We're all in for a real treat. Dong Liang is giving his defense this monday at 10am. Come listen to what he's done to save our world one exciton at a time.
Sonny
DEPT OF PHYSICS
DISSERTATION DEFENSE
Ph.D. Candidate: DONG LIANG
Research Advisor: JAMES S. HARRIS
Date: MondayNovember 12th, 2012
Time: 10:00 am (Refreshments at 9:45 am)
Location: Physics & Astrophysics Bldg. Room 102/103
Title: Nanostructures in III-V solar cells
Abstract:
From the physics and materials viewpoint, III-V materials are ideal
for highly efficient photovoltaic conversion. Their major limitations
are cost and resource availability. As industrial planar III-V solar
cells continue to set the all-time efficiency records, nanostructured
III-V solar cells are now being investigated in academia with the goal
to further improve the efficiency and lower the required materials
volume and cost. However, most previously investigated nano-structured
solar cells suffer from low efficiencies. In this talk, I will first
present optical enhancement in nanopyramid III-V ultra-thin
films which can potentially reduce the required materials by one order
of magnitude. I will then demonstrate significant efficiency
improvement in GaAs solar cell with an AlGaAs nanocone window layer,
leading to a 17% efficiency nanostructured single-junction solar cell.
The first part of my talk will focus on optical engineering and
absorption improvement in nanopyramid GaAs ultra-thin film. I will
demonstrate a double-sided nanopyramid GaAs film that is only 160 nm
thick, laminated in a flexible transparent superstrate. Without
additional antireflection coatings, this nanopyramid film absorbs over
80% more photons than a planar counterpart with equal thickness at
normal incidence and is equivalent to a 1um thick film. At large
incident angles, this enhancement can be even greater. With similar
light trapping design, III-V solar cell film thickness can be
potentially reduced from 3-4 um to 200-300 nm, which could
significantly reduce III-V cell cost.
The second part of my presentation will focus on efficiency
improvement in III-V nanostructured solar cells. Nanostructures
applied to different layers in III-V solar cells are demonstrated,
including antireflection, p-n junction and the window layer. Among
these, the nanostructured window layer leads to the best performances.
By applying a nanocone AlGaAs window layer with metal mesa grids to a
GaAs solar cell, light absorption, carrier confinement and lateral
conductance are improved at the same time, leading to a high energy
conversion efficiency of 17.0%, which is a 30% improvement from its
planar counterpart. High open circuit voltage of 0.982 V is achieved
with post-growth nanostructure processing that maintains low junction
area and low dark current. Fill factor also increases from 63% to 71%
with the metal mesa grids design that reduces series resistance and
maintains high shunt resistance. By comparing external quantum
efficiency, absorption spectra and 2D photon density mapping, charge
transferring mechanism in AlGaAs nanocone window layer is illustrated.
I will conclude my talk with some take-home design rules that can
optimize the optical and electrical trade-offs in nanostructured solar
cells.
--
Dong Liang
PhD candidate
Department of Physics
Stanford University
Regards,
Sonny
----
Department of Applied physics, Stanford University
research group: http://snow.stanford.edu/index.html
626-216-4597
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