chapter  4
30 Pages

Magnetic Field Effects in Organic Semiconducting Materials and Devices

WithBin Hu, Liang Yan

Intramolecular Excited States....................................................... 113 4.2 Magnetic Field Effects on Photoluminescence ...................................... 115

4.2.1 Intramolecular Excited States in MFPL ...................................... 116 4.2.2 Intermolecular Excited States in MFPL ...................................... 118

4.3 Magnetic Field Effects on Photocurrent ................................................. 121 4.4 Magnetic Field Effects on Electroluminescence .................................... 126

4.4.1 Electrofl uorescence-Based MFEL ................................................ 127 4.4.2 Electrophosphorescence-Based MFEL ........................................ 129

4.5 Magnetic Field Effects on Injection Current: Magnetoresistance ....... 132 4.5.1 Excited States-Based Magnetoresistance .................................... 133 4.5.2 Charge Transport-Based Magnetoresistance ............................. 135

4.6 Conclusion .................................................................................................. 136 References ............................................................................................................. 138

Magnetic fi eld effects (MFEs) are defi ned as a change caused by an applied magnetic fi eld in electroluminescence, photoluminescence, photocurrent, and electrical injection current in organic semiconducting materials. The amplitude of MFEs is given by the relative change in percent: = − ×B 0 0MFE (( )/ ) 100%,S S S where SB and S0 are the signal strengths with and without applied magnetic

fi eld. In general, MFEs can be attributed to two spin-dependent excited processes: (1) electron and hole pairing during the formation of excited states and (2) postelectronic developments of excited states. As a result, the excited sates are a critical issue in the determination of MFEs. An excited state is defi ned as an electron-hole (e-h) pair in organic semiconductors. In essence, electron and hole capture determines the formation of e-h pairs through intercharge electrical and magnetic interactions. It is noted that the electron and hole capture depends on the Coulombic capture radius ( )2CR e KT= ε and the physical contact time, where RC is the Coulombic capture radius, ε is the dielectric permittivity, K is the Boltzmann constant, and T is the temperature. When electrons and holes are moving in an organic semiconducting material, the Coulombic capture radius is determined by the electrical screening resulting from the dielectric fi eld background. The electron and hole contact time is dependent on charge mobility. As a result, internal dielectric fi eld and charge mobility can affect the formation of excited states and the resultant MFEs. Furthermore, due to electron spin multiplicities the electron and hole can have antiparallel or parallel spin orientation and form singlet or triplet excited state. As a consequence, the singlet and triplet excited states have zero and nonzero angular spin momentums, respectively. There exist two internal interactions, namely long-range Coulombic attraction ( (1/ ))r∝ and short-range spin-exchange interaction (∝ exp(−r)) between electron and hole in an excited state, where r is the e-h separation distance. The Coulombic attraction accounts for the binding energy between the electron and hole in an excited state while the spin-exchange interaction generates an energy difference between the singlet and triplet states. An external magnetic fi eld may infl uence the spin-dependent formation and postelectronic processes in excited states and consequently change electroluminescence, photoluminescence, photocurrent, and electrical current in organic semiconducting materials.