Understanding photophysical processes in lead halide perovskites is an important aspect of optimizing the performance of optoelectronic devices. The determination of exact charge carrier extraction rate constants remains elusive, as there is a large and persistent discrepancy in the reported absolute values. In this review, we concentrate on experimental procedures adopted in the literature to obtain kinetic estimates of charge transfer processes and limitations imposed by the spectroscopy technique employed. Time-resolved techniques (e.g., transient absorption-reflection and time-resolved photoluminescence spectroscopy) are commonly employed to probe charge transfer at perovskite/transport layer interfaces. The variation in sample preparation and measurement conditions can produce a wide dispersion of the measured kinetic parameters. The selected time window and the kinetic fitting model employed introduce additional uncertainty. We discuss here evaluation strategies that rely on multiexponential fitting protocols (regular or stretched) and show how the dispersion in the reported values for carrier transfer rate constants can be resolved.
A triplet-triplet annihilation-based upconversion (TTA-UC) system, employing a multichromophore assembly, is convenient to harvest low-energy photons for light energy conversion and optoelectronic applications. The primary donor in the TTA-UC system, typically a low-bandgap semiconductor, captures the low-energy photons and transfers triplet energy to an annihilator dye molecule, which in turn generates a high-energy singlet excited state via T-T annihilation. We have now succeeded in revealing kinetic and mechanistic details of multistep energy transfer processes in the CsPbI3-rubrene-perylene derivative (DBP) films by analyzing time-resolved emission and absorption measurements. The initial triplet energy transfer between CsPbI3 and rubrene occurs with an efficiency of 70% and a rate constant of 9 × 108 s-1. The rubrene triplets undergo T-T annihilation via simple second-order kinetics to form an excited singlet state exhibiting a delayed emission up to 10 µs, which is significantly greater than the intrinsic lifetime of 15 ns. The emitter DBP (a perylene derivative) captures the singlet energy quite effectively and delivers the upconverted emission in sync with the delayed emission of rubrene. The quadratic dependence of DBP emission on the excitation light intensity shows the importance of the T-T annihilation process in dictating the overall upconversion process. The kinetic parameters evaluated in this study, which divulge the critical steps dictating energy transfer in a TTA-UC system, should aid in the design of new light harvesting assemblies.
Extending the lifetime of photogenerated electrons in semiconductor systems is an important criterion for the conversion of light into storable energy. We have now succeeded in storing electrons in a photoirradiated colloidal molybdenum disulfide (MoS2) suspension, showcasing its unique reversible photoresponsive behavior. The dampened A and B excitonic peaks indicate the accumulation of photogenerated electrons and the minimization of interactions between MoS2 interlayers. The stored electrons were quantitatively extracted by titrating with a ferrocenium ion in the dark, giving ca. 0.2 electrons per MoS2 formula unit. The emergence of the photoinduced A1g* Raman mode and the decrease in zeta potential after irradiation suggest intercalation of counterions to maintain overall charge balance upon electron storage. These results provide insights into the mechanism of photogenerated electron storage in 2D materials and pave the way for the potential application of colloidal 2D materials in electron storage.
Lead halide perovskite nanocrystals, which offer rich photochemistry, have the potential to capture photons over a wide range of the visible and infrared spectrum for photocatalytic, optoelectronic, and photon conversion applications. Energy transfer from the perovskite nanocrystal to an acceptor dye in the form of a triplet or singlet state offers additional opportunities to tune the properties of the semiconductor-dye hybrid and extend excited-state lifetimes. We have now successfully established the key factors that dictate triplet energy transfer between excited CsPbI3 and surface-bound rhodamine dyes using absorption and emission spectroscopies. The pendant groups on the acceptor dyes influence surface binding to the nanocrystals, which in turn dictate the energy transfer kinetics, as well as the efficiency of energy transfer. Of the three rhodamine dyes investigated (rhodamine B, rhodamine B isothiocyanate, and rose Bengal), the CsPbI3-rose Bengal hybrid with the strongest binding showed the highest triplet energy transfer efficiency (96%) with a rate constant of 1 × 109 s-1. This triplet energy transfer rate constant is nearly 2 orders of magnitude slower than the singlet energy transfer observed for the pure-bromide CsPbBr3-rose Bengal hybrid (1.1 × 1011 s-1). Intriguingly, although the single-halide CsPbBr3 and CsPbI3 nanocrystals selectively populate singlet and triplet excited states of rose Bengal, respectively, the mixed halide perovskites were able to generate a mixture of both singlet and triplet excited states. By tuning the bromide/iodide ratio and thus bandgap energy in CsPb(Br1-xIx)3 compositions, the percentage of singlets vs triplets delivered to the acceptor dye was systematically tuned from 0 to 100%. The excited-state properties of halide perovskite-molecular hybrids discussed here provide new ways to modulate singlet and triplet energy transfer in semiconductor-molecular dye hybrids through acceptor functionalization and donor bandgap engineering.
