Organic semiconductors

We may, however, look at this in another way, which is more revealing for our present purposes. During the. Volume 40, Number 10, October 1963 / 547 ...
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& California Association of Chemistry Teachers

Norman J. Justerl

University of California 10s Angeles

Organic Semiconductors

A n organic semiconductor may he defined as a structure containing an appreciable number of carbon-carbon bonds and which is capable of supporting electronic conduction in the manner of inorganic semiconductors (14). The history of the study of photoconduction in poorly-conductive organic compounds goes back to the turn of the century. Photoconductivity in solid anthracene was discovered by Pocchettino (32) in 1906. At this time attempts were made to relate the phenomenon to the external photoelectric effect, hut Volmer (59) showed t h a t photoconduction in anthracene sets in a t 4000 A (ca. 3 ev), whereas actual ejection of electrons is only observed below 2250 A (5.5 ev). The existence of a small hut measurable dark conduction in anthracene was reported, and it was noted that the conductivity in the molten state was greater than that in the solid state. After the appearance of a spate of papers on the subject in the years just before 1914 (15), interest in organic semiconductors seems temporarily to have lapsed. I n the years between the wars, a certain amount of work was done on photoconduction in organic dyestuffs, with particular reference to the part played by these in the "sensitization" of photographic emulsions (37). In 1941 Szent-Gyorgi suggested that the transfer of pi-electrons from molecule to molecule might play an important role in the fundamental physical processes of living organisms and this provided considerable stimulus in the study of organic semiconductors (36). Szent-Gyorgi later reported the existence of photoconduction in certain dyed proteins as evidence supporting his suggestion (44). I n succeeding years there has appeared a considerable number of papers on the subjects of conduction and photoconduction in organic materials. In 1948 it was reported (15) that the conductivity of the phthalocyanines is an exPresented in part at the Fourth Summer CACT Conference, Aailorntu, California, August, 1962. The previous two papers on this topic (Electronic Interpretations of Physical Properties) 39,596 (1962) and 40,489 (1963). appeared in THIS JOURNAL, Visiting Professor, 1963, on leave from Pasadena City College.

ponential function of the temperature, as is the case for many inorganic semiconductors. More recently the use of anthracene for crystal counters in nuclear physics has promoted the study, first, of photoconduction and then of dark conduction in this material. During this time, the possible biophysical implications of organic semiconduction seem constantly to have been in men's minds (48). The theories of photosynthesis proposed by Calvin and his co-workers appeal to "photoconduction" in the sense that light has to be absorbed at one point, a t which it is used to promote some oxidation-reduction reaction. Indeed, photoconduction has been detected in chlorophyll in vitro. At any rate, as a result of an extensive series of investigations of a large number of materials, it is now clear that semiconduction (in the sense of a low conductivity which increases with increasing temperature) and photoconduction are common, if not perhaps universal, attributes of organic materials containing closed rings of alternating double and single bonds (delocalized electrons, conjugation), and of some other materials of less specific structure (such as proteins and technical polymers) as well. Nature of Organic Semiconduction

Let us state exactly what phenomena are in question. We are concerned with charge transfer in a bulk sample-preferably a single crystal-whether stimulated thermally or optically. For such transfer to occur, electrons (or possibly protons) must actually move from molecule to molecule-not just around a closed ring or down a long molecular chain. Unfortunately this distinction is seldom made clearly in the literature (I@, and so a few comments may be in order. The pielectrons in benzene may he considered, in the zeroth approximation that ignores exchange effects, to form a half-filled zone. If a steady magnetic field is applied in a direction perpendicular to the ring, the electronic states will shift in such a way as to give rise to a net diamagnetic moment associated with the ring. We may, however, look a t this in another way, which is more revealing for our present purposes. During the Volume 40, Number 10, October 1963

