Showing papers by "Prashant V. Kamat published in 2018"
TL;DR: In this article, the origin of light-induced halide phase segregation, its effects on photovoltaic response, and its effect on tandem solar cells are reviewed, and the effect of the halide migration has been investigated.
Abstract: Hybrid lead halide perovskites such as MAPbI3 (MA = CH3NH3+) and their mixed halide analogues represent an emerging class of materials for solar energy conversion. Intriguing aspects include sizable carrier diffusion lengths, large optical absorption coefficients, and certified power conversion efficiencies that now exceed 22%. Halide-composition-tunable band gaps also make MAPb(I1–xBrx)3 systems ideal candidates for tandem solar cells. Unfortunately, preventing the effective integration of MAPb(I1–xBrx)3 into working devices are intrinsic instabilities due to light-induced halide phase segregation. Namely, under illumination, mixed halide perovskites reversibly segregate into low-band-gap I-rich and high-band-gap Br-rich domains. Under electrical bias, halide migration has also been proposed as the source of undesirable charge injection barriers that degrade photovoltaic performance. In this Perspective, we review the origin of light-induced halide phase segregation, its effects on photovoltaic response,...
386 citations
154 citations
150 citations
TL;DR: In this article, the photodynamics of thiolated gold nanoclusters and efforts to exploit the Au particles in light energy conversion applications discussed in this Review show new opportunities to utilize them as photosensitizers.
Abstract: Few-atom gold nanoclusters (NCs) exhibit molecule-like properties due to a discrete electronic structure driven by the quantum confinement effect. Unlike plasmonic Au particles, these nonplasmonic particles of diameter less than 2 nm, commonly referred to as nanoclusters, possess a distinct excited-state behavior that can offer a new opportunity to employ them as a photosensitizer. Their size-dependent excited-state behavior enables establishing logical designing principles to build up efficient light energy conversion systems. The photodynamics of thiolated Au NCs and efforts to exploit the Au NCs in light energy conversion applications discussed in this Review show new opportunities to utilize them as photosensitizers. Current bottlenecks in implementing thiolated Au NCs in light conversion applications and new strategies and future directions to address these limitations are also discussed.
142 citations
TL;DR: In this paper, the effect of PbSO4-oleate capping on the excited state dynamics of lead halide perovskite nanocrystals has been investigated.
Abstract: The ease of halide ion exchange in metal halide nanocrystals offers an opportunity to utilize them in a layered or tandem fashion to achieve graded bandgap films. We have now successfully suppressed the halide ion exchange by capping CsPbBr3 and CsPbI3 nanocrystals with PbSO4–oleate to create a nanostructure assembly that inhibits the exchange of anions. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr3 or CsPbI3 for several days. Furthermore, the effect of PbSO4–oleate capping on the excited state dynamics has also been elucidated. The effectiveness of PbSO4–oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid toward the tandem design of perovskite light-harvesting assemblies.
104 citations
TL;DR: The focus of this study is to establish a stability window for electrochemical tests for all-inorganic CsPbBr3 and hybrid organic–inorganic MAPbI3 perovskites, and to understand the reduction and oxidation events that occur and to assess the damage done during these processes at extreme electrochemical conditions.
Abstract: The unique optoelectronic properties of lead halide perovskites have triggered a new wave of excitement in materials chemistry during the past five years. Electrochemistry, spectroelectrochemistry, and photoelectrochemistry could be viable tools both for analyzing the optoelectronic features of these materials and for assembling them into hybrid architectures (e.g., solar cells). At the same time, the instability of these materials limits the pool of solvents and electrolytes that can be employed in such experiments. The focus of our study is to establish a stability window for electrochemical tests for all-inorganic CsPbBr3 and hybrid organic–inorganic MAPbI3 perovskites. In addition, we aimed to understand the reduction and oxidation events that occur and to assess the damage done during these processes at extreme electrochemical conditions. In this vein, we demonstrated the chemical, structural, and morphological changes of the films in both reductive and oxidative environments. Taking all these result...
86 citations
TL;DR: In this paper, photoinduced halide ion segregation in mixed halide perovskites can introduce a detrimental effect on the photovoltaic performance of solar cells over extended light exposure.
Abstract: Photoinduced halide ion segregation in mixed halide perovskites can introduce a detrimental effect on the photovoltaic performance of perovskite solar cells over extended light exposure. The photoc...
77 citations
TL;DR: The electrophoretic deposition of layered films of perovskites in a controlled fashion opens up new ways to design tandem perovSKite solar cells and tunable display devices.
Abstract: The suppression of halide ion exchange between CsPbBr3 and CsPbI3 nanocrystals achieved through capping with PbSO4–oleate has enabled us to deposit different perovskite nanocrystals as aligned arrays on the electrode surfaces without intermixing of species. The electrophoretic deposition of PbSO4–oleate-capped CsPbX3 (X = Cl, Br, I) nanocrystals suspended in hexane solution on mesoscopic TiO2 films allows the design of controlled architecture with single or multiple layers of perovskite films. The hierarchy in the assembly of these nanocrystals is seen first through the linearly organized nanocrystals in hexane followed by the deposition of larger linear rods ∼500 nm in length. Since most of the photophysical properties of nanocrystals are retained in these aligned arrays, we can design films with tunable luminescence including white color. The electrophoretic deposition of layered films of perovskites in a controlled fashion opens up new ways to design tandem perovskite solar cells and tunable display de...
