfect single crystals by the Bridgman method. We have chosen not to measure the electrical conductivity as such, since it is in the most general case composed of at least four independent material properties: the electron and hole concentrations, n- and n +, and the electron and hole mobilities, # - and #+, respectively = e(n-#- + n+#+). (1) Instead, we decided to focus on the basic transport quantities, # - and/~+. These mobilities can, in principle, be measured separately by the time-of-flight technique, cf. [2], where the transit of electrons or holes over macroscopic sample dimensions (of typically 0.2-1 ram) is observed. However, most chemical impurities can very efficiently capture charge carriers during passage through the crystal, because the wide band gaps which are typical for this material class provide a wide energy range in which impurity states can act as traps. It is therefore crucial to study charge carrier transport properties at extremely low impurity concentrations. In addition, the concentration of physical (lattice) defects has to be kept low by careful annealing and handling of the crystals, since at the present level of chemical purity, physical defects can no longer be considered to give rise to only minor contributions for the total trapping behaviour. Traps which are energetically deep with respect to kT can completely prohibit the observation of carrier transits. Shallow traps can repeatedly hold charge carriers for short time intervals, until they are thermally released to the conduction states again. A strongly reduced, thermally activated "effective" mobility is therefore commonly observed in such cases, cf. [3]. The intrinsic transport properties, the "microscopic" mobilities (or "lattice mobilities") are obscured in such situations. Early mobility results have often suffered from these different trapping influences. (This is at least and definitely known to be so in cases where more efficient purification later led to higher, non-thermally activated mobility values.) Historically, the measured microscopic mobilities were often on the order of 1 cm2/Vs, cf. [4, 5] which is very small in comparison with the mobilities in standard inorganic semiconductors, such as silicon or germanium. An increase of the mobilities # with decreasing temperature T, # oc T", (n < 0), was found with some of the purer samples: This increase was usually restricted to narrow temperature intervals near room temperature (and therefore not very big). The reason is that even very shallow trapping influences, not seriously disturbing room-temperature transport, can limit the observability of transits at lower temperatures, since the time constants for thermal detrapping increase exponentially on cooling. The reproducibility of mobifity results was sometimes rather poor between different laboratories, which may also have been due to (unknown) impurities which were present in variable amounts in the different samples. There were only three cases reported in the literature where crystal quality permitted reaching lower temperatures: The electron-mobility tensor component #~;~, in anthracene was measured down to liq. N2 temperature [4, 6]; (surprisingly it exhibited the abnormality of remaining nearly temperatureindependent between 373 and 83 K, amounting to 0.4cmZ/Vs). In naphthalene the observation of a transition from a nearly temperature-independent electron-mobility tensor component #~;c.= 0.4 cm2/Vs to one rising exponentially below 120 K was accomplished in a measurement down to 31 K, where #~c. =2cmZ/Vs was reached [7]. Durene was the only example for which fairly high low-temperature mobilities have been reported (#+ = 55 cmZ/Vs at 120 K) [83. The aim of the work presented here was to improve purification procedures and to grow high-quality single crystals of naphthalene and perylene in order to find out if and to which maximum value intrinsic charge carrier mobilities continue to increase with decreasing temperature, and, if high charge carrier mobilities might eventually constitute a common lowtemperature property of aromatic crystals. On our way to gradually increased purities we found, among other things, that temperature-dependent charge carrier mobility measurements represent a very sensitive analytical tool for the assessment of purity of organic aromatic crystals and we wish to focus on this aspect in this publication. Purification Procedures We started