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Laura M. Herz - One of the best experts on this subject based on the ideXlab platform.
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Charge Carrier mobilities in metal halide perovskites fundamental mechanisms and limits
ACS energy letters, 2017Co-Authors: Laura M. HerzAbstract:Perovskite photovoltaic cells have seen a remarkable rise in power conversion efficiencies over a period of only a few years. Much of this performance is underpinned by the favorable Charge-Carrier mobilities in metal halide perovskites (MHPs), which are remarkably high for materials with such facile and versatile processing routes. This Perspective outlines the mechanisms that set a fundamental upper limit to Charge-Carrier mobility values in MHPs and reveals how they may be tuned through changes in stoichiometry. In addition, extrinsic effects such as grain size, energetic disorder, and self-doping are discussed for specific MHPs in the context of remedies designed to avoid them.
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Charge Carrier dynamics in organic inorganic metal halide perovskites
Annual Review of Physical Chemistry, 2016Co-Authors: Laura M. HerzAbstract:Hybrid organic-inorganic metal halide perovskites have recently emerged as exciting new light-harvesting and Charge-transporting materials for efficient photovoltaic devices. Yet knowledge of the nature of the photogenerated excitations and their subsequent dynamics is only just emerging. This article reviews the current state of the field, focusing first on a description of the crystal and electronic band structure that give rise to the strong optical transitions that enable light harvesting. An overview is presented of the numerous experimental approaches toward determining values for exciton binding energies, which appear to be small (a few milli-electron volts to a few tens of milli-electron volts) and depend significantly on temperature because of associated changes in the dielectric function. Experimental evidence for Charge-Carrier relaxation dynamics within the first few picoseconds after excitation is discussed in terms of thermalization, cooling, and many-body effects. Charge-Carrier recombination mechanisms are reviewed, encompassing trap-assisted nonradiative recombination that is highly specific to processing conditions, radiative bimolecular (electron-hole) recombination, and nonradiative many-body (Auger) mechanisms.
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hybrid perovskites for photovoltaics Charge Carrier recombination diffusion and radiative efficiencies
Accounts of Chemical Research, 2016Co-Authors: Michael B. Johnston, Laura M. HerzAbstract:Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers. Further optimization of such hybrid perovskite devices now needs to be supported by a better understanding of how light is converted into electrical currents and vice versa. This Account provides an overview of Charge-Carrier recombination and mobility mechanisms encountered in such materials. Optical-pump-terahertz-probe (OPTP) photoconductivity spectroscopy is an ideal tool here, because it allows the dynamics of mobile Charge Carriers inside the perovskite to be monitored following excitation with a short laser pulse whose photon energy falls into the range of the solar spectrum. We first review our insights gained from transient OPTP and photoluminescence spectroscopy on the mechanisms dominating Charge-Carrier recombination in these materials. We discuss that mono-molecular Charge-recombination predominantly originates from trapping of Charges, with trap depths being relatively shallow (tens of millielectronvolts) for hybrid lead iodide perovskites. Bimolecular recombination arises from direct band-to-band electron-hole recombination and is found to be in significant violation of the simple Langevin model. Auger recombination exhibits links with electronic band structure, in accordance with its requirement for energy and momentum conservation for all Charges involved. We further discuss Charge-Carrier mobility values extracted from OPTP measurements and their dependence on perovskite composition and morphology. The significance of the reviewed Charge-Carrier recombination and mobility parameters is subsequently evaluated in terms of the Charge-Carrier diffusion lengths and radiative efficiencies that may be obtained for such hybrid perovskites. We particularly focus on calculating such quantities in the limit of ultra-low trap-related recombination, which has not yet been demonstrated but could be reached through further advances in material processing. We find that for thin films of hybrid lead iodide perovskites with typical Charge-Carrier mobilities of ∼30cm(2)/(V s), Charge-Carrier diffusion lengths at solar (AM1.5) irradiation are unlikely to exceed ∼10 μm even if all trap-related recombination is eliminated. We further examine the radiative efficiency for hybrid lead halide perovskite films and show that if high efficiencies are to be obtained for intermediate Charge-Carrier densities (n ≈ 10(14) cm(-3)) trap-related recombination lifetimes will have to be enhanced well into the microsecond range.
