Photoexcitation dynamics and disorder effects in organic donor/acceptor systems
Organic semiconductors are a promising material class for applications in photovoltaics with photoconversion efficiencies beyond 10 % reported in recent years. However, despite this progress, the underlying photophysical processes of charge generation still need to be understood in greater detail. I...
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|Summary:||Organic semiconductors are a promising material class for applications in photovoltaics with photoconversion efficiencies beyond 10 % reported in recent years. However, despite this progress, the underlying photophysical processes of charge generation still need to be understood in greater detail. In contrast to most of their inorganic counterparts, absorption of light does not directly lead to the formation of free charges in organic systems. The primary photoexcitations in organic systems are Coulombically bound electron-hole pairs, so-called excitons. In order to promote exciton separation, the active layer in organic solar cells is therefore comprised of a donor/acceptor-blend, also known as bulk-heterojunction. In such a device, charge separation occurs at the donor/acceptor interfaces. In this context, so-called charge-transfer (CT) states are regarded as precursors for charges, signifying electron-hole pairs, which are still weakly correlated across the donor/acceptor interface. The strength of the Coulomb interaction involved is decisive for the photoconversion efficiency of an organic solar cell, as it may either promote exciton recombination or dissociation. The present work employs time-resolved photoluminescence (PL) spectroscopy to investigates radiative recombination losses, which naturally accompany the process of charge separation. The studies focus on two prototypical donor/acceptor systems P3HT/PC61BM and PTB7/PC71BM, respectively. First, luminescence decay in the neat polymers P3HT and PTB7 is characterized. The observed time-dependent red shift of the signatures is typical for organic systems and results from preferential exothermic hops of the excitons in a disordered density of states. The energetic relaxation of the emission in P3HT is consistent with an underlying Gaussian density of states. The relaxation in PTB7 is however stronger than expected, which might be due to the presence of a higher number of low-energetic tail states with respect to a typically expected Gaussian profile. In a next step, the PL of P3HT/PC61BM and PTB7/PC71BM mix films is studied. Beside the emission of so-called singlet excitons, which are also observed for the neat material, in both systems a CT signature is identified in the near-infrared. Both the CT intensity and also the drop of singlet emission intensity in blends with respect to the neat material are found to be correlated with the presence of an intimately mixed donor/acceptor phase. Furthermore, temperature-dependent PL studies show that in both material systems a high fraction of the CT emission is quenched with the help of thermal energy, suggesting that the CT binding energy is rather weak. In the final part of this work, the field-induced PL quenching in a PTB7/PCBM device is investigated under various temperatures. The decay of the PL intensity in an electric field arises from an enhanced dissociation rate of the excitons. The field-dependence of the PL quenching is thus related to the exciton binding energy. The binding energies are quantified employing a kinetic model known from literature, which is based on the assumption that exciton dissociation occurs via a multi-step hopping mechanism. The model gives an appropriate description of the data when (i) the underlying disorder is taken into account and when (ii) it is assumed that the Coulomb potential at the interface is effectively screened. The results suggest that the CT state in the PTB7/PCBM mix phase has a binding energy of about 50 meV, which is almost one order of magnitude below the binding energy of singlet excitons. The CT state can thus be regarded as a precursor for charges rather than a recombination center, as thermal energy present in the system largely promotes its dissociation. Overall, a methodological framework is presented in this work to identify and characterize the relatively weakly emitting CT states in organic donor/acceptor systems. The employed hopping model gives good agreement with the experimentally observed PL quenching over the whole investigated temperature range between 10 and 290 K. Moreover, it is demonstrated that approaches beyond the commonly applied Onsager-Braun model should be taken for an appropriate description of the charge separation process.|
|Physical Description:||166 pages.|