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The ability to manipulate low-energy triplet excited states into higher-energy emissive singlet states, a process known as photon upconversion (UC), has potential applications in bioimaging, photocatalysis, and in increasing the efficiency of solar cells. However, the overall UC mechanism is complex and can involve many intermediate states, especially when semiconductors such as lead halide perovskites are used to sensitize the required triplet states. Using a combination of pulse radiolytic and electrochemical techniques, we have now explored the transient features of rubrene─a commonly employed triplet annihilator in UC systems. The rubrene triplet, radical anion, and radical cation species yield unique spectra that can serve as spectral fingerprints to distinguish between transient species formed during UC processes. Using detailed kinetic studies, we have succeeded in establishing that the rubrene triplets are susceptible to self-quenching (

Transient chemistry of sensitizing dyes is important to obtain insights into the photochemical conversion processes of light harvesting assemblies. We have now employed transient absorption spectroscopy (pulsed laser and pulse radiolysis) to characterize the excited state and radical intermediates of a perylene derivative, (5,10,15,20-Tetraphenylbisbenz[5,6]indeno[1,2,3

Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO

Bipolar membranes (BPMs) which consist of a cation exchange layer (CEL) and anion exchange layer (AEL) are quite effective as membranes in gas phase electrolyzers. However, such membranes can also ...

Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr

Ternary semiconductors such as AgInS

ConspectusThe optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br0.5I0.5)3) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br0.5I0.5)3 films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br0.5I0.5)3 is considered to be an important step in inducing halide mobility in photoirradiated films. This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.

The instability of cesium lead bromide (CsPbBr3) nanocrystals (NCs) in polar solvents has hampered their use in photocatalysis. We have now succeeded in synthesizing CsPbBr3–CdS heterostructures with improved stability and photocatalytic performance. While the CdS deposition provides solvent stability, the parent CsPbBr3 in the heterostructure harvests photons to generate charge carriers. This heterostructure exhibits longer emission lifetime (τave = 47 ns) than pristine CsPbBr3 (τave = 7 ns), indicating passivation of surface defects. We employed ethyl viologen (EV2+) as a probe molecule to elucidate excited state interactions and interfacial electron transfer of CsPbBr3–CdS NCs in toluene/ethanol mixed solvent. The electron transfer rate constant as obtained from transient absorption spectroscopy was 9.5 × 1010 s−1 and the quantum efficiency of ethyl viologen reduction (ΦEV+˙) was found to be 8.4% under visible light excitation. The Fermi level equilibration between CsPbBr3–CdS and EV2+/EV+˙ redox couple has allowed us to estimate the apparent conduction band energy of the heterostructure as −0.365 V vs. NHE. The insights into effective utilization of perovskite nanocrystals built around a quasi-type II heterostructures pave the way towards effective utilization in photocatalytic reduction and oxidation processes.

Halide ion mobility in metal halide perovskites plays an important role in dictating the overall device performance and long-term stability of perovskite solar cells. Alloying with chloride (Cl), w...

Visible light irradiation of a mixed halide perovskite film in contact with a solvent (dichloromethane, DCM) in which the film otherwise is stable leads to selective expulsion of iodide (I) from th...

Two-dimensional (2D) lead halide perovskites represent an emerging class of materials given their tunable optoelectronic properties and long-term stability in perovskite solar cells. In order to as...

Two-dimensional (2D) lead halide perovskites with better chemical stability and tunable dimensionality offer new opportunities to design optoelectronic devices. We have probed the transient absorption behavior of 2D lead halide (bromide and iodide) perovskites of different dimensionality, prepared by varying the ratio of methylammonium:phenylethylammonium cation. With decreasing dimensionality (

In metal halide perovskite solar cells, electron transport layers (ETLs) such as TiO2 dictate the overall photovoltaic performance. However, the same electron capture property of ETL indirectly imp...