with the purest commercially available material (naphthalene "for scintillation", Merck, perylene "purum", Fluka). Naphthalene was prepurified by liquid chromatography. After vacuum sublimation, Ultrapure, High Mobility Organic Photoconductors 165 3x z I I I m~ c ,I ', 1 , i L the major part of the impurities was removed by an initial zone refining step, consisting of 100 zones passed across the naphthalene and perylene tubes at a speed of 10 mm/h. Two impurities in naphthalene, //-methylnaphthalene and thionaphthene, are hard to deplete below 1 ppm by zone refining, because at lower concentrations they exhibit distribution coefficients close to one. In this ease (as in others, such as pyrene) treatment with molten potassium [9] (of. [10]) was applied in order to chemically modify the distribution coefficients of these persistent impurities by transforming them to reaction products with a more favorable distribution coefficient, or to non-volatile potassium salts which remain in the residue in a subsequent sublimation step. The principle of potassium fusion is sketched in Fig. 2 [,10]. The material is sublimed through a sequence of adsorbents (teflon wool, t, charcoal, c, and 4 fi, molecular sieves, m) into a reaction flask, 2, which had been coated with a potassium mirror. The molten components are allowed to react for about 12 hours. This step is repeated twice. Subsequently, the purified material is collected in zone refining tubes, z, for the main zone refinement. Prezonerefined perylene was used without potassium treatment. In three following zone refining steps a total number of about 500 molten zones were passed in a similar way across the naphthalene and the perylene ingots. The first step consisted in a normal vertical zone refinement, using a multistage intermittent technique. Then reverse horizontal zone refinement followed. For this purpose, the material was transferred to a narrower zone refining tube after removal of the impure ends (~ 15% at each side). Finally, another normal vertical zone refining step was applied, using a zonerefining tube equipped with a break seal. This break seal can be opened under vacuum conditions, allowing transfer (under complete exclusion of air) of the purest ~00 '~ 300 / Fig. 2. Schematic representation of the assemblyused for prepurification of naphthalene by treatment with molten potassium metal. For explanation, see text and [9, 10]. After [-10] 5 I 200. 100 i tO i i i i 20 30 /+0 50 i 60 [crn3 Fig. 3. Distribution of the lifetime of free triplet excitons zr (monitored by delayedfluorescence)along a typicalnaphthalene zone refining ingot (whichis schematicallyrepresented, with the sequence of the molten zones indicated, below the figure). Crystals were grown from the 400 ms lifetime fractions fraction of the material to a separate ampoule for Bridgman crystal growth. As another precaution, yellow safety lights were used during the entire material handling and purification procedures, in order to avoid photochemical reactions of the material. To exclude the possibility of thermal decomposition by overheating, material transfer from one ampoule to another was always effected by sublimation in a stabilized temperature heat box, rather than by melting by a flame. For more details of the purification procedures and the apparatuses used, the reader may consult [-10]. The progress o f purification was controlled by gas chromatography, mass spectrometry, and (in the case of napththalene) with extreme sensitivity by liquid He fluorescence spectrometry, cf. [11] and references given therein. Under favourable conditions impurities with 166 w. Warta et al. concentrations down to ~ 10 -8 mol/mol can be detected with the latter method. A rather convenient, but unspecific method for quick purity check is the measurement of triplet exciton lifetimes by exciton-exciton annihilation and delayed fluorescence, cf. [11]. An example of observed triplet exciton lifetimes along a naphthalene zone refining ingot is given in Fig. 3. A remarkably high triplet exciton lifetime was reached in the purest fractions. Sample Preparation Bridgman crystals were grown by conventional techniques [10]. Thin slices of these crystals, necessary for the mobility measurements, were prepared by dissolution sawing with a string saw, equipped with a solvent-wetted cotton thread. (Solvents were xylene in the case of perylene and a cyclohexane + 10% xylene