Balakrishna Ananthoju

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 ~1 μm). The earth-abundant Cu2MSnS4/Se4 (M = Zn, Mn and Fe) nanoparticles have band gaps in the range of 1.04–1.51 eV with high optical-absorption coefficients (~104 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 cm2. 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.

Understanding the Li-storage in few layers graphene with respect to bulk graphite: experimental, analytical and computational study

In order to understand, clarify and provide confirmations in the contexts of the prevalent confusions concerning Li-storage in graphenic carbon (viz. the reduced dimensional scale of graphitic carbon), electrochemical lithiation/delithiation has been performed with CVD-grown fairly pristine well-ordered few-layer graphene films (FLG; ∼7 layers; as a model material). Chronopotentiograms and cyclic voltammograms recorded with the FLG present distinct features corresponding to the transformation between different Li-GICs (i.e., ‘staging’) below 0.3 V against Li/Li+, thus confirming that ‘classical’ Li-intercalation does occur even at such reduced dimensional (nano)scale. Nevertheless, even in this lower potential window (our main focus here), Li-storage in FLG involves contributions from both diffusion- and surface-controlled mechanisms. The Li-capacities recorded with FLG just within this lower potential window, and also upon subtracting any possible contribution from the Cu current collector, were still ∼3–4 times greater than those obtained with similarly grown thicker bulk graphite films (TBG: ∼450 nm; Li-capacity recorded: ∼380 mA h g−1). Contrary to the usual belief, the excess Li-capacity of FLG cannot be explained by the presence of extrinsic/intrinsic defects, which are nearly negligible in the FLG films under consideration. Simulation of Li-storage in graphene via DFT indicated that the excess capacity (after formation of the LiC6 configuration) is associated with additional stable Li-storage on the outer graphene surfaces in the forms of more than one Li-layer (but different from Li-plating) and segregation close to the ‘stepped’ (exposed) edges of the inner graphene layers (but not exactly at the edge sites). Overall, such predicted Li-storage mechanisms are in agreement with the experimentally observed contributions from both ‘classical’ Li-intercalation and surface-controlled processes (even at potentials below 0.3 V), which primarily account for the excess Li-capacities recorded with graphenic carbon.

Structural and optical properties of electrochemically grown highly crystalline Cu2ZnSnS4(CZTS) thin films

We have fabricated kesterite phase films of Cu2ZnSnS4 (CZTS) using a single bath electrodeposition process followed by sulfurisation. Morphological analysis using SEM shows that the films have densely packed micron size grains. XRD and Raman spectroscopy confirm that grown films are highly crystalline and kesterite in phase. Band gap of electrochemically deposited films is estimated to 1.51 eV by using UV-Vis absorption spectra. Photoluminescence measurements show a single emission peak at 1.5 eV which corresponds to band to band transition of CZTS film. The electrical conductivity measurements of the films show good ohmic behavior of the contacts and the average conductance value is 50 μS.

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.

Broad-band absorption enhancement in lower absorption coefficient region in CZTS solar cells with embedded aluminum nanoparticles into absorber layer

Finite-difference-time-domain (FDTD) electromagnetic calculations are performed for a CZTS solar cell with embedded 100 nm aluminum (Al) nanoparticles (NPs) in the absorber layer. We predict significantly enhanced absorption for wavelengths where absorption is intrinsically low. Overall, over 17% enhancement is predicted in absorbed photon flux and (maximum attainable) current density below the absorber band gap. These are due to near-field enhancement by localized surface plasmon resonance (LSPR) and far-field enhancement by scattering from Al NPs. The increase in absorbed photon flux is explained through Mie scattering calculations in Lumerical FDTD.

Efficiency enhancement in Cu2ZnSnS4 solar cells with silica nanoparticles embedded in absorber layer

Light trapping is essential to lower transmission losses in thin-film solar cells, particularly for wavelengths where the absorption is inefficient. We have embedded silica nanoparticles into a CZTS absorber layer resulting in localized as well as scattering field enhancement. UV-Vis absorption measurements show position (depth in the absorber layer) dependent improvement in the optical absorption with incorporation of silica nanoparticles; this has been explained using finite-difference-time-domain (FDTD) calculations of Mie scattering. The optical enhancement in turn leads to efficiency improvement as seen from electrical measurements; this has a slightly different position dependence that can also be understood theoretically. We observe maximal efficiency improvement, of about 21% compared to devices without nanoparticles (reference cell efficiency ~ 4.13% and for particles at middle ~ 5%), when nanoparticles are placed at the middle of the absorber layer.

Electrochemical capacitance properties of Mn3O4nanoparticles via energy efficient thermolysis

We demonstrate a facile process for the synthesis of amine-functionalized hausmannite ( Mn 3 O 4 ) nanoparticles. The process is solventless and uses only a mixture of oleylamine and manganese chloride to produce Mn 3 O 4 . The size of the nanoparticles is tuned by varying Mn to amine mole ratio. The prepared nanoparticles are monodisperse and show tetragonal structure as characterized by TEM and XRD. The cyclic voltammetry curves are nearly perfect rectangular which indicates excellent capacitive nature of the nanoparticles. A maximum specific capacitance of 125 Fg−1 is obtained for the nanocrystals in a potential range from −0.1 to 0.7 V with 1 M sodium sulfate solution.

Electrochemical capacitance properties of Mn[sub 3]O[sub 4] nanoparticles via energy efficient thermolysis

We demonstrate a facile process for the synthesis of amine-functionalized hausmannite ( Mn 3 O 4 ) nanoparticles. The process is solventless and uses only a mixture of oleylamine and manganese chloride to produce Mn 3 O 4 . The size of the nanoparticles is tuned by varying Mn to amine mole ratio. The prepared nanoparticles are monodisperse and show tetragonal structure as characterized by TEM and XRD. The cyclic voltammetry curves are nearly perfect rectangular which indicates excellent capacitive nature of the nanoparticles. A maximum specific capacitance of 125 Fg−1 is obtained for the nanocrystals in a potential range from −0.1 to 0.7 V with 1 M sodium sulfate solution.

Electrochemical capacitance properties of Mn3O4 nanoparticles via energy efficient thermolysis

We demonstrate a facile process for the synthesis of amine-functionalized hausmannite ( Mn 3 O 4 ) nanoparticles. The process is solventless and uses only a mixture of oleylamine and manganese chloride to produce Mn 3 O 4 . The size of the nanoparticles is tuned by varying Mn to amine mole ratio. The prepared nanoparticles are monodisperse and show tetragonal structure as characterized by TEM and XRD. The cyclic voltammetry curves are nearly perfect rectangular which indicates excellent capacitive nature of the nanoparticles. A maximum specific capacitance of 125 Fg−1 is obtained for the nanocrystals in a potential range from −0.1 to 0.7 V with 1 M sodium sulfate solution.