53 citations
TL;DR: In this article, AgInS2-ZnS solid solution quantum dots (QDs) prepared with varying Ag/Zn ratios demonstrate composition-dependent photophysical properties. Absorption and emission processes are extremely complex i...
Abstract: AgInS2–ZnS solid solution quantum dots (QDs) prepared with varying Ag/Zn ratios demonstrate composition-dependent photophysical properties. Absorption and emission processes are extremely complex i...
44 citations
TL;DR: In this article, the authors present a complete microscopic account of charge recombination processes in high efficiency hybrid perovskite (mixed cation and methylammonium lead iodide) solar cells.
Abstract: Hybrid perovskites represent a potential paradigm shift for the creation of low-cost solar cells. Current power conversion efficiencies (PCEs) exceed 22%. However, despite this, record PCEs are still far from their theoretical Shockley–Queisser limit of 31%. To increase these PCE values, there is a pressing need to understand, quantify and microscopically model charge recombination processes in full working devices. Here, we present a complete microscopic account of charge recombination processes in high efficiency (18–19% PCE) hybrid perovskite (mixed cation and methylammonium lead iodide) solar cells. We employ diffraction-limited optical measurements along with relevant kinetic modeling to establish, for the first time, local photoluminescence quantum yields, trap densities, trapping efficiencies, charge extraction efficiencies, quasi-Fermi-level splitting, and effective PCE estimates. Correlations between these spatially resolved parameters, in turn, allow us to conclude that intrinsic electron traps in the perovskite active layers limit the performance of these state-of-the-art hybrid perovskite solar cells.
41 citations
TL;DR: The interfacial electron transfer between excited CsPbBr3 perovskite nanocrystals and metal oxides was elucidated using transient absorption spectroscopy and found to occur with a rate constant in the range of 2-4 × 1010 s-1.
Abstract: Record-breaking efficiency achieved with quantum dot solar cells made of perovskite nanocrystals demands understanding of the excited-state interactions between perovskite nanocrystals and metal oxide electron transport layers. The interfacial electron transfer between excited CsPbBr3 perovskite nanocrystals and metal oxides (TiO2, SnO2, and ZnO) was elucidated using transient absorption spectroscopy and found to occur with a rate constant in the range of 2–4 × 1010 s–1. In an inert atmosphere, back electron transfer helps to maintain the stability of the perovskite nanocrystals. However, the presence of oxygen introduces instability as it scavenges away transferred electrons from the electron-transporting metal oxide, leaving behind holes to accumulate at CsPbBr3 nanocrystals, which in turn induce anodic corrosion. X-ray photoelectron spectroscopy measurements have enabled us to identify PbO as the major photodegraded product. The importance of the surrounding atmosphere and the supporting metal oxide in...
TL;DR: The ultrafast spectroelectrochemical experiments described here offer a convenient approach to probe the charging effects in perovskite solar cells and indicate the reversibility of the photoresponse to charging effects.
Abstract: The charging of a mesoscopic TiO2 layer in a metal halide perovskite solar cell can influence the overall power conversion efficiency. By employing CsPbBr3 films deposited on a mesoscopic TiO2 film, we have succeeded in probing the influence of electrochemical bias on the charge carrier recombination process. The transient absorption spectroscopy experiments conducted at different applied potentials indicate a decrease in the charge carrier lifetimes of CsPbBr3 as we increase the potential from −0.6 to +0.6 V vs Ag/AgCl. The charge carrier lifetime increased upon reversing the applied bias, thus indicating the reversibility of the photoresponse to charging effects. The ultrafast spectroelectrochemical experiments described here offer a convenient approach to probe the charging effects in perovskite solar cells.
TL;DR: In this paper, the authors reported the chemical transformations on the water-vapor/solid interface and catalytic performance of an OER catalyst consisting of Co3O4 nanoparticles on multiwalled carbon nanotubes (Co3O 4-MWCNT).
Abstract: The design and mechanistic understanding of efficient and low-cost catalysts for the oxygen evolution reaction (OER) are currently the focus of electrochemical water-splitting technology. Herein, we report the chemical transformations on the water-vapor/solid interface and catalytic performance of an OER catalyst consisting of Co3O4 nanoparticles on multiwalled carbon nanotubes (Co3O4–MWCNT). Using a specially constructed electrochemical cell incorporated to the lab-based ambient-pressure X-ray photoelectron spectroscopy (APXPS) to mimic operando conditions, we obtained experimental evidence for the formation of CoO(OH) as the catalytically active phase on a Co3O4–MWCNT OER catalyst. Under water and applied potential conditions, CoO(OH) is formed, enriching the surface of Co3O4 nanoparticles with subnanometer thickness, and oxidizing H2O into O2. However, immediately after the removal of the applied potential, the CoO(OH) phase is converted back to Co3O4. This back-conversion from CoO(OH) to Co3O4 is like...
