Tuning Energy Transfer Pathways in Halide Perovskite-Dye Hybrids through Bandgap Engineering.

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.

Energy Versus Electron Transfer: Managing Excited-State Interactions in Perovskite Nanocrystal-Molecular Hybrids.

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.

How Pendant Groups Dictate Energy and Electron Transfer in Perovskite-Rhodamine Light Harvesting Assemblies.

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 acceptors─rhodamine B (RhB), rhodamine isothiocyanate (RhB-NCS), and rose Bengal (RoseB)─contain 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 (∼30%) 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.

Excited State and Transient Chemistry of a Perylene Derivative (DBP). An Untold Story.

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.

Excited-State Transient Chemistry of Rubrene: A Whole Story.

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 (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.

Directing Energy Transfer in Halide Perovskite-Chromophore Hybrid Assemblies.

Directing the flow of energy and the nature of the excited states that are produced in nanocrystal-chromophore hybrid assemblies is crucial for realizing their photocatalytic and optoelectronic applications. Using a combination of steady-state and time-resolved absorption and photoluminescence (PL) experiments, we have probed the excited-state interactions in the CsPbBr3-Rhodamine B (RhB) hybrid assembly. PL studies reveal quenching of the CsPbBr3 emission with a concomitant enhancement of the fluorescence of RhB, indicating a singlet-energy-transfer mechanism. Transient absorption spectroscopy shows that this energy transfer occurs on the ∼200 ps time scale. To understand whether the energy transfer occurs through a Förster or Dexter mechanism, we leveraged facile halide-exchange reactions to tune the optical properties of the donor CsPbBr3 by alloying with chloride. This allowed us to tune the spectral overlap between the donor CsPb(Br1-xClx)3 emission and acceptor RhB absorption. For CsPbBr3-RhB, the rate constant for energy transfer (kET) agrees well with Förster theory, whereas alloying with chloride to produce chloride-rich CsPb(Br1-xClx)3 favors a Dexter mechanism. These results highlight the importance of optimizing both the donor and acceptor properties to design light-harvesting assemblies that employ energy transfer. The ease of tuning optical properties through halide exchange of the nanocrystal donor provides a unique platform for studying and tailoring excited-state interactions in perovskite-chromophore assemblies.

TiO2-Assisted Halide Ion Segregation in Mixed Halide Perovskite Films.

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 impacts halide ion mobility as evident from the TiO2-assisted halide ion segregation in mixed halide perovskite (MHP) films under pulsed laser excitation (387 nm, 500 Hz). This segregation is only observed when deposited on an ETL such as TiO2 but not on insulating ZrO2 substrate. Injection of electrons from excited MHP into the ETL (ket = 1011 s-1) followed by scavenging of electrons by O2 causes hole accumulation in the MHP film. Localization of holes on the iodide site in the MHP induces instability causing iodide from the lattice to move away toward grain boundaries. Suppression of segregation occurs when holes are extracted by a hole transport layer (spiro-OMeTAD) deposited on the MHP, thus avoiding hole build-up. These results provide further insight into the role of holes in the phase segregation of MHPs and hole mobility in perovskite solar cells.

Synthesis and Assembly of Dipolar Heterostructured Tetrapods: Colloidal Polymers with "Giant tert-butyl" Groups.

We report on the first synthesis of a heterostructured semiconductor tetrapod from CdSe@CdS that carries a single dipolar nanoparticle tip from a core-shell colloid of Au@Co. A four-step colloidal total synthesis was developed, where the key step in the synthesis was the selective deposition of a single AuNP tip onto a CdSe@CdS tetrapod under UV-irradiation. Synthetic accessibility to this dipolar heterostructured tetrapod enabled the use of these as colloidal monomers to form colloidal polymers that carry the semiconductor tetrapod as a side chain group attached to the CoNP colloidal polymer main chain. The current report details a number of novel discoveries on the selective synthesis of an asymmetric heterostructured tetrapod that is capable of 1D dipolar assembly into colloidal polymers that carry tetrapods as side chain groups that mimic "giant tert-butyl groups".

Physical Chemistry Education and Research in an Open-Sourced Future.

Proficiency in physical chemistry requires a broad skill set. Successful trainees often receive mentoring from senior colleagues (research advisors, postdocs, etc.). Mentoring introduces trainees to experimental design, instrumental setup, and complex data interpretation. In lab settings, trainees typically learn by customizing experimental setups, and developing new ways of analyzing data. Learning alongside experts strengthens these fundamentals, and places a focus on the clear communication of research problems. However, this level of input is not scalable, nor can it easily be shared with all researchers or students, particularly those that face socioeconomic barriers to accessing mentoring. New approaches to training will therefore progress the field of physical chemistry. Technology is disrupting and democratising scientific education and research. The emergence of free online courses and video resources enables students to learn in a style that suits them. Higher degrees of automation remove cumbersome and sometimes arbitrary technical barriers to learning new techniques, allowing one to collect high quality data quickly. Open sourcing of data and analysis tools has increased transparency, lowered barriers to access, and accelerated scientific dissemination. However, these advances also can lead to "black box" approaches to acquiring and analyzing data, where convenience replaces understanding and errors and misrepresentations become more common. The risk is a breakdown in education: if one does not understand the fundamentals of a technique or analysis, it is difficult to correctly discern the practical limits of an experiment, distinguish signal from noise, troubleshoot problems, or take full advantage of powerful analytical procedures. Our vision of the future of physical chemistry is built around democratized learning, where deep technical and analytical expertise from physical chemists is made freely available. Advancements in technical education through expert-generated educational resources and AI-based tools will enrich physical chemistry education. A holistic approach to education will prepare the physical chemists of 2050 to adapt to rapidly advancing technological tools, which accelerate the pace of research. Technical education will be enhanced by accessible open-source instrumentation and analysis procedures, which will provide instruments and analysis scripts specifically designed for education. High quality, comparable data from standardized open-source instruments will feed into accessible databases and analysis projects, providing others the opportunity to store and analyze both failed and successful experiments. The coupling of open-source education, hardware, and analysis will democratize physical chemistry while addressing risks associated with "black box" approaches.

