ZnS quantum dots in luminescent solar concentrators

Available online at www.sciencedirect.com

Solar Energy 83 (2009) 566–573 www.elsevier.com/locate/solener

Photo-stability and performance of CdSe/ZnS quantum dots in luminescent solar concentrators Meredith G. Hyldahl, Sheldon T. Bailey, Bruce P. Wittmershaus * School of Science, Penn State Erie, The Behrend College, 4205 College Dr., Erie, PA 16563-0203, USA Received 2 January 2008; received in revised form 4 August 2008; accepted 4 October 2008 Available online 12 November 2008 Communicated by: Associate Editor Darren Bagnall

Abstract The performances of luminescent solar concentrators (LSCs) made with two versions of quantum dots (QDs) with CdSe cores and ZnS shells are compared to LSCs containing the organic dye, LumogenÒ F Red 300 (LR), to assess the viability of QD LSCs. In addition to spectroscopic and light collection measurements, the photo-degradation response of the version I (vI) QD LSC is compared to the LR LSC. The measured fluorescence quantum yield of the version II (vII) QDs (57%) is about half that of LR (>90%) and twice that of the vI QDs (31%). Though the quantum yield for vII QDs is lower than LR, the vII QD LSC has nearly twice the short-circuit current of the LR LSCs or the vI QD LSCs when their respective red-peak optical densities are the same in 6.2  6.2  0.6 cm LSCs. This is a reflection of the main advantage of QDs for use in LSCs, that QDs collect considerably more sunlight than LR due to their broad absorption spectrum. Despite the fact that the QD LSCs absorbs more photons than the LR LSCs, the slow phase of the photo-degradation rate of the QD LSC is approximately five times slower than the LR LSC under nearly constant light exposure. Most surprising is the observation that the photo-degradation of the QD LSC’s absorption completely recovers during a prolonged dark cycle. In a normal day/ night cycle, this will benefit the performance of the QD LSC. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Fluorescence quantum yield; Nanoparticle; Photo-degradation; Recovery

1. Introduction Luminescent solar concentrators (LSCs) were proposed by Weber and Lambe (1976) as a more cost effective method of collecting solar energy than panels of photovoltaic cells (PVCs). An LSC is typically made from a plate of a transparent solid containing a fluorescent material. The fluorescent material absorbs sunlight and fluoresces light over a specific range of wavelengths. If the plate has an index of refraction of 1.5, 75% of the emitted light is then transferred to the edges of the plate by total internal reflection (TIR) where PVCs can be placed to collect it (Hermann, 1982; Popov and Yakimenko, 1995; Barashkov *

Corresponding author. Tel.: +1 814 898 6476; fax: +1 814 898 6213. E-mail address: [email protected] (B.P. Wittmershaus).

0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.10.001

and Gunder, 1996). The three main advantages of LSCs over using large panels of PVCs or mirror concentrators are low cost, wavelength down-conversion, and non-directional collection. Problems encountered with LSCs are incomplete absorption of sunlight across the solar spectrum, reflection losses, losses from TIR, re-absorption of fluoresced photons, and photo-degradation of the fluorescent material. The ideal fluorescent material would be cheap and have a broad absorption spectrum, a small overlap between absorption and fluorescence spectra, high photo-stability, and a high fluorescence quantum yield (FQY). Some of the materials that others have tried in the past include phosphors and organic dyes (Hermann, 1982; Reisfeld and Joergensen, 1982; Popov and Yakimenko, 1995; Barashkov and Gunder, 1996). Phosphors are inexpensive and very photo-stable, but

