Effect of charge blocking layerfor enhancing detectivityin near infrared organic photodetectors

620.1;

유기물 반도체 ;

University:서울대학교 대학원

关键词: 620.1;   

Others  :  http://s-space.snu.ac.kr/bitstream/10371/150721/1/000000155440.pdf

美国|英语

来源: Seoul National University Open Repository

【 摘 要 】

Potential of organic semiconductors was proved as the development of organic light emitting diodes (OLEDs) used in many areas of our life. Interest in organic photodetectors is also increasing thanks to an advantage of organic materials such as low process cost, using flexible substrate, wavelength selectivity by designing a molecular structure. Organic photodetectors (OPDs) can be applied in various fields depending on the absorption wavelength region. Near-infrared photodetectors have the various application of NIR camera, organ implant, motion detector, and remote controller. However, NIR OPDs have a drawback in terms of small bandgap causing high dark current density, namely noise in OPDs. Noise stand out particularly in the photoconductive mode of photodetectors in which an external voltage is applied. An increase in the dark current reduces the detectivity and it is important to improve the detectivity without decreasing the efficiency.For enhancing the detectivity of OPDs it is required to reduce dark current density and increase external quantum efficiency. The dark current density is originated from the injected carrier from the electrode and thermally generated carrier in the PN junction and leakage current in devices. Reducing the dark current by blocking this flow of charge is important to improve the detectivity. In this study, we introduced organic materials as charge blocking layers (CBLs) on inverted-structure NIR OPDs to investigate the origin of the dark current density. Depending on CBLs with various molecular energy levels, characteristics of OPDs is different. The deeper HOMO of the charge blocking layer is, the lower the dark current density is. With inserting 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) between anode and donor the dark current density was reduced up to 164 nA/cm2 and EQE also increased from 25.1% to 29.3%. High detectivity of 1.06 ×1012 cm2 Hz/W at 970 nm under a reverse bias of -3V was obtained. The low dark current density was obtained due to low HOMO and high LUMO level of CBP working as energetic barriers on the injected charge from the electrode and thermally generated charge at PN junction respectively.Other figure of merits including linear dynamic range, cut off frequency, and response time were compared depending on used CBL. The linear region was also longest in the device with CBP due to the low dark current density. However, using charge blocking layers resulted in increased response time compared with the device without CBL because the charge extraction was delayed due to the increase of device thickness and accumulation at the organic interface. Similarly, the cut-off frequency was reduced compared with the device without CBL but high mobility of CBL compensated loss of frequency response.In this study, we could know the effect of the charge blocking layer on the characteristics of the organic photodetector and it will be useful for the study of material and development of device structure in the future.