Our most recent papers...
484. Evolution of Chemical Composition, Morphology, and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite under Ambient Conditions
Huang, W.; Manser, J. S.; Kamat, P. V.; Ptasinska, S.
Chem. Mater. 2016, 28 (1) pp 303-311
The surface composition and morphology of CH3NH3PbI3 perovskite films stored for several days under ambient conditions were investigated by X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction techniques. Chemical analysis revealed the loss of CH3NH3+ and I- species from CH3NH3PbI3 and its subsequent decomposition into lead carbonate, lead hydroxide, and lead oxide. After long-term storage under ambient conditions, morphological analysis revealed the transformation of randomly distributed defects and cracks, initially present in the densely packed crystalline structure, into relatively small grains. In contrast to PbI2 powder, CH3NH3PbI3 exhibited a different degradation trend under ambient conditions. Therefore, we propose a plausible CH3NH3PbI3 decomposition pathway that explains the changes in the chemical composition of CH3NH3PbI3 under ambient conditions. In addition, films stored under such conditions were incorporated into photovoltaic cells, and their performances were examined. The chemical changes in the decomposed films were found to cause a significant decrease in the photovoltaic efficiency of CH3NH3PbI3.
483. Dynamics of Photogenerated Charge Carriers in WO3/BiVO4 Heterojunction Photoanodes
Grigioni, I.; Stamplecoskie, K. G.; Selli, E.; Kamat, P. V.
J. Phys. Chem. C 2015, 119 (36) pp 20792-20800
Bismuth vanadate (BiVO4) with a band gap of ∼2.4 eV has emerged as one of the visible photocatalysts that can absorb light below 520 nm. The electron/hole pairs that are generated following BiVO4 band gap excitation are effective for water splitting, especially when BiVO4 is combined with other metal oxides such as WO3. We report a solution processed method for designing transparent WO3/BiVO4 heterojunction electrodes and observe a synergistic effect on the photoelectrochemical activity of WO3/BiVO4, with the combined system performing dramatically better than either individual component. Using ultrafast transient absorption spectroscopy, we elucidated the electronic interaction between WO3 and excited BiVO4. Moreover, the photocatalytic reduction of thionine by WO3/BiVO4 as well as by each individual oxide component is used to track electron injection processes and determine the energetics of the studied systems. In the composite WO3/BiVO4 film a shifted quasi-Fermi level results, due to electronic equilibration between the two materials. The better performance of WO3/BiVO4 heterojunction electrodes is thus a consequence of the electron injection from BiVO4 into WO3, followed by back electron transfer from WO3 to the holes in BiVO4.
482. Understanding the Implication of Carrier Diffusion Length in Photovoltaic Cells (Viewpoint)
Hodes, G.; Kamat, P. V.
J. Phys. Chem. Lett. 2015, 6 (20) pp 4090-4092
The purpose of this Viewpoint is to dispel a commonly held misconception when comparing diffusion lengths and discuss how variation in the measuring techniques can bring about differences in the measured values. The diffusion length, Ld, of electrons or holes in a semiconductor is defined by the average distance the relevant charge moves in the semiconductor. It is influenced by the average distance the relevant charge moves in the semiconductor (for example, in photovoltaic cells, which is the topic of interest in this Viewpoint) and recombination/extraction from the semiconductor.
481. Evolution of Organic–Inorganic Lead Halide Perovskite from Solid-State Iodoplumbate Complexes
Manser, J. S.; Reid, B.; Kamat, P. V.
J. Phys. Chem C 2015, 119, 17065–17073
The optoelectronic properties of hybrid perovskites are a strong function of their physical structure, and understanding the fundamental steps involved in the formation of these films can aid in the optimization and rational design of devices with tailored properties. Here we investigate the structural and optical characteristics of CH3NH3PbI3 films prepared from solutions composed of stoichiometric and nonstoichiometric quantities of lead iodide and methylammonium iodide precursors. In the presence of excess organohalide salt, a precursor phase composed of various iodoplumbate complexes is stabilized. The complexes dominate the optical properties of as-deposited films. Upon thermal treatment, the iodoplumbate precursor phase gradually evolves into the final tetragonal perovskite structure. Employing transient absorption spectroscopy, we have succeeded in tracking this transformation and gain insight into the interplay between the solid-state precursor and perovskite phases at various stages of formation. Correlation between time-resolved spectroscopic data and structural character can aid in better defining the structure–property relationship of hybrid perovskite thin films.