mixture in the case of naphthalene.) Extreme care had to be taken especially for naphthalene, not to exert mechanical or thermal stress to the crystals. Otherwise structural defects are introduced which were found to strongly influence the observed mobilities. Mobility Measurements Mobilities were measured by the "time-of-flight" technique [2], (Fig.4), were charge carrier pairs are created near one surface of a crystal slice of typical dimensions 50mmZ• at time t=O by a pulse of ~, hv I- (+)~ I j l I I __l__ C - -[~ - I .._1_ crystctt amplifier l osciltoscope Fig. 4. Schematic representation of the set-up used for measuring electron and hole mobilities by the time-of-flight method; the transit of charge carriers through the crystal (left)causes a pulse j(t) whoserise and decaytime are dependent, among other things, on the stray capacitance C. The signal is amplified by a preamplifier (center) and then fed to an oscilloscope(right side). The duration of the exciting laser flash, displayed on the upper oscilloscopetrace, must be short compared with the transit time z to be measured strongly absorbed light (2<320nm in the case of naphthalene and 2 < 450 nm in the case of perylene). We used a commercial KrF excimer laser (Lambda Physik, type E M G 102), emitting single 2 = 2 4 8 n m pulses of 8 ns halfwidth, for naphthalene, and a homemade N2 laser, emitting 2 = 337 nm pulses of 0.8 ns halfwidth, for perylene. For the measurements, individual crystal platelets were sandwiched between a metal rear electrode and a semi-transparent front electrode [a vapor-deposited silver film in the case of perylene, or a conductive layer (Aurel A, Balzers) on quarz glass in the case of naphthalene]. A potential difference applied across the sample generates an electric field parallel to the normal to the platelet. Depending on its polarity, either holes or electrons are drawn through the bulk of the crystal. While they are moving with constant velocity, a constant current flows in the low resistance external circuit (RC ~ z, see below). As soon as the "carrier sheet" reaches the rear electrode, the photocurrent pulse amplitude drops to zero (under idea/conditions). The kink thus indicates the arrival time t = z. With the crystal thickness L, the drift velocity v = L/z can be determined, and from the definition equation of the mobility v = #E the latter material property can be derived. (To be more precise, # is, in general, a symmetric tensor of the second rank, and the quantity measured in a typical time-of-flight experiment is the velocity component parallel to the applied electric field E, v II- The quantity defined by the ratio of cause (E) and response (v It), the "mobility in the given direction" 1-12], #11, is in general a certain linear combination of all 6 independent tensor components.) Time-of-flight pulses were picked up at a (selectable) resistor R, preamplified (either by a Tektronix P 170 CF cathode follower plus 1121 preamplifier, or by a Tektronix P 6201 FET probe plus Avantek two stage preamplifier, GPD461+462), displayed single shot on a fast real time oscilloscope (Tektronix, type 7104 with 7A29 and 7B 15 plug in units), and photographed for evaluation. Temperature-dependent measurements were performed in a He gas-flow cryostate (home-made) with the sample in vacuum (perylene), or in another He gas flow cryostate (Oxford Instruments, type CF 204), with the sample in He heat exchange gas (a situation which reduces sublimation losses of the volatile naphthalene crystals at the higher temperatures, but limits the useful upper electric fields by gas discharge). Experimental Results We wish to demonstrate two examples of the exciting low-temperature results which we were able to achieve Ultrapure, High Mobility Organic Photoconductors i I i i ill ~ , , ,i , i L i L i ill , , ,,i , 167 i I I I O 300 "" ~'~,A o[] ~176176176 r~ 2.0 "E z~ Ella /-,.2K o ~ EIIo o- --~ ~1~/~--$--~'-- 10.5F p+... I00 /~176176 I15oll 30 ~= 1.0 / .t, ~ ~//~/,~j . -1.40 "., n=-2.90 0 y ///~// t 5 1t0 --.-D,-- t 15 E [kV/cm'l Fig. 6. Electric field dependence of the hole drift velocities in 1 o E = 3kV/cm a E= 5kV/cm [] E = 10kV/crn o E = 12kV/cm 10 ~..