Marshall Frye

(Advisor: Prof. Lauren Garten)

Will propose a doctoral thesis entitled,

Developing Narrow-Band Gap Polar Materials for Photovoltaics 

on

Monday, November 11 at 11:00 a.m. (EST)

Van Leer Room C340

and 

Virtually via Teams

Committee

• Prof. Lauren Garten - School of Materials Science and Engineering (advisor)
• Prof. Juan-Pablo Correa-Baena - School of Materials Science and Engineering
• Prof. Eric Vogel - School of Materials Science and Engineering
• Prof. Antonio Facchetti - School of Materials Science and Engineering
• Prof. Thomas Beechem - School of Mechanical Engineering, Purdue University

Abstract

Developing methods to stabilize materials that exhibit polar crystal structures and narrow band gaps is critically needed to increase the efficiency of polar photovoltaics. Polar photovoltaics differ from p-n junction photovoltaics as charge separation occurs in a single layer of material with a polar crystal structure. The efficiency of these cells is hindered by the wide (>3 eV) band gaps of many polar materials, reducing light absorption and power conversion efficiency. Furthermore, the dominant mechanisms driving the separation of photogenerated charge carriers in polar photovoltaics is not yet established. In this thesis, I will focus on the deposition of two polar materials with predicted narrow band gaps: the P63cm phase of ScFeO3 (h-ScFeO3) and the Pmn21 phase of SnSe. The unique polarization directions of the two materials (out-of-plane vs. in-plane) will enable systematic studies of structure-property relationships in polar photovoltaics. The growth of ScFeO3 on conductive substrates has remained challenging due to the metastability of the targeted phase. To address this, an iron oxide (FexO) interlayer was deposited on (111) platinum substrates, stabilizing h-ScFeO3 due to the flexible coordination of the iron oxide and the similar oxygen sublattice of FexO and ScFeO3. The stabilization of h-ScFeO3 on (111) platinum will facilitate ferroelectric and photovoltaic measurements. 

Thickness control of SnSe films is critical for polar photovoltaics as the crystal structure of SnSe transitions from centrosymmetric Pnma to Pmn21 when thickness is scaled down to a monolayer. In SnSe, mechanical strain will be used to modulate polarization of SnSe via the piezoelectric effect, enabling a study of the effect of polarization on photovoltaic properties. The d11 piezoelectric coefficient of monolayer SnSe is predicted to be 250.8 pm/V, (d11 of AlN=5.1 pm/V), but this has not yet been quantified. Piezoelectricity is predicted to be conserved for odd-numbered layers near the monolayer limit because of surface effects, but piezoelectricity is not possible for even-numbered layers due to the antipolar stacking. Therefore, to assess piezoelectricity and the polar photovoltaic efficiency, it is critical to deposit SnSe films with precise layer control. SnSe films will be deposited by pulsed laser deposition (PLD) to elucidate the impact of adatom energy and deposition rate on film thickness, crystallinity, and lateral size. The effect of thickness on the optical properties (e.g., band gap, carrier lifetime), optoelectronic response and piezoelectricity will then be quantified. The successful completion of this work will outline methods to deposit narrow band gap polar materials and provide insight into the fundamental properties of polar photovoltaics.