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process of application of the magnetic field, an emf is induced in the ring. In the presence of the field there is a net component of "crystal monentum," p, clockwise or counterclockwise as the case may be-that is to say a net current flowing around the ring. The electrons of positive p have slightly higher energy than those of negative p, but, since the separations between the individual electronic levels are quite large (several ev), there are no immediately accessible states into which the electrons of positive p may go. In other words, there is no dissipative mechanism; and, from the point of view of electron flow around the ring, benzene is not a semiconductor, but a superconductor. On this basis it is possible quantitatively to account for the observed diamagnetic anisotropy in benzene, and this has been extended to the higher conjugated aromatic hydrocarbons. The reason why many of these materials in fact show semicrmducting properties in the crystalline state cannot, therefore, be sought within the molecule itself. The "slow step" in the conduction process has to be the actual transfer of electrons from molecule to molecule; and this is a problem for which as yet no adequate theoretical treatment exists, although a modified electronic-band model has been successful in explaining some experimental transport properties, as will be shown below. A macromolecular material for which a single molecular orbital wave-function may be written for an entire crystal or crystal domain has seemed a promising approach to bulk charge transfer, therefore, in organic compounds (45). The major goal of present research programs is to understand the mechanisms of electronic conductivity in such materials and to relate these mechanisms to the physical-chemical structure of the solid. While the present state of this understanding may be compared to that of inorganic semiconductors of about 35 years ago (do), the intriguing possibility of "tailoring" the electrical properties through organic chemistry is reason enough for substantial research activity. I t is possible, currently, to develop a wide range of mechanical properties in organic solids through preparation of specific molecular structures. If this same variety can be extended to electrical parameters, the scope of semiconductor applications is likely to be expanded far heyond that made available by use of the two commercially important inorganic semiconductors, silicon and germanium, and of the inorganic semiconducting compounds like gallium arsenide. Various organic materials display feeble electrical conductivity that increases with increasing temperature, and may also demonstrate photoconductivity. Semiconducting organic solids fall ulto two major categories: well-defined substances (molecular crystals and crystalline complexes, isotactic and syndiotactic polymers, etc.) and disordered materials (atactic polymers and pyrolytic materials). Molecular Crystals

The classical example cf a molecular crystal is anthracene, which is probably the most extensively studied organic semiconductor. There is now experimental evidence that some of the transport properties of anthracene are not unlike those familiar from studies on silicon and germanium. Hole and electron mobilities of the order of 1 cm2/v-sec can be measured in zone548

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refmed single crystals. These drift-mobility experiments employ photoconductivity, since the crystal is a poor conductor in the dark. There is also experimental evidence that carriers are produced only on the surfaces or a t electrodes by excitation waves (or "excitons") generated a t the point of photon absorption. These experimental transport properties can be explained on the basis of a modified electronic-band model of solids. Calculation of the width of the energy bands proceeds by computing overlap integrals between pielectron molecular wave functions on adjacent molecules. Because the intermolecular binding is weak in molecular crystals, the overlap is small and leads to narrow energy bands, which implies small carrier mobilities. The calculation is also successful in accounting for observed mobility-anisotropies. This demonstration of the success of a band model in explaining transport properties in organic semiconductors is comforting in that many of the concepts developed for inorganic materials are immediately applicable. Less comforting is the equally important conclusion that in order to observe the basic properties of the customary "smalldomain" organic crystals, purification techniques as extensive as those applied to inorganic materials must he applied. Nevertheless one may still hope that useful semiconducting properties might be discovered among "large-domain" organic semiconductors without such careful processing. Zone-refined anthracene single crystals are nearly free from space charge effects due to carrier trapping and furthermore can show electrunic (n-type) conductivity. Only hole-conduction and many trapping effects were always observed in less pure crystals and have contributed to confusion in experimental results in the past. Mechanisms for Semiconduction

