Browsing by Subject "Solar cell"

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    Device Modeling and Characterization for CIGS Solar Cells
    (2013-06) Song, Sang
    We studied the way to achieve high efficiency and low cost of CuIn1-xGaxSe2 (CIGS) solar cells. The Fowler-Nordheim (F-N) tunneling currents at low bias decreased the shunt resistances and degraded the fill factor and efficiency. The activation energies of majority traps were directly related with F-N tunneling currents by the energy barriers. Air anneals decreased the efficiency from 7.74% to 5.18% after a 150�C, 1000 hour anneal. The decrease of shunt resistance due to F-N tunneling and the increase of series resistance degrade the efficiencies of solar cells. Air anneal reduces the free carrier densities by the newly generated Cu interstitial defects (Cui). Mobile Cui defects induce the metastability in CIGS solar cell. Since oxygen atoms are preferred to passivate the Se vacancies thus Cu interstitial defects explains well metastability of CIGS solar cells. Lattice mismatch and misfit stress between layers in CIGS solar cells can explain the particular effects of CIGS solar cells. The misfits of 35.08o rotated (220/204) CIGS to r-plane (102) MoSe2 layers are 1% ~ -4% lower than other orientation and the lattice constants of two layers in short direction are matched at Ga composition x=0.35. This explains well the preferred orientation and the maximum efficiency of Ga composition effects. Misfit between CIGS and CdS generated the dislocations in CdS layer as the interface traps. Thermionic emission currents due to interface traps limit the open circuit voltage at high Ga composition. The trap densities were calculated by critical thickness and dislocation spacing and the numerical device simulation results were well matched with the experimental results. A metal oxide broken-gap p-n heterojunction is suggested for tunnel junction for multi-junction polycrystalline solar cells and we examined the characteristics of broken-gap tunnel junction by numerical simulation. Ballistic transport mechanism explains well I-V characteristics of broken-gap junction. P-type Cu2O and n-type In2O3 broken-gap heterojunction is effective with the CIGS tandem solar cells. The junction has linear I-V characteristics with moderate carrier concentration (2�1017 cm-3) and the resistance is lower than GaAs tunnel junction. The efficiency of a CGS/CIS tandem solar cells was 24.1% with buffer layers. And no significant degradations are expected due to broken gap junction.
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    Growth and Characterization of Wide Bandgap CIAGS Solar Cell Material and Devices
    (2018-12) Hwang, Sehyun
    In this study, we present the development of copper-indium-aluminum-gallium-selenium (Cu(In1-x-yAlyGax)Se2, or CIAGS) as a wide bandgap top cell absorber for tandem photovoltaic (PV) applications. Realizing a tandem PV structure could lead to a breakthrough for high efficiency solar cells. CIAGS absorbers were grown in a single-step process using a custom-designed co-evaporation system under an ultra-high vacuum. The material properties of CIAGS thin films were analyzed in terms of grain morphology, elemental composition, and energy bandgap. The bandgap of CIAGS is tuned by controlling the elemental composition of group III elements. The relation between energy bandgap and elemental composition was empirically established for CIAGS absorbers with varying bandgaps. The CIAGS grown here targeted a bandgap of ~1.65 eV which is optimal for a tandem top cell. CIAGS solar cell devices were fabricated and characterized electrically by J-V measurements. The highest efficiency obtained was 12.8%, although the efficiency tends to decrease as the bandgap increases. Poor film adhesion or delamination is a major problem in wide bandgap CIAGS solar cells. Delamination occurs at the interface between the CIAGS absorber and the Mo back contact layer. We suggest two possible delamination mechanisms caused by interfacial molybdenum diselenide (MoSe2) in the wide bandgap CIAGS. The CIAGS/Mo interface was characterized mechanically (adhesion) and electrically (contact resistance). A TiN diffusion barrier to selenization improves the CIAGS/Mo interfacial adhesion and provides a potential solution to the delamination problem in the wide bandgap absorbers such as CIAGS.