A series of four perylene diimide (PDI) chromophores were prepared with increasing steric bulk on the imide substituents with the aim of retarding the effect of concentration quenching on photoluminescence, commonly observed with these dyes. Spectroscopic investigations of the compounds in dilute solution confirmed that the photophysical properties of the PDI core chromophore were not perturbed by the bulky substituents. Solid film samples containing the PDI compounds at various concentrations dispersed in a poly(methyl methacrylate) (PMMA) matrix were examined and compared to amorphous neat films as well as crystalline samples. The PDI compounds containing di-tert-butylphenyl (bPDI-3) and trityl (bPDI-4) substituents showed near unity photoluminescence quantum yield (PLQY) up to 20 mM in PMMA compared to 10% PLQY for the reference compound (bPDI-1) without molecular insulation. Surprisingly, high concentrations (>40 mM) of a phenyl substituted PDI compound (bPDI-2) with moderate molecular insulation formed emissive aggregates that showed a higher PLQY compared to the PDI derivatives with greater steric bulk. By examining the molecular structure and solid state packing in conjunction with a series of photophysical measurements, new insights into designing highly fluorescent dyes, particularly in the solid state, were obtained. The trityl substituted PDI compound (bPDI-4) was used in a luminescent solar concentrator with optical quantum efficiency of 54%, flux gain of 6.4 and geometric gain of 45.

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.

Indene-C70 bisadduct (IC70BA) is a high performance electron acceptor material consisting of a mixture of regioisomers. A single isomer of the IC70BA was isolated by careful chromatographic separation. The structure of this isomer was confirmed by various analytical techniques including single crystal X-ray analysis. The isomer showed superior performance to other isomer mixtures of IC70BA in bulk heterojunction solar cells using poly(3-hexylthiophene) as the donor material.

Photon upconversion is a process that creates high-energy photons under low photon energy excitation. The effect of molecular geometry on the triplet fusion upconversion process has been investigated in this work through the design and synthesis of four new 9,10-diphenylanthracene (DPA) derivatives by employing platinum octaethylporphyrin as the triplet sensitizer. These new emitter molecules containing multiple DPA subunits linked together via a central benzene core exhibit high fluorescence quantum yields. Interestingly, large differences in the triplet fusion upconversion performance were observed between the derivatives with the meta-substituted dimer showing the closest performance to the DPA reference. The differences are discussed in terms of the statistical probability for obtaining a high-energy singlet excited state from triplet fusion, f, for both inter- and intramolecular processes and the effect of magnetic field on the upconversion efficiency. These results demonstrate the challenges to be overcome in improving triplet fusion upconversion efficiency based on multichromophoric emitter systems.

Luminescent solar concentrators (LSCs) are an emerging technology to collect and channel light from a large absorption area into a smaller one. They are a complementary technology for traditional solar photovoltaics (PV), particularly suitable for application in urban or indoor environments where their custom colors and form factors, and performance under diffuse light conditions may be advantageous. Förster resonance energy transfer (FRET) has emerged as a valuable approach to overcome some of the intrinsic limitations of conventional single lumophore LSCs, such as reabsorption or reduced quantum efficiency. This review outlines the potential of FRET to boost LSC performance, using highlights from the literature to illustrate the key criteria that must be considered when designing an FRET‐LSC, including both the photophysical requirements of the FRET lumophores and their interaction with the host material. Based on these criteria, a list of design guidelines intended to aid researchers when they approach the design of a new FRET‐LSC system is presented. By highlighting the unanswered questions in this field, the authors aim to demonstrate the potential of FRET‐LSCs for both conventional solar‐harvesting and emerging LSC‐inspired technologies and hope to encourage participation from a diverse researcher base to address this exciting challenge.

Two dyes showing aggregation induced emission behaviour were examined for use as a light harvesting Förster resonance energy transfer pair in luminescent solar concentrators.

A high-performance large-area luminescent solar concentrator (LSC) is demonstrated, fabricated via careful choice of the fluorophore system, waveguide material, and coupled photovoltaic (PV) cells. Doctor-blading was used to deposit a PMMA–dye blend film on the surface of a PMMA planar waveguide to fabricate the large-area LSC with dimensions of 20 × 20 × 0.1 cm3. A perylene diimide-based donor–emitter fluorophore pair was chosen to both improve the photoluminescent quantum yield and reduce the luminescence reabsorption. Three types of photovoltaic cells were coupled to the LSC to optimize the device output performance. With the combination of the donor–emitter fluorophore pair chosen, the PMMA-based LSC deposition process and coupled perovskite PV cells, the LSC has an experimental flux gain of 7.4 at a geometric gain of 50 (G = 50) under 1 sun irradiation, with a power conversion efficiency of 2.6%.

Triplet–triplet annihilation upconversion (TTA-UC) is a process by which a lower energy photon can be upconverted to a higher energy state. The incorporation of TTA-UC materials into solid-state hosts has enabled advances in solar energy and many other applications. The choice of host system is, however, far from trivial and often calls for a careful compromise between characteristics such as high molecular mobility, low oxygen diffusion, and high material stability, factors that often contradict one another. Here, we evaluate these challenges in the context of the state-of-the-art of primarily polymer hosts and the advantages they hold in terms of material selection and tunability of their diffusion or mechanical or thermal properties. We encourage more collaborative research between polymer scientists and photophysicists in order to further optimize the current systems and outline our thoughts for the future direction of the field.