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Crystal engineering is a practical approach for tailoring material properties. This approach has been widely studied for modulating optical and electrical properties of semiconductors. However, the properties of organic molecular crystals are difficult to control following a similar engineering route. In this Letter, we demonstrate that engineered crystals of Alq3 and Ir(ppy)3 complexes, which are commonly used in organic light-emitting technologies, possess intriguing functional properties. Specifically, these structures not only process efficient low-energy induced triplet excitation directly from the ground state of Alq3 but also can show strong emission at the Alq3 triplet energy level at room temperatures. We associate these phenomena with local deformations of the host matrix around the guest molecules, which in turn lead to a stronger host-guest triplet-triplet coupling and spin-orbital mixing.

Solid-state bulk lead halide perovskite thin films have recently shown progress as triplet sensitizers in infrared-to-visible photon upconversion (UC) schemes. Common systems pair lead halide perov...

Sheng et al. (2021) report an increase in the power conversion efficiency of a methylammonium lead iodide perovskite solar cell, which they have attributed to the benefits of in situ perovskite-sensitized upconversion by triplet–triplet annihilation in the organic semiconductor rubrene. In the following, we will discuss why in the device structure presented here upconversion inherently cannot be the underlying cause of the observed effects. Rather, the process of upconversion can fundamentally only lead to a reduction of the device efficiency if the same material is used as the active layer in the photovoltaic devices and as the triplet sensitizer. Therefore, the improved photovoltaic performance must have a different root cause. We follow up with an alternative interpretation for the performance increase observed by the authors.

The processes of singlet fission and triplet fusion could allow state-of-the-art photovoltaic devices to surpass the Shockley–Queisser limit by optimizing the utilized solar spectrum by reducing thermal relaxation and inaccessible sub-bandgap photons, respectively. Triplet fusion demands precise control of the spin-triplet state population, and requires a sensitizer to efficiently populate the triplet state of an acceptor molecule. In this perspective, we highlight the established field of sensitized upconversion and further examine alternative triplet sensitization routes, including the possibility of bulk solid-state semiconductors as triplet sensitizers, which provide a new avenue for charge transfer-based triplet sensitization rather than excitonic triplet energy transfer.

The mechanism and elemental composition that form the basis for the improved optical and electronic properties in mixed-ion lead halide perovskite solar cells are still not well understood compared...

Photon upconversion via triplet-triplet annihilation creating light in the high energy regime of the visible spectrum could prove particularly useful in applications such as photocatalysis. Quantum-confined semiconductor nanocrystals have previously been shown to function as efficient triplet sensitizers in green-to-blue upconversion schemes, but an improvement in the apparent anti-Stokes shift of the upconversion scheme will be beneficial for future commercial applications. Additionally, both zero- and two-dimensional quantum-confined sensitizers have been investigated, but not one-dimensional nanomaterials. In this work, we fill a hole in the present field of photon upconversion by accomplishing both of these feats. Specifically, we introduce CdTe nanorods as a new class of triplet sensitizers for red-to-blue photon upconversion. When the triplet transmitter ligand (9-anthracenecarboxylic acid) and triplet annihilator (9,10-diphenylanthracene) are added to the nanorods, we observe efficient photon upconversion at a normalized upconversion efficiency of huc = 4.3% and a low threshold power of Ith = 93 mW/cm2. The introduction of one-dimensional triplet sensitizers could yield future research that effectively harnesses the unique properties of these materials allowing for new approaches for efficient photon upconversion, especially at large apparent anti-Stokes shifts.

