Screening of suitable cationic dopants for solar absorber material CZTS/Se: A first principles study

The earth abundant and non-toxic solar absorber material kesterite Cu2ZnSn(S/Se)(4) has been studied to achieve high power conversion efficiency beyond various limitations, such as secondary phases, antisite defects, band gap adjustment and microstructure. To alleviate these hurdles, we employed screening based approach to find suitable cationic dopant that can promote the current density and the theoretical maximum upper limit of the energy conversion efficiency (P(%)) of CZTS/Se solar devices. For this task, the hybrid functional (Heyd, Scuseria and Ernzerhof, HSE06) were used to study the electronic and optical properties of cation (Al, Sb, Ga, Ba) doped CZTS/Se. Our in-depth investigation reveals that the Sb atom is suitable dopant of CZTS/CZTSe and also it has comparable bulk modulus as of pure material. The optical absorption coefficient of Sb doped CZTS/Se is considerably larger than the pure materials because of easy formation of visible range exciton due to the presence of defect state below the Fermi level, which leads to an increase in the current density and P(%). Our results demonstrate that the lower formation energy, preferable energy gap and excellent optical absorption of the Sb doped CZTS/Se make it potential component for relatively high efficient solar cells

Photon induced non-linear quantized double layer charging in quaternary semiconducting quantum dots

Room temperature quantized double layer charging was observed in 2 nm Cu2ZnSnS4 (CZTS) quantum dots. In addition to this we observed a distinct non-linearity in the quantized double layer charging arising from UV light modulation of double layer. UV light irradiation resulted in a 26% increase in the integral capacitance at the semiconductor-dielectric (CZTS-oleylamine) interface of the quantum dot without any change in its core size suggesting that the cause be photocapacitive. The increasing charge separation at the semiconductor-dielectric interface due to highly stable and mobile photogenerated carriers cause larger electrostatic forces between the quantum dot and electrolyte leading to an enhanced double layer. This idea was supported by a decrease in the differential capacitance possible due to an enhanced double layer. Furthermore the UV illumination enhanced double layer gives us an AC excitation dependent differential double layer capacitance which confirms that the charging process is non-linear. This ultimately illustrates the utility of a colloidal quantum dot-electrolyte interface as a non-linear photocapacitor. (C) 2017 Elsevier Inc. All rights reserved

Cation/Anion Substitution in Cu2ZnSnS4 for Improved Photovoltaic Performance

Cations and anions are replaced with Fe, Mn, and Se in CZTS in order to control the formations of the secondary phase, the band gap, and the micro structure of Cu2ZnSnS4. We demonstrate a simplified synthesis strategy for a range of quaternary chalcogenide nanoparticles such as Cu2ZnSnS4 (CZTS), Cu2FeSnS4 (CFTS), Cu2MnSnS4 (CMTS), Cu2ZnSnSe4 (CZTSe), and Cu2ZnSn(S0.5Se0.5)(4) (CZTSSe) by thermolysis of metal chloride precursors using long chain amine molecules. It is observed that the crystal structure, band gap and micro structure of the CZTS thin films are affected by the substitution of anion/cations. Moreover, secondary phases are not observed and grain sizes are enhanced significantly with selenium doping (grain size similar to 1 mu m). The earth-abundant Cu2MSnS4/Se-4 (M = Zn, Mn and Fe) nanoparticles have band gaps in the range of 1.04-1.51 eV with high optical-absorption coefficients (similar to 10(4) cm(-1)) in the visible region. The power conversion efficiency of a CZTS solar cell is enhanced significantly, from 0.4% to 7.4% with selenium doping, within an active area of 1.1 +/- 0.1 cm(2). The observed changes in the device performance parameters might be ascribed to the variation of optical band gap and microstructure of the thin films. The performance of the device is at par with sputtered fabricated films, at similar scales

Insight into the mechanical integrity of few-layers graphene upon lithiation/delithiation via in situ monitoring of stress development

