Parag Vasekar

Moisture-Induced Surface Corrosion in AZO Thin Films Formed by Atomic Layer Deposition

Aluminum-doped zinc oxide (AZO) thin film is a viable alternative to tin-doped indium oxide, the dominant transparent conducting oxide used in solar cells. The durability of the AZO thin films grown by atomic layer deposition technique, which is known to form layers with atomic layer precision, is studied. The AZO films were subjected to the harsh environmental conditions of varying temperatures and humidity, and their changes in surface morphology and conductivity are investigated. Four different combinations of temperature (100°C and 20°C) and relative humidity (100% and 20%) were used. It was found that the films exposed to the high-moisture and temperature conditions resulted in surface corrosion and lowered conductivity. However, SEM cross-sectional images showed that the bulk of the film was unaffected. The corroded surface had contaminants deposited from the measurement chamber as observed from XPS elemental analysis. Detailed phase analysis showed the presence of zinc hydroxide and zinc carbonate inside the corroded regions.

Thin Film Solar Cells Using Earth-Abundant Materials

In a p-n junction photovoltaic (PV) cell, a photon of light produces an electron hole pair if the energy of the photon is at least equal to the band gap of the material constituting the p-n junction. The electrons and holes first diffuse toward the respective edge of the depletion re‐ gion, and then drift across the junction due to the built-in potential and are collected at the electrodes. Thus, materials with long minority carrier life times and high carrier mobilities are desired for high efficiency. Because electrons have higher mobility than the holes, a ptype semiconductor is used as light absorber in a p-n junction solar cell.

Development of zinc phosphide as a p-type absorber

In the present contribution, we report a simple and repeatable process for the synthesis of zinc phosphide on two different substrates, zinc foil and zinc evaporated on molybdenum-coated glass. Zinc phosphide has been an important candidate for optoelectronic applications and has also been explored in the lithium ion batteries. Zinc phosphide is synthesized from earth-abundant constituents, zinc and phosphorous. Trioctylphosphine (TOP) is used as a source of phosphorous which reacts with zinc and results in the growth of zinc phosphide. Zinc phosphide has been successfully synthesized in both continuous thin film and nanowires form around ~ 350°C. The synthesized zinc phosphide phase was characterized using SEM, EDS, XRD and XPS. Possible growth mechanism is discussed.

CZTS solar cells with 5.75% efficiency using sputtered precursors

Recent research trends in thin film solar cells are moving towards finding alternatives based on earth-abundant elements. Cu(Zn, Sn)(S, Se)2 is being explored these days by the thin film photovoltaics community which contains earth abundant materials like Zn and Sn. CZTS has near optimum bandgap around 1.5 eV and a large absorption coefficient in the order of 104 cm-1. We have identified the optimum sequence for deposition of sputtered precursors has Sn/Zn/Cu. It was also observed that formation of Molybdenum sulfide layer can be controlled by optimizing the sulfurization process and hence series resistance can be minimized. Current efficiencies are around 6%.

Fabrication of Cu 2 ZnSnS 4 thin film solar cell using chemical method

We report the growth, characterization and fabrication of the quaternary compound semiconductor Cu2ZnSnS4 (CZTS) thin film solar cell using low temperature chemical synthesis. The constituent materials required for this p-type absorber are earth abundant and available at low cost. In addition, CZTS has large absorption coefficient in the order of 104 cm-1 and has optimum band-gap energy of about 1.5 eV required for an efficient photo-energy conversion. We have adopted a low temperature chemical route followed by spin coating to synthesize CZT layers. Essentially, a metal compound solution of CZT is formed by dissolving Copper (II) acetate, zinc (II) acetate and tin (II) chloride in 2-methoxyethanol and monoethanolamine. The CZT layer, which is formed by spin coating and annealing at around 300°C, is then sulphurized by using a safe organic sulphur source called di-tert-butyl-disulfide (TBDS) for a controlled sulphur transfer at temperature around 400°C to form stoichiometric Cu2ZnSnS4. The Cu to Zn+Sn and Zn to Sn ratios for an optimally synthesized film were 0.87 and 1.2 respectively. Techniques like EDX, XRD and XPS were used for composition, crystallinity and phase analysis.