Carrier Generation Process on Photoconductive Polymer Films as Studied by Magnetic Field Effects on the Charge-Transfer Fluorescence and Photocurrent

Abstract

We have studied the magnetic field effects (MFEs) on the charge-transfer fluorescence and transient photocurrent of a 1,2,4,5-tetracyanobenzene-doped poly(N-vinylcarbazole) film, which reflect the recombination and escape yields of the carriers, respectively. The recombination yield dependence of the external magnetic field (B) clearly shows two types of the MFEs, growth with increasing B due to the hyperfine mechanism (HFM) and a negative dip due to the level-crossing mechanism (LCM). On the other hand, the escape yield indicates complementary MFEs with a sharp decrease in yield with increasing B and then a positive dip. Simultaneous observation of the HFM- and LCM-MFEs proves the stepwise hole-hopping mechanism rather the long-range hole-jumping one. The quantitative analysis of the recombination and escape MFEs is performed using the stochastic Liouville equations (SLE) for a one-dimensional lattice model in which the stepwise hole hops take place between the nearest neighbor carbazole units with spin conservation. The SLE analysis provides the recombination and hole transfer rate constants of 7.0 x 10(7) and 4.5 x 10(8) s(-1), respectively. The boundary site number for the ion pairs in the one-dimensional model is estimated by the best fit to the experimental results. The interionic distance of the boundary ion pair in the one-dimensional model including eight sites agrees with the thermalization distance in the Onsager model. Hence, it is concluded that the elementary processes in the Onsager model applied to molecular amorphous solids are the stepwise hole hops rather than a long-range hole jump.

Magnetic‐Field Effects in Organic Semiconducting Materials and Devices

“…Intermolecular excited states can also experience radiative emission, nonradiative multiphonon emission, and charge dissociation to release their energies. When an external magnetic field changes the singlet and triplet ratios in intermolecular excited states, MFEs can then be observed in excimers, [54][55][56] exciplexes, [9][10][11][12][57][58][59] charge-transfer complexes, [60][61][62][63][64] and donor-acceptor pairs [65] based on the fact that singlets and triplets have different contributions to radiative emissions and intermolecular interactions in excited states, due to their different spin configurations, involved in electronic transitions, and ionic natures, involved in intermolecular electrical dipole interactions.…”

“…[72][73][74][75][76][77][78][79][80][81] This phenomenon, named as MFEs on the photocurrent (MFE PC ), can be readily observed in organic semiconducting materials [82][83][84][85] and also in organic-material complex systems [60,86,87] with an amplitude of about 5%. It can be envisioned that the MFEs on photocurrents can elucidate not only photoexcited processes but also photovoltaic channels in organic semiconducting materials.…”

“…Second, there is still controversy about whether the negative MFE PC comes from triplet exciton-charge reaction, named charge reaction model, [82][83][84] or from the intersection between singlet and triplet (T m ¼ À1 ) levels, named level crossing model. [60,85,86] To explore the direct experimental evidence for polaron-pair model-based positive MFE PC , we recently examined the positive MFE PC before and after the polaron-pair states were removed upon introducing donor-acceptor interaction through doping. We found that removing polaron-pair states have led to a significant reduction in positive MFE PC .…”

Photovoltaic Processes of Singlet and Triplet Excited States in Organic Solar Cells

“…We note, in a first-order approximation, that the magnetic-field dependence of the photocurrent can be assumed by the modification of the singlet and triplet ratios in the polaron-pair states when the singlet and triplet excited states have different contributions to the photovoltaic channels of the dissociation and charge reaction. The modification of the singlet and triplet polaron-pair ratios can come from two different mechanisms: singlet-triplet intersystem crossing [20,21] through spin relaxation, or the singlet/triplet formation rate [10,22] through spin mixing. Firstly, an external magnetic field may change the intersystem crossing if: (i) the spinexchange related energy difference between the singlet and triplet manifolds is perturbed by external Zeeman splitting; and (ii) the spin-flip probability is magnetically modified by the applied field.…”

“…This can occur in polaron pairs, which have a relatively large e-h separation distance and a weak spinexchange energy relative to the excitons, in a low magnetic field. [10,21,23,24] Secondly, a low magnetic field may affect the singlet and triplet polaron-pair formation rates through spin mixing and consequently increase the singlet-to-triplet ratio in the polaron-pair states. [22,25] Nevertheless, the change in the singlet and triplet polaron-pair ratios can be reflected as a perturbation in the singlet and triplet exciton ratios when mutual relaxations between excitons and polaron pairs are considered.…”