By monitoring the stress developments in situ during lithiation/delithiation of few layers graphene films, we report here for the first time an interesting observation of stress release/reversal just within the potential ranges corresponding to the co-existences of pristine graphene, dilute stage I and stage IV Li-graphite intercalation compounds (Li-GICs). This, along with supporting observations related to the presence of cracks in the graphene films upon electrochemical cycling and enhanced I-D/I-G ratio in Raman spectra recorded upon cycling primarily within the potential regimes corresponding to the stress release/reversal, indicate possible occurrences of mechanical/structural degradation mainly during the initial stages of lithiation and later stages of delithiation. This observation is being supported by a geometric model which estimates the strains induced in the individual graphene layers at the interfaces between the different Li-GICs at the various stages of lithia don; and as a function of distance from the current collector. On a slightly different note, the experimentally recorded magnitude of net lithiation induced reversible in-plane stress development was similar to 11 GPa. This is in fair agreement with that expected based on the theoretical dilation of similar to 1% along the graphene layers upon lithiation as per the classical Li-intercalation mechanism (i.e., up to formation of LiC6). (c) 2015 Elsevier Ltd. All rights reserved

Improved structural and optical properties of Cu2ZnSnS4 thin films via optimized potential in single bath electrodeposition

Here we report on the preparation of high quality Cu2ZnSnS4 thin films using single bath electrodeposition process via an optimized deposition potential. X-ray diffraction and Raman analysis validated the formation of kesterite phase of CZTS without any secondary phases at an optimized deposition potential of -1.4V vs. Ag/AgCl. As a signature of highly pure crystalline films of CZTS kesterite phase, we observed a characteristic Raman peak at 338 cm(-1) that corresponds to the vibration of sulfur atoms. Elemental analysis using energy dispersion analysis of X-rays (EDX) reveals a near ideal composition ratio of 2:1:1:4 for these films, and indicates the formation of the ideal stoichiometric compound. Furthermore, X-ray photoelectron spectroscopy analysis of the grown films illustrates an appropriate chemical composition and valence states of the constituent elements without a trace of free sulfur. Using the chrono- amperometry data and the Scharifker and Hill model we found that the nucleation mechanism for CZTS thin film is instantaneous. Optical properties demonstrated the optimum band gap of 1.5 eV for kesterite CZTS film prepared from a precursor electrodeposited at -1.4 V vs. Ag/AgCl. Mott-Schottky electrical measurements confirm the p-type nature of the film with a carrier concentration of 10(17) cm(-3), a flat band potential of V-FB = 0.7V and space charge region width of 0.2 mu m. (C) 2014 Published by Elsevier Ltd

Electron-Selective TiO2/CVD-Graphene Layers for Photocorrosion Inhibition in Cu2O Photocathodes

Mitigating photocorrosion in the light absorber material used for photoelectrochemical solar water splitting is a subject of major research. In this work, a systematic investigation is carried out on suppressing the photocorrosion in an electrodeposited Cu2O photocathode using stable protective layers. The photocathode protected with chemical vapor deposited graphene offers significant stability, till 600 s during light chopping chronoamperometry. However, the presence of a few microcracks in the graphene layer cannot offer complete protection, and causes a gradual decay in the photocurrent. The addition of an ultrathin layer (approximate to 10 nm) of amorphous TiO2 on top of the graphene blocks the microcracks, thereby resulting in complete protection to the Cu2O absorber layer. The TiO2/graphene protected Cu2O photocathode generates -3 mA cm(-2) photocurrent at 0.0 V versus reversible hydrogen electrode under 1 sun in 1 m Na2SO4 electrolyte (pH 7), which is twice that compared to the bare Cu2O electrode. The enhancement in photocurrent can be attributed to the ease of separating the photogenerated charge carriers due to the suitable band alignment and electron selective nature of the protective TiO2/graphene layers

Enzymatic and non-enzymatic electrochemical glucose sensor based on carbon nano-onions

A high sensitive glucose sensing characteristic has been realized in carbon nano-onions (CNOs). The CNOs of mean size 30 nm were synthesized by an energy-efficient, simple and inexpensive combustion technique. These as-synthesized CNOs could be employed as an electrochemical sensor by covalently immobilizing the glucose oxidase enzyme on them via carbodiimide chemistry. The sensitivity achieved by such a sensor is 26.5 mu A mM (1) cm (2) with a linear response in the range of 1-10 mM glucose. Further to improve the catalytic activity of the CNOs and also to make them enzyme free, platinum nanoparticles of average size 2.5 nm are decorated on CNOs. This sensor fabricated using Pt-decorated CNOs (Pt@CNOs) nanostructure has shown an enhanced sensitivity of 21.6 mu A mM (1) cm (2) with an extended linear response in the range of 2-28 mM glucose. Through these attempts we demonstrate CNOs as a versatile biosensing platform. (C) 2018 Elsevier B.V. All rights reserved