We developed a three-step colloidal synthesis of near-infrared active Cu-In-Se (CISe)/ZnS core/shell nanocrystals (NCs) via a sequential partial cation exchange. In the first step binary highly copper deficient Cu2‒xSe NCs were synthesized, followed by a partial cation exchange of copper to indium ions yielding CISe NCs. In order to enhance the stability and the photoluminescence (PL) properties of the NCs, a subsequent ZnS shell was grown, resulting in CISe/ZnS core/shell NCs. These core/shell hetero-NCs exhibited a dramatic increase in size and a restructuring to trigonal pyramidal particles. The reaction parameters, e.g. the Cu:Se-ratio, the temperature and the time were carefully tuned enabling a distinct control over the size and the composition of the NCs. By varying only the size of the CISe/ZnS NCs (from 9 to 18 nm) the PL spectra could be tuned covering a wide range with maxima from 990 nm to 1210 nm. Thus, in these experiments we demonstrate a clear dependence of the optical properties of these materials on their size and extend the PL range of CISe-based nanoparticles further to the infrared part of the spectrum. Furthermore, the relatively large size of these NCs allows their detailed structural analysis via electron microscopy techniques, which is particularly challenging in the case of small particles and especially important to relate the size, composition and crystal structure to their optoelectronic properties.

Indiana University-Purdue University Indianapolis (IUPUI) / In this research thesis, copper indium gallium selenium (CIGS) nanoparticles were synthesized from metal chlorides, functionalized to disperse in water, and further used in layer by layer (LbL) nanoassembly of CIGS films. CIGS nanoparticles were synthesized through the colloidal precipitation in an organic solvent. The peak and average sizes of the synthesized particles were measured to be 68 nm and 75 nm in chloroform, and 30 nm and 115 nm in water, respectively. Two methods were used to disperse the particle in water. In the first method the stabilizing agent oleylamine (OLA) was removed through multiple cleaning processes, and in the second method ligand exchange was performed with polystyrene sulfonate (PSS). Zeta potential of CIGS nanoparticles dispersed in water was measured to be +61 mV. The surface charge of the nanoparticles was reversed by raising the pH of the solution, which was measured to be −43.3 mV at 10.5 pH. In a separate process, the CIGS nanoparticles dispersed in water were coated with PSS. The resulting dispersion was observed to be stable and the surface charge was measured to be −56.9 mV. The LbL deposition process of CIGS nanoparticles was characterized by depositing thin films on quartz crystal microbalance (QCM). LbL depositions was conducted using (i) oppositely charged CIGS nanoparticles, (ii) positively charged CIGS nanoparticles and PSS, and (iii) PSS-coated CIGS (CIGS-PSS) and polyethyleneimine (PEI). The average thickness of each bi-layer of the above mentioned depositions were measured to be 2.2 nm, 1.37 nm, and 10.12 nm, respectively. The results from the QCM have been observed to be consistent with the film thickness results obtained from atomic force microscopy (AFM). Various immersion times versus thickness of the film were also studied. For electrical characterization, the CIGS films were deposited on indium tindioxide (ITO)-coated glass substrates. Current versus voltage (I/V) measurements were carried out for each of the films using the Keithley semiconductor characterization instruments and micromanipulator probing station. It was observed that the conductivity of the films was increased with the deposition of each additional layer. The I/V characteristics were also measured under the light illumination and after annealing to study the photovoltaic and annealing effects. It was observed that under light illumination, the resistivity of a 12-layer CIGS film decreased by 93% to 0.54 MΩ.m, and that of the same number of layers of PSS-coated CIGS and PEI film decreased by 60% to 0.97 MΩ.m under illumination. The resistivity of an 8-layer CIGS and PSS film decreased by 76.4% to 0.1 MΩ.m, and that of the same layers of PSS-coated CIGS and PEI decreased by 87% to 0.07 MΩ.m after annealing. The functionalized nanoparticles and the LbL CIGS films were implemented in the solar cell devices. Several configurations of CIGS films (p-type), and ZnO and CdS films (n-type) were considered. Poly(3,4-ethylenedioxythiophene) (PEDOT), molybdenum (Mo), and ITO were used as back contacts and ITO was used as front contact for all the devices. The devices were characterized the Keithley semiconductor characterization instruments and micromanipulator probing station. For a CIGS and n-ZnO films device with PEDOT as back contact and ITO as front contact, the current