Mechanisms of Spontaneous and Amplified Spontaneous Emission in CH3 NH3 Pb I3 Perovskite Thin Films Integrated in an Optical Waveguide

In this paper, the physical mechanisms responsible for optical gain in CH3NH3PbI3 (MAPI) polycrystalline thin films are investigated experimentally and theoretically. Waveguide structures composed by a MAPI film embedded in between PMMA and silica layers are used as an efficient geometry to confine emitted light in MAPI films and minimize the energy threshold for amplified spontaneous emission (ASE). We show that photogenerated exciton density at the ASE threshold is as low as (2.4-12)×1016cm-3, which is below the Mott transition density reported for this material and the threshold transparency condition deduced with the free-carrier model. Such a low threshold indicates that the formation of excitons plays an important role in the generation of optical gain in MAPI films. The rate-equation model including gain is incorporated into a beam-propagation algorithm to describe waveguided spontaneous emission and ASE in MAPI films, while using the optical parameters experimentally determined in this work. This model is a useful tool to design active photonic devices based on MAPI and other metal-halide semiconductors

Localized versus delocalized states: Photoluminescence from electrochemically synthesized ZnO nanowires

We analyze the near-band-edge photoluminescence of electrochemically deposited ZnO nanowires and directly correlate the photoluminescence properties with the carrier concentration in the nanowires as determined from electrochemical impedance spectroscopy. We find a donor density of 81019 cm−3 in the as-deposited nanowires and show that the near-band-edge emission results from band-to-band recombination processes delocalized states. A photoluminescence band centered at 3.328 eV scales with the diameter of the nanowires and is assigned to recombination processes involving surface states. We show that annealing at 500 °C in air reduces the donor density in the nanowires by more than one order of magnitude, leading to sharp excitonic transitions in the electrochemically deposited nanowire

Low-Cost Flexible Nano-Sulfide/Carbon Composite Counter Electrode for Quantum-Dot-Sensitized Solar Cell

Cu2S nanocrystal particles were in situ deposited on graphite paper to prepare nano-sulfide/carbon composite counter electrode for CdS/CdSe quantum-dot-sensitized solar cell (QDSC). By optimization of deposition time, photovoltaic conversion efficiency up to 3.08% was obtained. In the meantime, this composite counter electrode was superior to the commonly used Pt, Au and carbon counter electrodes. Electrochemical impedance spectra further confirmed that low charge transfer resistance at counter electrode/electrolyte interface was responsible for this, implied the potential application of this composite counter electrode in high-efficiency QDSC

