Luminescent solar concentrator efficiency versus edge solar cell coverage

This Letter introduces an analytical approach to estimate the waveguiding efficiency of large-area luminescent solar concentrators (LSCs), where the edges are covered by a varied number of mirrors and solar cells. The model provides physically relevant description in the whole range of optical (absorption, scattering) and geometrical (size) parameters of rectangular LSCs. A 19 × 19 cm2 silicon quantum dot-based LSC has been fabricated to verify the theory. Within an experimental error, the predicted waveguiding efficiency matched well the measured one. A critical LSC size, beyond which a part of the device turns inactive, has been determined as N/α for N attached solar cells (one or two) and LSC material absorption coefficient α. This model provides a straightforward waveguiding analysis tool for large-area LSCs with different structural parameters relevant for both high concentration ratio and glazing applications.

Low-Loss, High-Transparency Luminescent Solar Concentrators with a Bioinspired Self-Cleaning Surface

Luminescent solar concentrators (LSCs) have emerged as a disruptive technology that can potentially enable carbon-neutral buildings. The issues with current LSCs, however, are low optical efficiencies and limited long-term outdoor stability. Here we simultaneously address them by developing an LSC with aggregation-induced-emission (AIE) molecules embedded in a polydimethylsiloxane (PDMS) matrix. The AIE-emitter displayed a near unity emission quantum yield when embedded in the PDMS and the apparent absorption-emission Stokes shift reached 0.59 eV, effectively suppressing the reabsorption loss of waveguided photons inside an LSC. Moreover, the surface texture of the PDMS matrix was engineered using a bioinspired nanolithography method with a natural lotus leaf as the template. This allowed the fabricated AIE-PDMS LSC to inherit the superhydrophobic, self-cleaning properties of the leaf and meanwhile to possess a light-trapping capability. Our 100 cm² LSC, when coupled with commercial Si PVs, delivered efficient solar power conversion, high visible transmittance, and high working stability.

Optical center of a luminescent solar concentrator

This Letter introduces a novel, to the best of our knowledge, approach to estimate the power conversion efficiency (PCE) of a square luminescent solar concentrator (LSC) by point excitations on the "optical centers." Predicted by theoretical calculations, photoluminescence emissions from these optical centers experience almost the same average optical path as those from the whole device under uniform illumination. This is experimentally verified by a 20 × 20 cm² silicon quantum dot-based LSC, with a negligible error between the predicted PCE and the measured one. This method provides a convenient way to estimate the photovoltaic performance of large-area LSC devices with basic laboratory instruments.

Solid state structure and properties of phenyl diketopyrrolopyrrole derivatives

Crystal structures of the title compounds show diverse packing by interactions of auxochromes giving materials with varied optoelectronic properties.

High-Performance Luminescent Solar Concentrators Based on Poly(Cyclohexylmethacrylate) (PCHMA) Films

In this study, we report on the use of poly(cyclohexylmethacrylate) (PCHMA) as an alternative to the commonly used poly(methylmethacrylate) (PMMA) for the design of efficient luminescent solar concentrators (LSCs). PCHMA was selected due to its less polar nature with respect to PMMA, a characteristic that was reported to be beneficial in promoting the fluorophore dispersibility in the matrix, thus maximizing the efficiency of LSCs also at high doping. In this sense, LSC thin films based on PCHMA and containing different contents of Lumogen F Red 305 (LR, 0.2–1.8 wt%) demonstrated optical efficiencies (η_opt) comprising between 9.5% and 10.0%, i.e., about 0.5–1% higher than those collected from the LR/PMMA systems. The higher LR/polymer interactions occurred using the PCHMA matrix maximized the solar harvesting characteristics of the fluorophore and limited the influence of the adverse dissipative phenomena on the fluorophore quantum efficiency. These effects were also reflected by varying the LSC film thickness and reaching maximum η_opt of about 11.5% in the case of PCHMA films of about 30 µm.

Synthesis, Photophysical and Electronic Properties of Mono-, Di-, and Tri-Amino-Substituted Ortho-Perylenes, and Comparison to the Tetra-Substituted Derivative

We synthesized a series of new mono-, di-, tri- and tetra-substituted perylene derivatives with strong bis(para-methoxyphenyl)amine (DPA) donors at the uncommon 2,5,8,11-positions. The properties of our new donor-substituted perylenes were studied in detail to establish a structure-property relationship. Interesting trends and unusual properties are observed for this series of new perylene derivatives, such as a decreasing charge transfer (CT) character with increasing number of DPA moieties and individual reversible oxidations for each DPA moiety. Thus, (DPA)-Per possesses one reversible oxidation while (DPA)4 -Per has four. The mono- and di-substituted derivatives display unusually large Stokes shifts not previously reported for perylenes. Furthermore, transient absorption measurements of the new derivatives reveal an excited state with lifetimes of several hundred microseconds, which sensitizes singlet oxygen with quantum yields of up to 0.83.

