References of "Singh, Ajay 50044271"
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See detailMethylamine Gas Treatment Affords Improving Semitransparency, Efficiency, and Stability of CH3NH3PbBr3-Based Perovskite Solar Cells
Singh, Ajay UL; Matteocci, Fabio; Zhu, Hongwei et al

in Solar RRL (2021)

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 ... [more ▼]

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 perovskite growth limit the exploitation of CH3NH3PbBr3-based perovskite solar cells. Herein, a posttreatment is carried out after the initial CH3NH3PbBr3 crystallization based on methylamine gas that drastically enhances the perovskite quality leading to a highly crystalline film with improved average visible transmittance (AVT) close to 56%. Opaque devices showed outstanding results in terms of open-circuit voltage and power conversion efficiency (PCE) reaching 1.54 V and 9.2%, respectively. These achievements are ascribed to a film with reduced morphological defects and better interface quality and reduced nonradiative pathways. For the first time, the fabrication of semitransparent CH3NH3PbBr3-based solar cells is demonstrated reaching a maximum PCE equal to 7.6%, an AVT of the full stack device of 52%, and an excellent light stability at maximum-power point tracking. [less ▲]

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See detailRole of cation-mediated recombination in perovskite solar cells
Singh, Ajay UL; Kaiser, Waldemar; Gagliardi, Alessio

in Solar Energy Materials and Solar Cells (2021)

The origin of the hysteresis in the current–voltage (J–V) characteristics in perovskite solar cells (PSCs) is one of the most debated topics of recent years. Hysteretic effects are connected with the slow ... [more ▼]

The origin of the hysteresis in the current–voltage (J–V) characteristics in perovskite solar cells (PSCs) is one of the most debated topics of recent years. Hysteretic effects are connected with the slow redistribution of ionic defects during the voltage sweep. Existing literature focuses on the potential screening due to accumulated ions, solely, while neglecting the possibility of charge trapping and subsequent recombination via ions. We investigate the role of cation-mediated recombination of ions using time-dependent drift–diffusion simulations in MAPbI3 PSCs. Slow-moving cations are considered as traps for the electrons. Trapped electrons can subsequently recombine non-radiatively with holes. We analyze the role of the cation-mediated trapping and its parameters (capture coefficient, cation energy, ion mobility) as well as the scan rate on the device performance. For shallow cation energies, a decrease in open-circuit voltage and slight enhancement in hysteresis is observed. Deep cation energies lead to a substantial deterioration of device performance and large hysteresis enhancement. The presented study emphasizes the importance of considering the interaction of ions with charge carriers beyond the simple electrostatic models to improve our understanding of PSCs. [less ▲]

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See detailTuning halide perovskite energy levels
Canil, Laura; Cramer, Tobias; Fraboni, Beatrice et al

in Energy and Environmental Science (2020)

Solar energy is playing a significant role in the development of a world powered by clean energy sources. In this context, halide perovskite solar cells (PSCs) are considered one of the most promising ... [more ▼]

Solar energy is playing a significant role in the development of a world powered by clean energy sources. In this context, halide perovskite solar cells (PSCs) are considered one of the most promising research lines thanks to their high efficiencies and flexibility, combined with an easy and cheap fabrication process. The possibility of combining different materials and compositions is an excellent advantage of PSCs. However, still, a big limit is posed by the need for a proper energy level alignment between the layers of materials comprising devices. Therefore, it is of utmost interest to develop methods allowing to tune the energy levels of the different materials. In semiconductors physics, a common technique to achieve this purpose is to functionalize the surface of the materials with dipolar molecules. Nevertheless, this has been rarely applied to perovskites because of the highly rough surface of the films. In this study, we show that it is possible to use this technique in hybrid organic–inorganic perovskite semiconductors systematically and tune the direction and magnitude of the shift by controlling the deposition process. These findings offer a toolbox to simplify the application of halide perovskites in optoelectronic devices. [less ▲]

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See detailGeneration of a multi-wavelength source spanning the entire C-band by nonlinear spectral broadening of dual-carrier electro-optic frequency combs
VIKRAM, B. S.; PRAKASH, ROOPA; NAGARJUN, K. P. et al

in OSA Continuum (2020)

