J. Phys. Chem. 1083, 8 7 , 552-557
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Absorption Spectroscopy of Amorphous Tetracene R. Jankowiak,+ K. D. Rockwltr, and H. Bassler' Fachbereich Physlkalische Chemle der Phillpps-Unlverslt 0-3550 Marburg, West Qermny (Recelveo: Ju& 19, 1982; I n Flnal Form: September 14, 1982)
Absorption spectra of amorphous tetracene (TC)filmsprepared by vapor condensation at temperatures between -4.2 and 90 K have been measured as a function of both formation and annealing temperature. They can be deconvoluted into a series of Gaussian profiles whose width decreases with increasing temperature. The inhomogeneous width of the lowest So SI0-0 band can quantitatively be explained in terms of random fluctuations of the van der Waals energy due to positional fluctuations of the molecules recently determined by electron diffraction studies. Appearance of site splitting analogous to the crystal Davydov splitting demonstrates persistence of crystallike short-range order even in highly disordered structures. Since diagonal static disorder splitting is comparable to the crystal exciton bandwidth, the Klafter-Jortner (KJ) theory cannot be applied to analyze the exciton-vibron excitation bands.
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Introduction The prospect of using noncrystalline organic solids in electronic or photoelectronic devices has a considerable impact on basic research aimed at elucidating their electronic transport properties. Experimental evidence has been accumulated that the key feature of prototypical disordered organic solids, such as vapor-deposited members of the acene series as well as polymeric materials, is presence of diagonal disorder.' It derive from static positional fluctuations of the molecules constituting the solid. One consequence is inhomogeneous broadening of the sharp absorption lines commonly observed with the crystalline counterpart structures. Provided that disorder broadening exceeds the width of the exziton bands in the perfect crystal, the restrictions of the k-selection rule on optical transitions in the solid are suspended and the inhomogeneous band profiles map the density-of-stateprofile in the disordered material. Recent absorption spectroscopy on amorphous tetracene and pentacene films2i3which were laid down onto substrates cooled to temperatures as low as 80 K indicate the existence of Gaussian density-of-state profiles for the lowest excited singlet states of the bulk material. On the low-energy side a structural defect present at a concentration on the percent level showed up. The defect band also exhibited Gaussian line shape of comparable width. This behavior sharply contrasts with amorphous inorganic solids, notably a-Si, where the density-of-states profiles in the forbidden gap display exponential tailse4 Since electronic transport in disordered systems is largely determined by tail state^,^ the shape of the distribution-of-states profile is of crucial importance for charge as well as exciton transport. Recent computer simulations have indicated that an exponential distribution of hopping states leads to dispersive charge carrier transport on a macroscopic time scale.6 The more rapid dilution of the density of states at the low-energy tail of a Gaussian distribution, on the other hand, has been shown to lead to dynamic energy equilibration once a hopping particle has visited only about 100 new sites.' As a consequence, transport becomes nondispersive shortly after a charge carrier has been started. In the Gaussian transport regime the carrier mobility is predicted to follow an exp[-(T,/ law. Verification of this temperature dependence for a molecularly doped polymer provided
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'On leave of absence from the Institute of Physics, Technical University of Gdansk, Poland.
strong argument for the density of hole transporting states in that particular system being in fact of the Gaussian type.s Apart from the need to obtain more direct experimental information on the density-of-states distribution in prototypical amorphous organic solids and, in particular, on its sensitivity to sample preparation, the inducement for performing the present study came from recent experimental as well as theoretical work on disordered organic single-component solids. Electron diffraction studies of Eiermann et al.9 on tetracene and pentacene films grown in situ at temperatures as low as 20 K confirmed the earlier notion that these systems are amorphous in the sence that long-range order is absent but short-range order derived from the molecular packing in the single-crystallinephase does exist. Quantitative analysis of the diffractograms allowed deducing the dependence of the relative degree of disorder on parameters governing film growth, in particular deposition temperature and rate, and making an estimate regarding ita absolute magnitude. To substantiate the earlier hypothesis that it is in fact the fluctuation of the molecular positions that determines the inhomogeneous broadening of the absorption bands, an elaboration of earlier spectroscopic work2to systems formed under the same conditions seemed desirable. On the theoretical side, Klafter and Jortner'O presented a theory of absorption band profiles in the spectral regime corresponding to simultaneous excitation of the first molecular singlet state and a molecular vibration. Their formalism distinguishes between excitation to either two-particle exciton-vibron band states or exciton-vibron continua on the basis of profile analyses and predicts specific disorder effects. As suggested by Klafter and Jortner,ll analysis of the vibronic (1)H. Bbsler, Phys. Status Solidi B , 107,9 (1981). (2)R. Hesse, W.Hofberger, and H. Biissler, Chem. Phys., 49, 201 11 wn\ ~-"--,. (3)K. 0.Lee and T. T. Can, Chem. Phys. Lett., 61,120 (1977). (4)N. F. Mott and E. A. Davies in "Electronic Processes in NonCrystalline Materials",2nd ed., Clarendon Press, Oxford, 1979. (5) See, e.g., T. Tiedje and A. Rose, Solid State Commun., 37, 49 (1980). (6)M. Silver, G.Schonherr, and H. Biissler,Phys. Reu. Lett., 48,352 (1982). (7)H. Biissler, G.SchBnherr, and M. Silver, Philos. Mag., [Part] E , 44., 369 ~-~ (19811. .----,(8)H. BLsler, G. Schonherr, M. Abkovitz, and D. Pai, Phys. Rev. E , 26, 3105 (1982). (9)R. Eiermann, G.M. Parkinson, H. Bbsler, and J. M. Thomas, J. Phys. Chem., preceding paper in this issue. (10)J. Klafter and J. Jortner, Chem. Phys., 47,25 (1980). (11)J. Klafter and J. Jortner, J . Chem. Phys., 77,2812,2816 (1982). ~~
0022-3654/83/2087-0552$01.50/00 1983 American Chemical Society
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Absorption Spectroscopy of Amorphous Tetracene
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983 553
satellites of the So SI absorption spectra of amorphous organic solids with variable degrees of disorder appears to be a straightforward tool to test applicability of their theory. This paper reports absorption spectra of the lowest singlet transition in amorphous tetracene and pentacene layers prepared by vapor deposition at temperatures 2 6 K. The results confirm the Gaussian density-of-states profile. The degree of disorder expressed in terms of the Gaussian width is in quantitative agreement with the prediction of recent structural work.g Persistence of the short-range order down to the lowest formation temperatures is documented by observation of the Davydov splitting. It will further be shown that the degree of disorder is too large for application of the Klafter-Jortner theory'" of two-particle excitations.
Experimental Section Layers of resublimed tetracene were formed by vapor deposition onto a glass substrate kept in thermal contact with a cold stage (Cryo-tip Model WMX-1) attached to the cold finger of a helium flow cryostat (Heli-Tran LT3-110 system). The temperature difference between the crystal cold finger and the center of the glass support was monitored by attaching a thermocouple to the center of the latter in the absence of a sample. It turned out to be
