Rowan W. MacQueen

Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion

Single-threshold solar cells are fundamentally limited by their ability to harvest only those photons above a certain energy. Harvesting below-threshold photons and re-radiating this energy at a shorter wavelength would thus boost the efficiency of such devices. We report an increase in light harvesting efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell due to a rear upconvertor based on sensitized triplet–triplet-annihilation in organic molecules. Low energy light in the range 600–750 nm is converted to 550–600 nm light due to the incoherent photochemical process. A peak efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in adapting photochemical UC for application in various single threshold devices.

Dye-Sensitized Solar Cell with Integrated Triplet-Triplet Annihilation Upconversion System.

Photon upconversion (UC) by triplet-triplet annihilation (TTA-UC) is employed in order to enhance the response of solar cells to sub-bandgap light. Here, we present the first report of an integrated photovoltaic device, combining a dye-sensitized solar cell (DSC) and TTA-UC system. The integrated device displays enhanced current under sub-bandgap illumination, resulting in a figure of merit (FoM) under low concentration (3 suns), which is competitive with the best values recorded to date for nonintegrated systems. Thus, we demonstrate both the compatibility of DSC and TTA-UC and a viable method for device integration.

Photochemical Upconversion Enhanced Solar Cells: Effect of a Back Reflector

Photochemical upconversion is applied to a hydrogenated amorphous silicon solar cell in the presence of a back-scattering layer. A custom-synthesized porphyrin was utilized as the sensitizer species, with rubrene as the emitter. Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement of ~2 % was observed at a wavelength of 720 nm. Without the scattering layer, the EQE enhancement was half this value, indicating that the effect of the back-scatterer is to double the efficacy of the upconverting device. The results represent an upconversion figure of merit of 3.5 × 10–4 mA cm–2 sun–2, which is the highest reported to date.

Highly efficient photochemical upconversion in a quasi-solid organogel

Despite the promise of photochemical upconversion as a means to extend the light-harvesting capabilities of a range of photovoltaic solar energy conversion devices, it remains a challenge to create efficient, solid-state upconverting materials. Until now, a material has yet to be found which is as efficient as a liquid composition. Here, a gelated photochemical upconversion material is reported with a performance indistinguishable from an otherwise identical liquid composition. The sensitizer phosphorescence lifetime, Stern–Volmer quenching constants and upconversion performance (6% under one-sun illumination) were all found to be unchanged in a quasi-solid gelated sample when compared to the liquid sample. The result paves the way to a new family of efficient photochemical upconversion materials comprised of macroscopically solid, but microscopically liquid gel, for application in photovoltaics and photocatalytic water-splitting.

Towards an aligned luminophore solar concentrator

Luminescent solar concentrators promise to reduce the cost of solar energy, but are hindered by a number of losses. Escape of luminescence through the large waveguide-air interfaces can be attenuated through alignment of the optical transition dipole of the luminophore along the waveguide surface normal, directing the maximum possible proportion of luminescence into waveguide modes. We demonstrate such alignment using a guest-host dye-doped liquid crystal sandwiched between conductive glass slides. Application of a potential while illuminating through a narrow edge caused a drop in the intensity of luminescence escaping the large surfaces, and an increase in the intensity of light escaping the narrow edges of the system. This is explained in terms of alignment of the transition dipoles of the dye. We discuss implementation in a luminescent solar concentrator.

A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs

Curcumin exhibits broad ground state light absorption and can act as a photoinitiator for the free radical photopolymerization of methacrylates under air upon exposure to different household LED bulbs. The effects of temperature and various additives on the photoinitiation efficiency of curcumin-based systems have been investigated. The curcumin-based system exhibits the highest photoinitiation efficiency at 25 °C. Additives also play an important role in the photoinitiation efficiency, and well-designed systems can even demonstrate higher efficiency than the commercial type I photoinitiator [phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (XBPO)] and type II photoinitiator [camphorquinone (CQ)]. Interestingly, the curcumin/diphenyliodonium hexafluorophosphate/triphenylphosphine combination is a capable multicolor photoinitiating system able to initiate free radical photopolymerization under air upon exposure to UV, blue, green, yellow, red, and warm white household LED bulbs. In addition, reversible addition–fragmentation chain transfer (RAFT) photopolymerization of N-isopropylacrylamide can also be achieved using a curcumin-based system under the irradiation of a blue LED bulb. The photochemical mechanisms associated with the generation of radicals from the investigated photoinitiating systems are investigated by different techniques (fluorescence, steady state photolysis, and electron spin resonance spin-trapping methods) and discussed in detail. More interestingly, the polymer sample produced through the photopolymerization process using the curcumin-based photoinitiating system demonstrates almost no toxicity to human fibroblast Hs-27 cells, endowing this photoinitiating system with great potential for the fabrication of biocompatible polymeric materials.

