Effects Of Solution-processed Hole Transport Layer And Injection Layer On The Performance Of Organic Light-emitting Devices

Organic light emitting diodes (OLEDs) have been broadly used for displaying and lighting. Although OLEDs possess excellent properties, the improvement of device performance and the investigation in device physics are still hotpots and focuses in this research field. Hole injection and transporting are two of key factors determining the performances of OLEDs. Hole injection or transport layers prepared by a solution method can save a lot of energy, In this paper, various hole transport materials, including V2O5, CuTCPc and PCDTBT, are prepared from solutions. Meanwhile, the role of these materials and their effects on device performance are also investigated. The experiment results are obtained and listed as follows:Firstly, the effect of indium tin oxide (ITO) modified by vanadium penoxide (V2O5) saturation solution was studied. It is found that the V2O5solution-treated devices have a much higher current density compared to the device with bare ITO and a higher current efficiency compared with UV-ozone-treated device. Series resistances were derived by fitting the current-voltage curves and monitoring the AC impedance. The influence of series resistances were also taken into account in this section.Secondly, the effects of a hole-transport layer2,9,16,23-tetracarboxylic copper phthalocyanine (CuTCPc) using a spin-coating way was investigated The insertion of the CuTCPc between ITO and N,N’-biphenyl-N,N’-bis (1-naphthyl)-(1,1’-biphenyl)-4,4’-diamine (NPB) significantly reduces the operating voltage and thereby increases the current efficiency. The hole is considered to be the main carrier in NPB/Alq3structure devices, but the increase of hole-transporting can still improve the efficiency of the devices. In this case, the use of the hole transport layer CuTCPc can greatly abate the average field intensity in the devices, thus diminish the negative impact of carrier imbalance.The effect of the device structure of NPB/A1Q3:C545T/AlQ3was also studied. Considering that the hole is the major carrier in this structure, the enhanced hole injection is attributed to the imbalance of different carriers and lower efficiency. However, in our study, it indicates that the current efficiency increases with the increasing of the hole concentration. We speculate that the increased current efficiency comes from a lower average electric field due to the hole transport layer CuTCPc in the devices.Finally, we doped PCDTBT (Poly [[9-(l-octylnonyl)-9H-carbazole-2,7-diyl]-2, 5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]), a kind of Poly carbazole compounds, into the commonly used hole transport layer (HTL) PVK. The doped PCDTBT in PVK can significantlyenhance the luminance and reduce the turn-on voltage of the devices. The proportion of the PCDTBT peak becomes gradually larger along with the increase of the doping concentration of PCDTBT, and the device with the largest doped concentration achieved five times higher brightness compared to the as-spun device.In order to prevent the luminescence of PCDTBT, we studied the devices with the structure of ITO/PCDTBT/NPB/Alq3/LiF/Al. We found that the turn-on voltage was reduced with increasing PCDTBT concentration. At a high voltage, the current density first increased and then decreased with the increase of PCDTBT concentration implying that the inserted PCDTBT layer improved the hole injection ability but reduced its hole transport ability.