103 results found
Calado P, Barnes PRF, 2019, Is it possible for a perovskite p-n homojunction to persist in the presence of mobile ionic charge?, Publisher: arXiv
Recently Cui et al. reported on the fabrication a p-n homojunction perovskitesolar cell (PSC) using stoichiometric control of sequentially-depositedperovskite layers. The authors propose that the junction leads to an enhancedelectric field in the perovskite absorber resulting in improved chargeseparation. In this response to Cui et al. 2019 we show that the experimentaldata presented in the paper does not directly support this claim. Furthermore,Cui et al.'s thesis is not compatible with the large body of existingliterature showing that mobile ionic defects present in methyl-ammonium leadiodide (MAPI) and its derivatives are highly mobile at room temperature. Usingdrift diffusion device simulations we show that large densities of mobile ioniccharge in the system are likely to the screen any beneficial effects of a p-nhomojunction.
Moia D, Gelmetti I, Calado P, et al., 2019, Ionic-to-electronic current amplification in hybrid perovskite solar cells: ionically gated transistor-interface circuit model explains hysteresis and impedance of mixed conducting devices, Energy and Environmental Science, Vol: 12, Pages: 1296-1308, ISSN: 1754-5692
Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings suggest a strategy to design thin film electronic components with large, tuneable, capacitor-like and inductor-like characteristics. The resulting simple equivalent circuit model, which we verified with time-dependent drift-diffusion simulations of measured impedance spectra, allows a general description and interpretation of perovskite solar cell behaviour.
Calado P, Burkitt D, Yao J, et al., 2019, Identifying dominant recombination mechanisms in perovskite solar cells by measuring the transient ideality factor, Physical Review Applied, Vol: 11, ISSN: 2331-7019
The light ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used to identify the dominant recombination mechanism in solar cells. Applying this ‘Suns-VOC’ technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), bias light intensity, and device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified from the signature of the transient ideality factor following application of a forward bias, Vpre, to the device in the dark. The transient ideality factor, is measured by monitoring the evolution of VOC as a function of time at different light intensities. The initial values of ideality found using this technique were consistent with estimates of ideality factor obtained from measurements of photoluminescence vs light intensity and electroluminescence vs current density. Time-dependent simulations of the measurement on modelled devices, which include the effects of mobile ionic charge, reveal that this initial value can be correlated to an existing zero-dimensional model while steady-state values must be analysed taking into account the homogeneity of carrier populations throughout the absorber layer. The analysis shows that Shockley Read Hall (SRH) recombination through deep traps at the charge collection interfaces is dominant in both architectures of measured device. Using transient photovoltage measurements directly following illumination on bifacial devices we further show that the perovskite/electron transport layer interface extends throughout the mesoporous TiO2 layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This method will be useful for identifying performance bottlenecks in new variants of perovskite and
Moia D, Giovannitti A, Szumska AA, et al., Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes, Energy & Environmental Science, ISSN: 1754-5692
<p>Solution processable p-type and n-type conjugated polymers with polar side chains enable fast charging in aqueous electrolytes and 1.4 V cell voltage.</p>
Hu Y, Spies LM, Alonso-Alvarez D, et al., 2018, Identifying and controlling phase purity in 2D hybrid perovskite thin films, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 6, Pages: 22215-22225, ISSN: 2050-7488
Shital S, Barnes PRF, Dutta V, 2018, Analysis of four-flux parameters of TiO2 films commonly used in DSSCs, SOLAR ENERGY, Vol: 173, Pages: 530-538, ISSN: 0038-092X
Du T, Kim J, Ngiam J, et al., 2018, Elucidating the origins of subgap tail states and open-circuit voltage in methylammonium lead triiodide perovskite solar cells, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X
Recombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, the impact of active layer crystallinity on the accumulated charge and open‐circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI) is demonstrated. It is shown that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high‐performance devices.
Du T, Kim J, Ngiam J, et al., 2018, Elucidating the origins of sub-gap tail states and open-circuit voltage in methylammonium lead triiodide perovskite solar cells, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X
Recombination via sub-gap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, we demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI). We show MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV. Using in-situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap states correlates with faster carrier trapping and more non-radiative recombination pathways. We provide fundamental insights into the origin of Voc in perovskite photovoltaics and demonstrate why highly crystalline perovskite films are paramount for high-performance devices.
