Dixit, Ambesh

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.

The phase pure monoclinic copper oxide (CuO) nanoparticles (NPs) are synthesized, and the observed strain strongly affects bandgap, leading to its reduction from 3.62 eV to 3.00 eV (±0.02 eV) by reducing the crystallite size from 92 to 37 nm (±5 nm), in contrast to the geometrical size effect, where bandgap (Eg) increases with decreasing particle size. This exciting characteristic is attributed to the dominant micro-strain effect over the size on electronic properties of CuO nanoparticle systems, proving a new way of strain mediated bandgap engineering in NP systems.

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.

KBiFe2O5 (KBFO) is an interesting multiferroic compound with the coexistence of ferroelectric and ferromagnetic phases with a narrow band gap. In this work, KBiFe2O5 (KBFO) is synthesized using a sol-gel technique, followed by calcination at 973 K. Rietveld refinement analysis of XRD data confirms the presence of monoclinic crystal structure with space group P2/c. The SEM micrograph of KBFO reflects the irregular shape of grains with slight agglomeration. The optical band gap measured by UV visible spectroscopy of KBFO is found to be 1.56 eV. The multiferroic behavior of KBFO has been confirmed by the electrical (dielectric) and magnetic (M-H loop) measurements. A weak ferromagnetic nature is observed due to canting of the magnetic moment of G-type anti-ferromagnetic ordering. The dielectric studies reveal the Maxwell-Wagner type dispersion and an anomaly near magnetic phase transition temperature. KBFO shows negative magneto-dielectric (MD) coupling as both ε and tan δ decreases with the increasing magnetic field. Impedance spectroscopy is performed to analyze the intrinsic and extrinsic effects in MD coupling. The Nyquist plot shows positive magneto-resistance and negative magneto-capacitance which represent strong MD coupling in KBFO. It also reveals that the bulk property (intrinsic effect) dominates over the extrinsic effect in the sample. These results affirm that KBFO is a new candidate for room-temperature multiferroicity with a suitable bandgap and strong MD coupling for photovoltaic applications.