Synthesis and characterization of bent fluorine-containing donor-π-acceptor molecules as intense luminophores with large Stokes shifts

Herein, we prepared novel bent fluorine-containing donor-π-acceptor (D-π-A) molecules from commercially available octafluorocyclopentene using a facile two-step procedure, revealing that the above molecules absorb UV-light and exhibit yellow photoluminescence (PL) with high PL efficiencies (ΦPL) in solution. The corresponding Stokes shifts exceeded 10 000 cm-1, and the maximum PL wavelength (λPL) strongly depended on solvent polarity or intermolecular interactions in the solid state. On the basis of a Lippert-Mataga plot, PL was confidently assigned to radiative relaxation from an intramolecular charge-transfer excited state. Moreover, the synthesized luminophores showed intense PL even in the crystalline state and exhibited alkoxy chain length-dependent PL behavior (e.g., high ΦPL, λPL = 486-540 nm).

First demonstration of the use of very large Stokes shift cycloparaphenylenes as promising organic luminophores for transparent luminescent solar concentrators

The use of [n]CPP derivatives as luminophores in LSC-devices minimises reabsorption losses.

Ultrabright Red-Emitting Photostable Perylene Bisimide Dyes: New Indicators for Ratiometric Sensing of High pH or Carbon Dioxide

New pH-sensitive perylene bisimide indicator dyes were synthesized and used for fabrication of optical sensors. The highly photostable dyes show absorption/emission bands in the red/near-infrared (NIR) region of the electromagnetic spectrum, high molar absorption coefficients (up to 100 000 m-1  cm-1 ), and fluorescence quantum yields close to unity. The absorption and emission spectra show strong bathochromic shifts upon deprotonation of the imidazole nitrogen atom, which makes the dyes promising as ratiometric fluorescent indicators. Physical entrapment of the indicators into a polyurethane hydrogel enables pH determination at alkaline pH values. It is also shown that a plastic carbon dioxide solid-state sensor can be manufactured by immobilization of the pH indicator in a hydrophilic polymer, along with a quaternary ammonium base. The influences of the plasticizer, different lipophilic bases, and humidity on the sensitivity of the sensor material are systematically investigated. The disubstituted perylene, particularly, features two deprotonation equilibria, enabling sensing over a very broad pCO2 range of 0.5 to 1000 hPa.

Gain investigation of Perylene-Red-doped PMMA for stimulated luminescent solar concentrators

Luminescent solar concentrators (LSCs) utilizing stimulated emission by a seed laser are a promising approach to overcome the limitations of conventional LSCs, with a significant reduction of the photovoltaic material. In our previous work, we demonstrated the principle of a stimulated LSC (s-LSC) and correspondingly developed a model for quantifying the output power of such a system, taking into account different important physical parameters. The model suggested Perylene Red (PR) dye as a potential candidate for s-LSCs. Here, we experimentally investigate the gain of PR-doped polymethyl methacrylate (PMMA) required for s-LSCs using a single pump wavelength (instead of the solar spectrum) as a proof of principle. The results found from the experiment are well matched with the previously developed numerical model except for gain saturation, which occurs at a comparatively small seed laser signal power. To investigate the gain saturation, two approaches were taken: investigating (i) spectral hole burning and (ii) triplet state absorption. Experimental investigation of spectral hole burning with PR dyes showed a small effect on the gain saturation. We developed a general state model considering triplet state absorption of the PR dyes for the second approach. The state model suggests that the PR dyes suffer from significant triplet state absorption loss, which obstructs the normal operation of the PR-based s-LSC system.

A joint experimental and theoretical study on the electro-optical properties of 1,6- and 1,7-fluorenyl disubstituted perylene diimide isomers

The electronic and optical properties of fluorenyl functionalized perylene diimides are influenced by the substituent position on the perylene core.

Construction of J-type aggregates as multi-functional interlayers for nonfullerene polymer solar cells

Light-absorbing J-aggregates: a new strategy to develop high performance cathode interlayers with solar concentrating functions.

Luminescent solar concentrators based on PMMA films obtained from a red-emitting ATRP initiator

PMMA_TPE_RED polymers containing 0.98–3.05 wt% of a red-emitting AIEgen were prepared and proposed as high performance luminescent solar concentrators.

Impact of curvature on the optimal configuration of flexible luminescent solar concentrators

Flexible luminescent solar concentrators (LSCs) could deliver integrated photovoltaics in all aspects of our lives, from architecture to wearable electronics. We present and experimentally verify a model for the optimization of the external optical efficiency of LSCs under varying degrees of curvature. We demonstrate differences between the optimization of flat and bent LSCs, showing that optimal fluorophore concentrations can differ by a factor of two.

