Studies On The Magnetic Field Effects And Exciton Behaviors In Organic Optoelectronic Devices

Organic magnetic field effects (MFEs) are used to describe the changes of current and electroluminescence intensity when none magnetic organic optoelectronic devices are submitted to an external magnetic field. The organic MFEs have attracted a broad interest in the past few years because of their scientific importance and wide application. Although large progresses have recently been made in this field, the mechanisms for the intriguing organic MFEs are still under intense debate due to the intrinsic electronic properties and disorder structures of organic solids.In this M.S. Dissertation, we describe the MFEs in three different organic optoelectronic systems, including normal organic light-emitting diodes (OLEDs), dye-doped OLEDs and bulk-heterojunction solar cells. The mechanisms underlying the organic MFEs are ilucidated by investigating the behaviors of excitons (or the other excited states) in the organic optoelectronic devices. The main works in this Dissertation are listed as following:(1) Organic optoelectronic devices with various structures and active materials were prepared by the methods of organic molecular beam deposition, co-evoporation, spin coating and thermal evaporation. The optoelectronic properties of these devices were also investiged.(2) Normal OLEDs with typiacl configuration of ITO/CuPc/NPB/Alq3/LiF/Al were fabricated in the third Chapter. The magneto-luminescence (MEL) in the devices was measured under different temperature and driving current. Results show that MEL increase first and then decrease with enlarging the magnetic field strength at low temperatures and large driving current levels. We propose a composite model to explain the observed MFEs and attribute the low-field increase in MEL mainly to the magnetic-field dependent singlet-to-triplet electron-hole pair conversion while the high-field decrease in MEL mostly to the magnetic-field mediated TTA process. This composite model is proven to be in consistent with the experiment results by studying the dependences of MEL on driving current and temperature.(3) DCM-doped OLEDs with structure of ITO/NPB/Alq₃:DCM/Alq₃/LiF/Al were prepared in the fourth Chapter. The MEL effects in the devices were also measured. High-field decrease in MEL was observed even at room temperature, suggesting that TTA process was exsited in the dye-doped OLEDs at room temperature. The contribution of TTA process to the electroluminescence intensity is quantitatively estimated based on a theoretical rate model.Our results reveal that TTA process yields considerable extra singlets, accounting for as high as 17% of total singlets at room temperature and causing the total singlet generation fraction in the doped device exceeding the classical 0.25 simple spin statistics limit.(4)The organic magnetoconductance (MC) effects in poly (3-hexylthiophene):[6,6]-phenyl-C61-butyricacid methylester based bulk heterojunction solar cells were studied in dark and under illumination. The correlations between the MC and current character were revealed in this study. Results show that the dark current always exhibits a negative MC whereas a sign change in MC under illumination occurs at the bias around the open circuit voltage V_oc. We suggest that the positive MC in photocurrent is due to the field dependent conversion of singlet electron-hole pairs to triplet states and the negative MC is associated with space charge limited current with traps.