Multispectral harvesting rare-earth oxysulphide based highly efficient transparent luminescent solar concentrator☆
Graphical abstract
Multi-wavelength sensitized Gd2O2S:Er,Yb nanophosphor embedded PMMA based LSC exhibits high power conversion efficiency and optical efficiency.
Introduction
In recent times, amongst various green technologies to supply for worldwide energy demand, sunlight harvesting based electrical systems implementing silicon (Si) photovoltaic cells (PV-cells) as prime collectors of solar energy have garnered much attention.1, 2, 3, 4, 5, 6, 7 Conventional solar concentrators comprising of enormous parabolic mirrors and Fresnel lenses worked via focussing direct radiation onto a compact solar cell, thus increasing the sunlight collection by a PV-cell.8 However, assembly of such complex optical concentration systems often comes with substantial space requirements, the constant tracking, and inability to focus diffused light, hence increasing the cost of operation while also being cumbersome.8 In the late 1970s, the LSC was conceptualized as a photon managing waveguide to enhance the power output of the PV-cells and hence offered the reduced cost per unit watt energy.9 Earlier LSC was a transparent slab made of glass coated with a fluorescent dye that absorbed incoming high energy photons and then downconverted them to a specific longer wavelength, which traveled towards the slab edges via total internal reflection (TIR).10 The area of the LSC exposed to sunlight is always higher than the edge of the LSC, where PV-cells are connected, thereby increasing the incident photon flux density onto small area PV-cells, which increases the power generation dramatically.11
Since the inception of LSC in 1976, the predominant area of research in LSC gathering peak attention has been the fluorophore or dye compound utilized for the conversion of light.5 Primarily this is because of the role that a fluorophore plays in determining the power conversion efficiency (ηPCE) and light concentration factor (C or τ). ηPCE is the relation between electrical power output and incident photon flux input, and the light concentration factor is an active optical flux gain coefficient of the LSC edge coupled PV-cell; both are vital in gauging the performance of an LSC.8 If τ is more than unity for a system, then the use of LSC can be hugely beneficial in increasing the photocurrent and photovoltage.5,12,13
Nevertheless, the LSCs with increased ηPCE as well as τ have been astoundingly challenging to obtain due to various shortcomings and limitations.8 The fluorophore used in the LSC fabrication itself imposes most of these limitations, such as reabsorption losses, narrow absorption region, and low photostability.14, 15, 16 However, the creation and design of new LSC geometries like stacked structures, bent LSCs, and others help in evading the impediments mentioned above in LSC applications. Yet, the maximum potential of an LSC device with reduced cost is still not achieved due to the shortcomings of the participating dye.17,18 Organic dyes are most famously used to improve LSC performance, but they suffer from very poor photostability, overlapping excitation-emission regions, and narrow absorption band.8,19, 20, 21, 22 These disadvantages can be alleviated by engineered quantum dots (QDs) such as lead-based perovskites QDs (PbS), heavy metal-based QDs (CdSe/CdZnS), core-shell structured g-QDs (CdSe/CdSeS/CdS) and others, which provide significant quantum efficiency and no emission reabsorption; but suffer from the substantial cost of preparation and very high toxicity.6,10,23, 24, 25, 26, 27, 28, 29, 30 On the other hand, non-heavy metal QDs such as carbon QDs, silica micro-/nanoparticles, and silicon-based QDs, steer clear of these shortcomings and are non-toxic but offer only Stoke's shift from UV to visible, and this still renders near-infrared (NIR) region non-utilized.7,8,31, 32, 33, 34, 35, 36, 37 Hence, the performance of an LSC is again falling short that can be overcome by the introduction of new fluorophores, which are photostable, non-toxic, and downconverts, as well as upconverts incoming radiation with coincidental emission band and solar cell's spectral response.
