Biswajit Kundu

Stability and Performance of CsPbI2Br Thin Films and Solar Cell Devices

In this manuscript, the inorganic perovskite CsPbI2Br is investigated as a photovoltaic material that offers higher stability than the organic-inorganic hybrid perovskite materials. It is demonstrated that CsPbI2Br does not irreversibly degrade to its component salts as in the case of methylammonium lead iodide but instead is induced (by water vapor) to transform from its metastable brown cubic (1.92 eV band gap) phase to a yellow phase having a higher band gap (2.85 eV). This is easily reversed by heating to 350 °C in a dry environment. Similarly, exposure of unencapsulated photovoltaic devices to water vapor causes current (JSC) loss as the absorber transforms to its more transparent (yellow) form, but this is also reversible by moderate heating, with over 100% recovery of the original device performance. NMR and thermal analysis show that the high band gap yellow phase does not contain detectable levels of water, implying that water induces the transformation but is not incorporated as a major component. Performances of devices with best efficiencies of 9.08% (VOC = 1.05 V, JSC = 12.7 mA cm-2 and FF = 68.4%) using a device structure comprising glass/ITO/c-TiO2/CsPbI2Br/Spiro-OMeTAD/Au are presented, and further results demonstrating the dependence of the performance on the preparation temperature of the solution processed CsPbI2Br films are shown. We conclude that encapsulation of CsPbI2Br to exclude water vapor should be sufficient to stabilize the cubic brown phase, making the material of interest for use in practical PV devices.

Heterovalent substitution in anionic and cationic positions of PbS thin-films grown by SILAR method vis-à-vis Fermi energy measured through scanning tunneling spectroscopy

We report the growth and characterization of doped-PbS thin-films deposited by a successive ionic layer adsorption and reaction (SILAR) method. Altervalent cation and aliovalent anion substitution by ions of mono- and trivalent elements as dopants have been achieved in the compound semiconductor. The heterovalent elements introduced free carriers into the semiconductors, the nature of which depended on its valency and the ions it substituted into the compound. The effect of such dopants on the Fermi energy of PbS has been followed by scanning tunneling spectroscopy (STS), and was found to have correspondence to the density of states (DOS) of a semiconductor. By locating the conduction and valence band-edges of the pristine and different doped-semiconductors, the STS studies provided a direct evidence of a shift in Fermi energy upon heterovalent cationic and anionic substitution in compound semiconductors.

Differential conductance (dI/dV) imaging of a heterojunction-nanorod

Through scanning tunneling spectroscopy, we envisage imaging a heterostructure, namely a junction formed in a single nanorod. While the differential conductance spectrum provides location of conduction and valence band edges, dI/dV images record energy levels of materials. Such dI/dV images at different voltages allowed us to view p- and n-sections of heterojunction nanorods and more importantly the depletion region in such a junction that has a type-II band alignment. Viewing of selective sections in a heterojunction occurred due to band-bending in the junction and is correlated to the density of states spectrum of the individual semiconductors. The dI/dV images recorded at different voltages could be used to generate a band diagram of a pn junction.

Scanning tunneling spectroscopy to probe site-selection in heterovalent doping: Zn(II)-doped Cu(I)In(III)S2 as a case study

We report scanning tunneling spectroscopy (STS) of a heterovalent-doped ternary compound semiconductors and their binary counterparts. The effect of dopants in the semiconductors that yielded a shift in Fermi energy has been found to be manifested in the density of states (DOS) spectrum. The shift infers the nature of doping, which the heterovalent dopants induce, and hence the site of the ternary system that the dopants occupy. For example, in the present case with Zn(II)-doped Cu(I)In(III)S2, the DOS spectra showed a shift in Fermi energy towards the conduction band and hence a n-type doping due to the introduction of electrons. Such a shift inferred that the bivalent dopants occupied the cuprous site. The results have been substantiated by STS studies of doped binary components, namely, Cu2S and In2S3 and shift in Fermi energy thereof. With the tuning in the Fermi energy, the homojunctions between undoped and doped semiconductors have a type-II band-alignment at the interface resulting in current recti...

Energy levels of metal porphyrins upon molecular alignment during layer-by-layer electrostatic assembly: scanning tunneling spectroscopy vis-à-vis optical spectroscopy

We report ultrathin-film formation of metal-porphyrins with their molecular plane aligned parallel to the substrate. Such an alignment has been achieved through application of an external magnetic field to a monolayer of the porphyrin derivatives followed by immobilization of the molecules with a layer of a polyion. The orientation of metal-porphyrin molecules in a monolayer responded to the magnetic field due to their anisotropic magnetic moment arising out of unpaired d-electrons of the central atom. In this work, we compared characteristics of different metal-porphyrin molecules in their aligned and unaligned forms. Scanning tunneling spectroscopy (STS) of a monolayer of the porphyrins were recorded that in turn yielded density of states (DOS) from which highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) could be located. We observed that the energies responded upon alignment of the molecules; transport gap of metal porphyrins representing both Soret and Q-bands decreased in the aligned films due to an interaction of the molecular planes with the electrode. We compared the transport gap from STS and optical gap representing Soret and Q-bands in films of aligned and unaligned molecules.

Temperature dependent electron delocalization in CdSe/CdS type-I core-shell systems: An insight from scanning tunneling spectroscopy

Core-shell nanocrystals having a type-I band-alignment confine charge carriers to the core. In this work, we choose CdSe/CdS core-shell nano-heterostructures that evidence confinement of holes only. Such a selective confinement occurs in the core-shell nanocrystals due to a low energy-offset of conduction band (CB) edges resulting in delocalization of electrons and thus a decrease in the conduction band-edge. Since the delocalization occurs through a thermal assistance, we study temperature dependence of selective delocalization process through scanning tunneling spectroscopy. From the density of states (DOS), we observe that the electrons are confined to the core at low temperatures. Above a certain temperature, they become delocalized up to the shell leading to a decrease in the CB of the core-shell system due to widening of quantum confinement effect. With holes remaining confined to the core due to a large offset in the valence band (VB), we record the topography of the core-shell nanocrystals by probing their CB and VB edges separately. The topographies recorded at different temperatures representing wave-functions of electrons and holes corresponded to the results obtained from the DOS spectra. The results evidence temperature-dependent wave-function delocalization of one-type of carriers up to the shell layer in core-shell nano-heterostructures.