480. CdSe/CdS Nanorod Photocatalysts: Tuning the Interfacial Charge Transfer Process through Shell Length
Bridewell, V. L.; Alam, R.; Karwacki, C. J.; Kamat, P. V.
Chem. Mater. 2015, 27, 5064-5071
CdSe/CdS core/shell semiconductor nanorods (NR) with rod-in-rod morphology offer new strategies for designing highly emissive nanostructures. The interplay between energetically matched semiconductors results in enhanced emission from the CdSe core. In order to further evaluate the cooperative role of these two semiconductors in a core/shell geometry, we have probed the photoinduced charge transfer between CdSe/CdS core/shell semiconductor NR and methyl viologen (MV2+). The quenching of the emission by the electron acceptor, MV2+, as well as the production of electron transfer product MV•+ depends on the aspect ratio (l/w) of the NR thus pointing out the role of CdS shell in determining the overall photocatalytic efficiency. Transient absorption measurements show that the presence of MV2+ influences only the bleaching recovery of the CdS shell and not of the CdSe core recovery. Thus, optimization of shell aspect ratio plays a crucial role in maximizing the efficiency of this photocatalytic system.
479. Spatial and temporal imaging of long-range charge transport in perovskite thin films by ultrafast microscopy
Guo, Z.; Manser, J. S.; Wan, Y.; Kamat, P. V.; Huang, L.
Nat. Commun. 2015, 6, Article No. 7471
Charge carrier diffusion coefficient and length are important physical parameters for semiconducting materials. Long-range carrier diffusion in perovskite thin films has led to remarkable solar cell efficiencies; however, spatial and temporal mechanisms of charge transport remain unclear. Here we present a direct measurement of carrier transport in space and in time by mapping carrier density with simultaneous ultrafast time resolution and ~50-nm spatial precision in perovskite thin films using transient absorption microscopy. These results directly visualize long-range carrier transport of ~220 nm in 2 ns for solution-processed polycrystalline CH3NH3PbI3, thin films. Variations of the carrier diffusion coefficient at the μm length scale have been observed with values ranging between 0.05 and 0.08 cm2 s−1. The spatially and temporally resolved measurements reported here underscore the importance of the local morphology and establish an important first step towards discerning the underlying transport properties of perovskite materials.
478. Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping
Christians, J. A.; Manser, J. S.; Kamat, P. V. J. Phys. Chem. Lett. 2015, 6, 2086–2095.
A need to understand the excited-state behavior of organic–inorganic hybrid perovskites, such as CH3NH3PbI3, has arisen due to the rapid development of perovskite solar cells. The photoinduced processes leading to the efficient charge separation observed in these materials remain somewhat elusive. This Perspective presents an overview of the initial attempts to characterize the excited-state and charge recombination dynamics in the prototypical material CH3NH3PbI3. While much has been accomplished in designing high-efficiency solar cells, the multifaceted nature of the CH3NH3PbI3 excited state offers ample challenges for the photovoltaic community to better comprehend. Building on this foundation may enable us to tackle the stability concerns that have shadowed the rise of perovskite solar cells. Furthermore, a better understanding of the excited-state properties can provide insight into the specific properties that have thrust this material to the forefront of photovoltaic research.
477. Synergistic Effects in the Coupling of Plasmon Resonance of Metal Nanoparticles with Excited Gold Clusters
Stamplecoskie, K. G.; Kamat, P. V. J. Phys. Chem. Lett. 2015, 6 (5), 1870–1875.
When molecules or clusters are within the proximity of metal particles, their electronic transitions can be drastically enhanced. We have now probed the off-resonance excitation of molecule-like, glutathione-capped gold clusters (Au-GSH) in the close proximity of larger (plasmonic) Au and Ag nanoparticles. The excited state absorption spectrum of Au-GSH* is obtained with monophotonic excitation. The characteristic absorption of Au-GSH* allows us to probe the influence of excited plasmonic nanoparticles coupled with the clusters. Although infrared (775 nm) lasers pulses do not produce Au-GSH*, the excited states of these clusters are formed when coupled with metal (Au, Ag) nanoparticles. Interestingly, the coupled excitation of Au-GSH/AgNP with 775 nm laser pulses also results in an enhanced field effect, as seen from increased plasmon response of the metal nanoparticles. Transient absorption measurements confirm the synergy between these two inherently different nanomaterials, causing them to display greater excitation features. Better understanding of metal cluster–metal nanoparticle interactions will have important implications in designing light harvesting systems, and optoelectronic devices.