Rational Design of A Chemical Bath Deposition Based Tin Oxide Electron Transport Layer for Perovskite Photovoltaics

Yongli Lu, Meng-Chen Shih, Shaun Tan, Matthias J. Grotevent, Lili Wang, Hua Zhu, Ruiqi Zhang, Joo-Hong Lee, Jin-Wook Lee, Vladimir Bulović, Moungi. G. Bawendi

Keywords: Chemical bath deposition, Tin oxide, Perovskite solar cells

Chemical bath deposition is widely used to deposit SnOx as an electron transport layer in perovskite solar cells (PSCs). The conventional recipe uses thioglycolic acid (TGA) to facilitate attachments of SnOx particles onto the substrate. However, nonvolatile TGA has been reported to harm the operational stability of PSCs. In this work, we introduced a volatile oxalic acid (OA) as an alternative to TGA. OA, a dicarboxylic acid, functions as a chemical linker for the nucleation and attachment of particles to the substrate in the chemical bath. Moreover, OA can be readily removed through thermal annealing followed by a mild H2O2 treatment, as shown by FTIR measurements. Synergistically, the mild H2O2 treatment selectively oxidizes the surface of the SnOx layer, minimizing nonradiative interface carrier recombination. EELS (electron-energy-loss-spectroscopy) confirms that the SnOx surface is dominated by Sn4+, while the bulk is a mixture of Sn2+ and Sn4+. This rational design of a CBD SnOx layer leads to devices with T85∼1,500 h, a significant improvement over the TGA-based device with T80∼250 h. Our champion device reached a power conversion efficiency of 24.6%. This work offers a rationale for optimizing the complex parameter space of CBD SnOx to achieve efficient and stable PSCs.

Nano Lett.

Lead Halide Perovskite Nanocrystals with Low Inhomogeneous Broadening and High Coherent Fraction through Dicationic Ligand Engineering

Matthias Ginterseder, Weiwei Sun, Wenbi Shcherbakov-Wu, Alexandra R. McIsaac, David B. Berkinsky, Alexander E. K. Kaplan, Lili Wang, Chantalle Krajewska, Tara Šverko, Collin F. Perkinson, Hendrik Utzat, William A. Tisdale, Troy Van Voorhis, and Moungi G. Bawendi

Keywords: Colloidal quantum dots, lead halide perovskites, ligand engineering, inhomogeneous broadening, Stokes shift, coherent emission

Lead halide perovskite nanocrystals (LHP NCs) are an emerging materials system with broad potential applications, including as emitters of quantum light. We apply design principles aimed at the structural optimization of surface ligand species for CsPbBr3 NCs, leading us to the study of LHP NCs with dicationic quaternary ammonium bromide ligands. Through the selection of linking groups and aliphatic backbones guided by experiments and computational support, we demonstrate consistently narrow photoluminescence line shapes with a full-width-at-half-maximum below 70 meV. We observe bulk-like Stokes shifts throughout our range of particle sizes, from 7 to 16 nm. At cryogenic temperatures, we find sub-200 ps lifetimes, significant photon coherence, and the fraction of photons emitted into the coherent channel increasing markedly to 86%. A 4-fold reduction in inhomogeneous broadening from previous work paves the way for the integration of LHP NC emitters into nanophotonic architectures to enable advanced quantum optical investigation.



Designing Highly Luminescent Molecular Aggregates via Bottom-Up Nanoscale Engineering

