Hot-Injection

Development of colloidal routes enable low-cost fabrication of inorganic solar cells through a wet-chemistry process of semiconductor nanocrystal colloidal solution has attracted a great deal of attention. It has been known that crystal structure, composition, and size of the nanocrystals may significantly affect their optoelectronic property and device performance. Although the structure of nanocrystals can be generally controlled by the capping ligands used, the reported Cu-In-S nanocrystals are still limited to the chalcopyrite structure. Nevertheless, composition of current Cu-In-S nanocrystals is mainly controlled by using a single ternary precursor. In spite of the use of a single-source precursor is convenient, this method is limited by the precursor availability and their tedious procedure.

In 2006, Nakumura et al. introduced Zn into CIS system with a kind of hot-injection method and enhanced their PL intensity. [75] The particle size varied from 2.6 to 4 nm with increasing Cu/Zn ratio, both the absorption and emission peaks also show red-shifted with increasing Cu/Zn ratio (Figure 2.19).

Figure 2.19 Raw material composition effects on optical properties: (a) UV-vis absorption and (b) PL emission spectra (excitation 500 nm). (Raw material composition Zn:Cu;In:S = 1:n:n:4 (n = 0.5-5).) [75]

In a chalcopyrite CIS, the Cu-S bond is weaker than that of In-S and the Cu vacancy is preferably generated. In addition, the Cu vancancies induced anti-site defect generation [76] and the ionic diameters of Cu and Zn are similar. On the basis of reasons above, Zn preferentially substituted to Cu and prevented anti-site defects.

Therefore, introduction of Zn ion into CIS nanocrystals was predicted to improve the PL intensity. Furthermore, the band-gap was controlled by this Zn content. The wide selectivity in components and composition by this method suggest a new series of chalcopyrite-type semiconductor nanocrystals with various properties.

Different crystal structures were obtained through a hot-injection method on Cu(dedc)2 and In(dedc)3 precursors (dedc is diethyldithiocarbamate.), with tunable [Cu]/[In] composition. [77] Zincblende CIS nanocrystals can be obtained by the co-decomposition of Cu(dedc)2 and In(dedc)3 in the presence of oleic acid. Similarly, wurtzite CIS nanocrystals can be prepared by the co-decomposition of Cu(dedc)2 and In(dedc)3 in the presence of dodecanethiol. The formation of CIS nanoparticles may

involve possible reaction routes illustrated in Scheme 2.2: the reaction of CuS or Cu2S from the decomposition of the Cu(dedc)2 in the presence of oleic acid or dodecanethiol with In2S3 from In(dedc)3, respectively, results in the formation of zincblende and wurtzite CIS nanocrystals.

Scheme 2.2 Schematic formation mechanism of CIS from Cu(dedc)2 and In(dedc)3

precursors. [77]

This study correlates the crystalline structure of the binary ZnS nanocrystals with those of ternary Cu-In-S nanocrystals, demonstrating the feasibility fabrication their alloyed or core/shell structures.

Chapter 3

Experiment 3.1 Chemicals

Copper(I) chloride (CuCl, 95%, analytical reagent), indium(III) chloride (InCl3, 98%, AR), sulfur powder (99.5%, chemically pure), Zinc stearate (10~12% as Zn, technical) and trioctylphosphine (TOP, 90%, technical grade) were purchased from Sigma-Aldrich Corp.; octadecene (ODE, 90%, technical grade) and oleylamine (70%, technical grade) were purchased from Tokyo chemical industry Co., Ltd.

Diethyldithiocarbamic acid zinc salt ([(C2H5)2NCSS]2Zn, technical grade), zinc nitrate hexahydrate (Zn(NO3)2．6H2O), methenamine (C6H12N4), mercaptopropionic acid (MPA) were purchased from Sigma-Aldrich Corp.

3.2 Characterization

The resulting powder collected from the solutions was examined using X-ray powder diffraction (XRD, M18XHF, Mac Science, Japan) to identify the crystallographic phase of nanocrystals, with Cu Kα radiation (λ=0.15405 nm) (40kV, 200 mA), 2θ ranging from 10^o to 70^o at a scanning rate of 10^o /min.

Transmission electron microscope (TEM)

TEM images were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. Samples for the TEM were prepared by depositing a drop of the samples dispersed in toluene onto a carbon grid. The excess liquid was wicked away with filter paper, and the grid was dried in air.

X-ray photoelectron spectroscopy (XPS)

XPS measurements were carried out using a Field Emission – Auger Electron Microprobe (Thermo VG Microlab 350) x-ray photoelectron spectrometer using an Mg Kα x-ray as the excitation source.

3.2.2 Optical measurement

Then UV-vis, PL emission, and excitation (PLE) spectroscopy was applied using a UV-vis spectrophotometer (UV-1600; Agelent 8453) and a spectrofluorometer (FP-6600; Jasco, Inc., Japan). For these observations, the products were precipitated with alcohol. They were further isolated by centrifugation to remove excess surfactants. The centrifuged particles were redispersed into an organic solvent such as toluene to yield a clear and colloidally stable suspension. The amounts of Zn-CIS absorbed on the ZnO films were measured with an UV-visible-NIR spectrophotometer (JASCO V-570) equipped with an integrating sphere (JASCO ISN-470).