General C-H Arylation Strategy for the Synthesis of Tunable Visible Light-Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores

Herein we report the discovery of the benzo[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emission covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a chemoselective process involving Tf₂O-mediated amide cyclodehydration, followed by intramolecular C-H arylation. Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent access to varied substitution. The effects of said substitution on the compounds' photophysical properties were rationalized by density functional theory calculations. For some compounds, emission wavelengths are directly correlated to the substituent's Hammett constants. Easily introduced nonconjugated reactive functional groups allow the labeling of biomolecules without modification of emissive properties. This work provides a straightforward platform for the synthesis of new moderately bright fluorescent dyes remarkable for their chemical stability, predictability, and unusually high excitation-emission differential.

Emissive Molecular Aggregates and Energy Migration in Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are light harvesting devices that are ideally suited to light collection in the urban environment where direct sunlight is often not available. LSCs consist of highly luminescent compounds embedded or coated on a transparent substrate that absorb diffuse or direct solar radiation over a large area. The resulting luminescence is trapped in the waveguide by total internal reflection to the thin edges of the substrate where the concentrated light can be used to improve the performance of photovoltaic devices. The concept of LSCs has been around for several decades, and yet the efficiencies of current devices are still below expectations for commercial viability. There are two primary challenges when designing new chromophores for LSC applications. Reabsorption of dye emission by chromophores within the waveguide is a significant loss mechanism attenuating the light output of LSCs. Concentration quenching, particularly in organic dye systems, restricts the quantity of chromophores that can be incorporated in the waveguide thus limiting the light absorbed by the LSC. Frequently, a compromise between increased light harvesting of the incident light and decreasing emission quantum yield is required for most organic chromophore-based systems due to concentration quenching. The low Stokes shift of common organic dyes used in current LSCs also imposes another optimization problem. Increasing light absorption of LSCs based on organic dyes to achieve efficient light harvesting also enhances reabsorption. Ideally, a design strategy to simultaneously optimize light harvesting, concentration quenching, and reabsorption of LSC chromophores is clearly needed to address the significant losses in LSCs. Over the past few years, research in our group has targeted novel dye structures that address these primary challenges. There is a common perception that dye aggregates are to be avoided in LSCs. It became apparent in our studies that aggregates of chromophores exhibiting aggregation-induced emission (AIE) behavior are attractive candidates for LSC applications. Strategic application of AIE chromophores has led to the development of the first organic-based transparent solar concentrator that harvests UV light as well as the demonstration of reabsorption reduction by taking advantage of energy migration processes between chromophores. Further developments led us to the application of perylene diimides using an energy migration/energy transfer approach. To prevent concentration quenching, a molecularly insulated perylene diimide with bulky substituents attached to the imide positions was designed and synthesized. By combining the insulated perylene diimide with a commercial perylene dye as an energy donor-acceptor emitter pair, detrimental luminescence reabsorption was reduced while achieving a high chromophore concentration for efficient light absorption. This Account reviews and reinspects some of our recent work and the improvements in the field of LSCs.

A push–pull silafluorene fluorophore for highly efficient luminescent solar concentrators

We report on the preparation of luminescent collectors based on poly(methyl methacrylate) (PMMA) thin films doped with a red-emitting 2-amino-7-acceptor-9-silafluorene, where the amino group is –N(CH₃)₂and the acceptor is –CHC(CN)₂.

Modeling of stimulated emission based luminescent solar concentrators

The efficiency improvement of luminescent solar concentrators (LSCs) necessary for practical realization is currently hindered by one major loss mechanism: reabsorption of emitted photons by the luminophores. Recently, we explored a promising technique for reducing reabsorption and also improving directional emission in LSCs utilizing stimulated emission, rather than only spontaneous emission, with an inexpensive seed laser. In this work, a model is developed to quantify the gain (i.e. the amount of amplification of a low power seed laser propagating through the solar-pumped concentrator) of stimulated-LSCs (s-LSCs) considering the effects of different important physical parameters. The net optical output power, available for a small PV cell, from the concentrator can also be determined from the model, which indicates the performance of s-LSCs. Finally, the performance of different existing material systems is investigated using literature values of the parameters required for the model, and a set of optimal parameters is suggested for practical realization of such a device.

