Organic-Semiconductor Quantum Dot Hybrids: Controlling Photoinduced Energy and Electron Transfer
Authors: Tkachenko Nikolai V. , Virkki Kirsi , Arvani Maedeh , Stranius Kati , Efimov Alexander , Lupo Donald 
Published: February 2020

Self-assembled monolayes of photoactive compounds on semiconductor surfaces have been a subject of active research for a few decades already [1]. Apart from fundamental interest in electronic interactions at organic-semiconductor interfaces, the driving force for this activity is wide range of promising applications such as photo-voltaics, molecular senor, and nano-electronics. Although many semiconductor were tested in designing such organic-semiconductor hybrids, the properties of bulk semiconductors cannot be tuned easily and most of the research efforts were focused at the organic counterpart by synthesis of organic compounds with varying electronic properties, and providing different orientation and distance form the semiconductor surface [2]. Recent invention of semiconductor quantum dots (QDs) has increased tunability of organic-semiconductor hybrids drastically by allowing to very the energetic properties of the semiconductor counterpart [3]. The latter is controlled by the size of QDs which limits the spatial confinement of electrons in QDs and has two effects. Firstly, the energies of valence and conduction bands can be tuned in a rather wide range which affect the electron transfer at the organic-semiconductor interface. Secondly, the optical properties, absorption and emission of QDs, also depend on the size. Thus QDs can play an active role in light harvesting by organic-QD hybrids.
The main objectives of this study is to gain information on energy and electron transfer in organic-QD hybrids when both parts can absorb light, and find bases to control the direction of electron transfer between organic and semiconductor counterparts. The QDs used in this study are CdSe core and core/shell QDs of varying size. Organic compounds are phthalocyanine, porphyrin and fullerene derivatives. The study was carried out using a range of spectroscopy techniques including steady state and ultra-fast time resolved methods. The steady state emission quenching and reduction of the emission lifetime of both QDs and organic compounds were used as primary tools to ensure hybrid formation and to evaluate the degree of electronic interactions. Femtosecond to nanosecond transient absorption spectroscopy [4] was used then for selected systems to identify the intermediate states formed and to obtain quantitative information on the reaction rate constants. In the visible part of the spectrum the responses were dominated by the QDs showing some bleaching and reshaping of the QD ground state absorption. However, in the red and near infrared (NIR) part of the spectrum the responses from organic chromophores had comparable or, in some cases, stronger contribution than that from the QDs. Therefore the NIR part of the spectrum was more informative for identification of the intermediate states and establishing the reaction mechanisms.
The results show that the photoinduced electron transfer from CdSe QDs to fullerene C60derivative takes place for QDs of different size. The photoinduced electron transfer to the QDs was observed for QD-phthalocyanine hybrids, in which case only large QDs (with emission in the red part of the spectrum) operated as electron acceptors, whereas for hybrids with smaller QDs only energy transfer was observed. This can be rationalized considering the dependence of the energy of QD conduction band lower edge on the size. The larger QDs have lower energy and are better electron acceptor. This property of QDs can be effectively used to control photophyiscal reactions in QD-organic hybrids.
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