AVS 66th International Symposium & Exhibition
    Applied Surface Science Division Thursday Sessions
       Session AS-ThA

Paper AS-ThA6
Active Control of Interfacial Chemistry for Thin Film Solar Cells

Thursday, October 24, 2019, 4:00 pm, Room A211

Session: Role of Surfaces and Interfaces in Energy Material and Industrial Problems
Presenter: Alexandra Koziel, Case Western Reserve University
Authors: A.C. Koziel, Case Western Reserve University
K.A. Montiel, Case Western Reserve University
L.G. Wilson, Case Western Reserve University
J.L.W. Carter, Case Western Reserve University
I.T. Martin, Case Western Reserve University
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Global energy demand requires the development of efficient and reliable thin film photovoltaics with inexpensive processing. As the efficiency of hybrid perovskite solar cells has skyrocketed, practical constraints of the technology have put the scalability and durability into scientific focus. The development of inorganic interfacial layers, such as metal oxides, is a potential pathway to overcoming the stability and cost limitations associated with organic interlayers in perovskite solar cells. Thin films are sensitive to both the growth conditions, and the composition and morphology of the previously deposited layer. Interfacial engineering of metal oxides using molecular modifiers provides a powerful tool to tune interlayer properties, which can result in improved performance and stability.

This work details the effect of underlying layers on the growth of CsGeI3, a novel all-inorganic perovskite absorber. The hole-transport layer (HTL) and the underlying substrate were systematically varied. Surface and bulk properties of the film stack were characterized at every growth step. The choice of HTL affects the absorber film morphology, and resulting device efficiency. Further, this approach reveals that the choice of substrate can affect the properties of layers through the entire device.

Two common HTLs, PEDOT:PSS and MoO3, were deposited on substrates with differing surfaces. Glass, ITO (indium tin oxide, a common thin film solar cell transparent electrode), and Si substrates were selected to explore how a range of surface structures, from amorphous to polycrystalline to crystalline, affects the subsequent layers. The vapor-deposited MoO3 was further modified with gas-phase treatments (UV-ozone and O2 plasma exposure) and small molecules (silanization). Specifically, an IPTMS ((3-iodopropyl) trimethoxysilane) silanization procedure was developed to produce an iodine-terminated surface, for improved adhesion of the CsGeI3 absorber layer. A suite of materials characterization methods were applied to the samples after each step of device fabrication to assess the evolution of morphology and composition. Bulk, surface, and interface characteristics were probed using UV-Vis absorption measurements, X-ray photoelectron spectroscopy, scanning electron microscopy, optical profilometry, and spectroscopic ellipsometry. Notably, the absorber film morphology and ultimately the stability of the film stack is sensitive to not only the HTL, but the nature of the material under the HTL (ITO vs. glass), demonstrating the influence of surface/interface properties across multiple layers in a device.