Abstract
AbstractThe power conversion efficiency of inorganic–organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical‐field‐induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in‐situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl‐C61‐butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter‐diffusion. By employing wide‐field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature‐dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X‐ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built‐in field modulation and interfacial barriers.
Highlights
Driven by the demand for low cost and crystalline silicon; they still experience problems such as hysteresis and photo/electrical-field-induced degradation
When a phenyl-C61-butyric acid methyl ester (PCBM) layer is inserted between the perovskite and the TiO2 electron transport layer (ETL), as shown in Figure1b, the power conversion efficiency (η) of solar cells is comparable with the reference device in reverse scan direction
We find that PCBM which can freely interact or interdiffuse within a perovskite film decreases the ionic migration, and reduces the hysteresis in PCBM
Summary
We analyzed the activation barrier of ion migration of a pure perovskite film and another processed on a PCBM layer An Arrhenius plot shows a linear relationship between ln(τ) and 1/T and suggests that the movement of ions is facilitated by a single hopping mechanism.[56] The constant of τ related with the “jumping rate” of ions Γ is used to describe the ionic migration within the bulk of the perovskite film.[57] It indicates that ions are moving faster in the perovskite film than the one on PCBM layer in all temperatures, consistent with the pervious PL microscopy observation. Immobile (i.e., polymerized) derivatives, instead may become of great interest as PSC/ETL interlayers
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