Princeton University

School of Engineering & Applied Science

Silicon Heterojunction Photovoltaics: Metal-Insulator-Semiconductor Schottky Barriers and Passivated Selective Contacts

Nicholas Strandwitz, Lehigh University
B205 Engineering Quadrangle
Wednesday, November 14, 2018 - 4:30pm


Interfaces between semiconductors and metal oxides are important for a wide array of applications including surface passivation layers on solar cells and gate dielectrics.  Recently, atomic layer deposition (ALD) has emerged as a unique tool for the formation and study of oxides on semiconductors.  ALD enables conformal deposition of a wide variety of compositions with precise thickness control through a binary chemical reaction sequence.  In this talk, I will address two important aspects of ALD oxide-semiconductor interfaces as they relate to photovoltaics: 1) Fixed negative charge in metal-insulator-semiconductor stacks, and 2) the prospect of passivating selective contacts. 

Metal-insulator-semiconductor Schottky barriers are a potentially low-cost photovoltaic configuration.  The PV figures of merit are strongly influenced by the interface composition and structure.  Our group has used ALD to create well-defined alumina-based insulators in which the thickness, fixed charge, and composition can be well-controlled. Based on a simple electrostatic model, fixed charge in the insulator can be used to modify barrier heights, and the fixed charge at ALD alumina-silicon interfaces can be tuned over a range of approximately 5E12 cm-2, making this system an ideal test-bed to understand the role of fixed charge experimentally.  We find little to no influence of the fixed charge characteristic of the alumina-silicon interface in our experiments, and that barrier heights appear to be dominated by interface dipoles.  We relate our results to previous experimental and theoretical work that relates dipole strength to differences in oxygen areal densities at the silicon oxide-aluminum oxide interface.  We also report preliminary PV figures of merit for our well-controlled MIS junctions.

Recently, so-called “selective contacts” have been examined to boost efficiency of p-n and heterojunction silicon photovoltaics (PV), and also to provide PV junctions without the need for traditional p-n junctions. These materials provide selective transport for one carrier while often reducing interfacial recombination rates.  In some cases, it has been shown that the passivation properties of these selective contact materials are not ideal. This work explores the introduction of aluminum oxide tunnel layers sandwiched between crystalline silicon and selective contact materials such as MoOx.  The potential impact of this configuration is to 1) increase in stability of the junction and 2) and increase in performance through minimization of carrier recombination at the contact.  We have examined the role of the alumina layer thickness on interface quality and specific contact resistance.  We have found that there is a tradeoff between contact resistance and decreases in interface recombination that a compromise between the two can potentially increase efficiency in MoOx contacted silicon PV devices.  



Dr. Strandwitz joined the Lehigh University faculty in January 2013. Dr. Strandwitz completed his BS in Engineering Science at The Pennsylvania State University in 2004 during which time he worked with Prof. Joseph Rose and Prof. Stephen Fonash.  He then earned his PhD from the Materials Department at University of California Santa Barbara with Professor Galen D. Stucky. Professor Strandwitz conducted postdoctoral work at California Institute of Technology working with Professor Nathan S. Lewis.  His research interests at Lehigh include new chemistries and techniques in atomic layer deposition and interfacial electronic properties between semiconductors and metal oxides.