Triplet-triplet annihilation-based photon upconversion (UC) using bulk perovskite sensitizers has been previously shown to facilitate efficient UC at low fluences. However, the fabrication of the UC devices has not been fully optimized; thus, there is room for improvement. Here, we apply techniques that have been successful in enhancing the performance of perovskite solar cells in order to also improve perovskite-sensitized UC devices. In particular, we investigate the use of a post-fabrication thermal annealing step, overstoichiometric vs stoichiometric addition of PbI2 to the perovskite precursors, methylammonium vs formamidinium cation-rich lead halide perovskite compositions, and the use of different solvents for the annihilator molecules on the perovskite/annihilator interface. We find that excess PbI2 does not significantly affect the UC process, while the perovskite composition is crucial for the yield of extracted carriers across the interface. Comparing toluene and chlorobenzene, we find that the solvent used to deposit the annihilator is also a key factor in the overall device performance. Moreover, we find that thermal annealing of the whole device architecture significantly improves the UC performance by a factor of three.

Silicon dominates contemporary solar cell technologies1. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap2. Reducing these thermalization losses and enabling better sensitivity to light is possible by sensitizing the silicon solar cell using singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton)3–5. Singlet exciton fission in the molecular semiconductor tetracene is known to generate triplet excitons that are energetically matched to the silicon bandgap6–8. When the triplet excitons are transferred to silicon they create additional electron–hole pairs, promising to increase cell efficiencies from the single-junction limit of 29 per cent to as high as 35 per cent9. Here we reduce the thickness of the protective hafnium oxynitride layer at the surface of a silicon solar cell to just eight angstroms, using electric-field-effect passivation to enable the efficient energy transfer of the triplet excitons formed in the tetracene. The maximum combined yield of the fission in tetracene and the energy transfer to silicon is around 133 per cent, establishing the potential of singlet exciton fission to increase the efficiencies of silicon solar cells and reduce the cost of the energy that they generate. A silicon and tetracene solar cell employing singlet fission uses an eight-angstrom-thick hafnium oxynitride interlayer to promote efficient triplet transfer, increasing the efficiency of the cell.

Compared to the visible and near-infrared, the short-wave infrared region (SWIR; 1000–2000 nm) has excellent properties for in vivo imaging: low autofluorescence, reduced scattering, and a low-abso...

Green-to-blue photon upconversion bears great potential in photocatalytic applications. However, current hybrid inorganic-organic upconversion schemes utilizing spherical CdSe nanocrystals are often limited by energetic polydispersity, low quantum yields and an additional tunneling barrier resulting from the necessity of surface-passivating inorganic shells. In this contribution, we introduce anisotropic CdSe nanoplatelets as triplet sensitizers. Here, quantum confinement occurs in only one direction, erasing effects stemming from energetic polydispersity. We investigate the triplet energy transfer from the CdSe nanoplatelets to the surface-bound triplet acceptor 9-anthracene carboxylic acid. We further focus on the influence of nanoplatelet stacking and singlet back transfer on the observed upconversion efficiency. We obtain an upconversion quantum yield of 5.4% at a power density of 11 W/cm2­ using the annihilator 9,10-diphenylanthracene, and a low efficiency threshold Ith of 237 mW/cm2.

In this review, we highlight the current advancements in the field of triplet sensitization by three-dimensional (3D) perovskite nanocrystals and bulk films. First introduced in 2017, 3D perovskite sensitized upconversion (UC) is a young fast-evolving field due to the tunability of the underlying perovskite material. By tuning the perovskite bandgap, visible-to-ultraviolet, near-infrared-to-visible or green-to-blue UC has been realized, which depicts the broad applicability of this material. As this research field is still in its infancy, we hope to stimulate the field by highlighting the advantages of these perovskite nanocrystals and bulk films, as well as shedding light onto the current drawbacks. In particular, the keywords toxicity, reproducibility and stability must be addressed prior to commercialization of the technology. If successful, photon interconversion is a means to increase the achievable efficiency of photovoltaic cells beyond its current limits by increasing the window of useable wavelengths.

