The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells

2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH₃ (CH₂ )₃ NH₃ )₂ (CH₃ NH₃ )_{n -1} Pb_n I_{3 n +1} perovskite cells with different numbers of [PbI₆ ]^4- sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (V_OC ) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device V_OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

A Double Support Layer for Facile Clean Transfer of Two-Dimensional Materials for High-Performance Electronic and Optoelectronic Devices

Clean transfer of two-dimensional (2D) materials grown by chemical vapor deposition is critical for their application in electronics and optoelectronics. Although rosin can be used as a support layer for the clean transfer of graphene grown on Cu, it has not been usable for the transfer of 2D materials grown on noble metals or for large-area transfer. Here, we report a poly(methyl methacrylate) (PMMA)/rosin double support layer that enables facile ultraclean transfer of large-area 2D materials grown on different metals. The bottom rosin layer ensures clean transfer, whereas the top PMMA layer not only screens the rosin from the transfer conditions but also improves the strength of the transfer layer to make the transfer easier and more robust. We demonstrate the transfer of monolayer WSe₂ and WS₂ single crystals grown on Au as well as large-area graphene films grown on Cu. As a result of the clean surface, the transferred WSe₂ retains the intrinsic optical properties of the as-grown sample. Moreover, it does not require annealing to form good ohmic contacts with metal electrodes, enabling high-performance field effect transistors with mobility and ON/OFF ratio ∼10 times higher than those made by PMMA-transferred WSe₂. The ultraclean graphene film is found to be a good anode for flexible organic photovoltaic cells with a high power conversion efficiency of ∼6.4% achieved.

Solid-State Fluorescence-based Sensing of TATP via Hydrogen Peroxide Detection

Fluorenylboronate ester chromophore-based thin films were investigated for the detection of triacetone triperoxide (TATP) vapors via the decomposition product, hydrogen peroxide. Sensing with a high level of sensitivity was achieved using a fluorescence "turn-on" mechanism based on the significant shifts in the absorption and photoluminescence spectra that occurs when the boronate esters were converted to phenoxides by hydrogen peroxide under basic conditions. The addition of an organic base was found to be critical for achieving fast conversion reactions and the formation of the phenoxide anions. Addition of a nitrile group to the fluorenyl boronate ester moiety improved the stability of the material to photooxidation, increased the photoluminescence quantum yields, and enhanced the absorption and emission shifts to longer wavelengths. In real-time sensing measurements, films comprising the cyanofluorenyl boronate ester moiety and tetra- n-butylammonium hydroxide had a response time to acid-decomposed TATP vapor of seconds and a limit of detection of 40 ppb in 60 s.

Calculating transition dipole moments of phosphorescent emitters for efficient organic light-emitting diodes

Quantum-chemical calculations show that the direction of the transition dipole moment of organometallic phosphorescent emitters is sensitive to molecular geometry.

Sensitive and fast fluorescence-based indirect sensing of TATP

Sensing of TATP vapours via the decomposition product, hydrogen peroxide, was achieved using a fluorescence “turn-on” mechanism through conversion of boronate esters to phenoxides under basic conditions in solid-state films. High sensitivity was achieved with two new fluorenylboronate esters comprising either 2,4-difluorophenyl or 4-(trifluoromethyl)phenyl substituents. The key to the sensitivity was the fact that the phenoxide anion products from the hydrogen peroxide oxidation absorbed at longer wavelengths than the starting boronate esters. Selective excitation of the phenoxide anions avoided the background fluorescence from the corresponding boronate esters. The use of the electron withdrawing substituents also led to greater photostability. The derivative containing the 4-(trifluoromethyl)phenyl moiety was found to give the most stable phenoxide, and demonstrated fast fluorescence “turn-on” kinetics with a lower limit of detection of ≈2.5 ppb in 60 s.

We report fast and sensitive fluorescence “turn-on” sensing of TATP via hydrogen peroxide detection using fluorinated fluorenylboronate ester derivatives.

Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration

Atomistic nonequilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunction films formed by vapor deposition as used in organic photovoltaics. Films comprising C₆₀ and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C₆₀. The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2-1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films were found to decrease with increasing TAPC content.

