- Email: [email protected]
Optimized excitation energy transfer in a three-dye luminescent solar concentrator
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 91 (2007) 67–75 www.elsevier.com/locate/solmat
Optimized excitation energy transfer in a three-dye luminescent solar concentrator Sheldon T. Baileya, Gretchen E. Lokeya, Melinda S. Hanesa, John D.M. Shearerb, Jason B. McLaffertya, Gregg T. Beaumonta, Timothy T. Baselerb, Joshua M. Layhueb, Dustin R. Broussardb, Yu-Zhong Zhangc, Bruce P. Wittmershausa, a School of Science, Pennsylvania State University: Erie, The Behrend College, Erie, PA 16563-0203, USA School of Engineering, Pennsylvania State University: Erie, The Behrend College, Erie, PA 16563-1701, USA c Molecular Probes Inc., 4849 Pitchford Ave., Eugene, OR 97402-9165, USA
b
Received 1 November 2005; received in revised form 17 July 2006; accepted 19 July 2006 Available online 26 September 2006
Abstract The spectral range of sunlight absorbed by a luminescent solar concentrator (LSC) is increased by using multiple dyes. Absorption, fluorescence, and fluorescence excitation spectra, and relative light output are reported for LSCs made with one, two, or three BODIPY dyes in a thin polymer layer on glass. Losses caused by multiple emission and reabsorption events are minimized by optimizing resonance excitation energy transfer between dyes. Increases in the outputs from the multiple-dye LSCs are directly proportional to increases in the number of photons absorbed. The output of the three-dye LSC is 45–170% higher than those of the single-dye LSCs. r 2006 Elsevier B.V. All rights reserved. Keywords: Luminescent solar concentrator; Energy transfer
1. Introduction A luminescent solar concentrator (LSC) is a thin, flat plate of highly fluorescent material that absorbs sunlight and concentrates most of the resulting fluorescence to its edges through total internal reflection (Fig. 1) [1–4]. For example, a clear polymer sheet with an index of refraction of 1.5, when doped with an organic dye, will trap approximately 75% of the dye’s fluorescence. Photovoltaic cells (PVCs), attached to the edges with an index matching material, will absorb the fluorescence converting it to electricity. The primary motivation in developing LSCs is to lower the cost of solar energy conversion. In a PVC-based collection system, expensive PVCs make up the solar collection area, whereas in a LSC system, the inexpensive plate material fills this area thus lowering the cost. LSCs Corresponding author. Tel.: +1 814 898 6476; fax: +1 814 898 6213.
E-mail address: [email protected] (B.P. Wittmershaus). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.07.011
are omnidirectional collectors and therefore do not require expensive positioning systems. The fluorescent materials used in LSCs are wavelength shifters, emitting photons of lower energy than those absorbed. This results in a loss of energy, but not of quanta, as long as the emitted photons have energies greater than the band gap energy of the PVC. Shifting ultraviolet and blue photons into the red helps as silicon PVCs are generally more efficient in the red. Many types of fluorescent materials have been studied for use in LSCs [1–4] including organic dyes, inorganic phosphors, and, more recently, quantum dots [5,6]. Different designs for LSCs have also been tried, such as uniformly doped polymer sheet and sol–gels, thin-films on glass and polymer plates, and even liquid LSCs [1–4,7–14]. Commercial application of LSCs has been hampered by three major problems. Many fluorescent materials do not absorb broadly over the visible region of the solar spectrum, thus limiting the amount of light converted. Also, numerous promising materials photodegrade too quickly, limiting the
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
68
SUN
Photovoltaic Cell Incident Light
15 µm
The work presented in this paper illustrates the viability of improving the range of absorption of LSCs through the use of multiple organic dyes connected in a FRET network (Fig. 1). Our results illustrate the full potential of a highly efficient FRET network composed of three dyes in a LSC. The efficiency of FRET between dyes is documented using two types of measurements. The performances of multipledye LSCs are compared to single-dye LSCs made using the same dyes at the same optical densities and concentrations found in the multiple-dye plates. 2. Materials and methods
White Scattering Surface Fig. 1. Light collection in a multiple-dye LSC. Cut-away edge view illustrates how incident sunlight is absorbed by the first dye, dye A (K), then transferred by fluorescence resonance energy transfer (FRET) ( ) to the next dye, dye B (’), and finally to the last dye in the transfer chain, dye C (m). Seventy five percent of the emission from dye C (-) is trapped within the plate and guided to the photovoltaic cell at the edge via total internal reflection. Light not absorbed in the first past through is reflected back into the plate by a white scattering surface just below the LSC plate.
useful lifetime of these devices. Finally, overlap in the absorption and fluorescence spectra of the fluorophore causes reabsorption losses that increase as the size of the LSC increases [11,12]. To try to resolve the problem of limited absorption, LSCs have been made using many dyes that absorb at different locations in the solar spectrum. One approach used many plates, each with its own dye, each connected separately to a PVC [15]. This design required as many plates as there were dyes, greatly increasing the quantity of materials needed and decreasing the concentration factor compared to a single plate. It is an effective design for indoor lighting applications of LSCs where a broad band output is needed [13,14]. Other approaches included multiple-layer films on one plate [16], each containing its own dye, and single plates randomly doped with many dyes [17,18]. These schemes all relied on fluorescence from one dye being reabsorbed by the other dyes. This increased reabsorption losses. All three of these designs also required that all the dyes have very high fluorescence quantum yields and excellent photostability. Schwartz et al. [17] first realized that while these multiple-dye designs did improve performance when compared to single-dye LSCs, the full advantage of using multiple dyes was not being realized. They proposed using fluorescence resonance excitation energy transfer (FRET) [19,20] between the dyes to optimize performance but were not able to prove the concept experimentally. The FRET strategy was applied to LSCs used for solar energy collection where PVCs convert the fluorescence into electricity. Use of FRET in LSCs for lighting applications [11–14] probably has limited, if any, value.
The organic dyes used in this research are three different derivatives of the parent molecule, 4,4-difluoro-4-bora3a,4a-diaza-s-indacene (BODIPYs), obtained from our collaborative partner, Molecular Probes Inc. They are BODIPY 494/505, BODIPY 535/558, and BODIPY 564/ 591 to be referred to in this paper as dyes A, B, and C, respectively, to remain consistent with previous work [21–23]. Their absorption coefficients are 65600, 76400, and 78000 cm1M1, respectively. This and other pertinent physical and spectral properties of these dyes have been previously reported [21,24–26]. These dyes are characterized by strong absorption peaks in the visible region, high fluorescence quantum yields, weak interaction among like molecules, and moderate-to-small overlaps of their individual absorption and fluorescence spectra making them excellent candidates for use in LSCs. LSCs can be made from a wide variety of materials. For this study, we focused our attention on LSCs made of thin films of a methyl acrylate/ethyl methacrylate co-polymer (n ¼ 1.49) that are solvent cast onto clear glass substrates (n ¼ 1.52). Thin films have the advantages of being simple to fabricate and able to accommodate the high dye concentrations needed for efficient FRET to occur. To make dye-doped polymer films, dyes were obtained in crystalline form, weighed, and then dissolved in toluene to make stock solutions that were stored in the dark until further use. Similarly, stock solutions of polymer in toluene were also made. After all solutions were thoroughly mixed, the concentrations of dye in each of the stock dye solutions were calculated using the Beer–Lambert Law. Solution absorption spectra were measured from 350 to 650 nm with a bandwidth resolution of 1 nm using a UV/visible absorption spectrometer (Cary Win-UV/VIS BIO 300 Spectrophotometer). Dye–polymer solutions were made by adding a specific volume of a stock dye solution to a specific volume of the stock polymer solution to yield the target dye concentration of 1 102 M in the polymer film. Six solutions were mixed in all, one for each individual dye and three for each combination of the different dyes. Each dye–polymer solution was stirred in a vortex mixer for 20 min and stored in the dark for 24 h before casting. Thin films were statically cast by pipetting a specified volume of a dye–polymer solution onto a glass substrate. After drying, the optical density (OD) of each film was measured.
