Sagar M. Jain

Resonance Raman and Excitation Energy Dependent Charge Transfer Mechanism in Halide-Substituted Hybrid Perovskite Solar Cells

Organo-metal halide perovskites (OMHPs) are materials with attractive properties for optoelectronics. They made a recent introduction in the photovoltaics world by methylammonium (MA) lead triiodide and show remarkably improved charge separation capabilities when chloride and bromide are added. Here we show how halide substitution in OMHPs with the nominal composition CH3NH3PbI2X, where X is I, Br, or Cl, influences the morphology, charge quantum yield, and local interaction with the organic MA cation. X-ray diffraction and photoluminescence data demonstrate that halide substitution affects the local structure in the OMHPs with separate MAPbI3 and MAPbCl3 phases. Raman spectroscopies as well as theoretical vibration calculations reveal that this at the same time delocalizes the charge to the MA cation, which can liberate the vibrational movement of the MA cation, leading to a more adaptive organic phase. The resonance Raman effect together with quantum chemical calculations is utilized to analyze the change in charge transfer mechanism upon electronic excitation and gives important clues for the mechanism of the much improved photovoltage and photocurrent also seen in the solar cell performance for the materials when chloride compounds are included in the preparation.

Micro-FTIR and Micro-Raman Studies of a Carbon Film Prepared from Furfuryl Alcohol Polymerization

The synthesis of a carbon film by the acid-catalyzed polymerization and resinification of furfuryl alcohol with a diluted solution of HCl is studied by combining micro-FTIR and micro-Raman spectroscopies. The detailed study of the evolution of spectra as a function of dosage of furfuryl alcohol and temperature shows that neutral and protonated species are formed at 80 degrees C, while upon gradually increasing the temperature up to 600 degrees C, the viscous polyfurfuryl alcohol resin is transformed into a carbon phase, containing a heterogeneous distribution of pores, with a size in the 100-2000 nm range, as shown by SEM and AFM analyses.

Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of opto-electronic properties and photovoltaic performance

Hybrid (organic-inorganic) lead trihalide perovskites have attracted much attention in recent years due to their exceptionally promising potential for application in solar cells. Here a controlled ...

Frustrated Lewis pair-mediated recrystallization of CH3NH3PbI3 for improved optoelectronic quality and high voltage planar perovskite solar cells

Films of the hybrid lead halide perovskite CH3NH3PbI3 were found to react with pyridine vapor at room temperature leading to complete bleaching of the film. In dry air or nitrogen atmosphere recrystallization takes place, leading to perovskite films with markedly improved optical and photovoltaic properties. The physical and chemical origin of the reversible bleaching and recrystallization mechanism was investigated using a variety of experimental techniques and quantum chemical calculations. The strong Lewis base pyridine attacks the CH3NH3PbI3. The mechanism can be understood from a frustrated Lewis pair formation with a partial electron donation of the lone-pair on nitrogen together with competitive bonding to other species as revealed by Raman spectroscopy and DFT calculations. The bleached phase consists of methylammonium iodide crystals and an amorphous phase of PbI2(pyridine)2. After spontaneous recrystallization the CH3NH3PbI3 thin films have remarkably improved photoluminescence, and solar cell performance increased from 9.5% for as-deposited films to more than 18% power conversion efficiency for recrystallized films in solar cells with planar geometry under AM1.5G illumination. Hysteresis was negligible and open-circuit potential was remarkably high, 1.15 V. The results show that complete recrystallization can be achieved with a simple room temperature pyridine vapor treatment of CH3NH3PbI3 films leading to high quality crystallinity films with drastically improved photovoltaic performance.

Vapor phase conversion of PbI2 to CH3NH3PbI3: spectroscopic evidence for formation of an intermediate phase

The formation of CH3NH3PbI3 (MAPbI3) from its precursors is probably the most significant step in the control of the quality of this semiconductor perovskite material, which is highly promising for photovoltaic applications. Here we investigated the transformation of spin coated PbI2 films to MAPbI3 using a reaction with MAI in vapor phase, referred to as vapor assisted solution process (VASP). The presence of a mesoporous TiO2 scaffold on the substrate was found to speed up reaction and led to complete conversion of PbI2, while reaction on glass substrates was slower, with some PbI2 remaining even after prolonged reaction time. Based on data from UV-visible spectroscopy, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy, the formation of an X-ray amorphous intermediate phase is proposed, which is identified by an increasing absorption from 650 to 500 nm in the absorption spectrum. This feature disappears upon long reaction times for films on planar substrates, but persists for films on mesoporous TiO2. Poor solar cell performance of planar VASP prepared devices was ascribed to PbI2 remaining in the film, forming a barrier between the perovskite layer and the compact TiO2/FTO contact. Good performance, with efficiencies up to 13.3%, was obtained for VASP prepared devices on mesoporous TiO2.

