Search

The development of highly efficient lightweight flexible perovskite solar cells (PSCs) opens the way to high-throughput roll-to-roll manufacturing processes and new applications such as building integration and mobile products. Flexible PSCs are generally realized on small areas (< 0.2 cm(2)), far from technology commercialization where modules-scale is necessary. In this work, we demonstrate highly efficient n-i-p PSCs grown on flexible substrates by proper interface engineering for improved electron extraction. We compared spin coated PEIE and vacuum deposited LiF as interlayers between Al-doped ZnO, as transport conductive oxide (TCO), and thermally evaporated C-60, as electron transport layer (ETL). Once interlayers are applied, we observed a favorable band bending at TCO interface which results in enhanced charge extraction and lower recombination losses. We achieved flexible PSCs with stabilized efficiencies of 14.8%, both with PEIE and LiF interfacial modifications., In addition, we developed a flexible perovskite mini-module with stabilized efficiency of 10.5% onto an aperture area larger than 10 cm(2). The monolithic interconnections are entirely obtained by highly accurate and reliable laser scribing methods. A geometric fill factor as high as similar to 94% is achieved, with a dead area width of similar to 250 mu m.

Hybrid halide perovskite materials have become one of the leading candidates for various optoelectronic applications. They are based on organic-inorganic structures defined by the AMX(3) composition, were A is the central cation that can be either organic (e.g., methylammonium, fonnamidinium (FA)) or inorganic (e.g., Cs+), M is a divalent metal ion (e.g., Pb2+ or Sn2+), and X is a halide anion (I-, Br-,( )or Cl-). In particular, FAPbI(3) perovskites have shown remarkable optoelectronic properties and thermal stabilities. However, the photoactive alpha-FAPbI(3) (black) perovskite phase is not thermodynamically stable at ambient temperature and forms the delta-FAPbI(3) (yellow) phase that is not suitable for optoelectronic applications. This has stimulated intense research efforts to stabilize and realize the potential of the alpha-FAPbI(3) perovskite phase. In addition, hybrid perovskites were proven to be unstable against the external environmental conditions (air and moisture) and under device operating conditions (voltage and light), which is related to various degradation mechanisms. One of the strategies to overcome these instabilities has been based on low-dimensional hybrid perovskite materials, in particular layered two-dimensional (2D) perovskite phases composed of organic layers separating hybrid perovskite slabs, which were found to be more stable toward ambient conditions and ion migration. These materials are mostly based on S(x)A(n-1)Pb(n)X(3n+1) composition with various mono- (x = 1) or bifunctional (x = 2) organic spacer cations that template hybrid perovskite slabs and commonly form either Ruddlesden-Popper (RP) or Dion Jacobson (DJ) phases. These materials behave as natural quantum wells since charge carriers are confined to the inorganic slabs, featuring a gradual decrease in the band gap as the number of inorganic layers (n) increases from n = 1 (2D) to n = co (3D). While various layered 2D perovskites have been developed, their FA-based analogues remain under-represented to date. Over the past few years, several research advances enabled the realization of FA-based layered perovskites, which have also demonstrated a unique templating effect in stabilizing the alpha-FAPbI(3) phase. This, for instance, involved the archetypical n-butylammonium and 2-phenylethylammonium organic spacers as well as guanidinium, 5-ammonium valeric acid, iso-butylammonium, benzylammonium, n-pentylammonium, 2-thiophenemethylammo-nium, 2-(perfluorophenyl)ethylammonium, 1-adamantylmethanammonium, and 1,4-phenylenedimethanammonium. FAPbBr(3)-based layered perovskites have also demonstrated potential in various optoelectronic applications, yet the opportunities associated with FAPbI(3)-based perovskites have attracted particular attention in photovoltaics, stimulating further developments. This Account provides an overview of some of these recent developments, with a particular focus on FAPbI(3)-based layered perovskites and their utility in photovoltaics, while outlining challenges and opportunities for these hybrid materials in the future.

Solution-processed lead sulfide quantum dots (PbS QDs) are very attractive as NIR-active semiconductors for the fabrication of cost-efficient optoelectronic devices. To control the thin film carrier transport, as well as stability, surface passivation is of crucial importance. Here, we present the successful surface passivation of PbS QDs by the formamidinium lead iodide (FAPbI(3)) ligand. An effective procedure for the fabrication of FAPbI(3)-passivated PbS QDs through a binary-phase ligand exchange protocol in hexane and n-methylformamide is demonstrated. It is shown that this solution-processed ligand exchange drastically changes the photoluminescence intensity, exciton recombination dynamics, and carrier lifetime of the nanocrystals. The solution casting of the ligand-exchanged nanocrystals into thin films results in the periodic ordering of QDs in a square superlattice with close contacts. Planar graphene/QD photodetectors fabricated with PbS QDs passivated with FAPbI(3) show substantially increased thermal stability as compared to similar devices using PbS QDs passivated with commonly used methylammonium lead iodide.