A key debate involving mixed-cation lead mixed-halide perovskite thin-films relates to the effects of process conditions on film morphology and local performance of perovskite solar cells. In this contribution, we investigate the influence of precursor concentration on the film thickness, grain size, and orientation of these polycrystalline thin-films. We vary the molar concentration of the perovskite precursor containing Rb, Cs, MA, FA, Pb, I, and Br from 0.4 to 1.2 M. We use optical and electrical probes to measure local properties and correlate the effect of crystallographic orientation on the inter- and intragrain charge-carrier transport. We find that, with increasing precursor concentration, the grain size of the polycrystalline thin-films becomes larger and more faceted. Films with small grains show mostly random grain orientation angles, whereas films with large grains are oriented with {100} planes around an angle of 20° relative to the surface normal. These films with oriented large grains also ...

Semiconductor nanocrystals are a promising class of materials for a variety of novel optoelectronic devices, since many of their properties, such as the electronic gap and conductivity, can be controlled. Much of this control is achieved via the organic ligand shell, through control of the size of the nanocrystal and the distance to other objects. We here simulate ligand-coated CdSe nanocrystals using atomistic molecular dynamics, allowing for the resolution of novel structural details about the ligand shell. We show that the ligands on the surface can lie flat to form a highly anisotropic 'wet hair' layer as opposed to the 'spiky ball' appearance typically considered. We discuss how this can give rise to a dot-to-dot packing distance of one ligand length since the thickness of the ligand shell is reduced to approximately one-half of the ligand length for the system sizes considered here; these distances imply that energy and charge transfer rates between dots and nearby objects will be enhanced due to the thinner than expected ligand shell. Our model predicts a non-linear scaling of ligand shell thickness as the ligands transition from 'spiky' to 'wet hair'. We verify this scaling using TEM on a PbS nanoarray, confirming that this theory gives a qualitatively correct picture of the ligand shell thickness of colloidal quantum dots., 17 Pages, 9 Figures

Inspired by the rapid rise in efficiencies of lead halide perovskite (LHP) solar cells, lead-free alternatives are attracting increasing attention. In this work, we study the photovoltaic potential of solution-processed antimony (Sb)-based compounds with the formula A3Sb2I9 (A = Cs, Rb, and K). We experimentally determine bandgap magnitude and type, structure, carrier lifetime, exciton binding energy, film morphology, and photovoltaic device performance. We use density functional theory to compute the equilibrium structures, band structures, carrier effective masses, and phase stability diagrams. We find the A-site cation governs the structural and optoelectronic properties of these compounds. Cs3Sb2I9 has a 0D structure, the largest exciton binding energy (175 ± 9 meV), an indirect bandgap, and, in a solar cell, low photocurrent (0.13 mA cm–2). Rb3Sb2I9 has a 2D structure, a direct bandgap, and, among the materials investigated, the lowest exciton binding energy (101 ± 6 meV) and highest photocurrent (1....

Recently, we and others have proposed screening criteria for “defect-tolerant” photovoltaic (PV) absorbers, identifying several classes of semiconducting compounds with electronic structures similar to those of hybrid lead–halide perovskites. In this work, we reflect on the accuracy and prospects of these new design criteria through a combined experimental and theoretical approach. We construct a model to extract photoluminescence lifetimes of six of these candidate PV absorbers, including four (InI, SbSI, SbSeI, and BiOI) for which time-resolved photoluminescence has not been previously reported. The lifetimes of all six candidate materials exceed 1 ns, a threshold for promising early stage PV device performance. However, there are variations between these materials, and none achieve lifetimes as high as those of the hybrid lead–halide perovskites, suggesting that the heuristics for defect-tolerant semiconductors are incomplete. We explore this through first-principles point defect calculations and Shock...

Recent advances in perovskite-sensitized photon upconversion via triplet-triplet annihilation (TTA) in rubrene have yielded several unanswered questions about the underlying mechanism and processes occurring at the interface. In particular, the near-infrared perovskite emission is not significantly quenched and a rapid reversible photobleach of the upconverted emission can be observed under fairly low excitation densities of 3.2 mW/cm2. In this contribution, we investigate the perovskite/organic interface in more detail and conclude that non-covalent interactions between the organic layer and perovskite result in surface trap passivation. In addition, band bending results in a charge space region at the perovskite/rubrene interface, which precharges the rubrene interface with holes. Upon initial illumination, electrons can rapidly transfer to the excited triplet state of rubrene, followed by efficient TTA upconversion. As the device is continuously illuminated, the precharged holes are consumed and a new equilibrium is reached, resulting in the previously investigated steady-state upconversion efficiency.

