NG Next: now online!

Northrop Grumman’s NG Next, home of my Basic Research group, has launched a public website so everyone can see who we are, what we’re working on, and where we’re going NEXT!

http://www.northropgrumman.com/ngnext/

Let me know what you think in the comments below – we’re still a new division, and suggestions can be translated into an improved website over the coming months.

NEXT position: moving to LA!

All good things must end – and that includes the wonderful postdoctoral position I’ve enjoyed at the AMOLF institute in Amsterdam over the past three years.

On March 6th I’ll begin my new position with NG-NEXT, a new basic research division within Northrop Grumman, as a research scientist. In a transition from solar energy back to my roots, I’ll begin working on experimental tests of carrier dynamics, and coupled plasmonics and nanophotonic systems.

If you’re in LA, let’s meet for a coffee, talk science, and ride bikes!

AMS Magazine

The solar revolution in Amsterdam – an interview with Dr. Mark Knight

Following a lively presentation and discussion at Science Café Amsterdam on Nanotechnology for a green energy future (hosted by RTLZ.nl), Mark was interviewed by AMS magazine to provide a perspective on solar research in the Netherlands. You can read the full article in the AMS Knowledge Special, starting on page 131.

 

Citation

AMS Magazine  (Annual Amsterdam business magazine): The Knowledge Special: “The solar revolution in Amsterdam”. AMS 6, 130-133, 2016/2017.

Photovoltaic materials

Background

Photovoltaics, which directly convert solar energy into electricity, offer a practical and sustainable solution to the challenge of meeting the increasing global energy demand. According to the Shockley-Queisser (S-Q) detailed-balance model, the limiting photovoltaic energy conversion efficiency for a single-junction solar cell is 33.7%, for an optimum semiconductor band gap of 1.34 eV. Parallel to the development of wafer-based Si solar cells, for which the record efficiency has continually increased during recent decades, a large range of thin-film materials have been developed with the aim to approach the S-Q limit. These materials can potentially be deposited at low cost, in flexible geometries, and using relatively small material quantities.

Science Fig. 1

What do we discuss?

We review the electrical characteristics of record-efficiency cells made from 16 widely studied photovoltaic material geometries and illuminated under the standard AM1.5 solar spectrum, and compare these to the fundamental limits based on the S-Q model. Cells that show a short-circuit current (Jsc) lower than the S-Q limit suffer from incomplete light absorption or incomplete collection of generated carriers, whereas a reduced open-circuit voltage (Voc) or fill factor (FF) reflects unwanted bulk or interfacial carrier recombination, parasitic resistance, or other electrical nonidealities. The figure shows the experimental values for Jsc and the Voc × FF product relative to the S-Q limiting values for the different materials. This graph enables a direct identification of each material in terms of unoptimized light management and carrier collection (Jsc/JSQ < 1) or carrier management (Voc × FF/VSQ × FFSQ < 1).

Monocrystalline Si cells (record efficiency 25.6%) have reached near-complete light trapping and carrier collection and are mostly limited by remaining carrier recombination losses. In contrast, thin-film single-crystalline GaAs cells (28.8%) show only minimal recombination losses but can be improved by better light management. Polycrystalline CdTe thin-film cells (21.5%) offer excellent light absorption but have relatively high recombination losses; perovskite cells (21.0%) and Cu(In,Ga)(Se,S)2 (CIGS) cells (21.7%) have poorer light management, although CIGS displays higher electrical quality.

Aside from these five materials (Si, GaAs, CdTe, CIGS, perovskite) with efficiencies of >20%, a broad range of other thin-film materials have been developed with efficiencies of 10 to 12%: micro/nanocrystalline and amorphous Si, Cu(Zn,Sn)(Se,S)2 (CZTS), dye-sensitized TiO2, organic polymer materials, and quantum dot solids. So far, cell designs based on these materials all suffer from both light management and carrier management problems. Organic and quantum dot solar cells have shown substantial efficiency improvements in recent years.

Outlook

The record-efficiency single-crystalline materials (Si, GaAs) have room for efficiency improvements by a few absolute percent. The future will tell whether the high-efficiency polycrystalline thin films (CdTe, CIGS, perovskite) can rival the efficiencies of Si and GaAs. Because the cost of photovoltaic systems is only partly determined by the cost of the solar cells, efficiency is a key driver to reduce the cost of solar energy, and therefore large-area photovoltaic systems require high-efficiency (>20%), low-cost solar cells. The lower-efficiency (flexible) materials can find applications in building-integrated PV systems, flexible electronics, flexible power generation systems, and many other (sometimes niche) markets. High-efficiency (>20%) materials find applications in large-area photovoltaic power generation for the utility grid as well as in small and medium-sized systems for the built environment. They will enable very large-scale penetration into our energy system, starting now and growing as the cost per kilowatt-hour is reduced further by a factor of 2 to 3. This can be achieved by nanophotonic cell designs, in which optically resonant and nonresonant structures are integrated with the solar cell architecture to enhance light coupling and trapping, in combination with continued materials engineering to further optimize cell voltage. Making big steps forward in these areas will require a coordinated international materials science and engineering effort.

Citation

Photovoltaic materials – present efficiencies and future challenges. A. Polman, M.W. Knight, E.C. Garnett, B. Ehrler, and W.C. Sinke. Science 352, aad4424 (2016).

Gallium plasmonics

Gallium has recently been demonstrated as a phase-change plasmonic material offering UV tunability, facile synthesis, and a remarkable stability due to its thin, self-terminating native oxide. However, the dense irregular nanoparticle (NP) ensembles fabricated by molecular-beam epitaxy make optical measurements of individual particles challenging. In this work we employ hyperspectral cathodoluminescence (CL) microscopy to characterize the response of single Ga NPs of various sizes within an irregular ensemble by spatially and spectrally resolving both in-plane and out-of-plane plasmonic modes. These modes, which include hybridized dipolar and higher-order terms due to phase retardation and substrate interactions, are correlated with finite difference time domain (FDTD) electrodynamics calculations that consider the Ga NP contact angle, substrate, and native Ga/Si surface oxidation.

This study measures the plasmonic size-tunability in single Ga NPs and demonstrates that the plasmonic modes of interacting Ga nanoparticles can hybridize to produce strong hot spots in the ultraviolet. The controlled, robust UV plasmonic resonances of gallium nanoparticles are applicable to energy- and phase-specific applications such as optical memory, environmental remediation, and simultaneous fluorescence and surface-enhanced Raman spectroscopies.

 

Citation

Gallium Plasmonics: Deep Subwavelength Spectroscopic Imaging of Single and Interacting Gallium Nanoparticles. M.W. Knight, T. Coenen, Y. Yang, B.J.M. Brenny, M. Losurdo, A.S. Brown, H.O. Everitt, and A. Polman. ACS Nano 9, 2049–2060 (2015).