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
The thin films were designed to maintain the same OD for each dye in every plate. For example, the LSC plate that contained dyes A, B, and C (ABC-LSC) had the same OD for dye A as the LSC plate that contained only dye A (A-LSC). The OD’s for the dyes were chosen to achieve high absorption of light at their peak values. They were reasonable and experimentally convenient choices for LSCs, but may not be the optimum ODs for maximum LSC performance. All absorption spectra were measured directly from films deposited on 10 cm 10 cm 0.2 cm glass substrates. For fluorescence measurements, the LSC plates could not be used because of reabsorption artifacts [27]. To prevent these artifacts, fluorescence and fluorescence excitation spectra were measured from thin films with ODs less than 0.05 that were cast on glass slides of dimensions 3 cm 1 cm 0.05 cm.These films were cast from the same dye–polymer solutions used for the 10 cm 10 cm 0.2 cm plates. The thicknesses of the films were calculated via the Beer–Lambert Law to be approximately 15 mm for the 10 cm 10 cm 0.2 cm LSC plates and 0.5 mm for films on glass slides used to measure fluorescence spectra. Fully corrected fluorescence and fluorescence excitation spectra were measured with a research-grade fluorimeter (Photon Technology International, Model QM-2) as previously described [21–23]. Fluorescence was measured from the front surface of the slides by orienting their normal to 301 from the excitation beam [27]. Appropriate long-pass filters were placed in the emission beam path to block any exciting light from being detected. Collection bandwidths for fluorescence spectra were 4 nm for excitation and 2 nm for emission with the numbers reversed for measuring fluorescence excitation spectra. The anisotropy of fluorescence was measured as previously described [27]. The absorption and fluorescence spectra of multiple-dye films were deconvoluted using a sum of least-squares fitting routine to obtain each dye’s contribution to the spectra. To measure the light output from a 10 cm 10 cm 0.2 cm LSC, one edge of the plate was placed against two PVCs (Solar World, SuperCell) that were positioned next to each other to form a detection area of 12-cm long by 2-cm high. The two PVCs were wired in parallel and connected to a digital voltmeter to detect open circuit current. The other three sides of each LSC were blackened with electrical tape to eliminate artifacts caused by reflections off of these surfaces. A light-tight box containing the PVCs shielded the cells from any excitation light that might have fallen directly on them to less than 0.001 mA generated. Current measurements from the PVC detector reflect relative changes in the light outputs at the edges of the LSCs and should not be confused with measurements of maximum power performance of the LSC–PVC system. The coupling of the PVCs to the LSC plates was not optimized, but was done in a repeatable manner to ensure good reproducibility. Any variations in intensity of emission across the edge of the plate and in the angle of refracted emission are
69
the same for all the LSCs examined within the accuracy of our measurement. A solar simulator (Spectral Energy Corp.) was used as the excitation source for measuring the outputs from the LSCs. It emitted an AM 1.5 global solar spectrum with a spatially and temporally averaged intensity of 68 mW/cm2. A titanium oxide white background of the same area was placed 2 mm below a LSC plate inserted into the PVC detector system. This allowed the incident light to make a second pass through the LSC, making the effective OD approximately double that of the plate’s measured absorption spectrum [3]. The percentage of incident light absorbed by a LSC was calculated using its absorption spectrum, its effective OD, and the AM 1.5 global solar spectrum [28]. The percent efficiency of FRET from one dye to another was determined by taking the number of excitations created in the dye being transferred to (the acceptor) and dividing it by the number of photons absorbed by the dye doing the transferring (the donor) [22,23]. The number of excitations created in the acceptor dye was measured by the fluorescence excitation spectrum of emission from the acceptor dye. The percent efficiency was calculated by normalizing the fluorescence excitation and absorption spectra at a wavelength where only the acceptor had appreciable absorption and then dividing the former by the latter. By definition, a photon absorbed directly by the acceptor was equivalent to 100% energy transfer, so this is why the two curves were normalized here [22,23]. A second measure of FRET was determined through the deconvolution of the fluorescence spectrum into components from each dye when using exciting light absorbed primarily by the donor. 3. Results The spectral properties of dyes A, B, and C in a polymer thin-film are presented in Fig. 2 and Table 1. The absorption spectrum for each of these dyes is characterized by a large primary peak and a small secondary peak to the blue of the primary peak with approximately 10% of its height (Fig. 2). Table 1 describes the individual absorption and fluorescence peaks of dyes A, B, and C. The absorption bandwidths are 30, 38, and 50 nm for dyes A, B, and C, respectively. The Stokes shifts between absorption and fluorescence peaks of the individual dyes increase with increasing peak wavelength with dye A being 9 nm, dye B, 19 nm, and dye C, 32 nm. The profiles of the absorption spectra of the dyes are independent of the concentration of the dyes in the film over the range examined. Even at high concentrations in the polymer thinfilm, the dyes remain very fluorescent. Fig. 2 illustrates that the spectral overlaps of the donor’s fluorescence spectrum and acceptor’s absorption spectrum are very large for the dye FRET combinations A–B and B–C. Of the three dyes, dye C has the smallest probability of absorbing its own fluorescence due to the small overlap of its absorption and
ARTICLE IN PRESS
Fluorescence (a.u.) and Absorption
1.6
2
1.2
1.5
0.8
1
0.4
0.5
0 350
400
450
500
550
600
Photon Flux (photons m-2 s-1 nm-1)
S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
70
0 650
Wavelength (nm) Fig. 2. Absorption and fluorescence spectra (with excitation at 445, 505, and 540 nm, respectively) of the individual dyes A, B, and C in a polymer thin) and fluorescence ( ) spectra are plotted with decreasing line widths to film plotted with the photon flux density of the sun. Absorption ( distinguish dyes A, B, and C, respectively. The solar photon flux ( ) is for AM 1.5 Global [28].
Table 1 Performance of single and multiple-dye LSC plates Dye(s)
labs (lem) (nm)
% light abs. (350–650 nm)
Iout74% (mA)
Iout/Iout (A-LSC)
% light abs./% light abs. (A-LSC)
A B C AB BC ABC
500 (507) 537 (555) 571 (602) 500, 537 (555) 540, 571 (602) 500, 540, 571 (602)
26.3 36.1 50.0 46.7 63.1 70.4
18.9 25.5 35.1 34.6 43.0 51.0
1.00 1.35 1.86 1.83 2.28 2.70
1.00 1.37 1.90 1.78 2.40 2.68
fluorescence spectra, with this overlap being progressively larger in dye B and dye A. Since the dyes being examined were in a solid matrix, fluorescence from these dye molecules was not randomly polarized by molecular motion. Artifacts caused by polarized emission passing through monochromators could be present [27]. The measured fluorescence anisotropies for each of the individual dyes in a polymer thin-film were in the range of 0.02–0.04. This negligible anisotropy in the emission of each dye was caused by rapid FRET between identical dye molecules facilitated by their high concentration. Likewise in the multiple-dye samples, polarization artifacts were not a problem because FRET ensured that the emission was also randomized. To optimize FRET between dyes in the multiple-dye plates, we started with thin films made with dyes A and B and measured the efficiency of transfer from dye A to dye B while varying their concentrations from 1 103 to 7 102 M. It was determined that maximum FRET was achieved when dyes A and B were at the concentration of 1 102M70.2 M. Very high FRET was also observed for thin films containing dyes B and C at this concentration. Thus for all data taken on polymer thin-films containing
one, two, or three dyes, the concentration of each dye was 1 102M70.2 M. The light outputs of the 10 cm 10 cm 0.2 cm LSCs containing only one dye are given in Table 1. The ODs of dyes A, B, and C are 0.8, 1.0 and 1.2, respectively. Of the three single-dye LSCs, the plate with dye C (C-LSC) has the largest output. The outputs of the B-LSC and C-LSC are 35 and 86% higher than that of A-LSC, respectively. The differences in outputs are consistent with the relative percentages of photons absorbed by each dye (Table 1). The two-dye LSC containing dyes A and B (AB-LSC) was matched to the single-dye LSCs by ensuring dyes A and B had optical densities of 0.8 and 1.0, respectively (Fig. 3). The AB-LSC absorbs 47% of the available photons in the region from 350 to 650 nm. Its output of 34.6 mA, is 83% greater than the A-LSC and 36% greater than the BLSC (Table 1). Fig. 3 illustrates that most of the fluorescence comes from dye B, even though the 465-nm excitation used to excite the plate is primarily absorbed by dye A. Deconvolution of the emission spectrum using a sum of least-squares fitting (Fig. 3) reveals that dye B contributes 9971% of the total emission while dye A contributed only 170.5%. This indicates excellent FRET
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
1.2
1.2 1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
400
450
500
550
Fluorescence (a.u.)
