Our most recent papers...
491. Structural Phase- and Degradation-Dependent Thermal Conductivity of CH3NH3PbI3 Perovskite Thin Films
Guo, Z.; Yoon, S. J.; Manser, J. S.; Kamat, P. V.; Luo, T.
J. Phys. Chem. C 2016, 120 (2) pp 6394-6401
Organic-inorganic lead halide perovskites have shown great promise in photovoltaics and optoelectronics. In these applications, device performance and reliability can be strongly influenced by thermal transport in the materials. Through laser pump-probe experiments, different microstructures of CH3NH3PbI3 perovskite thin films are found to give rise to different phonon scattering mechanism. The thermal conductivity in CH3NH3PbI3 neat film decreases with temperature. Even though this agrees with the behavior of its bulk crystalline counterparts, an apparent thermal conductivity change near the structural phase transition temperature of this perovskite (orthorhombic vs tetragonal) has only been observed in the spin-coated films. Analyses suggest that this may be attributed to either an energy landscape change related to organic cation disorder or the thickness change of ferroelectric domain walls formed in the neat perovskite films that affects the phonon scattering at the domain boundaries. In contrast, no thermal conductivity discontinuity has been observed in the CH3NH3PbI3 /Al2O3 mesostructured films, where the thermal conductivity first shows an increasing trend at low temperature (<80 K) and then stays nearly constant. Such a trend is typical in amorphous materials and nanostructured composites where phonon scatterings are due to morphological disorder and internal interfaces play key roles in the thermal transport. When exposed to the ambient environment, humidity induced degradation is found to have a significant impact on the overall thermal conductivity of the spin-coated CH3NH3PbI3 neat film.
490. Band Diagram and Effects of the KSCN Treatment in TiO2/Sb2S3/CuSCN ETA Cells
Itzhaik, Y.; Bendikov, T.; Hines, D.; Kamat, P. V.; Cohen, H.; Hodes, G.
J. Phys. Chem. C 2016, 120 (1) pp 31-41
Thiocyanate ion treatment, usually either LiSCN or KSCN, of the absorbing semiconductor before deposition of a CuSCN hole conducting layer is known to improve the performance of extremely thin absorber (ETA) solar cells by reducing the cell resistivity. However, in spite of several hypotheses, the mechanism behind this treatment outcome remains elusive. In this study, the interface between Sb2S3 and CuSCN in an ETA cell is investigated with surface spectroscopy and transient absorption spectroscopy to establish the mechanistic aspects of the KSCN treatment and the role it plays in improving the photovoltaic performance. The prominent factors that dictate the cell performance are (a) doping the interfacial CuSCN and thus preventing the formation of a sub-μm depleted layer and (b) passivating charge traps at the Sb2S(O)3 surface, which increases the rate of hole transfer from the absorber to the hole conductor. We further show that the treatment works just as well in improving photovoltaic performance when carried out after CuSCN deposition (post-treatment).
489. Making and Breaking of Lead Halide Perovskites
Manser, J. S.; Saidaminov, M. I.; Christians, J. A.; Bakr, O. M.; Kamat, P. V.
Acc. Chem. Res. 2016, 49 (2) pp 330-338
A new front-runner has emerged in the field of next-generation photovoltaics. A unique class of materials, known as organic metal halide perovskites, bridges the gap between low-cost fabrication and exceptional device performance. These compounds can be processed at low temperature (typically in the range 80-150 oC) and readily self-assemble from the solution phase into high-quality semiconductor thin films. The low energetic barrier for crystal formation has mixed consequences. On one hand, it enables inexpensive processing and both optical and electronic tunability. The caveat, however, is that many as-formed lead halide perovskite thin films lack chemical and structural stability, undergoing rapid degradation in the presence of moisture or heat. To date, improvements in perovskite solar cell efficiency have resulted primarily from better control over thin film morphology, manipulation of the stoichiometry and chemistry of lead halide and alkylammonium halide precursors, and the choice of solvent treatment. Proper characterization and tuning of processing parameters can aid in rational optimization of perovskite devices. Likewise, gaining a comprehensive understanding of the degradation mechanism and identifying components of the perovskite structure that may be particularly susceptible to attack by moisture are vital to mitigate device degradation under operating conditions.
This Account provides insight into the lifecycle of organic-inorganic lead halide perovskites, including (i) the nature of the precursor solution, (ii) formation of solid-state perovskite thin films and single crystals, and (iii) transformation of perovskites into hydrated phases upon exposure to moisture. In particular, spectroscopic and structural characterization techniques shed light on the thermally driven evolution of the perovskite structure. By tuning precursor stoichiometry and chemistry, and thus the lead halide charge-transfer complexes present in solution, crystallization kinetics can be tailored to yield improved thin film homogeneity. Because degradation of the as-formed perovskite film is in many ways analogous to its initial formation, the same suite of monitoring techniques reveals the moisture-induced transformation of low band gap methylammonium lead iodide (CH3NH3PbI3) to wide band gap hydrate compounds. The rate of degradation is increased upon exposure to light. Interestingly, the hydration process is reversible under certain conditions. This facile formation and subsequent chemical lability raises the question of whether CH3NH3PbI3 and its analogues are thermodynamically stable phases, thus posing a significant challenge to the development of transformative perovskite photovoltaics. Adequately addressing issues of structural and chemical stability under real-world operating conditions is paramount if perovskite solar cells are to make an impact beyond the benchtop. Expanding our fundamental knowledge of lead halide perovskite formation and degradation pathways can facilitate fabrication of stable, high-quality perovskite thin films for the next generation of photovoltaic and light emitting devices.