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exhibit only moderate FQYs and limited solar absorption (Barashkov and Gunder, 1996). Organic dyes are generally inexpensive and can have very high FQYs, but exhibit only poor-to-good photo-stability and a limited absorption spectrum (Popov and Yakimenko, 1995) except when combined (Bailey et al., 2007). The latest material that has been used for LSCs is the quantum dot (QD) (Chatten et al., 2003, 2004; Gallagher et al., 2007; Rowan et al., 2007; Sholin et al., 2007). The QDs being considered are fluorescent man-made nanoparticles, comprising either II–VI or III–V compounds (Reed and Jan, 1993). Their emission wavelength is directly proportional to their diameter (Alivisatos, 1996). The range of wavelengths emitted by a collection of QDs is determined by the uniformity of QD sizes in the sample. The more uniform the QD size within a sample, the narrower the emission spectrum’s bandwidth. A completely homogenous sample of QDs of the size used here would have a fluorescence spectrum with a FWHM of 13–18 nm (van Sark et al., 2002). Core-shell QDs have higher FQYs and better photo-stability than QDs without a shell (Xu et al. 2000; Evident Technologies, 2007). QDs have been suggested to be very photo-stable (Chatten et al., 2003, 2004; Gallagher et al., 2007; Rowan et al., 2007; Sholin et al., 2007). They have broad absorption spectra in the visible, significant overlap between absorption and fluorescence spectra, generally low-to-medium FQYs, and are fairly expensive to make. Previously published studies of QD LSCs have shown some promising results but the stabilities of the QD LSCs have not been quantified and their outputs have not been as good as LSCs made using organic dyes (Chatten et al., 2003, 2004; Gallagher et al., 2007) or a semiconducting fluorescent polymer (Sholin et al., 2007). This study was designed to assess the viability of different formulations of core-shell QDs as a material for LSCs not only in terms of conversion of sunlight into electricity but also photo-stability. Commercially available fluorescent materials were used so that other researchers may easily study them in the future. In addition to measurement and analysis of absorption and fluorescence spectra, measurements of the FQYs of the QDs were reported to provide more precise values than the manufacturer’s quoted FQY ranges. The photo-stabilities and outputs of the QD LSCs were determined and compared to LSCs made using the organic dye, Lumogen Red. This dye was chosen for the proximity of its absorption and emission peaks to those of the QDs, its high FQY and good photo-stability, and for historical comparison to previously published work where Lumogen Red LSCs were compared to other QD LSCs (Chatten et al., 2004; Gallagher et al., 2007). 2. Materials and methods 2.1. Materials Two versions of the same model of QDs composed of a CdSe core and a ZnS shell were used in this study (Evident

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Technologies, Model EC-C11-0620, 620-nm Maple Red–Orange Core Shell EvidotsÒ). Samples of the version I (vI) QDs were first bought commercially, and later were donated by Evident Technologies. These QDs were studied in anhydrous toluene and in a solid, UV-cure epoxy (supplied by Evident Technologies). The manufacture’s specifications for vI QDs are: emission peak = 620 + 10 nm, FWHM < 30 nm, diameter = 5.2 nm, molar extinction coefficient = 4.5  105 l/(mol  cm), molecular weight = 200 lg/nmol, and approximate FQY = 30–50% (Evident Technologies, 2007). The average diameter of the QDs is calculated from the emission peak of the sample. The QDs are nearly spherical with a mean aspect ratio of 1.22 as measured from transmission electron microscope images of representative samples (Evident Technologies, 2008). Version II (vII) of the same model QDs were in product development at the time of this study and were donated by Evident Technologies. The vII QDs, while having a higher FQY, cannot be embedded in a solid medium as yet and were only studied in anhydrous toluene. The ligands used in the QDs to keep them from aggregating also affect the FQY. The ligands used in the vII QDs cause the FQY to increase, but are not strong enough to overcome the forces that cause the QDs to aggregate, thus not allowing them to be incorporated into a solid LSC material (Evident Technologies, 2006). Since the vII QDs could not be put into a solid matrix, they were not used in the photo-degradation experiment. QDs are very susceptible to oxidation, which causes the absorption coefficient and emission intensity to drop drastically and the emission to blue-shift (van Sark et al., 2002). To minimize oxidation, all QD samples were prepared in a nitrogen atmosphere of less than 0.1% oxygen and sealed between glass plates when constructing the QD LSCs. QDs in solution were suspended in 99.8% pure anhydrous toluene (n = 1.497) from Sigma–Aldrich on the advice of Evident Technologies to limit the amount of oxygen that could reach the QDs. The organic dyes, rhodamine 101 and cresyl violet, used in the FQY measurements, were purchased from Exciton Dyes, Inc. Samples of LumogenÒ F Red 300 (LR) dye were both purchased from and donated by BASF. All dyes were used without further purification. 2.2. Absorption and fluorescence measurements Absorption and fluorescence spectra of both the versions of QDs in anhydrous toluene were measured. Absorption spectra were taken with the reference cuvette filled with anhydrous toluene using a dual beam Cary Win-UV/VIS BIO 300 spectrophotometer with all slits set to 2 nm. Fluorescence spectra were taken with a Photon Technology International Model QM-2 fluorimeter with all slits set to 1 nm in a manner similar to that previously described (Wittmershaus et al., 2001). The FQY measurements and analyses were performed for both the vI and vII QDs based on a procedure described previously (Wittmershaus et al., 2001). The sam-