Absorption
1
0 350
71
0 650
600
Wavelength (nm)
1.4 100
1.2
80
1 0.8
60
0.6 40
FRET (%)
Fluorescence Excitation (a.u.) and Absorption
Fig. 3. Absorption ( ) and fluorescence ( ) spectra of dyes A and B in a LSC plate. The absorption spectrum is deconvoluted into the contributions from dyes A and B ( ). An excitation wavelength of 465 nm was used to collect the emission data.
0.4 20
0.2 0 350
0 400
450
500
550
Wavelength (nm) Fig. 4. Absorption ( ) and fluorescence excitation ( ) spectra and percentage FRET ( excitation spectrum was measured observing emission from dye B at 580 nm.
from dye A to dye B in the AB-LSC. This is verified and further quantified by comparing the fluorescence excitation and absorption spectra associated with the AB-LSC when normalized where only dye B absorbs (540 nm) (Fig. 4). The two spectra are nearly identical. The average percentage FRET over the wavelength region of dye A’s absorption from 420 to 525 nm is 9871% (Fig. 4). The other two-dye LSC contains dyes B and C at optical densities of 1.0 and 1.2, respectively, again to correspond with the single-dye plates (Fig. 5). The BC-LSC absorbs 63% of the available photons in the region from 350 to 650 nm with an output of 43.0 mA (Table 1). This is 69% greater than the B-LSC and 23% greater than the C-LSC. Fig. 5 illustrates that nearly all of the fluorescence comes from dye C when the excitation light is primarily absorbed by dye B (475 nm). Only 170.5% of the emission is from
) of dyes A and B in a LSC plate. The fluorescence
dye B indicating excellent FRET from dye B to dye C. The absorption and fluorescence excitation spectra of the BCLSC are very similar (Fig. 6), but have larger variations compared to those of the AB-LSC (Fig. 4). The average percentage FRET over the wavelength region of dye B’s absorption from 460 to 570 nm is 9971% (Fig. 6), which agrees with the emission spectrum analysis. The three-dye LSC containing dyes A, B, and C (ABCLSC) (Fig. 7) has the same individual dye concentrations (1 102M702 M) and optical densities for dyes A, B, and C (0.8, 1.0, and 1.2, respectively) as the other LSCs examined. The ABC-LSC absorbs 70% of the available photons in the region from 350 to 650 nm. cWith an output of 51.0 mA (Table 1), the ABC-LSC’s output is 47% greater than the two-dye LSC, AB-LSC, and 19% greater than the BC-LSC. Fig. 7 illustrates that almost all of the
ARTICLE IN PRESS
Absorption
1.6
1.6
1.4
1.4
1.2
1.2
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0 350
400
450
500
550
600
Fluorescence (a.u.)
S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
72
0 650
Wavelength
100
1.6
80 1.2 60 0.8
FRET (%)
Fluorescence Excitation (a.u.) and Absorption
Fig. 5. Absorption ( ) and fluorescence ( ) spectra of dyes B and C in a LSC plate. The absorption spectrum is deconvoluted into the contributions from dyes B and C ( ). An excitation wavelength of 520 nm was used to collect the emission data.
40 0.4
0 350
20
0 400
450
500
550
600
Wavelength (nm) Fig. 6. Absorption ( ) and fluorescence excitation ( ) spectra and percentage FRET ( excitation spectrum was measured observing emission from dye C at 670 nm.
fluorescence originates from dye C, even though dye A is absorbing most of the excitation at 465 nm.The contributions to the total fluorescence from the ABC-LSC from dyes A, B, and C are 170.5%, 170.5% and 9871%, respectively. The absorption and fluorescence excitation spectra of the ABC-LSC match very well (Fig. 8). The average percentage FRET from dyes A and B to dye C over dyes A and B’s absorption from 420 to 570 nm is 9871% (Fig. 8). The results from the analysis of the emission spectrum and comparing the fluorescence excitation spectrum to the absorption spectrum are in excellent agreement and indicate that the ABC-LSC forms a very effective FRET network with nearly complete excitation transfer to the final element, dye C.
) of dyes B and C in a LSC plate. The fluorescence
4. Discussion 4.1. Performance of a multiple-dye LSC The performance values given in Table 1 document the advantages of multiple-dye LSCs with efficient FRET between dyes over that of single-dye LSCs. The ABC-LSC benefits from a wide spectral region of absorption and the highly efficient FRET process, which delivers almost all the excitations created into the last dye, dye C. It absorbs more than two and a half times more light in the 350- to 650-nm region with a commensurate increase in output current when compared to A-LSC and outperforms the best singledye LSC from this study (C-LSC) by 45%. The measurements of FRET between the dyes obtained by comparison
ARTICLE IN PRESS 1.6
1.6
1.4
1.4
1.2
1.2
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0 350
400
450
500
550
600
73
Fluorescence (a.u.)
Absorption
S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
0 650
Wavelength (nm)
1.6 100
1.4 1.2
80
1 60
0.8 0.6
40
FRET (%)
Fluorescence Excitation (a.u.) and Absorption
Fig. 7. Absorption ( ) and fluorescence ( ) spectra of dyes A, B, and C in a LSC plate. The absorption spectrum is deconvoluted into the ). An excitation wavelength of 465 nm was used to collect the emission data. contributions from dyes A, B, and C (
0.4 20 0.2 0 350
0 400
450
500
550
600
Wavelength (nm) Fig. 8. Absorption ( ) and fluorescence excitation ( ) spectra and percentage FRET ( excitation spectrum was measured observing emission from dye C at 670 nm.
of absorption and fluorescence excitation spectra and from deconvolutions of fluorescence spectra are in excellent agreement indicating nearly 100% FRET between the dyes in all of the multiple-dye LSC plates (Figs. 4–8). The matching of the relative output currents to the percentage of light absorbed when referenced to the performance of ALSC (Table 1) further verifies that the high FRET efficiencies in the multiple-dye LSC plates permit them to take full advantage of every photon absorbed. The multiple-dye plates created for this study experimentally achieve the potential of an idea first published by Swartz et al. [17]. This research group studied a solid LSC made with two dyes [17,18] and a liquid, three-dye LSC [18]. Although they were seeking to create LSCs with efficient FRET between the dyes, the authors commented that the concentrations of dyes in their LSCs (p104 M)
) of dyes A, B, and C in a LSC plate. The fluorescence
were not high enough for FRET ‘‘to be dominant’’ [17]. Evidence of some FRET occurring in the two-dye LSC was based solely on fluorescence spectra taken from the edges of the plate [17]. This data cannot be used to identify FRET between the dyes. The high optical density and long pathlength of the plate used to make this measurement would have ensured that nearly all the emission from the donor dye would be completely reabsorbed by the acceptor dye. This explains why the fluorescence spectrum measured from the edge of the LSC showed no emission from the donor dye. It probably was all reabsorbed. Emission measured from the surface had comparable contributions from both dyes [17], but likely had some distortion due to reabsorption effects making it impossible to determine how much, if any, FRET was occurring between the dyes.
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
74
4.2. Design considerations for a multiple-dye LSC There are many issues to address in designing an optimal LSC based on efficient FRET between dyes. The BODIPY dyes A, B, and C generally fit the criteria well. The exchange of excitation energy between dyes is described by Fo¨rster [19]. The efficiency of this transfer, E, goes as [19,20,27] E¼
R60
R60 , þ R6
(1)
where R is the distance between the donor and acceptor dye molecules and R0, the Fo¨rster radius, is defined as the distance between donor and acceptor at which E ¼ 0.5 or 50%. The Fo¨rster radius is [19,20,27]: R60 ¼
9000 lnð10Þk2 fD J ¼ 8:7853 1023 ðk2 n4 fD JÞA˚ 6 , 128p5 n4 N 0 (2)
where J is related to the overlap integral of the donor’s fluorescence and the acceptor’s absorption, k is the relative transition dipole orientation factor (k2 ¼ 2/3 for a random distribution), fD is the fluorescence quantum yield of the donor, n is the index of refraction, and N0 is Avogadro’s number. Eq. (2) shows that efficient FRET is favored by dyes that have high fluorescence quantum yields. This is not critical for the first few dyes in the transfer network, since the FRET process can dominate even for moderate yields, but the last dye must have a high yield since the only useful pathway for its excitations is fluorescence. The fluorescence quantum yields of dyes B and C in polystyrene microspheres are 95% and 10172%, respectively [21]. The fluorescence quantum yield of dye A is 55% in microspheres and 100% in ethanol [21]. The agreement between the relative output current ratios and relative absorption of sunlight with one-, two-, and three-dye plates (Table 1) indicates that dyes A, B, and C have approximately the same fluorescence quantum yields when in the co-polymer. We infer that all three dyes have high fluorescence quantum yields in the LSC plates based on the fact that dyes A, B, and C have high yields and that the plates appear to be visually very bright. Since the last dye in the transfer network is doing all the fluorescing, the absorption of its own emission must be minimized to mitigate reabsorption losses. Self-absorption in the previous dyes is irrelevant since the FRET pathway is energetically favored over fluorescence from these dyes. But FRET from dyes A and B to dye C does not prevent reabsorption losses in emission from dye C. To maximize the performance of a FRET LSC, the last dye in the network needs to have the overlap between its absorption and fluorescence spectra to be as small as possible [13,14]. Dye C has the largest Stokes shift of the three dyes and so it is a good candidate as the final dye in the network.