Mechanism of carrier accumulation in perovskite thin-absorber solar cells

[EN] Photovoltaic conversion requires two successive steps: accumulation of a photogenerated charge and charge separation. Determination of how and where charge accumulation is attained and how this accumulation can be identified is mandatory for understanding the performance of a photovoltaic device and for its further optimization. Here we analyse the mechanism of carrier accumulation in lead halide perovskite, CH3NH3PbI3, thin-absorber solar cells by means of impedance spectroscopy. A fingerprint of the charge accumulation in high density of states of the perovskite absorber material has been observed at the capacitance of the samples. This is, as far as we know, the first observation of charge accumulation in light-absorbing material for nanostructured solar cells, indicating that it constitutes a new kind of photovoltaic device, differentiated from sensitized solar cells, which will require its own methods of study, characterization and optimization.We thank the following agencies for supporting this research: Ministerio de Educacion y Ciencia under project HOPE CSD2007-00007, Generalitat Valenciana (ISIC/2012/008) and Universitat Jaume I project 12I361.01/1. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No. NRF-2012M1A2A2671721, NRF-2010-0014992 and NRF-2012M3A6A7054861 (the Global Frontier R&D Program on Center for Multiscale Energy System). H.-S.K. is grateful for the global Ph.D. fellowship grant funded by NRF (NRF-2011-0008467). We thank Mr. Dae-Yong Son for preparation of the ZrO2 paste. We thank Prof. A. Maquieira and Dr. M.J. Banuls from the Institute of Molecular Recognition and Technological Development (Polytechnic University of Valencia) for SEM measurements of CH3NH3PbI3-xClx samples.Kim, H.; Mora-Sero, I.; González-Pedro, V.; Fabregat-Santiago, F.; Juarez-Perez, EJ.; Park, N.; Bisquert Mascarell, J. (2013). Mechanism of carrier accumulation in perovskite thin-absorber solar cells. Nature Communications. 4:1-7. https://doi.org/10.1038/ncomms3242S174Nozik, A. J. Quantum dot solar cells. Physica E 14, 115–200 (2002).O'Regan, B. & Gratzel, M. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740 (1991).Hodes, G. Comparison of dye- and semiconductor-sensitized porous nanocrystalline liquid junction solar cells. J. Phys. Chem. C 112, 17778–17787 (2008).Mora-Seró, I. & Bisquert, J. Breakthroughs in the development of semiconductor-sensitized solar cells. J. Phys. Chem. Lett. 1, 3046–3052 (2010).Kim, H.-S. et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2, 591 (2012).Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338, 643–647 (2012).Noh, J. H., Im, S. H., Heo, J. H., Mandal, T. N. & Seok, S. I. Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells. Nano. Lett. 13, 1764–1769 (2013).Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. Low-temperature processed mesosuperstructured to thin-film perovskite solar cells. Energy Environ. Sci. 6, 1739–1743 (2013).Edri, E., Kirmayer, S., Cahen, D. & Hodes, G. High open-circuit voltage solar cells based on organic–inorganic lead bromide perovskite. J. Chem. Phys. Lett. 4, 897–902 (2013).Im, J.-H., Lee, C.-R., Lee, J.-W., Park, S.-W. & Park, N.-G. 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale 3, 4088–4093 (2011).Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051 (2009).Etgar, L. et al. Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells. J. Am. Chem. Soc. 134, 17396–17399 (2012).Hod, I. et al. Dye versus quantum dots in sensitized solar cells: participation of quantum dot absorber in the recombination process. J. Phys. Chem. Lett. 2, 3032–3035 (2011).Boix, P. P. et al. From flat to nanostructured photovoltaics: balance between thickness of the absorber and charge screening in sensitized solar cells. ACS Nano 6, 873–880 (2012).Unger, E. L. et al. Bilayer hybrid solar cells based on triphenylamine-thienylenevinylene dye and TiO2 . J. Phys. Chem. C 114, 11659–11664 (2010).Palomares, E., Clifford, J. N., Haque, S. A., Lutz, T. & Durrant, J. R. Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers. J. Am. Chem. Soc. 125, 475–482 (2003).Topoglidis, E., Campbell, C. J., Palomares, E. & Durrant, J. R. Photoelectrochemical study of Zn cytochrome-c immobilised on a nanoporous metal oxide electrode. Chem. Comm. 14, 1518–1519 (2002).Bisquert, J. Chemical capacitance of nanostructured semiconductors: its origin and significance for nanocomposite solar cells. Phys. Chem. Chem. Phys. 5, 5360–5364 (2003).Fabregat-Santiago, F., Garcia-Belmonte, G., Mora-Seró, I. & Bisquert, J. Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy. Phys. Chem. Chem. Phys. 13, 9083–9118 (2011).Bisquert, J., Grätzel, M., Wang, Q. & Fabregat-Santiago, F. Three-channel transmission line impedance model for mesoscopic oxide electrodes functionalized with a conductive coating. J. Chem. Phys. B 110, 11284–11290 (2006).Bisquert, J. Theory of the impedance of electron diffusion and recombination in a thin layer. J. Chem. Phys. B 106, 325–333 (2002).Fabregat-Santiago, F. et al. Electron transport and recombination in solid-state dye solar cell with Spiro-OMeTAD as hole conductor. J. Am. Chem. Soc. 131, 558–562 (2009).Dualeh, A., Moehl, T., Nazeeruddin, M. K. & Grätzel, M. Temperature dependence of transport properties of Spiro-MeOTAD as a hole transport material in solid-state dye-sensitized solar cells. ACS Nano 7, 2292–2301 (2013).Fabregat-Santiago, F., Garcia-Belmonte, G., Bisquert, J., Bogdanoff, P. & Zaban, A. Mott-Schottky analysis of nanoporous semiconductor electrodes in the dielectric state deposited on SnO2(F) conducting substrates. J Electrochem. Soc. 150, E293–E298 (2002).Kim, M.-J. et al. Unusual enhancement of photocurrent by incorporation of bronsted base thiourea into electrolyte of dye-sensitized solar cell. J. Chem. Phys. C 114, 19849–19852 (2010).Ito, S. et al. Photovoltaic characterization of dye-sensitized solar cells: effect of device masking on conversion efficiency. Prog. Photovolt: Res. Appl. 14, 589–601 (2006)

