We describe Linearly Polarized Luminescent Solar Concentrators (LP-LSCs) to replace conventional, purely absorptive, linear polarizers in energy harvesting applications. As a proof of concept, we align 3-(2-Benzothiazolyl)-N,N-diethylumbelliferylamine (Coumarin 6) and 4-dicyanomethyl-6-dimethylaminostiryl-4H-pyran (DCM) dye molecules linearly in the plane of the substrate using a polymerizable liquid crystal host. We show that up to 38% of the photons polarized on the long axis of the dye molecules can be coupled to the edge of the device for an LP-LSC based on Coumarin 6 with an order parameter of 0.52.
©2010 Optical Society of America
Linear polarizers selectively absorb one polarization component of incident light, making them an important element in a wide variety of optical systems. Here, we describe linear polarizers that do not waste the absorbed light as heat, but rather act as luminescent concentrators [1–8]. Such linearly-polarized luminescent solar concentrators (LP-LSCs) are a promising approach to light harvesting in portable devices with flat panel displays. LP-LSCs can replace or be used in combination with the linear polarizers present in existing display technologies, redirecting previously wasted light to solar cells mounted in the frame of the display. They allow for energy harvesting over the full surface of the display with minimal image distortion or reduction in display brightness.
Linear polarizers are typically composed of stretched poly(vinyl alcohol) films doped with iodine crystals or dichroic, black, organic dyes. They are specifically crucial to liquid crystal displays, but linear polarizers are generally useful for improving contrast ratios in many display technologies. Combined into a circular polarizer, linear polarizers absorb 50% of light incident on a display and ideally, the remaining incident light is absorbed when its polarization reverses rotation upon any reflection from within the display. Incidentally, polarizers also absorb at least 50% of the display’s own light. All this energy is wasted as heat.
Previous work on energy harvesting in displays has investigated the integration of solar cells within organic light emitting device (OLED)-based displays [9, 10]. But the wide bandgap III-V based solar cells best suited for indoor light harvesting are relatively expensive . In addition, solar cells must be positioned behind the display’s light source, and consequently, the solar cells are shadowed by the driving electronics in the backplane. Finally, the efficiency of solar cells decreases markedly under the low light levels typical of indoor applications .
2. Strategy and device layout
Conventional LSCs employ randomly oriented luminescent dye molecules that are embedded in a transparent waveguide. The dyes absorb diffuse light incident on the waveguide, and re-emit these photons isotropically at a lower energy . A fraction of the re-emitted photons are trapped within the waveguide through total internal reflection – identical to the operational principle of an optical fiber. These trapped photons are funneled to the edges of the concentrator where photovoltaic elements are placed to collect the photons and convert them to electrical energy. When the area of the face of the waveguide is larger than the area of the edges, the light can be concentrated, an important attribute for efficient energy harvesting under low light conditions .
Unlike conventional LSCs, Linearly Polarized Luminescent Solar Concentrators (LP-LSCs) have aligned dye molecules, similar to the work presented in Part I of our study on aligned LSCs. In that accompanying paper the dye molecules are aligned perpendicular to the plane of the waveguide to improve the fraction of re-emitted photons that are trapped in the waveguide . In this work we align the dye molecules in a linear fashion in the plane of the waveguide; see Fig. 1 . They have been demonstrated previously with the aim of selectively coupling incident light to solar cells mounted perpendicularly to the polarized axis of an LSC, thereby improving the geometric gain of the LSC at the cost of poor absorption off axis .Here, we consider LP-LSCs as replacements for conventional linear polarizers in energy harvesting applications. As in the previous demonstration, we employ dichroic dye molecules linearly aligned in the plane of the substrate . LP-LSCs preferentially absorb light for which the electric field is parallel to the dipole moment of the dye molecules, causing the optical transmission of the LP-LSC to be linearly polarized, just like in an ordinary polarizer. However, instead of dissipating the absorbed photons as heat, a linearly polarized LSC will funnel the captured photons to photovoltaic elements placed at the edges of the waveguide (see Fig. 1). Hence, this concept allows the photovoltaics to be located in the frame of the display, which minimizes their area, while leaving the entire front surface available for the display.
