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

Improved Solar Cell Efficiency 

Through the Use of an Additive 

Nanostructure-Based Optical 

Downshifter

Final Subcontract Report

January 28, 2010 — February 28, 2011

Juanita Kurtin

SpectraWatt, Inc.

Hillsboro, Oregon

Subcontract Report

NREL/SR-5200-51381

May 2011
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.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Improved Solar Cell Efficiency 

Through the Use of an Additive 

Nanostructure-Based Optical 

Downshifter

Final Subcontract Report

January 28, 2010 — February 28, 2011

Juanita Kurtin

SpectraWatt, Inc.

Hillsboro, Oregon

NREL Technical Monitor: Brian Keyes

Prepared under Subcontract No. NEU-0-99010-10

Subcontract Report

NREL/SR-5200-51381

May 2011
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL.

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1

Table of Contents

Table of Contents........................................................................................................................................ 1

Technology Summary................................................................................................................................. 2

Task 2: Quantum Dot Heterostructure Synthesis ......................................................................4

Task 3: Thin films of QDH in a polymer matrix .......................................................................6

Task 5: Achieve > 80% Light coupling efficiency ....................................................................8

Task 6: Reliability......................................................................................................................9

Conclusions............................................................................................................................................... 11

References................................................................................................................................................. 12

Supplementary Information ..................................................................................................................... 13
2

Technology Summary

Spectrawatt’s downshifting technology is based on a nanostructured material system which 

absorbs high energy (short wavelength) light and reemits it at a lower energy (long wavelength) 

with high efficiency. This system has shown unprecedented performance parameters including 

near unity quantum yield and high thermal stability.

The downshifter technology consists of a luminescent layer composed of chromophores 

embedded in a transparent matrix that is optically coupled to the solar cell. A fraction of the blue 

light incident on the luminescent material is absorbed by the chromophores (the fraction 

absorbed depends on chromophore loading) and re-emitted into the solar cell as red light (figure 

1). All other photons are absorbed by the solar cell directly as indicated in the figure. The 

chromophore emits isotropically into the transparent matrix. Photons that are re-emitted at angles 

less than the critical angle escape through the top of the solar cell (~12.8% for a material with an 

index 1.5). A fraction of the re-emitted photons are lost out of the edge and some are reabsorbed and go through these processes again. [See SI section III for further information.] All 

other photons end up in the solar cell either directly or through reflectance off of the top surface. 

The optical properties of the downshifter layer must be controlled to maximize the fraction of 

photons that end up in the solar cell and get converted to electricity. 

Figure 1: Operating principle of the luminescent downshifter

The goal of our materials system is to absorb the majority of the light below 500 nm where the 

solar cell is inefficient, and emit in the visible near 600 nm where the cell is the most efficient. 

The requirements for the downshifter are: 

• High PLQY – this ensures that every photon absorbed is re-emitted as a redder 

photon

• Low self-absorption – since the chromophores emit isotropically, the photons can 

travel a large distance before entering the solar cell

• Long-term reliability – solar cells must survive 20+ years under the sun
3

• High temperature stability – solar panels in the field can reach temperatures as hot as 

40 C above ambient

• Good compatibility between the chromophore and the host matrix – need to be able to 

avoid self-quenching through aggregation and need haze/clarity control.

• Optical compatibility between the downshifter and the PV module being modified.

• Good compatibility with the host matrix and the solar module.

A downshifter that meets our design rules can be seen in figure 2. The bulk of the absorption 

comes below 500nm while the emission is at ~600nm, a wavelength at which most solar cells are 

very efficient at converting light into energy (figure 2a). Such a system serves two purposes: first 

we are capturing photons that would otherwise be poorly converted, and secondly, by reducing 

overlap between absorption and emission spectra, we have very few self absorption events. The 

downshifter in figure 2 displays a PLQY of 90% in a film, which includes the losses due to the 

few self-absorption events that do occur. A comparison of our downshifter coupled to a CIGS 

cell and a standard encapsulated CIGS module is shown in figure 3b with a photograph of the 

two devices shown in the inset. The EQE of the two CIGS modules are shown. As can be seen in 

the figure, the response for the cell with the downshifter is much better in the blue. This 

difference accounts for ~1.5mA/cm2 (or a 5% relative boost). The nearly flat EQE below 500 nm 

at 65% agrees with our accounting of all the photons. 
4

Figure 2: Optical and device properties of the luminescent downshifter. The absorbance and 

emission show minimal absorption by the chromophore at the emission wavelength – reducing 

self-absorption events. (a) Laminated CIGS cell with a standard encapsulant 

and with our downshifter (b).

