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

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

 

Abstract

 

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%.

Jung-Yong