We demonstrate a multi-wavelength source with a high repetition rate of 25 GHz, spanning the entire C-band, of which 124 lines lie within 10 dB bandwidth. We exploit the spectral and temporal properties ... [more ▼]

We demonstrate a multi-wavelength source with a high repetition rate of 25 GHz, spanning the entire C-band, of which 124 lines lie within 10 dB bandwidth. We exploit the spectral and temporal properties of dual carrier electro-optic combs to simultaneously enhance self-phase modulation (SPM) based broadening and increase the stimulated Brillouin scattering (SBS) threshold. Dual carrier combs are generated through electro-optic modulation of spectrally separated narrow linewidth carriers. They are spectrally broadened in a highly nonlinear fiber after amplification with an in-house built erbium ytterbium co-doped fiber amplifier. The temporal profile of the dual carrier combs consists of significantly narrow pulses (1.4-1.9 ps FWHM) in comparison to the single laser comb (16.5 ps FWHM), increasing the peak power and enhancing the SPM effects. Further, the spectral power is distributed across the comb lines, increasing the SBS threshold and thus the power scalability of the system. These two factors together boost the bandwidth of the spectrally broadened multi-wavelength source. [less ▲]

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See detailOptical frequency comb based on nonlinear spectral broadening of a phase modulated comb source driven by dual offset locked carriers
Nagarjun, K.P.; Vikram, B. S.; Prakash, Roopa et al

in Optics Letters (2020)

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by ... [more ▼]

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by dual frequency offset locked carriers. The locking is achieved by setting up a heterodyne optical frequency locked loop to lock two phase modulated electro-optic 25 GHz frequency combs sourced from individual seed carriers offset by 100 GHz, to within 6.7 MHz of each other.We realize spectral broadening in highly nonlinear fiber after suitable amplification to obtain an equalized, nonlinearly broadened frequency comb.We obtain ~86 lines in a 20 dB band spanning over 2 THz. [less ▲]

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See detailInterface Electrostatics of Solid-State Dye-Sensitized Solar Cells: A Joint Drift-Diffusion and Density Functional Theory Study
Singh, Ajay UL; Radicchi, Eros; Fantacci, Simona et al

in The Journal of Physical Chemistry C (2019)

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the ... [more ▼]

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the dye and the chargetransport materials, the interface electrostatics becomes a key factor determining the overall performance of the cell. Liquid-electrolyte-based DSCs suffer from low stability, electrolyte leakage, and, in some cases, electrode corrosion. Replacing liquid electrolyte with a solid semiconducting material leads to poor interfacial contacts, hence the interface electrostatics becomes one of the limiting factors. In this work, we present a drift-diffusion and density functional theory (DFT) study of solid-state DSCs to investigate the electrostatics at the TiO2/organic dye/Spiro-OMeTAD interface and its impact on the adsorbed dye energy levels, its absorption spectrum, and the related charge injection. In our three-dimensional drift-diffusion model, we solve a set of drift-diffusion equations coupled to Poisson equation for electrons, holes, doping impurities, and interface traps simultaneously. After that, we use first-principles DFT modeling of dye-sensitized interfaces in the presence of the calculated electric fields. We find that interface traps located below the conduction band edge of mesoporous TiO2 influence the accumulation of photogenerated holes and built-in electric field near the interface. The built-in electric field leads to change in the energetics at the dye/TiO2 interface, leading to poor charge injection from excited dye into TiO2. The simulations were carried out for different electronic trap densities in TiO2 and different doping levels in the Spiro-OMeTAD hole-transport layer. This study helps to a better understanding of the interface electrostatics and its role in the charge injection mechanism of solid-state DSCs. [less ▲]

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See detailEfficiency of all-perovskite two-terminal tandem solar cells: A drift-diffusion study
Singh, Ajay UL; Gagliardi, Alessio

in Solar Energy (2019)

Organic-inorganic (hybrid) perovskite semiconductors offer a wide range of bandgaps, low-cost deposition, and wide optical absorption, making them an ideal candidate for new photovoltaic devices. All ... [more ▼]

