From Intrinsic Bipolar Transport to the Abnormal Curves of Mobility–E^1/2 in the Common Hole-Transporting Materials

Abstract

Carrier transport in organic semiconductors (OSCs) plays an essential role in device performance. OSCs are generally divided into hole-transporting (p-type) and electron-transporting (n-type) materials. The holes should transport in the HOMO energy level, and electrons should transport in the LUMO. Such simple model analysis would easily lead to the primary theoretical result that intrinsic bipolar transport should be a basic characteristic of OSCs. Five widely used hole-transporting materials were utilized for fabricating the corresponding hole-only and electron-only diodes. The results show that the five OSCs are all bipolar transporting materials with hole and electron mobility data in 1 order of magnitude. Considering the simple two electrode structure and that there are not any additional electrode modifications, the data in these experiments should be the real intrinsic bipolar transport characteristic in OSCs. In addition, the experimental mobility versus electric field (μ ∼ E 1/2) curves also showed that there were positive or negative slopes for mobilities of holes or electrons, which leads to further development of the bipolar transport model; it is believed that there is a third kind of traps in the HOMO and LUMO energy spaces, which should possess different work mechanism during the external electric field, and thus result in negative or positive slopes for μ ∼ E 1/2.

Concepts of the HOMO and LUMO Traps from the Carrier Dynamics of Organic Semiconductor Isomers α-NPB and β-NPB

“…The experimental data in this report and our previous report 18,19 all suggest there is another kind of trap, which is different with the structure trap, but closely correlated with the molecular structure itself. Compared with the geometric trap, such a kind of trap can be defined as HOMO and LUMO traps according to whether it exists in the energy space nearby the HOMO or LUMO.…”

“…9−15 Taking advantage of the traditional IS method, our group has incorporated the particle swarm optimization (PSO) algorithm to overcome some basic fitting problems, 16 and then carried out some research on the interface trap free energy, defect state, and bipolar transport of OSCs. 8,17,18 Lots of experimental data have proved that the IS method can easily determine the carrier mobility of dispersive materials (such as in polymer semiconductors) in greatly thicker film (such as 50 nm) in the diode structure. 19 In common viewpoint, the curve of the carrier mobility of OSCs versus electric field strength should be positive, which means that the charge carrier transferring will be increased with each increment of electric field.…”

“…This seems to be reasonable since the increment of electric field will accelerate the movement of charged carrier. In our recent report, 18 five common hole transporting materials have been investigated. As to the hole transfer, the curves of the carrier mobility versus electric field strength (μ dc ∼ E 1/2 ) are all positive.…”

“…And, in addition, for some dispersive materials, the turning point for the curve of photocurrent ∼ time is not obvious, which leads to the failure of the determining of the carrier mobility . In order to overcome such problems of TOF method, the impedance spectroscopy (IS) method, which uses the suitable mathematical equation, slowly becomes another option. − Taking advantage of the traditional IS method, our group has incorporated the particle swarm optimization (PSO) algorithm to overcome some basic fitting problems, and then carried out some research on the interface trap free energy, defect state, and bipolar transport of OSCs. ,, Lots of experimental data have proved that the IS method can easily determine the carrier mobility of dispersive materials (such as in polymer semiconductors) in greatly thicker film (such as 50 nm) in the diode structure …”