Date of Award
Doctor of Philosophy (PhD)
As the worldwide demand for renewable energy is increasing, growth of the global share of alternative energy sources would improve overall energy security as well as bring environmental benefits. So far, solar cells - the devices that convert direct sunlight into electricity - are dominated by silicon devices. Another alternative is thin film solar cell, whose main inspiration is to reduce the electricity production cost. Cu(In, Ga)Se2 (CIGS) solar cells are considered to have a great prospective because of reduced material and energy consumption during manufacturing. Many CIGS solar cell manufacturers are already exhibiting GW-scale production capacity. With the development of CIGS applications, it is essential to modify the properties of each of the constituent material to adapt to the new requirements.
Molybdenum is the most appropriate material used as the back contact for CIGS solar cells, and is commonly deposited by sputtering onto soda lime glass (SLG). Mo thin films act as the metal contact. The formation of an Ohmic contact at the Mo/CIGS interface is one of the most important properties apart from high conductivity, strong adhesion of the film as well as chemical and mechanical compatibility with the CIGS layer. A suitable amount of sodium is necessary for enhanced solar cell performance. When using soda lime glass (SLG) as a sodium source, the Mo layer acts like a barrier for sodium diffusion and the deposition process provides proper control of sodium supply from the SLG. Structural, thermal, and chemical properties of the Mo film have a direct influence on the growth and nucleation of the CIGS layer as well.
In the first part of the thesis, in-situ and ex-situ characterization techniques were used to understand the growth, as well as the morphology and structural properties of the Mo films grown on various substrates, namely Si (100) wafer, soda lime glass and borosilicate glass, at a fixed deposition power and pressure. Real time spectroscopic ellipsometry (RTSE) analysis demonstrated a Volmer-Weber growth mechanism for all films. Dielectric functions extracted from the ex-situ analysis illustrate a Drude oscillator, characteristic of metals. Resistivity values were extracted from this oscillator and correlated with Hall Effect and 4-point probe measurements. Substrates with sodium produced slightly less resistive films. AFM images showed that the films were deposited conformally on the substrates, and that the roughness of the films was inversely related to the resistivity values. XRD analysis showed that all the Mo films deposited were preferentially oriented along the (110) direction regardless of the substrates. SEM surface images showed good correlation with XRD analysis. Na depth profiles, obtained by SIMS analysis, were then compared for Mo/CIGS structures deposited on SLG and BSG. A clear difference between the two was seen with a much higher intensity of Na for the SLG substrates. Devices were then fabricated on both substrates and analyzed by J-V and QE measurements. Even though no change occurred for the current, a clear decrease in VOC and FF was observed for the BSG substrate compared to the SLG substrate.
The influence of the substrate temperature (TSS) at a fixed deposition power and pressure was studied in the second part of this thesis. Correspondence between the X-ray diffraction (XRD), four-point probe resistivity measurements, and SEM analysis are presented. Films deposited at a higher substrate temperature exhibit better crystallinity, lower sheet resistance and larger grain size. Secondary ion mass spectrometry (SIMS) demonstrated the influence of substrate temperature on sodium diffusion. X-ray photoelectron spectroscopy (XPS) analysis was performed on the films to understand the molybdenum oxidation states as a function of substrate temperature. Theoretical simulation models were developed to further understand the sodium diffusion, allowing extraction of Dboundary and Dgrain for the first time.
The third objective of this thesis was to focus on the effect of substrate temperature on the traditional bilayer molybdenum films used as the back contact for CIGS solar cells, where the first layer is deposited at comparatively higher pressure to fabricate porous films to allow better adhesion of the films and the 2nd layer is deposited at relatively lower pressure to produce denser films with better electrical properties. These films were subject to post-deposition annealing and both as-deposited and annealed films were investigated with XRD, SIMS, AFM and Hall Effect measurement. The films deposited at TSS of 100°C were found to be outliers after an in depth examination. Solar cells were fabricated using these different substrate temperatures to study the effect on the device parameters. The device analysis reveals that the room temperature device exhibits better device efficiency, mostly because of lower series resistance and reverse saturation current. Improvement in electrical properties for higher deposition temperature was not assisted by higher sodium diffusion in the film, therefore no noteworthy changes were witnessed from the devices performance, specifically for VOC and FF.
Ashrafee, Tasnuva. "Study of Modified Deposition Process for Cu(In,Ga)Se2 Solar Cell Back Contact" (2017). Doctor of Philosophy (PhD), dissertation, Electrical/Computer Engineering, Old Dominion University, https://digitalcommons.odu.edu/ece_etds/24
Available for download on Saturday, March 14, 2020