Ulugbek Barotov, Megan D. Klein, Lili Wang, Moungi G. Bawendi

Keywords: Absorption, Alcohols, Dyes and pigments, Excitons, Fluorescence

Coupling of excitations between organic fluorophores in J-aggregates leads to coherent delocalization of excitons across multiple molecules, resulting in materials with high extinction coefficients, long-range exciton transport, and, in particular, short radiative lifetimes. Despite these favorable optical properties, uses of J-aggregates as high-speed light sources have been hindered by their low photoluminescence (PL) quantum yields (QYs). Here, we take a bottom-up approach to design a novel J-aggregate system with a large extinction coefficient, a high QY, and a short lifetime. To achieve this goal, we first select a J-aggregating cyanine chromophore and reduce its nonradiative pathways by rigidifying the backbone of the cyanine dye. The resulting conformationally restrained cyanine dye exhibits strong absorbance at 530 nm and fluorescence at 550 nm with 90% QY and 2.3 ns lifetime. We develop optimal conditions for the self-assembly of highly emissive J-aggregates. Cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) reveal micron-scale extended structures with two-dimensional (2D) sheetlike morphology, indicating a long-range structural order. These novel J-aggregates have strong, red-shifted absorption at 600 nm, resonant fluorescence with no Stokes shift, 50% QY, and 220 ps lifetime at room temperature. We further stabilize these aggregates in a glassy sugar matrix and study their excitonic behavior using temperature-dependent absorption and fluorescence spectroscopy. These temperature-dependent studies confirm J-type excitonic coupling and superradiance. Our results have implications for the development of a new generation of organic fluorophores that combine high speed, high QY, and solution processing.



Interfacial Trap-Assisted Triplet Generation in Lead Halide Perovskite Sensitized Solid-State Upconversion

Lili Wang, Jason J. Yoo, Ting-An Lin, Collin F. Perkinson, Yongli Lu, Marc A. Baldo, Moungi G. Bawendi


Photon upconversion via triplet–triplet annihilation (TTA) has promise for overcoming the Shockley–Queisser limit for single-junction solar cells by allowing the utilization of sub-bandgap photons. Recently, bulk perovskites have been employed as sensitizers in solid-state upconversion devices to circumvent poor exciton diffusion in previous nanocrystal (NC)-sensitized devices. However, an in-depth understanding of the underlying photophysics of perovskite-sensitized triplet generation is still lacking due to the difficulty of precisely controlling interfacial properties of fully solution-processed devices. In this study, interfacial properties of upconversion devices are adjusted by a mild surface solvent treatment, specifically altering perovskite surface properties without perturbing the bulk perovskite. Thermal evaporation of the annihilator precludes further solvent contamination. Counterintuitively, devices with more interfacial traps show brighter upconversion. Approximately an order of magnitude difference in upconversion brightness is observed across different interfacial solvent treatments. Sequential charge transfer and interfacial trap-assisted triplet sensitization are demonstrated by comparing upconversion performance, transient photoluminescence dynamics, and magnetic field dependence of the devices. Incomplete triplet conversion from transferred charges and consequent triplet-charge annihilation (TCA) are also observed. The observations highlight the importance of interfacial control and provide guidance for further design and optimization of upconversion devices using perovskites or other semiconductors as sensitizers.



Evidence for the Dominance of Carrier-Induced Band Gap Renormalization over Biexciton Formation in Cryogenic Ultrafast Experiments on MoS2 Monolayer

Ryan E. Wood, Lawson T. Lloyd, Fauzia Mujid, Lili Wang, Marco A. Allodi, Hui Gao, Richard Mazuski, Po-Chieh Ting, Saien Xie, Jiwoong Park, and Gregory S. Engel

Keywords: Absorption, Diffusion, Electrical conductivity, Excitons, Monolayers

Transition-metal dichalcogenides (TMDs) such as MoS2 display promising electrical and optical properties in the monolayer limit. Due to strong quantum confinement, TMDs provide an ideal environment for exploring excitonic physics using ultrafast spectroscopy. However, the interplay between collective excitation effects on single excitons such as band gap renormalization/exciton binding energy (BGR/EBE) change and multiexciton effects such biexciton formation remains poorly understood. Using two-dimensional electronic spectroscopy, we observe the dominance of single-exciton BGR/EBE signals over optically induced biexciton formation. We make this determination based on a lack of strong PIA features at T = 0 fs in the cryogenic spectra. By means of nodal line slope analysis, we determine that spectral diffusion occurs faster than BGR/EBE change, indicative of distinct processes. These results indicate that at higher sub-Mott limit fluences, collective effects on single excitons dominate biexciton formation.


