I’m not usually susceptible to pangs of jealousy, but some papers presented at CLEO really make me think I chose the wrong career. I am referring to the veritable banquet of talks on anti-reflection and light trapping methods for solar cell enhancement.

Don’t get me wrong, as an investment analyst I am as much interested in the greenback as the green movement. By my calculations a 1% increase in the efficiency of a polysilicon solar panel (from 16% to 17%), or a 3% increase in the output of a multi-junction cell (from 35% to 38%), will be worth around $1 billion across Europe in additional PV revenues by 2013. Little wonder that companies such as Du Pont and GE have announced major investment programs into photovoltaic enhancement.

Table: Economic value of PV efficiency increases in European solar power generation

Source: Elucidare Limited

CLEO Symposium on Photonics for Advanced Energy Technology: Photonics for Power Generation and Delivery

Monday, May 17. 10.15 am to 12.00 pm

CLEO kicks off its photovoltaic programme with a morning session on advanced solar cell design and performance.

First up, Sarah Kurtz (Natl. Renewable Energy Lab, USA) will examine high-efficiency photovoltaic technology, in particular the latest breakthroughs in multi-junction techniques where near-perfect materials have achieved over 40% in efficiency under concentrated sunlight.

Next, Weiquan Yang (Univ. of Texas at Arlington, USA) et al will describe flexible solar cells based on stacked crystal-line semiconductor nanomembranes on plastic substrates. The reported efficiency of only 1.5% must be understood in the context that these cells are only for 1 um thick. Remember, the important factor is US$/Watt, not efficiency alone.

Keeping with the nano theme, Zongfu Yu (Stanford Univ., USA) et al will discuss the fundamental limits of nanophotonic light-trapping in solar cells. Light trapping is an efficiency enhancement technique whereby solar light can be captured within sub-wavelength thick absorption layers using periodic surface gratings. Via a rigorous electromagnetic approach, Yu reveals that the standard bulk optic limit developed by Yablonovitch can be exceeded in the nanophotonic regime, opening new avenues for highly efficient solar cells.

Finally, Jan-Gustav Werthen (JDS Uniphase Corp., USA) will give an overview of Power-over-Fiber, a technique for powering remote devices such as sensors and FFTH transceivers. I am slight puzzled that Werthen should describe this as an “emerging technology” because five years have passed since JDSU paid US$9.7 million to acquire Photonics Power Systems, the company that already had spent many years developing power-over-fiber technology. I guess he means this in relation to its application to FTTH networking, which truly is a more recent introduction, at least here in Europe.

Advanced Concepts in Photovoltaics

Monday, 17 May. 1.30 pm to 3.15 pm

In a session otherwise dedicated to the topic of plasmon-enhanced photon absorption, this programme commences with a talk by Chang-Hong Shen (Natl. Nano Device Labs, Taiwan) on thin-film flexible silicon solar cells fabricated at 100°C by high density plasmas.

Next up, Wenli Bai (Lehigh Univ., USA) introduces plasmonic back structures designed to enhance the efficiency of thin film solar cells. Metallic structures with one-dimensional periodic nanoridges attached to thin-film amorphous silicon (a-Si) solar cell are proposed to enhance the cell efficiency in a wide near-IR range.

Niraj N. Lal (Univ.  of Cambridge, UK) et al discusses nanovoid plasmonic-enhanced low-cost photovoltaics. Gold and silver nanovoid structures generate localised plasmon modes which are harnessed to enhance organic and amorphous silicon solar cell performance. According to Lal, higher absorption at plasmonic resonant wavelengths indicates significant potential for enhanced photocurrent and efficiency. Bravo, though with gold hitting a record high last week, I hope Lal will provide some insight into the cost effectiveness of this technique.

Fu-Ji Tsai (Natl. Taiwan Univ., Taiwan) et al presents work on absorption enhancement using localized surface plasmon-induced scattering with metal nanoparticles, while Yifen Liu (Iowa State Univ., USA) discusses grating-induced surface plasmon-polaritons for enhancing photon absorption  in organic photovoltaic devices. Using a grating electrode to launch laterally propagating surface plasmon-polaritons along the active layers of the organic photovoltaic device, Lui creates a standing wave to increase the device thickness and thereby enhance photon absorption.

Finally, Jing-Shun Huang (Natl. Taiwan Univ., Taiwan) discusses inverted polymer solar cells with paired metal oxide modifications through solution processing. The 5% conversion efficiency is respectable, in my opinion, for a polymer solar cell.

Photonic Crystals I

Tuesday, May 18, 8.00 am

For a wake-up introduction to the subject of photonic band gap materials and their use as light trapping crystals in solar energy harvesters, I recommend the tutorial session by Sajeev John (Univ. of Toronto, Canada). John will review the physics, microfabrication, and applications of these structures. Solar buffs like me should then excuse ourselves and trot along to the parallel session on novel materials for enhanced solar cell performance. See below.

Novel Materials for Enhanced Solar Cell Performance

Tuesday, May 18. 8.00 am to 9.45 am

In an invited speech, Jeff Nause (Cermet, Inc., USA) will discuss multijunction solar cells based on tunable bandgap ternary InGaN p-n junctions. Cermet is perhaps best known for its work on lattice matched, high efficiency solid state emitters grown on ZnO substrates.

Next up, Broderick (MIT, USA) et al will present work on Si-on-insulator thin film solar cells featuring textured back reflectors. Reportedly cells at all thicknesses demonstrated significant external quantum efficiency (EQE) enhancement due to the back reflector.

In the first of a series of papers on light trapping techniques, Simone Zanotto (Univ. degli Studi di Pavia, Italy) discusses absorption enhancement in thin-film silicon solar cells with a periodic etching in addition to an AR-coating. The authors claim an increased short-circuit current of up to 36.5%.

Here follows a series of anti-reflection and light-trapping papers from Taiwanese universities. Ping-chen Tseng (Chiao-Tung Univ., Taiwan) et al discussed antireflection and light trapping of periodic subwavelength surface structures formed by colloidal monolayer spheres deposited lithography on thin film solar cells; Shu-Chia Shiu (Natl. Taiwan Univ.) et al talks about reducing Si reflectance using high-density Si nanowires fabricated by metal-assisted etching; Min-An Tsai (Natl. Chiao-Tung Univ., Taiwan) et al presents results on efficient enhancement in a InGaP/GaAs dual-junction solar cell by broadband  and omnidirectional antireflection nanorod arrays. Finally, Shu-Chia  Shiu (Natl. Taiwan Univ., Taiwan.) introduces a new type of hybrid solar  cells  based  on  a heterojunction  between PEDOT:PSS and vertically-aligned n-type GaAs nanowire  arrays  is  investigated. Such solar cells exhibit a power conversion efficiency of 3.46 %.

Joint CLEO/QELS Post Session

Tuesday, May 18.

Don’t forget the poster session. Ragip Pala (Stanford Univ., USA) et al present a paper entitled “Plasmonic  Solar Cells with Broadband Absorption Enhancements”.  This describes a computational-experimental  study optimizing plasmon-enhanced absorption in thin film solar cells. Pala investigates the effect of different geometries where 2-dimensional periodic-aperiodic arrays of metal nanostructures sit above or below the active material. It will be interesting to compare and combine this work with the Yablonovitch paper by Zongfu Yu (see above).

Dr David Nugent is Founder and CEO of Elucidare Limited, a boutique technology development and investment advisory business.