Trapping effects on emission-limited current flow in narrow-band insulators

Abstract

The effect of charge carrier traps on emission-limited injected currents in narrow-band insulators is studied. At low voItages the physical mechanism responsible for the current is the trapping of injected carriers in the crystal image force potential we11 between the emitter and the potential maximum; the Schottky emission is the dominant conduction mechanism at high voltages. A theoretical model is presented and the voltage dependence of currents is derived including the effects of trapping and barrier lowering. It IS shown, in agreement with the model, that in injection experiments on anthracene and tetracene crystals the voltage ( U ) dependence of current (j) can be described by j mj,, exp (aUl/zb U -I P ) , where a and b are constants characterizing the crystals. The constant b depecdent on the trap concentration, determines a t what voltage the conductivity changes from trapping to Schottky barrier lowering mechanism. The hole diffusion length in five different anthracene and tetracene rrystals is found to be within the range 19 t o 310 A and the trap concentration in the vicinity of the injection contact, within the range 6 x x 1017 to 9 X lozo I / I C C J I e~O B a H H O BJIIIRIIHe JIOByUleK HOCIITeJIefi 3apHna H a 3~11CCIIOHHO-OrpaHII~IeHHL1,Ie IIH-il~eKUHOHIIbIe TOKII B Y3KO-30HHbIX II3OJlFITOpaX. B 06naCTII HLl3KHX IlaIIpHXeHHfi @II3Ll-9eCKIIM MeXaHH3MOM OTBeTCTBeHHbIM 38 TOR RBJIHeTCFI 3aXBaYLlBaHLle IIHmeKTHpOBaHmix Hocmenefi .repea JIOB~IIIKII, HaxonHUaecfi B o 6 n a c~~ KpbIcTanna, onpenenemoii RMOB ~~~K T~O C T~T I I~~C K O~O a e p~a n b a o r o noTeHr[nana a a~n~o s e~~o f i 3MMTePOM II MaH-CMMYMOM IlOTeHUIIaJla; B 06nac~m BbICOKLlX llarIpa~€!HII~ AOMIIHHPYIOUIIM MeXaHH3MOhf

Composite polypyrrole-containing particles and electrical properties of thin films prepared therefrom

“…When the voltage applied to the electrodes increases, x m moves towards the injecting electrode and f m decreases [46,47]. As the voltage increases further, the barrier height can no longer be lowered by the removal of opposite charge from the injecting electrode and the barrier lowering takes place by the interaction of the image force with the applied field (Schottky effect, [48]). The current becomes contact-or emission-limited (ELC) rather than bulk limited.…”

Space-charge-limited currents in organic films: Some open problems

“…The theory of contact-limited j ∼ U characteristics was elaborated by Godlewski and Kalinowski [58] who considered a potential barrier formed by a superposition of the image Coulombic, space-charge and external potentials in a sample containing traps. According to the model, the current is controlled by two processes: a lowering of the Schottky barrier at higher voltages and the trap control of the number of carriers arriving (at low voltages) at the interface barrier with the energy sufficient to cross it…”

Currents and photocurrents in organic materials determined by the interface phenomena

“…Probabilities of charge carrier transportation over barrier are limited by near-electrode work function (exp(À //kT)), lowering of the barrier by external electric field strength (exp(aF 0 1/2 )) and probability of reaching the maximum of the barrier without trapping (exp(À x m /l D )). The above limitations of the current flow at considered conditions can be described as follows [40]:…”

“…Generally, its extent does not exceed a few micrometers and it is followed from the nature of coulombic interaction, especially generated by image forces (up to about 1 Am), diffusion length of free charge carrier (Schubweg [8,40] and Debye length [13,36]), diffusion length of singlet and triplet excitons (from part of micrometer for singlet excitons to a few micrometers for triplet excitons [1,3,37 -39]), penetration depth of strongly absorbed light (starting from a part of micrometer) and other phenomena following these processes. It is worth to point out that the near-surface interaction between organic material and substrate can change structure of organic materials in both mentioned regions [30,41 -48,61,85 -94].…”

“…Current -voltage characteristics described by Eq. (27) have been observed in many organic materials [40,53,64 -67], such as anthracene [40,53], polyethylene (PET) [64,65], 8-hydroxyquinoline aluminium complex (Alq 3 ), aromatic diamine (TPD), quinacridone (QAC), perylene bismide pigment (PBP) [66], phthalocynines [67] and polysilanes [87].…”