Showing papers by "Sudha Mokkapati published in 2012"

Nanophotonic light trapping in solar cells

Abstract: Nanophotonic light trapping for solar cells is an exciting field that has seen exponential growth in the last few years. There has been a growing appreciation for solar energy as a major solution to the world’s energy problems, and the need to reduce materials costs by the use of thinner solar cells. At the same time, we have the newly developed ability to fabricate controlled structures on the nanoscale quickly and cheaply, and the computational power to optimize the structures and extract physical insights. In this paper, we review the theory of nanophotonic light trapping, with experimental examples given where possible. We focus particularly on periodic structures, since this is where physical understanding is most developed, and where theory and experiment can be most directly compared. We also provide a discussion on the parasitic losses and electrical effects that need to be considered when designing nanophotonic solar cells.

Optimal wavelength scale diffraction gratings for light trapping in solar cells

Abstract: Dielectric gratings are a promising method of achieving light trapping for thin crystalline silicon solar cells. In this paper, we systematically examine the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO2) gratings using numerical simulations. The square pyramid structure with silicon nitride coating provides the best light trapping among all the symmetric structures investigated, with 89% of the expected short circuit current density of the Lambertian case. For structures where the grating is at the rear of the cell, we show that the light trapping provided by the square pyramid and the checkerboard structure is almost identical. Introducing asymmetry into the grating structures can further improve their light trapping properties. An optimized Si skewed pyramid grating on the front surface of the solar cell results in a maximum short circuit current density, Jsc, of 33.4 mA cm−2, which is 91% of the Jsc expected from an ideal Lambertian scatterer. An optimized Si skewed pyramid grating on the rear performs as well as a rear Lambertian scatterer and an optimized TiO2 grating on the rear results in 84% of the Jsc expected from an optimized Si grating. The results show that submicron symmetric and skewed pyramids of Si or TiO2 are a highly effective way of achieving light trapping in thin film solar cells. TiO2 structures would have the additional advantage of not increasing recombination within the cell.

Semiconductor Nanolasers

Abstract: Recent progress in the field of semiconductor nanolasers is discussed. New designs have emerged that eliminate the need for a conventional Fabry-Perot cavity, bringing down the physical dimensions of the lasers below the diffraction limit. Semiconductor nanolasers are critical components for nanophotonics and offer possible integration with Si nanoelectronics.

Plasmonic quantum dot solar cells for enhanced infrared response

Abstract: Enhanced near infrared photoresponse in plasmonic InGaAs/GaAs quantum dot solar cells (QDSC) is demonstrated. Long wavelength light absorption in the wetting-layer and quantum-dot region of the quantum dot solar cell is enhanced through scattering of light by silver nanoparticles deposited on the solar cell surface. Plasmonic light trapping results in simultaneous increase in short-circuit current density by 5.3% and open circuit voltage by 0.9% in the QDSC, leading to an overall efficiency enhancement of 7.6%.

Polarization tunable, multicolor emission from core-shell photonic III-V semiconductor nanowires.

Abstract: We demonstrate luminescence from both the core and the shell of III–V semiconductor photonic nanowires by coupling them to plasmonic silver nanoparticles This demonstration paves the way for increasing the quantum efficiency of large surface area nanowire light emitters The relative emission intensity from the core and the shell is tuned by varying the polarization of the excitation source since their polarization response can be independently controlled Independent control on emission wavelength and polarization dependence of emission from core–shell nanowire heterostructures opens up opportunities that have not yet been imagined for nanoscale polarization sensitive, wavelength-selective, or multicolor photonic devices based on single nanowires or nanowire arrays

Designing single GaAs nanowire lasers

Abstract: Design parameters for a single GaAs nanowire laser are determined by calculating the threshold gain for nanowire guided modes as a function of nanowire diameter and length. The material gain as a function of carrier density is modelled using theoretical microscopic gain model. The laser power required to optically pump these nanowires to reach threshold gain is also determined. These calculations provide guidance to grow the optimal structures that can lase at low threshold at room temperature (RT).

Dielectric diffraction gratings for light-trapping in InGaAs-GaAs quantum well solar cells

Abstract: The enhancement in short-circuit current density (Jsc) provided by a TiO2 diffraction grating composed of rectangular strips at the rear of an In021Ga079As-GaAs quantum well solar cell (QWSC) is investigated using finite-difference time-domain (FDTD) simulations Optimisation of the grating height, periodicity and fill-factor yielded an enhancement of 53% in Jsc over the reference cell

Optoelectronic properties of GaAs nanowire photodetector

Abstract: A single GaAs nanowire (NW) photodetector (PD) is fabricated based on the back-to-back Schottky diode structure. Optoelectronic properties are characterized by measuring device photocurrent, and also the spectral response, which indicates our device is very sensitive and applicable as a PD.

Plasmonics for III–V semiconductor solar cells

Abstract: III–V semiconductors like GaAs and InGaN are very promising candidates for solar cells. While GaAs has near-ideal bandgap to reach the maximum possible efficiency limit for single junction solar cells, InGaN provides the ability to tune the bandgap of absorbing layers over a wide energy range. Since III–V semiconductors are mostly direct bandgap semiconductors, they are also very strong absorbers of light. Currently novel solar cell designs based on nanostructured absorbers like quantum dots1,2 or nanowires are under investigation to demonstrate high efficiency solar cells.

Effect of plasmonic nanoparticles on the quantum efficiency of III–V semiconductor nanowire emitters

Abstract: We study the possibility of increasing the quantum efficiency of III–V semiconductor nanowire emitters using plasmonics Results on the effect of plasmonic nanoparticle size, emitter-plasmonic nanoparticle distance and the initial quantum efficiency of the emitter on the quantum efficiency enhancement factor are presented