Education History

Electricity, one of our most widely used forms of energy, is primarily obtained from non-renewable energy sources like coal, oil and gas. These resources are finite. Nature has given us renewable energy sources in the form sunlight, wind and water. Sun gives us ~10²⁴ joules of energy per year. It is believed that covering 0.1% of earth surface with solar cells with an efficiency of 10% would probably satisfy world’s energy needs. Over the years of research activities have led to the development of solar cells consisting of various semiconducting materials such as Silicon (Si), Gallium Arsenide (GaAs), Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS), Dye-Sensitized Metal Oxides, Conductive Polymers and Quantum Dots etc. Several innovative ideas have been incorporated in material and process research. However, present solar technologies, some of them are in early stage of development, are capital-intensive and do not offer drastic reduction in cost per W_P, thus becoming inaccessible to domestic users.

Our projects focus on casting of solar quality Silicon Sheet (or wafer) for Silicon Solar Cells and on low-temperature fabrication of photo-anodes for nano-crystalline metal oxides Dye-Sensitized Solar Cells.

1. Casting of Solar Quality Silicon Sheet (or wafer)

Silicon solar cell is one of the matured and dominant amongst all solar cell technologies and is based on the use of Silicon wafers. Wafering is usually achieved by slicing ingots processed via Czochralski method (single crystalline) and various casting methods (multi-crystalline). Wires used to cut these ingots are as thick as wafers and hence, cutting crystals of any kind into slices is wasteful and expensive. Moreover, the quality and cost of Silicon wafers, in the case of most widely used multi-crystalline form; depend on the quality and cost of solidified ingots of Silicon and prevention of any reactions between highly reactive molten silicon and mold (or crucible) during casting process. Several other relatively less expensive methods, Vertical String Ribbon, Edge-defined Film-fed crystal Growth, Horizontal crystal growth and ribbon-growth-on substrates were explored to grow or cast silicon sheet (poly-crystalline/multi-crystalline) directly from silicon melt to rival the wire saw process on an industrial scale. Molds and/or mechanical supports are preferably made up of solid substrates such as silica, alumina, graphite or silicon carbide plus a coating of silicon nitride. Molten silicon cast on these involves directional solidification leading to multi-crystalline structure with numerous crystal defects.
Alternatively, we are developing and substituting the existing (mechanical) supports with novel substrates in order to directly cast a continuous solar quality silicon sheet (or wafer).

2. Low-Temperature Fabrication of Photo-anodes for Dye-Sensitized Solar Cells

Below is the schematic representations of a DSSC comprised of following components:  (1) Transparent Conductive Oxide (TCO) coated substrate: GLASS or PLASTIC.  (2) Granular TiO₂ forming a nanoporus structure.  (3) A dye, which is a visible light-absorbing substance spread on the TiO₂ surface. (4) A redox couple located in the space between the dye and the cathode. (5) A solvent for the redox couples (I^-/I^3-) e.g. an organic solvent or Room Temperature Ionic Liquid. (6) Counter electrode.

(a) Schematic representation of a dye-sensitized solar cell (DSSC). (b) Molecular structure of dye, N3. (c) Working principle of DSSC; Arrows 1-5 represent the primary pathways for device operation, and arrows 6-8 represent energy-loss via recombination and non-radiative process.

Photo-anode is one of the major components of DSSC where primary step of converting solar light into electricity takes place.

Photo-anode and processes involved

Conventional photo-anodes on inorganic substrates are made by first flocculating a TiO₂ suspension with a polymer. The resultant paste is then applied to the substrate. Spare polymer strands on TiO₂ particles adsorb and anchor the paste to the substrate. The flocculation process invariably leaves polymer material between particles, which, if left in place, would seriously hamper electron conduction from the TiO₂ particles to the substrate electrode. For GLASS substrate this problem is overcome by high temperature sintering (450°C) where the residual blocking polymer is burnt away and particles come into solid/solid contact. High temperature sintering is not possible for PLASTIC substrates and thus precludes the use of polymer flocculation in the manufacturing process. The inter-particles connectivity and strong adherence to TCO-plastic which involves low-temperature sintering process (<150°C) still remain a formidable challenge.