The exciton states of the simpler aromatic solids have been examined (8, 9, 11, 12, 15, 28) in the quest to understand organic semiconduction. I n these ccmpounds the absorption spectra of the crystals and of the corresponding free molecules are very similar. This suggests that the final state of an optical transition in the solid is essentially a molecular excited state. However, there are features of the crystal spectra which can he explained qualitatively and quantitatively if cne assumes that the molecular excitation is not confined to one molecule but is coherently shared by all molecules of a perfect crystal. In such a state-an excitou-the excitatiou energy and crystal momentum is definite but the position of the exciton is completely uncertain; i.e., it may he considered delocalized. In a classical description of an exciton in a molecular crystal, one may consider the excitation to be mobile, with the electrons involved in the excited state of a particular molecule remaining behind as the excitation passes. In contrast, the classical picture of the exciton in a conventional semiconductor (germanium, silicon, etc.) is that of an electron and hole hound together to form a hydrogen-like atom which moves freely through the crystal carrying along the excitation energy (41). In a more general formulation (IS) of the exciton problem, some of the kinetic energy terms are matrix elements in which there is a transfer between the initial and the final state, of both the excitation and the charges involved in this excitation from one molecule to the next.

Such charge exchange cannot be important if the electronic wave functions of neighboring atoms or molecules are isolated from one another, as is the case in crystals like anthracene. On the other hand, where the overlap is large, the charge exchange is very important, but then the use of atomic or molecular orbitals in building crystal wave functions becomes inaccurate. In such cases one may reformulate the problem, obtaining the well-known model of a bound electron-hole pair moving through the conventional semiconductor (41). One may also transfer excitation primarily through an electric interaction between the transition dipole moments of the molecules. These transition moments are the same as the matrix elements involved in optical absorption, so the interaction is large and the effective mass of the exciton is thereby small if the absorption is strong. The classical model (41) is most accurate where the binding between the electron and hole is weak, so that the exciton radius is large. With the electron far from the hole, the optical transition dipole moment will be small so that in crystals like germanium this second mechanism for motion of the excitation may he neglected. On the other hand, aromatic molecules generally have fairly strong optical absorptions, so the second mechanism is important; excitons corresponding to the second anthracene transition have a range of kinetic energies of the order of 1 ev, as have been found experimentally (see above). A model, similar to the one-electron hand model classic to inorganic semiconductors (19), may be applied to the perfect aromatic molecular crystal (35). The free electron will spend most of its time in the neighborhood of one molecule or another, and very little time in the interstitial regions; its wave function will be a linear combination of excited molecular orbitals. Because these orbitals are almost completely isclated from one another, the effective mass of the charge carrier will he large. Free hole states can be treated in a similar fashion. In the case of the excitons, the strong excitation transfer mechanism in aromatic crystals makes up for the weak pair transfer, so that the exciton effective mass may still be small. KOsuch alternative mechanism is known for the conducting states. With a large effectivecarrier mass, the range of kinetic energies will be of the order of, or much smaller than, IcT at room temperature. These kinetic energies may then be smaller than the perturbation energies, so that the model of a free carrier occasionally scattered by imperfections is not valid. With a sufficiently large effective mass in the perfect crystal, a better picture is provided by the "hopping model," in which one assumes that the carrier spends a long time on some molecule; the perturb:~tions arising from the presence of other molecules cause an occasional jump to a neighbor. For intermediate values of the effective mass, it is possible that neither the hopping nor the conventional band picture is valid. TO find the excitation energy for the lowest conducting state in the hopping model, one may start with a neutral crystal, remove an electron from one molecule, and place it on another a great distance away (11, 28). Considering this charge transfer to take place in several steps, we first remove the electron from its original molecule to a point far from the crystal; this requires the addition of energy I, the mclecular