Task 2: Quantum Dot Heterostructure Synthesis

The chromophore that generates the optical properties of figure 2 is a Quantum Dot 

Heterostructure (QDH). The QDH is based on a core/shell nanocrystal structure, in which a 

cadmium selenide (CdSe) core acts as the emitter, and a cadmium sulfide (CdS) shell is the 

absorber. [4] These two materials form a type I, or nested band structure as shown in figure 3a. 

This system is ideal for our purposes as it provides the downshift in photon energy, coupled with 

the ability to attain near unity photoluminescence quantum yields (PLQY). The quantum 

confinement of the CdSe core (figure 3b) allows us to tune the emission wavelength of the QDH 

simply by tuning its size. [5] The CdS, on the other hand, is a large rod shaped shell (figure 3c). 

The idea behind this architecture is to maximize the absorption ratio of CdS to CdSe as described 

above. The bigger this ratio, the more the absorption is due to CdS, reducing any self absorption. 

The shell is grown as a high aspect ratio rod to reduce the effects of strain when growing a thick 
5

epitaxial shell. If too much strain builds up, as often happens in a large spherical shell, cracking 

and grain boundaries can form, which will lead to traps and reduced PLQY. [6]

Figure 3: The quantum dot heterostructures. In the type I bandgap structure, an exciton excited in 

the wide bandgap material will recombine in the lower bandgap material, emitting a photon of 

lower energy (a). Transmission electron micrographs of the CdSe core (b), the CdSe/CdS QDH (c), 

and the encapsulated QDH (d).

Previous attempts to achieve stable PLQY >95% of high aspect ratio QDH have been less 

successful. [4] The QDH in these reports have suffered from incomplete surface passivation of 

the nanocrystal surface by the organic ligands. Because surface atoms make up a large portion of 

a nanocrystal, they are particularly sensitive to defects and external interactions on their surface 

[7]. These challenges are exacerbated in solid films – aggregation driven by ligand-ligand 

interactions leads to concentration quenching of the PLQY. 

One way to overcome this problem is by the addition of an additional inorganic shell. [8] By 

tuning synthetic conditions, we incorporated a single QDH into silica spheres as shown in figure 

3d. This shelling leaves the QDH surface well passivated, allowing PLQY values close to 100% 

in both solution and solid films. 
6

Task 3: Thin films of QDH in a polymer matrix

Incorporation of the silica-encased QDH into PV encapsulants must be minimally disrupted to 

existing fabrication sequences of solar cell modules and, preferably, at no cost beyond that of the 

requisite materials. On one hand, the matrix must satisfy specific requirements set by the PV 

industry standards such as high adhesion to the substrate, high transparency, structural strength 

and excellent outdoor performance. On the other hand, in order to maximize the benefits of using 

the luminescent downshifter, the host material must provide an environment in which the PLQY 

of the incorporated QDH is stable. To date, the PV industry has had the most success with 

transparent polymers. Despite a wide range of options, only few meet all conditions.

In most of the cases, creating organic/inorganic hybrid materials represents a challenging 

problem. The systems are characterized by poor compatibility, leading to phase separation and 

QDH aggregation which leads to a drastic drop in PLQY. Consequently, device performance is 

compromised. Poly(ethylene-co-vinyl acetate) (EVA) is the most commonly used polymer as an 

encapsulant in the PV industry. By modifying the side chains, we can alter the macromolecules 

to the point that the QDH are well dispersed in concentrated polymer solutions with a long shelflife. The hydrophilic-hydrophobic balance was controlled by mixing potassium hydroxide into 

EVA until the degree of hydrolysis was between 1 and 2%. 

Scheme 1: Chemical structures of Ethylene-Vinyl Acetate Copolymer (EVA) 

and partly hydrolyzed EVA (HEVA)

The combination of the silica-encased QDH with the hydrolyzed ethylene-vinyl acetate 

copolymer (HEVA) yields a film that displays a PLQY >90%. This combination leads to 

colloidal stability and preservation of optical properties for both host and luminescent materials. 

