Theoretical Exploration for Bipolar Transport and Negative Slope of NPB Isomers

Abstract

As a group of isomers, α-NPB and β-NPB have subtle molecular structural differences. If such slight difference can also lead to large change in carrier dynamics, it is enough to show that little difference of molecular structure has a significant impact on charge transport characteristics. In previous work, the carrier mobility of α-NPB and β-NPB was measured using impedance spectroscopy, and it was found that difference of hole and electron mobility of α-NPB and β-NPB was within one order of magnitude, which is consistent with the phenomenon of bipolar transport. In this report, after careful theoretical research, it can be shown that organic semiconductors have intrinsic bipolar transport characteristics, and a single electron model is proposed to explain it. Holes can be regarded as a group of hole electrons, which are more difficult to capture than an electron. This is why most organic semiconductors have higher hole mobility than that of electron mobility and are usually regarded as hole transport materials. What is even more peculiar is that the mobility curve in this research has a negative slope phenomenon, which seems to be contrary to the basic fact that the electric field accelerates the directional movement of carriers. After quantum chemical calculation research, it was found that the molecular structure of α-NPB and β-NPB changed significantly during the processes of electronic excitation and electronic state change. These instantaneous structural changes are bound to produce larger energy relaxation. Therefore, based on such results, the concept of vibrational relaxation for carrier dynamics in molecular semiconductors was proposed. When carriers move in molecules, they can be accelerated by the external electric field, while dipole vibration will reduce the kinetic energy of the carriers. If the energy loss of carriers due to vibration relaxation is greater than the energy increment produced by the external electric field, the carrier mobility curve will exhibit an overall negative slope characteristic. Other kinds of slopes could also be shown when the energy relationships between the vibration relaxation and the electric field acceleration are suitable.