Abstract
Annealing is one of the processing methods that are used for the fabrication of defect-free, photoactive perovskite films with compact grains in highly efficient and stable perovskite solar cells (PSCs). Thus, the annealing temperature is a key parameter for the control of the interdiffusion (of constituent elements) in photoactive films. In this paper, we present the results of a systematic study of the effects of annealing on the interdiffusion of constituent elements in efficient formamidinium-based PSCs. We also explore the effects of annealing-induced interdiffusion on layer microstructures, local strains, and the optoelectronic properties of perovskite films. We observe a dramatic upward diffusion of tin (Sn) and titanium (Ti) from fluorine-doped tin oxide and titanium dioxide (TiO2) to the perovskite films. We also observe a downward diffusion of lead (Pb) and iodine (I) from the perovskite films to the mesoporous layer of the electron transporting layer (ETL), after annealing at temperatures between 100 and 150 °C. The diffused I substitutes for Ti in the ETL, which improves the optoelectronic properties of the films, for annealing temperatures between 100 and 130 °C. The annealing-induced interdiffusion that occurs at higher temperatures (between 140 and 150 °C) results in higher levels of interdiffusion, along with increased local strains that lead to the nucleation of pores and cracks. Finally, the implications of the results are discussed for the design of PSCs with improved photoconversion efficiencies and stability.
Highlights
Over the past decade, lead-based organic–inorganic hybrid perovskites have received significant attention as absorber materials in perovskite solar cells (PSCs).1–11 Significant improvements have been made in the performance and stability of perovskite solar cells in the same period
We present the results of a systematic study of the effects of annealing on the interdiffusion of constituent elements in efficient formamidinium-based PSCs
Fluorine-doped tin oxide (FTO)-coated glass (∼7 Ω sq−1), lead iodide (PbI2) (99.999%), diisopropoxide bis(acetylacetonate), formamidinium iodide (FAI) (98%), methylammonium chloride (MACl), methylammonium bromide (MABr) (98%), dimethyformamide (DMF), dimethylsulfoxide (DMSO), titania paste, 1-butanol, ethanol, iso-propyl alcohol (IPA), 4-tert-butylpyridine, acetonitrile, lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI), 2, 2′, 7, 7′-tetrakis(N,N-di-p-methoxyphenylamine)-9, 9′-spirobifluorene (Spiro-OMeTAD), and anhydrous chlorobenzene were all purchased from Sigma-Aldrich (Natick, MA, USA)
Summary
Lead-based organic–inorganic hybrid perovskites have received significant attention as absorber materials in perovskite solar cells (PSCs). Significant improvements have been made in the performance and stability of perovskite solar cells in the same period. Significant improvements have been made in the performance and stability of perovskite solar cells in the same period These have been achieved via process engineering, compositional control, the use of different layered architectures, and the application of different deposition techniques to achieve power conversion efficiencies (PCEs) in excess of 25%.21. Such high PCE values have been attributed largely to the properties of the photoactive perovskite material, which includes: long-range charge carrier diffusion lengths, highly tunable bandgaps, low exciton binding energies, and high absorption over a wide range of wavelengths.. The materials for perovskite solar cells (PSCs) have been improved from the initial single cation methylammonium lead iodide (MAPbI3) to formamidinium lead iodide (FAPbI3), and a combination of inorganic cesium and organic materials. The resulting formamidinium-based perovskite solar cells have been shown to have a broader solar spectrum absorption, thereby giving rise to improved photoconversion efficiencies. In addition, by varying the combination of cations, we can narrow the bandgaps of the perovskite layers and improve their thermal stability.
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