Publication date: 1st April 2013
Despite the significant performance improvement of semiconductor-sensitized solar cells (SSSCs) in recent years, their efficiency still falls short of that of dye-sensitized solar cells (DSSCs).1 One of the dominant mechanisms leading to poor efficiency is associated with unwanted recombination processes at interfaces between the anode and the sensitizer semiconductor and between the anode and the hole conductor or electrolyte.2,3 Of the exploited methods, heterostructured sensitizer layers (like CdSe/ZnS) have shown significant promise in mitigating undesired recombination channels.4,5
Of particular interest in the context are colloidal type-II heterostructured quantum dots (QDs),6 composed of a core localizing one charge carrier and a shell localizing the other, in which charge separation is induced already within the sensitizer layer. This system, which is in many senses analogous to heterostructures constructed by successive layering,4 should significantly reduce recombination losses of carriers already injected to the electrode with those remaining in the QDs. Another advantage is the significant red shift of the absorption edge of the heterostructured QDs relative to its two constituents due to “spatially indirect” energy gap, leading to improved absorption characteristics. The “spatially indirect” energy gap is determined by the energy separation between the conduction band edge of one semiconductor and the valence band edge of the other semiconductor. This type of QDs enables higher degree of freedom in tailoring the absolute conduction and valence band positions.
In this work we explored type-II heterostructure CdTe/CdSe core/shell nanocrystals as sensitizers in QDSSCs with TiO2 anode and a polysulfide electrolyte. The absorption edge of this system is readily tunable in the near-infrared spectral range, and colloidal synthesis protocols for it are highly evolved, enabling delicate control over the relative composition. I will show that despite the generally lower absorption cross section for the spatially indirect transition in CdTe/CdSe QDs, its magnitude is at least as large as that for a direct transition in CdSe QDs in the strongly confined regime. Thus, even in the near-IR region light absorption by these QDs is significant. In addition, high internal quantum efficiency (~80%) is reached despite the fact that hole is confined to the CdTe core upon photon absorption and exciton generation.
Although we already succeeded in fabricating a relatively high efficiency QDSSC (almost 2.5% for a 1x1 cm2 electrode), this type-II system has a big potential if several directions for optimizing the performance will take place.
Schematic representation of the system consists of a type-II CdTe/CdSe core/shell QD adsorbed on a nanocrystalline titania and overcoated by a ZnS shell. Assumed band diagram of the system is also shown. Arrows show the charge directions after exciton generation.
1. Yang, Z.; Chen, C. Y.; Roy, P.; Chang, H. T. Quantum Dot-Sensitized Solar Cells Incorporating Nanomaterials. Chem. Commun. 2011, 47, 9561−9571. 2. Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778−17787. 3. Mora-Serό, I.; Giménez, S.; Fabregat-Santiago, F.; Gόmez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848−1857. 4. Lee, H.; Wang, M.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, M. K. Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Lett. 2009, 9, 4221−4227. 5. González-Pedro, V.; Xu, X.; Mora-Serό, I.; Bisquert, J. Modeling High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 4, 5783−5790. 6. Kim, S.; Fisher, B.; Eisler, H. J.; Bawendi, M. Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures. J. Am. Chem. Soc. 2003, 125, 11466−11467.
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