Materials and design strategies for solar microcells

My thesis describes how photovoltaic performance can be improved by careful engineering and incorporation of optical elements and materials external to the devices. These optical elements and materials include periodically nanostructured semi-transparent metallic reflectors that are integrated below microcells, a light trapping structure on the cell’s surface, a down-shifting material that coverts ultraviolet light into visible light, as well as optical elements that can spectrum split light between two subcells of different band gaps. The solar cells studied in this work are Si, InGaP, and GaAs solar microcells. Their fabrication and characterization – along with the rationale behind certain design principles –are discussed. For the case of the periodically structured backside reflector and the surface light trapping structures, Si solar microcells were studied and I was able to demonstrate that in both instances Fabry-Perot resonances trap impinging light inside the Si slab via excitation of cavity modes and are the main photocurrent enhancement mechanism. This was shown through experimental measurements and compared to computational modeling. For each of these systems, significant absorption enhancements were observed in the near infrared — as shown from simplified quantum efficiency measurements and absorption measurements — that agreed with the computational models. For the metallic backside reflector integrated behind a Si microcell, experimental and computational results show that surface plasmons make no significant contribution to the enhanced photocurrent. For the project discussing the ultraviolet downshifting material, a Eu-based inorganic complex has beneficial effects on the overall cell performance. Experimental measurements on InGaP microcells showed improved performance in the ultraviolet portion of the spectrum where iii the down-shifting material is active. The mechanisms by which the down-shifting material affects cell performance are reviewed and further improvements to solar devices implementing down-shifting materials are also discussed. Optical elements such as prisms and dichroic mirrors have previously been employed in spectrum splitting designs. Working with micro-scale solar cells, chromatic aberration can be used to spectrally split the sunlight. In the last part of my thesis, optical modeling results for a prism lens spectrum splitting configuration as well as an aspheric lens with a high degree of chromatic aberration are discussed. Experimental results are presented that show the aspheric lens can be incorporated into a multijunction spectrum splitting solar cell design. Experimental performances for InGaP and GaAs solar cells in a model spectrum splitting device are examined. Finally, a discussion of future directions of photovoltaic research – particularly with III-V materials – is reviewed.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Corcoran, Christopher

Contributors dc:contributor

• Nuzzo, Ralph G.
• Rogers, John A.
• Gewirth, Andrew A.
• Rockett, Angus A.

• Solar Energy
• Transfer Printing
• Silicon
• III-V
• Photovoltaics

Statement dc:rights

• Copyright 2014 Christopher Corcoran

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/50684

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/50684