ionization potential. Bringing the electron back to the crystal and placing it on the second molecule regains the energy A, the molecular electron affinity. We now have a widely separated ion pair, and each ion polarizes the surrounding molecules. The energy, W , of interaction of each ion with the polarization is the same for the two ions, approximately. The excitation energy is then I - A - 2W. There are other minor terms, of course. Either the electron on the negative ion or the hole left behind on the positive ion may now carry current, by hopping to adjacent molecules. The two charges are sufficiently far apart that they act independently of one another. Where the optical transition moments for molecular crystals are very small, therefore, one should use a hopping model for the excitation, especially for excitations to the molecular triplet state where the optical transition is highly forbidden. The hopping model is also applicable to such excitations as the 0-0 component of the first benzene transition, except possihly in a good crystal a t very low temperatures (a forbidden transition in the'free molecule, but weakly allowed in the crystal). Another excitation in which the hopping model is useful is one in which an electron is transferred to an adjacent molecule, leaving a hole behind (28). The attraction between the two charges prevents further separation, if no additional energy is supplied. To move this excitation through the crystal, one must also transfer the charge pair. The matrix element for this process is extremely small, and the hopping model should be used, probably even a t low temperatures (29). I n each of several aromatic crystals, the observed value of the activation energy for dark conductivity, E, turned out to be 1/2Et,where E , is the energy of the triplet state of the isolated molecule, measured from the ground state, and E is defined by the first of the following relations:

with T the absolute temperature, k the Boltzmann constant, rr the dark conductivity, and c, a constant. The quantity E', introduced to simplify later discussion, is defined as 2E. The similarity between the last member of the above relations and the expression for the dark conductivity of a conventional intrinsic semiconductor (19) led some investigators to assume that E' was the band gap in aromatic crystals. The observed equality between E' and E , led to the further assumption that the molecular triplet state was involved in the conduction. Since this state is nonconducting, the involvement must be indirect. The validity of the first assumption depends on whether the conductivity is intrinsic; this point has been dealt with by Garrett (16); others have shown that the triplet state actually can play no role in organic semiconduction (11, 12). I n thermal equilibrium, the population of each conducting state depends on the energy of that level, but not on the mechanisms of transitions between states. The observed value of E' therefore cannot be explained in terms of the excitation of one of the nonconducting states (in particular, the triplet state of a molecule), followed by a transition to the conducting state. If the equality of E' and E, is not a coincidence, it must arise from a relationship between the structures of the triplet and conducting states. It has been suggested Volume 40, Number 10, October 1963

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that it takes the same energy to raise an electron from the highest filled to the lowest unfilled molecular orbital (with the final spin state a triplet) as it does to place that electron in the same unoccupied state of a distant molecule. When the latter transfer is effected, either the electron or the hole left behind on the first molecule is free to move from one neighbor to another. The population of charge carriers would then have the temperature dependence indicated in the relation above, with the "band gap" E' numerically equal to E,. One might, as an approximation, use the same molecular wave functions, in the two cases, for the state which the electron enters. But even if the wave functions were identical, the two excitation energies would not be the same. For spinless electrons, the transfer of an electron to a distant molecule involves, among other energy terms, the Coulomb energy of separation of the electron from the positive charge left behind. This contribution, which is not negligible, does not appear in the energy of the triplet state. Second, when the spin is taken into account, we note that each of the two ions produced by the charge transfer is in a doublet state; the wide separation of the ions prevents these states from interacting to form a singlet and a triplet. Since the singlet and triplet molecular states differ greatly in energy, there is no reason to expect equality between the energy of the molecular triplet and that of the two ionic doublets. Next, let us assume that for some unknown reason, each set of measurements was carried out on a sample in which there was an equilibrium number of triplet excitations but an abnormally low number of charge carriers. For such a nonequilibrium case, one can set up a rather artificial model in which the current is indirectly determined by the population of the triplet states. This number, however, varies with temperature as e-B1'nT, not as e-EL'Zkz. If, for this or any other reason, the current were proportional to the triplet population, one should find E, and not E', equal to Et (in anthracene at room temperature the factor is so small that there is a negligible probability of finding even a single molecule in the triplet state); if even the triplet states are not in equilibrium, there should be n o relationship between E' and El. It should also be noted that the factor of in the exponent in the case of an intrinsic semiconductor appears because the possible distributions of electrons in the conduction band are independent of the distributions of holes in the valence band. This factor appears also in the case of an equilibrium distribution of ion pairs. However, in molecular excitations, once one determines a distribution of electrons among the excited states of the various molecules, the distribution of "holes" in the ground states is completely determined. I n such a case, the factor of 1/2 does not appear. These conclusions are based on some rather general statistical arguments, not restricted to solids. It is required only that the Fermi level (46) he far, compared to kT, from any of the o n e electron states. The spacial distribution of trapped charge in anthracene has been subjected to analysis (81, SS), and evidence has been presented that indicates that most of the free electrons are injected into the anthracene at the negative electrode and that anthracene is an extrinsic rather than intrinsic photoconductor. If correct, electron-injection is an exponential function of 550

