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Perovskite-sensitized triplet-triplet annihilation (TTA) upconversion (UC) holds potential for practical applications of solid-state UC ranging from photovoltaics to sensing and imaging technologies. As the triplet sensitizer, the underlying perovskite properties heavily influence the generation of spin-triplet states once interfaced with the organic annihilator molecule, typically polyacene derivatives. Presently, most reported perovskite TTA-UC systems have utilized rubrene doped with ∼1% dibenzotetraphenylperiflanthene (RubDBP) as the annihilator/emitter species. However, practical applications require a larger apparent anti-Stokes than is currently achievable with this system due to the inherent 0.4 eV energy loss during triplet generation. In this minireview, we present the current understanding of the triplet sensitization process at the perovskite/organic semiconductor interface and introduce additional promising annihilators based on anthracene derivatives into the discussion of future directions in perovskite-sensitized TTA-UC.

CdSe nanoplatelets (NPLs) have been reported as triplet sensitizers for photon upconversion (UC). However, their UC quantum yields lag behind more conventional systems. Here, we take advantage of their one-dimensional quantum confinement to decouple effects caused by the energetic driving force and lateral size. A surprising anti-correlation between the power threshold

One of the major limitations of solid-state perovskite-sensitized photon upconversion to date is that the only annihilator successfully paired with the perovskite sensitizer has been rubrene, raising the question of whether this appraoch of triplet sensitization is universal or limited in scope. Additionally, the inherent energetic mismatch between the perovskite bandgap and the rubrene triplet energy has restricted the apparent anti-Stokes shift achievable in the upconversion process. To increase the apparent anti-Stokes shift for upconversion processes, anthracene derivates are of particular interest due to their higher triplet energies. Here, we demonstrate successful sensitization of the triplet state of 1-chloro-9,10-bis(phenylethynyl)anthracene using the established formamidinium methylammonium lead triiodide perovskite FA

Photon upconversion (UC) stands for the conversion of low to high energy photons, a promising approach to improve solar cells. While high efficiencies can be obtained in liquid UC, will a solid UC device be able to reach such levels?

Triplet–triplet annihilation-based photon upconversion (TTA-UC) is a promising mechanism for harvesting lower energy photons by converting them to a higher energy. Photons generated from this process can be used for numerous applications, including photovoltaics, infrared sensing and imaging, biomedicine and photochemical reactions. Recently, bulk metal halide perovskite semiconductors have been introduced as triplet sensitizers for the TTA-UC process. While relatively efficient upconversion has been achieved at low fluences, the full potential of these materials as triplet sensitizers has not been unlocked. Here, we examine four pathways for device optimization and improvements, while discussing relevant works and potential further improvements. Finally, we discuss the outlook and bright future of such perovskite materials as triplet sensitizers and their important role in solid-state upconversion applications.

Crystal engineering is a practical approach for tailoring material properties. This approach has been widely studied for modulating optical and electrical properties of semiconductors. However, the properties of organic molecular crystals are difficult to control following a similar engineering route. In this Letter, we demonstrate that engineered crystals of Alq3 and Ir(ppy)3 complexes, which are commonly used in organic light-emitting technologies, possess intriguing functional properties. Specifically, these structures not only process efficient low-energy induced triplet excitation directly from the ground state of Alq3 but also can show strong emission at the Alq3 triplet energy level at room temperatures. We associate these phenomena with local deformations of the host matrix around the guest molecules, which in turn lead to a stronger host-guest triplet-triplet coupling and spin-orbital mixing.

Solid-state bulk lead halide perovskite thin films have recently shown progress as triplet sensitizers in infrared-to-visible photon upconversion (UC) schemes. Common systems pair lead halide perov...

Metal halide perovskite materials have recently upended the field of photovoltaics and are aiming to make waves across a multitude of other fields and applications. Recently, perovskite nanocrystals have been synthesized and are rapidly outpacing traditional semiconductor nanocrystals in application driven fields due to their inherent defect tolerance and facile tunability, resulting in high photoluminescent quantum yields and efficient devices. Future improvements to perovskite nanocrystals toward device driven applications must come at the perovskite surface. The last half decade has resulted in considerable progress in tailoring the perovskite nanocrystal/ligand surface toward maximizing the optoelectronic performance. Here, we review the current progress and discuss how further improvements could be made to further improve this bright class of materials.

The stability of organic–inorganic halide perovskite films plays an important role for their successful incorporation as absorber materials in solar cells under realistic operation conditions. While light-induced effects have been observed and traced to phase segregation, the impact of different stressors simultaneously is mostly unexplored. In this work, we investigate the combined influence of light and elevated temperature on the performance of mixed-cation mixed-halide perovskites. In particular, we compare the effect of different A-site cations on the photoluminescence (PL) properties and film stability when both stressors are used simultaneously. We find two pathways underlying the PL peak reduction and PL shift in the optical properties. For perovskite films composed of formamidinium and methylammonium as A-site cations, we can correlate the decrease in film performance to the formation of Pb(I,Br)2 and an increase in electron–phonon interactions. Similarly, Rb doping in the perovskite film exhibits comparable results. Contrary, using Cs as an additional A-site cation greatly enhances the overall performance and results in more stable film structures which indicates that Cs is effective in stiffening the perovskite lattice, which can be attributed to a better size match for the Pb(I,Br)3 sublattice as predicted by the Goldschmidt tolerance factor. These findings suggest that it is of importance to carefully select stressors when assessing performance related parameters of perovskite solar cells.

