Browsing by Author "Hemati, Azadeh"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    FABRICATION OF A THIN FILM SOLAR CELLS USING LAYER BY LAYER (LBL) NANOASSEMBLY OF COPPER INDIUM GALLUIM SELENIUM (CIGS) NANOPARTICLES
    (Office of the Vice Chancellor for Research, 2012-04-13) Ghane, Parvin; Hemati, Azadeh; Shrestha, Sudhir; Agarwal, Mangilal; Varahramyan, Kody
    Copper Indium Gallium Selenium (CIGS), a p-type semiconductor material with a tunable band gap, has been broadly studied for high efficiency solar cells as a viable sustainable energy source. Production of CIGS nanoparticles gives the ability of fabricating thin, light, and flexible solar cells. However, the current fabrication technologies of such devices are still very costly. This poster presents the synthesis and functionalization of CIGS nanoparticles and proposes Layer-by-Layer (LbL) nanoassembly process, as a low cost method, to fabricate thin films for solar cell applications. The results show that the synthesized CIGS particles have 1.3 ev band gap and 30 nm diameter in av-erage. These particles were later coated with polymers to provide alternative opposite surface charges suitable for LbL process. Deposition of 20 layers of the particles on indium tinoxide (ITO) coated glass formed a thin film with 220 nm thickness. The measured current voltage (I-V) characteristic of the film gave resistivity of 7.9 MΩ.m in dark and 2.25 MΩ.m under light illumina-tion. A prototype solar cell made out of the film resulted in short circuit cur-rent density (JSC) of 0.3 mA/cm2 and open circuit voltage (VOC) of 0.7 V.
  • Thumbnail Image
    Item
    LAYER BY LAYER NANOASSEMB​LY OF COPPER INDIUM GALLIUM SELENIUM (CIGS) NANOPARTIC​LES FOR SOLAR CELL APPLICATIO​N
    (2011-12) Hemati, Azadeh; El-Mounayri, Hazim; Agarwal, Mangilal; Varahramyan, Kody
    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.