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

Anna Pyayt, Ph.D.

Committee Member

Yusuf Emirov, Ph.D.

Keywords

CdTe Solar Cell, Doping, Elemental Vapor Transport, Minority Carrier Lifetime

Abstract

Cadmium Telluride is widely used as a thin-film solar cell material due to its nearly ideal optical bandgap (~ 1.5eV) and its high absorption coefficient (> 104 cm-1). In the past several decades, most of the research focused on p-type CdTe solar cells. In recent years, the research on CdTe devices is mainly based on CdSexTe1-x (CST) absorbers, leading to substantial advancements in conversion efficiency, comparable to that of silicon-based solar cells. Presently, p-type polycrystalline CST/CdTe thin-film solar cells exhibit a notable efficiency of 23.1%. The state-of-art CST/CdTe solar cell has attained an almost optimal short-circuit current density (JSC) of 31.66 mA/cm2 and a fill factor (FF) of 80.6%. Nevertheless, the open-circuit voltage (VOC) stands at 905mV, which remains below the practical level of 1 volt.

To enhance the open-circuit voltage (VOC), n-type polycrystalline CdTe thin films were investigated with the aim of achieving both high n-type doping concentrations and extended minority carrier lifetimes—both of which are critical for optimizing device VOC. Achieving high n-type doping in CdTe is generally more feasible compared to p-type doping.

The Elemental Vapor Transport (EVT) process was utilized to deposit n-type polycrystalline CdTe thin films. This method allowed for in situ control of the vapor phase stoichiometry, enabling the deposition of thin films under various Cd/Te vapor ratio conditions. Group-Ⅲ elements (i.e., In) and Group-Ⅶ elements (i.e., Cl) serve as n-type dopants, substituting for Cd and Te atoms respectively, facilitating in-situ extrinsic doping of CdTe through the EVT process. Several materials (ZnTe:Cu, ZnTe:N, ZnSe:Cu, and CuxZn1-xS) were studied as potential candidates for the p-type window layer.

Scanning Electron Microscopy (SEM) Energy-dispersive X-ray Spectrometry (EDS) and X-Ray Diffraction (XRD) were used to study the film properties of the n-CdTe absorber layer and the p-type window layer. The complete devices were characterized using Current-Voltage (J-V), Spectral Response (SR), Capacitance-Voltage (C-V), and Time-resolved Photoluminescence (TRPL) measurements.

The CdTe:Cl films show higher net n-type doping (~ 7×1016 cm-3) compare to the CdTe:In cells. The CdCl2 post-deposition heat treatment and surface treatments improve carrier collection. Higher Se incorporation in CdTe films results in the structure changing from zinc blend to wurtzite. The Se incorporation improves the minority carrier lifetime.

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