Ultrapure, high mobility organic photoconductors

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168 W. Warta et al. 100 10 1r-- --0 0 0 O0 O I I I I I IIII ' I ' I '1'1 9 ele~ 9 78 + +++ + 9 + ~ o o + Ella o -~ e 9 9 E = 6 kV/cm + E =20 kV/cm o E =60 kV/cm --L--log I I I 10 o + o o+ + _ + ++ + ++ \ ++ % \ I I IIII , I , I 50 200 TCK] 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 > 1,5 1,0 0,5 ~ ." . , I , I 10 20 o n o 84 o o Q I I @ @ o o o o I A 27K ~ o 35K o § 50K 9 9 7OK 9 ~ 100K I , I , I ~ I , 30 t.0 50 60 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. > E ,'4, 89 100 - T 10 o -- o I - +: i v, 1 - p o, o' -t-tog 10 i I .'. I I I I III ," ~ ~ n=-1,87 /z~ ~ it o 1 t r + /~ ~ ~ k o r' ++ + \ % E = 8 kV/cm + % I ' I 'l'l o E= llkV/cm + ++ ', E= 13kV/cm \ +E= 16kV/cm \ - \ o E= 22kV/cm o E= 27kV/cm I I I I IIIII , I , I,I,I 50 200 ~--- 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*=55(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/N b = 5 x 10 -4 mol/mol 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 (temperature- dependent) 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 169 r'-i ul r k..I 100 - 10- 1-- 0,1 - l 0,01 ~ tog I I I I n=-2,16 I I 30 100 "...: EIIc' \ / o d 4 o' f o' I I 3OO ~.,.- T rK] / x 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 Hoesterey- Letson 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. How 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 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 inter- preted 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: Ntr/Nb=l.7xlO -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. Conclusion 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 low- mobility 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. 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" (ultrapurific- ation and characterization) and by the "Deutsche Forschungs- gemeinschaft" (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)
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