density at 0 V and under light illumination was measured to be 60 nA/cm2 and the power density was measured to be 0.018 nW/cm2. For a CIGS and CdS films device with ITO as both back and front contact, the current density at 0 V and under light illumination was measured to be 50 nA/cm2 and the power density was measured to be 0.01 nW/cm2. For a drop-casted CIGS and CdS films device with Mo as back contact and ITO as front contact, the current density of 50 nA/cm2 at 0 V and power density of 0.5 nW/cm2 under light illumination was measured. For the LbL CIGS and chemical bath deposited CdS films device with ITO as both back and front contact, the current density of 0.04 mA/cm2 at 0 V and power density of 1.6 μW/cm2 under light illumination was measured. Comparing to Device-III, an increase by 99% in the power density was observed by using the CIGS LbL film in the device structure. The novel aspects of this research include, (i) functionalization of the CIGS nanoparticles to disperse in water including coating with PSS, (ii) electrostatic LbL deposition of CIGS films using oppositely charged nanoparticles and polymers, and (iii) the utilization of the fabricated LbL CIGS films to develop solar cells. In addition, the n-type cadmium sulfide film (CdS) and zinc oxide (ZnO) buffer layer were also deposited through LbL process after the respective particles were functionalized with PSS coating in separate experiments.

Indiana University-Purdue University Indianapolis (IUPUI) / Fabrication and analysis of Copper Indium Gallium di-Selenide (CIGS) nanoparticles-based thin film solar cells are presented and discussed. This work explores non-traditional fabrication processes, such as spray-coating for the low-cost and highly-scalable production of CIGS-based solar cells. CIGS nanoparticles were synthesized and analyzed, thin CIGS films were spray-deposited using nanoparticle inks, and resulting films were used in low-cost fabrication of a set of CIGS solar cell devices. This synthesis method utilizes a chemical colloidal process resulting in the formation of nanoparticles with tunable band gap and size. Based on theoretical and experimental studies, 100 nm nanoparticles with an associated band gap of 1.33 eV were selected to achieve the desired film characteristics and device performances. Scanning electron microcopy (SEM) and size measurement instruments (Zetasizer) were used to study the size and shape of the nanoparticles. Electron dispersive spectroscopy (EDS) results confirmed the presence of the four elements, Copper (Cu), Indium (In), Gallium (Ga), and Selenium (Se) in the synthesized nanoparticles, while X-ray diffraction (XRD) results confirmed the tetragonal chalcopyrite crystal structure. The ultraviolet-visible-near infra-red (UV-Vis-NIR) spectrophotometry results of the nanoparticles depicted light absorbance characteristics with good overlap against the solar irradiance spectrum. The depositions of the nanoparticles were performed using spray-coating techniques. Nanoparticle ink dispersed in ethanol was sprayed using a simple airbrush tool. The thicknesses of the deposited films were controlled through variations in the deposition steps, substrate to spray-nozzle distance, size of the nozzle, and air pressure. Surface features and topology of the spray-deposited films were analyzed using atomic force microscopy (AFM). The deposited films were observed to be relatively uniform with a minimum thickness of 400 nm. Post-annealing of the films at various temperatures was studied for the photoelectric performance of the deposited films. Current density and voltage (J/V) characteristics were measured under light illumination after annealing at different temperatures. It was observed that the highest photoelectric effect resulted in annealing temperatures of 150-250 degree centigrade under air atmosphere. The developed CIGS films were implemented in solar cell devices that included Cadmium Sulfide (CdS) and Zinc Oxide (ZnO) layers. The CdS film served as the n-type layer to form a pn junction with the p-type CIGS layer. In a typical device, a 300 nm CdS layer was deposited through chemical bath deposition on a 1 $mu$m thick CIGS film. A thin layer of intrinsic ZnO was spray coated on the CdS film to prevent shorting with the top conductor layer, 1.5 μm spray-deposited aluminum doped ZnO layer. A set of fabricated devices were tested using a Keithley semiconductor characterization instrument and micromanipulator probe station. The highest measured device efficiency was 1.49%. The considered solar cell devices were simulated in ADEPT 2.0 solar cell simulator based on the given fabrication and experimental parameters. The simulation module developed was successfully calibrated with the experimental results. This module can be used for future development of the given work.