3.3.3 Solar parameter measurement

Measurements of IV curves were made with a digital source meter (Keithley 2400, computer-controlled) with the device under one-solar AM 1.5 irradiation of a solar simulator (Newport-Oriel 91160) calibrated with a Si-based reference cell (Hamamatsu S1133) and a IR-cut filter (KG5) to correct the spectral mismatch of the lamp. The Zn-CIS/ZnO devices were operated in a back-side illumination fashion with the transparent counter electrode masked by a black plastic tape of the same size with a round hole to allow the actively illuminated area of 0.28 cm² for all measurements. Time-resolved measurements were performed with a tunable nanosecond optical-parametric-oscillator/Q-switch-pumped neodymium doped

yttrium aluminum garnet laser system NT341/1/UV, Ekspla. Emission transients were collected with a monochromator SpectraPro-300i, ARC, detected with a photomultiplier tube R928HA, Hamamatsu connected to a digital oscilloscope LT372, LeCroy, and transferred to a computer for kinetic analysis.

3.2.4 High frequency magnetic field set up

A high frequency (50-100 kHz) magnetic field (HFMF) was applied to the precursors to provide kinetic energy for Zn-CIS nanocrystal synthesis. The HFMF was created by a power supply, function generator, amplifier, and cooling water. Similar equipment was also reported in PNAS, vol. 103, 3540–3545 (2006). The strength of the magnetic field depended on the coils. In this study, the coil was 8 loops, the frequency was 50 kHz and the strength of magnetic field (H) was 2.5 kA/m. The temperature of HFMF generator was controlled by cycling cooling water at 25 ^oC.

Hysteresis loop analysis

Measurements of magnetization (M) versus applied field H and temperature T were carried out using a commercial SQUID (superconducting quantum interference device) magnetometer (MPMX-XL7).

3.3 Material fabrication

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 50^oC 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 240^oC 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 240^oC, solutions 3 and 4 were injected into the solution for 300s. Subsequently, the mixture was cooled to 90^oC 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 50^oC 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 240^oC 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 450^oC 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 50^oC 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.

Chapter 4

Synthesis and Characterization of Highly Luminescent CuInS

and CuInS

/ZnS (Core/Shell) Nanocrystals

4.1 Introduction

CuInS2 is one of the most important I-III-VI2 semiconductor materials for use in photovoltaic solar cells and it has many notable advantages such as an appropriate band gap, a high absorption coefficient, and good thermal, environmental, and electrical stability. [78-80] Also, some research aims to apply chalcopyrite materials in “spintronics” as tailorable ferromagnetic materials. [81-84] However, chalcopyrite NCs have not been systematically investigated and relatively few studies [70, 73, 75, 85-86] have achieved particle sizes small enough to exhibit quantum confinement (the Wannier-Mott bulk exciton of the CuInS2 is 8.1 nm [86]). Although Nairn et al.

combined single source precursors and photolysis methods to obtain CuInS2 NCs ~2 nm in diameter, the photoluminescence (PL) in these chalcopyrite NCs were not so strong as the well-developed II-VI group semiconductor NCs (CdSe, CdS et al.). [85]

Nakamura and coworkers modified the high-temperature organic solvent method and successfully introduced Zn into the CuInS2 NCs to achieve tunable band-gap energies (Eg) and PL. Compared to the pure CIS NCs prepared by a similar method, it was found that the quantum yield for the Zn-CIS alloy (~5%) was still much less than that of CdSe. [76]

Expansion of typical II-VI group semiconductor NCs such as CdSe and PbS are necessary because of the “Restriction of the Use of Certain Hazardous Substance (RoHS)” in Europe. Based on considerations of environmental and human toxicity, biological studies confine these NCs to specific fields. Luminescent CIS NCs are sought as new candidates for building blocks of nanomaterials and biological tabs

though quantum efficiency and photostability has been improved. Up to now, wide-band gap materials such as CdS, ZnS, and ZnSe have been widely used as shells to enhance the photoluminescence quantum yield by passivating surface nonradiative recombination sites. [87-89] However, no systematic studies have been reported on surface coatings on I-III-VI2 type chalcopyrite semiconductors and core-shell structures although Nakamura and co-workers mentioned that ZnS deposition on the surface of Zn-CIS alloy NCs could improve the PL intensity. [75]

In the CIS/ZnS NCs system, ZnS was chosen to coat the CIS to form CIS/ ZnS core shell NCs, the facile synthesis and luminescent properties of monodisperse CIS, and CIS/ZnS (core-shell) NCs were investigated. In addition, x-ray diffraction, x-ray photoelectron spectroscopy, dynamic light scattering, and transmission electron microscopy were used to analyze the composite NCs and determine their chemical composition, average size, size distribution, shape, and internal structure.