Organic-inorganic (hybrid) perovskite semiconductors offer a wide range of bandgaps, low-cost deposition, and wide optical absorption, making them an ideal candidate for new photovoltaic devices. All-perovskite two terminal (2T) tandem solar cells have the potential to achieve high efficiency and at the same time offer costeffective fabrication. In a 2T tandem cell, it is needed to optimize various device parameters such as bandgaps and thicknesses of the subcells, in order to make the best use of the available solar spectrum. In this study, we propose a drift-diffusion (DD) simulation model to optimize the bandgaps and thicknesses of the top and bottom cells in all-perovskite 2T tandem solar cell. Using our simulation model, we investigated the effect of interface and bulk traps, mobility, doping of the charge transport layers and contact workfunctions to the power conversion efficiency. We calculated up to 36.6% efficiency for an ideal device. We found that the traps at the interfaces and in the bulk perovskite films are the most important factor hampering the tandem cell efficiency. We predicted up to 29.8% efficiency for a device with recombination losses. By changing the mobility in the active material of the bottom cell we found that, the mobility plays an important role in determining the optimum thicknesses of the top and the bottom cells. Optimizing cathode workfunctions leads to a 3–4% improvement in the efficiency. Our study will help to understand the role of various factors limiting tandem cell efficiency and ways to optimize the device parameters to ensure the best performing all-perovskite 2T tandem solar cell. [less ▲]

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See detailFrequency Offset Locked, Dual Carrier Excitation of Phase modulated, Electro-optic Frequency Combs for Bandwidth Scaling and Nonlinear Spectral Broadening
Nagarjun, K. P.; Vikram, B. S.; Prakash, Roopa et al

in SPIE (2018, January 29)

DWDM with/without superchannel based photonic networks require the use of optical carriers with equalised amplitudes and frequency stabilization of adjacent carriers to realise reliable high bandwidth ... [more ▼]

DWDM with/without superchannel based photonic networks require the use of optical carriers with equalised amplitudes and frequency stabilization of adjacent carriers to realise reliable high bandwidth optical communication systems with high spectral efficiency and long reach. Cascading of electro-optic (EO) modulators is a versatile method for generating tuneable, high repetition rate frequency combs which can be used as sources for the carriers. However, the number of lines produced with this technique is limited by the number of phase modulators. Nonlinear spectral broadening is an attractive option for bandwidth scaling; however, bandwidth scaling of single carrier combs through four wave mixing suffers from unequal comb lines or power limitations due to Brillouin scattering. A simpler technique to increase the number of comb lines would involve using multicarrier excitations for comb generation which would result in a proportional increase in the comb lines. Further, dual-carrier excitation enables an excellent temporal profile for nonlinear spectral broadening. However, since the two carriers have uncorrelated drifts, the resultant frequency combs would be unsuitable for most applications. This issue can be overcome by frequency offset locking the two lasers. Here, we demonstrate frequency offset locking (MHz accuracy) of two diode lasers spaced by 100GHz by using an optical phase locked loop which locks one laser to a RF harmonic of the other. This allows for the generation of frequency comb lines locked to each other even post nonlinear broadening. Using this technique, we demonstrate a 25GHz frequency comb with >90 lines (2THz) in the C-band. [less ▲]

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See detailSimulation of ion migration in perovskite solar cells using a kinetic Monte Carlo/drift diffusion numerical model and analysis of the impact on device performance
Gagliardi, A.; Singh, Ajay UL; Kaiser, W.

in International Conference on Hybrid and Organic Photovoltaics Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18), Oral Presentation 099 (2018)

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of ... [more ▼]

