Randy P. Sabatini

Perovskite energy funnels for efficient light-emitting diodes

Organometal halide perovskites exhibit large bulk crystal domain sizes, rare traps, excellent mobilities and carriers that are free at room temperature-properties that support their excellent performance in charge-separating devices. In devices that rely on the forward injection of electrons and holes, such as light-emitting diodes (LEDs), excellent mobilities contribute to the efficient capture of non-equilibrium charge carriers by rare non-radiative centres. Moreover, the lack of bound excitons weakens the competition of desired radiative (over undesired non-radiative) recombination. Here we report a perovskite mixed material comprising a series of differently quantum-size-tuned grains that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence non-radiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 W sr-1 m-2. These represent the brightest and most efficient solution-processed near-infrared LEDs to date.

Hybrid organic–inorganic inks flatten the energy landscape in colloidal quantum dot solids

Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (Voc) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher Voc and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.

Tailoring the Energy Landscape in Quasi-2D Halide Perovskites Enables Efficient Green-Light Emission

Organo-metal halide perovskites are a promising platform for optoelectronic applications in view of their excellent charge-transport and bandgap tunability. However, their low photoluminescence quantum efficiencies, especially in low-excitation regimes, limit their efficiency for light emission. Consequently, perovskite light-emitting devices are operated under high injection, a regime under which the materials have so far been unstable. Here we show that, by concentrating photoexcited states into a small subpopulation of radiative domains, one can achieve a high quantum yield, even at low excitation intensities. We tailor the composition of quasi-2D perovskites to direct the energy transfer into the lowest-bandgap minority phase and to do so faster than it is lost to nonradiative centers. The new material exhibits 60% photoluminescence quantum yield at excitation intensities as low as 1.8 mW/cm2, yielding a ratio of quantum yield to excitation intensity of 0.3 cm2/mW; this represents a decrease of 2 orders of magnitude in the excitation power required to reach high efficiency compared with the best prior reports. Using this strategy, we report light-emitting diodes with external quantum efficiencies of 7.4% and a high luminescence of 8400 cd/m2.

Two-dimensional femtosecond stimulated Raman spectroscopy: Observation of cascading Raman signals in acetonitrile

A new methodology for two-dimensional Raman spectroscopy-termed two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS)-is presented and experimental results for acetonitrile are discussed. 2D-FSRS can potentially observe molecular anharmonicity by measuring the modulation of the frequency of a probed Raman mode, at frequency omega(hi), by the coherent motion of an impulsively driven mode, at frequency omega(low). In acetonitrile, the signal is generated by driving the CCN bend (379 cm(-1)) and CC stretch (920 cm(-1)) into coherence via impulsive stimulated Raman scattering and subsequently probing the stimulated Raman spectrum of the CC stretch, the CN stretch (2250 cm(-1)) and the CH stretch (2942 cm(-1)). The resultant signal can be generated by two alternative mechanisms: a fifth-order Raman process that would directly probe anharmonic coupling between the two modes, or a third-order cascade in which a third-order coherent Raman process produces a field that goes on to participate in a third-order stimulated Raman transition. The third-order cascade is shown to dominate the 2D-FSRS spectrum as determined by comparison with the predicted magnitude of the two signals, the 2D spectrum of a mixed isotope experiment, and the concentration dependence of the signal. In acetonitrile, theoretical calculations of the vibrational anharmonicity indicate that the third-order cascade signal should be 10(4) times larger than the fifth-order Raman signal. 2D-FSRS signals are observed between acetonitriles CCN bend, of E symmetry, and several different A(1) modes but are forbidden by symmetry in the fifth-order pathway. A 2D-FSRS spectrum of a 50:50 mixture of acetonitrile and d(3)-acetonitrile shows equivalent intensity for intramolecular coupling peaks and intermolecular coupling peaks, indicating that the observed signal cannot be probing molecular anharmonicity. Finally, the magnitudes of the 2D-FSRS peaks are observed to be proportional to the square of the number density, supporting the cascade mechanism.

Continuous-wave lasing in colloidal quantum dot solids enabled by facet-selective epitaxy

Colloidal quantum dots (CQDs) feature a low degeneracy of electronic states at the band edges compared with the corresponding bulk material, as well as a narrow emission linewidth. Unfortunately for potential laser applications, this degeneracy is incompletely lifted in the valence band, spreading the hole population among several states at room temperature. This leads to increased optical gain thresholds, demanding high photoexcitation levels to achieve population inversion (more electrons in excited states than in ground states—the condition for optical gain). This, in turn, increases Auger recombination losses, limiting the gain lifetime to sub-nanoseconds and preventing steady laser action. State degeneracy also broadens the photoluminescence linewidth at the single-particle level. Here we demonstrate a way to decrease the band-edge degeneracy and single-dot photoluminescence linewidth in CQDs by means of uniform biaxial strain. We have developed a synthetic strategy that we term facet-selective epitaxy: we first switch off, and then switch on, shell growth on the (0001) facet of wurtzite CdSe cores, producing asymmetric compressive shells that create built-in biaxial strain, while still maintaining excellent surface passivation (preventing defect formation, which otherwise would cause non-radiative recombination losses). Our synthesis spreads the excitonic fine structure uniformly and sufficiently broadly that it prevents valence-band-edge states from being thermally depopulated. We thereby reduce the optical gain threshold and demonstrate continuous-wave lasing from CQD solids, expanding the library of solution-processed materials that may be capable of continuous-wave lasing. The individual CQDs exhibit an ultra-narrow single-dot linewidth, and we successfully propagate this into the ensemble of CQDs.