Choosing dyes to maximize FRET in a LSC, while optimizing its output, requires attaining the broadest spectral absorbance by all of the dyes and maintaining adequate overlap of the donor’s fluorescence and the acceptor’s absorption in each transfer path (Eq. (2)). Since silicon PVCs ideally convert one photon of sunlight into one electron of current, the LSC must have a broad absorption, specifically at redder wavelengths where there are more solar photons. It does not matter that ultraviolet photons have more energy than red photons since the PVC only converts any absorbed photon into an electron with the band gap energy of silicon [3]. Fig. 3 illustrates that the absorption and fluorescence spectra of dyes A, B, and C are well matched to maximize overlap between the donor and acceptor molecules while providing good spectral coverage to optimize the absorption of light. FRET goes as the inverse sixth power of the distance between the donor and acceptor molecules so spacing is critical (Eq. (1)). The higher the dye concentration, the smaller R and the larger the rate of excitation hopping is from one molecule to the other. This minimizes emission from, or any other loss process in, the higher energy-level dyes and finally results in maximum fluorescence from the lowest energy-level dye molecules. To optimize transfer between different types of dye molecules in the network, it is advantageous for moderate transfer to exist between identical dye molecules, as found in the light-harvesting chlorophyll networks in green plants [29]. The data show no significant polarization of fluorescence from the single-dye slides made of dyes A, B, or C, at concentrations of 1 102 M. This lack of polarized emission from the dyes in a solid proves that there is significant FRET between molecules of the same dye as FRET randomizes fluorescence polarization [27]. Since these single-dye slides are made with the same concentrations used in the multiple-dye LSCs, significant FRET between identical dye molecules is likely in the multipledye LSCs. Another consideration is that the dyes and the polymer used to make the films must be soluble in the same solvent. The dyes must remain as monomers at the high concentrations needed for FRET in the solid polymer to prevent the quenching of excited states by the formation of aggregates. The results indicate that the dyes A, B, and C are highly soluble in toluene and there is no evidence of aggregation occurring in solutions or in the dried films at the concentrations being used. This is consistent with the general observation that BODIPY dyes are weakly interacting. Finally, the ultimate goal of the LSC is to lower the cost of solar energy collection. To this end, designing a multiple-dye LSC for commercial use will need to balance the gain in light absorbed by having multiple dyes to the cost of adding additional dyes to the system. This is one of many production level costs to consider when seeking the largest output to cost ratio in a LSC.
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
5. Conclusions The merit of the strategy of increasing the absorption of solar light by a LSC through the use of many dyes is verified by a marked increase in output of multiple-dye LSCs over single-dye LSCs. Highly efficient FRET between multiple dyes is shown to optimize the output of a multiple-dye LSC. Maximizing the fluorescence quantum yield and minimizing the reabsorption losses of the final dye in the network is of critical importance in defining the optimal performance of the FRET LSC. The challenge in making an efficient multiple-dye LSC is to find dyes that have suitable properties to make such a system work. They must be soluble at high concentrations in the same solvent used to cast the polymer and they must form a good FRET network. Acknowledgments We would like to express our appreciation to Molecular Probes Inc. for providing samples of their dyes for this research. We would also like to thank the many people from Molecular Probes who have contributed their time and ideas in valuable discussions about our research, particularly Drs. Richard and Rosaria Haugland and Dr. Iain Johnson. Our special thanks to Jerry McGraw for his valuable advice and time. This research was supported through grants from the National Science Foundation, the Division of Electronics and Communications Systems, Grants ECS-9906282 and ECS0424153. Additional support was received through undergraduate research grants awarded to STB, GEL, MSH, JDMS, JBM, GTB, and TTB from the Pennsylvania State University: Erie, The Behrend College. References [1] A.M. Hermann, Sol. Energy 29 (1982) 323. [2] R. Reisfeld, C.K. Joergensen, Struct. Bond. 49 (1982) 1. [3] N.N. Barashkov, O.A. Gunder, Fluorescent Polymers, Ellis Horwood Limited, New York, 1996.
75
[4] V.V. Popov, V.N. Yakimenko, J. Appl. Spectrosc. 62 (1995) 573. [5] A.J. Chatten, K.W.J. Barnham, B.F. Buxton, N.J. Ekins-Daukes, M.A. Malik, Sol. Energy Mater. Sol. Cells 75 (2003) 363. [6] A.J. Chatten, K.W.J. Barnham, B.F. Buxton, N.J. Ekins-Daukes, M.A. Malik, Semiconductors 38 (2004) 909. [7] R. Reisfeld, D. Shamrakov, C. Jorgensen, Sol. Energy Mater. Sol. Cells 33 (1994) 417. [8] A.F. Mansour, Polym. Test. 17 (1998) 153. [9] R. Reisfeld, Opt. Mater. 16 (2001) 1. [10] A.F. Mansour, M.G. El-Shaarawy, S.M. El-Bashir, M.K. El-Mansy, M.A. Hammam, Polym. Test. 21 (2002) 277. [11] P.D. Swift, G.B. Smith, J. Franklin, Light to light efficiencies in luminescent solar concentrators. in: Proceedings of the SPIE Conference on Solar Optical Materials, 1999, pp. 21–28. [12] P.D. Swift, G.B. Smith, Appl. Opt. 42 (2003) 5112. [13] A.A. Earp, G.B. Smith, J. Franklin, P. Swift, Sol. Energy Mater. Sol. Cells 84 (2004) 411. [14] A.A. Earp, G.B. Smith, P.D. Swift, J. Franklin, J. Sol. Energy 76 (2004) 655. [15] A. Goetzberger, W. Greubel, Appl. Phys. 14 (1977) 123. [16] P.S. Friedman, SPIE Role of Electro-Optics in Photovoltaic Energy Conversion 248 (1980) 98. [17] B.A. Swartz, T. Cole, A.H. Zewail, Opt. Lett. 1 (1977) 73. [18] J.S. Batchelder, A.H. Zewail, T. Cole, Appl. Opt. 20 (1981) 3733. [19] T. Fo¨rster, in: O. Sinanoslu (Ed.), Modern Quantum Chemistry, Part III, Academic Press, New York, 1965, p. 93. [20] B.W. Van Der Meer, G. Coker, S.-Y. Chen, Resonance Energy Transfer: Theory and Data, VCH Publishers, Inc., New York, 1994. [21] B.P. Wittmershaus, J.J. Skibicki, J.B. McLafferty, Y.-Z. Zhang, S. Swan, J. Fluorescence 11 (2001) 119. [22] D.V. Roberts, B.P. Wittmershaus, Y.-Z. Zhang, S. Swan, M.P. Klinosky, J. Luminescence 79 (1998) 225. [23] B.P. Wittmershaus, T.T. Baseler, G.T. Beaumont, Y.-Z. Zhang, J. Luminescence 96 (2002) 107. [24] R.P. Haugland, H.C. Kang, US Patent No. 4 774 (1988) 339. [25] R.P. Haugland, H.C. Kang, US Patent No. 5 248 (1993) 782. [26] R.P. Haugland, in: B. Herman, K. Jacobson (Eds.), Optical Microscopy for Biology, Wiley-Liss, New York, 1990, p. 143. [27] J. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1983. [28] American Society for Testing and Materials, Solar Spectral Irradiance Standard Curves, international standard curve for Terrestrial Global Air Mass 1.5 ASTM G173-03, 2003, http://rredc.nrel.gov/ solar/spectra/am1.5/ [29] R. van Grondelle, J.P. Dekker, T. Gillbro, V. Sundstrom, Biochim. Biophys. Acta 1187 (1994) 1.