Trap or Triplet? Excited-State Interactions in 2D Perovskite Colloids with Chromophoric Cations.

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 ∼109 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.

Photoinduced Halide Segregation in Ruddlesden-Popper 2D Mixed Halide Perovskite Films.

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 <1% as the dimensionality is decreased for 2D perovskite film from n = 10 to 1. The segregation rate constant (ksegregation ), which decreases from 5.9 × 10-3  s-1 (n = 10) to 3.6 × 10-4  s-1 (n = 1), correlates well with nearly an order of magnitude decrease observed in charge-carrier lifetime (τaverage  = 233 ps for n = 10 vs τavg  = 27 ps for n = 1). The tightly bound excitons in 2D perovskites make charge separation less probable, which in turn decreases the halide mobility and resulting phase segregation. The importance of controlling the dimensionality of the 2D architecture in suppressing halide ion mobility is discussed.

Electrochemically induced iodine migration in mixed halide perovskites: suppression through chloride insertion.

The role of chloride in improving the stability of mixed halide perovskites (MAPbClxBr0.5(1-x)I0.5(1-x))3 is probed using spectroelectrochemistry. The injection of holes into mixed halide perovskite films through applied anodic bias results in the selective migration of iodine with ultimate expulsion into the electrolyte. Increasing the Cl content (x = 0 to 0.1) in the mixed halide perovskite suppresses the iodine mobility and thus decreases the rate of its expulsion into the solution. Implications of iodine mobility induced by hole accumulation and its impact on overall stability is discussed.

Modulation of Photoinduced Iodine Expulsion in Mixed Halide Perovskites with Electrochemical Bias.

Hole trapping at iodine (I) sites in MAPbBr1.5I1.5 mixed halide perovskites (MHP) is responsible for iodine migration and its eventual expulsion into solution. We have now modulated the photoinduced iodine expulsion in MHP through an externally applied electrochemical bias. At positive potentials, electron extraction at TiO2/MHP interfaces becomes efficient, leading to hole buildup within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (kexpulsion = 0.0030 s-1). Conversely, at negative potentials (-0.3 V vs Ag/AgCl) electron-hole recombination is facilitated within MHP, slowing down iodine expulsion by an order of magnitude (kexpulsion = 0.00018 s-1). The tuning of the EFermi level through external bias modulates electron extraction at the TiO2/MHP interface and indirectly controls the buildup of holes, ultimately inducing iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.

Charge Carrier Recombination Dynamics of Two-Dimensional Lead Halide Perovskites.

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 (n = ∞ → 1), we observe a blue shift in transient absorption bleach in agreement with the trend observed with the shift in the excitonic peak. The lifetime of the charge carriers decreased with decreasing layer thickness. The dependence of charge carrier lifetime on the 2D layers as well as the halide ion composition shows the dominance of excitonic binding energy on the charge carrier recombination in 2D perovskites. The excited-state behavior of 2D perovskites discussed in this study shows the need to modulate the layer dimensionality to obtain desired optoelectronic properties.

Probing Perovskite Photocatalysis. Interfacial Electron Transfer between CsPbBr3 and Ferrocene Redox Couple.

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 vacuum) has now been used as a model redox relay system to investigate photocatalytic properties of CsPbBr3 perovskite nanocrystals. The photocatalytic reduction of ferrocenium (Fc+) to ferrocene (Fc0) with CsPbBr3 nanocrystals was dictated by the surface interactions. Whereas a rapid quenching and subsequent recovery of CsPbBr3 emission is seen at low Fc+ concentrations, the quenched emission was sustained at higher Fc+ concentrations. The photoinduced interfacial electron transfer between CsPbBr3 and ferrocenium (Fc+) studied using transient absorption spectroscopy occurred with a rate constant of 1.64 × 1010 s-1. Better understanding of interfacial processes using redox probes can lead to the improvement in photocatalytic performance of perovskite nanocrystals.

Colloidal Random Terpolymers: Controlling Reactivity Ratios of Colloidal Comonomers via Metal Tipping.

We report on a versatile synthetic method of preparing colloidal copolymers and terpolymers composed of dipolar Au@Co core-shell nanoparticles (NPs) in the backbone, along with semiconductor CdSe@CdS nanorod (NR), or tetrapod (TP) side chain groups. A seven-step colloidal total synthesis enabled the synthesis of well-defined colloidal comonomers composed of a dipolar Au@CoNP attached to a single CdSe@CdS NR, or TP, where magnetic dipolar associations between Au@CoNP units promoted the formation of colloidal co- or terpolymers. The key step in this synthesis was the ability to photodeposit a single AuNP tip onto CdSe@CdS NR or TP that enables selective seeding of a dipolar CoNP onto the AuNP seed. We show that the variation of the AuNP size directly controlled the size and dipolar character of the CoNP tip, where the size modulation of the Au and Au@CoNP tips is analogous to control of comonomer reactivity ratios in classical copolymerization processes.