Journal Information
Journal ID (publisher-id): chemical
Title: Journal of the Korean Chemical Society
Translated Title (ko): 대한화학회지
ISSN (print): 1017-2548
ISSN (electronic): 2234-8530
Publisher: Korean Chemical Society대한화학회
Over the last two decades, there have been numerous efforts to prepare well-defined semiconducting nanocrystalline quantum dots (QDs) by the formation of colloidal nanoparticles in solution or by in-situ deposition of those over substrates.1,2 The as-prepared QDs have been utilized in many optoelectronic applications as well as in the investigation of fundamental photophysical properties depending on their different sizes in the realm of quantum confinement.3,4 Recently, QD sensitizer-based solar cells have attracted much attention due to its intrinsic advantages such as strong light-absorption, easy tuning of band gap, and etc.5 In particular, QDs could play a crucial role as a photo-sensitizer in the structure of dye-sensitized solar cells (DSSCs) instead of molecular dyes.6 Besides direct attachment of colloidal QDs,7,8 those QD sensitizers could be grown in-situ on the surface of mesoporous metal oxide films by various chemical bath deposition (CBD) techniques.9−11 Among the many CBD processes, successive ionic layer adsorption and reaction (SILAR) is considered one of the most effective ways to control the deposition of QDs precisely by alternating dipping of the substrate into each precursor solution.12−14
In this communication, we suggest that there is a facile route to converting SILAR-deposited QD sensitizer over mesoporous TiO2 film into other ones by just using a simple cation-exchange process as a post-treatment. As a proof-of-concept experiment, the well-known CdS QD was deposited firstly over TiO2 film by repeating the SILAR process five times.15 Then, the as-prepared CdS QD-sensitized TiO2 electrode was just dipped for 30 seconds into another chemical bath containing a salt of metal cation of target QDs. The typical yellow color of CdS QDs was changed immediately into the different one of the target QDs as shown in the color change of electrodes and their corresponding absorption changes in Fig. 1. This very rapid change of color could be explained roughly by different solubility products (Ksp) of participating metal cations and sulfide anion. The Ksp value of CdS is much larger than those of all the other metal sulfides tested here,16 which means that Cd2+ is subject to be more soluble than other cations when present together with sulfide anion in an aqueous solution. Therefore, Cd2+ is diffused out from CdS QD and other cations (Bi3+, Ag+, Cu2+, and Pb2+) are diffused in to make new metal sulfide QD-sensitizers over the mesoporous TiO2 film. However, more precisely, we have to consider the relative thermodynamic stabilities between the reactant and product phases, and the effect of cation solvation (or ligation) and nanostructures to estimate the driving force for an ion-exchange reaction.17−19
Figure1.
Absorption spectra of SILAR-deposited CdS QD and its converted ones (Bi2S3, Ag2S, CuS, and PbS) by a cation-exchange process over mesoporous TiO2 film/FTO electrode and a picture of as-obtained electrodes (inset).
This kind of conversion by a cation-exchange reaction was successfully demonstrated before in freely-moving colloidal QDs in solution,17−20 but not yet in QDs fixed onto the surface of mesoporous metal oxide films. Here, we have confirmed that the process of cation-exchange is also possible over the electrode-anchored QD by contacting it with a solution with a target cation that shows a lower solubility product with a common anion (S2−). This simple but very effective way of conversion from the preformed SILAR-deposited CdS QD into new one could be useful in preparing QD sensitizers of which preparations were known to be difficult by a direct SILAR process. In the preparation of Sb2S3-sensitized solar cells, only time-consuming and inconvenient bulk chemical bath deposition was proven to be effective in preparing Sb2S3 over mesoporous metal oxide films.21,22 The more straightforward and controllable SILAR was rarely applied maybe due to the ineffective adsorption and reaction of Sb3+ with (+3) positive charges. To prepare an Sb2S3-sensitized TiO2 electrode, CdS was deposited over the TiO2 film by a typical SILAR process where cadmium acetate was used to induce more adsorption than in the case of cadmium nitrate.23 The orange-colored TiO2/CdS was observed with Cd(acetate)2 (Fig. 2) while a yellow one was prepared with Cd(nitrate)2 (Fig. 1). This SILAR-deposited orange TiO2/CdS electrode was dipped into a SbCl3 solution to induce the cation-exchange process between Cd2+ and Sb3+. After this dipping, the color of the electrode changed to a little bit deeper one but got to dark brown with the well-known annealing process up to 300 ℃ under nitrogen atmosphere for the crystallization of Sb2S3 (Fig. 2).24 The energy levels of main components were estimated in the inset of Fig. 2 from the Tauc plots (Fig. S1) and reported data,25 which indicate a favorable charge transfer after light absorption.