Application of an A-A'-A-Containing Acceptor Polymer in Sequentially Deposited All-Polymer Solar Cells

PNNT has been prepared as a polymeric electron acceptor for organic solar cells. The polymer has an A-A'-A acceptor motif linked alternatively with thiophene and vinyl moieties. The A'-unit is a naphthalene diimide, while the A groups are thiazoles. PNNT films were found to have an estimated electron affinity of ≈4.3 eV and an electron mobility of the order of 10^-4 cm² V^-1 s^-1. Its relatively low solubility in common chlorinated solvents at ambient temperature allowed the manufacture of sequentially deposited (SD) devices, which were found to have significantly higher efficiency than that of bulk heterojunction (BHJ) solar cells containing the same materials. Grazing-incidence wide-angle X-ray scattering measurements indicated that the SD films retained the ordering of the individual polymers to a greater extent compared to the BHJ films. The best SD devices were found to have a power conversion efficiency of up to 4.5%, with stable performance under thermal stress.

Interface Engineering of Solution-Processed Hybrid Organohalide Perovskite Solar Cells

Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage ( V_OC) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI₃ perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.

Real-time fluorescence quenching-based detection of nitro-containing explosive vapours: what are the key processes?

The detection of explosives continues to be a pressing global challenge with many potential technologies being pursued by the scientific research community. Luminescence-based detection of explosive vapours with an organic semiconductor has attracted much interest because of its potential for detectors that have high sensitivity, compact form factor, simple operation and low-cost. Despite the abundance of literature on novel sensor materials systems there are relatively few mechanistic studies targeted towards vapour-based sensing. In this Perspective, we will review the progress that has been made in understanding the processes that control the real-time luminescence quenching of thin films by analyte vapours. These are the non-radiative quenching process by which the sensor exciton decays, the analyte-sensor intermolecular binding interaction, and the diffusion process for the analyte vapours in the film. We comment on the contributions of each of these processes towards the sensing response and, in particular, the relative roles of analyte diffusion and exciton diffusion. While the latter has been historically judged to be one of, if not the primary, causes for the high sensitivity of many conjugated polymers to nitrated vapours, recent evidence suggests that long exciton diffusion lengths are unnecessary. The implications of these results on the development of sensor materials for real-time detection are discussed.

Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the T_g of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.

Relating Structure to Efficiency in Surfactant-Free Polymer/Fullerene Nanoparticle-Based Organic Solar Cells

Nanoparticle dispersions open up an ecofriendly route toward printable organic solar cells. They can be formed from a variety of organic semiconductors by using miniemulsions that employ surfactants to stabilize the nanoparticles in dispersion and to prevent aggregation. However, whenever surfactant-based nanoparticle dispersions have been used to fabricate solar cells, the reported performances remain moderate. In contrast, solar cells from nanoparticle dispersions formed by precipitation (without surfactants) can exhibit power conversion efficiencies close to those of state-of-the-art solar cells processed from blend solutions using chlorinated solvents. In this work, we use small-angle neutron scattering measurements and transient absorption spectroscopy to investigate why surfactant-free nanoparticles give rise to efficient organic solar cells. We show that surfactant-free nanoparticles comprise a uniform distribution of small semiconductor domains, similar to that of bulk-heterojunction films formed using traditional solvent processing. This observation differs from surfactant-based miniemulsion nanoparticles that typically exhibit core-shell structures. Hence, the surfactant-free nanoparticles already possess the optimum morphology for efficient energy conversion before they are assembled into the photoactive layer of a solar cell. This structural property underpins the superior performance of the solar cells containing surfactant-free nanoparticles and is an important design criterion for future nanoparticle inks.

The Molecular Origin of Anisotropic Emission in an Organic Light-Emitting Diode

Atomistic nonequilibrium molecular dynamics simulations have been used to model the induction of molecular orientation anisotropy within the emission layer of an organic light-emitting diode (OLED) formed by vapor deposition. Two emitter species were compared: racemic fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃) and trans-bis(2-phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)₂(acac)). The simulations show that the molecular symmetry axes of both emitters preferentially align perpendicular to the surface during deposition. The molecular arrangement formed on deposition combined with consideration of the transition dipole moments provides insight into experimental reports that Ir(ppy)₃ shows isotropic emission, while Ir(ppy)₂(acac) displays improved efficiency due to an apparent preferential alignment of the transition dipole vectors parallel to the substrate. The simulations indicate that this difference is not due to differences in the extent of emitter alignment, but rather differences in the direction of the transition dipoles within the two complexes.

