Spray-assisted silar deposition of cadmium sulphide quantum dots on metal oxide films for excitonic solar cells
Graphical abstract
Introduction
Inorganic semiconductor nanocrystals (NCs), also known as quantum dots (QDs) have attracted a remarkable interest over the last two decades as appealing materials to be exploited in a variety of domains [1], [2], [3]. This attention is especially motivated by their optical and electronic properties, easily tunable through the modulation of their dimensions. The first paper dealing with their preparation appeared in 1993 [4] and since then many efforts have been carried out devoted to both increase the basic knowledge on these systems and the ways to exploit them.
In particular, in the field of third generation photovoltaics (PV), semiconductor QDs appear as the ultimate frontier as light harvesters [3], since they have band gap easily tunable with sizes, they can energetically sensitize metal oxides, they can expand the solar spectrum region in which light is collected [5] and they could in principle overcome the Queisser–Shockley limit, thanks to the claimed multiexciton generation effect, the presence of intraband transitions and other processes, which are not present in single junction cells [6]. Unfortunately, up to now the performances provided by quantum dot sensitized solar cells (QDSCs) are still lower as compared with those shown by classical dye sensitized solar cells [7]; QDSCs are indeed photoelectrochemical systems very different from their homologues DSCs, although taking advantage from a very similar architecture, and many things are to be learnt from the very beginning [8], [9]. However, this has not reduced the tremendous interest for QDSCs and many efforts are currently carried out to understand these systems and building up devices with improved performances. Very recently, a remarkable 5.4% photoconversion efficiency has been obtained with Mn-doped CdS/CdSe QDs [10]. From the technological viewpoint, the same research group set-up a one-step coating approach to design relatively high efficiency semiconductor-sensitized solar cells, based on QD-sensitized TiO2 paint [11].
Over the last three years, the technique known as SILAR (successive ionic layer adsorption and reaction) has emerged as an effective strategy to directly generate and grow semiconductor QDs on a host surface. This approach is particularly promising in the field of QDSCs for which increased photoconversion efficiencies (PCEs) have been obtained [12], [13], [14].
SILAR technique presents several appealing advantages as compared with the classical synthetic approaches, based on the well-known hot injection method. It is indeed a very simple and fast strategy to generate semiconductor nanocrystals, envisaging a certain number of immersion/washing/drying cycles into ionic solutions of selected precursors. According to the number of cycles and the chosen precursors, it is also easily possible growing QDs of various sizes and building up networks of different materials, thus opening the path towards the so-called rainbow QDSCs. SILAR approach provides for polydisperse QDs and allows an intimate contact between them and the metal oxide surface used as host (typically, a thin film of nanoparticulate TiO2).
On the other hand, some limits related to the SILAR technique in the field of PV have also been highlighted: a faster recombination with respect to colloidal QDs and a decrease in the rate of photogenerated electron injection with the number of SILAR cycles [15]. Despite the mentioned limits, SILAR remains an interesting way to easily exploit QDs for PV.
A real scenario technological exploitation of these materials asks for development of synthetic routes able to provide low cost, high production yield, high reproducibility, precise control of composition and size, low by-products amount. This latter point is of particular relevance, taking into account that the chosen materials are often composed of heavy metals (such as cadmium) and, in view of the possibility to scale up the production, waste should be as reduced as possible.
In this work we propose an innovative synthetic route to fabricate semiconductor quantum dots of CdS on polycrystalline TiO2 scaffold via SILAR technique mediated by spray deposition (SD-SILAR). The approach has been recently attempted in literature for generation of CdS QDs, exploiting secondary sulphide sources (such as thiourea), needing thermal decomposition to provide for S2− ions [16]. This mandatory treatment requires temperatures as high as 400–450 °C and in some cases a post-growth annealing devoted to either eliminate big CdS aggregates or improve QD attachment on TiO2 scaffold [17], resulting in undesired CdO formation.
In the present study, growth dynamics at low temperatures (RT to 60 °C) and QD features obtained by applying the two SILAR approaches are systematically studied and compared, demonstrating improved functional features of the QDs obtained via SD-SILAR with respect to traditional SILAR technique based on impregnation and reaction in a hot application such as a QDSC. Moreover, we will show that the SD-SILAR approach uses quite reduced amount of precursor materials, without any waste.
CdS QDs have been chosen as system models, since they are easily generated and grown on metal oxide surface with simple procedures, without any need for operating in inert atmosphere, and they are among the most exploited semiconductor sensitizers in QDSCs.
Section snippets
Preparation of photoanode
The photoanodes were prepared by tape casting commercial TiO2 pastes (from Dyesol) on FTO glass (sheet resistance ∼10 Ω □−1), followed by 30 min annealing at 450 °C in air. Substrates with various thicknesses were applied, ranging from 2 μm up to 11.5 μm. In the case of thin substrates (up to 5 μm) transparent TiO2 layer was applied (Dyesol 18 NR-T, composed of anatase nanoparticles 20 nm in diameter). The 11.5-μm thick layers have a bi-layered structure in which a 4-μm thick scattering layer
Results and discussion
CdS QDs are deposited on TiO2 thin film by means of both classical immersion SILAR (hereafter bath-SILAR) and SD-SILAR. Every sample has been characterized by ultraviolet–visible (UV–Vis) spectrophotometry after each deposition cycle. It is well known that this technique is a powerful tool to investigate semiconductor QDs since their main spectral features (position of the first excitonic peak and related absorbance value) are directly correlated with nanocrystal size and amount, respectively
Conclusions
This work demonstrates the potential hold by spray deposition applied to SILAR technique in generating and growing semiconductor quantum dots on nanoparticulate metal oxides. With respect to traditional bath-SILAR, spray deposition was found to provide for higher amount of smaller nanocrystals, resulting in increased functional performances of the corresponding solar cells.
The last, although not less relevant, characteristic of the proposed approach worth to be mentioned is the reduced amount
Acknowledgments
IC acknowledges Regione Lombardia under “X-Nano” Project (“Emettitori di elettroni a base di nano tubi di carbonio e nanostrutture di ossidi metallici quasi monodimensionale per lo sviluppo di sorgenti a raggi X”) for funding. GSS thanks OIKOS srl for financial support. AV acknowledges the European Commission under the contracts WIROX (no. 295216) and F-LIGHT (no. 299490) for partial funding.
M.N. Poli and M.S. Crottini are acknowledged for technical support.
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