Spin-Orbital Coupling Effects on Magnetoresistance in Organic Light-Emitting Diodes
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0965-S03-08
Spin-Orbital Coupling Effects on Magnetoresistance in Organic Light-Emitting Diodes Bin Hu and Yue Wu Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996 ABSTRACT The magnetoresistance of conjugated polymer poly [2-methoxy-5-(2’-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) based organic light-emitting diodes (OLED) was investigated at both forward and reverse bias at liquid nitrogen temperature. We find that the reverse bias yields a largely increased magnetoresistance when the electron-hole capture zone is away from the metal electrode as compared to the forward bias with the electron-hole capture zone close to the metal electrode. The electroluminescence suggests that the deposited metal atoms enhance the spin-orbital coupling at the polymer/metal interface and consequently lead to electron-hole capture zone-dependent magnetic field effects in organic semiconductor devices. INTRODUCTION Conjugated polymers as soluble semiconductors have been used to fabricate a wide range of optoelectronics, especially organic light emitting diodes (OLED), due to their excellent mechanic and optic properties. Recently, a new functionality was discovered that an external magnetic field can affect the resistance of non-ferromagnetic OLEDs [1-9], namely magnetoresistance. The mechanism for the magnetoresistance is still controversial [6,8], however, it is commonly accepted that an external magnetic field can modify the relative ratio between the singlet and triplet states through four spin-dependent processes: singlet and triplet dissociation [10], singlet→triplet conversion (intersystem crossing) [11], triplet→singlet conversion (triplet-triplet annihilation) [12], and triplet-charge reaction [13]. The spin-orbital coupling or hyperfine interaction essentially forms a mechanism to flip electron spin polarizations and thus has an important impact on the singlet↔triplet conversions. In present study, we investigated magnetoresistance of poly [2-methoxy-5-(2’-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) based OLED at both forward and reverse bias to elucidate the possibility that the magnetoresistance is related to the magnetic field effect on the spindependent processes. We found that at forward bias the deposited metal electrode enhances the polymer spin-orbital coupling and reduces the magnetoresistance when the electron-hole recombination zone is close to metal electrode as compared with that of reverse bias with recombination zone far away from the electrode.
EXPERIMENTAL The MEH-PPV purchased from Aldrich was spin cast from chloroform solutions on precleaned ITO glass substrates in nitrogen gas. The polymeric OLEDs were fabricated using indium tin oxide (ITO) and aluminum (Al) to sandwich the MEH-PPV film with a thickness of 80 nm, yielding a device architecture of ITO/MEH-PPV/Al. The Al electrode was prepared from thermal depositions at the vacuum of 2x10-6 Torr. Fabricated OLEDs were put between the two poles of an electromagnet to investigate the magnetic field effe
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