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Charge Carrier dynamics and mobilities in formamidinium lead mixed halide perovskites
Advanced Materials, 2015Co-Authors: Waqaas Rehman, Christian Wehrenfennig, Giles E. Eperon, Michael B. Johnston, Henry J. Snaith, Rebecca L Milot, Jessica L Boland, Laura M. HerzAbstract:The mixed-halide perovskite FAPb(Bry I1-y )3 is attractive for color-tunable and tandem solar cells. Bimolecular and Auger Charge-Carrier recombination rate constants strongly correlate with the Br content, y, suggesting a link with electronic structure. FAPbBr3 and FAPbI3 exhibit Charge-Carrier mobilities of 14 and 27 cm(2) V(-1) s(-1) and diffusion lengths exceeding 1 μm, while mobilities across the mixed Br/I system depend on crystalline phase disorder.
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temperature dependent Charge Carrier dynamics in ch3nh3pbi3 perovskite thin films
Advanced Functional Materials, 2015Co-Authors: Rebecca L Milot, Giles E. Eperon, Michael B. Johnston, Henry J. Snaith, Laura M. HerzAbstract:The photoluminescence, transmittance, Charge-Carrier recombination dynamics, mobility, and diffusion length of CH3NH3PbI3 are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that Charge localization effects are absent in this range. The monomolecular Charge-Carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies Ea associated with ionization are found to increase markedly from the room-temperature tetragonal (Ea ≈ 20 meV) to the higher-temperature cubic (Ea ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as Charge-Carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The Charge-Carrier diffusion length gradually decreases with rising temperature from about 3 μm at -93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions. The photoconductivity in CH3NH3PbI3 thin films is investigated from 8 to 370 K across three structural phases. Analysis of the Charge-Carrier recombination dynamics reveals a variety of starkly differing recombination mechanisms. Evidence of Charge-Carrier localization is observed only at low temperature. High Charge mobility and diffusion length are maintained at high temperature beyond the tetragonal-to-cubic phase transition at ≈310 K.
James R Durrant - One of the best experts on this subject based on the ideXlab platform.
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Charge Carrier trapping recombination and transfer in hematite α fe2o3 water splitting photoanodes
Chemical Science, 2013Co-Authors: Monica Barroso, Stephanie R Pendlebury, Alexander J Cowan, James R DurrantAbstract:Hematite is currently considered one of the most promising materials for the conversion and storage of solar energy via the photoelectrolysis of water. Whilst there has been extensive research and much progress in the development of hematite structures with enhanced photoelectrochemical (PEC) activity, relatively limited information has been available until recently concerning the dynamics of photogenerated Charge Carriers in hematite and their impact upon the efficiency of water photoelectrolysis. In this perspective we present an overview of our recent studies of the dynamics of photoinduced Charge Carrier processes in hematite, derived primarily from transient absorption spectroscopy of nanostructured photoanodes. The relationship between PEC activity and transient measurements are discussed in terms of a phenomenological model which rationalizes the observations and in particular the impact of external potential bias on the relative rates of Charge Carrier trapping, recombination and interfacial transfer in hematite photoanodes for water oxidation.
Vladimir Dyakonov - One of the best experts on this subject based on the ideXlab platform.
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improved Charge Carrier lifetime in planar perovskite solar cells by bromine doping
Scientific Reports, 2016Co-Authors: David Kiermasch, Philipp Rieder, Kristofer Tvingstedt, Andreas Baumann, Vladimir DyakonovAbstract:The Charge Carrier lifetime is an important parameter in solar cells as it defines, together with the mobility, the diffusion length of the Charge Carriers, thus directly determining the optimal active layer thickness of a device. Herein, we report on Charge Carrier lifetime values in bromine doped planar methylammonium lead iodide (MAPbI3) solar cells determined by transient photovoltage. The corresponding Charge Carrier density has been derived from Charge Carrier extraction. We found increased lifetime values in solar cells incorporating bromine compared to pure MAPbI3 by a factor of ~2.75 at an illumination intensity corresponding to 1 sun. In the bromine containing solar cells we additionally observe an anomalously high value of extracted Charge, which we deduce to originate from mobile ions.