Giovannitti A, Maria I, Hanifi D, et al., 2018, The role of the side chain on the performance of n-type conjugated polymers in aqueous electrolytes, Chemistry of Materials, Vol: 30, Pages: 2945-2953, ISSN: 0897-4756
We report a design strategy that allows the preparation of solution processable n type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol based side chain. A series of random copolymers are prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90 and 100 with respect to the alkyl side chains. These are characterized in order to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions and performance in OECTs when operated in aqueous electrolytes. We observe that glycol side chain percentages of >50 % are required to achieve volumetric charging while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.
Calado P, Barnes PRF, Azzouzi M, et al., 2018, Driftfusion
An open source drift diffusion code based in MATLAB for simulating solar cells.
Shital S, Swami SK, Barnes P, et al., 2018, Monte Carlo simulation for optimization of a simple and efficient bifacial DSSC with a scattering layer in the middle, SOLAR ENERGY, Vol: 161, Pages: 64-73, ISSN: 0038-092X
Jong U-G, Yu C-J, Ri G-C, et al., 2018, Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites, Publisher: ROYAL SOC CHEMISTRY
Moss B, Lim KK, Beltram A, et al., 2017, Comparing photoelectrochemical water oxidation, recombination kinetics and charge trapping in the three polymorphs of TiO2, Scientific Reports, Vol: 7, ISSN: 2045-2322
In this article we present the first comparative study of the transient decay dynamics of photo-generated charges for the three polymorphs of TiO2. To our knowledge, this is the first such study of the brookite phase of TiO2 over timescales relevant to the kinetics of water splitting. We find that the behavior of brookite, both in the dynamics of relaxation of photo-generated charges and in energetic distribution, is similar to the anatase phase of TiO2. Moreover, links between the rate of recombination of charge carriers, their energetic distribution and the mode of transport are made in light of our findings and used to account for the differences in water splitting efficiency observed across the three polymorphs.
Calado P, Telford AM, Bryant D, et al., 2016, Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis, Nature Communications, Vol: 7, ISSN: 2041-1723
Ionic migration has been proposed as a possible cause of photovoltaic current-voltage hysteresis in hybrid perovskite solar cells. A major objection to this hypothesis is that hysteresis can be reduced by changing the interfacial contact materials; this is unlikely to significantly influence the behaviour of mobile ionic charge within the perovskite phase. Here we show that the primary effects of ionic migration can in fact be observed regardless of whether the contacts were changed to give devices with or without significant hysteresis. Transient optoelectronic measurements combined with device simulations indicate that electric-field screening, consistent with ionic migration, is similar in both high and low hysteresis CH3NH3PbI3 cells. Simulation of the photovoltage and photocurrent transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the perovskite-contact interfaces. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias which screen the ionic charge, reducing hysteresis.
Belisle RA, Nguyen WH, Bowring AR, et al., 2016, Interpretation of inverted photocurrent transients in organic lead halide perovskite solar cells; proof of the field screening by mobile ions and determination of the space charge layer widths, Energy & Environmental Science, Vol: 10, Pages: 192-204, ISSN: 1754-5706
In Methyl Ammonium Lead Iodide (MAPI) perovskite solar cells, screening of the built in field by mobile ions has been proposed as part of the cause of the large hysteresis observed in the current/voltage scans in many cells. We show that photocurrent transients measured immediately (e.g. 100 μs) after a voltage step can provide direct evidence that this field screening exists. Just after a step to forward bias, the photocurrent transients are reversed in sign (i.e. inverted), and the magnitude of the inverted transients can be used to find an upper bound on the width of the space charge layers adjacent to the electrodes. This in turn provides a lower bound on the mobile charge concentration, which we find to be 1 x 1017/cm3. Using a new photocurrent transient experiment, we show that the space charge layer thickness remains approximately constant as a function of bias, as expected for mobile ions in a solid electrolyte. We also discuss additional characteristics of the inverted photocurrent transients that imply either an unusually stable deep trapping, or a photo effect on the mobile ion conductivity.
Barnes PRF, Vaissier V, Garcia Sakai V, et al., 2016, How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study, Scientific Reports, Vol: 6, ISSN: 2045-2322
Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results.