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ples used for the FQY measurements were prepared in two different ways. The absorption measurements used low concentration samples (2  107 M–2  108 M) in 10-cm path-length quartz cuvettes. These low concentration samples were then pipetted into 1-cm path-length glass cuvettes for fluorescence measurements. The FQY reference samples were rhodamine 101 and cresyl violet. These reference dyes were chosen because of their well defined FQYs and absorption maxima near that of the QDs. The FQY of rhodamine 101 is 100% in methanol (n = 1.326) (Karstens and Kobbs, 1980) and the FQY of cresyl violet is 57% in ethanol (n = 1.359) (Magde et al., 1979). Average FQY results are based on the FQY measurements for both reference dyes using a fluorescence spectrum of the sample excited at three different wavelengths; one wavelength that produced a complete spectrum and two wavelengths that were common between all three samples. The values reported here are the average FQYs for the two versions of QDs from 18 separate FQY measurements made on each version, using nine measurements with each reference dye. 2.3. LSC performance measurements For performance characterization, two experiments were run; the output efficiency of both versions of QDs in LSCs and the degradation/dark-cycle recovery rates for only the vI QD LSC. Three types of LSCs were used in the output efficiency experiments: liquid, epoxy, and polymer. The liquid LSCs were made by inserting the fluorescent material dissolved in toluene between two, 6.2  6.2  0.3 cm plates of B270 glass (n = 1.523). A well was made between the two plates using an index-matching epoxy (Norland, NOA #76) around the edges. The inside well dimensions were 5.8  5.8  0.026 cm. Toluene was used for the liquid cells because its index of refraction, at 1.497, is close to that of the glass. The epoxy LSCs were made in the same manner with epoxy containing vI QDs placed in the well and cured with UV light. The polymer LSCs were made by dipping 6.2  6.2  0.6 cm plates of B270 glass into a solution of one part polymer (Paraloid B72) to four parts toluene with a LR concentration of 1.68  103 g/ml (Grande and Moss, 1983; Grande et al., 1983). The withdrawal rate of the glass from the dipping solution was 4.4 mm/s, which yielded a dried film with a uniform thickness of approximately 9 lm on one side. Film thickness was determined using the optical density of LR in the film and its known concentration and extinction coefficient. The film on the other side of the glass as well as on the edges was scraped off. The film was dried at room temperature for at least one week before being used in measurements. The absorptions of the LSCs at their respective absorption peaks were: ALRtol = 0.287 (573 nm), ALRpoly = 0.292 (577 nm), AQDIepx = 0.368 (610 nm), AQDItol = 0.238 (610 nm), AQDIItol = 0.205 (588 nm). PVCs (SG-7x6ic, efficiency = 18.3%, Solar World) were cut to the dimensions of the LSC’s edges and secured with another

glass index-matching epoxy (Kemxert Corp., KOA #350). One edge of each LSC had a photovoltaic cell that was wired to a digital multimeter to measure short-circuit current and the other three edges had unwired photovoltaic cells attached to them to mimic the absorption properties of a true LSC. The LSCs were placed with their largest open surface perpendicular to the exciting light. A white background plate was placed 1 mm underneath each LSC to scatter unabsorbed light back through the LSC to maximize absorption. The short-circuit current measured from one edge was then multiplied by a factor of four to find the full output from all edges. The solid LSCs were tested with excitation from direct sunlight and all LSCs were tested under a solar simulator (Spectral Energy Corp.). The excitation irradiance was measured using a calibrated radiometer (International Light, Model IL1400A). To report the data for the liquid LR LSC under sunlight, the ratio of the Paraloid B72 LR LSC’s output under sunlight to its output under the solar simulator was multiplied by the output of the liquid LR LSC under the solar simulator. To obtain the data for the liquid QD LSCs, the same procedure was performed between the vI QDs in epoxy and the vI QDs in toluene as well as between the vI QDs in epoxy and the vII QDs in toluene. LSC performance is reported in six different ways: percentage of sunlight collected (% sun coll.), short-circuit current (Isc), short-circuit current at normalized optical density (Iscn), the ratio of the normalized short-circuit current to the normalized short-circuit current of the vII QD LSC (Iscn/(Iscn vII QD)), electrical gain (gp) and electrical efficiency (ge). The percentage of sunlight collected is the percentage of the photons emitted by the sun from 280 to 4000 nm that the QDs or LR collects at a normalized optical density (OD). Short-circuit current is the average edge output of the LSC in milliamps adjusted to an irradiance of 95 mW/cm2. The normalized short-circuit current is Isc multiplied by the ratio of the percentage of photons collected by the sample at its real OD divided by the percentage of photons collected by the sample at the same OD as the vI QDs in epoxy (0.368). The electrical gain (gp) is the ratio of Iscn for the LSC to Isc for the PVC along the edge of the LSC assuming the open circuit voltage remains constant, and electrical efficiency is given by ge ¼ g p

