Origins of Photoluminescence Instabilities at Halide Perovskite/Organic Hole Transport Layer Interfaces

Abstract

Metal halide perovskites are promising for optoelectronic device applications; however, their poor stability under solar illumination remains a primary concern. While the intrinsic photostability of isolated neat perovskite samples has been widely discussed, it is important to explore how charge transport layersemployed in most devicesimpact photostability. Herein, we study the effect of organic hole transport layers (HTLs) on light-induced halide segregation and photoluminescence (PL) quenching at perovskite/organic HTL interfaces. By employing a series of organic HTLs, we demonstrate that the HTL’s highest occupied molecular orbital energy dictates behavior; furthermore, we reveal the key role of halogen loss from the perovskite and subsequent permeation into organic HTLs, where it acts as a PL quencher at the interface and introduces additional mass transport pathways to facilitate halide phase separation. In doing so, we both reveal the microscopic mechanism of non-radiative recombination at perovskite/organic HTL interfaces and detail the chemical rationale for closely matching the perovskite/organic HTL energetics to maximize solar cell efficiency and stability.

Mixed-Halide Perovskites with Halogen Bond Induced Interlayer Locking Structure for Stable Pure-Red PeLEDs

“…However, their success is typically attributed to blocking possible migration pathways and failure to fundamentally address the ion migration problems. Developing effective protocols for suppressing halide migration and gaining a comprehensive understanding of the underlying mechanisms still represents a significant challenge for the whole community, particularly for developing spectra-stable mixed-halide PeLEDs. , …”

“…Developing effective protocols for suppressing halide migration and gaining a comprehensive understanding of the underlying mechanisms still represents a significant challenge for the whole community, particularly for developing spectra-stable mixed-halide Pe-LEDs. 23,24 To our current knowledge, the mobile halide anions migrate out of the perovskite lattice through halide vacancies. 25 Different from the traditional strategy that to passivate the vacancy, we propose to explore an alternative method that to in situ anchors the mobile halide anions into the perovskite lattice.…”

Mitigating Iodine Diffusion by a MoO3–Organic Composite Hole Transport Layer for Stable Perovskite Solar Cells

“…To further understand iodine permeation through HTLs and subsequent electrode corrosion, we characterized the stability of HTLs against iodine permeation using a metal corrosion test, modified from the widely used calcium corrosion test (Figure S4). ,, As silver is easily oxidized by iodine to form silver iodide (AgI), ,, it was used as the metal electrode in the iodine corrosion test. The tested HTLs on top of the silver sensor have a thickness of 100 nm, similar to that of the HTLs in PSCs.…”

“…Since a hole transport layer (HTL) capable of efficiently extracting holes from an iodide-based perovskite can also readily be oxidized by iodinefacilitating iodine diffusion through HTLs toward the metal electrodesit is important to develop effective and stable HTLs that also limit iodine diffusion. − The benchmark PSC HTL 2,2′,7,7′-tetrakis­[ N , N -di­(4-methoxyphenyl)­amino]-9,9′-spirobifluorene (spiro-OMeTAD) has an appropriate highest occupied molecular orbital (HOMO) level for hole extraction and a shallow lowest unoccupied molecular orbital (LUMO) level for electron blocking, but the low conductivity of pristine spiro-OMeTAD makes p -doping a necessity, which however is accompanied by additional stability issues. − Conventional spiro-OMeTAD doping with lithium bis­(trifluoromethane)­sulfonimide (LiTFSI) requires an oxidation step by exposing devices to ambient atmosphere, increasing the risk of device degradation. , Moreover, the hygroscopic nature of LiTFSI and the inhomogeneous and unstable morphology of doped spiro-OMeTAD also contribute to PSC instability, where the morphological deformation of doped spiro-OMeTAD further promotes iodine diffusion through large pinholes. ,, Therefore, it is crucial to explore alternative HTLs and doping approaches in order to enhance device stability and mitigate the adverse effects of iodine diffusion.…”

“…In this study, we employed 4,4′,4″-tris­[(3-methylphenyl)­phenylamino]­triphenylamine (m-MTDATA) as an HTL and demonstrate that p -doping with MoO 3 can provide benefits. The small-molecule m-MTDATA can be processed either via evaporation or solution and possesses suitable HOMO and LUMO levels for hole extraction and electron blocking, respectively. − However, the shallow HOMO level of m-MTDATA of −5.0 eV means a vulnerability to oxidation by iodine, which we have previously shown to accelerate iodine diffusion through the HTL. , Therefore, we preoxidize (i.e., p -dope) m-MTDATA with MoO 3 in this work to avoid further oxidation reactions between m-MTDATA and iodine. By oxidizing m-MTDATA with MoO 3 , we realize increased hole conductivity and enhanced iodine blocking capability, simultaneously.…”