School of Chemistry - Research Publications
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ItemChemical vapor deposition growth of phase-selective inorganic lead halide perovskite films for sensitive photodetectors(ELSEVIER SCIENCE INC, 2021-01)Inorganic lead halide perovskites are attractive optoelectronic materials owing to their relative stability compared to organic cation alternatives. The chemical vapor deposition (CVD) method offers potential for high quality perovskite film growth. The deposition temperature is a critical parameter determining the film quality owing to the melting difference between the precursors. Here, perovskite films were deposited by the CVD method at various temperatures between 500−800 °C. The perovskite phase converts from CsPb2Br5 to CsPbBr3 gradually as the deposition temperature is increased. The grain size of the perovskite films also increases with temperature. The phase transition mechanism was clarified. The photoexcited state dynamics were investigated by spatially and temporally resolved fluorescence measurements. The perovskite film deposited under 750 °C condition is of the CsPbBr3 phase, showing low trap-state density and large crystalline grain size. A photodetector based on perovskite films shows high photocurrent and an on/off ratio of ∼2.5 × 104.
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ItemRevealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells(American Chemical Society, 2020-06-10)Layered perovskite films, composed of two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs), show improved stability compared to their conventional three-dimensional (3D) counterparts in perovskite solar cells (PSCs). However, 2D PSCs exhibit a lower power conversion efficiency (PCE), which has been attributed to compositional inhomogeneity and nonuniform alignment of the 2D perovskite phases. Methylammonium chloride (MACl) has been adopted as an additive to improve the PCE and the operational stability of 2D PSCs, although the role of MACl in performance enhancement is unclear. In this work, time- and spatially resolved fluorescence and absorption techniques have been applied to study the composition and charge carrier dynamics in MACl-doped BA2MA4Pb5I16 (⟨n⟩ = 5) layered perovskite films. The inhomogeneous phase orientation distribution in the direction orthogonal to the substrate for undoped layered perovskite films undergoes reorganization upon MACl doping. Based on structural and crystallographic analyses, it is revealed that MACl can facilitate the crystallization of small-n 2D perovskite phases at the cost of consuming an increased amount of BA cations. Consequently, an increase in the thickness of large-n 2D perovskite phases accompanies their enhanced perpendicular alignment ([101] crystalline orientation) to the substrate, which facilitates charge carrier transport and collection by electrodes. The defect passivation of the MACl-doped layered perovskite film provided by the small-n phase is also beneficial to the photovoltaic performance of the PSC device. A maximum PCE (∼14.3%) was achieved at 6 mol % MACl doping, with this optimum level influenced by the increased interfacial roughness of the layered perovskite film caused by the edges of small-n perovskite flakes emerging on the front surface.
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ItemMorphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell(AMER CHEMICAL SOC, 2021-01-14)The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.
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ItemCompetitive Triplet Formation and Recombination in Crystalline Films of Perylenediimide Derivatives: Implications for Singlet Fission(AMER CHEMICAL SOC, 2020-05-28)Developing photostable compounds that undergo quantitative singlet fission (SF) is a key challenge. As SF necessitates electron transfer between neighboring molecules, the SF rate is highly sensitive to intermolecular coupling in the solid state. We investigate SF in thin films for a series of perylenediimide (PDI) molecules. By adding different substituents at the imide positions, the packing of the molecules in the solid state can be changed. The relationship between SF parameters and the stacked geometry in PDI films is investigated, with two-electron direct coupling found to be the main SF mechanism. Time-resolved emission and transient absorption data show that all of the PDI films undergo SF although with different rates and yields varying from 35 to 200%. The results show that PDI1 and 2, which are stacked PDI pairs twisted out of alignment along the highest occupied molecular orbital to lowest unoccupied molecular orbital transition, exhibit faster and more efficient SF up to 200% yield. We demonstrate that both triplet formation and decay rates are highly sensitive to the ordering of the molecules within a film. The results of this study will assist in the design of optimized structures with a fast SF rate and low recombination rate that are required for useful light harvesting applications.
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ItemFRET-enhanced photoluminescence of perylene diimides by combining molecular aggregation and insulation(Royal Society of Chemistry, 2020-07-14)The photoluminescence quantum yield (ϕPL) of perylene diimide derivatives (PDIs) is often limited by aggregation caused quenching (ACQ) at high concentration or in the neat solid-state. Energy transfer in high dye concentration systems is also a key factor in determining ϕPL as a result of energy funneling to trap sites in the sample. By tuning the substituents, we present two classes of PDIs with aggregation and insulation of the PDI core. By combining these fluorophores in a polymer film, we demonstrate highly emissive samples (85% ϕPL) at high concentration (140 mM or 20% w/w). Experimental and theoretical studies provide insight into why such a combination is necessary to achieve high ϕPL. While insulated fluorophores maintain respectable ϕPL at high concentration, an improved ϕPL can be achieved in the presence of appropriately oriented fluorophore aggregates as emissive traps. The theoretical calculations show that the relative orientation of aggregated monomers can result in energetic separation of localized states from the charge-transfer and bi-excitonic states thereby enabling high ϕPL.
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ItemQuantifying the Relaxation Dynamics of Higher Electronic Excited States in Perylene.(American Chemical Society, 2023-08-31)Gating logical operations through high-lying electronic excited states presents opportunities for developing ultrafast, subnanometer computational devices. A lack of molecular systems with sufficiently long-lived higher excited states has hindered practical realization of such devices, but recent studies have reported intriguing photophysics from high-lying excited states of perylene. In this work, we use femtosecond spectroscopy supported by quantum chemical calculations to identify and quantify the relaxation dynamics of monomeric perylene's higher electronic excited states. The 21B2u state is accessed through single-photon absorption at 250 nm, while the optically dark 21Ag state is excited via the 11B3u state. Population of either state results in subpicosecond relaxation to the 11B3u state, and we quantify 21Ag and 21B2u state lifetimes of 340 and 530 fs, respectively. These lifetimes are significantly longer than the singlet fission time constant from the perylene 21B2u state, suggesting that the higher electronic states of perylene may be useful for gating logical operations.
