Our most recent papers...
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.
476. Best Practices in Perovskite Solar Cell Efficiency Measurements. Avoiding the Error of Making Bad Cells Look Good (Viewpoint)
Christians, J. A.; Manser, J. S.; Kamat, P. V. J. Phys. Chem. Lett. 2015, 6 (5), 852–857.
Perovskite solar cells employing hybrid organic–inorganic halide perovskites (e.g., CH3NH3PbI3) have taken the photovoltaic community by storm. In the short time since being deemed its own class of emerging photovoltaic technologies by the National Renewable Energy Laboratory (October, 2013), the certified record efficiency of perovskite solar cells has increased nearly 50%, from 14.1 to 20.1% (http://www.nrel.gov/ncpv/). In addition, several groups have reported reproducible efficiencies in excess of 16%. These devices show great promise for commercial applications as they combine low-cost fabrication techniques with earth-abundant materials yet still deliver efficiencies rivaling traditional photovoltaic technologies. The possibility of using them in building facades or as a top cell in a tandem perovskite–Si architecture only increases their desirability. However, there is currently a dire need in the field for increased care on the part of authors in reporting their photovoltaic performance and on the side of reviewers and the scientific community at large in discriminating and evaluating reported results. Our hope is that this Viewpoint brings to light some of the issues pertaining to perovskite solar cells and provides the field with best practices for measuring and reporting perovskite solar cell performance.
475. Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air
Christians, J. A.; Miranda Herrara, P. A.; Kamat, P. V. J. Am. Chem. Soc. 2015, 137(4),PP 1530-1538.
Humidity has been an important factor, in both negative and positive ways, in the development of perovskite solar cells, and will prove critical in the push to commercialize this exciting new photovoltaic technology. The interaction between CH3NH3PbI3 and H2O vapor is investigated by characterizing the ground state and excited state optical absorption properties, and probing morphology and crystal structure. These systematic undertakings elucidate the complex interaction inherent in this system, demonstrating that H2O exposure does not simply only CH3NH3PbI3 to revert to PbI2. It is shown that, in the dark, H2O is able to complex with the perovskite, forming a hydrate product similar to (CH3NH3)4PbI6•2H2O. This causes a decrease in absorption across the visible region of the spectrum and a distinct change in the crystal structure of the material. Femtosecond transient absorption spectroscopic measurements show the effect that humidity has on the ultrafast excited state dynamics of CH3NH3PbI3. More importantly, the deleterious effects of humidity on complete solar cells, specifically on photovoltaic efficiency and stability, are explored in light of these spectroscopic understandings.
474. All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Assembly for Photolytic Solar Fuels Production
Chen, Y.-S.; Manser, J. S.; Kamat, P. V. J. Am. Chem. Soc. 2015, 137 (2), 974–981.
The quest for economic, large scale hydrogen production has motivated the search for new materials and device designs capable of splitting water using only energy from the sun. Here we introduce an all solution-processed tandem water splitting assembly composed of a BiVO4 photoanode and a single-junction CH3NH3PbI3 hybrid perovskite solar cell. This unique configuration allows efficient solar photon management, with the metal oxide photoanode selectively harvesting high energy visible photons and the underlying perovskite solar cell capturing lower energy visible-near IR wavelengths in a single-pass excitation. Operating without external bias under standard AM 1.5G illumination, the photoanode-photovoltaic architecture, in conjunction with an earth-abundant cobalt phosphate catalyst, exhibits a solar-to-hydrogen conversion efficiency of 2.5% at neutral pH. The design of low-cost tandem water splitting assemblies employing single-junction hybrid perovskite materials establishes a potentially promising new frontier for solar water splitting research.
473. Predicting the Rate Constant of Electron Tunneling Reactions at the CdSe-Linker-TiO2 Interface
Hines, D. A.; Forrest, R. P.; Corcelli, S. A.; Kamat, P. V. J. Phys. Chem. B 2015, 119 (24), 7439-7446.
Current interest in quantum dot solar cells (QDSCs) motivates an understanding of the electron transfer dynamics at the quantum dot (QD) – metal oxide (MO) interface. Employing transient absorption spectroscopy, we have monitored the electron transfer rate (ket) at this interface as a function of the bridge molecules that link QDs to TiO2. Using mercaptoacetic acid (MAA), 3-mercaptopropionic acid (3-MPA), 8-mercaptooctanoic acid (8-MOA) and 16-mercaptohexadecanoic acid (16-MHA) we observe an exponential attenuation of ket with increasing linker length, which has been attributed to the tunneling of the electron through the insulating linker molecule. We model the electron transfer reaction using both rectangular and trapezoidal barrier models that have been discussed in the literature. The one electron reduction potential (equivalent to the lowest unoccupied molecular orbital or LUMO) of each molecule as determined by cyclic voltammetry (CV) was used to estimate the effective barrier height presented by MAA, 3-MPA, 8-MOA and16-MHA at the CdSe-TiO2 interface. The electron transfer rate (ket) calculated for each CdSe-TiO2 interface using both models showed the results in agreement with the experimentally determined trend. This demonstrates that electron transfer between CdSe and TiO2 can be viewed as electron tunneling through a layer of organic linking molecules and provides a useful method for predicting electron transfer rate constants.