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Transport Anisotropy in Nanocrystal Superlattices

Subject Area Physical Chemistry of Solids and Surfaces, Material Characterisation
Term from 2015 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 281729251
 
Final Report Year 2023

Final Report Abstract

This Emmy-Noether Research group has explored novel ways to realize band-like transport in quantum dot (QD) solids. We have utilized three complimentary methodologies – namely transport measurements at cryogenic temperatures, in-situ conductance measurements during electrochemical cycling and optical spectroscopy – for a rigorous determination of the dominant transport mechanism. This has allowed to differentiate between temperature-activated hopping, small polaron hopping and band-like transport. At first, n-type CdSe QDs capped with halides were explored to establish a reference system. These materials exhibit very large field-effect mobilities (> 10 cm2/V-s) such that they are likely candidates for band-like transport to be operative. However, due to a mandatory annealing step during device fabrication, there is growing skepticism as to whether the reported mobilities reflect transport through QDs at all and not through a bulk subphase. Addressing this question was one of the aims of this project. The next goal was the assessment of the transport mechanism in so-called “Coupled Organic- Inorganic Nanostructures (COIN). Here, an organic semiconductor (OSC) is used to couple adjacent QDs chemically and electronically to provide channels for efficient carrier transport. We have investigated several n-type CdSe QD based COINs and studied size-dependent transport with the three key methods detailed above. The question to be answered was to which degree OSCs are suitable to allow for band-like transport in COINs. Again, temperatureresolved transport measurements were used to determine the operative transport mechanism, while spectroelectrochemistry played a pivotal role in revealing the role of the OSC molecular orbitals for transport. The long-term perspective, also beyond the funding period, was to establish COINs as the leading concept for hybrid nanostructured materials with band-like transport behavior and excellent opportunities for optoelectronic applications, such as photovoltaics, light-emitting diodes or photodetectors. The third project task expanded the COIN concept to QDs functionalized with photo-switchable linker molecules. The photochromism was exploited to switch this COIN between an “OFF” and an “ON” state by an external optical stimulus via the change of the HOMO-LUMO gap of the photochromic linker. In the ON state, the LUMO energy is in resonance with the first excited electron state of the QDs to allow for efficient carrier transport. Switching into the other conformer (the OFF state) moves the LUMO out of resonance and blocks the channel for carrier transport. We have explored the transport mechanism in this photoswitchable COIN with our three key methodologies and investigated to which degree transport can be modulated by the photochromism. During the extension of the funding period to a sixth year, we have demonstrated for the first time that transport anisotropy is present in superlattices of QDs. In this context, we have derived general synthetic strategies for the design of other nanocrystal superlattices with direction-dependent charge carrier transport.

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