4d g ; L PVC

where gPVC is the efficiency of the photovoltaic cell being used, d is the plate thickness, and L its length (Sidrach de Cardona et al., 1985). Two different types of LSCs were used in the photo-degradation and dark-cycle recovery experiments: epoxy and polymer. The LSC with vI QDs in epoxy was made in the manner previously described with the glass plates replaced by 2.5  7.6  0.12 cm glass microscope slides and with no photovoltaic cells attached to the edges. The epoxy well dimensions were 2.5  7.2  0.03 cm. Two

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3. Results 3.1. Absorption and fluorescence properties The fluorescence and absorption spectra of the vI and II QDs in toluene solutions used for the FQY measurements are given in Fig. 1. The vI QDs solution has a final red absorption peak at 610 nm while the vII QDs have a shoul-

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LSCs made of Paraloid B72 and LR were used as references. These polymer LSCs were made by solvent casting a film containing LR and 1 part Paraloid B72 in 8 parts toluene onto the face of a 2.5  7.6 cm microscope slide and covering the dried film with another glass slide. The FQY of LR is >90% in polymer (BASF, 1997). The QD LSC was made in epoxy using the epoxy procedure. The starting OD of the QD LSC was 0.437 at 610 nm and the starting OD of the LR LSC was 0.456 at 577 nm. The QD and the LR LSCs were photo-degraded in a Q-Sun Xenon Test Chamber (Q-Panel Lab Products). The Q-Sun delivers constant light weathering with an adjustable intensity xenon arc lamp in a constant temperature environment. During the degradation, the chamber was kept at a constant temperature such that the black panel thermocouple sensor stayed at 50o C. The samples were photo-degraded with the Q-Sun irradiance monitor for 430 nm set at an intensity of 1.50 W/cm2 (69.2 mW/cm2 of sunlight) for a total of 617 h and tested periodically. Two reference LR LSCs were left in the dark for use as controls. Before measuring absorption after a period of degradation, the samples were left in the dark for exactly 5 min to allow for uniform recovery. The absorption measurements were taken every nanometer between 550 and 650 nm. Every sample was tested against a blank consisting of two microscope slides with a film of either epoxy or paraloid B72 between them. After the absorption measurements were completed, the samples were returned to the Q-Sun exactly 10 min from their removal, for further degradation. Dark-cycle recovery of photo-degradation was measured by absorption as well. The QD LSC was removed from the Q-Sun after the final degradation cycle and placed in the spectrometer within 30 s. The absorption of the final red peak of the QD LSC (610 nm) was monitored every second for 30 min. The sample was then placed in the dark and the absorption spectrum from 550 to 650 nm was measured after increasing periods of recovery time to account for the gradual slowing of the recovery starting with a dark cycle of 30 min and ending with a dark cycle of 130 h. Recoveries of the LR LSCs were measured simultaneously with the QD LSCs, and in a similar manner. Curve fits and their uncertainties for the time-dependence of photo-degradation and dark-cycle recovery were performed using the software program, KaleidaGraph (5th ed., Synergy Software). The general curve fit routine was chosen which employs the Levenberg–Marquardt algorithm to determine the best fit.

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Wavelength (nm) Fig. 1. The absorption ( ) and emission (- - - - -) of vI QDs and the ) and emission ( ) of vII QDs used in the FQY absorption ( measurements. The excitation wavelength for the vI QDs emission was 555 nm and for the vII QDs was 530 nm. Inset shows the final red peak region of the spectra.