Compositional engineering related to the organic and inorganic cations (A-site) in halide perovskites solar cells has helped improve efficiency and long-term durability. However, this compositional complexity can lead to phase segregation that weakens the optoelectronic performance. Here, we show the halide distribution and cation distribution by means of synchrotron-based nanoprobe x-ray fluorescence. We find that the halide distribution homogenizes upon the addition of CsI and RbI precursors. The halide homogenization coincides with long-lived charge carrier decays. Additionally, we observe Rb-rich areas that phase segregate within the film and the Rb aggregates are identified to be recombination active using X-ray and E-beam induced current microscopy.

The mechanism and elemental composition that form the basis for the improved optical and electronic properties of perovskite solar cells are still not well understood. Here, we use synchrotron-based X-ray fluorescence to map the elemental distribution of ions in perovskite thin films with three different film thicknesses. To create a link to the electronic properties, we perform time-resolved photoluminescence spectroscopy and Kelvin probe force microscopy. We find that the elemental composition of the thin films is highly dependent on their thickness which resulted in different photoluminescence behaviors. In particular, we find that the I/Pb ratio is altered for single grains revealing the difference in grain composition. Our approach sheds light onto the fundamental properties occurring at the microscale which help to further engineer perovskite thin films and interfaces.

High internal quantum efficiency semiconductor nanocrystal (NC)-based photon upconversion devices are currently based on a single monolayer of active NCs. Devices are therefore limited in their external quantum efficiency based on the low number of photons absorbed. Increasing the number of photons absorbed is expected to increase the upconversion efficiency, yet experimentally increasing the number of layers does not appreciably increase the upconverted light output. We unravel this mystery by combining kinetic modeling and transient photoluminescence spectroscopy. The inherent energetic disorder stemming from the polydispersity of the NCs means that the kinetics are governed by a stochastic transfer matrix. By drawing the rates from a probabilistic distribution and constructing a reaction network with realistic connectivity, we are able to fit complex photoluminescence traces with a very simple model. We use this model to explain the thickness-dependent performance of the upconversion devices and can attribute the reduced efficiencies to the low excitonic diffusivity of the exciton within the NC layers and increased back transfer of the created singlets from the organic annihilator rubrene. We suggest some avenues for overcoming these limitations in future devices.

Photon upconversion via triplet–triplet annihilation could allow for the existing efficiency limit of single junction solar cells to be surpassed. Indeed, efficient upconversion at subsolar fluences has been realized in bulk perovskite-sensitized systems. Many questions have remained unanswered, in particular, regarding their behavior under photovoltaic operating conditions. Here, we investigate the impact of repeated and continuous illumination on bilayer perovskite/rubrene upconversion devices. We find that variations of the underlying perovskite carrier recombination dynamics greatly impact the upconversion process. Trap filling and triplet sensitization are in direct competition: more saturated trap states in the perovskite and, thus, longer underlying perovskite photoluminescence lifetimes allow for an increased number of carriers to diffuse to the perovskite/rubrene interface and undergo charge extraction to the triplet state of rubrene. As a result, the upconversion efficiency is greatly influenced by the underlying trap density: the upconverted photoluminescence intensity increases by two orders of magnitude under continuous illumination for 4 h. This shows that the upconversion efficiency is difficult to define for this system. Importantly, these results indicate that perovskite-sensitized upconversion devices exhibit peak performance under continuous illumination, which is a requirement for their successful integration into photovoltaics to help overcome the Shockley–Queisser limit in single junction solar cells.

The conversion of energy within materials has many applications. If one could convert the energy of a blue photon into two lower-energy particles, or into two red photons, solar cells could be fabricated much more efficiently. On the other hand, if one could combine the energy of two particles to form higher-energy particles, it could be used to activate a chemical reaction, for example, releasing a drug or sensitizing a standard silicon camera for infrared light.

