- Grain boundary premelting
- Polycrystalline Semiconductors
- Recombination processes at grain boundaries in polycrystalline semiconductors
- polycrystalline semiconductors
Under illumination, the population of the interface states is altered considerably from its dark level and as a result, V d decreases to that value which maximizes recombination equal concentrations of electrons and holes at the boundary. Published in: International Electron Devices Meeting. Article :. Need Help? Additionally, we have compared the experimentally found density located in part of the DOS to the simulations and again find reasonable agreement SI, section 2.
Both curves are asymmetric and show exponentially distributed traps with increasing E 0 slower decay for decreasing degree of crystallinity. In case of a planarized core with vanishing dipole moment the DOS decays much faster as shown in Fig. Synthesis of molecules with a smaller dipole moment should therefore suppress the traps at the grain boundaries, which then, in turn, would of course make the molecular structure more prone to dynamic disorder see above.
Grain boundary premelting
This observation is in agreement with previous experimental results showing that higher angle grain boundaries lead to deeper traps 40 , Having determined the lateral energy landscape has also allowed us to estimate the mobility of charge carriers in the polycrystalline thin film. The different regimes of charge transport in high-mobility organic semiconductors can range from band-like transport completely delocalized charges to hopping localized charges From our experimental observations above e. Therefore, we employ a Jortner hopping rate based on microscopically computed parameters details are given in the SI section 3.
Since we have additionally determined the lateral energy landscape, it is now also possible to assess the relative impact of energy valleys and activation barriers on the overall charge carrier mobility. This was achieved by visualizing the charge carrier trajectory and estimating the mobility through Kinetic Monte Carlo KMC simulations based on Jortner hopping rates for single electrons where all parameters are calculated from ab-initio see SI section 3. All visited molecules are marked white. In the inset, one can see that an electron travelling along the a -direction is reflected by an energetic barrier a , breaks through a grain boundary at a low-energy site b , is again reflected at a barrier c , is trapped at a grain boundary d and finally is trapped by three energy barriers e.
Mobilities with corresponding errorbars for different degrees of crystallinity in Fig. We have addressed the influence of the energetic disorder at the grain boundaries on the transport properties by comparing three disorder scenarios in Fig. Surprisingly, introducing only the valleys but not the barriers reduces the mobility only slightly blue to green , while including also the barriers has a much stronger effect green to red.
This shows that the high-energy states introduced by the grain boundaries play an important role in charge transport by blocking pathways in the monolayer see also inset of Fig. The observation that mostly the high-energy barriers limit the charge carrier mobility is an important point that justifies the applicability of the presented charge transport calculations: While our calculations have been obtained with single carrier occupancy, our experiments that are performed at finite charge carrier density.
However, as has been argued previously 14 , 23 , traps at grain boundaries are typically populated with charges in realistic devices.
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Such a filled trap in turn leads to the repulsion of other charges and consequently to a potential well. At finite charge carrier density however, charges in the gate electrode will screen this potential well mostly, leading to a strong suppression of the potential well formed at the filled trap. Energy barriers on the contrary, are however not filled i. This means, that the values obtained in our modeling where we did not consider finite density effects, are also to be expected to hold in the finite density regime of realistic devices.
The role of valleys on the other hand seems to be limited — both in the single particle modeling results obtained here and in experiment due to screening as described above. From our extensive theoretical and experimental study of charge transport in thin films of PDI1MPCN2 we can derive several conclusions relevant for future experiments. We have used the temperature dependence of the linear charge carrier mobility for thin films of various crystallinity to derive the charge carrier density dependent activation energy and from it the density of states.
The charge carrier density dependent activation energy was found to be inconsistent with the model of trapped charges causing potential wells for charges contributing to the mobility. We could describe the density of states by the combination of a Gaussian part at energies close to the transport level and an exponential at comparably lower energies. Neither the charge carrier mobility nor the density of grain boundaries was found to correlate with the features of the DOS.
Also the direct evaluation of the activation energy could not explain our data. Possibly, the model of charge carrier trapping does not capture the entire physics. We therefore used a combination of density-functional theory and kinetic Monte-Carlo simulations of charge transport in polycrystalline monolayers of the PDI1MPCN2 to understand our charge transport results. We found, that due to the misalignment of molecules at grain boundaries energy barriers are created that decisively determine the charge carrier mobility.
On the other hand we identified, that valleys in the energy landscape only play a minor role. This result is significant, since previously only valleys or energy barriers created by filled traps have been considered to hinder charge transport. Furthermore, the lateral energy landscape turned out to be more decisive than the transfer integral between the molecules. Finally, we have also identified in the dipole moment due to the twist angle of the PDI1MPCN2 core a molecular factor that has significant impact on the disorder at the grain boundaries.
Since the twisted core was found to suppress dynamic disorder and therefore lead to high mobilities within highly crystalline regions, it will be an interesting future challenge to identify molecules that have small dipole moments yet cores that suppress thermal disorder. Furthermore, we believe that our results are also of interest for the improvement of organic solar cells and organic light-emitting diodes, since in these devices typically a significant number of grain boundaries are present.
Charge transport measurements were performed in a Lakeshore variable temperature probe station in high vacuum. The dielectric constant of the bare Al 2 O 3 films and the SAM-treated substrates were determined to be 7. Crystals are grown in a spherical fashion in accordance with the experimental unit cell, where the only unknown is the molecular orientation. We have assumed an orientation which complies with isotropic transport and van der Waals energies see SI, where we also compare to an orientation leading to non-isotropic transport.
Furthermore, isotropic and uniform growth speed was assumed for the crystal grains. Relaxation of side chains at the grain boundaries was taken into account by shortening them to isopropyl in order to capture sterical effects. Second, the fitted exponential term see equation above is subtracted from the measured DOS for all energies. The resulting DOS at small negative energies shallow traps can then be attributed to a Gaussian.
The extracted values of the fit depend on the borders that are chosen for the fit in our case e. Zhan, X. Rylene and related diimides for organic electronics. Venkateshvaran, D. Approaching disorder-free transport in high-mobility conjugated polymers. Nature , — Takimiya, K. Thienoacene-based organic semiconductors.
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Recombination processes at grain boundaries in polycrystalline semiconductors
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Levinson, J. Conductivity behavior in polycrystalline semiconductor thin film transistors. Molecular microelectrostatic view on electronic states near pentacene grain boundaries. B 75 , Liu, C. A unified understanding of charge transport in organic semiconductors: The importance of attenuated delocalization for the carriers. Mladenovic, M. We would particularly like to thank the invited speakers for their talks, as well as for undertaking the task of refereeing the submitted papers.