Swapan Kumar Mandal

Selected publications

1. Mat. PHYSICA B

   Morphology controlled (CH3NH3)3Bi2Cl9 thin film for lead free perovskite solar cell

   Chandra, Paramesh, and Mandal, Swapan K.

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   Traditional perovskite solar cells have made tremendous progress in terms of efficiency in the past few years but the presence of toxic lead (Pb) has restricted their commercialization. We report here a process to fabricate MABiCl perovskite thin film based solar cell. The process shows some intriguing outcomes like uniform surface morphology of the perovskite layer, high open-circuit voltage and good fill-factor. The film shows improved uniform morphology and surface coverage with flakes (dimension 570 nm × 360 nm, thickness ∼ 100–200 nm). The solar cell device shows an open-circuit voltage of about 496 mV and a fill factor 0.55. The improved morphology of the perovskite layer with reduced number of defects and charge traps possibly decreases the probability of recombination of photoexcited charge carriers. The result indicates that the process can be a good alternative to prepare lead free perovskite solar cells with improved performance.

2. APA

   A dielectric study of Br-doped lead-free methylammonium bismuth chloride (CH3NH3)3Bi2BrxCl9−x

   Chandra, Paramesh, Saha, Saroj, and Mandal, Swapan K.

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   We report here the synthesis and dielectric characteristics of lead-free methylammonium bismuth chloride (MABiCl) and bromine (Br) doped methylammonium bismuth chloride (MABiBrCl) powders. The dielectric characteristics of the samples (pressed powder pellet) measured in the frequency range 1 Hz to 1 MHz and temperature range 333–403 K indicate complex electrical transport in these halide perovskite materials. We have observed that the transport is dominated by longrange hopping of carriers and localized charges above and below a critical frequency respectively. The experimental data are fitted with the theoretical models considering grains, grain boundary, and contacts effects and made a comprehensive analysis. The activation energy obtained from ac electrical conductivity measurements is found to be relatively higher for the Br doped MABiBrCl than undoped MABiCl, which might explain some of the properties reported on solar cells and optoelectronic devices.

3. PRB

   Role of f − d exchange interaction and Kondo scattering in the Nd-doped pyrochlore iridate ( Eu 1 − x Nd x ) 2 Ir 2 O 7

   Mondal, Sampad, Modak, M., Maji, B., Ray, Mayukh K., Mandal, S., Mandal, Swapan K., Sardar, M., and Banerjee, S.

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   We report a study of magnetization, resistivity, magnetoresistance, and specific heat of the pyrochlore iridate (Eu1−xNdx)2Ir2O7 with x=0.0, 0.5 and 1.0, where spin-orbit coupling, electronic correlation, magnetic frustration, and Kondo scattering coexist. The metal insulator transition temperature (TMI) decreases with increase in Nd content, but always coincides with the magnetic irreversibility temperature (field-induced moment). Resistivity below TMI does not fit with either activated (gap) or any power-law (gapless) dependence. The Curie constant shows the surprising result that Nd induces singlet correlation (reduction of paramoment) in the Ir sublattice. Magnetoresistance is negative at low temperatures below 10 K and increases strongly with increase in x, and varies quadratically with field switching over to a linear dependence above 50 kOe. Low-temperature specific heat shows a Schottky peak, coming from Nd moments, showing the existence of a doublet split in the Nd energy level, arising from the f−d exchange interaction. All materials show the presence of a linear specific heat in the insulating region. The coefficient of linear specific heat for x=0.0 does not vary with the external magnetic field, but varies superlinearly for x=1.0 materials. We argue that linear specific heat probably rules out weakly correlated phases such as Weyl fermions. We propose that with the introduction of Nd at the Eu site, the system evolves from a chiral spin liquid with gapless spinon excitations with a very small charge gap to a Kondo-type interaction superposed on a chiral spin liquid coexisting with long-range antiferromagnetic ordering. A huge increase of magnetoresistance with increase in Nd concentrations shows the importance of Kondo scattering in the chiral spin-liquid material by rare-earth moments.

4. PRB

   Evolution of magnetic and transport properties in the Cu-doped pyrochlore iridate Eu 2 ( Ir 1 − x Cu x ) 2 O 7

   Mondal, Sampad, Modak, Mantu, Maji, Bibekananda,  Mandal, Swapan, Ghosh, B., Saha, Surajit, Sardar, Mahdi, and Banerjee, Sangam

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   We have investigated the effect of Cu substitution in Eu2(Ir1-xCux)2O7 with the help of magnetic and transport property measurements. Cu substitution affects the spin orbit coupling (SOC) and interatomic electron correlation (U) in the transition metal site which in turn modifies the physical properties. XPS measurement reveal that each Cu2+ converts Ir4+ to double amount of Ir5+ ions. We observe metal to insulator transition (T_MI ) around 120 K. At lower temperature in the insulating phase below 50 K the temperature dependence of resistance follows power law dependence and the magnitude of the exponent increases with Cu concentration. The temperature dependent thermopower was observed to follow the electrical resistivity till 50 K, except for a sudden drop in thermopower at temperature less than 50 K was observed. We find negligible Hall voltage in the metallic regime of the samples but a sudden Hall voltage is developed at very low temperatures below 50 K. We observe bifurcation in zero field cooled and field cooled (ZFC-FC) magnetization measurement, exchange bias and negative magnetoresistance and in all these the magnitude increases with increase in Cu doping concentration. We observe linear specific heat and the coefficient of it decreases with Cu doping. All these physical properties observed will be explained in this manuscript by invoking antiferromagnetic all-in-all-out spin (AFM AIAO) structure and onset of spin liquid (SL) phase.