Nayan Chakravarty

A photonic-plasmonic structure for enhancing light absorption in thin film solar cells

We describe a photonic-plasmonic nanostructure, for significantly enhancing the absorption of long-wavelength photons in thin-film silicon solar cells, with the promise of exceeding the classical 4n2 limit for enhancement. We compare identical solar cells deposited on the photonic-plasmonic structure, randomly textured back reflectors and silver-coated flat reflectors. The state-of-the-art back reflectors, using annealed Ag or etched ZnO, had high diffuse and total reflectance. For nano-crystalline Si absorbers with comparable thickness, the highest absorption and photo-current of 21.5 mA/cm2 was obtained for photonic-plasmonic back-reflectors. The periodic photonic plasmonic structures scatter and reradiate light more effectively than a randomly roughened surface.

Surface plasmon enhancement of optical absorption of thin film a-Si:H solar cells

We design and fabricate a-Si:H solar cells with plasmonic crystal back reflectors demonstrating enhanced absorption of near infrared photons. Rigorous simulations predict plasmonic nanoparticles on ITO can increase absorption in the near infrared. Rigorous simulations also predict plasmonic crystal back reflectors with a pitch of ∼750 nm to optimize the enhanced absorption. Such periodic back reflectors have been fabricated with photolithography and a-Si:H solar cells have been grown on them by PECVD. The solar cells display ∼9% improvement in short circuit current and external quantum efficiency and a absorption enhancement factor exceeding 8 above 650 nm.

Device physics of nanocrystalline silicon solar cells

We investigate the electronic properties of nanocrystalline silicon solar cells. It is shown that the material behaves very similarly to crystalline silicon but with mobility and minority carrier lifetimes being significantly lower than in c-Si. Mobility is shown to increase with grain size. Minority carrier lifetime was measured and shown to be inversely proportional to defect density. The recombination defects are shown to be approximately 0.4 eV below the conduction band. It is also shown that the fundamental optical absorption itself depends upon the grain size. We also show that a gradient of ppm levels of doping can improve the performance of solar cells by introducing a built infield. We also show that a post-deposition H anneal can reduce the defects and improve performance of devices made at higher temperatures.

Superlattice solar cells on nanoimprinted photo-plasmon substrates

Thin film solar cells absorb only a small portion of the sunlight in one pass, which make light trapping a very important requirement for these solar cells. We report here the use of the two novel techniques (namely photonic and plasmonic) used in conjunction to achieve higher light trapping. When the thin film structure is deposited on a photo-plasmonic structure, which combines the light diffraction properties of a photonic structure with the reemission of light by a plasmonic structure, the current increases even more. We have obtained a 50% increase in current in a thin film of 0.9 micrometer by on a nanoimprinted photo-plasmon structure. In this study we also show the use of silicon alloys in a superlattice structure to further improve the photon absorption. The superlattice increases the quantum efficiency in the 600–700 nm range, and a properly designed photo-plasmon structure increases it in the 700–1000 nm range. A major advantage of the superlattice structure is that the crystallinity of the device remains constant throughout the thickness, without the need for changing hydrogen/silane dilution ratio. By using a-(Si, Ge) layers in the superlattice, one can shift the shoulder in optical absorption to longer wavelengths. Increasing the Ge content systematically increases the current in the cell.