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Author(s): Benjamin D. Ravetz [sup.1] , Andrew B. Pun [sup.1] , Emily M. Churchill [sup.1] , Daniel N. Congreve [sup.2] , Tomislav Rovis [sup.1] , Luis M. Campos [sup.1] Author [...], Recent advances in photoredox catalysis have made it possible to achieve various challenging synthetic transformations, polymerizations and surface modifications.sup.1-3. All of these reactions require ultraviolet- or visible-light stimuli; however, the use of visible-light irradiation has intrinsic challenges. For example, the penetration of visible light through most reaction media is very low, leading to problems in large-scale reactions. Moreover, reactants can compete with photocatalysts for the absorption of incident light, limiting the scope of the reactions. These problems can be overcome by the use of near-infrared light, which has a much higher penetration depth through various media, notably biological tissue.sup.4. Here we demonstrate various photoredox transformations under infrared radiation by utilizing the photophysical process of triplet fusion upconversion, a mechanism by which two low-energy photons are converted into a higher-energy photon. We show that this is a general strategy applicable to a wide range of photoredox reactions. We tune the upconversion components to adjust the output light, accessing both orange light and blue light from low-energy infrared light, by pairwise manipulation of the sensitizer and annihilator. We further demonstrate that the annihilator itself can be used as a photocatalyst, thus simplifying the reaction. This approach enables catalysis of high-energy transformations through several opaque barriers using low-energy infrared light.Photoredox transformations are achieved with infrared light by using triplet fusion upconversion that converts infrared into visible light, enabling the use of photoredox chemistry on larger scales and through barriers that are impenetrable by visible light.

Perovskites have proven to be a promising candidate for highly efficient solar cells, light-emitting diodes, and x-ray detectors, overcoming limitations of inorganic semiconductors. However, they are notoriously unstable. The main reason for this instability is the migration of mobile ions through the device during operation as they are mixed ionic–electronic conductors. Here, we show how measuring the capacitance in both the frequency and the time domain can be used to study ionic dynamics within perovskite-based devices, quantifying activation energy, diffusion coefficient, sign of charge, concentration, and the length of the ionic double layer in the vicinity of the interfaces. Measuring the transient of the capacitance furthermore allows for distinguishing between ionic and electronic effects. [ABSTRACT FROM AUTHOR]

Singlet exciton fission is a mechanism that could potentially enable solar cells to surpass the Shockley-Queisser efficiency limit by converting single high-energy photons into two lower-energy triplet excitons with minimal thermalization loss. The ability to make use of singlet exciton fission to enhance solar cell efficiencies has been limited, however, by the sparsity of singlet fission materials with triplet energies above the bandgaps of common semiconductors such as Si and GaAs. Here, we employ a high-throughput virtual screening procedure to discover new organic singlet exciton fission candidate materials with high-energy (>1.4 eV) triplet excitons. After exploring a search space of 4482 molecules and screening them using time-dependent density functional theory, we identify 88 novel singlet exciton fission candidate materials based on anthracene derivatives. Subsequent purification and characterization of several of these candidates yield two new singlet exciton fission materials: 9,10-dicyanoanthracene (DCA) and 9,10-dichlorooctafluoroanthracene (DCOFA), with triplet energies of 1.54 eV and 1.51 eV, respectively. These materials are readily available and low-cost, making them interesting candidates for exothermic singlet exciton fission sensitization of solar cells. However, formation of triplet excitons in DCA and DCOFA is found to occur via hot singlet exciton fission with excitation energies above ∼3.64 eV, and prominent excimer formation in the solid state will need to be overcome in order to make DCA and DCOFA viable candidates for use in a practical device. [ABSTRACT FROM AUTHOR]

Transient microscopy is of vital importance in understanding the dynamics of optical excited states in optoelectronic materials, as it allows for a direct visualization of the movement of energy carriers in space and time. Important information on trap‐state dynamics can be obtained using this technique, typically observed as a slow‐down of energy transport as carriers are trapped at defect sites. To date, however, studies of the trap‐state dynamics have been mostly limited to phenomenological descriptions of the early time‐dynamics. Here, it is shown how long‐acquisitiontime transient photoluminescence microscopy can be used to provide a detailed map of the trapstate landscape in 2D perovskites, in particular when used in combination with transient spectroscopy. An anomalous evolution of the studied exciton distribution is observed, which cannot be explained with existing models for trap limited exciton transport that only account for a single trap type. Instead, using a continuous diffusion model and performing Brownian dynamics simulations, it is shown that this behavior can be explained by accounting for a distinct distribution of traps in this material. These results highlight the value of transient microscopy as a complementary tool to more common transient spectroscopy techniques in the characterization of excited state dynamics in semiconductors. [ABSTRACT FROM AUTHOR]