We demonstrate the beneficial effect of a pre-annealing step prior to the boron diffusion on passivation and contact resistivity of industrially LPCVD deposited poly-Si/SiOX hole-selective contacts. We investigate the influence of the pre-annealing temperature on passivation quality, measured as implied open-circuit voltage and recombination current density, and on changes in crystallinity, characterized by Raman spectroscopy. A clear increase in passivation quality is observed on planar and textured surfaces as well as for various poly-Si thicknesses (100-230 nm) and thermal SiOX growth temperatures (600-800 degrees C). On planar surfaces and without the use of atomic hydrogenation, we report an increase in iVOC of around 5 mV with every additional increase of pre-annealing temperature by 50 degrees C (>900 degrees C) leading to an iVOC of 720 mV (J0 = 9.3 fA/cm2). After atomic hydrogenation, the effect of the pre-annealing is less pronounced. Nevertheless a gain in iVOC (reduction in J0) of 5-10 mV (2-5 fA/cm2) is achieved when comparing samples without pre-annealing with samples after a pre-annealing at 1050 degrees C. On textured surfaces on the other hand, this trend is more pronounced after atomic hydrogenation, for which a pre-crystallisation at 1050 degrees C leads to an iVOC (J0) of 705 mV (16.8 fA/cm2), which is a gain (reduction) of 24 mV (21.7 fA/cm2) compared to samples without a pre-annealing step.

Indoor photovoltaics are promising to enable self-powered electronic devices for the Internet of Things. Here, reported is a triple-anion CH3NH3PbI2-xBrClx perovskite film, of which the bandgap is specially designed for indoor light harvesting to achieve a record high efficiency of 36.2% with distinctive high open circuit voltage (V-oc) of 1.028 V under standard 1000 lux fluorescent light. The involvement of both bromide and chloride suppresses the trap-states and nonradiative recombination loss, exhibiting a remarkable ideality factor of 1.097. The introduction of chloride successfully restrains the halide segregation of iodide and bromide, stabilizing the triple-anion perovskite film. The devices show an excellent long-term performance, sustaining over 95% of original efficiency under continuous light soaking over 2000 h. These findings show the importance and potential of I/Br/Cl triple-anion perovskite with tailored bandgap and suppressed trap-states in stable and efficient indoor light recycling.

Citrazinic acid is used as a convenient starting material for both tridentate 2,6-di(pyrazol-1-yl)-pyridine and tetradentate 6,6'-di(pyrazol-1-yl)-2,2'-bipyridine ligands containing carboxylic groups useful for further anchoring of sensitizer on TiO2 for dye-sensitized solar cells (DSCs). Using 2,6-dichloro-4-carboxypyridine, the synthesis of the terdentate ligands was improved compared to previously used 2,6-dibromo-4-carboxypyridine or 2,6-dichloro-4-ethylcarboxylate pyridine. Controlling the reaction conditions, it is possible to efficiently obtain the monosubstituted 2-chloro-6-pyrazol-1-yl-4-carboxypyridine, a key intermediate for the preparation of tetradentate 6,6'-di(pyrazol-1-yl)-4,4'-dicarboxy-2,2'-bipyridine ligand. (C) 2010 Elsevier Ltd. All rights reserved.

We study dynamical processes at CdTe surfaces using ab initio metadynamics simulations. The c(2X2) to (2X1) transition of the Te-terminated (001) surface is found to involve a "c(2X2) + (2X1)" intermediate, consistent with experiment, and crossing of similar to 0.5 eV free-energy barriers at 400 K. Higher free-energy barriers (1.6-2.5 eV) are estimated for desorption of a Cd ion from Cd-terminated (001) and a CdTe unit from either Te-terminated (001) or (110) surfaces. Cd and Te exhibit a very different behavior. Concomitant to desorption, Te surface diffusion is observed as well as Te dimerization and bulk-surface Cd exchange events. (C) 2010 American Institute of Physics. [doi:10.1063/1.3499430]

Nanometer wide silicon filaments embedded in an amorphous silicon oxide matrix are grown at low temperatures over a large area. The optical and electrical properties of these mixed-phase nanomaterials can be tuned independently, allowing for advanced light management in high efficiency thin-film silicon solar cells and for band-gap tuning via quantum confinement in third-generation photovoltaics.

The commercialization of perovskite solar cells is mainly limited by their operational stability. Interlayer modification by thin interface materials between the perovskite and the charge transport layers is one of the most effective methods to promote the efficiency and stability of perovskite devices. However, the commonly used interlayer materials do not fulfill all the demands, including good film quality, excellent stability, and passivation capability without interfering with the charge transport. In this work, a water stable haloplumbate [TBA]PbI3 for interfacial modification that meets these demands is proposed, which is formed on the perovskite surface in situ by tetra-butylammonium iodide treatment. Benefiting from its passivation effect and robustness, the modified devices result in a power conversion efficiency of 22.90% with enhanced environmental and operational stability. In addition, the self-limiting effect of the reaction contributes to the controllability of device fabrication and the repeatability of device performance.

A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed-ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA(+)) and formamidinium (FA(+)) as a monovalent cation. We compare and quantify the methods of stabilizing FA(-) based perovskites involving the additional blending of the smaller inorganic cations cesium (Cs+) and rubidium (Rb+), which can lead to an improvement in phase purity of black cubic perovskite modification. Even under excess lead iodide conditions, the presence of a separate PbI2 phase as well as hexagonal phases, which are very common for formamidinium-containing perovskites, can be drastically reduced or even completely prevented. In this aspect, adding both Cs+ and Rb+ showed greater effectivity than only adding Cs+, enabling an increase in the percentage of the cubic phase within the material from 45% in the double-cation FA:MA mixture to 97.8% in the quadruple composition. The impact of admixing inorganic cations on the perovskite crystal structure resulted in enlarged homogeneous crystallite sizes and a less pronounced orientational order and indicated also minor modifications of unit cell sizes. Finally, we discuss the impact of the phase purity on charge-carrier recombination dynamics and solar cell performance.