488. Two Distinct Transitions in CuxInS2 Quantum Dots. Bandgap versus Sub-Bandgap Excitations in Copper-Deficient Structures
Jara, D. H.; Stamplecoskie, K. G.; Kamat, P. V.
J. Phys. Chem. Lett. 2016, 7 pp 1452-1459
Cu-deficient CuInS2 quantum dots (QDs) synthesized by varying the [Cu]:[In] ratio allow modulation of optical properties as well as identification of the radiative emission pathways. Absorption and emission spectral features showed a strong dependence on the [Cu]:[In] ratio of CuxInS2 QDs, indicating two independent optical transitions. These effects are pronounced in transient absorption spectra. The bleaching of band edge absorption and broad tail absorption bands in the subpicosecond-nanosecond time scale provide further evidence for the dual optical transitions. The recombination process as monitored by photoemission decay indicated the involvement of surface traps in addition to the bandgap and sub-bandgap transitions. Better understanding of the origin of the optical transitions and their influence on the photodynamics will enable utilization of ternary semiconductor quantum dots in display and photovoltaic devices.
487. How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites
Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V.
J. Phys. Chem. Lett. 2016, 7 pp 1368-1373
Varying the halide ratio (e.g., Br-:I-) is a convenient approach to tune the bandgap of organic lead halide perovskites. The complexation between Pb2+ and halide ions is the primary step in dictating the overall composition, and optical properties of the annealed perovskite structure. The complexation between Pb2+ and Br- is nearly 7 times greater than the complexation between Pb2+ and I-), thus making Br- a dominant binding species in mixed halide systems. Emission and transient absorption measurements show a strong dependence of excited state behavior on the composition of halide ions employed in the precursor solution. When excess halide (X = Br- and I-) are present in the precursor solution (0.3 M PbX2 and 0.9 M CH3NH3X), the exclusive binding of Pb2+ with Br- results in the formation of CH3NH3PbBr3 perovskites as opposed to mixed halide perovskite.
486. Modulation of Cu2-xS Nanocrystal Plasmon Resonance through Reversible Photoinduced Electron Transfer
Alam, R.; Labine, M.; Karwacki, C. J.; Kamat, P. V.
ACS Nano 2016, 10 (2) pp 2880-2886
Copper sulfide (Cu2-xS) nanocrystals with nonstoichiometric composition exhibit plasmon resonance in the near-infrared region. Compositional changes and varying electron density markedly affect the position and intensity of the plasmon resonance. We report a photochemically induced phenomenon of modulating the plasmon resonance in a controlled fashion. As photogenerated reduced methyl viologen radicals transfer electrons to Cu2-xS in inert solutions, we observe a decrease in localized surface plasmon resonance (LSPR) absorbance at 1160 nm. Upon exposure to air, the plasmon resonance band recovers as stored electrons are scavenged away by oxygen. This cycle of electron charge and discharge of Cu2-xS nanocrystals is reversible and can be repeated through photoirradiation in N2 saturated solution followed by exposure of the suspension to air. The spectroscopic studies that provide mechanistic insights into the reversible charging and discharging of plasmonic Cu2-xS are discussed.
485. Spatially Non-uniform Trap State Densities in Solution-Processed Hybrid Perovskite Thin Films
Draguta, S.; Thakur, S.; Morozov, Y.; Wang, Y.; Manser, J. S.; Kamat, P. V.; Kuno, M.
J. Phys. Chem. Lett. 2016, 7 (4) pp 715-721
The facile solution-processability of methylammonium lead halide (CH3NH3PbI3) perovskites has catalyzed the development of inexpensive, hybrid perovskite-based optoelectronics. It is apparent, though, that solution-processed CH3NH3PbI3 films possess local emission heterogeneities, stemming from electronic disorder in the material. Herein we investigate the spatially resolved emission properties of CH3NH3PbI3 thin films through detailed emission intensity versus excitation intensity measurements. These studies enable us to establish the existence of nonuniform trap density variations wherein regions of CH3NH3PbI3 films exhibit effective free carrier recombination while others exhibit emission dynamics strongly influenced by the presence of trap states. Such trap density variations lead to spatially varying emission quantum yields and correspondingly impact the performance of both methylammonium lead halide perovskite solar cells and other hybrid perovskite-based devices. Of additional note is that the observed spatial extent of the optical disorder extends over length scales greater than that of underlying crystalline domains, suggesting the existence of other factors, beyond grain boundary-related nonradiative recombination channels, which lead to significant intrafilm optical heterogeneities.
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.