Impact of Conformality and Crystallinity for Ultrathin 4 nm Compact TiO sub(2) Layers in Perovskite Solar Cells

• Published in: Advanced materials interfaces Vol. 3; no. 21; p. np
• Main Authors: Roelofs, Katherine E, Pool, Vanessa L, Bobb-Semple, Dara A, Palmstrom, Axel F, Santra, Pralay K, Van Campen, Douglas G, Toney, Michael F, Bent, Stacey F
• Format: Journal Article
• Language: English
• Published: 01.11.2016
• Subjects: Atomic layer deposition; Crystallinity; Diffraction; Perovskites; Photovoltaic cells; Solar cells; Titanium dioxide; X-ray sources
• ISSN: 2196-7350; 2196-7350
• DOI: 10.1002/admi.201600580

The rapid rise in power conversion efficiencies of lead-based perovskite solar cells (PSCs) has established their promise, motivating scale-up and commercialization. As a thin film solar cell, interfacial properties are key to the performance of PSCs; there is a need to further understand which properties are critical to control. In this work, the impact of the compact TiO sub(2) (com-TiO sub(2)) film properties on the performance of mesoporous PSCs is studied. Com-TiO sub(2) layers are fabricated by spray pyrolysis as well as by atomic layer deposition (ALD), using ALD as a proof-of-concept method to achieve highly conformal films of precisely controlled thickness. The com-TiO sub(2) layers' conformality and surface roughness are examined by microscopy techniques; their relative crystallinity is studied by grazing-incidence X-ray diffraction with a synchrotron source and an in situ annealing chamber; and completed devices are analyzed by J-V and impedance spectroscopy techniques. The results show that an ultraconformal com-TiO sub(2) layer can be thinned to 4 nm and match the performance of a approximately 50 nm spray-pyrolysis TiO sub(2) layer. This work concludes that film conformality is the primary consideration for creating a high-performing com-TiO sub(2) layer, overriding any effects of film crystallinity-a finding that can guide fabrication choices for the scale-up of PSCs. Highly conformal approximately 4 nm compact TiO sub(2) layers by atomic layer deposition are demonstrated to match the performance of approximately 50 nm spray-pyrolysis TiO sub(2) layers in mesoporous perovskite solar cells. TiO sub(2) films are characterized by grazing-incidence X-ray diffraction using a synchrotron source. Film thickness and conformality are found to be more critical to a high-performing compact TiO sub(2) layer than the degree of crystallinity.