Publication date: 1st April 2013
Among various kinds of light harvesting materials, semiconductor nanocrystals – the so-called quantum dots (QDs) – have attracted increasingattention due to their superb features, suchashigh extinction coefficient, a tunable absorption edge, and the possibility of multiple exciton generation and collection.1-4
The base stone for application of QDs in photovoltaics is their long-term stability, which is mostly limited by the degradation of QDs’ surface.5One of the ways to avoid the photodegradation is using core-shell QDs (CSQDs). In this case the surface is shielded from oxidation by a protective shell consisting of a wider band gap semiconductor.6Therefore, CSQDs feature superior thermal, chemical, and photochemical stability compared with organically-capped QDs.7
Usually the core and the shell of QD are prepared in sequence (core is formed first then covered with the shell). However, thecore-shell interface then suffers from surface defects due tothe lattice mismatch between the core and the shell layers.8Recent reports have revealed that the number of interfacial defects in CSQDs can beminimizedby using a gradual transition between core and shell material (gradient CSQDs).9The gradual change in chemical composition results in (i) enhanced fluorescence quantum yield and (ii) reduces the Auger recombination rate compared with step-like CSQDs.10
The shell layer inevitably affects also electron injection from QD to metal oxide – a crucial process for light harvesting. On one hand, shell layer of CSQDs acts as a potential barrier and slows down the electron injection. On the other hand, the shell protects the QD and reduces the unwanted back-recombination. Therefore it is of a big interest to find a suitable shell thickness, which will find a balance between the two effects.
We will present a detailed study of the electron injection process from gradient CSQDs CdSe/ZnS to ZnO NPs. We will focus on the dependence of electron injection rate on shell layer thickness. By combining injection kinetics with a study of photoluminescence quantum yield, we will demonstrate that there exists an optimum shell thickness, where electron injection stays efficient, yet QD surface is already well passivated.
(A). The dependence of injection efficiency ηinj form gradient CSQDs to ZnO NPs and the QY on the shell thickness. (B) Energetic scheme for gradient CSQDs-ZnO NPs and step-like CSQDs systems.
1. Kamat, P. V., Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. 2. Žídek, K.; Zheng, K.; Abdellah, M.; Lenngren, N.; Chábera, P.; Pullerits, T., Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSe–ZnO System: Electron Injection versus Auger Recombination. Nano Lett. 2012, 12, 6393-6399. 3. Kamat, P. V., Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 908-918. 4. Žídek, K.; Zheng, K.; Ponseca, C. S.; Messing, M. E.; Wallenberg, L. R.; Chábera, P.; Abdellah, M.; Sundström, V.; Pullerits, T., Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study. J. Am. Chem. Soc. 2012, 134, 12110-12117. 5. Zidek, K.; Zheng, K.; Chabera, P.; Abdellah, M.; Pullerits, T., Quantum dot photodegradation due to CdSe-ZnO charge transfer: Transient absorption study. Appl. Phys. Lett. 2012, 100, 243111-4. 6. Baranov, A. V.; Rakovich, Y. P.; Donegan, J. F.; Perova, T. S.; Moore, R. A.; Talapin, D. V.; Rogach, A. L.; Masumoto, Y.; Nabiev, I., Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots. Phys. Rev. B 2003, 68, 165306. 7. Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X., Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125, 12567-12575. 8. Bae, W. K.; Char, K.; Hur, H.; Lee, S., Single-Step Synthesis of Quantum Dots with Chemical Composition Gradients. Chem. Mater. 2008, 20, 531-539. 9. Ratchford, D.; Dziatkowski, K.; Hartsfield, T.; Li, X.; Gao, Y.; Tang, Z., Photoluminescence dynamics of ensemble and individual CdSe/ZnS quantum dots with an alloyed core/shell interface. J. App. Phys. 2011, 109, 103509-6. 10. Cragg, G. E.; Efros, A. L., Suppression of Auger Processes in Confined Structures. Nano Lett. 2009, 10, 313-317.
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