Development of a multifunctional TiO2/MWCNT hybrid composite grafted on a stainless steel grating

Functional materials have a promising potential for the fabrication of new devices with improved properties to meet many requirements, including environmental issues. Along this idea, a multiphase structure made using a TiO2/MWCNT hybrid nanoscaffold grafted on a metal grating (stainless steel type), acting as a strong, highly durable and heat/thermal inert support, is proposed. The method, consisting firstly of the fabrication of a porous scaffold via catalytic-CVD of a MWCNT forest on stainless steel, followed by the grafting of nanocrystalline TiO2 via the sol–gel method and then calcination, is simple and effective. Morphology, structure and optical properties have been investigated using XRD, SEM, AFM, and HRTEM techniques as well as Raman and UV-visible spectroscopy, and porosity analysis. Interestingly, the TiO2/MWCNT hybrid composite exhibits enhanced photocatalytic activity as compared to nanocrystalline TiO2, obtained by adopting the same preparation. More interestingly, the hybrid system exhibits additional functionalities, such as magnetic, surface and optical properties. The multifunctional approach allows for the combination of enhanced photodegradation with magnetic properties, which can make the recovery of a solution from a photocatalyst easier. Furthermore, it will be shown that, by moving from MWCNT/stainless steel to TiO2/MWCNT/stainless steel composites, the surface character changes from hydrophobic to hydrophilic in nature. Grafting on the stainless steel support allows for the addition of a broad range of features, including combined strength and corrosion resistance in aqueous solutions at ambient temperature, together with enhancement of electrical, optical and photocatalytic properties.

Acetylene oligomerization on the surface of TiO2: a step forward in the in situ synthesis of nanostructured carbonaceous structures on the surface of photoactive oxides

The interaction of acetylene with TiO2 (Evonik P25) was studied as a function of gas pressure and contact time by in situ vibrational (infrared and Raman) and electronic (UV-vis) spectroscopy and by gas chromatography-mass spectrometry analysis (GC-MS) of the reaction products. At low pressure the reaction proceeds through surface adsorption of acetylene and cyclotrimerization to benzene. At higher pressure (P > 100 mbar) the adsorption step is followed by the progressive formation of more complex oligomerization products containing an increasing number of conjugated double bonds. The final products obtained after mild heating at 373 K in acetylene excess absorb in the visible spectral region and confer to the system a strong blue color. These colored species are stable for many days in pure oxygen or air and cannot be extracted with common solvents, appearing strongly anchored to the TiO2 surface. The formation of saturated –CH3 and –CH2 groups during the oligomerization process and the spectroscopic and GC-MS results show that the colored species consist of polycyclic aromatic hydrocarbons containing a considerable number of condensed rings. Following these results, the controlled oligomerization of acetylene can represent a route for direct production of graphene-like species tightly anchored to the TiO2 surface, i.e. of composite materials of potential interest in photocatalysis or photovoltaic applications.

Pb-Sn-Cu Ternary Organometallic Halide Perovskite Solar Cells

Exploiting organic/inorganic hybrid perovskite solar cells (PSCs) with reduced Pb content is very important for developing environment-friendly photovoltaics. Utilizing of Pb-Sn alloying perovskite is considered as an efficient route to reduce the risk of ecosystem pollution. However, the trade-off between device performance and Sn substitution ratio due to the instability of Sn2+ is a current dilemma. Here, for the first time, the highly efficient Pb-Sn-Cu ternary PSCs are reported by partial replacing of PbI2 with SnI2 and CuBr2 . Sn2+ substitution results in a redshift of the absorption onset, whereas worsens the film quality. Interestingly, Cu2+ introduction can passivate the trap sites at the crystal boundaries of Pb-Sn perovskites effectively. Consequently, a power conversion efficiency as high as 21.08% in inverted planar Pb-Sn-Cu ternary PSCs is approached. The finding opens a new route toward the fabrication of high efficiency Pb-Sn alloying perovskite solar cells by Cu2+ passivation.

Toluene Alkylation with Methanol to p-Xylene over Heteropoly Acids Supported by Clay

Abstract The alkylation of toluene with methanol for the selective formation of p-xylene was systematically studied. Very few studies have been reported on the use of superacids such as heteropolyacids on cheap supports, such as clay. This article deals with the use of different heteropoly acids (HPAs), viz, Dodeca-Tungstophosphoric acid [H3PO4·12WO3·xH2O] (TPA), Dodeca-Molybdo phosphoric acid ammonium salt hydrate [H12Mo12N3O40P+aq] (DMAA), Dodeca-Molybdo Phosphoric acid (PMA) on clay (Montmorillonite, K-10) and as such plain clay. This comparative study reveals that 20%PMA/Clay shows 62% toluene conversion and 100% selectivity toward p-xylene.

Organic photovoltaic cells – promising indoor light harvesters for self-sustainable electronics

Photovoltaic cells are attracting significant interest for harvesting indoor light for low power consumption wireless electronics such as those required for smart homes and offices, and the rapidly-growing Internet of Things. Here, we explore the potential of solution processable, small molecule photovoltaic cells as indoor power sources. By optimizing the solvent vapour annealing (SVA) time for the photovoltaic layer, a balance between its crystallization and phase separation is obtained, resulting in a record power conversion efficiency (PCE) of over 28% under fluorescent lamps of 1000 lux, generating a maximum power density of 78.2 μW cm−2 (>10% PCE under AM1.5G). This high indoor performance surpasses that of silicon based photovoltaic cells, and is similar to that of gallium arsenide photovoltaic cells. Besides, the ratios of the voltage at the maximum power point (MPP) to the open circuit voltage are similar from indoor lighting to one sun conditions, which is unique and allows a less power consuming method to track the MPP for a broad range of light intensities (potentially attractive for wearable photovoltaics). New insight into the effect of SVA on the indoor and one sun performance is provided using advanced optoelectronic characterization techniques, which show that the mobility-lifetime products as a function of charge carrier density can be correlated well with the performance at different light levels. Our results suggest that organic photovoltaic cells could be promising as indoor power sources for self-sustainable electronics.