From seconds to femtoseconds: solar hydrogen production and transient absorption of chalcogenorhodamine dyes.

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO2 catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophores triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO2. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production.

Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

Significance The light-driven generation of H2, the reductive side of water splitting, requires a light absorber or photosensitizer (PS) for electron-hole creation and photoinduced electron transfer. To increase the effectiveness of charge transfer chromophores as PSs, this report describes the attachment of a strongly absorbing organic dye (dipyrromethene-BF2, commonly known as Bodipy) to Pt diimine dithiolate charge transfer chromophores and examination of systems containing these dyads for the light-driven generation of H2. The use of these dyads increases system activity under green light irradiation (530 nm) relative to systems with either chromophore alone, validating such an approach in designing artificial photosynthetic systems. One dyad system exhibits both high activity and substantial durability (40,000 turnovers relative to PSs over 12 d). New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine dithiolate (PtN2S2) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN2S2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN2S2 moieties), indicating little electronic coupling between them. Connection to TiO2 nanoparticles is carried out by sonication leading to in situ attachment to TiO2 without prior hydrolysis of the ester linking groups to acids. For H2 generation studies, the TiO2 particles are platinized (Pt-TiO2) so that the light absorber (the dyad), the electron conduit (TiO2), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H2O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO2 via a phosphonate or carboxylate attachment shows excellent light-driven H2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼40,000 turnover numbers of H2 over 12 d (with respect to dye).

Field-emission from quantum-dot-in-perovskite solids

Quantum dot and well architectures are attractive for infrared optoelectronics, and have led to the realization of compelling light sensors. However, they require well-defined passivated interfaces and rapid charge transport, and this has restricted their efficient implementation to costly vacuum-epitaxially grown semiconductors. Here we report solution-processed, sensitive infrared field-emission photodetectors. Using quantum-dots-in-perovskite, we demonstrate the extraction of photocarriers via field emission, followed by the recirculation of photogenerated carriers. We use in operando ultrafast transient spectroscopy to sense bias-dependent photoemission and recapture in field-emission devices. The resultant photodiodes exploit the superior electronic transport properties of organometal halide perovskites, the quantum-size-tuned absorption of the colloidal quantum dots and their matched interface. These field-emission quantum-dot-in-perovskite photodiodes extend the perovskite response into the short-wavelength infrared and achieve measured specific detectivities that exceed 1012 Jones. The results pave the way towards novel functional photonic devices with applications in photovoltaics and light emission.

Efficient Bimolecular Mechanism of Photochemical Hydrogen Production Using Halogenated Boron-Dipyrromethene (Bodipy) Dyes and a Bis(dimethylglyoxime) Cobalt(III) Complex

A series of Boron-dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in conjunction with [Co(III)(dmgH)2pyCl] (where dmgH = dimethylglyoximate, py = pyridine) as the catalyst and triethanolamine (TEOA) as the sacrificial electron donor. The Bodipy dyes are fully characterized by electrochemistry, X-ray crystallography, quantum chemistry calculations, femtosecond transient absorption, and time-resolved fluorescence, as well as in long-term hydrogen production assays. Consistent with other recent reports, only systems containing halogenated chromophores were active for hydrogen production, as the long-lived triplet state is necessary for efficient bimolecular electron transfer. Here, it is shown that the photostability of the system improves with Bodipy dyes containing a mesityl group versus a phenyl group, which is attributed to increased electron donating character of the mesityl substituent. Unlike previous reports, the optimal ratio of chromophore to catalyst is established and shown to be 20:1, at which point this bimolecular dye/catalyst system performs 3-4 times better than similar chemically linked systems. We also show that the hydrogen production drops dramatically with excess catalyst concentration. The maximum turnover number of ∼ 700 (with respect to chromophore) is obtained under the following conditions: 1.0 × 10(-4) M [Co(dmgH)2pyCl], 5.0 × 10(-6) M Bodipy dye with iodine and mesityl substituents, 1:1 v:v (10% aqueous TEOA):MeCN (adjusted to pH 7), and irradiation by light with λ > 410 nm for 30 h. This system, containing discrete chromophore and catalyst, is more active than similar linked Bodipy-Co(dmg)2 dyads recently published, which, in conjunction with our other measurements, suggests that the nominal dyads actually function bimolecularly.

Small-Band-Offset Perovskite Shells Increase Auger Lifetime in Quantum Dot Solids

Colloidal quantum dots (CQDs) enable low-cost, high-performance optoelectronic devices including photovoltaics, photodetectors, LEDs, and lasers. Continuous-wave lasing in the near-infrared remains to be realized based on such materials, yet a solution-processed NIR laser would be of use in communications and interconnects. In infrared quantum dots, long-lived gain is hampered by a high rate of Auger recombination. Here, we report the use of perovskite shells, grown on cores of IR-emitting PbS CQDs, and we thus reduce the rate of Auger recombination by up to 1 order of magnitude. We employ ultrafast transient absorption spectroscopy to isolate distinct Auger recombination phenomena and study the effect of bandstructure and passivation on Auger recombination. We corroborate the experimental findings with model-based investigations of Auger recombination in various CQD core-shell structures. We explain how the band alignment provided by perovskite shells comes close to the optimal required to suppress the Auger rate. These results provide a step along the path toward solution-processed near-infrared lasers.