Inorganic trivalent rare-earth (RE) or lanthanide (Ln3+) based luminophores are known to offer attractive wavelength conversion capabilities due to abundant excitation lines available in the UV and NIR region.38, 39, 40 To date, many RE-phosphors extending dopant dependent downconversion (DC) or upconversion (UC) have been fabricated mostly with oxide, fluoride, and coordination polymer host lattices.41, 42, 43, 44, 45 However, both oxide and fluoride host-based RE-phosphors are useful for many other applications but are not suitable for implementation in LSC; the fluorides are very toxic and unstable in ambient conditions, whereas oxides have low luminescence efficiency.38 Contrary to this, the oxysulphide hosts are very lucrative as they extend very high luminescence efficiency, no toxicity, and no ambient degradation.38 The excitation and emission wavelengths in an inorganic RE phosphor are decided by the main emitting centre or the dopant and usually provide either DC or UC of the incident spectrum. Earlier reports indicate that Gd2O2S:Er,Yb is a multimode emitting and multispectral harvesting fluorophore that simultaneously absorbs multiple UV as well as numerous NIR wavelengths and emits concurrently in UV, visible and NIR regions thereby increasing the photon flux in the visible spectrum.38,44 To the best of our knowledge, so far, there are no reports on LSCs incorporated with a single RE-oxysulphide based nanophosphor that offers simultaneous Stoke's and anti-Stoke's shift and yet suffers from no reabsorption losses meanwhile being non-toxic and photostable.
We demonstrate the fabrication process optimization of an LSC slab using a low-cost method of solvation-gelation-solidification of poly-methyl meth-acrylate (PMMA) polymer crystals. The Gd2O2S:Er,Yb nanofluorophore, which can simultaneously harvest UV and NIR while emitting in the visible region, was diffused in the slab using the method of secondary dispersion. The LSC slabs had top area ∼25 cm2 and were designed as a function of variable edge width and constant phosphor dispersion ratio and vice-versa to achieve the performance optimization. The electrical performance of the PV-cells attached to the LSC edge gave a window to calculate the efficiency and flux gain of the designed LSCs. The high power conversion efficiency and flux gain of more than unity indicate the successful “concentrator effect” and deem this as very preferable for large scale implementation in BIPV.
Section snippets
LSC waveguide fabrication methodology
The Gd2O2S:Er,Yb nanofluorophore, was synthesized using the molten-salt synthesis technique, which is reported in our earlier work.38 The PMMA slabs were fabricated using the method of solvation-gelation-solidification of PMMA crystals, and the nanophosphor was mixed using the technique of secondary dispersion.46,47 The solvents used for the dissolution of PMMA were benzene, acetone, tetrahydrofuran (THF) and toluene. The details of the synthesis methodology and precursors used are mentioned in
Results and discussion
The Gd2O2S:Er,Yb nanofluorophore exhibits supreme fluorescence under multiple excitation wavelengths (UV, visible, and NIR) and yet incurs no reabsorption losses due to non-overlapping excitation and emission bands as reported in our earlier work.38 Fig. S2(a) Fig. S2(a) (see ESI) depicts the single wavelength excitation and emission of usual fluorophores as reported in other works1, 3, 4, 7, 8, 9, 14, 15, 22, 27, 37. Fig. S2(b) represents the optical behavior of the Gd2O2S:Er,Yb
Conclusions
In this study, we demonstrate a very high-efficiency transparent LSC embedded with nanocrystals of inorganic Gd2O2S:Er,Yb nanofluorophore. The Gd2O2S:Er,Yb fluorophore harvests multiple wavelengths concurrently (285, 380, 450, 808, 980, and 1064 nm) from the complete spectrum and emitted in the visible region via downconversion as well as upconversion process. No overlap between the excitation and emission profile of the considered fluorophore is observed, and hence, no reabsorption losses are
Acknowledgments
The authors are very grateful to the Department of Science and Technology (DST) India for the funding under project No. DST/TMD/CERI/C-24(g) entitled “Luminescent solar concentrator based hybrid sunlight harvesting system for daylight saving”. The authors are also thankful to the University Grants Commission (UGC) India for research fellowship.
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Foundation item: Project supported by Department of Science and Technology, Government of India (DST/TMD/CERI/C-24(g)).