Directing energy flow in light harvesting assemblies of nanocrystal-chromophore hybrid systems requires a better understanding of factors that dictate excited-state processes. In this study, we explore excited-state interactions within the CsPbI3-cyanine dye (IR125) hybrid assembly through a comprehensive set of steady-state and time-resolved absorption and photoluminescence (PL) experiments. Our photoluminescence investigations reveal the quenching of CsPbI3 emission alongside the simultaneous enhancement of IR125 fluorescence, providing evidence for a singlet energy transfer. The evaluation of both photoluminescence (PL) quenching and PL decay measurements yield â¼94% energy transfer efficiency for the CsPbI3-IR125 hybrid assembly. Transient absorption spectroscopy further unveils that this singlet energy transfer process operates on an ultrafast time scale, occurring within 400 ps with a rate constant of energy transfer of 1.4 × 1010 s-1. Our findings highlight the potential of the CsPbI3-IR125 hybrid assembly to extend the emission of halide perovskites into the infrared region, paving the way for light energy harvesting and display applications.
Photocatalytic reactions at semiconductor nanocrystal surfaces are useful for synthesizing value-added chemicals using sunlight. Semiconductor nanocrystals dispersed in a rigid framework, such as polymer film, can mitigate issues such as aggregation, product separation, and other challenges that are usually encountered in suspensions or slurries. Using a cation exchange technique, we successfully embedded AgInS2 nanoparticles into a Nafion matrix, termed AgInS2-Nafion. This was achieved through a galvanic exchange between In and Ag in In2S3 present within the Nafion film, enabling an adjustable Ag:In ratio for optimized photophysical properties. As in the case of colloidal suspension, the AgInS2 particles embedded in Nafion exhibit a long absorption tail, a broad emission band with a large Stokes shift, and emission lifetimes extending into the microseconds that are characteristic of donor-acceptor pairs, DAP. Remediation of surface states with the treatment of 3-mercaptopropionic acid resulted in significant enhancement in the emission yield. Charge carrier generation through bandgap excitation as well as activation of DAP states which reside within the bandgap is probed through transient absorption spectroscopy. The photocatalytic activity of AgInS2-Nafion was probed by using thionine as an electron acceptor. The electron transfer rate constant from excited AgInS2 to thionine as observed from transient absorption spectroscopy was determined to be â¼6.3 × 1010 s-1. The design of a photoactive membrane offers new ways to carry out photocatalytic processes with greater selectivity.
Movement of energy within light-harvesting assemblies is typically carried out with separately synthesized donor and acceptor species, which are then brought together to induce an interaction. Recently, two-dimensional (2D) lead halide perovskites have gained interest for their ability to accommodate and assemble chromophoric molecules within their lattice, creating hybrid organic-inorganic compositions. Using a combination of steady-state and time-resolved absorption and emission spectroscopy, we have now succeeded in establishing the competition between energy transfer and charge trapping in 2D halide perovskite colloids containing naphthalene-derived cations (i.e., NEA2PbX4, where NEA = naphthylethylamine). The presence of room-temperature triplet emission from the naphthalene moiety depends on the ratio of bromide to iodide in the lead halide sublattice (i.e., x in NEA2Pb(Br1-xIx)4), with only bromide-rich compositions showing sensitized emission. Photoluminescence lifetime measurements of the sensitized naphthalene reveal the formation of the naphthalene triplet excimer at room temperature. From transient absorption measurements, we find the rate constant of triplet energy transfer (kEnT) to be on the order of â¼109 s-1. At low temperatures (77 K) a new broad emission feature arising from trap states is observed in all samples ranging from pure bromide to pure iodide composition. These results reveal the interplay between sensitized triplet energy transfer and charge trapping in 2D lead halide perovskites, highlighting the need to carefully parse contributions from competing de-excitation pathways for optoelectronic applications.
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 (Kapp) 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.