To make the films compatible with standard solar panel manufacturing conditions, several crosslinking additives were added to the film. 

The samples that yielded the 5% relative boost to CIGS cells were tested for their temperature, 

light and humidity stability to ensure their durability and performance maintenance over the 

extended lifetime expected of solar products, as well as the wide range of temperatures and 

environmental conditions experienced by an outdoor installation. Details of characterization 

methods are available in the experimental section.

Task 2 & 3 Experimental

All manipulations were carried out under non-inert atmosphere except the QDH synthesis. The 

materials (all reagent grade) were purchased from Aldrich and used as is. Polymers were 

characterized by NMR, FTIR and gel permeation chromatography (GPC). NMR spectra were 

CH3

O CH3

A

O

x y x y z

H3C

OH

CH3

O CH3

O

H H a b
7

recorded on a Bruker 500 spectrometer. 1H resonances were referenced internally to the 

deuterated solvent resonances. Polymers molecular weights were determined by GPC using a 

Viscotek GPC MAX liquid chromatography equipped with a triple detector array. GPC columns 

were calibrated versus polystyrene standards (American Polymer Standards). A flow rate of 1.0 

mL/min was used with ACS grade THF as the eluent.

QDH synthesis and shelling

The CdSe seeds and CdS shells were synthesized with a modified version of the recipe reported 

in reference 7. The silica shells were formed with modification of the method described in 

reference 11.

Polymer modification 

Synthesis of hydrolyzed ethylene-vinyl acetate copolymer (HEVA) was obtained from a 

copolymer with a vinyl acetate content of 40% by mole. The degree of hydrolysis of the vinyl 

acetate component was controlled by the amount of KOH and the reaction time. A typical 

procedure is described here. 25 g EVA was dissolved in 250 mL tetrahydrofuran. Separately, a 

0.4 M solution of KOH in methanol was prepared. 2 mL of KOH solution was added to the 

stirring solution of polymer according to the desired degree of hydrolysis. The reaction was 

carried out for 30 min for a degree of hydrolysis of 2% and 2 h for a degree of hydrolysis of 5%. 

The hydrolyzed polymer was isolated from solution by precipitation in methanol or isopropanol. 

The organic solvents were decanted from precipitated polymer and the product washed with 

isopropanol or water to remove remaining salt and base, potassium acetate (KOAc) and KOH 

respectively. After filtration, the polymer was dried for 24 hours in a constant temperature 

vacuum oven at 40°Cunder vacuum. The total yield was 97%. FTIR was used to quickly identify 

the presence of hydroxyl groups in the polymeric product. In addition, NMR was used to 

determine the degree of hydrolyzation on the basis of chemical shifts integration of the protons 

in vinyl acetate and vinyl alcohol groups. When compared with starting material (EVA40), the 

GPC analysis of a HEVA with 5% degree of hydrolysis indicated only a slightly drop in 

molecular weight (from ca. 28 kg/mol to 24 kg/mol) but a significant narrowing in polydisperse 

index (1.83 to 1.35). 

Film preparation

The QDH-polymer nanocomposite films were prepared by drop-casting from QDH-polymer 

dispersion in toluene prepared as follows. A known volume of QDH dispersion in toluene is 

added to a 10 % wt. toluene solution of HEVA and stirred/hold on the shaker overnight. The 

amount of QDH dispersion added to polymer solution depends on the type of particle (Rabs), the 

concentration of the QDH dispersion and the targeted O.D. Prior to dropcasting we add initiator 

(2% by wt) and accelerator (0.2% wt). Typically, films were prepared by dropcasting from a 650 

μL of QDH-polymer dispersion on the 25 mm glass rounds. Larger films can be prepared simply 

by scaling the casting volume to the surface area. Slow evaporation of the toluene in ventilated 

fume hood yielded highly transparent homogeneous luminescent films readily available for the 

PLQY and UV-vis measurements, lamination, reliability tests and device integration.