der at 585 nm. The emission peaks are at 624 nm for vI (FWHM = 20 nm) and 616 nm for vII (FWHM = 44 nm). Table 1 summarizes the FQY results for vI and vII QDs. The vII QDs, with a FQY of 57%, are about twice the yield of the vI QDs at 31%. There is good agreement in FQY measurements for both versions of QDs between the two different reference dyes; the measurements fall within their uncertainties. The excitation wavelengths used to obtain these results varied from 490 to 570 nm. The average FQYs for both versions of QDs were observed to be approximately 10% lower for excitation at 570 versus 490 nm, however, this difference falls within the standard deviation of the final average FQY values. No significant variation in the profile of fluorescence spectra from both versions of QDs was observed over this range of excitation wavelengths. The absorption and fluorescence of LR and the vI QDs in their solid matrices shown in Fig. 2 illustrate the main reason for the use of QDs as an LSC material over a single organic dye; the high absorption of QDs across the visible spectrum. This is most evident when comparing the solar photon flux to the absorption spectra of QD and LR LSCs (Fig. 2). The spectral properties of the vI QDs in epoxy used for the degradation study and of those used for the Table 1 The FQYs of version I and version II QDs in toluene using rhodamine 101 and cresyl violet as reference dyes and the averages of the FQYs against both references. The uncertainties are the standard deviations from all the measurements taken at different exciting wavelengths. (%)

Cresyl violet

Rhodamine 101

Average

/I /II

31 ± 2 55 ± 4

30 ± 2 59 ± 4

31 ± 2 57 ± 4

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Wavelength(nm) Fig. 2. The absorption of vI QDs in epoxy ( ) and LR in Paraloid ) and their respective emissions; QDs (- - - - -) and LR B72 ( ). The excitation wavelengths for the emission spectra were 555 ( and 441 nm, respectively. The solar photon flux spectrum is also shown ) (Amer. Soc. for Testing and Materials, 2003). Inset shows the ( final red peak region of the spectra.

LSC output measurements are identical. The emission spectra of the LR LSC has a peak at 602 nm (FWHM ffi 56 nm); near to that of the vI QDs at 624 nm (FWHM = 20 nm). The absorption and emission spectra of LR in Paraloid B72 are similar in profile to LR in toluene, with the former having the absorption red peak shifted 3 nm to the blue and the far blue peak of the absorption shifted 2 nm to the red. 3.2. LSC performance

Fig. 3. The photo-degradation of absorption of the LR LSC (- -N- -) ) at 610 nm under monitored at 577 nm and the vI QDs LSC ( nearly constant light exposure in the Q-Sun light chamber. Solid and dashed lines are sums of two exponentials used to fit the data. The inset shows an expanded view of the initial degradation.

ciency of the vII QD LSC and the efficiency of the vII QD LSC was a factor of 1.5–2 times greater than those of the LR LSCs. Degradation tests were done on the LR and vI QD solid matrix LSCs (absorption and emission spectra are shown in Fig. 2). The bleaching of the absorption spectra reflecting degradation of the vI QD and LR LSCs are shown in Fig. 3 for nearly constant light exposure. Both LR and vI QD LSCs have a fast decay phase during the first day of exposure giving way to the predominant slow decay phase. Fitting both decays in absorption to the equation AðtÞ ¼ C 1 ek1 t þ C 2 ek2 t ;

Table 2 shows output performance of all the LSCs evaluated in the study. The most current (and highest electrical efficiency) was delivered by the vII QD LSC. The electrical efficiencies of the vI QD LSCs were about half of the effiTable 2 The values used to evaluate the output efficiency of the LSCs are: the percentage of photons from the sun absorbed (280–4000 nm) by each LSC at a normalized red peak optical density (% sun coll.), the short-circuit current (Isc), the short-circuit current normalized for red-peak optical density (Iscn), the ratio of the current to the current output by vII QDs in toluene LSC (Iscn/(Iscn vII QD)) using normalized OD, the electrical gain (gp), and the electrical efficiency (ge) of all the LSCs in the sun. Isc was found for a sunlight irradiance of 95 mW/cm2. LSC

% Sun coll. (%)

Isc (mA)

Iscn (mA)

Iscn/ (Iscn vII QD)

gp

ge (%)