Figure2.
Absorption spectra of (1) SILAR-deposited CdS QD and (3) its converted one to Sb2S3 by a cation-exchange reaction over mesoporous TiO2 film/slide glass while (2) and (4) are annealed ones from (1) and (3), respectively. A corresponding picture of as-obtained substrates and energy levels of main components are shown in the inset.
With this transformed Sb2S3 sensitizer, photovoltaic tests were conducted with a cobalt redox couple, Co(bpy)32+/3+ as a hole mediator in the structure of DSSCs. The overall power conversion efficiency increased from 0.55% (Jsc: 3.05 mA/cm2, Voc: 0.38 V, FF: 0.48) to 0.84% (Jsc: 5.22 mA/cm2, Voc: 0.35 V, FF: 0.46) after conversion from CdS to Sb2S3. In the lower intensities than the standard 1 sun, Sb2S3-sensitized cell showed about 1.0% efficiency (Table S1). This increase could be attributed mainly to the enhanced short-circuit current (Jsc) although open-circuit voltage (Voc) and fill factor (FF) decreased slightly in this conversion. As can be seen in the incident photon-to-current conversion efficiency (IPCE) in Fig. 3, the Sb2S3-sensitizer showed a better conversion efficiency response over a broader region up to ~750 nm, which was limited to ~650 nm in the case of CdS. The direct SILAR-deposited Sb2S3-sensitizer only showed about half of the value (0.40%) of this one converted from CdS (see Table S1). This lower efficiency could be attributed to a difficulty of controlling the direct adsorption of Sb3+ ion over mesoporous metal oxide films, which should be checked more in the research of SILAR-deposited QD sensitized solar cells. Although the overall efficiency of this cation-exchanged Sb2S3-sensitized cell is relatively lower compared to the reported CBD-based ones,26,27 there is still much room left for further optimization and improvements in many parameters of the experimental process. In particular, the relatively low Voc of 0.38 V was one main reason for total low efficiency, and this was confirmed by the measurement of the Voc decay curve as shown in Fig. 4. The faster decay of Sb2S3 sensitizer than the initial CdS one indicates that TiO2/Sb2S3 is suffering a faster recombination after electron injection from Sb2S3 to TiO2, thus leading to a lower Voc due to a shorter electron life-time. If we could passivate the interface of TiO2/Sb2S3, a higher Voc would be possible for better performance.28,29
Figure3.
IPCE spectra of solar cells with a QD-sensitizer of (1) SILAR-deposted CdS and (2) its converted one to Sb2S3 while (3) was from a direct SILAR-deposited Sb2S3.
Figure4.
Open-circuit voltage (Voc) decay data from (1) SILAR deposited CdS- and (2) its converted Sb2S3-sensitized cells.
In this study, we have successfully demonstrated that it is also possible to prepare new QD sensitizers anchored onto the surface of mesoporous TiO2 film via effective SILAR deposition firstly and a simple/rapid cation-exchange reaction secondly. As a model system, Sb2S3 QD sensitizer was prepared from the preformed CdS QD by SILAR and tested for its photovoltaic performances. This new way of surface-attached QD preparation could find many useful applications in optoelectronic, photonic, and catalytic reactions and devices by constructing tailor-made substrates with metal oxide film/semiconducting QDs.