PLQY measurement

The PLQY was quantitatively measured by optically exciting samples and measuring the number 

of photons absorbed and emitted by the sample. This was accomplished by using a spectrally 

calibrated CCD detector incorporating an integrating sphere. The sphere ensures photons 
8

emitted in all directions are collected, the CCD allows separation of spectral components due to 

excitation and emission, and spectral calibration allows the calibrated response of the system to 

be spectrally flat. Dividing the integrated emission counts by the integrated absorbed photon 

counts gives the PLQY.

Task 5: Achieve > 80% Light coupling efficiency

I. Photon Accounting

Early on in the project, our team devised a method to rapidly prototype the downshifter concept 

and to characterize the realistic efficiency ceiling and losses. The quantum dot heterostructures 

were not yet optimized to achieve high quantum yields and solid-solution films of well-dispersed 

QDHs had not yet been developed. The method involved fabricating test structures comprised of 

a glass well, or corral, coupled to an underying solar cell which was filled with a liquid solution 

containing luminescent material. Because the optics of liquid downshifters are identical to solid 

solution downshifters, we could rapidly screen materials, and prototype concepts. For the lossanalysis, a laser dye, Coumarin 6, in toluene was chosen to fill a well on a well understood mc-Si 

cell. Coumarin 6 was chosen because its PLQY in toluene is near unity. Figure S3 shows 

representative test structures.

Figure S1: Pictures of the Coumarin 6 liquid wells.

Various experiments were performed to characterize all optical and electronic losses. The 

equations below show how the product of all photophysical efficiencies yields the EQE in the 

region the downshifter absorbs.

Here, R is the reflection off the cell+well structure, Fabs is the fraction of light absorbed by the 

downshifter material, Fescape is the amount of light that is emitted within the escape cone, PLQY 

is the efficiency of converting absorbed photons to emitted ones, FSA is the fraction of photons 

that undergo self absorption loss (through reduced quantum yield and increased light 

outcoupling), Fother characterizes all other losses fundamental or specific to the measurement 

(edge loss, grid loss, etc.) and IQEcell is the internal quantum efficiency of the underlying cell.

ܧܳܧ௧௢௧௔௟ ൌ ܧܳܧ஽ௌ ൅ ܧܳܧி௜௟௧௘௥௘ௗ ஼௘௟௟

ܧܳܧ஽ௌ ൌ ሺ1െܴሻ · ܨ௕௔௦ · ܲܮܻܳ · ൫1 െ ܨ௘௦௖௔௣௘൯ · ሺ1െܨௌ஺ሻ · ሺ1െܨ௢௧௛௘௥ሻ · ܫܳܧ௟௟௘௖

ܧܳܧி௜௟௧௘௥௘ௗ ஼௘௟௟ ൌ ሺ1െܴሻ · ሺ1െܨ௕௔௦ሻ · ܫܳܧ௟௟௘௖
9

The table below summarizes the measured losses using a multicrystalline Si cell as the 

underlying solar cell.

Table 1. Measured losses.

Primary losses come from reflection off the top glass, and photons that are emitted within the 

escape cone, which from a medium with refractive index n=1.5, equals 12.8%. Other losses are 

specific to the cell and measurement: side escape, due to the small area devices tested, grid 

losses, which depend on the grid and busbar geometry and IQE of the underlying cell. The 

degree of self absorption strongly depends on the overlap between absorption and emission 

spectra of the downshifter material. The close agreement between measured and predicted 

EQEs clearly demonstrates that all photons have been accounted for. 

The above photon accounting has been verified through the use of a 3D Ray-Tracing program 

BLAM was coded in the open-source mathematics computing platform Octave [ref website]. The 

program was developed in response to the apparent complications of complex geometries, 

inclusion of materials with varying optical properties, and multiple self-absorption events. The 

program is designed to take simple inputs in the form of optical density, absorption ratio (ratio of 

absorption at peak to absorption at emission wavelength), material geometries and indices of 

refraction, and compute statistics regarding photon fates, i.e. top-loss, side-loss, absorption into 

underlying cell, etc., based on Snell’s and Beers’ Law. The program takes inputs for sinks, 

reflectivity (Lambertian, specular, distribution functions), and optionally displays an overlay of 

path-lengths. The space is sampled randomly for a specified number of photons by computation 

of a probability matrix based on Beer’s law.