Cubic phase stabilization is extremely important for the application of cesium lead halide inorganic perovskite materials into fabrication of perovskite solar cells. Methods on how to stabilize inorganic perovskite phase have been discussed in this review. The increase of surface energy of perovskite grains can stabilize the cubic perovskite phase simply by decreasing the grain size or reducing the dimension of perovskite crystals. The substitution of cations or anions in the perovskite crystal lattice by different elements, followed by the rule of Goldschmidt tolerance factor, can also produce stable inorganic perovskites with enhanced electronic and optical properties. Meanwhile, an abundant choice of appropriate elements for the substitution can fundamentally enrich the library of stable inorganic perovskites.

Methylammonium lead iodide doped with cesium (MA(1-x)Cs(x)PbI(3), x = 0, 0.03, 0.05, 0.1) thin films were prepared with and without chlorobenzene (CB). X-ray diffraction analysis applying the Rietveld refinement method is carried out, and the effect of Cs and CB on the lattice parameters, crystallite size, lattice microstrain, and dislocation density of the formed films is examined. SEM images with different magnifications are used to investigate the films' morphology and homogeneity. Adding CB reduced the number of pinholes and the grain size in the formed films. The UV-Vis-NIR spectroscopy technique was used to investigate the transmittance, reflectance, and absorbance spectra for Cs doped MAPbI(3) with and without CB. In general, for Cs doped films, adding CB increases the film transmittance. The refractive index of 3% Cs doped film has the highest value among the films without CB. The optical bandgap, refractive index, and optical conductivity for films with CB are higher than those without CB for all doping Cs values. The influence of Cs doping and CB addition on the photoluminescence (PL) emitted spectra was studied using a PL set-up with a laser source of 532 nm.

Large area photovoltaic devices based on interpenetrating networks of donor and acceptor molecules have been fabricated by using the doctor blade technique. Devices containing blends of poly (para phenylenevinylene, PPV) derivatives as donor materials with various C60 derivatives as acceptors are compared. It is shown that among the various combinations of materials short circuit currents are maximal for blends of PPV and a highly soluble fullerene derivative (PCBM monoadduct), whereas the open circuit potentials stay almost the same for all devices. For the best combination of donor - acceptor materials it has been shown that upscaling to 10×15 cm is possible by using the doctor blade technique without loosing efficiency.

Perovskite solar modules have been attracting increasing attention due to their market potential, yet publications concerned with theintrinsic scale-up potential of different perovskite compositions remain relatively scarce. On the other hand, while great success is being made towards improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs) by cesium cation (Cs+) doping of the perovskite, more attention is being paid to the perovskite phase stabilization effect of Cs+ doping, and less to other properties that are critical to understand and futher improve the PSC's. In this work, moderately-Cs-doped MAPbI(3) was employed as a model perovskite material in order to exclude the phase stabilization effect. Our systematic study revealed the influence of Cs+ in organic-inorganic hybrid perovskites on the crystal structure, crystallization process, trap state density, band structure and charge (i.e., ions or photo-carriers) transport. Markedly, it has been observed that Cs+ doping can greatly increase the carrier diffusion length in the perovskite films, thus improving the potential to scale-up PSC's.The PCE of small area devices (0.09 cm(2)) was increased to 21.72% from 19.73%, with decreased hysteresis behavior and increased operational stability (T-85 = 1000 h) after Cs+ doped, where T-85 refers to the retention of 85% of the initial PCE. Moreover, a PCE of 21.08% was obtained for a Cs+-containing perovskite module with an active area > 30 cm(2), which demonstrates a better "reproducibility " than the reference sample (MAPbI(3)-based perovskite modules, PCE = 18.26%).

Light-induced degradation (LID) of crystalline silicon (c-Si) surfaces passivated with hydrogenated amorphous silicon (a-Si:H) is investigated. The initial passivation decays on polished c-Si(100) surfaces on a time scale much faster than usually associated with bulk a-Si:H LID. This phenomenon is absent for the a-Si:H/c-Si(111) interface. We attribute these differences to the allowed reconstructions on the respective surfaces. This may point to a link between the presence of so-called "fast" states and (internal) surface reconstruction in bulk a-Si:H.

Metal halide perovskite solar cells (PSCs) have been considered to be one of the most promising next-generation energy harvesters over the past decades due to remarkably rapid improvement of power conversion efficiency in photovoltaics. However, energy harvesters based on the solar energy source have an intrinsic environment limitation for indoor applications. A feasible solution to the limitation is to add non-solar energy harvesting functions to the solar energy harvesters. Here, the piezoelectric properties of two types of metal halide PSCs are investigated, the 3D only and the 3D/2D structure, showing PCEs of 21.3% and 23.2%, respectively. Piezo-response force microscopy and synchrotron-based X-ray diffraction demonstrate that both types of PSC sample have piezoelectricity. Remarkably, the 3D/2D structure has considerably higher piezoelectric amplitude than the 3D-only. The deep level transient spectroscopy results reveal that the enhancement in the piezoelectricity of the 3D/2D structure originates from Pb-Br defects. This study unravels the role of defects in the piezoelectricity of metal halide PSCs and provides a direction to develop the multi-function energy harvesters based on the PSCs.