The processes of singlet fission and triplet fusion could allow state-of-the-art photovoltaic devices to surpass the Shockley–Queisser limit by optimizing the utilized solar spectrum by reducing thermal relaxation and inaccessible sub-bandgap photons, respectively. Triplet fusion demands precise control of the spin-triplet state population, and requires a sensitizer to efficiently populate the triplet state of an acceptor molecule. In this perspective, we highlight the established field of sensitized upconversion and further examine alternative triplet sensitization routes, including the possibility of bulk solid-state semiconductors as triplet sensitizers, which provide a new avenue for charge transfer-based triplet sensitization rather than excitonic triplet energy transfer.

The mechanism and elemental composition that form the basis for the improved optical and electronic properties in mixed-ion lead halide perovskite solar cells are still not well understood compared...

The role electron-transport layers (ETLs) play in perovskite solar cells (PSCs) is still widely debated. Conduction band alignment at the perovskite/ETL interface has been suggested to be an import...

Recently, we and others have proposed screening criteria for “defect-tolerant” photovoltaic (PV) absorbers, identifying several classes of semiconducting compounds with electronic structures similar to those of hybrid lead–halide perovskites. In this work, we reflect on the accuracy and prospects of these new design criteria through a combined experimental and theoretical approach. We construct a model to extract photoluminescence lifetimes of six of these candidate PV absorbers, including four (InI, SbSI, SbSeI, and BiOI) for which time-resolved photoluminescence has not been previously reported. The lifetimes of all six candidate materials exceed 1 ns, a threshold for promising early stage PV device performance. However, there are variations between these materials, and none achieve lifetimes as high as those of the hybrid lead–halide perovskites, suggesting that the heuristics for defect-tolerant semiconductors are incomplete. We explore this through first-principles point defect calculations and Shock...

Traditionally, PbS/CdS quantum dots (QDs) have been synthesized via a cation exchange method, making fine control over shell growth challenging. We show here that colloidal atomic layer deposition (c-ALD) allows for the sequential growth of single monolayers of the shell, thus creating a 'true' CdS shell on PbS QDs.

Nanometer-sized metal clusters are prime candidates for photoactivated catalysis, based on their unique tunable optical and electronic properties, combined with a large surface-to-volume ratio. Due to the very small optical cross sections of such nanoclusters, support-mediated plasmonic activation could potentially make activation more efficient. Our support is a semi-transparent gold film, optimized to work in a back-illumination geometry. It has a surface plasmon resonance excitable in the 510–540 nm wavelength range. Ptn clusters (size distribution peaked at n = 46 atoms) have been deposited onto this support and investigated for photoactivated catalytic performance in the oxidative decomposition of methylene blue. The Pt cluster catalytic activity under illumination exceeds that of the gold support by more than an order of magnitude per active surface area. To further investigate the underlying mechanism of plasmon-induced catalysis, the clusters have been imaged with optically-assisted scanning tunneling microscopy under illumination. The photoactivation of the Pt clusters via plasmonic excitation of the support and subsequential electronic excitation of the clusters can be imaged with nanometer resolution. The light-induced tunneling current on the clusters is enhanced relative to the gold film support.

The process of upconversion leads to emission of photons higher in energy than the incident photons. Near-infrared-to-visible upconversion, in particular, shows promise in sub-bandgap sensitization of silicon and other optoelectronic materials, resulting in potential applications ranging from photovoltaics that exceed the Shockley–Queisser limit to infrared imaging. A feasible mechanism for near-infrared-to-visible upconversion is triplet–triplet annihilation (TTA) sensitized by colloidal nanocrystals (NCs). Here, the long lifetime of spin-triplet excitons in the organic materials that undergo TTA makes upconversion possible under incoherent excitation at relatively low photon fluxes. Since this process relies on optically inactive triplet states, semiconductor NCs are utilized as efficient spin mixers, absorbing the incident light and sensitizing the triplet states of the TTA material. The state-of-the-art system uses rubrene with a triplet energy of 1.14 eV as the TTA medium, and thus allows upconversion of light with photon energies above ∼1.1 eV. In this perspective, we review the field of lead sulfide (PbS) NC-sensitized near-infrared-to-visible upconversion, discuss solution-based upconversion, and highlight progress made on solid-state upconversion devices.

Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics, and other important optoelectronic properties. We capture nanometer resolution images of electronically excited PbS quantum dots (QDs) by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ∼1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited-state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the QDs under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the QDs. Excitation of adjacent QD pairs reveals orbital alignment, evidence for electronic coupling between dots. Electronic structure modeling of a small PbS QD, when scaled for size, reveals Stark tuning and variation in the transition moment of different parity states, supporting the simple one-electron experimental interpretation in the hot carrier limit. The calculations highlight the sensitivity of orbital density to applied field, laser wavelength, and structural fluctuations of the QD.

Single-molecule experiments based on Forster resonant energy transfer (FRET) or on single molecule absorption spectroscopy (SMA) are now capable of studying energy funneling, exciton blockade, singlet fission, and a variety of other processes that involve multiple photoactive groups interacting on a single molecular backbone. Here, we present synthesis and optical characterization of a new dendron functionalized with two green donor dyes (Cy3) and one red acceptor dye (Cy5) through flexible linkers. We describe in detail the synthesis of the conjugated network and the flexible dye coupling. Characterization of the dendron and of control molecules with fewer donors or no acceptor by ensemble absorption and emission spectroscopy shows that the system is capable of light harvesting, producing an intramolecular FRET signal from the acceptor greater than expected from a single donor. We also investigate intramolecular energy transfer upon UV excitation of the conjugated backbone. The photophysical behavior of this light harvesting dendron can be rationalized by a simple Forster/superexchange model. Simulations and scanning tunneling microscopy of single dendron molecules show that the dyes can fold over onto the dendron, creating a heterogeneous distribution of conformations suitable for single molecule studies of light harvesting.