Task 6: Reliability

Light exposure and Damp Heat (DH) accelerated testing (85C, 85%RH) testing over thousands 

of hours was performed as a step in the refinement of both QDH optimization and film (QDH + 

HEVA + additives) formulation. A variety of atmospheric conditions, e.g. vacuum, nitrogen, 

desiccation, were applied to films in development to isolate primary degradants and establish a 

range of acceptable operating conditions for the downshifter. Interestingly, the films are in 

general insensitive to ambient or elevated humidity, and only demonstrate decreased 

performance under the most desiccated atmosphere over the course of weeks, a condition not 

realized in their packaged operating environment for solar applications. Shown below in figure 4 
10

is electrical data for a QDH/HEVA film incorporated into a typical thin-film CIGS package as an 

EVA replacement (exact configuration is proprietary), compared with an identical cell with plain 

encapsulant. The data is normalized to facilitate direct comparison of the samples with and 

without downshifter, demonstrating no appreciable difference in performance over >1000 sun 

hours. This power maintenance far exceeds performance criteria of IEC standards and 

demonstrates apparent viability for products carrying a twenty year performance guarantee.

Figure 4: Pseudo-normalized power output vs. time of thin-film CIGS solar cell under one-sun 

illumination with and without QDH/HEVA film as an EVA topsheet replacement. 

Fluctuations in the power output are due to lighting and electrical instabilities and fluctuations in 

operating temperature due to ambient temperature variation. This is demonstrated by the stability 

of the ratio of power output with and without the QDH/HEVA film to within 0.5% relative. 

Overall, the DS film does not result in a change in the reliability of the underlying device.

QDH films and devices were measured at a variety of temperatures to determine suitable 

operating temperatures. While this aspect of film performance presented the greatest challenge in 

process optimization, the apparent instability of comparable material systems at high temperature 

also provides the largest window for improvement. 

In conclusion we have produced chromophores with nearly 100% PLQY specifically tailored for 

the downshifting application. These can be well dispersed in a variety of host materials common 

to the Solar industry. On CIGS solar cells, our downshifter adds about 1.5mA of current (5% 

relative increase on top of 30mA/cm2).

0 250 500 750 1000

Sun Hours

0.850

0.875

0.900

0.925

0.950

0.975

1.000

Normalized Maximum Power Output

With DS film

Without DS film

0 250 500 750 1000

0.980

0.985

0.990

0.995

1.000

1.005

Power Ratio
11

Reliability measurements

QDH solutions, films, and devices were tested at each stage of development for their ability to 

maintain quantum efficiency under the range of operating conditions relevant for solar 

applications. Furthermore, experimental conditions were employed to elucidate the origin of 

degradation, i.e. isolate the degradant and accelerating conditions. 

Solar light-soaking: solution, film and active solar cell samples were exposed to one Sun 

conditions while temperatures of the sample were actively controlled. Furthermore, solid 

samples were often isolated with the use of cell containment units that allowed the samples to be 

in a carefully controlled atmosphere, most often vacuum, high-purity nitrogen, or constantmoisture air. Solution and film samples were monitored periodically for changes in both 

absorbance and luminescence with UV-Vis and Photoluminescence measurements, yielding a 

relative performance change against an absolute value measured in an integrating sphere. 

Electrical samples were continuously monitored by custom active loading electronics and 

maintained at their Maximum Power Point (MPP).

85/85 Humidity Testing: Films and devices were exposed to accelerated humidity and 

temperature testing, as is standard solar cells, e.g. as specified in UL1703 and IEC61646. 

Samples were placed inside a Hastest environmental test chamber and left at a 

temperature/humidity condition of 85C and 85% RH for extended periods of time (hundreds to 

thousands of hours). Samples were periodically removed, cooled, and tested: UV-Vis / PL 

measurements for optical films and IV characterization under one sun for devices. 

Temperature stability: luminescent films were heated through a range of temperatures in order to 

trace their performance through a range of realizable operating conditions for solar applications. 

Films were mounted in a combined UV-Vis Photoluminescence measurement system, between 

thermoelectric unit coupled with a heat-spreading aluminum plate and plastic insulator. 

Temperature was monitored via a digital temperature sensor, and samples were cycled between 

ambient (~20C) and 80-100C.

Conclusions

In conclusion, SpectraWatt successfully integrated several processes from existing literature to 

synthesize and QDH and protect them from the environment. Taken together, these procedures 

enabled the production of a highly efficient downconverting material. 