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voltage, which indicates that the anthracene-metal contact may form a p-n junction (19). ChargbTransfer Complexes

A charge-transfer complex is an intermolecular addition compound, composed of an electron-donor (e.g., an aromatic hydrocarbon) and an electron-acceptor (e.g., a halogen). I n passing from anthracene to the chargetransfer complexes, we go from one extreme in which the optical evidence for delocalized charge is limited or absent to the other in which electronic delocalization effects are so strong as to overwhelm the optical properties associated with the private molecular energy levels. The metastable (Per~lene)&)~and (Pyrene)(I,), complexes (84) and the unstable bromine-perylene complex (5) show relatively high conductivities (0.01 to 1 mho/cm a t room temperature); and tbey are optically opaque in thicknesses (down) to 1000 A in the near-infrared, visible, and near-ultraviolet. The activation energy for conduction of these complexes is fairly low (0.14 to 0.01 ev). Lahes (26, 87) showed that the unpaired-spin concentration in these complexes varied and that, on changing the temfrom loT8to 10'9 perature, the density of unpaired spins changed in the same way as did the conductivity (below 200°K, where the activation energy for conduction in pyrene became altered, the unpaired-spin concentration became temperature-independent). This suggests, as with anthracene, that the major part of the temperature-dependence of the conductivity comes from a change in the carrier emcentration rather than from any change in mobility. The absolute values for mobility demanded by a comparison of conductivity and spin-resonance data are in the vicinity of 10W2 em2 volt-' sec-'. Therefore, in these materials, the carrier density at room temperature must be of the order of 1014to 1015times that prevailing in anthracene, each carrier having, however, a mobility of the order of 100 times less. Quite large conductivities, as high as 100 mho/cm, can be developed in other molecular crystals in which molecular complexes consisting of chemical groups which are electron donors are combined with groups which act as electron acceptors. The electron exchange between such groups has the effect of producing many nearly free carriers and yielding high conductivity even with very small carrier mobilities (usually less than 10W2 cm2 v-sec). The electrical properties of some molecular complexes are nearly metallic-like in many respects, showing degeneracy effects and small or negligible activation energies for conductivity. There is some preliminary evidence that the band model is applicable here also and that again the crucial point is the overlap of electronic wave functions between molecules. The other features of the baud structure--e.g., the width of the forbidden gap-are related to the energy levels of the isolated molecule, although intermolecular interactions may not be neglected. It is interesting to note the analogy between the donor-acceptor action of chemical groups in the molecular complexes and the behavior of impurity atoms in germanium and silicon. Recently an interesting new set of compounds having higher electronic conductivity than any others reported has been announced (1, 2, 4, 6, 7, 16, 17, 25, SO, 43) by workers a t

DuPont. These are charge-transfer complex salts of the radical anion of tetracyanoquinodimethane,

and have the generalized structure M+(TCNQ7) (TCNQ), where TCNQ is tetracyanoquinodimethane and TCNQ7 is the radical anion