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ItemA sandwich-like structural model revealed for quasi-2D perovskite films(Royal Society of Chemistry, 2021-04-28)The excellent performance and stability of perovskite solar cells (PSCs) based on quasi-2D Ruddlesden–Popper perovskites (RPPs) holds promise for their commercialization. Further improvement in the performance of 2D PSCs requires a detailed understanding of the microstructure of the quasi-2D perovskite films. Based on scanning transmission electron microscopy (STEM), time-resolved photoluminescence, and transient absorption measurements, a new sandwich-like structural model is proposed to describe the phase distribution of RPPs. In contrast to the conventional gradient distribution, it is found that small-n RPPs are sandwiched between large-n RPP phase layers at the front and back sides owing to crystallization initiated from both interfaces during film formation. This sandwich-like distribution profile facilitates excitons funneling from the film interior to both surfaces for dissociation while free carriers transport via large-n channels that permeate the film to ensure efficient charge collection by the corresponding electrodes, which is favorable for high-performance photovoltaics. This discovery provides a new fundamental understanding of the operating principles of 2D PSCs and has valuable implications for the design and optimization strategies of optoelectronic devices based on quasi-2D RPPs films.
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ItemBrownian Tree‐Shaped Dendrites in Quasi‐2D Perovskite Films and Their Impact on Photovoltaic Performance(Wiley, 2022-05)Quasi-2D Ruddlesden–Popper perovskites (RPPs) are candidates for constructing perovskite solar cells (PSCs) with superior stability due to their tolerance to the external environment. Fully understanding the film growth mechanism and structure is crucial to further improve the performance of 2D-PSCs while maintaining device stability. In this work, the origin of Brownian tree-shaped dendrites formed in hot-cast methylammonium chloride (MACl)-doped BA2MAn−1PbnI3n+1 ( = 5) quasi-2D perovskite films are reported. Investigations based on optical, electronic, atomic force, and fluorescence microscopies reveal that the dendrites are assembled from large-n RPPs-dominated grains, while the nondendritic film area is composed of small-n RPPs grains and associated with film surface pits caused by the evaporation of MACl. It is proposed that these dendrites are grown by the diffusion-limited aggregation of the MA-rich intermediate phase domains that initially crystallize from the precursor. The formation of these dendrites in quasi-2D perovskite films upon MACl doping is accompanied by improved organization and crystallinity of the 2D RPPs, which benefits the photovoltaic performance. This work provides new insights into the formation mechanism of quasi-2D perovskite films that should assist device engineering strategies to further improve the performance of 2D PSCs.
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ItemSpontaneous Formation of a Ligand-Based 2D Capping Layer on the Surface of Quasi-2D Perovskite Films(AMER CHEMICAL SOC, 2022-11-23)Two-dimensional (2D) Ruddlesden-Popper phase perovskites (RPPs) are attracting growing attention for photovoltaic applications due to their enhanced stability compared to three-dimensional (3D) perovskites. The superior tolerance of 2D RPPs films to moisture and oxygen is mainly attributed to the hydrophobic nature of the introduced long-chain spacer cations (ligands). In this work, it is revealed that a thin capping layer, consisting of self-assembled butylammonium ligands, is spontaneously formed on the top surface of a quasi-2D perovskite film prepared by conventional one-step hot casting. Based on morphological and crystallographic analyses of both the top/bottom surfaces and the interior of quasi-2D perovskite films, the formation process of the 2D capping layer and the assembly of RPPs, comprising both large and small slab thickness (large-n, small-n), is elucidated. The vertical orientation of RPPs that is required for sufficient charge transport for 2D perovskite solar cells (PSCs) is further verified. We propose that the surface capping layer is directly responsible for the long-term stability of 2D PSCs. This work provides detailed insight into the microstructure of quasi-2D RPPs films that should assist the development of strategies for unlocking the full potential of 2D perovskites for high-performance PSCs and other solid-state electronic devices.
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ItemBrownian Tree-Shaped Dendrites in Quasi-2D Perovskite Films and Their Impact on Photovoltaic Performance(WILEY, 2022-05)Abstract Quasi‐2D Ruddlesden–Popper perovskites (RPPs) are candidates for constructing perovskite solar cells (PSCs) with superior stability due to their tolerance to the external environment. Fully understanding the film growth mechanism and structure is crucial to further improve the performance of 2D‐PSCs while maintaining device stability. In this work, the origin of Brownian tree‐shaped dendrites formed in hot‐cast methylammonium chloride (MACl)‐doped BA2MAn−1PbnI3n+1 (<n> = 5) quasi‐2D perovskite films are reported. Investigations based on optical, electronic, atomic force, and fluorescence microscopies reveal that the dendrites are assembled from large‐n RPPs‐dominated grains, while the nondendritic film area is composed of small‐n RPPs grains and associated with film surface pits caused by the evaporation of MACl. It is proposed that these dendrites are grown by the diffusion‐limited aggregation of the MA‐rich intermediate phase domains that initially crystallize from the precursor. The formation of these dendrites in quasi‐2D perovskite films upon MACl doping is accompanied by improved organization and crystallinity of the 2D RPPs, which benefits the photovoltaic performance. This work provides new insights into the formation mechanism of quasi‐2D perovskite films that should assist device engineering strategies to further improve the performance of 2D PSCs.