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

A series of carbazole based sensitizers with either phenyl based donors (TBC, TMC, OMC, PC, TBR, TMR, OMR and PR) or aryl amine based donors (OMNC, CNC and HNC) as well as one without a donor group (CC) have been synthesized to understand the influence of the strength of the donor moiety on the optical, electrochemical and photovoltaic properties. Two different acceptor moieties such as cyano acrylic acid and rhodanine acetic acid were introduced and evaluated. Different substituents on the phenyl group have a significant impact on the light harvesting ability of the sensitizers. Among phenyl based donors, anisole based carbazole (OMC) shows the highest short circuit current (JSC) of 4.96 mA cm(-2) with overall power conversion efficiency (PCE) of 2.69%. In the case of the sensitizers with aryl amine based donors, the increasing bulkiness of the donor group lead to increasing open circuit potential. Transient photocurrent and photovoltage measurements signify the importance of a bulky donor fragment in determining the open circuit potential of the dyes. Sensitizers with hexyloxy substituted phenyl amine as the donor group shows a JSC of 6.84 mA cm(-2) with PCE of 3.33%. The overall investigation provides vital information about the influence of donor groups on the optoelectronic properties of the sensitizers for its photovoltaic applications.

Gallium arsenide p-i-n radial junctions were fabricated by molecular beam epitaxy. The current-voltage characteristics of single nanowires were measured in the dark and under various illumination conditions including 1.5 AM. The total efficiency was 4.5%. Spatially resolved and power dependent photocurrent measurements indicate that the p-i-n junction is homogeneous along the nanowire. Electroluminescence measurements show an emission peak at about 1.4 eV, further corroborating the good quality of the nanowire. These results constitute an important progress for the use of nanowires in photovoltaic applications.

A series of imidazolium salts with the nitrile functional group attached to the alkyl side chain, viz. [C(n)CNmim][X] (where C(n)CNmim is the 1-alkylnitrile-3-methylimidazolium cation and C-n = (CH2)(n), n = 1-4; X = Cl, PF6, and BF4) and [C(3)CNdmim][X] (where C(n)CNdmim is the 1-alkylnitrile-2,3-dimethylimidazolium cation and C-n = (CH2)(n), n = 3; X = Cl, PF6, and BF4), have been prepared and characterized using spectroscopic methods. The majority of the nitrile-functionalized imidazolium salts can be classed as ionic liquids since they melt below 100 degreesC. Four of the imidazolium salts have been characterized in the solid state using single-crystal X-ray diffraction analysis to reveal an extensive series of hydrogen bonds between H atoms on the cation and the anion. The relationship between the solid-state structure and the melting point is discussed. Key physical properties (density, viscosity, and solubility in common solvents) of the low melting ionic liquid have been determined and are compared with those of the related 1-alkyl-3-methylimidazolium and 1-alkyl-2,3-dimethylimidazolium ionic liquids. It was envisaged that these ionic liquids could act as both solvent and ligand for catalyzed reactions, and this application is demonstrated in hydrogenation reactions, which show that retention of the catalyst in the ionic liquid during product extraction is extremely high.

Transient absorption spectroscopy on the mu s-s time scale is used to monitor the yield and decay dynamics of photogenerated holes in nanocrystalline hematite photoanodes. In the absence of a positive applied bias, these holes are observed to undergo rapid electron-hole recombination. The application of a positive bias results in the generation of long-lived (3 +/- 1 s lifetime) photoholes.

Recent years have witnessed considerable progress in the development of solar cells based on lead halide perovskite materials. However, their intrinsic instability remains a limitation. In this context, the interplay between the thermal degradation and the hydrophobicity of perovskite materials is investigated. To this end, the salt 1-(4-ethenylbenzyl)-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylimidazolium iodide (ETI), is employed as an additive in hybrid perovskites, endowing the photoactive materials with high thermal stability and hydrophobicity. The ETI additive inhibits methylammonium (MA) permeation in methylammonium lead triiodide (MAPbI(3)) occurring due to intrinsic thermal degradation, by inhibiting out-diffusion of the MA(+) cation, preserving the pristine material and preventing decomposition. With this simple approach, high efficiency solar cells based on the unstable MAPbI(3) perovskite are markedly stabilized under maximum power point tracking, leading to greater than twice the preserved efficiency after 700 h of continuous light illumination and heating (60 degrees C). These results suggest a strategy to tackle the intrinsic thermal decomposition of MAI, an essential component in all state-of-the-art perovskite compositions.