Energy and electron transfer processes allow for efficient manipulation of excited states within light harvesting assemblies for photocatalytic and optoelectronic applications. We have now successfully probed the influence of acceptor pendant group functionalization on the energy and electron transfer between CsPbBr3 perovskite nanocrystals and three rhodamine-based acceptor molecules. The three acceptorsârhodamine B (RhB), rhodamine isothiocyanate (RhB-NCS), and rose Bengal (RoseB)âcontain an increasing degree of pendant group functionalization that affects their native excited state properties. When interacting with CsPbBr3 as an energy donor, photoluminescence excitation spectroscopy reveals that singlet energy transfer occurs with all three acceptors. However, the acceptor functionalization directly influences several key parameters that dictate the excited state interactions. For example, RoseB binds to the nanocrystal surface with an apparent association constant (Kapp = 9.4 × 106 M-1) 200 times greater than RhB (Kapp = 0.05 × 106 M-1), thus influencing the rate of energy transfer. Femtosecond transient absorption reveals the observed rate constant of singlet energy transfer (kEnT) is an order-of-magnitude greater for RoseB (kEnT = 1 × 1011 s-1) than for RhB and RhB-NCS. In addition to energy transfer, each acceptor had a subpopulation of molecules (â¼30%) that underwent electron transfer as a competing pathway. Thus, the structural influence of acceptor moieties must be considered for both excited state energy and electron transfer in nanocrystal-molecular hybrids. The competition between electron and energy transfer further highlights the complexity of excited state interactions in nanocrystal-molecular complexes and the need for careful spectroscopic analysis to elucidate competitive pathways.
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-cd:1',2',3'-lm]perylene (DBP). The distinguishable transient absorption features for the singlet and triplet excited states and radical anion and radical cation provide spectral fingerprints to identify the reaction intermediates in photochemical energy and electron transfer processes of composite systems involving DBP. For example, identifying these transients in the energy transfer processes of the rubrene-DBP system would aid in establishing their role as annihilator-emitter for triplet-triplet annihilation up-conversion (TTA-UC). The transient characterization thus serves as an important mechanistic fingerprint for elucidating mechanistic details of systems employing DBP in optoelectronic applications.
Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO3/BiVO4 heterojunction photoanodes perform much better than the separate oxide components, though internal charge recombination undermines their PEC performance when both oxides absorb light. Here we exploit the BiVO4 layer to sensitize WO3 to visible light and shield it from direct photoexcitation to overcome this efficiency loss. PEC experiments and ultrafast transient absorption spectroscopy performed by frontside (through BiVO4) or backside (through WO3) irradiating photoanodes with different BiVO4 layer thickness demonstrate that irradiation through BiVO4 is beneficial for charge separation. Optimized electrodes irradiated through BiVO4 show 40% higher photocurrent density compared to backside irradiation.
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 rubreneâa 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 (kquench = 3.6 × 108 M-1 s-1), and as the triplets decay, an additional transient feature is observed in the transient absorption spectra. This new feature indicates a net electron transfer process occurs to form the radical cation and anion as the triplets recombine. Taken together, this work provides a comprehensive picture of the excited state and transient features of rubrene and will be crucial for understanding the mechanism(s) of photon upconversion systems.
Photons , Electron Transport , Kinetics , Naphthacenes
Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr3 with relatively high emission yield and strong light absorption can transfer singlet and triplet energy to surface-bound acceptor molecules. They can also induce photocatalytic reduction and oxidation by selectively transferring electrons and holes across the nanocrystal interface. This perspective discusses key factors dictating these excited-state pathways in perovskite nanocrystals and the fundamental differences between energy and electron transfer processes. Spectroscopic methods to decipher between these complex photoinduced pathways are presented. A basic understanding of the fundamental differences between the two excited deactivation processes (charge and energy transfer) and ways to modulate them should enable design of more efficient light harvesting assemblies with semiconductor and molecular systems.
Electrons , Nanoparticles , Calcium Compounds , Nanoparticles/chemistry , Oxides , Titanium/chemistry
Ternary semiconductors such as AgInS2, with their interesting photocatalytic properties, can serve as building blocks to design light harvesting assemblies. The intraband transitions created by the metal ions extend the absorption well beyond the bandgap transition. The interfacial electron transfer of AgInS2 with surface bound ethyl viologen under bandgap and sub-bandgap irradiation as probed by steady state photolysis and transient absorption spectroscopy offers new insights into the participation of conduction band and trapped electrons. Capping AgInS2 with CdS shifts emission maximum to the blue and increases the emission yield as the surface defects are remediated. CdS capping also promotes charge separation as evident from the efficiency of electron transfer to ethyl viologen, which increased from 14% to 29%. The transient absorption measurements that elucidate the kinetic aspects of electron transfer processes in AgInS2 and CdS capped AgInS2 are presented. The improved performance of CdS capped AgInS2 offers new opportunities to employ them as photocatalysts.