Petrus ML, Hu Y, Moia D, et al., 2016, The Influence of Water Vapor on the Stability and Processing of Hybrid Perovskite Solar Cells Made from Non-Stoichiometric Precursor Mixtures, Chemsuschem, Vol: 9, Pages: 2699-2707, ISSN: 1864-564X
We investigated the influence of moisture on methylammonium lead iodide perovskite (MAPbI3) films and solar cells derived from non-stoichiometric precursor mixtures. We followed both the structural changes under controlled air humidity through in situ X-ray diffraction, and the electronic behavior of devices prepared from these films. A small PbI2 excess in the films improved the stability of the perovskite compared to stoichiometric samples. We assign this to excess PbI2 layers at the perovskite grain boundaries or to the termination of the perovskite crystals with Pb and I. In contrast, the MAI-excess films composed of smaller perovskite crystals showed increased electronic disorder and reduced device performance owing to poor charge collection. Upon exposure to moisture followed by dehydration (so-called solvent annealing), these films recrystallized to form larger, highly oriented crystals with fewer electronic defects and a remarkable improvement in photocurrent and photovoltaic efficiency.
Moia D, Szumska A, Vaissier V, et al., 2016, Interdye Hole Transport Accelerates Recombination in Dye Sensitized Mesoporous Films, Journal of the American Chemical Society, Vol: 138, Pages: 13197-13206, ISSN: 1520-5126
Charge recombination between oxidized dyes attached to mesoporous TiO2 and electrons in the TiO2 was studied in inert electrolytes using transient absorption spectroscopy. Simultaneously, hole transport within the dye monolayers was monitored by transient absorption anisotropy. The rate of recombination decreased when hole transport was inhibited selectively, either by decreasing the dye surface coverage or by changing the electrolyte environment. From Monte Carlo simulations of electron and hole diffusion in a particle, modeled as a cubic structure, we identify the conditions under which hole lifetime depends on the hole diffusion coefficient for the case of normal (disorder free) diffusion. From simulations of transient absorption and transient absorption anisotropy, we find that the rate and the dispersive character of hole transport in the dye monolayer observed spectroscopically can be explained by incomplete coverage and disorder in the monolayer. We show that dispersive transport in the dye monolayer combined with inhomogeneity in the TiO2 surface reactivity can contribute to the observed stretched electron-hole recombination dynamics and electron density dependence of hole lifetimes. Our experimental and computational analysis of lateral processes at interfaces can be applied to investigate and optimize charge transport and recombination in solar energy conversion devices using electrodes functionalized with molecular light absorbers and catalysts.
Flasque M, Van Nhien AN, Moia D, et al., 2016, Consequences of solid electrolyte interphase (SEI) formation upon aging on charge-transfer processes in dye-sensitized solar cells, Journal of Physical Chemistry C, Vol: 120, Pages: 18991-18998, ISSN: 1932-7455
Solid electrolyte interphase (SEI) layers form on sensitized-TiO2 photoanodes and platinum counter electrodes when dye-sensitized solar cells (DSSCs) are subjected to an accelerated aging protocol (e.g., heating at 85 °C in the dark for 500 h). To understand how this impacts device operation, we conducted an electrochemical impedance spectroscopy study and found that the SEI induces an additional electron-transfer process from the TiO2 to the electrolyte. This is materialized by the onset of a new charge-transfer semicircle at higher frequencies, predominantly visible under bias voltages similar to and greater than the open-circuit voltage. Our results emphasize the detrimental role of SEI formation on device performance and lifetime. Additionally, nanosecond transient absorption spectroscopy showed that SEI formation reduced the rate of oxidized dye regeneration. We also found that a proportion of the photogenerated holes on the dyes were transferred to the SEI itself. A prolonged aging duration led to the electrode’s mesoporosity network being entirely clogged by the SEI, thus impeding efficient transport of the electrolyte redox couple and being responsible for a further decline in photovoltaic performances.
Leguy AM, Goñi AR, Frost JM, et al., 2016, Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites, Physical Chemistry Chemical Physics, Vol: 18, Pages: 27051-27066, ISSN: 1463-9084
We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm(-1). Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.