As a proof of concept, we create and characterize a LP-LSC based on the rod shaped dye molecule, Coumarin 6 (3-(2-Benzothiazolyl)-N,N-diethylumbelliferylamine), within a polymerizable nematic liquid crystal – identical to a typical guest host system [15, 16]. Ultimately, to improve the harvesting of indoor radiation across the visible spectrum, it may be preferable to create a linearly polarized LSC that hosts several dye molecules that cascade in energy. For this purpose we also create and characterize a LP-LSC that employs two dye molecules, 4-dicyanomethyl-6-dimethylaminostiryl-4H-pyran (DCM) and Coumarin 6.
3. Experimental section
The LP-LSCs are created on a 1-mm-thick glass substrate with a refractive index, n = 1.7 (SF10, Schott). The glass substrates are cut with a dicing saw to obtain the desired dimension. The geometric gain of a LSC, G, is defined as the ratio of the face area versus PV area and is given by G = (L / 4d), where L is the length of the LSC and d is the thickness (assuming PV elements are placed on all four edges of a square collector). For measurements of optical quantum efficiency (the fraction of photons coupled to the edges of the LSC), the glass substrates are cut to squares of 2 × 2 cm, while for measurements of the external quantum efficiency (the fraction of incident photons converted to current in solar cells) the substrates are cut to a substrate size of 7.6 × 9.5 cm. The glass is thoroughly cleaned with a detergent solution, DI water and solvents.
To create the alignment layer, a polyimide acid (SE410, Nissan Chemical Industries, LTD) is diluted to a ratio 1:1 with Solvent 25 (Nissan Chemical Industries, LTD), and spin-cast on the clean substrates in air with a ramp of 1000 rpm/sec and a spin speed of 2500 rpm for 30s. Subsequently, the samples are baked on a hotplate in still air for 10 min at 80°C and 60 minutes at 180°C. The coated samples are hand-rubbed with a velvet cloth to introduce alignment in the liquid crystal layer.
The polymerizable nematic liquid crystal host chosen for in this study is Paliocolor 242 (BASF) (see Fig. 2 for the chemical structure). The dyes used for the experiments are Coumarin 6 and DCM (both purchased from Sigma Aldrich). Their structures are also given in Fig. 2. We selected these dyes because they are known to possess a relatively high dichroic ratio  and their photoluminescence efficiency is reasonably high (measured to be 78% and 60%, respectively). Coumarin 6 also possesses a large Stokes shift, which makes this dye especially suitable in an LSC [7, 18, 19]. Some of our experiments are performed with Coumarin 6 as the sole dopant (at a 1% solid weight content), while a second set of experiments contains both Coumarin 6 and DCM (both dyes at a 1% solid weight content).
In the following procedure all percentages of the separate components are given in weights relative to the total weight of the mixture. In a vial, a solution is prepared that contained Paliocolor (30%), Coumarin 6 (0.30%) or both DCM and Coumarin 6 (both at 0.30%). To these powders, toluene is added (68.95%) and gently stirred. As a surfactant BYK-361 (BYK-Chemie) is used (0.15%), which is taken from a pre-prepared solution of 5% BYK-361 dissolved in toluene. Lastly, Irgacure 184 (0.60%, Ciba Chemicals) is added as a photo initiator. When the components are well dissolved the solution is spin-cast on the pre-rubbed substrates. The samples are dried for 3 minutes at room temperature in still air, after which they are placed for 4 minutes on a hotplate at 80 °C (also in still air). The samples are cooled down to room temperature for 1 minute before placing them under a UV lamp (365 nm) for 3 minutes to cure. This spin speed is adopted to yield a film thickness that results in a peak absorption of 78% for light that is polarized parallel to the rubbing direction for the Coumarin 6 LP-LSCs. We plan to optimize the absorption in LP-LSC to maximize the performance of the LP-LSC in future work. This film-thickness is estimated to be 1.1 microns thick through optical modeling. All thin film absorption measurements are obtained using an Aquila spectrophotometer.