Solar Energy Materials & Solar Cells 91 (2007) 67–75 www.elsevier.com/locate/solmat
Optimized excitation energy transfer in a three-dye luminescent solar concentrator Sheldon T. Baileya, Gretchen E. Lokeya, Melinda S. Hanesa, John D.M. Shearerb, Jason B. McLaffertya, Gregg T. Beaumonta, Timothy T. Baselerb, Joshua M. Layhueb, Dustin R. Broussardb, Yu-Zhong Zhangc, Bruce P. Wittmershausa, a School of Science, Pennsylvania State University: Erie, The Behrend College, Erie, PA 16563-0203, USA School of Engineering, Pennsylvania State University: Erie, The Behrend College, Erie, PA 16563-1701, USA c Molecular Probes Inc., 4849 Pitchford Ave., Eugene, OR 97402-9165, USA
b
Received 1 November 2005; received in revised form 17 July 2006; accepted 19 July 2006 Available online 26 September 2006
Abstract The spectral range of sunlight absorbed by a luminescent solar concentrator (LSC) is increased by using multiple dyes. Absorption, fluorescence, and fluorescence excitation spectra, and relative light output are reported for LSCs made with one, two, or three BODIPY dyes in a thin polymer layer on glass. Losses caused by multiple emission and reabsorption events are minimized by optimizing resonance excitation energy transfer between dyes. Increases in the outputs from the multiple-dye LSCs are directly proportional to increases in the number of photons absorbed. The output of the three-dye LSC is 45–170% higher than those of the single-dye LSCs. r 2006 Elsevier B.V. All rights reserved. Keywords: Luminescent solar concentrator; Energy transfer
1. Introduction A luminescent solar concentrator (LSC) is a thin, flat plate of highly fluorescent material that absorbs sunlight and concentrates most of the resulting fluorescence to its edges through total internal reflection (Fig. 1) [1–4]. For example, a clear polymer sheet with an index of refraction of 1.5, when doped with an organic dye, will trap approximately 75% of the dye’s fluorescence. Photovoltaic cells (PVCs), attached to the edges with an index matching material, will absorb the fluorescence converting it to electricity. The primary motivation in developing LSCs is to lower the cost of solar energy conversion. In a PVC-based collection system, expensive PVCs make up the solar collection area, whereas in a LSC system, the inexpensive plate material fills this area thus lowering the cost. LSCs Corresponding author. Tel.: +1 814 898 6476; fax: +1 814 898 6213.
E-mail address: [email protected] (B.P. Wittmershaus). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.07.011
are omnidirectional collectors and therefore do not require expensive positioning systems. The fluorescent materials used in LSCs are wavelength shifters, emitting photons of lower energy than those absorbed. This results in a loss of energy, but not of quanta, as long as the emitted photons have energies greater than the band gap energy of the PVC. Shifting ultraviolet and blue photons into the red helps as silicon PVCs are generally more efficient in the red. Many types of fluorescent materials have been studied for use in LSCs [1–4] including organic dyes, inorganic phosphors, and, more recently, quantum dots [5,6]. Different designs for LSCs have also been tried, such as uniformly doped polymer sheet and sol–gels, thin-films on glass and polymer plates, and even liquid LSCs [1–4,7–14]. Commercial application of LSCs has been hampered by three major problems. Many fluorescent materials do not absorb broadly over the visible region of the solar spectrum, thus limiting the amount of light converted. Also, numerous promising materials photodegrade too quickly, limiting the
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
68
SUN
Photovoltaic Cell Incident Light
15 µm
The work presented in this paper illustrates the viability of improving the range of absorption of LSCs through the use of multiple organic dyes connected in a FRET network (Fig. 1). Our results illustrate the full potential of a highly efficient FRET network composed of three dyes in a LSC. The efficiency of FRET between dyes is documented using two types of measurements. The performances of multipledye LSCs are compared to single-dye LSCs made using the same dyes at the same optical densities and concentrations found in the multiple-dye plates. 2. Materials and methods
White Scattering Surface Fig. 1. Light collection in a multiple-dye LSC. Cut-away edge view illustrates how incident sunlight is absorbed by the first dye, dye A (K), then transferred by fluorescence resonance energy transfer (FRET) ( ) to the next dye, dye B (’), and finally to the last dye in the transfer chain, dye C (m). Seventy five percent of the emission from dye C (-) is trapped within the plate and guided to the photovoltaic cell at the edge via total internal reflection. Light not absorbed in the first past through is reflected back into the plate by a white scattering surface just below the LSC plate.
useful lifetime of these devices. Finally, overlap in the absorption and fluorescence spectra of the fluorophore causes reabsorption losses that increase as the size of the LSC increases [11,12]. To try to resolve the problem of limited absorption, LSCs have been made using many dyes that absorb at different locations in the solar spectrum. One approach used many plates, each with its own dye, each connected separately to a PVC [15]. This design required as many plates as there were dyes, greatly increasing the quantity of materials needed and decreasing the concentration factor compared to a single plate. It is an effective design for indoor lighting applications of LSCs where a broad band output is needed [13,14]. Other approaches included multiple-layer films on one plate [16], each containing its own dye, and single plates randomly doped with many dyes [17,18]. These schemes all relied on fluorescence from one dye being reabsorbed by the other dyes. This increased reabsorption losses. All three of these designs also required that all the dyes have very high fluorescence quantum yields and excellent photostability. Schwartz et al. [17] first realized that while these multiple-dye designs did improve performance when compared to single-dye LSCs, the full advantage of using multiple dyes was not being realized. They proposed using fluorescence resonance excitation energy transfer (FRET) [19,20] between the dyes to optimize performance but were not able to prove the concept experimentally. The FRET strategy was applied to LSCs used for solar energy collection where PVCs convert the fluorescence into electricity. Use of FRET in LSCs for lighting applications [11–14] probably has limited, if any, value.
The organic dyes used in this research are three different derivatives of the parent molecule, 4,4-difluoro-4-bora3a,4a-diaza-s-indacene (BODIPYs), obtained from our collaborative partner, Molecular Probes Inc. They are BODIPY 494/505, BODIPY 535/558, and BODIPY 564/ 591 to be referred to in this paper as dyes A, B, and C, respectively, to remain consistent with previous work [21–23]. Their absorption coefficients are 65600, 76400, and 78000 cm1M1, respectively. This and other pertinent physical and spectral properties of these dyes have been previously reported [21,24–26]. These dyes are characterized by strong absorption peaks in the visible region, high fluorescence quantum yields, weak interaction among like molecules, and moderate-to-small overlaps of their individual absorption and fluorescence spectra making them excellent candidates for use in LSCs. LSCs can be made from a wide variety of materials. For this study, we focused our attention on LSCs made of thin films of a methyl acrylate/ethyl methacrylate co-polymer (n ¼ 1.49) that are solvent cast onto clear glass substrates (n ¼ 1.52). Thin films have the advantages of being simple to fabricate and able to accommodate the high dye concentrations needed for efficient FRET to occur. To make dye-doped polymer films, dyes were obtained in crystalline form, weighed, and then dissolved in toluene to make stock solutions that were stored in the dark until further use. Similarly, stock solutions of polymer in toluene were also made. After all solutions were thoroughly mixed, the concentrations of dye in each of the stock dye solutions were calculated using the Beer–Lambert Law. Solution absorption spectra were measured from 350 to 650 nm with a bandwidth resolution of 1 nm using a UV/visible absorption spectrometer (Cary Win-UV/VIS BIO 300 Spectrophotometer). Dye–polymer solutions were made by adding a specific volume of a stock dye solution to a specific volume of the stock polymer solution to yield the target dye concentration of 1 102 M in the polymer film. Six solutions were mixed in all, one for each individual dye and three for each combination of the different dyes. Each dye–polymer solution was stirred in a vortex mixer for 20 min and stored in the dark for 24 h before casting. Thin films were statically cast by pipetting a specified volume of a dye–polymer solution onto a glass substrate. After drying, the optical density (OD) of each film was measured.