Cappel UB, Moia D, Bruno A, et al., 2016, Evidence for photo-induced charge separation between dye molecules adsorbed to aluminium oxide surfaces, Scientific Reports, Vol: 6, ISSN: 2045-2322
Excited state dynamics and photo-induced charge transfer of dye molecules have been widely studied due to their relevance for organic and dye-sensitised solar cells. Herein, we present a femtosecond transient absorption spectroscopy study of the indolene dye D131 when adsorbed to inert Al2O3 substrates for different surface concentration of the dye. Surprisingly, we find that at high surface concentrations, the first singlet excited state of the dye is converted into a new state with an efficiency of about 80%. We assign the absorption features of this state to the oxidised dye and discuss the possibility of photo-induced charge separation between neighboring dye molecules. Our study is the first to show that this process can be highly efficient without the use of donor and acceptor molecules of different chemical structures.
Kafizas A, Wang X, Pendlebury SR, et al., 2016, Where Do Photogenerated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime, JOURNAL OF PHYSICAL CHEMISTRY A, Vol: 120, Pages: 715-723, ISSN: 1089-5639
Carnie MJ, Troughton J, Regan BO, et al., 2015, Identifying recombination mechanisms through materials development in perovskite solar cells
© 2015 IEEE. Through materials and device developments and by using measurements such as transient photovoltage decay and impedance spectroscopy we have begun to identify recombination mechanisms in perovskite solar cells. At the forefront of our developments is a transparent, indium free, cathode which allows measurements to be made whilst illuminating from both the photoanode side and the cathode side of the device. Recombination is consistently faster when illuminated from the cathode side and we conclude that in this case, as charge carriers are generated closer to the perovskite/SPIRO-OMeTAD interface, interfacial recombination is a significant contributor to voltage losses within the device.
Vaissier V, Frost JM, Barnes PRF, et al., 2015, Influence of Intermolecular Interactions on the Reorganization Energy of Charge Transfer between Surface-Attached Dye Molecules, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 24337-24341, ISSN: 1932-7447
The parameters controlling the kinetics ofintermolecular charge transfer are traditionally estimatedfrom electronic structure calculations on the charge donorand charge acceptor in isolation. Here, we show that thisprocedure results in inaccuracies for hole transfer between apair of organic dye molecules by comparing charge-constraineddensity functional theory (DFT) calculations on a dyecation/neutral dye pair to the conventional DFT calculationson the isolated molecules. We quantify the error made in thereorganization energy of hole exchange between dye molecules(λi). We choose three indolene-based organic dyes with application to dye-sensitized solar cells, namely, D149, D102, and D131,for which experimental values of λ are available. We find that, although highly system dependent, the intermolecular interactionbetween the charge donor and acceptor can lead to a 0.25 eV change in λi, illustrating the limitations of the widely used originalmethod in predicting the rate of charge transfer.
Moia D, Leijtens T, Noel N, et al., 2015, Dye Monolayers Used as the Hole Transporting Medium in Dye-Sensitized Solar Cells, ADVANCED MATERIALS, Vol: 27, Pages: 5889-5894, ISSN: 0935-9648
Brivio F, Frost JM, Skelton JM, et al., 2015, Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide, PHYSICAL REVIEW B, Vol: 92, ISSN: 1098-0121
Leguy AMA, Azarhoosh P, Alonso MI, et al., 2015, Experimental and theoretical optical properties of methylammonium lead halide perovskites, Nanoscale, Vol: 8, Pages: 6317-6327, ISSN: 2040-3372
Moia D, Cappel UB, Leijtens T, et al., 2015, The Role of Hole Transport between Dyes in Solid-State Dye-Sensitized Solar Cells, Journal of Physical Chemistry C, Vol: 119, Pages: 18975-18985, ISSN: 1932-7447
Bryant D, Wheeler S, ORegan BC, et al., 2015, Observable Hysteresis at Low Temperature in “Hysteresis Free” Organic–Inorganic Lead Halide Perovskite Solar Cells, The Journal of Physical Chemistry Letters, Vol: 6, Pages: 3190-3194, ISSN: 1948-7185
Eames C, Frost JM, Barnes PRF, et al., 2015, Ionic transport in hybrid lead iodide perovskite solar cells, NATURE COMMUNICATIONS, Vol: 6, ISSN: 2041-1723
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