An intermediate band dye-sensitised solar cell using triplet–triplet annihilation

A new mechanism of charge photogeneration is demonstrated for the first time, based on organic molecular structures. This intermediate band approach, integrated into a dye-sensitised solar cell configuration is shown to generate charges upon illumination with low energy photons. Specifically 610 nm photoexcitation of Pt porphyrins, through a series of energy transfer steps and triplet-triplet annihilation, excites a higher energy absorption onset molecule, which is then capable of charge injection into TiO2. Transient absorption measurements reveal further detail of the processes involved.

Action spectrum experiment for the measurement of incoherent photon upconversion efficiency under sun-like excitation

Photon upconversion (UC) processes result in the emission of photons at higher energies than those absorbed. Among the several recent novel applications of UC, the most widely studied is its use with photovoltaic (PV) cells. Photon UC can sensitize PV cells to portions of the solar spectrum at lower energy than the band gap, which are wasted in a normal single-junction cell, and so begins to address one of the major sources of PV cell efficiency loss. Developing a rigorous but practical method of quantifying upconversion efficiency is therefore an important objective. This task is complicated by the nonlinearity of upconversion efficiency at application-relevant light intensities, meaning the excitation conditions under which efficiency is measured must also be specified. A first-principles approach to determining upconversion efficiency, based on the quantum yields of the underlying photochemical processes, is rigorous in principle but difficult in practice. Absolute photometric measurements that treat the upconverter as a black box are similarly difficult, and measure optical losses alongside the photochemical performance. The widely-utilized relative actinometry method, based on comparisons to a known fluorescence standard, fails as a rigorous method without explicit consideration of the generation profile and reabsorption. In response to these issues, we report an upconverter action spectrum experiment, which is based on continuous-wave photoluminescence techniques. The experiment is used to determine the upconversion efficiency of a photochemical upconverter employing triplet–triplet annihilation (TTA). Full specification of the excitation conditions is made, allowing the efficiency measurement to be linked to well-defined solar excitation conditions. We measure the TTA–UC performance of the PQ4PdNA : rubrene system over a range of excitation conditions corresponding to 0.09–3.22 multiples of AM1.5G solar illumination. At 1 sun, we obtain a TTA yield of 1.1%.

Star-shaped fluorene–BODIPY oligomers: versatile donor–acceptor systems for luminescent solar concentrators

Luminescent solar concentrators (LSCs) are waveguides doped with luminescent centers that can spectrally and spatially concentrate sunlight. They can reduce the cost of photovoltaic energy production and are attractive prospects for photobioreactors and building-integrated applications. Reabsorption, caused by non-zero overlap between the absorption and emission spectra of the light-emitting centers, often limits LSC efficiency. Donor–acceptor energy-transfer complexes are one method to mitigate reabsorption by shifting the emission away from the main absorption peak. Here we introduce versatile star-shaped donor–acceptor molecules based on a central BODIPY energy acceptor with oligofluorene donor side units. Varying the oligofluorene chain length alters the relative oscillator strengths of the donor and acceptor, changing the severity of reabsorption for a given donor density, but also changing the luminescence yield and emission spectrum. We performed comprehensive device measurements and Monte Carlo ray tracing simulations of LSCs containing three oligofluorene–BODIPY donor–acceptor systems with different oligofluorene chain lengths, and then extended the simulation to study hypothetical analogs with higher donor–acceptor ratios and different terminal acceptors. We found that the measured structures permit waveguide propagation lengths on a par with state-of-the-art nanocrystalline emitters, while the proposed structures are viable candidates for photobioreactor and energy production roles and should be synthesized.

Nanostructured upconverters for improved solar cell performance

Triplet-triplet annihilation photon upconversion (TTA-UC) is a promising candidate for mitigating sub-band gap absorption losses in solar cells. In TTA-UC, sensitiser dyes absorb sub-band gap photons, cross to a triplet state, and transfer triplet excitons to emitter dyes. Two triplet-excited emitters can undergo TTA, raising one emitter to a higher-energy bright singlet state. The quadratic efficiency of TTA-UC at device-relevant light intensities motivates a push towards the higher chromophore densities achievable in the solid phase. We have begun this process by tethering tetrakisquinoxalino palladium porphyrin to 20nm silica nanoparticles using peptide chemistry techniques, achieving a total-volume concentration of 1.5mM. The phosphorescence kinetics of the tethered porphyrins was measured to quantify quenching by rubrene emitter. Upconverter performance was measured in a solar cell enhancement experiment.