The properties of efficient solar cells fabricated with triple-cation perovskite placed between a mesoporous titania layer and a spiro-OMeTAD layer are studied by using devices either prepared under water-free drybox conditions or fabricated under ambient room humidity. The morphological studies indicate that the content of unreacted PbI2 phase in the perovskite structure is much higher near the interface with titania than near the interface with spiro-OMeTAD. The stationary emission spectra and transient bleach peaks of perovskites show additional long-wavelength features close to the titania side. Time-resolved techniques ranging from femtoseconds to seconds reveal further differences in charge dynamics at both interfaces. The population decay is significantly faster at the titania side than at the spiro-OMeTAD side for the cells prepared under ambient conditions. An increased hole injection rate correlates with higher photocurrent seen in the devices prepared under drybox conditions. The charge recombination loss on the millisecond time scale is found to be slower at the interface with titania than at the interface with spiro-OMeTAD. The ideality factor of the cells is found to increase with increasing DMSO content in the precursor solution, indicating a change in recombination mechanism from bulk to surface recombination. We also found that the charge dynamics are not uniform within the whole perovskite layer. This feature has significant implications for understanding the operation and optimizing the performance of solar devices based on mixed cation perovskites.

Molecular additives are widely utilized to minimize non-radiative recombination in metal halide perovskite emitters due to their passivation effects from chemical bonds with ionic defects. However, a general and puzzling observation that can hardly be rationalized by passivation alone is that most of the molecular additives enabling high-efficiency perovskite light-emitting diodes (PeLEDs) are chelating (multidentate) molecules, while their respective monodentate counterparts receive limited attention. Here, we reveal the largely ignored yet critical role of the chelate effect on governing crystallization dynamics of perovskite emitters and mitigating trap-mediated non-radiative losses. Specifically, we discover that the chelate effect enhances lead-additive coordination affinity, enabling the formation of thermodynamically stable intermediate phases and inhibiting halide coordination-driven perovskite nucleation. The retarded perovskite nucleation and crystal growth are key to high crystal quality and thus efficient electroluminescence. Our work elucidates the full effects of molecular additives on PeLEDs by uncovering the chelate effect as an important feature within perovskite crystallization. As such, we open new prospects for the rationalized screening of highly effective molecular additives., Multidentate molecular additives are widely used to passivate perovskite, yet the role of chelate effect is still unclear. Here, the authors investigate a wide range of additives with different coordination number and functional moieties to establish correlation between coordination affinity and perovskite crystallisation dynamics.

The presence of bulk and surface defects in perovskite light harvesting materials limits the overall efficiency of perovskite solar cells (PSCs). The formation of such defects is suppressed by adding methylammonium chloride (MACl) as a crystallization aid to the precursor solution to realize high-quality, large-grain triple A-cation perovskite films and that are combined with judicious engineering of the perovskite interface with the electron and hole selective contact materials. A planar SnO2/TiO2 double layer oxide is introduced to ascertain fast electron extraction and the surface of the perovskite facing the hole conductor is treated with iodine dissolved in isopropanol to passivate surface trap states resulting in a retardation of radiationless carrier recombination. A maximum solar to electric power conversion efficiency (PCE) of 21.65% and open circuit photovoltage (V-oc) of approximate to 1.24 V with only approximate to 370 mV loss in potential with respect to the band gap are achieved, by applying these modifications. Additionally, the defect healing enhances the operational stability of the devices that retain 96%, 90%, and 85% of their initial PCE values after 500 h under continuously light illumination at 20, 50, and 65 degrees C, respectively, demonstrating one of the most stable planar PSCs reported so far.

Singlet fission (SF) is a promising multiexciton-generating process. Its demanding energy splitting criterion - that the S-1 energy must be at least twice that of T-1 - has limited the range of materials capable of SF. We propose heteroatom oxidation as a robust strategy to achieve sufficient S-1/T-1 splitting, and demonstrate the potential of this approach for intramolecular SF.

Probing local emission properties of organic-inorganic lead halide perovskite material can provide evidence regarding the photovoltaic performance of perovskite solar cells. Herein, cathodoluminescence, which has the potential to resolve emission characteristics in the nanoregime, has been exploited to carry out temperature-dependent studies on individual well-faceted CH3NH3PbI3-xBrx perovskite single crystals. The spatial distribution of emission recorded at 4 and 300 K reveals that the periphery of the perovskite crystals radiates predominantly, which establishes that such an unusual emission characteristic is independent of the crystallographic phase of CH3NH3PbI3-xBrx crystals. Investigation based on scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy deduces that the asymmetric cathodoluminescence is associated with the nonhomogeneous distribution of methylammonium cations in CH3NH3PbI3-xBrx single crystals. These results emphasize the unraveling of a correlation between the composition and spectroscopic properties of perovskite crystals in the nanoregime, which eventually can influence the overall photovoltaic performance of the devices based on them.

A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H-2 production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.

A primary method for the production of 2D nanosheets is liquid-phase delamination from their 3D layered bulk analogues. Most strategies currently achieve this objective by significant mechanical energy input or chemical modification but these processes are detrimental to the structure and properties of the resulting 2D nanomaterials. Bulk poly(triazine imide) (PTI)-based carbon nitrides are layered materials with a high degree of crystalline order. Here, we demonstrate that these semiconductors are spontaneously soluble in select polar aprotic solvents, that is, without any chemical or physical intervention. In contrast to more aggressive exfoliation strategies, this thermodynamically driven dissolution process perfectly maintains the crystallographic form of the starting material, yielding solutions of defect-free, hexagonal 2D nanosheets with a well-defined size distribution. This pristine nanosheet structure results in narrow, excitation-wavelength-independent photoluminescence emission spectra. Furthermore, by controlling the aggregation state of the nanosheets, we demonstrate that the emission wavelengths can be tuned from narrow UV to broad-band white. This has potential applicability to a range of optoelectronic devices.