4. Device characterization
4.1 Absorption and photoluminescence
The absorption and photoluminescence spectra of the LP-LSCs are presented in Fig. 3 . The absorption is measured both parallel (solid red line) and perpendicular to the rubbing direction (dotted blue line).From these measurements the order parameter, S, defined by S = (A ||-A ┴)/(A || + 2A ┴) , is determined. Here, A || is defined as the absorbance of the sample for incident light polarized parallel to the rubbing direction, and A ┴ is the absorbance for light polarized perpendicular to the rubbing direction. The samples that contained solely Coumarin 6 are found to have an order parameter of 0.52, while the samples that contained both Coumarin 6 and DCM are measured to have an order parameter of 0.45. These ratios are not ideal for commercial applications, but serve instead as a proof of principle. The absorption of the C6-DCM sample is extended relative to the C6 sample but still exhibits dichroism. Optimized dye systems are expected to have considerably better order parameters, but this is outside of the scope of this work.
The photoluminescence (PL) spectra of the C6 and DCM-C6 sample are plotted as dotted green lines in Fig. 3. These spectra are obtained from the face of the device under excitation by a λ = 408 nm laser. The photoluminescence efficiency of the C6 samples is measured with an integrating sphere and found to be 78%, close to literature values . The luminescence of the C6-DCM sample originates from the DCM dyes, confirming that energy is effectively funneled to the DCM molecules through Förster transfer [7, 22]. The PL efficiency of the C6-DCM samples is measured to be 60%, also in close agreement with literature . The similarity between literature and measured PL efficiencies suggests that the dyes are not quenched nor degraded during our device fabrication procedure.
4.2 The optical quantum efficiency
The optical quantum efficiency, OQE, defined as the fraction of photons coupled to the edges of the LSC, is a key performance parameter. It is characterized within an integrating sphere (see Fig. 4a for a schematic representation of the set-up). We discriminate between edge and facial LSC emission by selectively blocking the edge emission with a black marker, which has been tested to block above 98% of the internal reflection. The OQE is determined for light that is polarized parallel and perpendicular to the rubbing direction, i.e. polarized along the long or short axis of the dye molecules. These spectrally resolved measurements use a 150W Xenon lamp that is coupled into a monochromator and chopped at 73 Hz. The photoluminescence for the OQE measurements is detected by a Si photodetector mounted directly on the integrating sphere and which in turn is connected to a lock-in amplifier.
The spectrally resolved OQE of the C6 and the C6-DCM samples is shown in Fig. 4b. The samples used for these measurements are identical to the samples used for the absorption measurements presented in Fig. 3. The C6 sample has a peak OQE of 38% for light that is polarized parallel to the rubbing direction (red dotted line). This efficiency number is the product of the peak absorption of 78% for this polarization (see Fig. 3), a PL efficiency of 78% and a trapping efficiency of 62%. The trapping efficiency is determined within the integrating sphere and discussed in our companion paper . For light that is polarized perpendicular to the rubbing direction (dotted green line) the maximum OQE is measured to be 17%. This lower number results from the lower absorption (33%) within the sample for this polarization (See Fig. 3). The sample that contains both C6 and DCM has a peak OQE of 34% for light that is polarized parallel to the rubbing direction, as can be seen in the right panel of Fig. 4b. This number is the product of an absorption of 93% (Fig. 3 – right panel), a measured photoluminescent efficiency of 60%, and a trapping efficiency of 62%. The perpendicular polarization results in a peak OQE of 18% (dotted green line). When unpolarized light is used as an excitation source the OQE is measured to be 25%, a perfect average of the OQEs measured for the parallel and perpendicular polarized excitation sources (dotted blue line).
