Effects of fast back-fusion of charge transfer excimers on magneto-photocurrent in organic light emitting diodes

Abstract

We report the magnetic field dependence of the magneto-photocurrent (MPC) in organic light emitting diodes made of homo-polymer organic layers and compare it to the measured magneto-conductance (MC) in the same diodes. We find that the response 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 charge transfer excimers that are responsible for MPC: sub-ns fast fusion back to singlet excitons and slow (ns to μs) dissociation to free charges. In contrast, MC(B) is determined by long lived (>10 ns) polaron pairs having singlet and triplet dissociation rates of the same order.

Magnetic Field Effects of Charge Transfer Excitons in Organic Semiconductor Devices

“…The difference between MPC 1 and MPC 2 (blue line) is that MPC 2 is calculated with singlet decay rate 10 5 times faster than in the former case, g S;2 ¼ 10 5 g S;1 . The rest of the parameters are Reprinted from, [64] with the permission of AIP Publishing.…”

“…The case described above has been demonstrated in MPC response of homopolymer OLED devises [64] . The CT in photoexcited homopolymer photodiode (i. e. OLED device) is not efficient.…”

“…The case described above has been demonstrated in MPC response of homopolymer OLED devises. [64] The CT in photoexcited homopolymer photodiode (i. e. OLED device) is not efficient. This is because the CTE (or PP) energy level is not below the SE level, see Figure 1a, as it is in the case of donor-acceptor composition (Figure 1b).…”

“…[65] CT excimer is an excimer in which an electron and a hole are fully situated on different molecules. [64,65] It is presumed that the CT excimer has very fast fusion rate back to the SE. Figure 12 shows MC (a) and MPC (b) responses measured in the same OLED device.…”

“…The inserts show the corresponding data up to B = 8 T. ΔB indicates the experimentally obtained HWHM. The triangle magenta dots in insert (b) are the experimental MPL response measured simultaneously with MPC.Reprinted from,[64] with the permission of AIP Publishing.…”