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
The thin films were designed to maintain the same OD for each dye in every plate. For example, the LSC plate that contained dyes A, B, and C (ABC-LSC) had the same OD for dye A as the LSC plate that contained only dye A (A-LSC). The OD’s for the dyes were chosen to achieve high absorption of light at their peak values. They were reasonable and experimentally convenient choices for LSCs, but may not be the optimum ODs for maximum LSC performance. All absorption spectra were measured directly from films deposited on 10 cm 10 cm 0.2 cm glass substrates. For fluorescence measurements, the LSC plates could not be used because of reabsorption artifacts [27]. To prevent these artifacts, fluorescence and fluorescence excitation spectra were measured from thin films with ODs less than 0.05 that were cast on glass slides of dimensions 3 cm 1 cm 0.05 cm.These films were cast from the same dye–polymer solutions used for the 10 cm 10 cm 0.2 cm plates. The thicknesses of the films were calculated via the Beer–Lambert Law to be approximately 15 mm for the 10 cm 10 cm 0.2 cm LSC plates and 0.5 mm for films on glass slides used to measure fluorescence spectra. Fully corrected fluorescence and fluorescence excitation spectra were measured with a research-grade fluorimeter (Photon Technology International, Model QM-2) as previously described [21–23]. Fluorescence was measured from the front surface of the slides by orienting their normal to 301 from the excitation beam [27]. Appropriate long-pass filters were placed in the emission beam path to block any exciting light from being detected. Collection bandwidths for fluorescence spectra were 4 nm for excitation and 2 nm for emission with the numbers reversed for measuring fluorescence excitation spectra. The anisotropy of fluorescence was measured as previously described [27]. The absorption and fluorescence spectra of multiple-dye films were deconvoluted using a sum of least-squares fitting routine to obtain each dye’s contribution to the spectra. To measure the light output from a 10 cm 10 cm 0.2 cm LSC, one edge of the plate was placed against two PVCs (Solar World, SuperCell) that were positioned next to each other to form a detection area of 12-cm long by 2-cm high. The two PVCs were wired in parallel and connected to a digital voltmeter to detect open circuit current. The other three sides of each LSC were blackened with electrical tape to eliminate artifacts caused by reflections off of these surfaces. A light-tight box containing the PVCs shielded the cells from any excitation light that might have fallen directly on them to less than 0.001 mA generated. Current measurements from the PVC detector reflect relative changes in the light outputs at the edges of the LSCs and should not be confused with measurements of maximum power performance of the LSC–PVC system. The coupling of the PVCs to the LSC plates was not optimized, but was done in a repeatable manner to ensure good reproducibility. Any variations in intensity of emission across the edge of the plate and in the angle of refracted emission are
69
the same for all the LSCs examined within the accuracy of our measurement. A solar simulator (Spectral Energy Corp.) was used as the excitation source for measuring the outputs from the LSCs. It emitted an AM 1.5 global solar spectrum with a spatially and temporally averaged intensity of 68 mW/cm2. A titanium oxide white background of the same area was placed 2 mm below a LSC plate inserted into the PVC detector system. This allowed the incident light to make a second pass through the LSC, making the effective OD approximately double that of the plate’s measured absorption spectrum [3]. The percentage of incident light absorbed by a LSC was calculated using its absorption spectrum, its effective OD, and the AM 1.5 global solar spectrum [28]. The percent efficiency of FRET from one dye to another was determined by taking the number of excitations created in the dye being transferred to (the acceptor) and dividing it by the number of photons absorbed by the dye doing the transferring (the donor) [22,23]. The number of excitations created in the acceptor dye was measured by the fluorescence excitation spectrum of emission from the acceptor dye. The percent efficiency was calculated by normalizing the fluorescence excitation and absorption spectra at a wavelength where only the acceptor had appreciable absorption and then dividing the former by the latter. By definition, a photon absorbed directly by the acceptor was equivalent to 100% energy transfer, so this is why the two curves were normalized here [22,23]. A second measure of FRET was determined through the deconvolution of the fluorescence spectrum into components from each dye when using exciting light absorbed primarily by the donor. 3. Results The spectral properties of dyes A, B, and C in a polymer thin-film are presented in Fig. 2 and Table 1. The absorption spectrum for each of these dyes is characterized by a large primary peak and a small secondary peak to the blue of the primary peak with approximately 10% of its height (Fig. 2). Table 1 describes the individual absorption and fluorescence peaks of dyes A, B, and C. The absorption bandwidths are 30, 38, and 50 nm for dyes A, B, and C, respectively. The Stokes shifts between absorption and fluorescence peaks of the individual dyes increase with increasing peak wavelength with dye A being 9 nm, dye B, 19 nm, and dye C, 32 nm. The profiles of the absorption spectra of the dyes are independent of the concentration of the dyes in the film over the range examined. Even at high concentrations in the polymer thinfilm, the dyes remain very fluorescent. Fig. 2 illustrates that the spectral overlaps of the donor’s fluorescence spectrum and acceptor’s absorption spectrum are very large for the dye FRET combinations A–B and B–C. Of the three dyes, dye C has the smallest probability of absorbing its own fluorescence due to the small overlap of its absorption and
ARTICLE IN PRESS
Fluorescence (a.u.) and Absorption
1.6
2
1.2
1.5
0.8
1
0.4
0.5
0 350
400
450
500
550
600
Photon Flux (photons m-2 s-1 nm-1)
S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
70
0 650
Wavelength (nm) Fig. 2. Absorption and fluorescence spectra (with excitation at 445, 505, and 540 nm, respectively) of the individual dyes A, B, and C in a polymer thin) and fluorescence ( ) spectra are plotted with decreasing line widths to film plotted with the photon flux density of the sun. Absorption ( distinguish dyes A, B, and C, respectively. The solar photon flux ( ) is for AM 1.5 Global [28].
Table 1 Performance of single and multiple-dye LSC plates Dye(s)
labs (lem) (nm)
% light abs. (350–650 nm)
Iout74% (mA)
Iout/Iout (A-LSC)
% light abs./% light abs. (A-LSC)
A B C AB BC ABC
500 (507) 537 (555) 571 (602) 500, 537 (555) 540, 571 (602) 500, 540, 571 (602)
26.3 36.1 50.0 46.7 63.1 70.4
18.9 25.5 35.1 34.6 43.0 51.0
1.00 1.35 1.86 1.83 2.28 2.70
1.00 1.37 1.90 1.78 2.40 2.68
fluorescence spectra, with this overlap being progressively larger in dye B and dye A. Since the dyes being examined were in a solid matrix, fluorescence from these dye molecules was not randomly polarized by molecular motion. Artifacts caused by polarized emission passing through monochromators could be present [27]. The measured fluorescence anisotropies for each of the individual dyes in a polymer thin-film were in the range of 0.02–0.04. This negligible anisotropy in the emission of each dye was caused by rapid FRET between identical dye molecules facilitated by their high concentration. Likewise in the multiple-dye samples, polarization artifacts were not a problem because FRET ensured that the emission was also randomized. To optimize FRET between dyes in the multiple-dye plates, we started with thin films made with dyes A and B and measured the efficiency of transfer from dye A to dye B while varying their concentrations from 1 103 to 7 102 M. It was determined that maximum FRET was achieved when dyes A and B were at the concentration of 1 102M70.2 M. Very high FRET was also observed for thin films containing dyes B and C at this concentration. Thus for all data taken on polymer thin-films containing
one, two, or three dyes, the concentration of each dye was 1 102M70.2 M. The light outputs of the 10 cm 10 cm 0.2 cm LSCs containing only one dye are given in Table 1. The ODs of dyes A, B, and C are 0.8, 1.0 and 1.2, respectively. Of the three single-dye LSCs, the plate with dye C (C-LSC) has the largest output. The outputs of the B-LSC and C-LSC are 35 and 86% higher than that of A-LSC, respectively. The differences in outputs are consistent with the relative percentages of photons absorbed by each dye (Table 1). The two-dye LSC containing dyes A and B (AB-LSC) was matched to the single-dye LSCs by ensuring dyes A and B had optical densities of 0.8 and 1.0, respectively (Fig. 3). The AB-LSC absorbs 47% of the available photons in the region from 350 to 650 nm. Its output of 34.6 mA, is 83% greater than the A-LSC and 36% greater than the BLSC (Table 1). Fig. 3 illustrates that most of the fluorescence comes from dye B, even though the 465-nm excitation used to excite the plate is primarily absorbed by dye A. Deconvolution of the emission spectrum using a sum of least-squares fitting (Fig. 3) reveals that dye B contributes 9971% of the total emission while dye A contributed only 170.5%. This indicates excellent FRET
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
1.2
1.2 1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
400
450
500
550
Fluorescence (a.u.)