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oxygen doping of p3ht pcbm blends influence on trap states Charge Carrier mobility and solar cell performance
Organic Electronics, 2010Co-Authors: Julia Schafferhans, Andreas Baumann, Carsten Deibel, Alexander Wagenpfahl, Vladimir DyakonovAbstract:Abstract We investigated the influence of oxygen on the performance of P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) solar cells by current–voltage, thermally stimulated current (TSC) and Charge extraction by linearly increasing voltage (CELIV) measurement techniques. The exposure to oxygen leads to an enhanced Charge Carrier concentration and a decreased Charge Carrier mobility. Further, an enhanced formation of deeper traps was observed, although the overall density of traps was found to be unaffected upon oxygen exposure. With the aid of macroscopic simulations, based on solving the differential equation system of Poisson, continuity and drift-diffusion equations in one dimension, we demonstrate the influence of a reduced Charge Carrier mobility and an increased Charge Carrier density on the main solar cell parameters, consistent with experimental findings.
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Charge Carrier concentration and temperature dependent recombination in polymer fullerene solar cells
Applied Physics Letters, 2009Co-Authors: Alexander Foertig, Andreas Baumann, Vladimir Dyakonov, Daniel Rauh, Carsten DeibelAbstract:We performed temperature dependent transient photovoltage and photocurrent measurements on poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester bulk heterojuction solar cells. We found a strongly Charge Carrier concentration and temperature dependent Langevin recombination prefactor. The observed recombination mechanism is discussed in terms of bimolecular recombination. The experimental results were compared with Charge Carrier extraction by linearly increasing voltage measurements done on the same blend system. We explain the Charge Carrier dynamics, following an apparent order larger than two, by dynamic trapping of Charges in the tail states of the Gaussian density of states.
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influence of Charge Carrier mobility on the performance of organic solar cells
Physica Status Solidi-rapid Research Letters, 2008Co-Authors: Carsten Deibel, Alexander Wagenpfahl, Vladimir DyakonovAbstract:The power conversion efficiency of organic solar cells based on donor–acceptor blends is governed by an interplay of polaron pair dissociation and bimolecular polaron recombination. Both processes are strongly dependent on the Charge Carrier mobility, the dissociation increasing with faster Charge transport, with raised recombination losses at the same time. Using a macroscopic effective medium simulation, we calculate the optimum Charge Carrier mobility for the highest power conversion efficiency, for the first time accounting for injection barriers and a reduced Langevin-type recombination. An enhancement of the Charge Carrier mobility from 10–8 m2/V s for state of the art polymer–fullerene solar cells to about 10–6 m2/V s, which yields the maximum efficiency, corresponds to an improvement of only about 20% for the given parameter set. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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influence of Charge Carrier mobility on the performance of organic solar cells
arXiv: Materials Science, 2008Co-Authors: Carsten Deibel, Alexander Wagenpfahl, Vladimir DyakonovAbstract:The power conversion efficiency of organic solar cells based on donor--acceptor blends is governed by an interplay of polaron pair dissociation and bimolecular polaron recombination. Both processes are strongly dependent on the Charge Carrier mobility, the dissociation increasing with faster Charge transport, with raised recombination losses at the same time. Using a macroscopic effective medium simulation, we calculate the optimum Charge Carrier mobility for the highest power conversion efficiency, for the first time accounting for injection barriers and a reduced Langevin-type recombination. An enhancement of the Charge Carrier mobility from $10^{-8}$m$^2$/Vs for state of the art polymer:fullerene solar cells to about $10^{-6}$m$^2$/Vs, which yields the maximum efficiency, corresponds to an improvement of only about 20% for the given parameter set.