~ " ~ ~. f N'~ \ 30 100 --mm,,=- T[K3 300 Fig. 5. Electron and hole mobilities in naphthalene for the electric field E parallel to the crystallographic a axis, for different field strengths (marked by differentsymbols)between 3 and 12kV/cm. Crystal thickness was 1010(10)~tm.The + symbols in the lower right comer represent previously available data 1-13] with the ultrapurified crystal material. In Fig. 5 mobilities of holes and electrons in the crystallographic a direction in naphthalene are plotted versus temperature in a log/log plot. In this type of plot a straight line indicates a #oc T" temperature dependence of the mobilities. It is seen that for the electron mobility we could establish a T - 1.4 dependence down to 27 K. The hole mobilities could be followed even down to 4.2 K. In this temperature region high mobilities are reached. The highest observed value is /~+=400 cmZ/Vs. To demonstrate the progress which the described ultrapurification of the material has allowed us to achieve, the best data o f # + available so far [13] are inserted as crosses and connected by a dashed line. We can now clearly attribute the decrease of the hole mobilities in these earlier measurements [13] to impurity trapping. In the region of high mobilities at low temperatures a remarkable new effect appears which has not been reported for any organic material before [14]: the mobilities become electric field-dependent, they decrease with rising field. We have represented in Fig. 5 mobilities for various electric field strengths. At each temperature the different symbols refer to different magnitudes of the electric field. To demonstrate this extraordinary behaviour more clearly, we have also naphthalene at three temperatures for Ella. The broken lines were drawn to connect the experimentalpoints and extrapolated to v~rlft~0 for E~0. They reflect a strongly sub-Ohmic behaviour. Low fieldmobilitieswould be represented by the lowfieldslope of these curves (ifthere were experimentallyaccessible points). Ohmic behaviour would be indicated by the tangential (full) lines plotted the original drift velocity data versus the electric field (Fig. 6). There the strong deviation from Ohmic behaviour is demonstrated most clearly. Already at 31 K the observed drift velocities fall well below the straight line through the origin, representing Ohm's law. Below about 10 K the drift velocities tend to saturate with increasing field. As a second example we will present similar observations which were made with the electron mobility in perylene. For the a direction an exact /zocT -1'78 dependence is obeyed down to about 40 K for low fields (Fig. 7). For very high fields a field dependence of the mobility can be observed already at about 100 K. The drift velocity versus field plot (Fig. 8) again demonstrates strong non-Ohmic behaviour. The strong non-Ohmic behaviour of the drift velocities in these two substances closely resembles the observations which were reported before for conventional inorganic semiconductors (such as silicon and germanium), and explained as hot carrier effects, cf. [-15, 163. Below 3 0 K the perylene electron mobilities of the example described (Fig. 7) begin to become influenced by residual shallow trap states (clearly visible in the figure only for the low electric field points obtained at E=6kV/cm). Such trap influence was found more pronounced in another crystal which was made from less purified perylene (for which the reverse horizontal zone refining step was left out). The low temperature behaviour in crystal samples of the two different perylene batches differed in that the electron mobilities in the less extensively purified material displayed a 168 W. Warta et al. I I I I I IIII ' I ' I '1'1 I .'. I I I III I I 9 ele~ 100 100 9 + 78 +++ --0 0 0 O0 ," ~ ~ - + /z~ 9 it + 1 t r O ~ o o + > -~ o e o + ~ ~ k o 9 E o o ,'4, 89 + r' i o+ + 10 _ + T ++ + 9 E = 6 kV/cm + E =20 kV/cm o E =60 kV/cm ++ \ ++ % \ ++ 10 o -- + \ oI % E = - +: i v, 1r---L--log I I I 10 I I IIII , 50 I , 200 I 27K o 35K § 50K 9 9 7OK 9 ~ 100K ~ I 60 o o o o 84 o o 1,0 @ Q @ o 0,5 ." ~ . , I , I 10 20 I 30 , I t.0 o' I o E= llkV/cm ', E = 13kV/cm +E= 16kV/cm o E= 22kV/cm o E= 27kV/cm I I I +% +++ \ \ \ IIIII , 50 I , - I,I,I 200 TEK] Fig. 9. Electron mobility as a function of temperature in perylene in an oblique crystallographic direction [~ E, a =45(1) ~ g E, b =66(1) ~ ~ E , c * = 5 5 ( 1 ) ~ sample thickness was 370(10)pm]. The broken line is a fit with the Hoesterey-Letson type shallow trapping model [3] with the parameters trap depth, Etr = 17.5 meV, and trap concentration, Ntr/Nb = 5 x 10 - 4 m o l / m o l I A o~ n o, 8 kV/cm ~--- o 1,5 p 10 