Absorption
1
0 350
71
0 650
600
Wavelength (nm)
1.4 100
1.2
80
1 0.8
60
0.6 40
FRET (%)
Fluorescence Excitation (a.u.) and Absorption
Fig. 3. Absorption ( ) and fluorescence ( ) spectra of dyes A and B in a LSC plate. The absorption spectrum is deconvoluted into the contributions from dyes A and B ( ). An excitation wavelength of 465 nm was used to collect the emission data.
0.4 20
0.2 0 350
0 400
450
500
550
Wavelength (nm) Fig. 4. Absorption ( ) and fluorescence excitation ( ) spectra and percentage FRET ( excitation spectrum was measured observing emission from dye B at 580 nm.
from dye A to dye B in the AB-LSC. This is verified and further quantified by comparing the fluorescence excitation and absorption spectra associated with the AB-LSC when normalized where only dye B absorbs (540 nm) (Fig. 4). The two spectra are nearly identical. The average percentage FRET over the wavelength region of dye A’s absorption from 420 to 525 nm is 9871% (Fig. 4). The other two-dye LSC contains dyes B and C at optical densities of 1.0 and 1.2, respectively, again to correspond with the single-dye plates (Fig. 5). The BC-LSC absorbs 63% of the available photons in the region from 350 to 650 nm with an output of 43.0 mA (Table 1). This is 69% greater than the B-LSC and 23% greater than the C-LSC. Fig. 5 illustrates that nearly all of the fluorescence comes from dye C when the excitation light is primarily absorbed by dye B (475 nm). Only 170.5% of the emission is from
) of dyes A and B in a LSC plate. The fluorescence
dye B indicating excellent FRET from dye B to dye C. The absorption and fluorescence excitation spectra of the BCLSC are very similar (Fig. 6), but have larger variations compared to those of the AB-LSC (Fig. 4). The average percentage FRET over the wavelength region of dye B’s absorption from 460 to 570 nm is 9971% (Fig. 6), which agrees with the emission spectrum analysis. The three-dye LSC containing dyes A, B, and C (ABCLSC) (Fig. 7) has the same individual dye concentrations (1 102M702 M) and optical densities for dyes A, B, and C (0.8, 1.0, and 1.2, respectively) as the other LSCs examined. The ABC-LSC absorbs 70% of the available photons in the region from 350 to 650 nm. cWith an output of 51.0 mA (Table 1), the ABC-LSC’s output is 47% greater than the two-dye LSC, AB-LSC, and 19% greater than the BC-LSC. Fig. 7 illustrates that almost all of the
ARTICLE IN PRESS
Absorption
1.6
1.6
1.4
1.4
1.2
1.2
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0 350
400
450
500
550
600
Fluorescence (a.u.)
S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
72
0 650
Wavelength
100
1.6
80 1.2 60 0.8
FRET (%)
Fluorescence Excitation (a.u.) and Absorption
Fig. 5. Absorption ( ) and fluorescence ( ) spectra of dyes B and C in a LSC plate. The absorption spectrum is deconvoluted into the contributions from dyes B and C ( ). An excitation wavelength of 520 nm was used to collect the emission data.
40 0.4
0 350
20
0 400
450
500
550
600
Wavelength (nm) Fig. 6. Absorption ( ) and fluorescence excitation ( ) spectra and percentage FRET ( excitation spectrum was measured observing emission from dye C at 670 nm.
fluorescence originates from dye C, even though dye A is absorbing most of the excitation at 465 nm.The contributions to the total fluorescence from the ABC-LSC from dyes A, B, and C are 170.5%, 170.5% and 9871%, respectively. The absorption and fluorescence excitation spectra of the ABC-LSC match very well (Fig. 8). The average percentage FRET from dyes A and B to dye C over dyes A and B’s absorption from 420 to 570 nm is 9871% (Fig. 8). The results from the analysis of the emission spectrum and comparing the fluorescence excitation spectrum to the absorption spectrum are in excellent agreement and indicate that the ABC-LSC forms a very effective FRET network with nearly complete excitation transfer to the final element, dye C.
) of dyes B and C in a LSC plate. The fluorescence
4. Discussion 4.1. Performance of a multiple-dye LSC The performance values given in Table 1 document the advantages of multiple-dye LSCs with efficient FRET between dyes over that of single-dye LSCs. The ABC-LSC benefits from a wide spectral region of absorption and the highly efficient FRET process, which delivers almost all the excitations created into the last dye, dye C. It absorbs more than two and a half times more light in the 350- to 650-nm region with a commensurate increase in output current when compared to A-LSC and outperforms the best singledye LSC from this study (C-LSC) by 45%. The measurements of FRET between the dyes obtained by comparison
ARTICLE IN PRESS 1.6
1.6
1.4
1.4
1.2
1.2
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0 350
400
450
500
550
600
73
Fluorescence (a.u.)
Absorption
S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
0 650
Wavelength (nm)
1.6 100
1.4 1.2
80
1 60
0.8 0.6
40
FRET (%)
Fluorescence Excitation (a.u.) and Absorption
Fig. 7. Absorption ( ) and fluorescence ( ) spectra of dyes A, B, and C in a LSC plate. The absorption spectrum is deconvoluted into the ). An excitation wavelength of 465 nm was used to collect the emission data. contributions from dyes A, B, and C (
0.4 20 0.2 0 350
0 400
450
500
550
600
Wavelength (nm) Fig. 8. Absorption ( ) and fluorescence excitation ( ) spectra and percentage FRET ( excitation spectrum was measured observing emission from dye C at 670 nm.
of absorption and fluorescence excitation spectra and from deconvolutions of fluorescence spectra are in excellent agreement indicating nearly 100% FRET between the dyes in all of the multiple-dye LSC plates (Figs. 4–8). The matching of the relative output currents to the percentage of light absorbed when referenced to the performance of ALSC (Table 1) further verifies that the high FRET efficiencies in the multiple-dye LSC plates permit them to take full advantage of every photon absorbed. The multiple-dye plates created for this study experimentally achieve the potential of an idea first published by Swartz et al. [17]. This research group studied a solid LSC made with two dyes [17,18] and a liquid, three-dye LSC [18]. Although they were seeking to create LSCs with efficient FRET between the dyes, the authors commented that the concentrations of dyes in their LSCs (p104 M)
) of dyes A, B, and C in a LSC plate. The fluorescence
were not high enough for FRET ‘‘to be dominant’’ [17]. Evidence of some FRET occurring in the two-dye LSC was based solely on fluorescence spectra taken from the edges of the plate [17]. This data cannot be used to identify FRET between the dyes. The high optical density and long pathlength of the plate used to make this measurement would have ensured that nearly all the emission from the donor dye would be completely reabsorbed by the acceptor dye. This explains why the fluorescence spectrum measured from the edge of the LSC showed no emission from the donor dye. It probably was all reabsorbed. Emission measured from the surface had comparable contributions from both dyes [17], but likely had some distortion due to reabsorption effects making it impossible to determine how much, if any, FRET was occurring between the dyes.
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
74
4.2. Design considerations for a multiple-dye LSC There are many issues to address in designing an optimal LSC based on efficient FRET between dyes. The BODIPY dyes A, B, and C generally fit the criteria well. The exchange of excitation energy between dyes is described by Fo¨rster [19]. The efficiency of this transfer, E, goes as [19,20,27] E¼
R60
R60 , þ R6
(1)
where R is the distance between the donor and acceptor dye molecules and R0, the Fo¨rster radius, is defined as the distance between donor and acceptor at which E ¼ 0.5 or 50%. The Fo¨rster radius is [19,20,27]: R60 ¼
9000 lnð10Þk2 fD J ¼ 8:7853 1023 ðk2 n4 fD JÞA˚ 6 , 128p5 n4 N 0 (2)
where J is related to the overlap integral of the donor’s fluorescence and the acceptor’s absorption, k is the relative transition dipole orientation factor (k2 ¼ 2/3 for a random distribution), fD is the fluorescence quantum yield of the donor, n is the index of refraction, and N0 is Avogadro’s number. Eq. (2) shows that efficient FRET is favored by dyes that have high fluorescence quantum yields. This is not critical for the first few dyes in the transfer network, since the FRET process can dominate even for moderate yields, but the last dye must have a high yield since the only useful pathway for its excitations is fluorescence. The fluorescence quantum yields of dyes B and C in polystyrene microspheres are 95% and 10172%, respectively [21]. The fluorescence quantum yield of dye A is 55% in microspheres and 100% in ethanol [21]. The agreement between the relative output current ratios and relative absorption of sunlight with one-, two-, and three-dye plates (Table 1) indicates that dyes A, B, and C have approximately the same fluorescence quantum yields when in the co-polymer. We infer that all three dyes have high fluorescence quantum yields in the LSC plates based on the fact that dyes A, B, and C have high yields and that the plates appear to be visually very bright. Since the last dye in the transfer network is doing all the fluorescing, the absorption of its own emission must be minimized to mitigate reabsorption losses. Self-absorption in the previous dyes is irrelevant since the FRET pathway is energetically favored over fluorescence from these dyes. But FRET from dyes A and B to dye C does not prevent reabsorption losses in emission from dye C. To maximize the performance of a FRET LSC, the last dye in the network needs to have the overlap between its absorption and fluorescence spectra to be as small as possible [13,14]. Dye C has the largest Stokes shift of the three dyes and so it is a good candidate as the final dye in the network.