LR film LR in toluene vI QDs in epoxy vI QDs in toluene vII QDs in toluene

9.8 9.4 20.5 23.3 23.3

132 153 137 122 226

158 181 137 142 277

0.562 0.643 0.494 0.512 1.00

0.297 0.340 0.258 0.267 0.522

2.27 2.60 1.97 2.04 3.98

gives fit constants for the LR LSC of C1 = 0.007 ± 0.002, k1 = (1.4 ± 0.7)  103 min1, C2 = 0.453 ± 0.002, k2 = (1.6 ± 0.2)  106 min1 and for the vI QD LSC of C1 = 0.0144 ± 0.0005, k1 = (9.3 ± 0.8)  103 min1, C2 = 0.4225 ± 0.0003, k2 = (3.0 ± 0.3)  107 min1. The slow phase of the vI QD LSC photo-degradation is about a factor of five times slower than that of the LR LSC, despite the fact that the LR LSC only absorbs 42% of the photons that the vI QD LSC absorbs. The relative amplitude of the fast phase of degradation for the vI QD LSC is larger than that for the LR LSC. Therefore, on a short time scale (<300 h), they would appear to degrade by roughly a similar amount if characterized by only a single degradation rate. However, QDs have another advantage over LR: a full dark-cycle recovery. The dark-cycle recovery of the QD LSC is shown in Fig. 4. The dark-cycle recovery was fit using the following equation: AðtÞ ¼ Ao  ½C 3 ðek3 t Þ þ C 4 ðek4 t Þ þ C 5 ðek5 t Þ

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Fig. 4. The recovery time of the vI QD LSC in epoxy. The QD LSC had ) and was degraded for 617 h. an initial OD at 610 nm of 0.437 ( (Fig. 3) before beginning the dark-cycle recovery measurements. The ) is 0.437 minus a sum of equation of the line fitted to the data ( three exponential components. The insets show close-up views of the midand fast-phase recoveries.

where Ao is the sample’s absorption before degradation (in this case, Ao = 0.437). Three phases of recovery are displayed: a quick phase (C3 = (3.01 ± 0.05)  103, k3 = (0.58 ± 0.01) min1) a middle phase (C4 = (9.23 ± 0.09)  103, k4 = (1.79 ± 0.05)  103 min1) and a slow phase (C5 = (9.79 ± 0.09)  103, k5 = (4.32 ± 0.07)  105 min1). This dark-cycle recovery was discovered by leaving the QD LSC in the dark for three months after a previous degradation. The absorption of the QD LSC was then measured only to discover that the absorption peak had fully recovered. The QD LSC was subsequently degraded by small amounts (5%) four times with a full recovery each time. It should be noted that the LR LSC was degraded alongside the vI QD LSC sample and exposed to the same dark-cycle periods. The LR LSC showed no indication of absorption recovery during these studies. 4. Discussion Despite only a 57% FQY, the best performance was displayed by the vII QD LSC with a short-circuit current 1.5 times that of the LR LSCs. The vII QD LSC collected about 99% of the light that the vI QD LSCs collected (Table 2). The doubling of efficiency (from 1.97 to 3.98) can be completely explained by the doubling of the FQY (31–57%) of the QDs between vI and vII. The QD LSCs collected a factor of approximately 2.5 more photons that the LR LSCs collected. Even though the FQY of LR is >90%, the LR LSCs’ outputs were only 0.7 times that of the vII QD LSC. The vI QD LSCs, on the other hand, only have about 0.3 times the FQY of the LR LSCs so their outputs were about 0.8 times the LR LSC outputs. That LR is a better LSC material than the vI QDs is consistent with

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other findings involving CdSe QDs compared to LR (Gallagher et al., 2007), a red perylene dye (Chatten et al., 2004) and the organic dye, Rhodamine B (Sholin et al., 2007). The performances of the vI QD LSCs are comparable to the best QD LSCs of these previous studies. The vII QDs, on the other hand, appear to be a superior material to LR for LSCs. If the FQY of QDs could be brought up to 80–90%, the fluorescent output from these QDs would be double the output of LR. It is important to note that these efficiencies and outputs are based on the dimensions and ODs of the specific LSCs used in this study and that the criteria for matching LSCs was that the reddest peak of absorption of the LR and QD LSCs examined be the same OD. The dimensions of the LSCs and concentration of the LR and QDs in them have not been optimized to produce the best possible LSCs for these materials. Another very important result for the performance of the QDs as an LSC material was the sample photo-degradation. It has been proposed that the QDs are very stable (Chatten et al., 2004; Evident Technologies, 2007; Gallagher et al., 2007; Rowan et al., 2007; Sholin et al., 2007). Our results do, indeed, show that under nearly constant illumination the QD LSC was more stable than the LR LSC if observed for longer than 300 h. During very long light exposure, most of the degradation of the QD and LR LSCs will occur during a slow phase. The rate of the slow phase of the QD LSC (3.0  107 min1) is five times slower than the rate of the slow phase of the LR LSC (1.6  106 min1). These values reflect true LSC performance under nearly constant solar illumination. The characteristic time for the slow phase of the QD LSC is 3.3  106 min (6.3 years) and for the LR LSC, it is 6.2  105 min (1.2 years). However, to determine the respective rates of slow phase photo-degradation for these materials, these rates must be corrected for the fact that the LR LSC absorbs less than half of the photons that the QD LSC absorbs (Table 2). With this correction the slow phase degradation rate of QDs is almost 13 times slower than that of LR under equal amounts of photon absorption. In our study, the QD LSC was sealed from oxygen and degradation was done in constant illumination with brief, 10-minute testing intervals. Gallagher et al. (2005) reported a 2–4 times faster degradation rate for their QDs, but these QDs were in solution and it was reported that oxygen may have been introduced into the system. The slight bluing of the QD spectra in their study also indicates that significant photo-oxidation was occurring (van Sark et al., 2001). Van Sark et al. (2002) also noted that photo-oxidation increases the degradation rate by a factor of four. We have observed no bluing of the absorption spectra of the QDs in our study. The difference between the QD photo-degradation rate given by Gallagher et al. (2005) and that reported in our study can be explained by a difference in the amount of oxygen present in the samples. Even with our precautions aimed at minimizing the presence of oxygen, it is difficult to ensure the complete absence of oxygen in our QD

