Award Date

May 2023

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

First Committee Member

Shubhra Bansal

Second Committee Member

Brendan O'Toole

Third Committee Member

Hui Zhao

Fourth Committee Member

Kwang Kim

Fifth Committee Member

Mohamed Trabia

Sixth Committee Member

Rama Venkat

Number of Pages



Cu(In,Ga)(S,Se)2 or CIGS is a thin-film semiconductor that has shown a device efficiency of 23.35% and 24.2% for single-junction and perovskite/CIGS tandem solar cells, respectively. CIGS offers promising properties such as tunable bandgap and ease of processing making them great candidates for thin-film tandem devices. However, knowledge of the effect of material defects, buffer materials, and post-deposition treatment (PDT) on degradation and metastability behavior in these devices is not well understood.In this dissertation, metastability and long-term degradation of CIGS thin-film solar cells have been investigated under combinatorial stress factors of heat, light, and voltage bias to systematically understand the effect of buffer layers and PDT. In-situ measurements have been developed to record device characteristics during heat-light soaking (HLS) and potential-induced degradation (PID) tests. State-of-art devices from ZSW Germany have been tested with a variety of buffer layers [CdS vs. Zn(O,S)] and variations in Na and RbF-PDT. Device parameters measured have been used to calculate the activation energy of the underlying mechanisms of degradation supported by composition profiles and first principal calculations. The stability of CIGS devices under HLS conditions has been positively affected by the integration of sodium (Na) or rubidium-fluoride post-deposition treatment (RbF PDT). However, these methods have not been effective in preventing the degradation of solar devices under the PID stress test. Low-sodium devices have achieved the highest efficiency gain by reversing the voltage polarity during PID recovery. CIGS devices with a Zn(O,S) alternative buffer layer experienced degradation after HLS stress tests and PID, regardless of the junction bias. However, successful recovery from PID has been achieved in this device type.


Alkali treatment; Alternative buffer layer; CIGS; light‑soaking tests; Potential induced degradation; Thin-film solar cells


Engineering Science and Materials | Materials Science and Engineering | Mechanical Engineering | Oil, Gas, and Energy

File Format


Degree Grantor

University of Nevada, Las Vegas




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