NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Optical Materials, Adhesive and 

Encapsulant, III-V, and Optical 

Characterization Evaluation

Cooperative Research and 

Development Final Report

CRADA Number: CRD-07-216

NREL Technical Contact: Michael Kempe

CRADA Report

NREL/TP-7A10-55834

November 2012
NOTICE

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1 

Cooperative Research and Development Final Report

In accordance with Requirements set forth in Article XI.A(3) of the CRADA document, this document is 

the final CRADA report, including a list of Subject Inventions, to be forwarded to the Office of Science 

and Technical Information as part of the commitment to the public to demonstrate results of federally 

funded research.

CRADA Number: CRD-07-216

CRADA Title: Optical Materials, Adhesive and Encapsulant, III-V, and Optical Characterization 

Evaluation

Parties to the Agreement: SolFocus 

Joint Work Statement Funding Table showing DOE commitment: 

Estimated Costs NREL Shared Resources

Year 1 $ 00.00

Year 2 $ 00.00

Year 3 $ 00.00

TOTALS $ 00.00

Abstract of CRADA work: 

SolFocus is currently developing solar technology for utility scale application using Winston collector 

based concentrating photovoltaics (CPV). Part of that technology development includes small mirror 

dishes and front surface reflectors, and bonding the separate parts to the assembly. Mirror panels must 

meet rigid optical specifications in terms of radius of curvature, slope errors and specularity. The 

reflective surfaces must demonstrate long term durability and maintain high reflectivity. Some bonded 

surfaces must maintain adhesion and transparency under high concentrations and high temperatures. 

Others will experience moderate temperatures and do not require transparency. NREL researchers have 

developed methods and tools that address these related areas.

Summary of Research Results: 

Cheryl Kennedy began testing the durability of SolFocus mirrors in 2007 under the CRADA. The initial 

mirrors were of poor quality. She attended a SolFocus tiger team meeting to analyze their mirror 

reliability issues in August 2008, visiting the SolFocus headquarters and a nearby demonstration system. 

Kennedy presented common mirror reliability issues and SolFocus presented their current mirror 

process issues. The root cause of the reliability issues was determined after discussion with the tiger 

team. SolFocus used this information to change their mirroring process and began producing reliable 

solar mirrors.

Over the course of about 6 years a total of 19 different candidate adhesive materials were investigated 

by Mike Kempe for use in the CPV system of Solfocus. These materials were used to bond two pieces of 
2 

glass in either a lap shear (3 materials) or a butt joint configuration (16 sample sets). Samples were 

exposed for up to 8000 h in an Atlas Ci4000 WeatherOmeter (WOM) set up with an ambient 

temperature of 600C, a black panel temperature of 950C, a relative humidity of 60%, and an irradiance of 

114 W/m2 between 300 and 400 nm. Some materials were found to maintain excellent adhesion over 

the course of this exposure, and others were found to decompose, delaminate, or creep because of 

exposure. Solfocus has used this data to aid in selection of their adhesives.

Additionally, cell assemblies were similarly stressed in the Atlas Ci4000 WOM. This allowed the 

simultaneous application of heat, humidity, light, and voltage to the cells. These cells were also fitted 

with a lens to provide concentrated light, though only over a small portion of the cell. While this did not 

duplicate the high intensity light of the actual use condition, it did highlight some concerns with the 

durability of the cell materials.

Lastly some receiver assemblies were subjected to a rapid thermo cycling test between -40 and 850C for 

a thousand cycles. Cells were heated by the application of current directly to the cells, and cooled by 

blowing air chilled with dry ice over them. This experiment helped establish the high durability of their 

receivers to this sort of stress.

Sarah Kurtz joined the SolFocus advisory board in 2007 under the CRADA. She joined an Advisory Board 

meeting in fall of 2007, visiting the SolFocus headquarters and a nearby demonstration system. 

SolFocus presented their current product and modifications that they were planning for the product for 

comment/discussion by the board members.

Subject Inventions Listing: 

There were no inventions made during the course of these experiments.

Report Date: 

8/23/12

Responsible Technical Contact at Alliance/NREL: 

Michael Kempe

This document contains NO confidential, protectable, or proprietary information.