Microwave assisted synthesis of CuInGaSe2 quantum dots and spray deposition of their composites with graphene oxide derivatives
Graphical abstract
Introduction
Copper indium gallium selenide (CIGSe) materials are I-III-VI p-type semiconductors that possess high optical absorption coefficients and low band gap energy. Because of these features, bulk CIGSe has been successfully applied in thin film solar cells with ca. 20% of efficiency [1,2]. Lately, there has been a growing interest in the synthesis of nano-sized CIGSe and other quaternary quantum dots (QDs), aiming at further improving the efficiency of the solar cells and the reduction of costs by using lower amounts of materials.
The synthesis of CIGSe can be performed using vacuum or non-vacuum techniques. Significant energy input is typically required in vacuum techniques, such as evaporation or sputtering. Therefore, alternative, non-vacuum, solution-based techniques have drawn crescent interest. Examples of non-vacuum tecnhniques used to synthesize CIGS from chemical precursors include refluxing [3], solvothermal [4], precipitative [5] and colloidal [6] methods, as well as the microwave assisted method [[7], [8], [9]]. In these solution-based techniques, organic solvents and high temperatures are often used. The reaction time, temperature, solvent and other preparation conditions are key parameters to minimize the size distribution of the CIGSe nanoparticles and the formation of undesired by-products.
Amongst the different non-vacuum techniques, the microwave assisted method generally offers advantage regarding a record reaction time, combined with relatively low temperatures, at low energy consumption. Microwave energy is considered as a heat source that leads to faster chemical reactions in comparison to conventional heating methods. The reaction rate is affected by the microwave field and the quality of the generated material depends on the type of reactant, applied power, reaction time and temperature [8]. The use of microwave heating for the preparation of CIGSe nanopowder has been previously reported [7,8].
Solution processing techniques are also appealing because of their potential low cost and high yield, and possibility of incorporation in a roll-to-roll (R2R) line. One example of solution processing deposition method is the spray coating technique. This deposition technique has numerous applications in industry, like in the painting, coating and graphic industry. There has also been a grown interest in the use of spray deposition for other fields, such as electronics and photovoltaics [[10], [11], [12], [13], [14]]. Recently, Kramer et al. [10] simulated a R2R processing line for the spray deposition of PbS QDs. The films were sprayed onto substrates mounted on a rapidly rotating drum and then successfully used as the active layer of solar cells with a depleted heterojunction architecture. Furthermore, the use of ultrasonic spray nozzles might allow the deposition of uniform films and improve reproducibility over larger areas [11,12].
Herein, we report the synthesis of CuInGaSe2 QDs using a microwave assisted method, and the solution processing via spray deposition of thin films based on the combination of these QDs with graphene oxide (GO) or reduced graphene oxide (rGO). Because of the electronic and optical characteristics of these materials, nanocomposites based on the combination of GO and rGO with quantum dots can be used in a variety of applications, ranging from biomedical optical imaging [15], to light-controlled conductive switching [16] and photocatalytic CO2 reduction [17], for example. In this work, we demonstrate the possibility of exploring such films for energy conversion by using the rGO-QD thin film as a counter electrode in dye-sensitized solar cells (DSSCs). DSSCs have drawn a lot of attention because of their easy fabrication process. A lab-scale DSSC may be assembled in a bench top, from solution-processed materials, without the need of expensive machinery or techniques [18]. A typical DSSC consists of a dye-sensitized TiO2 photoanode, a liquid electrolyte (typically containing the I−/I3− redox couple) and a Pt counter electrode. Amongst these components, there is interest in finding a replacement for Pt, due to the high costs associated with this noble metal. Graphene-based materials have been explored as alternatives, because these materials possess high electrical conductivity and chemical and mechanical robustness [19]. Their composites with cupper chalcogenide-based quantum dots have also been strategically used as counter electrodes and are particularly advantageous when combined with electrolytes containing different redox couples, in substitution to the corrosive I−/I3− [20,21].
Section snippets
Materials
Copper (I) chloride (CuCl), indium chloride (InCl3), gallium chloride (GaCl3), selenium powder, tri-n-octylphosphine (TOP), oleylamine (OLA), mercapto-propionic acid (MPA), ethane dithiol (EDT), TiO2 (DSL 18NR-T, Dyesol), N749 (Dyesol), expansible graphite (Grafexp, Nacional de Grafite), hydrazine, acetonitrile, hexane, methanol, ethanol and acetone were used as received.
Synthesis of CIGSe QD using microwave assisted method (MAM)
In a typical synthesis, 2 ml of 1 M TOPCuCl, 0.5 ml of 1.0 M TOPInCl3 and 0.5 ml of 1.0 M TOPGaCl3 were added to 5 ml of OLA
Synthesis of CIGSe QDs
The optical properties of as-synthesized CIGSe are shown in Fig. 1a. A large tailing of the UV–Vis absorption curve was observed. This profile is similar to that reported in previous studies using microwave-assisted synthesis of CIGSe [7] or other solution-based synthetic routes [24]. A broadening of the absorption spectrum and band tail effects may be related to the presence of intrinsic defects, since the breadth of the absorption tail is a strong function of the disorder. The absorption band
Conclusion
Pyramidal shaped-like CIGSe QDs were synthesized using a microwave-assisted technique. The nanocrystals of 10 nm average size possessed a large absorption tailing and emitted light at 650 nm. The combination of these nanoparticles with GO or rGO gave composites which were processed from aqueous solutions using an automated ultrasonic spray-coating system. The optical, morphological and electrical properties of the thin films of GO-QD and rGO-QD were investigated, revealing that the presence of
Declaration of competing interest
There is no conflict of interest.
Acknowledgements
The authors acknowledge financial support from CNPq (458413/2014-3), PIBIC/CTI and PCI/CTI, in Brazil, and the National Research Foundation (NRF), CANERGIE and Carsten trust fund, in South Africa. The authors also acknowledge LME/LNNano/CNPEM for the support in SEM analyses.
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