We demonstrate a passive down-conversion imaging system that converts broadband ultraviolet light to narrow-band green light while preserving the directionality of rays, and thus enabling direct down-conversion imaging. At the same time our system has high transparency in the visible, enabling superimposed visible and ultraviolet imaging. The frequency conversion is performed by a subwavelength-thickness transparent downconverter based on highly efficient CsPbBr3 nanocrystals incorporated into the focal plane of a simple telescope or relay-lens geometry. The resulting imaging performance of this down-conversion system approaches the diffraction limit. This demonstration sets the stage for the incorporation of other high-efficiency perovskite nanocrystal materials to enable passive multi-frequency conversion imaging systems. [ABSTRACT FROM AUTHOR]

The efficiency of solar cells may be increased by utilizing photons with energies below the band gap of the absorber. This may be enabled by upconversion of low energy photons into high energy photons via triplet–triplet annihilation (TTA) in organic chromophores. The quantum yield of TTA is often low due to competing processes. The singlet pathway, where a high energy photon is emitted, is one of three possible outcomes of an encounter between two triplet excitons. The quintet pathway is often too high in energy to be accessible, leaving the triplet pathway as the main competing process. Using many-body perturbation theory in the GW approximation and the Bethe–Salpeter equation, we calculate the energy release in both the singlet and triplet pathways for 59 chromophores of different chemical families. We find that in most cases the triplet pathway is open and has a larger energy release than the singlet pathway. Thus, the energetics perspective explains why there are so few TTA emitters and why the quantum yield of TTA is typically low. That said, our results also indicate that the performance of emitters from known chemical families may be improved by chemical modifications, such as functionalization with side groups, and that new chemical families could be explored to discover more TTA emitters. [ABSTRACT FROM AUTHOR]

The ability to upconvert light is useful for a range of applications, from biological imaging to solar cells. But modern technologies have struggled to upconvert incoherent incident light at low intensities. Here, we report solid state photon upconversion employing triplet-triplet exciton annihilation in an organic semiconductor, sensitized by a thermally activated-delayed fluorescence (TADF) dye. Compared to conventional phosphorescent sensitizers, the TADF dye maximizes the wavelength shift in upconversion due to its small singlet-triplet splitting. The efficiency of energy transfer from the TADF dye is 9.1%, and the conversion yield of sensitizer exciton pairs to singlet excitons in the annihilator is 1.1%. Our results demonstrate upconversion in solid state geometries and with non-heavy metal-based sensitizer materials. [ABSTRACT FROM AUTHOR]

Triplet excitons are ubiquitous in organic optoelectronics, but they are often an undesirable energy sink because they are spin-forbidden from emitting light and their high binding energy hinders the generation of free electron-hole pairs. Harvesting their energy is consequently an important technological challenge. Here, we demonstrate direct excitonic energy transfer from 'dark' triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90 ± 13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near-infrared light-emitting applications, where effective molecular phosphors are lacking at present. In particular, this route to 'brighten' low-energy molecular triplet excitons may permit singlet exciton fission sensitization of conventional silicon solar cells. [ABSTRACT FROM AUTHOR]

Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications. [ABSTRACT FROM AUTHOR]

Singlet exciton fission splits one singlet exciton into two triplet excitons. Using a joint analysis of photocurrent and fluorescence modulation under a magnetic field, we determine that the triplet yield within optimized tetracene organic photovoltaic devices is 153%±5% for a tetracene film thickness of 20 nm. The corresponding internal quantum efficiency is 127%±18%. These results are used to prove the effectiveness of a simplified triplet yield measurement that relies only on the magnetic field modulation of fluorescence. Despite its relatively slow rate of singlet fission, the measured triplet yields confirm that tetracene is presently the best candidate for use with silicon solar cells. [ABSTRACT FROM AUTHOR]

A correction to the article "A transferable model for singlet-fission kinetics" that was published in the 2014 issue is presented.