Transparent Luminescent Solar Concentrators for Large‐Area Solar Windows Enabled by Massive Stokes‐Shift Nanocluster Phosphors

Abstract

Integrating solar-harvesting systems into the building envelope is a transformative route to improving building effi ciency, capturing large areas of solar energy, and lowering effective solar cell installation costs by piggybacking on the installation, framing, and maintenance of the existing building envelope. However, the widespread adoption of such a pathway is typically hampered by diffi culties associated with mounting traditional photovoltaic (PV) modules in non-standard confi gurations on and around buildings due to added structural cost, architectural impedance, and most importantly, aesthetics. To overcome these hurdles we have developed a luminescent solar concentrator (LSC) employing novel nanocrystal-polymer blends that allow for selective ultraviolet light harvesting that results in a high degree of visible light transmittance. These transparent LSCs offer a different route to large area scaling with high defect tolerances compared to other transparent photovoltaic devices. These systems have signifi cant potential in 1) energy scavenging electronics displays, 2) autonomous electrochromicwindows, 3) visible-blind detectors, and 4) coatings for improved UV response and protection of traditional solar installations.The concept of luminescent solar concentrators (LSCs) has existed for over 30 years, and with recent advances in phosphorescent and fl uorescent luminophores effi ciencies, the power conversion effi ciency for LSC modules have increased to 7.1% using multi-dye systems with GaAs photovoltaics. [ 1 ] Although optical funneling limits the overall system conversion effi ciency of LSCs to less than 10% (without LSC stacking), [ 2 ] it can dramatically reduce the area of expensive solar cells needed, driving down the overall module cost. [3][4][5][6][7][8] Recently, there has been refocused interest in applying LSCs as architectural windows and skins. [9][10][11] However, the limited aesthetic appeal and coloration from chromophore absorption and emission (glow) in the visible spectrum of these systems has hindered widespread adoption of such devices in window applications (see Figure 1 ).In general, windows are installed to provide natural lighting with a view; that is, most people prefer not to work behind strongly colored or glowing glass. A high level of untinted-transparency is therefore necessary for ubiquitous adoption.Previous efforts in constructing transparent or semitransparent power-producing surface have focused on 1) optically-thin photovoltaics that have signifi cant tinting or limited transmission, [ 9 , 12 ] 2) LSCs incorporating colored chromophores that have either absorption or emission in the visible spectrum, again creating signifi cant tinting, [ 13 , 14 ] or 3) focusing-optics systems using direct-light only that requires bulky solar tracking or optics. [ 15 , 16 ] Recently, selectively absorbing near-infrared photovoltaics (PV) have been fabricated by exploiting the excitonic character of molecular and organic semiconductors with efficiencies in the range of 2-4% over ...

Limits of Visibly Transparent Luminescent Solar Concentrators

“…[92] f) Emission linewidth comparison of different emitter species. [3,4,11,46,76,92] Note that small W (<5 nm), and a range of S (5-500 nm) have been reported. on variations of these cyanine salts with system efficiency of 0.4% efficiency combined with an AVT of 86% and little tinting.…”

“…For example, given a particular molecule emitter with a defined absorption, the smaller the W, the higher voltage PV that can be selected. For reference, a range of emission widths have been demonstrated for organic molecules (30-100 nm), [21,[37][38][39][40] nanoclusters (50-300 nm), [4] J-aggregates (15-40 nm), [41,42] nanocrystals (15-30 nm), [43] rareearth ions (<5-20 nm), [44,45] and single diameter carbon nanotubes (8-15 nm), [46] suggesting that such losses can, in fact, be minimized. Nonetheless, for the most efficient emitters, this value of W is typically around 20-100 nm.…”

“…[11] c) (TBA) 2 Mo 6 Cl 14 nanoclusters. [4] d) Eu(TTA) 3 (TTPO) 2 rare-earth ion complex. [76] e) single-walled carbon nanotube.…”

“…Recent demonstrations of clear transparent LSCs have further reinvigorated research into these systems. [3][4][5][6][7] By converting windows, architectural glazing, and displays into LSC waveguides, it is possible to transform these passive absorption efficiency of the luminophore, η PL is the luminescence efficiency of the luminophore, η Trap is the photon trapping (or waveguiding) efficiency, and η RA is the efficiency of suppressing reabsorption. [1,17] The light trapping efficiency can be approximated for simple waveguides with isotropic emitters as 1 1/ T Tr ra ap p S ub 2 n η η = − and R = (n Sub − 1) 2 /(n Sub + 1) 2 excited at normal incidence (the only case considered here), where n Sub is the index of refraction of the waveguiding substrate (see Figure 2b).…”

“…This new approach to visibly transparent LSCs was developed by exploiting the nature of excitonic semiconductors to optimize the use of this light-exposed surface area without compromising its existing functionality, thereby avoiding the aesthetic tradeoffs that can hinder adoption. Recent work has demonstrated devices that selectively harvest different parts of the solar spectrum including i) ultraviolet (UV) absorption, with massive downconverted luminescence deeper into the NIR (>400 nm) [4] and ii) near-infrared (NIR) absorption, with even deeper luminescence in the NIR. [3] This work has been subsequently followed by a number of other groups.…”