Mastering the complexity of mixed ionic–electronic conduction in hybrid perovskite solar cells is a most critical challenge in the quest for further developing and, eventually, commercializing this ...technology. In this Perspective, we refer to the literature invoking ion transport in hybrid perovskite devices when interpreting their long time scale behavior. We present an overview on the defect chemistry of methylammonium lead iodide (MAPbI3), and we extend the discussion to some of the questions about composition that are currently being addressed in the field. We further consider the entangled relation between ionic and electronic charge carriers in mixed conducting solar cells at equilibrium as well as out-of-equilibrium conditions. We review and suggest research directions that can bridge defect chemistry with device physics of hybrid perovskite solar cells and enrich the know-how concerning this exciting intersection.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
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.
Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of ...reactive side‐products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox‐active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side‐reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high‐performance, state‐of‐the‐art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H2O2), a reactive side‐product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox‐active organic semiconductors based on donor–acceptor copolymers that prevents the formation of H2O2 during device operation. This study elucidates the previously overlooked side‐reactions between redox‐active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte‐gated devices in application‐relevant environments.
Faradaic side‐reactions of redox‐active materials, that can produce harmful side‐products, should be minimized when employed in bioelectronic devices for studying biological systems. This work sheds light on side‐reactions with oxygen in state‑of‑the‑art materials for electrochemical transistors, forming hydrogen peroxide (H2O2), and provides design rules toward high‐performance materials that prevent adverse reactions by tailoring the energy levels of the redox‐active material.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Interfacial space charges significantly influence transport and recombination of charge carriers in optoelectronic devices. Due to the mixed ionic‐electronic conducting properties of halide ...perovskites, not only electronic effects, but also ionic interactions at their interfaces need to be considered in the analysis of space charges. Understanding of these interactions and their control is currently missing. This study elucidates the ionic effects on space charge formation at the interface between methylammonium lead iodide (MAPI) and alumina, and its modulation through surface modification using organic molecules. Embedding insulating alumina nanoparticles within MAPI films leads to enhancement of the electronic conductivity. This effect is consistent with the formation of an interfacial inversion layer in MAPI and can only be explained on the basis of ionic interactions. Such an effect is attenuated by surface modification of the oxide via the chemisorption of organic molecules. Finally, the same trend is observed in solar cells, where reducing the potential of the distributed space charges within the composite active layer improves device performance. These findings emphasize the necessity of taking into account ionic interactions to control the space charge formation at interfaces involving mixed ionic‐electronic conductors, an essential aspect in the performance optimization of halide perovskite‐based devices.
The space charge equilibrium at MAPI/alumina interfaces is dictated by ionic equilibration. Ionic space charges are modulated via surface‐adsorbed organic molecules. These hinder ionic adsorption from MAPI to the contact phase, reducing the interfacial space charge potential and the concentration of accumulated electrons. Suppressing the formation of distributed space charges in the active layer of solar cells improves device performance.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Electron transfer kinetics between donor and acceptor molecules in electrolytes has been described by Marcus theory using reorganization energy (λ), electronic coupling (H), and free energy ...difference (ΔG°). In solution, the molecules can collide freely, while collision occurs only at the exposed area of the molecules when the donors or the acceptors are anchored onto an electrode, altering the values of λ and H. To date, these structural effects of electrode-bound molecules have not been considered in detail. To study geometrical effects, we fabricate TiO2 electrodes with nine different donor-(π-bridge)-acceptor type molecules and measure the kinetics of electron transfer from five different Co complexes in electrolytes. For densely adsorbed electrodes, the molecules with larger donor moieties have faster reduction kinetics and the kinetics are independent of the length of the π-bridge. When the amount of the adsorbed molecules is reduced, the kinetics become faster and the kinetics depend on the π-bridge length. These phenomena can be partially correlated to the increased exposed area of the molecules to the electrolyte. By fitting the data, we obtain lower λ values for lower dye-loading conditions, which is not expected if only the effect of solvent molecules is considered. Obtained H values with various geometries suggest that it is important not only to increase the exposed area but also to expose the point giving high H values to increase the kinetics. One example found is designing molecules with small molecular orbitals to increase H values, though this would also give large λ values.