Low-magnetic field effect and electrically detected magnetic resonance measurements of photocurrent in vacuum vapor deposition films of weak charge-transfer pyrene/dimethylpyromellitdiimide (Py/DMPI) complex

Abstract

Magnetic field effect (MFE) and electrically detected magnetic resonance (EDMR) measurements of photocurrent have been conducted to clarify the excited-state dynamics in films of an organic weak charge-transfer (CT) complex, Pyrene/Dimethylpyromellitdiimide (Py/DMPI), fabricated by vacuum vapor deposition. Low-field MFE measurements of the photocurrent were carried out using an interdigitated platinum electrode made on a quartz substrate as well as the re-examination of the photocurrent and MFE in the range of 3-200 mT. The spin-dependent carrier dynamics leading to the low-field MFE are reasonably simulated as the low-field effect due to the hyperfine mechanism in the radical-pair intersystem crossing, which was solved through the Liouville equations of the density matrix for the stepwise hopping model in the doublet electron-hole pair (DD pair mechanism). Single-crystal time-resolved electron spin resonance measurement was also carried out to justify the MFE mechanism. The averaged trap depth (Etrap) of the triplet exciton was estimated to be +640 ± 89 cm-1 (Etrap/kB = +921 ± 128 K) by the temperature dependence of the signal intensity. This finding gave confidential experimental evidence for the majority of the trapped triplet exciton (3ext). The EDMR experiment directly revealed the evidence of the weakly coupled electron-hole pairs. The effective activation energies (ΔE) for the separation from the photoinduced CT state to the mobile carries are 1200-1900 cm-1 (ΔE/kB = 1700-2700 K). A systematic protocol to clarify the photo-generated carrier dynamics in weak CT complexes is demonstrated, and our findings from this method give not only further support for the two types of collision mechanisms assumed in our previous work but also the detailed information of the carrier dynamics of the weak CT complex, including the activation energy and trapping/detrapping process, which have significant influence on the performance of the organic devices.