3.3.1 Synthesis of CuInS2 nanocrystals
The CIS nanocrystals were prepared using a high-temperature organic solvent process.
First, sulfur was dissolved in TOP. The solution was diluted with ODE to form a clear solution (solution 1). Then, CuCl and InCl3 were dissolved in oleylamine at 50oC to form another solution (solution 2). These two material solutions were mixed to produce a raw material solution. A small aliquot of raw material solution was put into
a test tube and soaked directly in an oil bath that had been preheated to 240oC and aged for 300s. The prepared solutions were limpid, reflecting their high colloidal stability.
3.3.2 Synthesis of CuInS2@ZnS core/shell nanocrystals
Preparation of the CIS@ZnS core/shell nanocrystals was achieved by modifying the above reaction system and surface coating of ZnS. Zinc stearate (ZnSt) and sulfur were each dissolved in TOP (solutions 3 and 4). After the colloidal solution of CIS nanocrystals was preheated to 240oC, solutions 3 and 4 were injected into the solution for 300s. Subsequently, the mixture was cooled to 90oC to form the ZnS shell on the surface of CIS nanocrystals. Repeating the injection step were also used to grow the multiple ZnS layers on the CIS nanocrystals. The product was diluted with toluene and PL spectra were measured.
3.3.3 Systhesis of ZnO nanowire
The ZnO thin films were deposited on F-doped SnO2 (FTO, 30 Ω/sq, Sinonar, Taiwan) substrates by RF magnetron sputtering as followed from our previous report.[126]
The seeded substrates were then suspended horizontally in a reagent solution containing 0.016 M zinc nitrate and 0.025 M methenamine and heated to 95°C to initiate nanowire growth.
3.3.4 Synthesis of Zn-CIS quantum dots
The Zn-CIS QDs were prepared using a high-temperature organic solvent process. 0.5 mmol diethyldithiocarbamic acid zinc salt was dissolved in 6 ml TOP. The solution was diluted with 24 ml ODE to form a clear solution (solution 1). Then, 0.2 mmol CuCl and InCl3 were dissolved in 6 ml oleylamine at 50oC to form another solution
(solution 2). Here, amine coordinates the Cu and In ions to produce amine complexes.
These two material solutions were mixed to produce a raw material solution with a ratio Zn:Cu:In:S = 1:n:n:4 (n=1 in this composition), the composition ratios of Cu and In were varied from 1 to 3. Those of Zn and S precursors were fixed in concentration, indicated as Zn:Cu:In:S = 1:n:n:4 (n=1-3). A small aliquot of raw material solution was put into a test tube and soaked directly in an oil bath that had been preheated to 240oC and aged for 300s. The prepared solutions were limpid, reflecting their high colloidal stability.
3.3.5 ZnS coating of Zn-CIS quantum dots
A colloidal solution of ca. 20 mg of Zn-CIS nanocrystals with an average diameter of 3.6 nm in 4 mL of toluene was placed in a three-neck flask under purified argon flow.
After addition of 2.5 mL of TOPO, the mixture was heated to 190 °C and then kept at this temperature till a complete heptane evaporation. Zinc stearate (316 mg) was dissolved in 2.5 mL of toluene upon gentle heating (ca. 60 °C). After cooling to room temperature, the resulting 0.2 M solution was mixed with 2.5 mL of a 0.2 M solution of Se in TOP. By means of a syringe pump this mixture was injected within 1 h into the reaction flask containing the core nanocrystals at 190-200 °C. Periodically small aliquots were removed in order to monitor the shell growth. After the addition was completed the crystals are annealed at 190 °C for an additional 1-1.5 h.
3.3.6 Synthesis of hydrophilic quantum dots by ligand exchange
Purified QDs were dissolved in a minimum amount of chloroform and excess mercaptopropionic acid (MPA) was added until the solution became cloudy, and stirred at 55°C for one night in argon atmosphere. After that, 1mL of tetrahydrofuran (THF) was added to stop the surface exchange reaction. After cooling to room
temperature, a suspension of potassium t-butoxide in THF was added to neutralize the carboxyl acid function, and then centrifuged to remove the by-products with THF.
The THF washing process was repeated for 2-3 times, and finally distilled water was added to disperse hydrohilized Zn-CIS NCs into water.
3.3.7 Assembling Zn-CIS quantum dots on ZnO Nanowires
Fire ZnO plate at 450oC for 30 minutes and then after cooling in air for 5 minutes, transferred ZnO plate to ZCIS solution and left for 3 days to ensure saturated adsorption of Zn-CIS QD onto the ZnO nanowires.
3.3.8 Fabrication of solar cells
For characterization of the photovoltaic performance of our devices, the Zn-CIS/ZnO films were served as working electrode (anode); a fluorine doped tin oxide glass (typical size 1.0×2.0 cm2) coated with platinum (Pt) particles by sputtering was used as a counter electrode (cathode). The two electrodes were assembled into a cell of sandwich type and sealed with a hot-melt film (SX1170, Solaronix, thickness 25 μm);
a thin layer of electrolyte was introduced into the space between the two electrodes and the device was fabricated accordingly. A typical redox electrolyte contained 0.1 M lithium iodide (LiI), 0.01 M iodine (I2), 0.5 M 4-tert-butylpyridine (TBP), 0.6 M butyl methyl imidazolium iodide (BMII), and 0.1 M guanidinium thiocynate (GuNCS) in a mixture of acetonitrile (CH3CN, 99.9%) and valeronitrile (n-C4H9CN, 99.9%) (v/v = 15/1)
3.3.9 Synthesis of Zn-CIS nanocrystals in the presence of high frequency magnetic field
0.5 mmol diethyldithiocarbamic acid zinc salt was dissolved in 6 ml TOP. The
solution was diluted with 24 ml ODE to form a clear solution (solution 1). Then, 0.2 mmol CuCl and InCl3 were dissolved in 6 ml oleylamine at 50oC to form another solution (solution 2). Here, amine coordinates the Cu and In ions to produce amine complexes. These two solutions were mixed to produce the raw material solution. A small aliquot of raw material solution was put into a test tube and exposed to HFMF with an input power of 90 W (Figure 6.1). The color of the mixture solution was changed with different durations of HFMF exposure from yellow (30 sec), red (45 sec) to black (120 sec). The resulting precipitated powders were collected via centrifugation at 6000 rpm, removed from the solution, and repeated three times to remove excess surfactants which were precipitated using methanol.
3.3.10 Fabrication of Zn-CIS thin film solar cells
The Mo coated soda lime glass substrates used here was fabricated by dc magnetron sputtering at Ar pressures 1.5 mTorr resulting in a 200 nm layer. Deposition of the CIS absorber layer on top of the Mo substrates is used drop casting by the nanoink solution and subsequent thermal treatments to remove the organics and sinter the films under Ar and Se atmospheres at 500 oC respectively. A ~ 50 nm CdS layer is then deposited by a chemical bath deposition (CBD) technique. The CBD bath contains 183 ml of deionized H2O, 25 ml of 0.015 M CdSO4 solution, 12.5 of 1.5 M thiourea solution, and 31.25 ml of stock NH4OH (Aldrich). Next, A ~50 nm high resistivity i-ZnO film capped with a ~300 nm high conductivity ITO layer are deposited by RF magnetron sputtering. The ZnO film is sputtered in a mixture of 10%
O2 in Ar at sputtering pressure of 10 mTorr with no intentional heating. The ITO layer is sputtered with neither O2 nor intentional heating at sputtering pressure of 1 mTorr.
After sputtering of the oxide layers, the final device is baked in air at 200 oC over night.