The structural impact of water sorption on device-quality melanin thin films

The melanins are a class of pigmentary bio-macromolecules ubiquitous in the biosphere. They possess an intriguing set of physico-chemical properties and have been shown to exhibit hybrid protonic-electronic electrical conductivity, a feature derived from a process termed chemical self-doping driven by the sorption of water. Although the mechanism underlying the electrical conduction has been established, how the sorbed water interacts with the melanin structure at the physical level has not. Herein we use neutron reflectometry to study changes in the structure of synthetic melanin thin films as a function of H₂O and D₂O vapour pressure. Water is found to be taken up evenly throughout the films, and by employing the contrast effect, the existence of labile protons through reversible deuterium exchange is demonstrated. Finally, we determine a sorption isotherm to enable quantification of the melanin-water interactions.

Dependence of Organic Interlayer Diffusion on Glass-Transition Temperature in OLEDs

Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T_g) of the material with the lowest T_g. Diffusion occurred when the material with the lowest T_g formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T_g's. If the supercooled liquid formed at a temperature significantly lower than the T_g of the higher-T_g material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T_g's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.

A Triarylamine-Based Anode Modifier for Efficient Organohalide Perovskite Solar Cells

Organohalide lead perovskite solar cells have emerged as a promising next-generation thin-film photovoltaic technology. It has been clearly recognized that interfacial engineering plays a critical role in cell performance. It has been also proposed that the open-circuit voltage is dependent on the ionization potential of the hole transport layer at the anode. In this communication, we report a simple modification of the anode with a triarylamine-based small molecule (1), which avoids the need to use standard hole transport materials and delivers a relatively high open-circuit voltage of 1.08 V and a power conversion efficiency of 16.5% in a simple planar architecture.

Efficient organic photovoltaic cells on a single layer graphene transparent conductive electrode using MoOx as an interfacial layer

The large surface roughness, low work function and high cost of transparent electrodes using multilayer graphene films can limit their application in organic photovoltaic (OPV) cells. Here, we develop single layer graphene (SLG) films as transparent anodes for OPV cells that contain light-absorbing layers comprised of the evaporable molecular organic semiconductor materials, zinc phthalocyanine (ZnPc)/fullerene (C60), as well as a molybdenum oxide (MoO_x) interfacial layer. In addition to an increase in the optical transmittance, the SLG anodes had a significant decrease in surface roughness compared to two and four layer graphene (TLG and FLG) anodes fabricated by multiple transfer and stacking of SLGs. Importantly, the introduction of a MoO_x interfacial layer not only reduced the energy barrier between the graphene anode and the active layer, but also decreased the resistance of the SLG by nearly ten times. The OPV cells with the structure of polyethylene terephthalate/SLG/MoO_x/CuI/ZnPc/C60/bathocuproine/Al were flexible, and had a power conversion efficiency of up to 0.84%, which was only 17.6% lower than the devices with an equivalent structure but prepared on commercial indium tin oxide anodes. Furthermore, the devices with the SLG anode were 50% and 86.7% higher in efficiency than the cells with the TLG and FLG anodes. These results show the potential of SLG electrodes for flexible and wearable OPV cells as well as other organic optoelectronic devices.

A thiocarbonyl-containing small molecule for optoelectronics

We report the synthesis, characterization, and device properties of a novel thiocarbonyl iso-DPP derivative, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dithione.

Electrochemically tuneable multi-colour electrochemiluminescence using a single emitter

A single starting component electrochemiluminescence system from which red, green, blue or white emission can be obtained, depending on the applied potential or the mode of the ECL experiment, is described. The convoluted ECL spectral responses observed at different potentials are readily explained using a 3D-ECL technique, where the ECL spectral profile is continuously monitored as a function of potential during voltammetric scanning. The 3D plots obtained using this technique implicate cross-annihilation ECL reactions involving the complex itself and stable products resulting from its electrolysis. Combining this information with knowledge of the energetic requirements of the various reactions involved, suggests a mechanism involving traces of two emissive products, related to the loss of a methyl group from the triazole moiety. These products, while barely detectable electrochemically, are sufficiently emissive to influence and even dominate the ECL emission under some conditions.