while M + is a cation, usually organic, such as the triethylammonium ion. In a few cases, compounds lacking neutral TCNQ have been formed; for example, 5,8-dihydroxyquinoline with TCNQ gave the anionradical salt, CsH7N02H+TCNQ7, resistivity (powder) = 14 ohm-cm However, quinoline with TCXQ gave CsHrNH+(TCNQ-) (TCKQ), resistivity (single crystal) equals 0.01 ohm-cm. These complexes have an electron density of one free electron per molecule, and a resistivity of the order 0.01-100 ohm-cm, depending on direction (the electrical resistivity is highly anisotropic along the three principal crystal axes). For comparison, the resistivity of hyperpure siliccn is ohm-cm and about 1000 ohm-cm, graphite about most organic compounds 10'0-10'4 ohm-cm. (The lowest resistivities previously reported for organic compounds were those for the metastable halogenperylene complexes (5, 24).) The electron mobilitv seems to be very low, although so far only incomplete data (5) have been published. The thermoelectric power of about -100pV/'C is characteristic of a good metallic conductor. A preliminary theoretical interpretation of these phenomena has been developed (6-7). In both types of TCNQ salts the band formed from the lowest normally unfilled MO of TCNQ is partially filled, being half filled for the first series of salts (M+TCYQ7) and 1/4 filled for the second. As a result of the relatively weak intermolecular interactions characteristic of mclecular crystals, this band will be quite narrow. In addition, both the bandwidth and the exchange coupling of the odd electrons (6, 7) appear to be strongly influenced by the nature of the cation, M+, and by whether or not TCNQo is present in the crystal. I n the quinolinium salt (see Table I), the electrons of the (TCNQ') radicals appear to form a conventional degenerate system similar to a metal. This is evidenced by the very large electrical conductivity, the virtual absence of activation energy for conductivity, and a temperature-independent paramagnetism. The activation energies of the other salts are also low and are isotropic within experimental error although the electrical conductivities are highly anisotropic (the triethylammonium salt has room-temperature values of 4.0, 0.05, and 0.001 mho/cm in the three principal crystal directions, and these varyexponentiallywith temperature with an activation energy of 0.14 ev). Preliminary crystallographic data show that this salt he-

longs to the monoclinic C2 space group with four TCNQ formula units per unit cell. The TCNQ units appear to be arranged in infinite face-to-face stacks with the direction of highest conductivity lying along these stacks and approximately normal to the planes of the TCKQ molecules. The thermoelectric power (see above) is in the direction of highest conductivity and this indicates that electrons are the majority carriers in this materials. No Hall voltage has been detected in this salt and this, together with the observed conductivity and the assumption that all of the electrons in the radical-anions participate in carrying the current, leads to the conclusion that the electron mobility in it lies between 0.02 and 0.04 cm2/v-sec. It is possible that charge carrier motion in these materials takes place by an activated hopping process, in which case it is questionable whether a Hall effect should be observed at all, but I feel this interpretation is not as likely. TCKQ is a strong pi-acceptor. ESR studies (6, 7) show a spin correlation between magnetic electrons in these salts, giving rise to a ground singlet state and a thermally accessible triplet state. Typical S-T separations (J)found are: 0.034 ev for the triethylammoninm salt, 0.062 ev for the triphenyl phosphonium and the triphenylarsenium salts, and 0.41 ev for the morpholiniiim salt. The importance of paramagnetic impurities in studies of the magnetic properties of organic solids must be emphasized. A per cent or so of doubletstate impurity species can dominate resonances near g = 2.002 in molecular solids obeying S-T (81- Sa) statistics. (For a strong donor and strong acceptor, as here, complete electron transfer could occur and the system could be expected to exhibit paramagnetic characteristics in the ground electronic state. In the solid, interactions are more extensive than in solution (single molecules in solution) and than paimise and could provide an electron-conduction mechanism.) In these salts, M + is diamagnetic and TCKQ- or (TCNQ)%-are paramagnetic, and EPR characteristics are attributable to the presence of entities residing in the triplet rather than doublet electronic states. The best model to date (6, 7) shows hl+ forming insulating layers separating adjacent two-dimensional layers of the stacked TCKQ's (face-to-face stack). Formally there are 2 "molecules" per unit cell, i.e., 2 hI+, 2 TCKQand 2 TCNQo. Evidence has been presented foi. a model in which triplet electrons exist in an extended pisystem which allows effective distance of separation of electrons larger than for naphthalene and indicates Toble 1.

Electronic Properties of Some Rodical Anion Salts

Material

a'

(C?H,),NH(TCNQ.]* (TCNQ) IIm[TCNQ. le a

Electrical conductivity, mbo/cm at 300° I