Two-dimensional (2D) hybrid perovskite solar cells have induced widespread research interest owing to their higher stability as compared to three-dimensional (3D) analogues. The photovoltaic performance, however, is still limited. Here, we report a perovskite film dominated by 3D components that are modulated by formamidinium thiocyanate in the 2D PEA(2)FA(3)Pb(4)I(13) perovskite composition. We demonstrate that a dimensionality transformation from a 2D/3D to predominantly 3D perovskite phase in the presence of SCN-based treatment enhances the optical absorption in visible and near-infrared regions. As a result, the power conversion efficiency improved 4.5 times with respect to the control to above 14%. The unencapsulated cell exhibited excellent moisture stability during the shelf lifetime measurement over 70 days under ambient conditions.

Dion-Jacobson (DJ) iodoplumbates based on 1,4-phenylenedimethanammonium (PDMA) have recently emerged as promising light absorbers for perovskite solar cells. While PDMA is one of the simplest aromatic spacers potentially capable of forming a DJ structure based on (PDMA)A(n)(-1)Pb(n)I(3)(n)(+1) composition, the crystallographic proof has not been reported so far. Single crystal structure of a DJ phase based on PDMA is presented and high-field solid-state NMR spectroscopy is used to characterize the structure of PDMA-based iodoplumbates prepared as thin films and bulk microcrystalline powders. It is shown that their atomic-level structure does not depend on the method of synthesis and that it is ordered and similar for all iodoplumbate homologues. Moreover, the presence of lower (n) homologues in thin films is identified through UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction measurements, complemented by cathodoluminescence mapping. A closer look using cathodoluminescence shows that the micron-scale microstructure corresponds to a mixture of different layered homologues that are well distributed throughout the film and the presence of layer edge states which dominate the emission. This work therefore determines the formation of DJ phases based on PDMA as the spacer cation and reveals their properties on a multi-length scale, which is relevant for their application in optoelectronics.

Low-temperature annealing of hydrogenated amorphous silicon (a-Si:H) is investigated. An identical energy barrier is found for the reduction of deep defects in the bulk of a-Si:H films and at the interface such layers form with crystalline Si (c-Si) surfaces. This finding gives direct physical evidence that the defects determining a-Si:H/c-Si interface recombination are silicon dangling bonds and that also kinetically this interface has no unique features compared to the a-Si:H bulk.

Solar hydrogen production via watersplitting with hematite (Fe2O3) has been limited by poor light absorption and a small hole diffusion length. These drawbacks can be overcome by using a high-surface-area host to support a thin layer of hematite-allowing photogenerated holes to be produced in high proximity to the semiconductor-liquid junction. Here we demonstrate the effectiveness of this concept using a nanostructured host scaffold (WO3) prepared by atmospheric pressure CVD to support a thin layer of Fe2O3 nanoparticles deposited by a similar method. A 20% increase in the photocurrent was observed in host-guest electrodes as compared to control films with the same amount of hematite (equivalent to a 60 nm film) deposited without the host scaffold. The improvement is attributed to an increase in the absorbed photon conversion efficiency (APCE). especially,for longer wavelengths where the photon penetration depth is large in hematite. For light with a wavelength of 565 nm, the APCE improves to 8.0%, as compared to 5.7% with the control films because of the host/guest architecture.

By properly tuning the photon energy of a femtosecond laser pump, we disentangle, in carbon nanotube–Si (CNT/Si) heterojunctions, the fast relaxation dynamics occurring in CNT from the slow repopulation dynamics due to hole charge transfer at the junction. In this way we are able to track the transfer of the photogenerated holes from the Si depletion layer to the CNT layer, under the action of the built-in heterojunction potential. This also clarifies that CNT play an active role in the junction and do not act only as channels for charge collection and transport.

In the past decades, atomically controlled multicomponent nanomaterials have served as platforms to advance the understanding of scientific phenomena along with providing practical solutions for various applications. Colloidal chemistry and atomic layer deposition techniques are two of the most powerful techniques to tailor make materials with tunable composition and morphology. In this Review, we focus on how the combination of these two highly versatile chemistries enables an atomic-scale of multicomponent nanomaterials in a wide compositional range. We discuss very recent studies from us and others, which encourage the scientific community to explore the tunability offered by such combination even further.

The superweak tetrakis(nonafluoro-tert-butoxy)aluminate coordinating anion was employed to introduce pseudo-gas-phase conditions to the 2-[5-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3-pentadien-1-yl]-1,3,3-trimethyl-3H-indolium chromophore. The resulting formation of a photoactive organic-inorganic hybrid salt has led to a highly stabilized excited state of the organic chromophore mainly due to the minimized lattice energy and Coulomb interactions. These highly beneficial features caused by the well dispersed negative charge of the anion have led to an enhanced neat spin-casted film fluorescence intensity, prolonged fluorescence lifetime, smooth thin film surfaces and a record power photovoltaic efficiency of 3.8 % when compared to organic salts of this particular chromophore containing anions with localised negative charge. Clear evidence is given that a superweak coordinating anion is an emerging key parameter in cyanine dye photochemistry. This approach can be seen as a general guideline to prepare highly efficient ionic dyes for organic semiconductor applications.

