Dual-source vacuum deposition of pure and mixed halide 2D perovskites: thin film characterization and processing guidelines

The dual-source vacuum deposition of 2D perovskite films of the type PEA2PbX4, (PEA = phenethylammonium and X = I−, Br−, or a combination of both) is presented. Low-temperature deposited 2D perovskite films showed high crystallinity with the expected trend of bandgap as a function of halide type and concentration. Importantly, we observed an unavoidable halide cross-contamination among different deposition runs, as well as a strong dependence of the material quality on the type of halide precursors used. These findings should be taken into account in the development of vacuum processing for low-dimensional mixed halide perovskites

Vacuum Deposited Triple-Cation Mixed-Halide Perovskite Solar Cells

Hybrid lead halide perovskites are promising materials for future photovoltaics applications. Their spectral response can be readily tuned by controlling the halide composition, while their stability is strongly dependent on the film morphology and on the type of organic cation used. Mixed cation and mixed halide systems have led to the most efficient and stable perovskite solar cells reported, so far they are prepared exclusively by solution-processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. In this report, thermal vacuum deposition with multiple sources (3 and 4) is used to prepare for the first time, multications/anions perovskite compounds. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability. The importance of the control over the film morphology is highlighted, which differs substantially when these compounds are vacuum processed. Avenues to improve the morphology and hence the performance of fully vacuum processed multications/anions perovskite solar cells are proposed

Vacuum-Deposited 2D/3D Perovskite Heterojunctions

Low-dimensional (quasi-) 2D perovskites are being extensively studied in order to enhance the stability and the open-circuit voltage of perovskite solar cells. Up to now, thin 2D perovskite layers on the surface and/or at the grain boundaries of 3D perovskites have been deposited solely by solution processing, leading to unavoidable intermixing between the two phases. In this work, we report the fabrication of 2D/3D/2D perovskite heterostructures by dual-source vacuum deposition, with the aim of studying the interaction between the 3D and 2D phases as well as the charge transport properties of 2D perovskites in neat 2D/3D interfaces. Unlike what is normally observed in solution-processed 3D/2D systems, we found a reduced charge transport with no direct evidence of surface passivation, in spite of larger open-circuit voltage. This is likely due to a nonfavorable orientation of the 2D perovskite with respect to methylammonium lead iodide and to the formation of 2D phases with very low dimensionality (pure 2D)

Red Light-Emitting Electrochemical Cells Employing Pyridazine-Bridged Cationic Diiridium Complexes

A rigid dinuclear Ir(III) complex showing high photoluminescence quantum yield in pure films was successfully used to fabricate light-emitting electrochemical cells with and without ionic liquid additives. The devices showed nearly instantaneous electroluminescence after biasing and maximum quantum yield approaching 1%. The lifetime of the devices was found to be limited to approximately 20 hours, which we correlated with the irreversible oxidation of the complex as seen from electrochemical measurements. This work validates the use of highly luminescent dinuclear iridium complexes in light-emitting electrochemical cells. Future studies will pursue materials with more efficient photoluminescence as well as improved electrochemical stability

CF3 Substitution of [Cu(P^P)(bpy)][PF6] complexes: Effects on Photophysical Properties and Light-emitting Electrochemical Cell Performance

We report [Cu(P^P)(N^N)][PF 6 ] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos), N^N = CF 3 -substituted 2,2'-bipyridines (6,6'-(CF 3 ) 2 bpy, 6-CF 3 bpy, 5,5'-(CF 3 ) 2 bpy, 4,4'-(CF 3 ) 2 bpy, 6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy). We present the effects of CF 3 substitution on structures, and electrochemical and photophysical properties. The HOMO–LUMO gap is tuned by the N^N ligand; the largest redshift in the MLCT band is for [Cu(P^P)(5,5'-(CF 3 ) 2 bpy)][PF 6 ]. In solution, the compounds are weak yellow to red emitters. The emission properties depend on the substitution pattern but this cannot be explained by simple electronic arguments. For powders, [Cu(xantphos)(4,4'-(CF 3 ) 2 bpy)][PF 6 ] has the highest PLQY (50.3%) with an emission lifetime of 12 µs. Compared to 298 K solution behaviour, excited state lifetimes lengthen in frozen Me-THF (77 K) indicating thermally activated delayed fluorescence (TADF). TD-DFT calculations show that the energy gap between the lowest-energy singlet and triplet excited states (0.12–0.20 eV) permits TADF. LECs with [Cu(POP)(6-CF 3 bpy)][PF 6 ], [Cu(xantphos)(6-CF 3 bpy)][PF 6 ] or [Cu(xantphos)(6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy)][PF 6 ] emit yellow electroluminescence. A LEC with [Cu(xantphos)(6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy)][PF 6 ] had the fastest turn-on time (8 min); the LEC with the longest lifetime ( t 1/2 = 31 h) contained [Cu(xantphos)(6-CF 3 bpy)][PF 6 ]; these LECs reached maximum luminances of 131 and 109 cd m –2

