Subodh G. Mhaisalkar

Low-temperature solution-processed wavelength-tunable perovskites for lasing

Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coefficients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show electrically driven lasing.

High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer Rutile TiO2 Nanorod and CH3NH3PbI3 Perovskite Sensitizer

We report a highly efficient solar cell based on a submicrometer (~0.6 μm) rutile TiO2 nanorod sensitized with CH3NH3PbI3 perovskite nanodots. Rutile nanorods were grown hydrothermally and their lengths were varied through the control of the reaction time. Infiltration of spiro-MeOTAD hole transport material into the perovskite-sensitized nanorod films demonstrated photocurrent density of 15.6 mA/cm(2), voltage of 955 mV, and fill factor of 0.63, leading to a power conversion efficiency (PCE) of 9.4% under the simulated AM 1.5G one sun illumination. Photovoltaic performance was significantly dependent on the length of the nanorods, where both photocurrent and voltage decreased with increasing nanorod lengths. A continuous drop of voltage with increasing nanorod length correlated with charge generation efficiency rather than recombination kinetics with impedance spectroscopic characterization displaying similar recombination regardless of the nanorod length.

Flexible, low-temperature, solution processed ZnO-based perovskite solid state solar cells

A ZnO compact layer formed by electrodeposition and ZnO nanorods grown by chemical bath deposition (CBD) allow the processing of low-temperature, solution based and flexible solid state perovskite CH3NH3PbI3 solar cells. Conversion efficiencies of 8.90% were achieved on rigid substrates while the flexible ones yielded 2.62%.

Perovskite Materials for Light-Emitting Diodes and Lasers

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Lead-Free Halide Perovskite Solar Cells with High Photocurrents Realized Through Vacancy Modulation

Lead free perovskite solar cells based on a CsSnI3 light absorber with a spectral response from 950 nm is demonstrated. The high photocurrents noted in the system are a consequence of SnF2 addition which reduces defect concentrations and hence the background charge carrier density.

Laminated Carbon Nanotube Networks for Metal Electrode-Free Efficient Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells.

Cobalt Sulfide Nanosheet/Graphene/Carbon Nanotube Nanocomposites as Flexible Electrodes for Hydrogen Evolution

Flexible three-dimensional (3D) nanoarchitectures have received tremendous interest recently because of their potential applications in wearable electronics, roll-up displays, and other devices. The design and fabrication of a flexible and robust electrode based on cobalt sulfide/reduced graphene oxide/carbon nanotube (CoS2 /RGO-CNT) nanocomposites are reported. An efficient hydrothermal process combined with vacuum filtration was used to synthesize such composite architecture, which was then embedded in a porous CNT network. This conductive and robust film is evaluated as electrocatalyst for the hydrogen evolution reaction. The synergistic effect of CoS2 , graphene, and CNTs leads to unique CoS2 /RGO-CNT nanoarchitectures, the HER activity of which is among the highest for non-noble metal electrocatalysts, showing 10 mA cm(-2) current density at about 142 mV overpotentials and a high electrochemical stability.

Electrospun α-Fe2O3 nanorods as a stable, high capacity anode material for Li-ion batteries

α-Fe2O3 nanorods are synthesized by electrospinning of polyvinylpyrrolidone (PVP)/ferric acetyl acetonate (Fe(acac)3) composite precursors and subsequent annealing at 500 °C for 5 h. X-ray diffraction and Raman spectroscopy analyses confirm the formation of a hematite structure as the predominant phase. The electron microscopy studies show that the electrospun α-Fe2O3 nanorods are composed of agglomerates of nano-sized particles and the average diameter of the nanorods is found to be 150 nm. Li-storage and cycling properties are examined by galvanostatic cycling in the voltage range 0.005–3 V vs. Li at various current densities and it is complemented by cyclic voltammetry. The electrospun α-Fe2O3 nanorods exhibit a high reversible capacity of 1095 mA h g−1 at 0.05 C, are stable up to 50 cycles and also show high rate capability, up to 2.5 C. The high rate capability and excellent cycling stability can be attributed to the unique morphology of the macroporous nanorods comprised of inter-connected nano-sized particles, thus making electrospun α-Fe2O3 a promising anode material for Li-ion batteries.

Synthesis and electrochemical properties of electrospun V2O5 nanofibers as supercapacitor electrodes

Vanadium pentoxide (V2O5) nanofibers (VNF) were synthesized through a simple electrospinning method, and their application as supercapacitor electrodes demonstrated. The effect of annealing temperature on the microstructure and morphology of VNF was investigated systematically through scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area measurements. Electrochemical properties of the synthesized products as electrodes in a supercapacitor device were studied using cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy in aqueous electrolyte of different pH and also in an organic electrolyte. The highest specific capacitance was achieved for VNF annealed at 400 °C, which yielded 190 F g−1 in aqueous electrolyte (2 M KCl) and 250 F g−1 in organic electrolyte (1 M LiClO4 in PC) with promising energy density of 5 Wh kg−1 and 78 Wh kg−1 respectively.

Band-gap tuning of lead halide perovskites using a sequential deposition process

Band-gap tuning of mixed anion lead halide perovskites (MAPb(I1−xBrx)2 (0 ≤ x ≤ 1)) has been demonstrated by means of a sequential deposition process. The optical properties of perovskite hybrids can be flexibly modified by changing (mixing) the concentration of halogen precursors. The concentrations of precursor solution as well as the conversion time play an important role in determining the band-gap of perovskites. A systematic shift of the absorption band edge to shorter wavelengths is observed with increasing Br content in the perovskite films, which results in the decrement of the photocurrent. Nanorod like morphological features are also observed for perovskite films with an iodide to bromide molar ratio of <0.7.