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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.

Sheng et al. (2021) report an increase in the power conversion efficiency of a methylammonium lead iodide perovskite solar cell, which they have attributed to the benefits of in situ perovskite-sensitized upconversion by triplet–triplet annihilation in the organic semiconductor rubrene. In the following, we will discuss why in the device structure presented here upconversion inherently cannot be the underlying cause of the observed effects. Rather, the process of upconversion can fundamentally only lead to a reduction of the device efficiency if the same material is used as the active layer in the photovoltaic devices and as the triplet sensitizer. Therefore, the improved photovoltaic performance must have a different root cause. We follow up with an alternative interpretation for the performance increase observed by the authors.

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.

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.

Perovskite solar cells (PSCs) are very promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature, planar PSCs are of particularly interest for large-scale deployment due to their inherent suitability for flexible substrates and potential for silicon/perovskite tandems. So far, planar PSCs have been prone to large current-voltage hysteresis and low stabilized power output due to a number of issues associated with this kind of device configuration. We find that the suppression of the yellow-phase impurity (∂-FAPbI3) present in formamidium-based perovskites, by RbI addition, contributes to low hysteresis, higher charge carrier mobility, long-lived carrier lifetimes and a champion stabilized power output of 20.3% using SnOx as the electron selective contact. We study the effects of these impurities on the transient behavior that defines hysteresis and its relation to ionic movement. In addition, we find that the formation of a RbPbI3 phase does not significantly affect the charge carrier lifetimes and consequently the performance of the devices. This brings new physical insights onto the role of different impurities in perovskite solar cells, which make these materials so remarkable., US Department of Energy, Office of Science, Office of Basic Energy Sciences (award no. DE-SC0001088)

Correction for ‘Colloidal atomic layer deposition growth of PbS/CdS core/shell quantum dots’ by Michel Nasilowski et al., Chem. Commun., 2017, 53, 869–872.