Leon Hsu

Modeling of InGaN/Si tandem solar cells

We investigate theoretically the characteristics of monolithic InGaN/Si two-junction series-connected solar cells using the air mass 1.5 global irradiance spectrum. The addition of an InGaN junction is found to produce significant increases in the energy conversion efficiency of the solar cell over that of one-junction Si cells. Even when Si is not of high quality, such two-junction cells could achieve efficiencies high enough to be practically feasible. We also show that further, though smaller, improvements of the efficiency can be achieved by adding another junction to form an InGaN/InGaN/Si three-junction cell.

Ideal transparent conductors for full spectrum photovoltaics

In current technologies, state-of-the-art transparent conducting oxides exhibit good conductivity (∼5 × 103 S/cm) and transparency up to only λ ∼ 1000 nm, restricting the use of such thin films to photovoltaics that are not utilizing the infrared part of the solar spectrum. We have found that among metal oxides, high electron mobility CdO satisfies the essential requirements for a low resistance and high infrared transmission transparent contact. With appropriate intentional doping, we have achieved ideal uncompensated CdO with extremely high conductivity (>104 S/cm) and an excellent transmission window in the range from 400 to >1500 nm, making this material an ideal TCO for photovoltaics with low band gap absorbers.

Electron mobility in InN and III-N alloys

We have calculated electron mobilities in InN and its III-nitride alloys using a variational procedure and taking into account the standard scattering mechanisms of Coulomb scattering, alloy disorder, and optical and acoustic phonons. The effects of the nonparabolicity of the conduction band and resulting energy-dependent effective mass have also been included. Scattering from charged Coulombic centers and alloy disorder are the dominant scattering mechanisms that limit the mobilities in currently available materials. Phonons play a role only in relatively pure (n 400 K). In addition, our calculations are in good agreement with experimental Hall mobilities obtained through controlled doping studies performed on InN, InGaN, and InAlN by high energy irradiation.

Transparent conductors for full spectrum photovoltaics

Low transparency of conventional transparent conducting oxides (TCOs) in the IR region (λ~1000 nm) due to absorption and reflection by the high concentration of free carrier restricts the applications of materials to photovoltaics that do not use the infrared part of the solar spectrum. In order to maintain low resistivity with reduced free carrier absorption, TCOs with high mobility are required. We have found that among metal oxides, CdO has all the essential requirements for low resistance and high infrared transmission: low electron scattering rates and high electron mobility. In this work we report the electrical and optical properties of CdO, undoped and doped with various n-type dopants, grown using pulsed laser deposition (PLD). We found that nominally undoped CdO films have Hall mobilities in the range of 50-150 cm2/Vs with carrier concentrations of 3×1020-2×1019 cm-3 corresponding to a high compensation ratio k=0.7. However, with appropriate intentional doping, ideal uncompensated CdO with extremely high conductivity (>;104 S/cm) and an excellent transmission window in the range from 400 to >;1500 nm can be achieved. These properties make CdO an ideal TCO for photovoltaics with low band gap absorbers including Si and high efficiency multijunction cells.

Publishing and refereeing papers in Physics Education Research

At the 2007 Physics Education Research Conference, a workshop on publishing and refereeing was held with a panel of editors from four different publishing venues: the physics education research section of the American Journal of Physics, the Journal of the Learning Sciences, Physical Review Special Topics–Physics Education Research, and the Physics Education Research Conference Proceedings. These editors answered questions from participants regarding publishing in their respective venues, as well as writing referee reports that would be useful to both journal editors and authors. This paper summarizes the discussion.