4.2 The external quantum efficiency
Next, we examine the performance of the LP-LSCs as a function of concentration factor, G, which we defined above as the ratio of the facial area to the PV area (assuming PV elements are placed on all four edges). A GaAs solar cell from Spectrolab with an external quantum efficiency (electrons out per photon in) at the emission wavelength of Coumarin 6 of 85% is cut into 3.8 cm × 0.34 cm strips. Two of these cells are connected in series, and attached to one of the short edges of the concentrator with index matching fluid (Norland Products)(see Fig. 5a ). The other 3 edges are blackened out with a black marker to prevent indirect luminescence from reaching the solar cell. We test the performance of the external quantum efficiency for LP-LSCs based on Coumarin 6 for which the dipoles are aligned parallel or perpendicular to the solar cell (see Fig. 5a).
Concentration factor-dependent measurements are obtained by directing an excitation beam perpendicular to the LP-LSC so as to create an excitation spot of ~1 mm2, while the distance, d, between the spot and the solar cell is varied; see Fig. 5a. This is an experimentally convenient technique to simulate the performance of a uniformly-illuminated LP-LSCs that collects power from all four edges at different geometric gains. This experimental technique provides a lower bound for performance since the average path-length is slightly longer than a uniformly illuminated LSC . The measured photocurrent is divided by a geometric factor that corrects for the fraction of the total trapped power emitted by a dipole within an angle subtended by the solar cell at each illumination spot position, d; see Fig. 5a. This geometric correction factor must also account for non-isotropic emission from the aligned dipoles. Following the approach for calculating the time-averaged power density of a Hertzian dipole as outlined in our accompanying paper , two different correction factors are derived for a dipole that is either oriented parallel (in the direction) or perpendicular (in the direction) to the solar cell. Integrating the expression for the power density distribution of a dipole oriented along the y-axis over the solid angle from to and from θC to π - θC, where θC is the critical angle and subsequently normalizing by the total power emitted by the dipole which is trapped in the waveguide, yields the following expression for the correction factor for the parallel dipole, :
The external quantum efficiency (EQE) as a function of geometric gain, G, for the LP-LSCs is presented in Fig. 5b. The results obtained for dipoles that are aligned parallel to the solar cell are depicted as a red dotted line, while the perpendicular dipoles are presented as green dots. The EQE is measured at λ = 465 nm. It is observed that the parallel dipoles have a more pronounced roll-off in efficiency with increasing concentration factor, resulting from stronger overlap between the emissive and absorptive dipoles along the path that the photons have to travel towards the solar cell. A uniformly illuminated LP-LSC will have a performance that is the average of the two curves weighted by and , each evaluated at ϕ = 45° (blue line in Fig. 5). The theory yields a ratio of emission to the edges parallel and perpendicular to the dipoles of 2.4:1, respectively. Due to stronger self-absorption for light coupled to the parallel edges, the measured ratio is 2:1.
5. Conclusions and outlook
To conclude, we propose that Linearly Polarized Luminescent Solar Concentrators (LP-LSCs) may replace conventional linear polarizers when energy harvesting in displays is desired. Previous studies have suggested that indoor light harvesting using solar cells can generate in excess of 10 μW/cm2 under typical office lighting conditions (400 lux) . LP-LSCs should be able to generate similar or higher power densities because they concentrate light, which is an advantage for photovoltaic conversion under low light intensity . In addition, they can exploit the full surface area of a display. The technology can be integrated within displays with two modifications to display functionality: there will be some luminescence emitted through the face of the display, and the LP-LSC must be optically isolated to permit optical waveguiding. Although not pursued here, the use of infrared dyes is a promising solution to the first concern, and low index cladding layers may be necessary to prevent facial scattering of guided radiation at LP-LSC surfaces. Future work must also improve the order parameter and extend the wavelength coverage of LP-LSCs to enable energy harvesting across the full visible spectrum. Aligned luminescent polymers and energy transfer from aligned hosts to isotropic guests offer two promising research directions. But the development of efficient dichroic near infrared dye molecules is especially desirable because infrared dye molecules can capture the red portion of the visible spectrum and infrared emission will minimize image distortion due to radiation escaping from the face of the waveguide.
This material is based upon work supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088.
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