Search

We investigated a single heterojunction solar cell with stannite phase Cu2HgSnS4 as a new potential absorber material using the hybrid density functional theory for materials parameter and macroscopic device simulation studies for the photovoltaic response. The lattice constants are optimized using PBEsol and GGA exchange–correlation approach with mBJ parameterization and considering spin–orbit coupling effect on heavy element Hg, while strong correlation of Cu and Hg 3d electrons are taken into account by Hubbard parameter U = 0.52 Ry. The computed bandgap is ~1.33 eV. The effective mass of electrons and holes in the respective band edges (electron in conduction band and the hole in the valence band) is 0.25 m0 and 0.91 m0, respectively. Further, the computed materials optoelectronic parameters are used to optimize the device performance by introducing a variation in minority carrier lifetime, defect concentration in Cu2HgSnS4 absorber, Cu2HgSnS4/CdS interface, and the absorber thickness to achieve a realistic photovoltaic response. The optimal conversion efficiency is 11.6% after taking more realistic parameters in the considered single-junction solar cell. However, the maximum photovoltaic response >17% can be achieved by controlling absorber and interface defects together with optimal carrier concentration.

Quantum Dot Sensitized Solar Cells are considered as the potential third generation solar cells due to their suitable optoelectronic properties for photovoltaic response. The possibility of size and composition tunability makes quantum dots as relevant absorber materials to match the wider solar spectrum more efficiently. In conjunction, the possibility of multiple electron-hole pair generations at the cost of single photon i.e. multiple carrier generation is showing potential to overcome the theoretical single junction power conversion efficiency limitations. Quantum dot sensitized solar cells are showing power conversion efficiencies up to 12%, very close to its counterpart dye sensitized solar cells. However, QDSSCs efficiencies are still lagging behind the conventional solid state single junction solar cells. In this review, we will discuss the initial evolution of quantum dot sensitized solar cells with their microscopic working principles. The review will also address development of key building blocks and factors such as various interfaces in QDSSCs, carrier transport and recombination across different interfaces, affecting the power conversion efficiency. Further, fundamental concepts of carrier multiplication and possible theoretical models for multiple exciton generation are discussed towards their impact on the power conversion efficiencies of quantum dot sensitized solar cells.

We used density functional theory based calculations to investigate the structural and optoelectronic properties of copper-based ternary chalcogenide Cu-M-X2 (M: Sb, Bi & X: S, Se). These form orthorhombic crystallographic structure with Pnma space group. The relative thermodynamic stability of these structures is supported by their phonon band dispersions. The calculated electronic band structure is indirect for all these compounds in conjunction with a close direct band gap transition. The calculated effective mass of electrons and holes are (0.074, 0.732), (0.053, 0.297), (0.039, 0.655) and (0.031, 0.514) for CuSbS2, CuSbSe2, CuBiS2 and CuBiSe2, respectively. Interestingly, a very high optical absorption coefficient above 105 cm−1 above band gap values is noticed for these materials, making them suitable for ultrathin solar cell absorbers.

Cadmium telluride (CdTe) quantum dots are integrated with mesoporous titanium dioxide “TiO2” electrode using an in-situ sensitization approach for the first time, where water soluble N-Acetyl Cysteine capped CdTe quantum dots (diameter 4–5 nm) are grown hydrothermally inside mesoporous TiO2 electrode matrix. This in-situ sensitization approach has shown the effective sensitization over the conventional reported sensitization processes. The in-situ CdTe sensitized TiO2 electrodes are further unified with PbS counter electrode to realize the quantum dot sensitized solar cell (QDSSC). The fabricated CdTe QDSSC has shown the optimal short circuit current density 3.35 ± 0.21 mA/cm2 and open circuit voltage 0.58 ± 0.01 V. The observed current density is the highest among such cell configurations, reported till date. The in-situ sensitized CdTe QDs are further treated with zinc sulfide for surface passivation to realize the reduced back recombination, if any. These ZnS passivated CdTe QDs sensitized solar cells in similar device configurations resulted into the significant reduction in the short circuit current density (20%) with the minimal change in open circuit voltage (5%). The impedance spectroscopy measurements suggest that the transmission resistance “Rtr” has increased after ZnS treatment for these electrodes, however, the recombination resistance Rrec has not changed much. The observed high Rtr might be responsible for the observed current deterioration after ZnS surface passivation. These studies suggest that in-situ QD sensitization may provide a new method of integrating QDs into mesoporous TiO2 matrix for effective QDSSC device performance without any additional surface passivation.

