Studies on optimizing Cadmium Telluride solar cells through thickness reduction and the addition of a Nickel Oxide electron-blocking layer

Open Access
Author:
Waggoner, Shawn Allen
Graduate Program:
Engineering Science and Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 30, 2016
Committee Members:
  • Wook Jun Nam, Thesis Advisor
  • Osama O Awadelkarim, Committee Member
  • Mark William Horn, Committee Member
Keywords:
  • CdTe
  • NiO
  • ALD
  • solar cell
  • PV
  • HTL
  • EBL
Abstract:
Cadmium Telluride (CdTe) solar cells are the current leader in thin film photovoltaic technologies: with record device efficiencies comparable to any other thin film photovoltaic, and a lower cost per watt-peak than any other industrial photovoltaic. Yet device efficiency and cost must be further improved to continue making these devices economically competitive. This thesis discusses cost reduction of CdTe solar cells by creating ultrathin (< 1 µm thick) CdTe absorber layers through Magnetron Sputter Deposition. Previous research on these devices has demonstrated excellent stability, but significantly reduced efficiency when compared to CdTe devices with thicker (typically > 3 µm thick) absorber layers. To avoid this efficiency reduction, research is conducted on adding a < 10 nm NiO layer between the CdTe and the metal back contact. Ideally this NiO layer would act as electron-blocking / hole-transporting layer (HTL), which is typically necessary with ultrathin solar absorbers. NiO layers were grown by Atomic Layer Deposition, with surface topography characterized through Atomic Force Microscopy (ALD), crystal structure characterized through Grazing-Incidence X-Ray Diffraction, and optical absorption characterized through UV-Vis Spectrometry. These methods demonstrated that the ALD NiO films were conformal and continuous, p-type, and had an electronic band-gap of 3.55 eV. 500 nm CdTe solar cells developed with 3.5 nm NiO also yielded promising results: Fill Factor was enhanced by 6.74%, and Photon Conversion Efficiency (PCE) was enhanced by 1.12% with the addition of a 3.5 nm NiO layer. Beyond these experimental tests, additional simulation work using Analysis of Microelectronic and Photonics Structures (AMPS) software demonstrated that CdTe simulations gained 247 mV of open-circuit voltage, and a PCE increase of 6.13% with the addition of a 3.5 nm layer of NiO. The results demonstrate that NiO could act as an effective HTL, and that further experimental work should be conducted on solar cells utilizing ultrathin CdTe.