Lead-free halide double perovskites offer an environmentally friendly alternative to lead halide perovskites for designing optoelectronic solar cell devices. One simple approach to synthesize such double halide perovskites is through metal ion exchange. CsPbBr

In-situ polymer capping of cesium lead bromide (CsPbBr3) nanocrystals with polymethyl acrylate is an effective approach to improve the colloidal stability in the polar medium and thus extends their use in photocatalysis. The photoinduced electron transfer properties of polymethyl acrylate (PMA)-capped CsPbBr3 nanocrystals have been probed using surface-bound viologen molecules with different alkyl chains as electron acceptors. The apparent association constant ( K app) obtained for the binding of viologen molecules with PMA-capped CsPbBr3 was 2.3 × 107 M−1, which is an order of magnitude greater than that obtained with oleic acid/oleylamine-capped CsPbBr3. Although the length of the alkyl chain of the viologen molecule did not show any impact on the electron transfer rate constant, it influenced the charge separation efficiency and net electron transfer quantum yield. Viologen moieties with a shorter alkyl chain length exhibited a charge separation efficiency of 72% compared with 50% for the longer chain alkyl chain length viologens. Implications of polymer-capped CsPbBr3 perovskite nanocrystals for carrying out photocatalytic reduction in the polar medium are discussed.

2D lead halide perovskites, which exhibit bandgap tunability and increased chemical stability, have been found to be useful for designing optoelectronic devices. Reducing dimensionality with decreasing number of layers (n = 10-1) also imparts resistance to light-induced ion migration as seen from the halide ion segregation and dark recovery in mixed halide (Br:I = 50:50) perovskite films. The light-induced halide ion segregation efficiency, as determined from difference absorbance spectra, decreases from 20% to

Polysulfide electrolyte that is employed as a redox electrolyte in quantum dot sensitized solar cells provides stability to the cadmium chalcogenide photoanode but introduces significant redox limitations at the counter electrode through undesirable surface reactions. By designing reduced graphene oxide (RGO)-Cu

The instability of cesium lead bromide (CsPbBr3) nanocrystals (NCs) in polar solvents has hampered their use in photocatalysis. We have now succeeded in synthesizing CsPbBr3–CdS heterostructures with improved stability and photocatalytic performance. While the CdS deposition provides solvent stability, the parent CsPbBr3 in the heterostructure harvests photons to generate charge carriers. This heterostructure exhibits longer emission lifetime (τave = 47 ns) than pristine CsPbBr3 (τave = 7 ns), indicating passivation of surface defects. We employed ethyl viologen (EV2+) as a probe molecule to elucidate excited state interactions and interfacial electron transfer of CsPbBr3–CdS NCs in toluene/ethanol mixed solvent. The electron transfer rate constant as obtained from transient absorption spectroscopy was 9.5 × 1010 s−1 and the quantum efficiency of ethyl viologen reduction (ΦEV+˙) was found to be 8.4% under visible light excitation. The Fermi level equilibration between CsPbBr3–CdS and EV2+/EV+˙ redox couple has allowed us to estimate the apparent conduction band energy of the heterostructure as −0.365 V vs. NHE. The insights into effective utilization of perovskite nanocrystals built around a quasi-type II heterostructures pave the way towards effective utilization in photocatalytic reduction and oxidation processes., The insights into effective utilization of perovskite nanocrystals built around a CsPbBr3–CdS heterostructure pave the way towards their utilization in photocatalytic reduction and oxidation processes.

Lead-free double perovskites such as Cs2AgBiBr6 are gaining attention because of their environmental friendliness compared to the lead halide perovskites. In order to establish their photoactivity,...

Interfacial charge transfer between a semiconductor nanocrystal and a molecular relay is an important step in nanomaterial photocatalysis. The ferrocene redox couple (Fc+/Fc0, E0 = −4.9 eV vs vacuu...

MAPbBr3 and MAPbI3 films cast onto glassslides and physically paired together undergo halide exchange to formmixed halide films. The change in halide composition in these two∼130 nm thick films occurs concurrently with Br– diffusing toward the MAPbI3 film and I– diffusing toward the MAPbBr3 film. The diffusion of thesehalide species, which is tracked through changes in the absorption,offers a direct measurement of thermally activated halide diffusionin perovskite films. The increase in the rate constant of halide diffusionwith increasing temperature (from 8.3 × 10–6 s–1 at 23 °C to 3.7 × 10–4 s–1 at 140 °C) follows an Arrhenius relationshipwith activation energy of 51 kJ/mol. The thermally activated halideexchange shows the challenges of employing layers of different metalhalide perovskites in stable tandem solar cells.