The effects of thermal disorder on the electronic properties of organic/inorganic halide perovskites were investigated using ab initio molecular dynamics simulations. It was generally found that band gap variations due to effects of thermal disorder are the largest in materials with the smallest lattice constant. The factors that may lead to departure from this trend include the degree of rotational and translational motion of the organic cation and the strength of its dipole. It was found that the contribution of the flexible organic part to the band gap variations is considerably smaller than the contribution of the inorganic part of the material. The results of our simulations indicate that band gap variations in halide perovskites fall within the range exhibited in inorganic semiconductors.

Molecular heterojunctions, such as the one based on copper phthalocyanine (CuPc) and carbon fullerene (C-60) molecules, are commonly employed in organic photovoltaic cells as electron donor-acceptor pairs. We have investigated the different atomic structures and electronic and optical properties of the C-60/CuPc heterojunction through first-principles calculations based on density functional theory (DFT) and time-dependent DFT. In general, configurations with the CuPc molecule "lying down" on C-60 are energetically more favorable than configurations with the CuPc molecule "standing up". The lying-down configurations also facilitate charge transfer between the two molecules, due to the stronger interaction and the larger overlap between electronic wavefunctions at the interface. The energetically preferred structure consists of CuPc placed so that the Cu atom is above a bridge site of C-60, with one N-Cu-N bond of CuPc being parallel to a C-C bond of C-60. We also considered the structure of a periodic CuPc monolayer deposited on the (001) surface of a face-centered cubic (fcc) crystal of C-60 molecules with the lying-down orientation and on the (111) surface with the standing-up configuration. We find that the first arrangement can lead to larger open circuit voltage due to an enhanced electronic interaction between CuPc and C-60 molecules.

Biphasic and multiphasic processes have emerged as effective methods to reduce the amount of volatile organic solvents used in the manufacture of chemicals. In this article some of the research conducted in my laboratory in this domain utilizing ionic liquids is described. Other dimensions to our research interests in ionic liquids are also discussed including their conversion to porous nanomaterials.

The optoelectronic properties, morphology, and consequently the performance of metal halide perovskite solar cells are directly related to the crystalline phases and intermediates formed during film preparation. The gas quenching method is compatible with large-area deposition, but an understanding of how this method influences these properties and performance is scarce in the literature. Here, in situ grazing incidence wide angle X-ray scattering is employed during spin coating deposition to gain insights on the formation of MAPbI(3)and Cs(x)FA(1-)(x)Pb(I0.83Br0.17)(3)perovskites, comparing the use of dimethyl sulfoxide (DMSO) and 2-methyl-n-pyrrolidone (NMP) as coordinative solvents. Intermediates formed using DMSO depend on the perovskite composition (e.g., Cs content), while for NMP the same intermediate [PbI2(NMP)] is formed independently on the composition. For MAPbI(3)and Cs(x)FA(1-)(x)Pb(I0.83Br0.17)(3)with a small amount of Cs (10% and 20%), the best efficiencies are achieved using NMP, while the use of DMSO is preferred for higher (30% and 40%) amount of Cs. The inhibition of the 2H/4H hexagonal phase when using NMP is crucial for the final performance. These findings provide a deep understanding about the formation mechanism in multication perovskites and assist the community to choose the best solvent for the desired perovskite composition aiming to perovskite-on-silicon tandem solar cells.

Print-light-synthesis is a new concept for the large-scale in situ fabrication of nanoparticles and structures on large substrates. Here, Pt nanoparticles are synthesized on indium tin oxide (ITO) coated glass slides by using combined inkjet printing and photonic curing operated under ambient conditions and with low material usage. A formulated chloroplatinic acid based ink provides stable and reproducible jetting, optimized ink–substrate interaction and fast ink drying to create well-defined thin precursor films acting as nanoreactors. The precursor loading is precisely controlled by the printing parameters. Then, a short light pulse from a Xe flash lamp fully converts the printed Pt precursor containing film into pure Pt nanoparticles. The optimum precursor coverage is ≈1 µg mm−2 Pt consuming as less as ≈50 nL mm−2 of ink. Neither reducing nor capping agents are used resulting in pure Pt nanoparticles (30 nm average size) and micrometer-size aggregates. The nanostructures are well-adhered to the ITO substrate and show a stable electrochemical performance for the oxygen reduction reaction. The fast and cost-effective process optimization in terms of ink formulation, substrate pre-treatment, inkjet printing resolution, and post-processing for the rapid fabrication of Pt nano- and microparticle-coated ITO electrodes is presented and discussed.

Replacing silver metallization with earth-abundant materials in Si solar cells is a critical step towards the sustainable growth of photovoltaics. In this work, we investigate fire-through processes using Al for the metallization of solar cells with p-type passivating contacts, with the goal to achieve a thin depth of contact, oppositely to what is used for standard back-surface field Al contacts. The interactions taking place during firing between the different elements in our contact stack (Al, SiNx:H, SiCx(p) and tunnel SiOx) are studied. We discuss how Al and SiNx react during firing by performing contact formation through different nitride layers, and show that increasing the N content of the silicon nitride can reduce Al penetration depth. Finally, we also investigate the mechanisms behind metal-induced passivation degradation and identify possible ways of mitigating them by employing an adapted SiOx/SiCx stack to allow for Ag-free metallization of tunnel oxide passivating contacts.