Radiation calorimeter is a device for assessment of the incident local concentrated solar irradiance (CSI) onto a receiver surface. This is required to evaluate the performance of a concentrated solar thermal system using, for instance a volumetric receiver, for applications. In this paper a low-temperature water cooled radiation calorimeter (RADCAL) is presented. This is designed as a cavity based on the concept of blackbody to maximize the absorption of the incident CSI on its absorber surfaces. Solar selective coatings are deposited on its reflector and absorber surface to achieve the same and are characterized using standard methodologies. This depicts an absorptivity of 0.95 and a reflectivity of 0.87 in the desired range of the solar spectrum and is theoretically verified. The absorption of energy results in temperature rise of RADCAL, which is controlled by an external water jacket to limit its value at 100 °C for mitigating the use of pressurized water. At any instant and eventually at the steady-state the CSI is estimated following the conservation of energy principle. The developed RADCAL is experimentally evaluated up to a CSI of 800 Suns (1 Sun = 1 kW/m2) using Joule heating. The entire heat transfer process is analysed with the developed unsteady state one-dimensional mathematical model. A comparative assessment of the measured and calculated RADCAL body temperature provides the underlying uncertainty and confirms its design basis. Furthermore, the design and given consideration allows its use in arid desert condition with dust and wind. Thus, RADCAL is likely to serve in future for evaluating concentrated solar thermal system in arid deserts.

We synthesized zinc titanate nanopowder in different crystal structures by varying calcination temperature and time and phase evolution is qualitatively investigated using reference intensity ratio (RIR) approach. The electronic properties showed 2.9 eV as indirect band transition, and 3.59 ± 0.02 eV as direct band transition for the highest temperature annealed rhombohedral ZnTiO3 and cubic spinel Zn2TiO4 zinc titanate materials. The intermediate temperature annealed (∼850 °C) material consists of ZnTiO3, Zn2TiO4, and Zn2Ti3O8 zinc titanate nanopowder, showing similar two electronic transitions. Zinc titanate with dominant rhombohedral phase ZnTiO3 (900 °C annealed) and dominant Zn2Ti3O8 phase (750 °C annealed) are used to synthesize mesoporous electrodes for integrating the ZnS passivated CdS quantum dots. Finally, Cu2S counter electrode is integrated with polysulfide electrolyte to realize 1 cm2 area quantum dot sensitized solar cells (QDSSCs). QDSSCs with dominant ZnTiO3 phase are showing lower photovoltaic performance (short circuit current 0.76 mA/cm2, open circuit voltage 0.55 V) with respect to that of dominant Zn2Ti3O8 phase with 2.2 mA/cm2 short circuit current and 0.69 V open circuit voltage, respectively. The observed difference is attributed to differences in electronic and morphological properties of the synthesized zinc titanate powders.

Thermal phase diagrams are computed for binary mixtures of different fatty acids with 1-dodecanol to design new eutectic phase change materials with melting temperatures and latent heatof fusion ranging from 15.5 to 19 °C and 183 to 188 kJ kg −1 , respectively. The lauric (LA), myristic (MA) and palmitic (PA) fatty acids with 1-dodecanol eutectic compositions are identified and experimentally validated using differential scanning calorimetric measurements. The measured eutectic compositions are 29:71, 17:83, and 10:90 wt% for LA-DE, MA-DE, and PA-DE binary systems respectively. The melting temperatures and latent heat of fusion for these eutectic compositions are 17, 18.43, 20.08 °C and 175.3, 180.8, 191 kJ kg −1 respectively. The computed and measured composition weight fractions, melting temperature and latent heat of fusion values are in good agreement. The suitable melting temperature and considerably large latent heat of fusion for these designed and developed new eutectic phase change materials make them suitable for latent heat thermal energy storage systems for low temperature solar thermal applications such as building heating/cooling and body cooling clothing applications.

ZrOx/ZrC–ZrN/Zr absorber–reflector tandem layered structures were fabricated on stainless steel (SS) and copper (Cu) substrates using DC/RF magnetron sputtering system. ZrC–ZrN absorber layer was grown on the Zr infrared reflector, in conjunction with the top ZrOx anti-reflecting layer. Absorbing properties, of ZrC–ZrN absorber layer, were optimized by varying nitrogen flow during deposition of this layer. The optimized ZrN fraction in a ZrC–ZrN layer showed additional plasmonic absorption in ∼1.0–2.5 μm wavelength range, together with other intrinsic absorptions, providing enhanced solar absorption in 0.3–2.5 μm wavelength range. The detailed structural, micro-structural, surface and optical characterization showed the strong structure – solar thermal property correlation. We observed that absorber–reflector tandem structures, fabricated at ∼12.5 sccm nitrogen flow rate, exhibit the best solar thermal response among investigated structures with absorptance α ∼ 0.88 and 0.85 and emittance e27°C ∼ 0.04 and 0.1 on stainless steel and copper substrates. Thermal studies showed high temperature stability at ∼700 °C and 600 °C in vacuum for these solar selective coatings on SS and Cu substrates, whereas at or below 200 °C in the air. These studies suggest that ZrOx/ZrC–ZrN/Zr absorber–reflector tandem structures may be a good choice for high temperature applications under vacuum/inert conditions.