Soumyo Chatterjee

p–i–n Heterojunctions with BiFeO3 Perovskite Nanoparticles and p- and n-Type Oxides: Photovoltaic Properties

We formed p-i-n heterojunctions based on a thin film of BiFeO3 nanoparticles. The perovskite acting as an intrinsic semiconductor was sandwiched between a p-type and an n-type oxide semiconductor as hole- and electron-collecting layer, respectively, making the heterojunction act as an all-inorganic oxide p-i-n device. We have characterized the perovskite and carrier collecting materials, such as NiO and MoO3 nanoparticles as p-type materials and ZnO nanoparticles as the n-type material, with scanning tunneling spectroscopy; from the spectrum of the density of states, we could locate the band edges to infer the nature of the active semiconductor materials. The energy level diagram of p-i-n heterojunctions showed that type-II band alignment formed at the p-i and i-n interfaces, favoring carrier separation at both of them. We have compared the photovoltaic properties of the perovskite in p-i-n heterojunctions and also in p-i and i-n junctions. From current-voltage characteristics and impedance spectroscopy, we have observed that two depletion regions were formed at the p-i and i-n interfaces of a p-i-n heterojunction. The two depletion regions operative at p-i-n heterojunctions have yielded better photovoltaic properties as compared to devices having one depletion region in the p-i or the i-n junction. The results evidenced photovoltaic devices based on all-inorganic oxide, nontoxic, and perovskite materials.

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.

Tin(IV) Substitution in (CH3NH3)3Sb2I9: Toward Low-Band-Gap Defect-Ordered Hybrid Perovskite Solar Cells

The prevailing issue of wide optical gap in defect-ordered hybrid iodide perovskites has been addressed in this effort by heterovalent substitution at the metal site. With the introduction of Sn4+ in the (CH3NH3)3Sb2I9 structure, we have successfully lowered the pristine optical gap (2 eV) of the perovskite to a close-to optimum one (1.55 eV). Upon such heterovalent substitution, a gradual shift in the type of electronic conduction of the perovskites was observed. As evidenced from scanning tunneling spectroscopy and correspondingly density-of-state spectra, a significant shift of Fermi energy toward the conduction band edge occurred with an increase in the tin content in the host perovskite. This shift has resulted in tuning of the type of electronic conductivity from p-type to n-type and more importantly led to a better band alignment with the selective contacts of p-i-n heterojunctions. However, tin inclusion affected the surface roughness of the perovskite film in an adverse manner. Hence, the tin content was optimized by considering both the factors, namely, the band gap of the material and the surface roughness of thin films. In an energy-level-optimized planar heterojunction device, the short-circuit current density excelled with a power conversion efficiency of 2.69%.

Solar Cells: Materials Beyond Silicon

Considering India’s inability to produce semiconductor grade silicon and correspondingly manufacturing of silicon solar cell modules, the prospects of other upcoming materials in third-generation solar cells have been discussed. Our outlook on solar cells based on conjugated organics, nanostructures of compound semiconductors, hybrids between two types of semiconductors, oxides, and newly-found organic-inorganic perovskites has been presented.