4.2 Formation and microstructure of CuInS

Figure 4.1 shows the XRD patterns of the as-prepared CIS and CIS/ZnS core/shell NCs. The powder patterns for ZnS and CuInS2 are also shown for comparison in the bottom and top inset. In Figure 4.1(a), there is an intense peak at 2θ=27.8^o, oriented along the (112) direction and other prominent peaks observed at 46.5^o (204) and 54.8^o ((312)/(116)), which are signatures of the chalcopyrite structure of CIS. The location of the pattern is in good agreement with the JCPDS reference diagrams in the bottom inset (JCPDS No. 32-0339). Similar features are observed for CIS/ZnS core–shell NCs but the latter exhibit an enhanced intensity in XRD peaks, which could result from the contribution of both ZnS and CIS. In the range between 15^o and 25^o, a broad halo appears, probably due to an amorphous contribution. This may indicate that the organic ligand TOP is still present, and may

200103

not have been removed completely in processing.

Figure 4.1 (a)XRD patterns of pure CIS and (b) CIS/ZnS (coe/shell) NCs as well as the standard data for ZnS (vertical bars, JCPDS card No. 10-0434) and CIS. (vertical

bars, JCPDS card No. 32-0339).

Figure 4.2 shows the TEM images of the as-prepared CIS NCs and particle size distribution. In Figure 4.2(a), it can be seen that the resultant CIS NCs are uniform spherical-shaped particles with a very narrow size distribution (Fig 4.2(d) and 4.2(e) show the result of DLS analysis). Figure 4.2b shows a high-resolution TEM (HRTEM) image of the CIS NCs. It demonstrates the high crystallinity of the CIS NCs. The distances (0.31 nm) between the adjacent lattice fringes are the interplanar distances of CIS (112) plane, agreeing well with the (112) d spacing of the literature value of 0.319 nm (JCPDS NO.32-0339). Furthermore, after the growth of the ZnS shell, the image in Figure 4.2(c) shows slight differences in contrast between the core and the outer part, whereas it is hard to identify the structural differences between the core and the shells by comparing the contrast in the TEM image. However, the shell thickness can be roughly determined by the size difference between the average size of CIS/ZnS core/shell NCs (4.6±0.5nm), which is larger than that of the core CIS NCs

2 nm

(2.4±0.5nm). The difference in particle size indicates that the shell thickness is around 1.1 nm.

Figure 4.2 (a)TEM image, and (b) HRTEM image of CIS, as well as (c) HRTEM image of CIS/ZnS NCs. Particle size distribution histograms of (d) CIS, and (e) CIS/ZnS NCs.

The X-ray photoelectron spectra (XPS) were employed to investigate the surface compositions and chemical state of CIS and CIS/ZnS core/shell NCs. The results are shown in Figure 4.3(a) and 4.3(b), respectively, indicating the presence of Cu, In, Zn, and S, as well C from reference and O impurity. The spectra could be considered to be primarily contributed from the shell material, since XPS is a surface-sensitive technique. The peak of the Zn 2p3/2 in 3b is larger than that of the CIS/ZnS NCs, which can explain the formation of the ZnS shell to some extent.

0 200 400 600 800 1000

S 2p Zn 2p

Cu 2p

In 3s

O 1s

In 3d

C 1s

(a) (b)

In te n s it y ( a .u .)

Binding energy (ev)

Figure 4.3 XPS of CIS (a) and CIS/ZnS (b) core/shell NCs.

4.3 Photoluminescence (PL) of the CuInS

Nanocrystals.

Figure 4.4 displays the photoluminescence behavior of as-prepared CIS NCs dispersed in a toluene solution. It was observed that the emission spectrum under excitation wavelength 366 nm is a slightly broad band centered around 450 nm. It has been previously reported that addition of TOP enhances the 450 nm emission band and quenches the 550 nm band. [74] The peak intensity of this emission band is significantly higher than that of bulk CuInS2 (>810 nm) due to the size-dependent quantum confinement effects. However, the weak intensity of PL is due to the surface defects in these nanoparticles, which is in agreement with the report of Castro et al.

who attributed this emission band to broad-band, defect-related transitions. [73] In addition, the PL intensity of the CIS NCs can be considerably improved by surface passivation with an inorganic or organic (amine) shell. ZnS is the shell material of choice as it provides good confinement of both electrons and holes in the core. Figure 4.5 demonstrates the photoluminescence emission spectra of CIS and CIS/ZnS

300 400 500 600 700

core/shell NCs. To identify if the enhancement of PL is contributed from pure ZnS or the ZnS shell on the CIS NCs, PL properties of a pure ZnS sample was also measured for comparison with CIS and CIS/ZnS core/shell NCs. It can be seen that the PL emission intensity was much enhanced with a ZnS coating and a small shift in the PL spectra to the red (lower energies) was observed after surface coating due to partial

core/shell NCs. To identify if the enhancement of PL is contributed from pure ZnS or the ZnS shell on the CIS NCs, PL properties of a pure ZnS sample was also measured for comparison with CIS and CIS/ZnS core/shell NCs. It can be seen that the PL emission intensity was much enhanced with a ZnS coating and a small shift in the PL spectra to the red (lower energies) was observed after surface coating due to partial