Effect of substrate surface free energy on the optoelectronic and morphological properties of organolead halide perovskite solar cell materials (Presentation Recording)

Abstract

Here, we show how the surface free energy of the electron-collecting oxide contact has a very pronounced effect on the nucleation free energy of solution-processed organolead halide perovskite thin films, which influences the crystal size/orientation, band-edge energies, conductivity and, ultimately, the performance of solar cell devices. While a great deal of the research community’s attention has been focused on the perovskite deposition methodology (e.g., starting precursors, annealing conditions, etc.), we demonstrate how the surface free energy of the oxide contact itself can be modified to control morphology and optoelectronic properties of the resulting hybrid perovskite thin films. The surface free energy of high-quality oxide contacts deposited by chemical vapor deposition (CVD) and atomic layer deposition (ALD) is modified by functionalization with a variety of self-assembled monolayers. We explore a number of deposition methodologies (e.g., a variety of single step and sequential step approaches) and their effect on the morphological and electronic properties of the resulting perovskite thin films deposited on these modified oxide contacts. Standard atomic force microscopy (AFM) and its conductive analog (cAFM) show how the oxide surface free energy ultimately affects the nanoscale morphology and charge transport characteristics of these semiconductor films. Photoelectron spectroscopy is used to elucidate the chemical composition (e.g., X-ray photoelectron spectroscopy - XPS), band edge energies (e.g., ultraviolet photoelectron spectroscopy - UPS), and the presence of gap states above the valence band (high sensitivity UPS measurements near the Fermi energy) of the hybrid perovskite materials as a function of the oxide surface free energy.