TCK] I 1 - -t-tog I Fig. 7. Electron mobilities in perylene for the electric field E parallel to the crystallographic a direction for different field strengths (marked by different symbols) between 6 and 60 kV/cm. The crystal slice had a thickness of 252(3)Ixm > I 'l'l ~ /~ Ella ' n=-1,87 , I 50 , E [kV/cm] Fig. 8. Electric field dependence of the electron drift velocities in perylene at 5 temperatures for Ella maximum at somewhat higher temperature and a more pronounced relative decrease below the maximum. In a crystal slice of the less purified material (cut in an oblique crystallographic orientation) shallow trap-limited electron mobilities were followed down to 14 K (Fig. 9) in order to be able to try a fit by the Hoesterey-Letson multiple shallow trapping formula [3] for obtaining the trap parameters. In this model the carrier mobilities fall with decreasing temperature because the carriers stay for increasingly longer time intervals in the trap states before they are thermally reactivated to move freely in the band for a short while. These trap-influenced "effective mobilities" #elf are governed by the underlying microscopic (lattice) mobility #o(T), the density of trap states Ntr, relative to band states Nb, and a Boltzmann factor with the trap depth Etr: #af(T) = #o(T) [1 + (Nt~/Nb) exp (Et~/kT)] -1 (2) Before we apply this formula to interpret the results of Fig. 9, we wish to emphasize that between 40 and 300 K the perylene mobilites were reproducible within experimental error between crystals fro m different batches, which [besides the fact that the temperature dependence was found to obey a #o oc T n (n < 0) law] supports their interpretation as true lattice mobilities. This good reproducibility is also demonstrated by the fact that it was possible to closely fit the experimental data of 15 series of measurements in 10 different crystallographic directions by a (temperaturedependent) second rank tensor [17]. We find for the less purified crystal, by fitting [19] the Hoesterey-Letson equation (2) to the experimental points (Fig. 9) between 20 and 14 K, that there is only a very shallow trap left with a trap activation energy of Err = 17.5 meV and a concentration Ntr/N b = 5 x 10-4 mol/mol. A trap with these parameters can only exert notable influence on the (macroscopic) Ultrapure, High Mobility Organic Photoconductors I I I earlier measurement in the higher temperature range reported in the literature [18], which we have inserted into Fig. 10 as circles. These previously reported mobilities are very small and exhibit a thermally activated behaviour, i.e. decreasing values with decreasing temperature, opposite to the microscopic mobilities reported in this paper for the respective temperature range. They can be quantitatively interpreted by fitting the shallow trapping equation (2) to the experimental points (dashed line), with po(T) from our trap-free microscopic mobility data, and the trap parameters: N t r / N b = l . 7 x l O -3 mol/mol and Err = 270 meV. The span of orders of magnitude in the mobilities between impure and ultrapure crystal material which we demonstrate here for naphthalene and perylene, clearly indicates the high sensitivity of charge carrier mobility data against impurities and imperfections. I 100 n=-2,16 "...: r'-i ul EIIc' 10r k..I \ 1-- l / 0,1 - x / o d Conclusion 4 In summary, we reported on extensive purification procedures which allowed us to grow ultrapure organic molecular crystals. A characteristic feature of these ultrapure crystals are their relatively high, electric-field-dependent microscopic (lattice) mobilites at low temperatures. These experimental results not only close, in a sense, the "mobility gap" which existed so far between high-mobility inorganic and lowmobility organic semiconductors; they also demonstrate that charge carrier mobility measurements are a useful and sensitive tool for the characterization of high purity organic crystals. In this paper we have emphasized the latter aspect. The other aspects will be treated in more detail elsewhere. o'f o'I 0,01 ~ tog I 30 I I 100 3OO ~.,.