The recent spotlight on supported nanoparticles (NPs) has attracted attention in the field of catalysis and fuel cell technology. Supported NPs can be used as model catalysts to gain a fundamental understanding of the catalytic properties at the interface. Here, especially the wet-chemical preparation of platinum NPs in alkaline ethylene glycol is a powerful approach to synthesize stable particles with a narrow size distribution in the nanometer regime. We combine high resolution imaging by scanning tunneling microscopy with electrochemical characterization by cyclic voltammetry to gain insights into the underlying degradation mechanism of supported platinum NPs, paving the way toward a rational design of supported catalysts with controlled activity and stability.

Traditionally, PbS/CdS quantum dots (QDs) have been synthesized via a cation exchange method, making fine control over shell growth challenging. We show here that colloidal atomic layer deposition (c-ALD) allows for the sequential growth of single monolayers of the shell, thus creating a 'true' CdS shell on PbS QDs.

Nanometer-sized metal clusters are prime candidates for photoactivated catalysis, based on their unique tunable optical and electronic properties, combined with a large surface-to-volume ratio. Due to the very small optical cross sections of such nanoclusters, support-mediated plasmonic activation could potentially make activation more efficient. Our support is a semi-transparent gold film, optimized to work in a back-illumination geometry. It has a surface plasmon resonance excitable in the 510–540 nm wavelength range. Ptn clusters (size distribution peaked at n = 46 atoms) have been deposited onto this support and investigated for photoactivated catalytic performance in the oxidative decomposition of methylene blue. The Pt cluster catalytic activity under illumination exceeds that of the gold support by more than an order of magnitude per active surface area. To further investigate the underlying mechanism of plasmon-induced catalysis, the clusters have been imaged with optically-assisted scanning tunneling microscopy under illumination. The photoactivation of the Pt clusters via plasmonic excitation of the support and subsequential electronic excitation of the clusters can be imaged with nanometer resolution. The light-induced tunneling current on the clusters is enhanced relative to the gold film support.

Lead halide perovskites have emerged as promising absorber materials over the last decade to increase the efficiency of photovoltaics beyond its current limits. However, to further optimize the performance of perovskites more detailed studies need to be performed, which allow for the correlation of film morphology and local electronic properties at the nanoscale. Here, we present a scanning tunneling microscopy (STM) approach to probe the effect of an applied electric field of a methylammonium formamidinium lead triiodide perovskite thin film on the film response by current–voltage spectroscopy, current imaging tunneling spectroscopy, differential conductance mapping, and x-ray absorption spectroscopy by means of synchrotron x-ray STM. We find a strong correlation between the measurement conditions and the obtained current–voltage characteristics when imaging under opposite bias polarities. In particular, we find similarities to already observed poling effects for lead halide perovskites, which result in either a positively or negatively charged interface due to ion and vacancy migration. Our results provide insight into the influence of measurement conditions such as bias polarity on the performance assessment of perovskite thin films by STM.

Lead halide-based perovskite thin films have attracted great attention due to the explosive increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet-triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross sections throughout the visible spectrum, as well as the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the additional tunneling barrier owing from the passivating ligands required for colloidal sensitizers. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination.

Multiexcitons in emerging semiconducting nanomaterials play a critical role in potential optoelectronic and quantum computational devices. We describe photon resolved single molecule methods to directly probe the dynamics of biexcitons and triexcitons in colloidal CdSe quantum dots. We confirm that biexcitons emit from a spin-correlated state, consistent with statistical scaling. Contrary to current understanding, we find that triexciton emission is dominated by band-edge 1Se1S3/2 recombination rather than the higher energy 1Pe1P3/2 recombination.