Synthesis of Perylene Imide Diones as Platforms for the Development of Pyrazine Based Organic Semiconductors

There is a great interest in peryleneimide (PI)-containing compounds given their unique combination of good electron accepting ability, high abosorption in the visible region, and outstanding chemical, thermal, and photochemical stabilities. Thus, herein we report the synthesis of perylene imide derivatives endowed with a 1,2-diketone functionality (PIDs) as efficient intermediates to easily access peryleneimide (PI)-containing organic semiconductors with enhanced absorption cross-section for the design of tunable semiconductor organic materials. Three processable organic molecular semiconductors containing thiophene and terthiophene moieties, PITa, PITb, and PITT, have been prepared from the novel PIDs. The tendency of these semiconductors for molecular aggregation have been investigated by NMR spectroscopy and supported by quantum chemical calculations. 2D NMR experiments and theoretical calculations point to an antiparallel π-stacking interaction as the most stable conformation in the aggregates. Investigation of the optical and electrochemical properties of the materials is also reported and analyzed in combination with DFT calculations. Although the derivatives presented here show modest electron mobilities of ∼10^-4 cm²V^-1s^-1, these preliminary studies of their performance in organic field effect transistors (OFETs) indicate the potential of these new building blocks as n-type semiconductors.

Solution-Processable BODIPY-Based Small Molecules for Semiconducting Microfibers in Organic Thin-Film Transistors

Electron-deficient π-conjugated small molecules can function as electron-transporting semiconductors in various optoelectronic applications. Despite their unique structural, optical, and electronic properties, the development of BODIPY-based organic semiconductors has lagged behind that of other π-deficient units. Here, we report the design and synthesis of two novel solution-proccessable BODIPY-based small molecules (BDY-3T-BDY and BDY-4T-BDY) for organic thin-film transistors (OTFTs). The new semiconductors were fully characterized by (1)H/(13)C NMR, mass spectrometry, cyclic voltammetry, UV-vis spectroscopy, photoluminescence, differential scanning calorimetry, and thermogravimetric analysis. The single-crystal X-ray diffraction (XRD) characterization of a key intermediate reveals crucial structural properties. Solution-sheared top-contact/bottom-gate OTFTs exhibited electron mobilities up to 0.01 cm(2)/V·s and current on/off ratios of >10(8). Film microstructural and morphological characterizations indicate the formation of relatively long (∼0.1 mm) and micrometer-sized (1-2 μm) crystalline fibers for BDY-4T-BDY-based films along the shearing direction. Fiber-alignment-induced charge-transport anisotropy (μ∥/μ⊥ ≈ 10) was observed, and higher mobilities were achieved when the microfibers were aligned along the conduction channel, which allows for efficient long-range charge-transport between source and drain electrodes. These OTFT performances are the highest reported to date for a BODIPY-based molecular semiconductor, and demonstrate that BODIPY is a promising building block for enabling solution-processed, electron-transporting semiconductor films.

Luminescent solar concentrators utilizing stimulated emission

Luminescent solar concentrators (LSCs) are an emerging technology that aims primarily to reduce the cost of solar energy, with great potential for building integrated photovoltaic (PV) structures. However, realizing LSCs with commercially viable efficiency is currently hindered by reabsorption losses. Here, we introduce an approach to reducing reabsorption as well as improving directional emission in LSCs by using stimulated emission. Light from a seed laser (potentially an inexpensive laser diode) passes through the entire area of the LSC panel, modifying the emission spectrum of excited dye molecules such that it is spectrally narrower, at wavelengths that minimize reabsorption to allow net gain in the system, and directed towards a small PV cell. A mathematical model, taking into account thermodynamic considerations, of such a system is presented which identifies key parameters and allows evaluation in terms of net effective output power.

Perylene diimide derivatives as red and deep red-emitters for fully solution processable OLEDs

We report on the photophysical characterization of two solution-processable red-emissive perylene diimide molecules and on their use in fully solution assembled OLEDs.

Cost-effective solar concentrators based on red fluorescent Zn(ii)–salicylaldiminato complex

A red light and concentrated zinc complex embedded in PMMA yields a cost-effective and efficient luminescent solar concentrator.

Ray-trace simulation of CuInS(Se)₂ quantum dot based luminescent solar concentrators

To enhance the performance of luminescent solar concentrator (LSC), there is an increased need to search novel emissive materials with broad absorption and large Stokes shifts. I-III-VI colloidal CuInS2 and CuInSe2 based nanocrystals, which exhibit strong photoluminescence emissions in the visible to near infrared region with large Stokes shifts, are expected to improve performance in luminescent solar concentrator applications. In this work, the performance of CuInS(Se)2 quantum dots in simple planar LSC is evaluated by applying Monte-Carlo ray-trace simulation. A systematic parameters study was conducted to optimize the performance. An optimized photon concentration ratio of 0.34 for CuInS2 nanocrystals and 1.25 for CuInSe2 nanocrystals doping LSC are obtained from the simulation. The results demonstrated that CuInSe2 based nanocrystals are particularly interesting for luminescent solar concentrator applications, especially to combine with low price Si solar cells.

Zero-reabsorption doped-nanocrystal luminescent solar concentrators

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn(2+)-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.