Self-assembled Plasmonic Nanostructures for Thin Film Photovoltaics

Abstract

The aim of this thesis is to explore the optical properties of localized surface plasmon resonance sustained by self-assembled metallic nanoparticles (NPs) for the light trapping application in thin lm photovoltaics (PV).

Photovoltaics is able provide safe and clean electricity of the future, inparticular, thin lms solar cells have a potential to increase the competitiveness of PV through a substantial reduction of the manufacturing cost. However, an essential step it to develop an e cient, reliable and inexpensive light trapping scheme in order to maximize absorption of the near-infrared radiation in the cell and balance the reduced volume of semiconductor material. Recently there is a growing interest in the application of subwavelength metallic NPs for light trapping as they can scatter light e ciently over a broad wavelength range of the solar spectrum, due to the to the phenomena known as localized surface plasmon (LSP) resonance.

A systematic study of the correlation between the structural and the optical properties of self-assembled silver nanostructures fabricated on soda-lime glass by a solid-state dewetting (SSD) process, which consist in thermallyinduced morphology transformation from a thin lm to an array of islands or nanoparticles is reported. It is shown that four distinct types of morphology tend to form in speci c ranges of fabrication parameters, which is quantitatively summarized by a proposed structural-phase diagram and allows to identify the fabrication conditions in which preferable, uniformly spaced and circular NPs are obtainable. The optical properties of the NPs stay in qualitative agreement with the trends predicted by Mie theory, and correlate with the surface coverage (SC) distributions and the mean SC size.

As a step forward towards the implementation in thin lm photovoltaics, the NPs are incorporated on the rear side of thin silicon fillm in two distinct arrangements, namely superstrate and substrate. In superstrate configration,The coupling e ciency increases with NPs' average size, decreases with increasing distance between silicon, and is signi cantly smaller for spherical than for hemispherical NPs, which stay in qualitative agreement with theoretical predictions. A novel procedure, involving a combination of phothermal de ection spectroscopy and fourier transform photocurrent spectroscopy, employed for substrate con guration lms allowed for the quanti cation of useful and parasitic absorption. It is demonstrated that the optical losses in the NPs are insigni cant in the 500-730nm wavelength range, beyond which they increase rapidly with increasing illumination wavelength. Furthermore, a broadband enhancement of 89.9% of useful absorption has been achieved.

Susequantly, a successful implementation of a plasmonic light trapping scheme implemented in a thin lm a-Si:H solar cell in plasmonic back re ector (PBR) con guration. The optical properties of the PBRs are systematically investigated according to the morphology of the self-assembled silver nanoparticles (NPs), which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs, it is shown that the photocurrent enhancement achieved in the a-Si:H light trapping window (600-800 nm) stays in linear relation with the PBRs di use re ection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of Jsc and Voc are achieved in comparison to those previously reported in the literature for the same type of devices. Furthermore an attempt on implementation of the plasmonic light trapping in the industrial a-Si/ c-Si double junction solar cells is reported.