By harvesting sub-bandgap photons, we have a path to overcome the Shockley-Queisser limit in photovoltaics (PVs). We investigate semiconductor nanocrystal (NC) sensitized upconversion via triplet-triplet annihilation (TTA) in organic semiconductors (OSCs). Since this process relies on optically inactive triplet states in the OSCs, we utilize PbS NCs to directly sensitize the triplet state via energy transfer. This is possible due to the strong spin-orbit coupling in PbS NCs, resulting in rapid spin-dephasing of the exciton. Current technology allows for upconversion of light with a photon energy above $\sim 1.1$ eV. However, while internal efficiencies are rapidly improving, the low external device efficiencies render them impractical for applications, as devices are based on a single monolayer of NCs. Our results show simply increasing the PbS NC film thickness does not show improvement in the efficiency due to poor exciton transport between PbS NCs. Here, we present a new strategy to increase the external upconversion efficiency by utilizing thin tinbased halide perovskites as the absorbing layer. Resonant energy transfer from the perovskite to the PbS NCs allows for subsequent sensitization of the triplet state in rubrene.

Mixed-cation lead mixed-halide perovskite thinfilms are a promising alternative to methylammonium lead iodide due to higher long-term stability, photostability and bandgap tenability. However, the crystallinity of the absorber layer plays an important role for charge carrier collection as well as for an efficient transport between the different device layers. In this contribution, we investigate the influence of grain size by changing the molar concentration of the perovskite precursor containing Rb, Cs, MA, FA, Pb, I and Br on the charge carrier dynamics. We find that with increasing molar precursor concentration, the grain sizes increase and the perovskite grains become more oriented with an improved charge carrier lifetime. In particular, films with small grains show mostly random grain orientation angles, whereas films with larger grains are oriented with {100} planes around an angle of 20° relative to the surface normal. These findings may be the crucial factor in engineering high-quality films leading to high power conversion efficiencies.

The process of upconversion leads to emission of photons higher in energy than the incident photons. Near-infrared-to-visible upconversion, in particular, shows promise in sub-bandgap sensitization of silicon and other optoelectronic materials, resulting in potential applications ranging from photovoltaics that exceed the Shockley–Queisser limit to infrared imaging. A feasible mechanism for near-infrared-to-visible upconversion is triplet–triplet annihilation (TTA) sensitized by colloidal nanocrystals (NCs). Here, the long lifetime of spin-triplet excitons in the organic materials that undergo TTA makes upconversion possible under incoherent excitation at relatively low photon fluxes. Since this process relies on optically inactive triplet states, semiconductor NCs are utilized as efficient spin mixers, absorbing the incident light and sensitizing the triplet states of the TTA material. The state-of-the-art system uses rubrene with a triplet energy of 1.14 eV as the TTA medium, and thus allows upconversion of light with photon energies above ∼1.1 eV. In this perspective, we review the field of lead sulfide (PbS) NC-sensitized near-infrared-to-visible upconversion, discuss solution-based upconversion, and highlight progress made on solid-state upconversion devices.

Perovskite solar cells (PSCs) are very promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature, planar PSCs are of particularly interest for large-scale deployment due to their inherent suitability for flexible substrates and potential for silicon/perovskite tandems. So far, planar PSCs have been prone to large current-voltage hysteresis and low stabilized power output due to a number of issues associated with this kind of device configuration. We find that the suppression of the yellow-phase impurity (∂-FAPbI3) present in formamidium-based perovskites, by RbI addition, contributes to low hysteresis, higher charge carrier mobility, long-lived carrier lifetimes and a champion stabilized power output of 20.3% using SnOx as the electron selective contact. We study the effects of these impurities on the transient behavior that defines hysteresis and its relation to ionic movement. In addition, we find that the formation of a RbPbI3 phase does not significantly affect the charge carrier lifetimes and consequently the performance of the devices. This brings new physical insights onto the role of different impurities in perovskite solar cells, which make these materials so remarkable., US Department of Energy, Office of Science, Office of Basic Energy Sciences (award no. DE-SC0001088)