Organic-inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a Sn-119 solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of Sn-119 can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/CI compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. Sn-119 NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr(3) and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact.

Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated is described. Statistical analysis is applied to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (VIM), are identified in different perovskite films. It is found that most passivating agents inhibit HIM, but not VIM. However, VIM can be reduced by deposition of imidazolium iodide on the perovskite surface. This method can be used to study perovskite-based devices efficiency and stability by providing molecular level mechanistic understanding of passivation approaches leading to performance improvement of perovskite solar cells via rational design.

We present a fragment-based decomposition analysis tool (FB-ECDA) for the electronic coupling of charge transfer processes. This tool provides insight on the sophisticated relationship between molecular packing, electronic coupling, and the molecular transport network present in organic amorphous semiconductors. On the basis of atomic orbitals, FB-ECDA decomposes the total electronic coupling into individual electronic coupling terms arising from each molecular building blocks in a straightforward manner. The usefulness of this approach is demonstrated by revealing the structure-packing-property relationships for two series of molecules differing by the number of arm substituents and acene core length. Overall, we provide insight on the design of organic semiconductors exhibiting efficient charge transport network through achieving a subtle balance between molecular packing and electronic structure. We expect FB-ECDA to be a valuable tool for understanding sophisticated charge transfer processes in amorphous systems and guiding the rational design of organic semiconductors.

Hybrid organic-inorganic halide perovskites represent a promising next-generation photovoltaic material with drawbacks for structure stability and composition concerns. Demonstrations of ion migration and molecular dynamics suggest room for structural contraction and subsequent property adjustments. Here, we deploy dielec-tric and infrared spectroscopy under external pressure to probe the full structural phase diagram and dielectric response of methylammonium lead halide perovskites CH3NH3PbX3 (X = I, Br, or Cl). Ion migration can be fully suppressed by pressure beyond 4 GPa. The low-temperature orthorhombic phase transition can be gradually enhanced and stabilized at ambient conditions with increasing pressure. A slow relaxation mode, presumably the motion of the CH3NH+3 cation, is observed at lower pressure and is absent in the orthorhombic phase for every halide.

In this work, we present the influence of thermal annealing on morphological, structural, electrical, optical, and photoelectrochemical properties of double- (DW) and single-wall (SW) TiO2 nanotube (TNT) layers. High-aspect ratio TNT layers with a thickness of ∼5 μm and ∼20 μm with an average inner diameter of ∼250 nm and ∼130 nm, respectively, were prepared via electrochemical anodization of Ti foil in two different fluoride containing ethylene glycol-based electrolytes. The inner wall of the native DW TNT layers was quantitatively removed via a mild pre-annealing followed by a selective etching treatment in piranha solution, yielding SW TNT layers. The obtained SW TNT layers annealed at 300–500 °C possess enhanced conductivity by 1–2 orders compared to their DW counterparts. The photoelectrochemical properties of SW TNT layers are enhanced compared to their DW counterparts in the annealing temperature range 300–800 °C. In principle, the SW TNT layers could be potentially favored to the DW TNT layers in electrical and photoelectrochemical applications due to their more superior intrinsic properties.

The strategy of mixing cations and halides in hybrid organic lead halide perovskites has recently been shown to be very successful in achieving high efficiency with long-term perovskite solar cell stability. In this study, the effect of photoexcitation on the crystal structure of a perovskite film based on triple cations and double halides is investigated and correlated to the recombination behavior in the material. Under continuous photon excitation, a gradual increase in photoluminescence is observed from the perovskite film, coupled with a minute red-shift. The X-ray diffraction pattern also shows a gradual shift toward lower angles under light soaking, suggesting a photoinduced-structural change. Noticeably, a lattice expansion occurs in a preferred orientation, which accordingly influences the crystal symmetry. The trap density of a complete perovskite solar cell is measured under continuous light. The traps are observed in the intermediate frequency, corresponding to the dielectric polarization of the bulk perovskite. A gradual decrease of the trap density is observed under light soaking accompanied by a substantial increase in photocurrent, which is in accordance with the increased recombination resistance monitored during the light soaking. The photoinduced-structural response of the perovskite material and its resulting beneficial recombination behavior is addressed.

Hybrid lead halide perovskites typically form polycrystalline films that have multiple grain sizes and surface defects. A key engineering challenge toward commercialization is therefore the production of homogeneous, defect-free largearea devices achieving high efficiency. New market opportunities may arise from vacuum-deposited perovskites if detailed understanding and control of crystal formation are available. Of the many factors that make reproducibility of device performance difficult, two variables are identified that have not yet been considered in detail: deposition speed and underlayer material selection. Herein, it is demonstrated that small changes in the perovskite growth rate (0.18-0.72 angstrom.s(-1)) substantially affect the preferred crystal orientation. Further, varying underlayer interfaces greatly influence the composition of the final perovskite and thus its energetic profile. The research aids control in fine-tuning the perovskite film at the nanometer scale, which enables the reproducible fabrication of vertically aligned and micrometer-sized grain features, highly demanded for in high-quality semiconductors.