Charge Photogeneration in m-LPPP

In low-dimensional systems, such as quantum-confined semiconductors and conjugated polymers, the first step of optical absorption is the creation of bound electron-hole pairs, known as excitons [34]. Charge photogeneration (CPG) occurs when excitons break into positive and negative carriers. This process is of essential importance both for the imderstanding of the fundamental physics of these materials and for applications in photovoltaic devices and photodetectors. Since exciton dissociation can be affected by an external electric field, field-induced spectroscopy is a powerful tool for studying CPG. 

We have studied the temporal dynamics of CPG in m-FPPP by performing field-assisted pump-probe experiments on LED structures, as described in Section 8.3.2. The narrow line-width PA assigned to polarons (see Section 8.5.2) is a fingerprint of charge generation in m-LPPP. Monitoring the dynamics of these PA band enables us, for the first time, to directly observe the CPG dynamics in a conjugated polymer with sub-picosecond time resolution [40]. 

8 Photophysics of Methyl-Substituted PPP-Type Ladder Polymers 

The interpretation of our CPG data is complicated by the presence of comparatively fast radiative and nonradiative decay channels for the singlet exciton, which compete with the field-induced dissociation. In order to provide a clear picture of the observed mechanism and disentangle it from the singlet exciton decay dynamics, we define the following phenomenological time-dependent parameter  

The small and weakly time-dependent CPG that persists at longer delays can be explained by the slower diffusion of excitons approaching the localization edge [15]. An altemative and intriguing explanation is, however, field-induced on-chain dissociation, a process that does not depend on the local environment but on the nature of the intrachain state. The one-dimensional Wannicr exciton model describes the excited state [44]. Dissociation occurs because the electric field reduces the Coulomb barrier, thus enhancing the escape probability. This picture is interesting, but so far we do not have any clear proof of its validity. 

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