Hybrid interfaces combining inorganic and organic materials underpin the operation of many optoelectronic and photocatalytic systems and allow for innovative approaches to photon up- and down-conversion. However, the mechanism of exchange-mediated energy transfer of spin-triplet excitons across these interfaces remains obscure, particularly when both the macroscopic donor and acceptor are composed of many separately interacting nanoscopic moieties. Here, we study the transfer of excitons from colloidal lead sulfide (PbS) nanocrystals to the spin-triplet state of rubrene molecules. By reducing the length of the carboxylic acid ligands on the nanocrystal surface from 18 to 4 carbon atoms, thinning the effective ligand shell from 13 to 6 Å, we are able to increase the characteristic transfer rate by an order of magnitude. However, we observe that the energy transfer rate asymptotes for shorter separation distances (≤10 Å) which we attribute to the reduced Dexter coupling brought on by the increased effective dielectric constant of these solid-state devices when the aliphatic ligands are short. This implies that the shortest ligands, which hinder long-term colloidal stability, offer little advantage for energy transfer. Indeed, we find that hexanoic acid ligands are already sufficient for near-unity transfer efficiency. Using nanocrystals with these optimal-length ligands in an improved solid-state device structure, we obtain an upconversion efficiency of (7 ± 1)% with excitation at λ = 808 nm.

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiO x |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics., National Science Foundation (U.S.) (Grant CBET-1605495), United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0001088), National Science Foundation (U.S.) (Grant DMF-08019762)

The relationship between charge-carrier lifetime and the tolerance of lead halide perovskite (LHP) solar cells to intrinsic point defects has drawn much attention by helping to explain rapid improvements in device efficiencies. However, little is known about how charge-carrier lifetime and solar cell performance in LHPs are affected by extrinsic defects (i.e., impurities), including those that are common in manufacturing environments and known to introduce deep levels in other semiconductors. Here, we evaluate the tolerance of LHP solar cells to iron introduced via intentional contamination of the feedstock and examine the root causes of the resulting efficiency losses. We find that comparable efficiency losses occur in LHPs at feedstock iron concentrations approximately 100 times higher than those in p-type silicon devices. Photoluminescence measurements correlate iron concentration with nonradiative recombination, which we attribute to the presence of deep-level iron interstitials, as calculated from first-principles, as well as iron-rich particles detected by synchrotron-based X-ray fluorescence microscopy. At moderate contamination levels, we witness prominent recovery of device efficiencies to near-baseline values after biasing at 1.4 V for 60 s in the dark. We theorize that this temporary effect arises from improved charge-carrier collection enhanced by electric fields strengthened from ion migration toward interfaces. Our results demonstrate that extrinsic defect tolerance contributes to high efficiencies in LHP solar cells, which inspires further investigation into potential large-scale manufacturing cost savings as well as the degree of overlap between intrinsic and extrinsic defect tolerance in LHPs and "perovskite-inspired" lead-free stable alternatives.

We analyze the optical, chemical, and electrical properties of chemical vapor deposition (CVD) grown hexagonal boron nitride (h-BN) using the precursor ammonia-borane ($H_3N-BH_3$) as a function of $Ar/H_2$ background pressure ($P_{TOT}$). Films grown at $P_{TOT}$ less than 2.0 Torr are uniform in thickness, highly crystalline, and consist solely of h-BN. At larger $P_{TOT}$, with constant precursor flow, the growth rate increases, but the resulting h-BN is more amorphous, disordered, and $sp^3$ bonded. We attribute these changes in h-BN grown at high pressure to incomplete thermolysis of the $H_3N-BH_3$ precursor from a passivated Cu catalyst. A similar increase in h-BN growth rate and amorphization is observed even at low $P_{TOT}$ if the $H_3N-BH_3$ partial pressure is initially greater than the background pressure $P_{TOT}$ at the beginning of growth. h-BN growth using the $H_3N-BH_3$ precursor reproducibly can give large-area, crystalline h-BN thin films, provided that the total pressure is under 2.0 Torr and the precursor flux is well-controlled., 45 pages including the supporting information, in print at Chemistry of Materials 2016