Scalable Synthesis of InAs Quantum Dots Mediated through Indium Redox Chemistry

Matthias Ginterseder, Daniel Franke, Collin F. Perkinson, Lili Wang, Eric C. Hansen, and Moungi G. Bawendi

Keywords: Absorption, Indium arsenide, Nanoparticles, Precursors, Quantum dots

Next-generation optoelectronic applications centered in the near-infrared (NIR) and short-wave infrared (SWIR) wavelength regimes require high-quality materials. Among these materials, colloidal InAs quantum dots (QDs) stand out as an infrared-active candidate material for biological imaging, lighting, and sensing applications. Despite significant development of their optical properties, the synthesis of InAs QDs still routinely relies on hazardous, commercially unavailable precursors. Herein, we describe a straightforward single hot injection procedure revolving around In(I)Cl as the key precursor. Acting as a simultaneous reducing agent and In source, In(I)Cl smoothly reacts with a tris(amino)arsenic precursor to yield colloidal InAs quantitatively and at gram scale. Tuning the reaction temperature produces InAs cores with a first excitonic absorption feature in the range of 700–1400 nm. A dynamic disproportionation equilibrium between In(I), In metal, and In(III) opens up additional flexibility in precursor selection. CdSe shell growth on the produced cores enhances their optical properties, furnishing particles with center emission wavelengths between 1000 and 1500 nm and narrow photoluminescence full-width at half-maximum (FWHM) of about 120 meV throughout. The simplicity, scalability, and tunability of the disclosed precursor platform are anticipated to inspire further research on In-based colloidal QDs.



Quantum coherences reveal excited-state dynamics in biophysical systems

Lili Wang, Marco A. Allodi, Gregory S. Engel


Ultrafast, multi-dimensional spectroscopic measurements of photosynthetic light-harvesting complexes have revealed quantum coherences with timescales comparable to those of energy-transfer processes. These observations have led to a debate regarding the states that give rise to the coherences and whether the presence of the coherences has implications for photosynthetic light harvesting. In these experiments, laser pulses create a coherent superposition of quantum states with a defined phase relationship across an ensemble, which gives rise to the quantum coherence and associated quantum beating signal. Dephasing of these quantum coherences, seen as a decay of the beating signal, is among the most sensitive probes of the interactions between a system and its surrounding environment. In this Review, we discuss the proposed origin and assignment of the observed quantum coherences in photosynthetic systems as electronic, vibronic or vibrational. We describe the latest experimental efforts towards unravelling the nature of the coherences, in particular ultrafast, two-dimensional electronic spectroscopy, as well as the accompanying theoretical and computational results. We discuss how measuring coherences can inform us about the excited-state dynamics of biophysical and chemical systems relevant to natural light harvesting and how these measurements reveal electronic structure beyond that captured by simplistic models.



Origin of Broad Emission Spectra in InP Quantum Dots: Contributions from Structural and Electronic Disorder

Eric M. Janke, Nicholas E. Williams, Chunxing She, Danylo Zherebetskyy, Margaret H. Hudson, Lili Wang, David J. Gosztola, Richard D. Schaller, Byeongdu Lee, Chengjun Sun, Gregory S. Engel, Dmitri V. Talapin*

Keywords: Absorption, Diseases and disorders, Lattices, Phonons, Quantum dots

The ensemble emission spectra of colloidal InP quantum dots are broader than achievable spectra of cadmium- and lead-based quantum dots, despite similar single-particle line widths and significant efforts invested in the improvement of synthetic protocols. We seek to explain the origin of persistently broad ensemble emission spectra of colloidal InP quantum dots by investigating the nature of the electronic states responsible for luminescence. We identify a correlation between red-shifted emission spectra and anomalous broadening of the excitation spectra of luminescent InP colloids, suggesting a trap-associated emission pathway in highly emissive core–shell quantum dots. Time-resolved pump–probe experiments find that electrons are largely untrapped on photoluminescence relevant time scales pointing to emission from recombination of localized holes with free electrons. Two-dimensional electronic spectroscopy on InP quantum dots reveals multiple emissive states and increased electron–phonon coupling associated with hole localization. These localized hole states near the valence band edge are hypothesized to arise from incomplete surface passivation and structural disorder associated with lattice defects. We confirm the presence and effect of lattice disorder by X-ray absorption spectroscopy and Raman scattering measurements. Participation of localized electronic states that are associated with various classes of lattice defects gives rise to phonon-coupled defect related emission. These findings explain the origins of the persistently broad emission spectra of colloidal InP quantum dots and suggest future strategies to narrow ensemble emission lines comparable to what is observed for cadmium-based materials.


