Integration and characterization of individual radial junction silicon nanowires for photovoltaic applications

Open Access
Author:
Wang, Xin
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 18, 2014
Committee Members:
  • Theresa Stellwag Mayer, Dissertation Advisor
  • Theresa Stellwag Mayer, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Douglas Henry Werner, Committee Member
  • Tom Mallouk, Committee Member
Keywords:
  • Radial junction
  • silicon nanowire
  • Vapor-Liquid-Solid growth
  • Al catalyst
  • Au catalyst
  • Heterojunction thin intrinsic layer
Abstract:
Considerable attention has been devoted to silicon (Si) nanowire arrays for photovoltaic energy conversion applications. Vertical arrays of Si nanowires with radial p-n/p-i-n junctions decouple the light absorption direction and photogenerated carrier collection direction, which enables the use of lower-quality Si materials without sacrificing device performance. Moreover, vapor-liquid-solid (VLS) technique offers the possibility of growing Si nanowires on inexpensive substrates such as glass or metal foil instead of crystalline Si substrates, which can reduce the cost of Si-based solar cells. The radial junction dark current plays an important role in the photovoltaic performance of these devices because Si nanowire arrays have significantly larger electrically active junction area relative to illuminated area of the solar cell. Previously reported Si nanowire solar cell devices suffer from the high dark current, low open circuit voltage, and low energy conversion efficiency. Therefore, it is necessary to understand the mechanisms responsible for the high dark current in radial junction Si nanowires and to develop strategies to improve junction properties through optimization of the VLS growth and shell coating processes. This thesis reports on the electrical and photovoltaic properties of individual radial junction Si nanowire devices to evaluate the effectiveness of different nanowire VLS growth and shell coating conditions. A process was developed to fabricate radial p+-n+ and p+-i-n+ junction Si nanowire devices that combines deterministic bottom-up assembly of individual nanowires with conventional top-down device nanofabrication. The effect of the metal catalyst, coating morphology and shell layer passivation were investigated in this research. In chapter 3, VLS growth using Al as catalyst was used to synthesize the p-type Si nanowires. Radial junctions were formed by low pressure chemical vapor deposition (LPCVD) of n+ or i-n+ Si layers on the p-type nanowires. In chapter 4, p-type nanowires were synthesized by VLS growth using Au as catalyst. The n-type shells were deposited by LPCVD at different growth temperatures to investigate the effect of coating morphology and pretreatment conditions on the electrical properties of the junctions. In chapter 5, radial heterojunction with intrinsic thin-layer (HIT) nanowire junctions were formed by plasma enhanced chemical vapor deposition (PECVD) of amorphous hydrogenated Si (a-Si:H) i-n layers on Au-catalyzed VLS-grown p-type wires. The electrical properties and photovoltaic performance of devices fabricated from individual radial junction nanowires were evaluated by current-voltage (I-V) measurement in the dark and under Air Mass 1.5 Global (AM 1.5G) illumination. Variable-temperature I-V measurements were used to analyze the mechanisms responsible for the dark current and to determine the conditions that produce the highest quality radial nanowire junctions for photovoltaic applications.