- 169 I T rK] Fig. 10. Electron mobilities in a 306(3)I~m thick perylene crystal for the electric field E parallel to the crystallographic c* direction. The open circles in the lower right comer represent previously available mobility data [18]. The dashed line is a HoestereyLetson type shallow trapping model fit with the trap parameters Etr=270meV and Nt~/Nb=1.7 x 10-3 mol/mol, revealing that the purity of the crystals used in these older measurements was insufficient for obtaining the true (microscopic) lattice mobilities transport behaviour at the lowest temperatures (T < 40 K). The origin of this trap is unknown, but it is likely that it reflects residual chemical impurities, which act either directly, or indirectly by X-trap formation (perturbation of the energy levels of one or more neighbouring molecules). But we cannot exclude that the nominally less purified crystal contained accidentally more physical imperfections (lattice defects, such as vacancies or dislocations) and that these physical imperfections, and not residual chemical defects dominated in influencing transport at the lowest temperatures. H o w sensitive mobilities are to deeper traps (most probably caused by chemical impurities) can be demonstrated by comparing our results for the crystallographic c'-direction (i.e., perpendicular to the cleavage plane) in perylene, plotted in Fig. 10, with results of an Acknowledgements. We wish to express our gratitude to M. Gerdon, Chr. Herb, W. Tuffentsammer and G. Pampel for their great engagement in purification, crystal growth and purity control. We are also grateful to our colleagues at the mechanical workshop who, among other things, designed and built one of the He gas flow cryostates. The excimer laser was kindly placed at our disposal by Prof. M. Pilkuhn and his eoworkers. This work was supported by the "Stiftung Volkswagenwerk" (ultrapurification and characterization) and by the "Deutsche Forschungsgemeinschaft" (charge carrier transport). References 1. P. Kollat: Computer plot using the atomic coordinates of A. Camerman and J. Trotter: Proc. R. Soc. (London)279 A, 129 (1964) 2. R.G. Kepler: Phys. Rev. 119, 1226 (1960) 170 3. D.C. Hoesterey, G.M. Letson: J. Phys. Chem. Sol. 24, 1609 (1963) 4. N. Karl: In Festktrperprobleme/Advances Solid State Phys. 14, 261 (Vieweg, Braunschweig 1974) 5. L.B. Schein, D.W. Brown: Mol. Cryst. Liq. Cryst. 87, 1 (1981) 6. K.H. Probst, N. Karl: Phys. Stat. Solidi A27, 499 (1975) 7. L.B. Schein, A.R. McGhie: Phys. Rev. B20, 1631 (1979) 8. Z. Burshtein, D.F. Williams: Phys. Rev. B 15, 5769 (1977) 9. W. Tuffentsammer: Stuttgarter Kristallabor (unpublished) 10. N. Karl: High purity organic molecular crystals, in Crystals 4 (Springer, Berlin, Heidelberg, New York 1980) 11. N. Karl: J. Crystal Growth 51, 509 (1981) 12. J.F. Nye: Physical Properties of Crystals, Their Representation by Tensors and Matrices (Clarendon Press, Oxford 1972) 13. J. Berrthar, M. Schott: Mol. Cryst. Liq. Cryst. 46, 223 (1978) 14. Electric field-dependent electron mobilities #~. reported for anthracene at 140K by S.Nakano, Y. Maruyarna: Solid State Commun. 35, 671 (1980) could not be reproduced by L.B. Schein, R.S. Narang, R.W. Anderson, K.E. Meyer, A.R. McGhie: Chem. Phys. Lett. 100, 37 (1983) even not at still higher electric fields 15. K. Seeger: Semiconductor Physics, Springer Ser. Solid-State Phys. 40 (Springer, Berlin, Heidelberg 1982) W. Warta et al. 16. E.M. Conwell: In Solid State Physics, Suppl. 9 (Academic, New York 1967) 17. R. Stehle, N. Karl: To be published 18. Y. Maruyama, T. Kobayashi, H. Inokuchi, S. Iwashima: Mol. Cryst. Liq. Cryst. 20, 373 (1973) 19. For the fit an underlying microscopic mobility #o(T) was assumed which was obtained from an extrapolation of the T > 4 5 K mobilities using the (E-~0) tangent, n = - 1 . 8 7 , drawn in Fig. 9. It turns out that the trap activation energy obtained from the data between 14 and 20K is rather insensitive to the exact go(T) dependence. Even assuming a constant value, #o = 100 cm2/Vs, would cause only a minor change of the resulting trap depth (to 16.1 meV). Between these two extreme assumptions, the resulting trap concentration changes from 5 x 10 - 4 to 2 • 10- 4 mol/mol. - The discrepancy between the calculated and the measured mobility curves in the range between 20 and 45 K results from the fact that the measured mobifities in this high-mobility region are strongly dependent on the applied electric field strength. - For very shallow traps, as found here, the possibility of field-activated detrapping (Pool-Frenkel effect) can require a revision of the simple Hoesterey-Letson description. However, this effect can be estimated to decrease the thermal activation energy by only a few meV for the fields used here