Excitations Partition into Two Distinct Populations in Bulk Perovskites

Lili Wang, Nicholas P. Brawand, Márton Vörös, Peter D. Dahlberg, John P. Otto, Nicholas E. Williams, David M. Tiede, Giulia Galli, Gregory S. Engel

Keywords: bulk carrier dynamics, organolead halide perovskites, polaron formation, transient absorption

Organolead halide perovskites convert optical excitations to charge carriers with remarkable efficiency in optoelectronic devices. Previous research predominantly documents dynamics in perovskite thin films; however, extensive disorder in this platform may obscure the observed carrier dynamics. Here, carrier dynamics in perovskite single-domain single crystals is examined by performing transient absorption spectroscopy in a transmissive geometry. Two distinct sets of carrier populations that coexist at the same radiation fluence, but display different decay dynamics, are observed: one dominated by second-order recombination and the other by third-order recombination. Based on ab initio simulations, this observation is found to be most consistent with the hypothesis that free carriers and localized carriers coexist due to polaron formation. The calculations suggest that polarons will form in both CH3NH3PbBr3 and CH3NH3PbI3 crystals, but that they are more pronounced in CH3NH3PbBr3. Single-crystal CH3NH3PbBr3 could represent the key to understanding the impact of polarons on the transport properties of perovskite optoelectronic devices.


Disentanglement of excited-state dynamics with implications for FRET measurements: two-dimensional electronic spectroscopy of a BODIPY-functionalized cavitand

John P. Otto, Lili Wang, Igor Pochorovski, Samuel M. Blau, Alán Aspuru-Guzik, Zhenan Bao, Gregory S. Engel, Melanie Chiu

Keywords: bulk carrier dynamics, organolead halide perovskites, polaron formation, transient absorption

Förster Resonance Energy Transfer (FRET) is the incoherent transfer of an electronic excitation from a donor fluorophore to a nearby acceptor. FRET has been applied as a probe of local chromophore environments and distances on the nanoscale by extrapolating transfer efficiencies from standard experimental parameters, such as fluorescence intensities or lifetimes. Competition from nonradiative relaxation processes is often assumed to be constant in these extrapolations, but in actuality, this competition depends on the donor and acceptor environments and can, therefore, be affected by conformational changes. To study the effects of nonradiative relaxation on FRET dynamics, we perform two-dimensional electronic spectroscopy (2DES) on a pair of azaboraindacene (BODIPY) dyes, attached to opposite arms of a resorcin[4]arene cavitand. Temperature-induced switching between two equilibrium conformations, vase at 294 K to kite at 193 K, increases the donor–acceptor distance from 0.5 nm to 3 nm, affecting both FRET efficiency and nonradiative relaxation. By disentangling different dynamics based on lifetimes extracted from a series of 2D spectra, we independently observe nonradiative relaxation, FRET, and residual fluorescence from the donor in both vase to kite conformations. We observe changes in both FRET rate and nonradiative relaxation when the molecule switches from vase to kite, and measure a significantly greater difference in transfer efficiency between conformations than would be determined by standard lifetime-based measurements. These observations show that changes in competing nonradiative processes must be taken into account when highly accurate measurements of FRET efficiency are desired.


Crystal structure of 4′-allyl-4,5,6,7,2′,7′-hexa­chloro­fluorescein allyl ester unknown solvate

Lili Wang, Alexander S. Filatov, Gregory S. Engel

Keywords: crystal structure; fluorescein; hydrogen bonding; Cl…π inter­action.

