PhD Orals - Jung-Yong Lee, July 23, 2009, 9:30am

Jung-Yong Lee ljy at
Mon Jul 20 14:40:36 PDT 2009

*Metal Nanostructures in Optoelectronic Devices*

*Jung-Yong Lee*

Ph.D. Oral Examination

Department of Electrical Engineering

Stanford University

Advisor: Peter Peumans

Date: Thursday, July 23, 2009

Time: 9:30am (Refreshments at 9:15am)

Location: Paul G. Allen Building Auditorium


In the first part, we consider the trade-off between optical absorption and
internal quantum efficiency in thin-film photovoltaic (PV) cells. Metal
nanoparticles (MNPs) have been used to alleviate this trade-off by enhancing
optical absorption in thinner films. We extended Mie’s approach to estimate
the absorption and scattering efficiency of MNPs embedded in an absorbing
medium. This theory predicts the spatially localized and spectrally
broadband optical absorption enhancement. Experimentally, we show that
incorporating MNPs in organic photovoltaic cells leads to an increase in
power conversion efficiency by up to 40%.

In the second part of this talk, we focus on transparent conductors, which
are essential components of thin-film optoelectronic devices. Sputtered
Indium-Tin-Oxide (ITO) is currently the most commonly used transparent
electrode material, but it has a number of shortcomings. I will discuss the
possibility of using metal gratings and metal nanowires as a replacement for
ITO. It is shown that random silver nanowire meshes have electrical and
optical properties that are comparable to those of ITO (15 Ω/☐ sheet
resistance and 85% solar spectral weighted transmission). We also show that
silver nanowire meshes are compatible with flexible substrates and stable
when encapsulated. We demonstrate organic photovoltaic cells grown on silver
nanowire meshes with characteristics similar to those grown on ITO.
Semi-transparent organic photovoltaic cells using laminated silver nanowire
meshes are presented both as an important application in itself and as a
stepping stone toward multi-terminal multi-junction (MTMJ) PV cells. As
opposed to conventional multi-junction (MJ) PV cells, MTMJ PV cells are not
restricted by current matching constraints. Therefore, MTMJ PV cells are
more robust to fabrication imperfections and spectral changes.  We discuss a
MTMJ PV cell using P3HT:PCBM bottom cell and CuPc/C60 top cell. In organic
light emitting devices (OLEDs), random metal nanowires meshes provide a
simple and efficient way to extract more photons out of the devices while
also providing a transparent contact.

Finally, we analyze the performance limits of silver nanowire meshes by
modeling the sheet resistance of random wire networks, and show that it is
dominated by the wire-wire contact resistance (estimated to be 200 Ω). We
show that if the contact resistance can be reduced (<< 10 Ω), or if the
nanowire length can be increased (>>20µm), the sheet resistance of silver
nanowire meshes can be reduced to <2 Ω/☐ for a solar spectral weighted
transmission >85%.

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