Amongst the most frequently used transparent conducting oxides are fluorine-doped tin oxide (SnO₂:F, FTO) and indium tin oxide (In₂O₃:SnO₂, ITO). FTO coating being more stable at high sintering temperature than ITO is the material of choice for glass-based DSSC. To obtain high cell performance, TCO-coated glass substrate must have low electrical resistivity, high conductivity, high transmittance and high heat stability where the optimized sheet resistance of the FTO-glass plate used has been considered a compromise between the above mentioned properties.

We are interested in developing a novel process for materials synthesis and film making and, multi-scale mathematical models for scale-up, process evaluation and process design. Experimental process involves low temperature fabrication of titania (and other metal oxides) photoelectrodes suitable for GLASS as well as PLASTIC substrate. Such process will allow us to use ITO-coated glass substrate instead of FTO-coated glass. As we know that carrier concentration in ITO are higher than those in FTO by almost a factor of two, because the solubility of tin in In₂O₃ is far superior to that of fluorine in SnO₂.

Our research focuses on applying methods and measurement techniques developed in the areas of colloid science and complex fluids to develop more robust and stable dye-sensitized metal oxide photoanode system. A two-step non-sintering, low-temperature procedure for the film deposition will ensure the following:

1. Predominance of coagulation over flocculation of particles
2. Strong inter-particle connectivity
3. Crystallization ability at low temperature
4. Strong adherence to the transparent conducting oxide (TCO) coated substrate, while providing a complete system with high conductivity and high porosity to accept the dye.
5. Transparency and long-term heat/light stability
6. Chemical and mechanical stability,
7. No stripping and damaging of photo-anode film surfaces, especially with an increase in cell area in the case of plastic substrate.
8. Roll-to-roll manufacturing

Other Activities/Awards

1. Placed First in the University of Bihar (B.S. program), Muzaffarpur (India)
2. Joint University Grants Commission – Council of Scientific and Industrial Research (UGC-CSIR) National Level Fellowship: (1992-1997), India
3. National Merit Scholarship (1984-1989), Bihar, India.
4. Coordinated Lab. Courses for B.S. (CHM 101) and M.S. (CHM 443) at IIT, Kanpur, India.
5. Member, American Chemical Society

Publications

Book:
Craig A. Grimes, Oomman Varghese, Sudhir Ranjan. “Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis”. Springer (November 2007)
           
Research Papers (Selected):

1. Sudhir Ranjan, S.-Y. Lin, K.-C. Hwang, Y. Chi, W.-L. Ching, C.-S. Liu, Y.-T. Tao, C.-H. Chien, S. M. Peng, G.H. Lee. Realizing green phosphorescent light-emitting materials from rhenium(I) pyrazolato diimmine complexes. Inorg. Chem. 42, 1248-1255 (2003).
2. Sudhir Ranjan and S. K. Dikshit. Synthesis, spectroscopic, electrochemical and photophysical behaviour of ruthenium (II) and copper (I) isocyano-bridged complexes with polypyridine ligands: 2,2’-bipyridine and 1,10-phenanthroline. Transition Met. Chem. 27, 668-675 (2002).
3. Y. Chi, Sudhir Ranjan, T. Y. Chou, C. S. Liu, S. M. Peng, G. H. Lee. Preparation and characterization of the volatile alkaline-earth metal complexes with multiply coordinated aminoalkoxide ligands. J. Chem. Soc., Dalton Trans.  2462-2466 (2001).
4. Y. Chi, Sudhir Ranjan, P.-W. Chung, C. S. Liu, S. M. Peng, G. H. Lee. Synthesis and characterization of two novel tetra-nuclear sodium ketoiminate complexes: structural evidence for formation of Na…F and Na-C (olefin) bonding interactions. J. Chem. Soc., Dalton Trans. 343-347 (2000).
5. Sudhir Ranjan and S. K. Dikshit. Synthesis, spectroscopic, photophysical and electrochemical properties of cyano-bridged copper(I)-ruthenium(II) complexes. Polyhedron 17, 3071-3082 (1998)