In the title compound, 4′-allyl-4,5,6,7,2′,7′-hexa­chloro­fluorescein allyl ester {systematic name: prop-2-en-1-yl 2,3,4,5-tetra­chloro-6-[2,7-di­chloro-6-hy­droxy-3-oxo-4-(prop-2-en-1-yl)-3H-xanthen-9-yl]benzoate}, C26H14Cl6O5, accompanied by unknown solvate molecules, the dihedral angle between the xanthene ring system (r.m.s. deviation = 0.046 Å) and the penta­substituted benzene ring is 71.67 (9)°. Both allyl groups are disordered over two sets of sites in statistical ratios. The scattering contributions of the disordered solvent mol­ecules (both Ph2O and CHCl3, as identified by NMR) were removed with the PLATON SQUEEZE algorithm [Spek (2015). Acta Cryst. C71, 9–18]. In the crystal, tetra­meric supra­molecular aggregates linked by O—H⋯O hydrogen bonds occur; these further inter­act with neighboring aggregates through C—Cl⋯π inter­actions arising from the benzene rings, forming infinite two-dimensional sheets. Each C6Cl4 ring shifts in the direction perpendicular to the two-dimensional sheet, exhibiting a helical chain in which every C6Cl4 ring is utilized as both a donor and an acceptor of Cl⋯π contacts. Thus, these two-dimensional sheets pack in a helical fashion, constructing a three-dimensional network.



Scalable Ligand-Mediated Transport Synthesis of Organic–Inorganic Hybrid Perovskite Nanocrystals with Resolved Electronic Structure and Ultrafast Dynamics

Lili Wang, Nicholas E. Williams, Edward W. Malachosky, John P. Otto, Dugan Hayes, Ryan E. Wood, Philippe Guyot-Sionnest, Gregory S. Engel

Keywords: Excitons, Ligands, Perovskites, Solution phase, Ultrafast phenomena

Colloidal perovskite nanocrystals support bright, narrow PL tunable over the visible spectrum. However, bandgap tuning of these materials remains limited to laboratory-scale syntheses. In this work, we present a polar-solvent-free ligand-mediated transport synthesis of high-quality organic–inorganic perovskite nanocrystals under ambient conditions with photoluminescence quantum yields up to 97%. Our synthesis employs a ligand-mediated transport mechanism that circumvents the need for exquisite external control (e.g., temperature control, inert-gas protection, dropwise addition of reagents) required by other methods due to extremely fast reaction kinetics. In the ligand-mediated transport mechanism, multiple equilibria cooperatively dictate reaction rates and enable precise control over NC size. These small nanocrystals exhibit high photoluminescence quantum yields due to quantum confinement. Nanosecond transient absorption spectroscopy experiments reveal a fluence-independent PL decay originating from exciton recombination. Two-dimensional electronic spectroscopy resolves multiple spectral features reflecting the electronic structure of the nanocrystals. The resolved features exhibit size-dependent spectral positions, further indicating the synthesized nanocrystals are quantum-confined.


Controlling quantum-beating signals in 2D electronic spectra by packing synthetic heterodimers on single-walled carbon nanotubes

Lili Wang, Graham B. Griffin, Alice Zhang, Feng Zhai, Nicholas E. Williams, Richard F. Jordan, Gregory S. Engel

Keywords: Carbon nanotubes and fullerenes, Light harvesting, Photobiology, Spectrophotometry

In multidimensional spectroscopy, dynamics of coherences between excited states report on the interactions between electronic states and their environment. The prolonged coherence lifetimes revealed through beating signals in the spectra of some systems may result from vibronic coupling between nearly degenerate excited states, and recent observations confirm the existence of such coupling in both model systems and photosynthetic complexes. Understanding the origin of beating signals in the spectra of photosynthetic complexes has been given considerable attention; however, strategies to generate them in artificial systems that would allow us to test the hypotheses in detail are still lacking. Here we demonstrate control over the presence of quantum-beating signals by packing structurally flexible synthetic heterodimers on single-walled carbon nanotubes, and thereby restrict the motions of chromophores. Using two-dimensional electronic spectroscopy, we find that both limiting the relative rotation of chromophores and tuning the energy difference between the two electronic transitions in the dimer to match a vibrational mode of the lower-energy monomer are necessary to enhance the observed quantum-beating signals.