Graduation Year
2024
Document Type
Dissertation
Degree
Ph.D.
Degree Name
Doctor of Philosophy (Ph.D.)
Degree Granting Department
Electrical Engineering
Major Professor
Chris Ferekides, Ph.D.
Committee Member
Andrew Hoff, Ph.D.
Committee Member
Arash Takshi, Ph.D.
Committee Member
Abla Zayed, Ph.D.
Committee Member
Yusuf Emirov, Ph.D.
Keywords
semiconductor, bandgap, spin coating, annealing, composition
Abstract
CdTe thin-film solar cells are one of the major interests in the solar industry for their favorable properties. For example, CdTe has a near-ideal bandgap (1.45 eV) for absorption of the solar spectrum, and a high absorption coefficient ( ̴ 105 cm-1) and can capture 99% of light with only 2 µm of film thickness. Moreover, CdTe can be doped both n and p-type. Additionally, low production costs contribute to more affordable panel prices (0.40 $/W) [1]. The highest efficiency achieved to-date for this material is 23.1% demonstrated by First Solar, which is still lower than the theoretical limit [2]. In recent years, a CST (CdSeXTe1-X)/CdTe device architecture has helped increase the short circuit current density (JSC) near the ideal JSC for CdTe (31.7 mA/cm2). The fill factor (FF) has reached 80% [3]. JSC and FF are already close to their theoretical limits. Therefore, only increases in the open-circuit voltage (VOC) can help increase the efficiency further. The current level of VOC for polycrystalline p-type CST/CdTe solar cells is approximately 900 mV. For monocrystalline CdTe solar cells, VOC higher than 1000 mV has been achieved [4]. Achieving a similar VOC for polycrystalline cells is the biggest remaining challenge. A net p-doping higher than 1016 cm-3 and minority carrier lifetimes of tens of nanoseconds are required to reach this goal [5]. However, even with both conditions fulfilled, the maximum VOC remains around 900 mV for the polycrystalline cells.
Copper (Cu) has been typically used for the formation of back contact. It plays an important role in increasing the p-doping concentration [6]. A specific quantity of Cu is essential to enhance cell performance, but exceeding this amount is detrimental to performance and also affects the stability of the cell in the long term by shunting [7]. The annealing temperature of the back contact controls Cu diffusion into the absorber. Under optimal Cu conditions, p-type doping is typically in the 1014 cm-3 range, which is insufficient for demonstrating VOC higher than 1000 mV.
p-type doping of CdTe is challenging [8]. Group V elements such as Phosphorous (P), Antimony (Sb), and Arsenic (As) can act as p-type dopants by replacing the Te atom [9]. Incorporating CST as part of the absorber layer has recently proved beneficial because it enhances the JSC, and Se has been shown to increase carrier lifetime. Therefore, doping CST layer can also serve as an alternative way of increasing the overall doping density for the absorber.
For single crystal n-type CdTe solar cells, VOC higher than 1000 mV is observed. Therefore, an n-type CdTe architecture can be used to reach the goal of 1000 mV VOC because it is easier to increase the n-doping concentration of CdTe (1016-1017 cm-3) while maintaining a high minority carrier lifetime [10]. Group III elements, such as Indium (In) or group VII elements, such as Chlorine (Cl) can be used as n-dopants.
CdTe thin films can be deposited either under Cd or Te-rich conditions. The as-deposited films are essentially stoichiometric, a small deviation in the stoichiometric is difficult to measure [11]. Cd-rich films can be intrinsically n-type, and Te-rich films p-type. Cd-rich films can have Tellurium vacancies (VTe) or Cadmium interstitials (Cdi). VTe are shallow donors [12]. When Cdi ionize, they will contribute electrons, resulting in n-type conductivity for non-extrinsically doped films. Also, when CdTe films are grown under Cd-rich conditions, they exhibit fewer mid-gap defects, resulting in a higher lifetime [13].
For designing high-efficiency n-type CdTe solar cells, a suitable transparent p-heterojunction partner is important. The performance, especially the VOC and JSC depends significantly on a heterojunction partner. The required properties for a suitable transparent heterojunction partner are high work function (small ∆EV) to match n-type CdTe, high bandgap for lower light absorption, and good lattice matching for lower recombination. Only a limited amount of research has been done on single crystal n-type CdTe films and no significant research on suitable transparent p-type heterojunction partners exists. Single crystalline n-type CdTe cells utilized a-Si heterojunction as a p-type partner/contact [14].
There are several potential candidates for the p-type transparent heterojunction partners such as CuXZn1-XS, CuSCN, CuMO2, CdXMg1-XTe, CuI, and NiO. This work focused on CuI and CuSCN as p-type transparent heterojunction partners. From their properties and simulations, both CuI and CuSCN seemed suitable as transparent p-type heterojunction partners for n-type CdTe. Moreover, they can also work as a transparent p-type back contact for the p-type CST/CdTe solar cells to form bifacial solar cells and utilize light from both the front and back side of the cells, thus, higher power density can be achieved.
In this work, the CST layer was studied by varying Se composition, CST thickness, As doping, and CdCl2 treatment conditions. The device performance was optimized to find the best condition for higher efficiency. The effect of the CuI and CuSCN on the performance of the n-type and p-type CdTe solar cells was investigated. The thickness, annealing time and temperature, and surface treatment on the cell characteristics—VOC, JSC, and FF—were also studied.
Scholar Commons Citation
Alom, Md Zahangir, "Performance Optimization and Application of p-type Transparent Semiconductors in the CdTe Solar Cells" (2024). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/10591