The violent youth of bright and massive cluster galaxies and their maturation over 7 billion years

In this study, we investigate the formation and evolution mechanisms of the brightest cluster galaxies (BCGs) over cosmic time. At high redshift (z ∼ 0.9), we selected BCGs and most massive cluster galaxies (MMCGs) from the Cl1604 supercluster and compared them to low-redshift (z ∼ 0.1) counterparts drawn from the MCXC meta-catalogue, supplemented by Sloan Digital Sky Survey imaging and spectroscopy. We observed striking differences in the morphological, colour, spectral, and stellar mass properties of the BCGs/MMCGs in the two samples. High-redshift BCGs/MMCGs were, in many cases, star-forming, late-type galaxies, with blue broad-band colours, properties largely absent amongst the low-redshift BCGs/MMCGs. The stellar mass of BCGs was found to increase by an average factor of 2.51 ± 0.71 from z ∼ 0.9 to z ∼ 0.1. Through this and other comparisons, we conclude that a combination of major merging (mainly wet or mixed) and in situ star formation are the main mechanisms which build stellar mass in BCGs/MMCGs. The stellar mass growth of the BCGs/MMCGs also appears to grow in lockstep with both the stellar baryonic and total mass of the cluster. Additionally, BCGs/MMCGs were found to grow in size, on average, a factor of ∼3, while their average Sérsic index increased by ∼0.45 from z ∼ 0.9 to z ∼ 0.1, also supporting a scenario involving major merging, though some adiabatic expansion is required. These observational results are compared to both models and simulations to further explore the implications on processes which shape and evolve BCGs/MMCGs over the past ∼7 Gyr

Properties of Electrodeposited CuSCN 2D Layers and Nanowires Influenced by Their Mixed Domain Structure

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Influence of cysteine adsorption on the performance of CdSe quantum dots sensitized solar cells

The surface of mesoporous TiO2 electrodes has been functionalized with cysteine using different solutions for the subsequent attachment of colloidal CdSe QDs solved in toluene. The adsorption behavior of both cysteine and CdSe QDs on the TiO2 nanoparticles has been investigated by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) Spectroscopy. The photoelectrochemical properties of the CdSe QDs sensitized TiO2 electrode have been characterized by using UV-vis spectroscopy, photocurrent-potential measurement, open-circuit voltage decay, and cyclic voltammetry. It is found that the amount of adsorbed cysteine becomes excessive when highly concentrated aqueous solutions were adopted instead of toluene solutions, leading to a blocking for the diffusion and adsorption of the CdSe QDs. This is translated into a decrease of light harvesting and short circuit current (Jsc) in the solar cell devices. On the other hand, it is suggested that the large amount of adsorbed cysteine on the surface of the TiO2 electrode could reduce the electron recombination at the interface between the TiO2 electrode and the electrolyte. As a consequence, the charge transfer resistance and fill factor became larger, and the electron lifetime became longer. © 2010 Elsevier B.V. All rights reserved

Effect of buffer layer on minority carrier lifetime and series resistance of bifacial heterojunction silicon solar cells analyzed by impedance spectroscopy

By combining information on solar cell layer structure and electrical response analyzed by impedance spectroscopy, relevant knowledge is obtained about photogenerated carriers recombination and extraction. The inclusion of a-Si:H buffer layers on the response of bifacial heterojunction silicon solar cells prepared by hot-wire chemical vapor deposition is studied. Impedance analysis indicates that the effect of the buffer layer is twofold: (a) effective minority carrier lifetime is improved by one order of magnitude, confirmed by alternative quasi-steady-state photoconductance, and (b) whole series resistance is increased. Both effects seem to compensate each other so as to get similar efficiency and fill factor.Peer ReviewedPostprint (published version