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samples, particularly in epoxy. The work of Van Sark et al. (2002) certainly indicates that photo-degradation of QDs can be affected by residual oxygen present in the solid matrix. But assigning a single rate to describe photo-degradation of the QD LSC does not reflect the complexity of the process since dark-cycle recovery must be taken into account. In order to characterize the photo-recovery of the QD LSC, it was degraded five times and then allowed to recover with very reproducible results (Fig. 4). During this process, we noticed that there was more than one phase of recovery, so, when the final full light photo-degradation study was done, 10-min test-period interval was allowed for the fast-cycle recovery to complete. It was experimentally impossible to test the sample without allowing some recovery to occur. With this recovery taken into account, a degradation test under constant illumination will degrade the QDs faster than if they were given a dark period in which to recover some of their absorption. The recovery shown in a 12 h ‘‘night” is approximately 30–40% of the total degradation. If the QD LSCs are in a real-life situation, with days and nights, they should degrade at a slower rate than the constant light rate quoted here. The complexity of the time-dependence of the recovery required the use of at least three exponential components to adequately fit the data. The cause of this complex is not understood but may be the result of heterogeneity in various parameters, such as, QD size, local polymer structure, localized oxygen or aggregation. The discovery that the QDs not only have a dark-cycle recovery but also given enough time, that the recovery is 100% is a major advantage for QDs as an LSC material. For instance, a cloudy, colder winter season would allow a QD LSC to recover most of its original output performance from the photo-degradation it had suffered over the summer months. The QD LSC could simply be covered and placed out of service for some time to allow it to recharge. This is a promising development that warrants more research in examining the mechanism(s) behind recovery and whether these back-reactions can be enhanced. The spectral properties of the QDs exhibit some unusual features for a fluorescent material. The most noteworthy of these is the continuous rise in the absorption of the QDs toward the higher energy region of the solar spectrum (Fig. 1). This means that the QDs can absorb as much as 2.5 times the sunlight that LR can absorb (Table 2) if they have the same OD at their far red peaks. This is noteworthy since LR is a very highly absorbing dye due to its three different absorption peaks in the visible region (Fig. 2). The absorption and fluorescence spectra of the QDs used and the Stokes shift between the spectra can easily be controlled by varying the size of the QD and the distribution of sizes of QDs in the sample. As the QD diameter gets larger, the final absorption peak shifts further to the red (Alivisatos, 1996). A wider distribution of diameters of QDs translates to a longer Stokes shift (Chatten et al.,