Ultimately the gains realized from downshifting compete with gains achieved in the blue. In the 

case of Silicon downshifting is outperformed by technologies like selective emitters. In the case 

of CIGS thinning of CdS or selection of an alternate window layer frustrate the application of 

downshifting technology. For superstrate CdTe, the optical losses associated with the thick top 

glass, TCO and high temperature of the process pose significant challenges to the application of 

the technology.

Despite these challenges we achieved very respectable gains on CIGS (0.5% absolute) which for 

some players may well prove to be a decisive advantage.
12

References

1. Richards, B.S., and McIntosh, K.R. Progress In Photovoltaics: Research and Applications 

15: 27-34 (2007).

2. D. V. Talapin, J. H. Nelson, E. V. Shevchenko, S. Aloni, B. Sadtler, and A. P. Alivisatos, 

Nano Lett. 7, 2951-2959 (2007)

3. A. P. Alivisatos, J. Phys. Chem 100, 13226–13239 (1996)

4. B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, 

K. F. Jensen, and M. G. Bawendi, J. Phys. Chem. B 101, 9463-9475 (1997)

5. L. Manna, E. C. Scher, L. S. Li, and A. P. Alivisatos, J. Am. Chem. Soc 124, 7136–7145 

(2002)

6. L. Jing, C. Yang, R. Qiao, M. Niu, M. Du, D. Wang, and M. Gao, Chemistry of Materials 

1861 (n.d.)

7. D. Valerini, A. Creti, M. Lomascolo, L. Manna, R. Cingolani, and M. Anni, Phys. Rev. B 

71, 235409 (2005)

8. P. Jin, J. Zheng, M. Ikezawa, X. Liu, S. Lv, X. Kong, J. Zhao, Y. Masumoto, J. Phys. 

Chem. C 113, 13545 (2009)

9. Y. Chan, J. P. Zimmer, M. Stroh, J.S. Steckel, R.k. Jain, M.G. bawendi, Advanced 

materials vol. 16 pp. 2092-2097 (2004).
13

Supplementary Information 

I. Synthesis Optimization 

As mentioned in the body of the document, the re-absorption of the emitted photons can be 

minimized by increasing the ratio of the CdS to the CdSe. Increasing this ratio allows the 

downshifter to absorb all of the light below 500 nm without absorbing much in the region where 

the solar cell is already performing well. A good way to define the degree of self-absorption is by 

the absorbance at 400 nm as compared to the absorbance at the exciton peak. E.g. if the ratio is 

50:1 and the downshifter requires an optical density of 1 at 400 nm, the optical density at the 

emission wavelength would be 0.02. Figure S1 shows how the ratio affects the absorbance. Note 

how for equivalent absorbance at 600 nm, the absorbance below 500 nm increases with 

increasing ratio 

Figure S2: Absorbance of the QDH with increasing CdS to CdSe ratio. 

II. Emission Tuning 

One of the nice properties of the QDH is that the emission peak can be varied by varying the size 

of the CdSe core size. Tuning this emission is extremely useful for several of the applications of 

a luminescent downshifter. Figure S2 shows 3 of the particles we have made that span the colors 

green to red. 
14

Figure S3: Emission tuning of the QDH with varying CdSe particle size 
F1146-E(10/2008)

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4. TITLE AND SUBTITLE

Improved Solar Cell Efficiency Through the Use of an Additive 

Nanostructure-Based Optical Downshifter: Final Subcontract Report, 

January 28, 2010--February 28, 2011

5a. CONTRACT NUMBER

DE-AC36-08GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

J. Kurtin

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NREL/SR-5200-51381

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PV101199

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Hillsboro, OR 9724

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NEU-0-99010-10

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14. ABSTRACT (Maximum 200 Words)

This final report summarizes all SpectraWatt’s progress in achieving a boost in solar cell efficiency using an optical

downshifter. Spectrawatt’s downshifting technology is based on a nanostructured material system which absorbs 

high energy (short wavelength) light and reemits it at a lower energy (long wavelength) with high efficiency. This 

system has shown unprecedented performance parameters including near unity quantum yield and high thermal 

stability.

15. SUBJECT TERMS

Photovoltaics; Si; CIGS; Downshifter

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