Low dimensional perovskite: synthetic methods and application in optoelectronics

Through the end of contribute to the next generation of photovoltaics and light emitting devices the preparation of no-expensive and high quality materials with high PLQY and reduced non-radiative recombination is crucial. For such a reason the thesis is focus on the develop of high optoelectronic potential hybrid perovskite material and on their processing methods. In the different Chapters, various low dimensional perovskite synthetic approaches and their application are investigated. In the Chapter 2, a low dimension quasi-2D perovskite with a remarkable PLQY was prepared thanks to interplay of different fabrication approaches such us altered stoichiometry, solution engineered deposition and the use of electron donor passivating agent. In the Chapter 3, quasi-2D and pure 2D perovskite were fabricated by vacuum-deposition method in order to overcame the limitation of the solution process approach. High crystallinity, homogenous, compact films were prepared. However, different challenges were face up during the deposition process. In the Chapter 4, the 2D perovskites fabricated by dual-source vacuum deposition in the Chapter 3 were apply to study the effect and the advantages of a wide bandgap 2D perovskite material on the reduction of non-radiative recombination of a polycrystalline 3D perovskite in a heterostructure system

Amplified Spontaneous Emission Threshold Dependence on Determination Method in Dye-Doped Polymer and Lead Halide Perovskite Waveguides

Nowadays, the search for novel active materials for laser devices is proceeding faster and faster thanks to the development of innovative materials able to combine excellent stimulated emission properties with low-cost synthesis and processing techniques. In this context, amplified spontaneous emission (ASE) properties are typically investigated to characterize the potentiality of a novel material for lasers, and a low ASE threshold is used as the key parameter to select the best candidate. However, several different methods are currently used to define the ASE threshold, hindering meaningful comparisons among various materials. In this work, we quantitatively investigate the ASE threshold dependence on the method used to determine it in thin films of dye-polymer blends and lead halide perovskites. We observe a systematic ASE threshold dependence on the method for all the different tested materials, and demonstrate that the best method choice depends on the kind of information one wants to extract. In particular, the methods that provide the lowest ASE threshold values are able to detect the excitation regime of early-stage ASE, whereas methods that are mostly spread in the literature return higher thresholds, detecting the excitation regime in which ASE becomes the dominant process in the sample emission. Finally, we propose a standard procedure to properly characterize the ASE threshold, in order to allow comparisons between different materials

Deep-blue thermally activated delayed fluorescence (TADF) emitters for light-emitting electrochemical cells (LEECs)

The authors acknowledge the University of St Andrews for financial support. The authors also acknowledge financial support from the European Union H2020 project INFORM (grant 675867), the Spanish Ministry of Economy and Competitiveness (MINECO) via the Unidad de Excelencia María de Maeztu MDM-2015-0538, MAT2014-55200 and the Generalitat Valenciana (Prometeo/2016/135). MLP acknowledges support from a Grisolia grant (GRISOLIA/2015/A/146).Two deep blue thermally activated delayed fluorescence (TADF) emitters ( imCzDPS and imDPADPS ) that contain charged imidazolium groups tethered to the central luminophore were designed and synthesized as small molecule organic emitters for light-emitting electrochemical cell (LEEC) electroluminescent devices. The emission profile in doped thin films (5 wt% in PMMA) is very blue and narrow (λPL: 414 nm and 409 nm; full width at half maximum (FHWM): 62 nm and 46 nm for imCzDPS and imDPADPS , respectively) with good photoluminescence quantum efficiencies (ФPL: 44% and 49% for imCzDPS and imDPADPS , respectively). In neat films, emission maxima occur at 440 nm and 428 nm for imCzDPS and imDPADPS , respectively with comparable ΦPL values of 44 and 61%, respectively. Both emitters exhibit biexponential emission decay kinetics (nanosecond prompt and microsecond delayed fluorescence) in both MeCN solution and thin film, characteristic of TADF behaviour. While imDPADPS did not show any emission in the LEEC device, that of imCzDPS gave an electroluminescence (EL) maximum at 470 nm and CIE coordinates of (0.208, 0.250), which makes this device amongst the bluest reported to date. However, the maximum device luminance achieved was 2.5 cd m-2 and this poor brightness was attributed to the electrochemical instability of the emitter in LEEC architecture, as evidenced by the additional peak around 550 nm observed in the EL spectrum.PostprintPeer reviewe