title: Role of 4-tert-Butylpyridine as a Hole Transport Layer Morphological Controller in Perovskite Solar Cells.
abstract: Hybrid organic-inorganic materials for high-efficiency, low-cost photovoltaic devices have seen rapid progress since the introduction of lead based perovskites and solid-state hole transport layers. Although majority of the materials used for perovskite solar cells (PSC) are introduced from dye-sensitized solar cells (DSSCs), the presence of a perovskite capping layer as opposed to a single dye molecule (in DSSCs) changes the interactions between the various layers in perovskite solar cells. 4-tert-Butylpyridine (tBP), commonly used in PSCs, is assumed to function as a charge recombination inhibitor, similar to DSSCs. However, the presence of a perovskite capping layer calls for a re-evaluation of its function in PSCs. Using TEM (transmission electron microscopy), we first confirm the role of tBP as a HTL morphology controller in PSCs. Our observations suggest that tBP significantly improves the uniformity of the HTL and avoids accumulation of Li salt. We also study degradation pathways by using FTIR (Fourier transform infrared spectroscopy) and APT (atom probe tomography) to investigate and visualize in 3-dimensions the moisture content associated with the Li salt. Long-term effects, over 1000 h, due to evaporation of tBP have also been studied. Based on our findings, a PSC failure mechanism associated with the morphological change of the HTL is proposed. tBP, the morphology controller in HTL, plays a key role in this process, and thus this study highlights the need for additive materials with higher boiling points for consistent long-term performance of PSCs.
authors.affiliation: Department of NanoEngineering, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Shen
authors.initials: S
authors.lastname: Wang
authors.affiliation: Department of NanoEngineering, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Mahsa
authors.initials: M
authors.lastname: Sina
authors.affiliation: Department of NanoEngineering, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Pritesh
authors.initials: P
authors.lastname: Parikh
authors.affiliation: Department of NanoEngineering, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Taylor
authors.initials: T
authors.lastname: Uekert
authors.affiliation: Department of NanoEngineering, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Brian
authors.initials: B
authors.lastname: Shahbazian
authors.affiliation: Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory , P.O. Box 999, Richland, Washington 99352, United States.
authors.forename: Arun
authors.initials: A
authors.lastname: Devaraj
authors.affiliation: Department of NanoEngineering, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Ying Shirley
authors.initials: YS
authors.lastname: Meng
keywords: Fourier transform infrared spectroscopy; Perovskite solar cells; atom probe tomography; focused ion beam; hole transport layer; transmission electron microscopy