Choosing dyes to maximize FRET in a LSC, while optimizing its output, requires attaining the broadest spectral absorbance by all of the dyes and maintaining adequate overlap of the donor’s fluorescence and the acceptor’s absorption in each transfer path (Eq. (2)). Since silicon PVCs ideally convert one photon of sunlight into one electron of current, the LSC must have a broad absorption, specifically at redder wavelengths where there are more solar photons. It does not matter that ultraviolet photons have more energy than red photons since the PVC only converts any absorbed photon into an electron with the band gap energy of silicon [3]. Fig. 3 illustrates that the absorption and fluorescence spectra of dyes A, B, and C are well matched to maximize overlap between the donor and acceptor molecules while providing good spectral coverage to optimize the absorption of light. FRET goes as the inverse sixth power of the distance between the donor and acceptor molecules so spacing is critical (Eq. (1)). The higher the dye concentration, the smaller R and the larger the rate of excitation hopping is from one molecule to the other. This minimizes emission from, or any other loss process in, the higher energy-level dyes and finally results in maximum fluorescence from the lowest energy-level dye molecules. To optimize transfer between different types of dye molecules in the network, it is advantageous for moderate transfer to exist between identical dye molecules, as found in the light-harvesting chlorophyll networks in green plants [29]. The data show no significant polarization of fluorescence from the single-dye slides made of dyes A, B, or C, at concentrations of 1 102 M. This lack of polarized emission from the dyes in a solid proves that there is significant FRET between molecules of the same dye as FRET randomizes fluorescence polarization [27]. Since these single-dye slides are made with the same concentrations used in the multiple-dye LSCs, significant FRET between identical dye molecules is likely in the multipledye LSCs. Another consideration is that the dyes and the polymer used to make the films must be soluble in the same solvent. The dyes must remain as monomers at the high concentrations needed for FRET in the solid polymer to prevent the quenching of excited states by the formation of aggregates. The results indicate that the dyes A, B, and C are highly soluble in toluene and there is no evidence of aggregation occurring in solutions or in the dried films at the concentrations being used. This is consistent with the general observation that BODIPY dyes are weakly interacting. Finally, the ultimate goal of the LSC is to lower the cost of solar energy collection. To this end, designing a multiple-dye LSC for commercial use will need to balance the gain in light absorbed by having multiple dyes to the cost of adding additional dyes to the system. This is one of many production level costs to consider when seeking the largest output to cost ratio in a LSC.
ARTICLE IN PRESS S.T. Bailey et al. / Solar Energy Materials & Solar Cells 91 (2007) 67–75
5. Conclusions The merit of the strategy of increasing the absorption of solar light by a LSC through the use of many dyes is verified by a marked increase in output of multiple-dye LSCs over single-dye LSCs. Highly efficient FRET between multiple dyes is shown to optimize the output of a multiple-dye LSC. Maximizing the fluorescence quantum yield and minimizing the reabsorption losses of the final dye in the network is of critical importance in defining the optimal performance of the FRET LSC. The challenge in making an efficient multiple-dye LSC is to find dyes that have suitable properties to make such a system work. They must be soluble at high concentrations in the same solvent used to cast the polymer and they must form a good FRET network. Acknowledgments We would like to express our appreciation to Molecular Probes Inc. for providing samples of their dyes for this research. We would also like to thank the many people from Molecular Probes who have contributed their time and ideas in valuable discussions about our research, particularly Drs. Richard and Rosaria Haugland and Dr. Iain Johnson. Our special thanks to Jerry McGraw for his valuable advice and time. This research was supported through grants from the National Science Foundation, the Division of Electronics and Communications Systems, Grants ECS-9906282 and ECS0424153. Additional support was received through undergraduate research grants awarded to STB, GEL, MSH, JDMS, JBM, GTB, and TTB from the Pennsylvania State University: Erie, The Behrend College. References [1] A.M. Hermann, Sol. Energy 29 (1982) 323. [2] R. Reisfeld, C.K. Joergensen, Struct. Bond. 49 (1982) 1. [3] N.N. Barashkov, O.A. Gunder, Fluorescent Polymers, Ellis Horwood Limited, New York, 1996.
75
[4] V.V. Popov, V.N. Yakimenko, J. Appl. Spectrosc. 62 (1995) 573. [5] A.J. Chatten, K.W.J. Barnham, B.F. Buxton, N.J. Ekins-Daukes, M.A. Malik, Sol. Energy Mater. Sol. Cells 75 (2003) 363. [6] A.J. Chatten, K.W.J. Barnham, B.F. Buxton, N.J. Ekins-Daukes, M.A. Malik, Semiconductors 38 (2004) 909. [7] R. Reisfeld, D. Shamrakov, C. Jorgensen, Sol. Energy Mater. Sol. Cells 33 (1994) 417. [8] A.F. Mansour, Polym. Test. 17 (1998) 153. [9] R. Reisfeld, Opt. Mater. 16 (2001) 1. [10] A.F. Mansour, M.G. El-Shaarawy, S.M. El-Bashir, M.K. El-Mansy, M.A. Hammam, Polym. Test. 21 (2002) 277. [11] P.D. Swift, G.B. Smith, J. Franklin, Light to light efficiencies in luminescent solar concentrators. in: Proceedings of the SPIE Conference on Solar Optical Materials, 1999, pp. 21–28. [12] P.D. Swift, G.B. Smith, Appl. Opt. 42 (2003) 5112. [13] A.A. Earp, G.B. Smith, J. Franklin, P. Swift, Sol. Energy Mater. Sol. Cells 84 (2004) 411. [14] A.A. Earp, G.B. Smith, P.D. Swift, J. Franklin, J. Sol. Energy 76 (2004) 655. [15] A. Goetzberger, W. Greubel, Appl. Phys. 14 (1977) 123. [16] P.S. Friedman, SPIE Role of Electro-Optics in Photovoltaic Energy Conversion 248 (1980) 98. [17] B.A. Swartz, T. Cole, A.H. Zewail, Opt. Lett. 1 (1977) 73. [18] J.S. Batchelder, A.H. Zewail, T. Cole, Appl. Opt. 20 (1981) 3733. [19] T. Fo¨rster, in: O. Sinanoslu (Ed.), Modern Quantum Chemistry, Part III, Academic Press, New York, 1965, p. 93. [20] B.W. Van Der Meer, G. Coker, S.-Y. Chen, Resonance Energy Transfer: Theory and Data, VCH Publishers, Inc., New York, 1994. [21] B.P. Wittmershaus, J.J. Skibicki, J.B. McLafferty, Y.-Z. Zhang, S. Swan, J. Fluorescence 11 (2001) 119. [22] D.V. Roberts, B.P. Wittmershaus, Y.-Z. Zhang, S. Swan, M.P. Klinosky, J. Luminescence 79 (1998) 225. [23] B.P. Wittmershaus, T.T. Baseler, G.T. Beaumont, Y.-Z. Zhang, J. Luminescence 96 (2002) 107. [24] R.P. Haugland, H.C. Kang, US Patent No. 4 774 (1988) 339. [25] R.P. Haugland, H.C. Kang, US Patent No. 5 248 (1993) 782. [26] R.P. Haugland, in: B. Herman, K. Jacobson (Eds.), Optical Microscopy for Biology, Wiley-Liss, New York, 1990, p. 143. [27] J. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1983. [28] American Society for Testing and Materials, Solar Spectral Irradiance Standard Curves, international standard curve for Terrestrial Global Air Mass 1.5 ASTM G173-03, 2003, http://rredc.nrel.gov/ solar/spectra/am1.5/ [29] R. van Grondelle, J.P. Dekker, T. Gillbro, V. Sundstrom, Biochim. Biophys. Acta 1187 (1994) 1.