Hole Transfer in Dye-Sensitized Cesium Lead Halide Perovskite Photovoltaics: Effect of Interfacial Bonding

Abstract

Lead halide perovskites have gained attention as an active material in solid-state dye-sensitized photovoltaics due to their high absorption of visible light and long charge-transport lengths. In perovskite-based dye-sensitized photovoltaic architectures the perovskite material is typically paired with a hole-transport material, such as spiro-OMeTAD, which extracts a hole from the photoexcited perovskite to generate free charge carriers. In this study, we explored two competing charge-transfer pathways at the interface between lead halide perovskite and spiro-OMeTAD: “through-bond” and “through-space”. For the through-bond pathway we use a segment of spiro-OMeTAD that contains methoxy linker groups, which will be referred to as “dye with methoxy linker groups” (DML). For the through-space pathway we use a segment of spiro-OMeTAD with removed linker groups, triphenylamine, which will be referred to as “dye”. Four atomistic models were studied: (I) a periodic cesium lead iodide (CsPbI3) perovskite nanowire (NW) that is paired with the dye molecule, (II) a periodic CsPbI3 perovskite NW paired with the DML molecule where the linker groups form coordination bond to the surface of the nanowire, (III) a CsPbI3 perovskite thin film (TF) paired with the dye molecule, and (IV) a CsPbI3 perovskite TF paired with the DML molecule. Charge-transfer dynamics, providing rates of electron/hole relaxation and relaxation pathways, are calculated using reduced density matrix formalism using Redfield theory. It was found that the terminal surface of the perovskite (Pb–I vs Cs–I) has important implications for energetic alignment at the perovskite–dye interface due to band bending. Computed charge-transfer rates match well with upper and lower bounds of reported experimental results where “fast” picosecond rates correspond to through-bond pathway and “slow” nanosecond rates correspond to through-space.

Effect of ligand groups on photoexcited charge carrier dynamics at the perovskite/TiO2 interface

“…27 Certain materials have shown the ability to accept these separated charges from the perovskite when placed in contact with the perovskite. [28][29][30] This increases the efficiency of charge separation and extraction, thereby improving the power conversion efficiency (PCE) of perovskite-based photovoltaic devices. 31 Materials that accept electrons have been dubbed electron transport materials (ETMs) and hole accepting materials are likewise called hole transport materials (HTMs).…”

“…There is ongoing research into the origin of the hysteretic behavior of their I-V curves 1 as well as the ultrafast (<1 ps) charge injection into ETMs/HTMs. [28][29][30] This work focuses on the effects of ligands adsorbed to a POT ETM. More specically, the ground state properties and photoexcited charge carrier dynamics of two different perovskite/ETM structures are explored.…”

Nonradiative Relaxation Dynamics of a Cesium Lead Halide Perovskite Photovoltaic Architecture: Effect of External Electric Fields

“…21 Different types of surface terminations enhance or suppress the formation of binding modes, which affect the efficiency of charge transfer between perovskite and donor or acceptor. 31 Although two types of surface terminationCsBr and PbBr 2 of CsPbBr 3 perovskiteare available, the former one might serve as an "insulating layer", while the latter one provides a channel for charge. In the current setup, the atomistic model consists of three components: PbBr 2 -terminated Cs 18 Pb 27 Br 72 slab as the optical absorber, tris(3-methoxyphenyl)amine molecule, i.e., (C 6 H 4 OMe) 3 N, as the HTL, and polyoxotitanate cluster, i.e., (HO) 6 Ti 6 O 6 (OAc) 6 , as the ETL.…”

“…It is therefore interesting to see how the instable ionically bound crystal structures respond to local and external electric field and polarization. Despite a large number of atomistic studies on photoinduced charge carrier dynamics of cesium lead halide perovskites have been conducted, only a few focused on the dynamics at the interfaces of the perovskite layer and ETL/HTL. − In this work, we consider a thin film composed of cesium lead halide perovskite slab interfacing with ETL and HTL, relevant to the practical solar cell architecture. With density functional theory (DFT) calculations, we apply external electric fields normal to the surface of the perovskite slab and find that the strengths of electric fields can modulate optoelectronic properties of the thin film.…”

“…The realistic conditions of synthetic protocols lead to the distribution of adsorption configurations at lead halide perovskite surfaces . Different types of surface terminations enhance or suppress the formation of binding modes, which affect the efficiency of charge transfer between perovskite and donor or acceptor . Although two types of surface terminationCsBr and PbBr 2 of CsPbBr 3 perovskiteare available, the former one might serve as an “insulating layer”, while the latter one provides a channel for charge.…”