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for ...molecular sensitized semiconductors in electrolytes must account for the restricted geometry of these systems compared to “free” molecules in solution. Here, we explore the extent to which dielectric properties of the surrounding medium can explain the rates of charge transfer processes, measured using transient absorption spectroscopy, involving photo-oxidized thiophene–carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes. We observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. The activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which we attribute to energetic disorder in the molecular monolayer. The recombination rate in different solvents scales with the hole hopping rate. It can also be varied by adding inert salts in the electrolyte and by controlling the access of cations in solution to the oxide surface. Finally, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules in solvents with low polarity is verified, but the kinetics are limited by the ionic dissociation. Our study highlights the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
We measured the rate of hole hopping between dye molecules on titanium dioxide nanocrystals using cyclic voltammetry. Dyes commonly used in the field of dye sensitized solar cells exhibited efficient ...intermolecular charge transport, showing apparent diffusion coefficient values between 10 super(-8) up to over 10 super(-7) cm super(2) s super(-1) at room temperature. From temperature dependent measurements, we observed that hole transport across dye monolayers is a thermally activated process with Arrhenius activation energies between about 170 and 370 meV depending on the dye. Analysis of the data in terms of non-adiabatic Marcus theory of charge transfer enabled the estimation of the reorganization energy (740-1540 meV) and of an effective electronic coupling for the different systems. The measured reorganization energies show reasonable agreement with values obtained from density functional theory based calculations, validating our computational approach. Finally, we interpret the experimental and calculated data with reference to the chemical structure of the dyes and to the packing of the dyes on the surface of the TiO sub(2) and suggest that delocalization of the HOMO and rigidity of the conjugated molecular structure result respectively in lower outer and inner sphere reorganization energies.
Full text
Available for:
IJS, KILJ, NUK, UL, UM, UPUK
In dye-sensitized solar cells (DSSCs) photogenerated positive charges are normally considered to be carried away from the dyes by a separate phase of hole-transporting material (HTM). We show that ...there can also be significant transport within the dye monolayer itself before the hole reaches the HTM. We quantify the fraction of dye regeneration in solid-state DSSCs that can be attributed to this process. By using cyclic voltammetry and transient anisotropy spectroscopy, we demonstrate that the rate of interdye hole transport is prevented both on micrometer and nanometer length scales by reducing the dye loading on the TiO2 surface. The dye regeneration yield is quantified for films with high and low dye loadings (with and without hole percolation in the dye monolayer) infiltrated with varying levels of HTM. Interdye hole transport can account for >50% of the overall dye regeneration with low HTM pore filling. This is reduced to about 5% when the infiltration of the HTM in the pores is optimized in 2 μm thick films. Finally, we use hole transport in the dye monolayer to characterize the spatial distribution of the HTM phase in the pores of the dyed mesoporous TiO2.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Dye‐sensitized TiO2 can be used as the active layer of solar‐cell devices without an additional hole‐transporting material. In this architecture, holes are transported through the dye monolayer.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Measured hole diffusion coefficients in dye monolayers are larger than can be explained by a charge hopping model with a static distribution of parameters describing intermolecular hole transfer. We ...show that large amplitude fluctuations of the tethered dye configurations on the surface could explain the observed diffusion rates by enabling charges trapped in particular configurations to escape as the dye orientation changes. We present a multiscale model of hole transport that includes the effect of dynamic rearrangement of the monolayer of anchored dyes. Conformations of pairs of indolene dye molecules (both D102 and D149) were generated by a rigid molecular packing algorithm and Car–Parrinello molecular dynamics to mimic the conformational and configurational disorder of a dye monolayer adsorbed to an anatase (101) titanium dioxide surface. The electronic coupling (J ij ) for each pair of neighboring dyes was calculated to build distributions representing the disorder in a real system. These values were used as inputs to Marcus’ non-adiabatic equation for charge transfer to calculate the rate of hole hopping for each pair. Hole diffusion was simulated with a continuous time random walk, accounting for different time scales of molecular rearrangement (changes in the dye geometry). The dynamic nature of configurational disorder was captured by reassigning the values of J ij , drawn from the aforementioned distributions, after a fixed renewal time. We found hole diffusion coefficients of 3.3 × 10–8 and 9.2 × 10–8 cm2 s–1 for D102 and D149, respectively, for a renewal time of 10–7 s. This is in good agreement with the corresponding measured coefficients for D102 and D149 of 9.6 × 10–8 and 2.5 × 10–7 cm2 s–1, whereas the diffusion coefficients are underestimated by at least a factor of 15 if the dynamics are ignored. Fast rearrangement of dye monolayer configuration may explain the high lateral hole diffusion coefficients determined experimentally. Our results indicate that both chemical structure and the availability of different packing configurations must be considered when designing conductive molecular monolayers.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
PDF