Journal Article•
TL;DR: In this article, the authors used a specially constructed electrochemical cell incorporated to the lab-based ambient-pressure X-ray photoelectron spectroscopy (APXPS) to mimic operando conditions, and obtained experimental evidence for the formation of CoO(OH) as the catalytically active phase on a Co₃O₄-MWCNT OER catalyst.
Abstract: The design and mechanistic understanding of efficient and low-cost catalysts for the oxygen evolution reaction (OER) are currently the focus of electrochemical water-splitting technology. Herein, we report the chemical transformations on the water-vapor/solid interface and catalytic performance of an OER catalyst consisting of Co₃O₄ nanoparticles on multiwalled carbon nanotubes (Co₃O₄–MWCNT). Using a specially constructed electrochemical cell incorporated to the lab-based ambient-pressure X-ray photoelectron spectroscopy (APXPS) to mimic operando conditions, we obtained experimental evidence for the formation of CoO(OH) as the catalytically active phase on a Co₃O₄–MWCNT OER catalyst. Under water and applied potential conditions, CoO(OH) is formed, enriching the surface of Co₃O₄ nanoparticles with subnanometer thickness, and oxidizing H₂O into O₂. However, immediately after the removal of the applied potential, the CoO(OH) phase is converted back to Co₃O₄. This back-conversion from CoO(OH) to Co₃O₄ is likely driven by locally concentrated protons (H⁺) in water vapor, which shows the necessity of an electrochemical bias to preserve the catalytically active phase. These results reveal the surface chemical identities of the Co₃O₄–MWCNT OER catalyst, which are in agreement with those obtained from in-situ APXPS studies of liquid/solid interfaces consisting of Co₃O₄ catalyst and disagree with those obtained from ex-situ ultrahigh vacuum (UHV) XPS. Thus, our results demonstrate the possibility of performing surface chemical analysis in simplified electrochemical systems and further reinforce the importance of performing mechanistic studies of electrochemical devices under in-situ conditions.
TL;DR: The electrochemical post-treatment protocol is shown to be important to maximize the photo-voltaic performance of the solar cells and the proposed electrochemical deposition methodology expands the pool of tech-niques available for hole transporting layer deposition in perovskite solar cells.
Abstract: Electrochemistry, as an analytical tool is gaining its foothold in the characterization of organic metal lead halide per-ovskites, however electrosynthetic applications are yet to be explored fully. Electrochemical deposition of hole-transport layers with desired properties could be an attractive tool to assemble complex solar cell architectures. We have now successfully electropolymerized PEDOT (Poly(3,4-ethylenedioxythiophene)), a hole-transporting material on the surface of methyl-ammonium lead iodide perovskite in a controlled fashion and evaluated its performance in perovskite solar cells. The electrochemical post-treatment protocol is shown to be important to maximize the photo-voltaic performance of the solar cells. The proposed electrochemical deposition methodology expands the pool of tech-niques available for hole transporting layer deposition in perovskite solar cells.
TL;DR: In this article, a few atom gold clusters modified with organic ligands were used to induce triplet energy transfer to carotene and oxygen with 13% quantum efficiency, and a singlet oxygen generation with the efficiency of 13% was observed.
Abstract: Glutathione-capped gold clusters prepared in an aqueous medium are known to exhibit excellent photosensitizing properties. We have now successfully transferred these gold clusters in an organic medium while retaining all the characteristic excited state properties. These gold clusters can be further modified with organic ligands such as 2-Phenylethanethiol (PET). The gold clusters in organic medium exhibit enhanced emission yield (
$$\Phi _{\mathrm{f}} = 0.15$$
) compared to that in an aqueous medium (
$$\Phi _{\mathrm{f}} = 0.08$$
). The excited state lifetimes of $$3.7\,\upmu \hbox {s}$$
(untreated) and $$1.5\,\upmu \hbox {s}$$
(PET treated) in toluene are also greater than the lifetime observed in aqueous solution (
$$0.77\,\upmu \hbox {s}$$
). By employing laser flash photolysis we are able to induce triplet energy transfer to $$\upbeta $$
-carotene and oxygen. A singlet oxygen generation with the efficiency of 13% was observed in these experiments. The excited state properties of glutathione-capped gold clusters further shows its importance as a photosensitizer in light energy conversion and biomedical applications SYNOPSIS A few atom gold clusters modified with organic ligands exhibit high photoactivity by generating singlet oxygen with 13% quantum efficiency.
TL;DR: The original version of this Article contained an error in the spelling of the author Joseph S. Manser, which is now corrected in both the PDF and HTML versions of the Article.
Abstract: The original version of this Article contained an error in the spelling of the author Joseph S. Manser, which was incorrectly given as Joseph M. Manser. This has now been corrected in both the PDF and HTML versions of the Article.