Effects of Spin-Pair Lifetime on Magnetic Field Effects in Organic Light Emitting Diodes

  Daniel Nikiforov [1,2]  ,  Bagrat Khachatryan [1,2]  ,  Nir Tessler [3]  ,  Eitan Ehrenfreund [1,2]  
[1] Physics Department, Technion-Israel Institute of Technology, Haifa 32000, ISRAEL
[2] Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, ISRAEL
[3] Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, ISRAEL

Magnetic field effects in organic semiconductors (OSs) are related to spin configurations of spin-pairs formed in the material. In organic light emitting diodes (OLEDs) made of homo-polymer organic layers the spin-pair reside on two chemically identical polymers. In these layers we distinguish between two types of spin-pairs: a polaron pair (PP) and a charge transfer excimer (CTE); both are composed of electron-hole (e-h) pairs held together by Coulomb force. PPs can be formed from free charge carriers injected into the OS layer via electrodes under forward bias applied on the OLED device. On the other hand, CTEs are generated from photoexcited singlet excitons (SEs). Absorbed light creates molecular excitations, SEs, in the OS material; in homo-polymers a resonance interaction between a photoexcited molecule and a neighboring non-excited molecule creates an excimer. Molecules composing the excimer can share not only energy but also charge; a CTE is an excimer in which the charges are fully situated on different molecules. A CTE may fuse back to an excimer and finally to a SE producing photoluminescence (PL) or it may dissociate into free charges giving rise to a photocurrent. We compare magnetic field dependence of conductance and photocurrent (MC and MPC, respectively) in the OLED devices and find that the response of MPC(B) is very different from MC(B) in at least two respects. (a) The low field (B<50 mT) response of MPC(B) is narrower by a factor of ~5 from that of MC(B); (b) At high fields (B>4 T) MPC(B) has a stronger dependence on B, d(MPC)/dB~5d(MC)/dB. We attribute these differences to a unique feature of CTEs that are responsible for MPC: sub-ns fast fusion back to SEs and slow (ns to s) dissociation to free charges. In contrast, MC is determined by long lived (>10 ns) PPs having singlet and triplet dissociation rates of the same order.