Publication date: 1st April 2013
Quantum dot sensitized solar cells (QDSSCs) represent an important class of photovoltaics, owing to the ease of fabrication due to the use of low cost components and processes, possibility to use non-toxic materials, as well as their very rapidly growing quantum efficiency.
The choice of the method of deposition of the quantum dots onto the transparent wide bandgap n-type semiconductor is crucial for the efficient work of the QDDSSC. Among various possibilities, linker-assisted deposition allows for the best control over the n-type material/QD distance and the state of the surface of the QDs. At the same time it is known that widely used mercaptocarboxylic acids often etch the fragile surface of nanostructured n-type oxides. We will present new bifunctional linker molecules for more efficient functionalisation of the TiO2 and ZnO. We have thoroughly studied the mechanism of the functionalisation process by XPS, Raman, IR and contact angle techniques. This has allowed to optimize the functionalisation conditions and avoid damaging of the oxides.
As the quantum dots for the sensitization of the functionalized wide bandgap nanostructured semiconductors we have chosen copper indium sulfide (CuInS2) nanocrystals of various sizes with optional passivating shell of CdS or ZnS. These nanocrystals advantageously possess very high absorption coefficients coupled with an appropriate bandgap of ca. 1.5 eV, moreover they do not contain toxic elements and are relatively stable against photooxidation. Their deposition process onto ZnO and TiO2 has been studied and optimized by scanning electron microscopy, and absorption and photoluminescence spectroscopy. After the deposition, the initial long ligands passivating the surface of CuInS2 quantum dots, such as aliphatic amines or thiols, can be replaced by shorter molecules, such as e.g. dithiols. The same method has also served to increase the thickness of the absorber layer by layer-by-layer deposition. We study also the influence of the initial CdS or ZnS passivation shell and compare it with the SILAR shell growth onto the assembled quantum dots.
The nanostructured oxides sensitized by CuInS2 were then integrated in the QDSSCs by using either liquid and solid electrolytes. With liquid electrolytes we have obtained so far the photovoltaic conversion efficiency (PCE) of 0.36%, while with the solid ones it is about 50 times lower. We will discuss possible reasons of the lower PCE of the solid state solar cells in terms of possible interaction of the deposited quantum dots with commonly used p-type semiconductors CuSCN and spiro-OMeTAD.
Cross-section of a ZnO/CuInS2/spiro-OMeTAD solar cell: cross-section
1. Sanchez, S.; Aldakov, D.; Rouchon, D.; Rapenne, L.; Delamoreanu, A.; Lévy-Clément, C.; Ivanova, V. “Sensitization of ZnO nanowire arrays with CuInS2 for extremely thin absorber solar cells” J. Renewable Sustainable Energy 2013, 5, 011207. 2. Aldakov, D.; Lefrançois, A.; Reiss, P. “Ternary and quaternary metal chalcogenide nanocrystals: synthesis, properties and applications” J. Mater. Chem. C, 2013, advance article.
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