Unequilibrated Charge Carrier Mobility in Organic Semiconductors Measured Using Injection Metal–Insulator–Semiconductor Charge Extraction by Linearly Increasing Voltage

Abstract

The charge carrier mobility in tris(4-carbazoyl-9-ylphenyl)amine (TCTA), a host and hole transport material typically used in organic light-emitting diodes (OLEDs), is measured using charge carrier electrical injection metal–insulator–semiconductor charge extraction by linearly increasing voltage (i-MIS-CELIV). By employing the injection current i-MIS-CELIV method, charge transport at time scales shorter than the transit times typically observed in standard MIS-CELIV is measured. The i-MIS-CELIV technique enables the experimental measurement of unequilibrated and pretrapped charge carriers. Through a comparison of injection and extraction current transients obtained from i-MIS-CELIV and MIS-CELIV, it is concluded that hole trapping is negligible in evaporated neat films of TCTA within the time-scales relevant to the operational conditions of optoelectronic devices, such as OLEDs. Furthermore, photocarrier generation in conjunction with i-MIS-CELIV (photo-i-MIS-CELIV) to quantify the properties of charge injection from the electrode to the semiconductor of the MIS devices is utilized. Based on the photo-i-MIS-CELIV measurements, it is observed that the contact resistance does not limit the injection current at the TCTA/molybdenum oxide/silver interface. Therefore, when TCTA is employed as the hole transport/electron-blocking layer in OLEDs, it does not significantly reduce the injection current and remains compatible with the high injection current densities required for efficient OLED operation.