Jia Hong Pan - One of the best experts on this subject based on the ideXlab platform.
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Charge Carrier trapping, recombination and transfer during TiO2 photocatalysis: An overview
Catalysis Today, 2019Co-Authors: Ruifeng Qian, Jenny Schneider, Detlef W. Bahnemann, Huixin Zong, Guanda Zhou, Ting Zhao, Jing Yang, Jia Hong PanAbstract:Abstract Heterogeneous photocatalysis mediated by semiconducting TiO2 has attracted continuous interest during the past decades and has shown great potentials in environmental remediation and solar energy conversion. Basically, photocatalysis is initiated by the TiO2 excitation. The generated Charge Carriers undergo trapping, recombination, and interfacial transfer before proceeding the redox reaction at TiO2 surface. Monitoring the Charge Carrier dynamics is of particulate importance for understanding the underlying mechanism and designing efficient photocatalysts. This review overviews the recent progress in characterization of Charge Carrier dynamics. We will present the analytic techniques for monitoring the fate of Charge Carriers at each elementary photocatalytic step, including Charge Carrier generation, trapping and recombination inside the photocatalyst, as well as the interfacial Charge transfer. The Charge Carrier dynamics at TiO2/H2O interface, hole transfer reactions for O2 production, and photocatalytic oxidation of organic compounds and nitric oxides, and electron transfer reactions for photocatalytic reduction of viologens and metal ions are addressed, aiming at a deeper understanding of photocatalytic process.
Detlef W. Bahnemann - One of the best experts on this subject based on the ideXlab platform.
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Charge Carrier trapping, recombination and transfer during TiO2 photocatalysis: An overview
Catalysis Today, 2019Co-Authors: Ruifeng Qian, Jenny Schneider, Detlef W. Bahnemann, Huixin Zong, Guanda Zhou, Ting Zhao, Jing Yang, Jia Hong PanAbstract:Abstract Heterogeneous photocatalysis mediated by semiconducting TiO2 has attracted continuous interest during the past decades and has shown great potentials in environmental remediation and solar energy conversion. Basically, photocatalysis is initiated by the TiO2 excitation. The generated Charge Carriers undergo trapping, recombination, and interfacial transfer before proceeding the redox reaction at TiO2 surface. Monitoring the Charge Carrier dynamics is of particulate importance for understanding the underlying mechanism and designing efficient photocatalysts. This review overviews the recent progress in characterization of Charge Carrier dynamics. We will present the analytic techniques for monitoring the fate of Charge Carriers at each elementary photocatalytic step, including Charge Carrier generation, trapping and recombination inside the photocatalyst, as well as the interfacial Charge transfer. The Charge Carrier dynamics at TiO2/H2O interface, hole transfer reactions for O2 production, and photocatalytic oxidation of organic compounds and nitric oxides, and electron transfer reactions for photocatalytic reduction of viologens and metal ions are addressed, aiming at a deeper understanding of photocatalytic process.
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Fractal Charge Carrier Kinetics in TiO2
The Journal of Physical Chemistry C, 2017Co-Authors: Fabian Sieland, Jenny Schneider, Detlef W. BahnemannAbstract:Charge Carrier recombination kinetics of TiO2 powder samples were analyzed in the time domain ranging from 50 ns to 1 ms. The transient reflectance signals of the Charge Carriers observed by laser flash photolysis spectroscopy do not fit to simple second order kinetics as expected for the recombination of trapped electrons and holes. The deviation from second order reaction dynamics could rather be explained by the segregation of Charge Carriers and the fractal dimension of the semiconductor agglomerates. According to the fractal reaction kinetics, the time dependent rate coefficient kf (kf = k2,ft–h) has been employed instead of the second order rate constant k2, where the fractal parameter h describes the dimension of the system. This model could successfully be used to describe Charge Carrier signals in all observed time domains. Moreover, the model was compared with the concept developed by Shuttle et al., which proposes that the Charge Carrier signals decay following a power-law. The benefits of the ...