Quantum Dot-Sensitizers Prepared by SILAR and Cation-Exchange Processes

All chemicals were used as received; Cadmium nitrate tetrahydrate [Cd(NO₃)₂·4H₂O, ≥99.0%], cadmium acetate dihydrate [Cd(CH₃COO)₂·2H₂O, reagent grade, 98.%], sodium sulfide nonahydrate (Na₂S·9H₂O), sodium sulfide (Na₂S), antimony(III) chloride (SbCl₃, ACS reagent, ≥99.0%), silver nitrate (AgNO₃, ACS reagent, ≥99.0%), bismuth nitrate pentahydrate [Bi(NO₃)₃·5H₂O, ACS reagent, ≥98.0%], copper nitrate hydrate [Cu(NO₃)₂·xH₂O, 99.999%], lead nitrate [Pb(NO₃)₂, 99.999%], and triethanolamine (TEA, reagent grade, 98%) were purchased from Sigma-Aldrich. Ethanol and methanol were of HPLC-grade.

0.20 M Co(II) complex of [Co(bpy)₃](PF₆)₂, 0.05 M Co(III) complex of [Co(bpy)₃](PF₆)₃, and 0.10 M, lithium perchlorate (LiClO₄) were dissolved in acetonitrile. Here, bpy stands for bipyridine.

For a thin TiO₂ blocking layer, fluorine-doped tin oxide (FTO)-coated glass electrode (Solaronix, 8 Ω/□) was treated with 40 mM TiCl₄ aqueous solution for 30 min at 70 ℃ and then washed with pure water. The electrode was gradually heated to 500 ℃. Using screen-printing machine and commercial TiO₂ pastes, about 5 μm thick TiO₂-blend layer (Dyesol, 18NR-AO; TiO₂ particle size 20~450 nm) which is responsible for adsorbing most of sensitizers in the electrode and ~4 μm scattering layer (Dyesol, WER2-O; TiO₂ particle size 150~250 nm) were deposited sequentially and then sintered up to 500 ℃ to make a mesoporous TiO₂ film. Finally, the typical TiCl₄ post-treatment was applied as the same manner with the pre-treatment of TiCl₄. A platinium (Pt)-counter electrode was prepared by following procedure; a 10 nM solution of chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, Sigma-Aldrich) in ethanol was dropped onto the FTO electrode and dried in air for 20 min. Then, the electrode was annealed gradually to 450 ℃ for 30 min under atmospheric conditions.

To grow CdS QD sensitizer, the typical SILAR process was done by alternative dipping of as-prepared FTO/TiO₂ electrodes in 0.05 M Cd(NO₃)₂ and 0.05 M Na₂S·9H₂O aqueous solutions, respectively for 1 minute, which was repeated five times. Between dipping, a washing step was included for 1 minute in pure water. After depositing CdS by following the SILAR process, the cation-exchange process was done by dipping the as-deposited CdS/TiO₂/FTO electrode into 0.10 M AgNO₃, Bi(NO₃)₃, Cu(NO₃)₂, or Pb(NO₃)₂ aqueous solution for 30 seconds [0.10 M Bi(NO₃)₃, was saturated due to its low solubility in water]. The yellow color of CdS/TiO₂/FTO electrode was changed immediately by a cation-exchange for new target QDs.

SILAR and cation exchange processes were used to prepare Sb₂S₃ QD sensitizer. For SILAR-growing of CdS, a 0.10 M Cd(CH₃COO)₂· 2H₂O and 1 M triethanolamine in ethanol/water (1:1, v/v), and a 0.10 M Na₂S·9H₂O in methanol/water (1:1, v/v) were used for each cationic and anionic source. The mesoporous TiO₂ film-covered FTO electrode was dipped into Cd²⁺ solution, pure ethanol (then dried in air), the S^2- solution, and then pure methanol (then dried in air) successively for 1 min each. Such immersion cycle was repeated 9 times. After this SILAR process, the electrode of TiO₂/CdS was immersed into a 0.10 M SbCl₃ in ethanol for 10 min to induce cation exchange process at room temperature. A post-treatment of annealing in N₂ atmosphere was applied by increasing temperature from 100 ℃ to 300 ℃ for 20 min. The color change was observed on the TiO₂/Sb₂S₃ electrode during the annealing procedure, which was orange before annealing and gradually turned dark brown as the temperature increased. But, in the case of CdS, there was only a slight change in the intensity of color.

A 0.020 M SbCl₃ in ethanol and 0.020 M Na₂S in ethanol were used for each cationic and anionic source for depositing Sb₂S₃ by the SILAR process, which was repeated 5 times with a dipping time of 90 seconds each. Between the dipping in each precursor solution, the electrode was washed in pure ethanol in 3 minutes. The as-prepared Sb₂S₃-deposited electrode was annealed to 300 ℃ for 20 minutes under nitrogen.

To make QD-sensitized solar cells with a counter electode of the typical platinized FTO glass, the as-prepared QD-sensitized photoanode and Pt-cathode were combined by hot-press machine through the Surlyn film, and the electrolyte solution was injected through a pre-drilled hole through the counter electrode.

The current-voltage and open-circuit voltage decay characteristics were analyzed under a standard illuminating condition (1 sun, 100 mW cm^-2) using a solar simulator (Peccell, PEC-L01) and a potentiostat (IVIUM, Compactstat). Various irradiance intensities from 0.1 to 1.0 sun could be provided with optical attenuators. The incident photon-to-current efficiency (IPCE) data were collected by a light source (ABET 150W Xenon lamp, 13014) with a monochromator (DONGWOO OPTORN Co., Ltd., MonoRa-500i) and a potentiostat (IVIUM, Compactstat) based on DC method without chopper and light bias. Optical absorbance was measured with a UV-VIS spectrophotometer (PerkinElmer Lambda 25).

Figure S1.

Tauc plots of (a) CdS- and (b) Sb₂S₃-QD sensitized TiO₂ films.