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of deposition techniques, a simple integration with both organic and inorganic materials and, most important of all, a high light power conversion efficiency. Perovskite materials have also challenged the scientific community due to the many different physical processes that concur to set the optical and electrical properties: from ferroelectricity [1], to ion migration [2], defects and different recombination processes [3]. An important aspect of perovskite films is the presence of grain and grain boundaries. Although many progresses have been obtained in the quality of the film, still grain boundaries within the perovskite film in fabricated devices are present. The effect of these grain boundaries have been investigated by many groups, we refer here to just one reference [4], but the effect of these grain boundaries to free charges and ion migration is still under debate. In [4] it has been demonstrated as the change in average size of grains have an important impact on cell performance and ionic diffusion. In the present work we theoretically investigate the effect of ion migration with the presence of grain boundaries. We make a multiscale simulation tool based on kinetic Monte Carlo (for ion dynamics) [5] and drift diffusion based on finite elements (for the electrical part) [6] to understand how the different ion diffusion mechanisms can impact the final cell performance. References [1] A. Pecchia et al., Nano Lett., 16, 988 (2016) [2] J. M. Azpiroz et al., Energy & Environmental Science, 8, 2118-2127 (2015) [3] L. M. Herz, Annual Rev. Phys. Chemistry, 67, 65-89 (2016) [4] B. Roose et al., Nano Energy, 39, 24-29 (2017) [5] T. Albes, A. Gagliardi, Physical Chemistry Chemical Physics, 19 (31), 20974-20983 (2017) [6] A. Gagliardi and A. Abate, ACS Energy Lett., 3, 163 (2018) [less ▲]

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See detailSilicon-Based Inorganic–Organic Hybrid Nanocomposites for Optoelectronic Applications
Kanaujia, Pawan K.; Singh, Ajay UL; Prakash, G. Vijaya

in Energy Technology (2017)

A nanocomposite consisting of a unique combination of optically active materials, that is, an inorganic–organic (IO) layered hybrid and porous silicon (PS), is prepared using a simple but effective three ... [more ▼]

A nanocomposite consisting of a unique combination of optically active materials, that is, an inorganic–organic (IO) layered hybrid and porous silicon (PS), is prepared using a simple but effective three-step electrochemical method. Xray diffraction, energy-dispersive X-ray spectroscopy, and photoluminescence spectral analysis suggest that the IO hybrid (R-MX4-type 2D perovskite, where R represents an organic compound, M a divalent metal ion, and X a halide) is conveniently incorporated into the interstitial spaces of nanoporous silicon. The IO-based 2D perovskite emits a strong and narrow room-temperature exciton line at 520 nm due to effects related to dielectric confinement. Similarly, n-type porous silicon emits in a broad range in the deep-red region at 700 nm, which is attributable to the quantum confinement effects related to the nanoporosity in silicon. Because of the contributions from both entities, the completely space-filled IO–PS nanocomposite shows an orange–yellow emission. The proposed methodology can be easily extended to a large number of such IO–PS functional nanocomposites and is thus expected to be used in optoelectronic applications such as light-emitting diodes (LEDs), solar cells, and other optical elements. [less ▲]

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See detailSolar pumped Nd:YAG laser systems
Singh, Ajay UL; Gupta, Mayank; Mehta, Dalip Singh

in Tezpur University (2016, November)

Solar radiation from sunlight is abundant on our earth during daytime. In order to utilise it, here we report a new design and development of solar pumped Nd:YAG laser system. Sun Light is collected using ... [more ▼]

Solar radiation from sunlight is abundant on our earth during daytime. In order to utilise it, here we report a new design and development of solar pumped Nd:YAG laser system. Sun Light is collected using a large Fresnel lens concentrator of diameter 46 cm, which is then used for side pumping of the Nd:YAG laser rod. Horizontal cavity design with a variable cavity length of 14 cm to 25 cm is proposed in which it accommodates mirror, output coupler, laser rod as well the cooling mechanism. Water cooling is employed to remove extra heat generated around the laser rod. The developed system is low cost and portable. Experimental results of the measurement are solar radiation spectrum, laser line width and output power of laser. Successfully Nd:YAG laser emissions with Full Width Half Maximum of about 1.67 nm at 1064 nm wavelength and Output power of the system has been achieved up to 200 mW. The potential applications of the solar powered laser are laboratory experiments, generation of second harmonic and imaging. [less ▲]

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See detailSolar Lasers: Another Dimension in Renewable Energy Applications
Singh, Ajay UL

Learning material (2016)

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