2003, 2004) and a wider final red peak (van Sark et al., 2001). This tuning is a handy feature in an LSC when attempting to choose which QDs emit in the highest efficiency region of the PVCs used. Unlike dyes, a QD sample is not homogeneous but rather a collection of dots over a certain size distribution. Any individual dot has much narrower absorption and fluorescence peaks than the collection (van Sark et al., 2002). Chatten et al. (2003, 2004) have proposed that this is actually an advantage over organic dyes as the spread of sizes of QDs can be adjusted to minimize losses due to re-absorption of fluorescence caused by overlap between absorption and fluorescence spectra. Considering their conclusion, the large overlaps in absorption and fluorescence spectra apparent in Figs. 1 and 2 may not be as detrimental as they appear. We chose the 620-nm emitting QDs to be as close as possible to the emission of the LR so that we could make useful comparisons between them. The width of the emission peak in the QDs used (FWHM = 20–44 nm) suggests that there is a distribution of sizes within the sample (van Sark et al., 2001). One feature of note about the QDs is the slight difference in the absorption profiles between the versions of the QDs. The vI QDs show a definite red peak whereas vII QDs exhibit no defined peak whatsoever. These spectra are both consistent with spectra others have found for QDs (de Mello Donega´ et al., 2003; Gallagher et al., 2005; Gallagher et al., 2007; Rowan et al., 2007; Sholin et al., 2007). From previous studies (Alivisatos, 1996; de Mello Donega´ et al., 2003; Gallagher et al., 2005), it appears that the peak becomes less defined as the size of the QDs increase. Evident Technologies supplied both versions of QDs used in this study. They quoted the FQY of the vI QDs as 30–50% and the FQY of the vII QDs as 60–80%. The measured FQYs of the QDs were 31% and 57%, respectively (Table 1); both on the low side of their ranges. In order for QDs to be a viable material for LSCs, the FQY would need to be raised to compensate for the cost of the materials. This does not seem impossible given the doubling of the FQY between the vI and vII QDs. Recent published studies have focused on the issues variability in FQY measurements of QDs and how FQY might be raised (Qu and Peng, 2001; de Mello Donega´ et al., 2003). De Mello Donega´ et al. (2003) stress the importance of considering the surface chemistry, growth temperature and Cd:Se ratio in raising FQY.

5. Conclusion The broad absorption, high photo-stability, and recovery from photo-degradation of QDs make them a promising fluorescent material for use in LSCs. QDs can outperform single organic dyes in the amount of sunlight they absorb and their useful lifetime. The benefit of an LSC is mainly making solar energy collection cheaper than panels of PVCs so the high cost of QDs cause them to be

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less desirable as an LSC material. The problems of high cost and mediocre FQYs of QDs need to be addressed. Acknowledgements We would like to express our appreciation of Evident Technologies for donating critical samples for completing our studies and giving valuable advice on handling the QDs. Samples of Lumogen F Red 300 were kindly donated by BASF with the help of Dr. Steve Goldstein. Our special thanks to Jerry Magraw, Paul Fontecchio, Alex Mooney, and Israel Schultz for their contributions. This research was supported through a Grant from the National Science Foundation, the Division of Electronics and Communications Systems, grant ECCS-0424153. MGH gratefully acknowledges summer support through an undergraduate research grant from the Lord Corporation, Erie, PA. Additional support was received through undergraduate research grants awarded to MGH and STB from Penn State Erie, The Behrend College. References Alivisatos, A.P., 1996. Perspectives on the physical chemistry of semiconductor nanocrystals. J. Phys. Chem. 100, 13226–13239. American Society for Testing and Materials, 2003, Solar Spectral Irradiance Standard Curves, International Standard Curve for Terrestrial Global Air Mass 1.5 ASTM G173–03. . Bailey, S.T., Lokey, G.E., Hanes, M.S., Shearer, J.D.M., McLafferty, J.B., Beaumont, G.T., Baseler, T.T., Layhue, J.M., Broussard, D.R., Zhang, Y.–Z., Wittmershaus, B.P., 2007. Optimized excitation energy transfer in a three-dye luminescent solar concentrator. Solar Energy Mat. and Solar Cells 91, 67–75. Barashkov, N.N., Gunder, O.A., 1996. Fluorescent Polymers. Ellis Horwood Limited, New York. BASF, 1997. . Chatten, A.J.K., Barnham, W.J., Buxton, B.F., Ekins-Daukes, N.J., Malik, M., 2003. A new approach to modeling quantum dot concentrators. Solar Energy Mat. and Solar Cells 75, 363–371. Chatten, A.J.K., Barnham, W.J., Buxton, B.F., Ekins-Daukes, N.J., Malik, M., 2004. Quantum dot solar concentrators. Semiconductors 38, 909–917. de Mello Donega´, C., Hickey, S.G., Wuister, S.F., Vanmaekelbergh, D., Meijerink, A., 2003. Single-step synthesis to control the photoluminescence quantum yield and size dispersion of CdSe nanocrystals. J. Phys. Chem. B 107, 489–496. Evident Technologies, 2006 and 2008, personal communications. Evident Technologies, 2007. .

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