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Inorganic-organic hybrid perovskite materials have been a topic of interest for the last few years due to their superior optoelectronic properties. However, the optical properties of perovskite materials are strongly dependent on the film morphology. A textured film morphology is expected to have higher light absorption as well as light out-coupling efficiency compared to a smooth film. There have been numerous methods for controlling and optimizing the film morphology to achieve high efficiency in solar cells and light emitting diodes. Here we have demonstrated that controlled anti-solvent treatment at low temperature can lead to Stranski-Krastanov growth in CH3NH3PbBr3 thin films with superior optical and electronic properties for light emitting diode applications. We have studied their photoluminescence properties at the micro- to nano-scale via fluorescence microscopy, hyper-spectral imaging and scanning near-field optical microscopy. We have demonstrated that the nanostructured micro-islands are highly emissive because of large quasi-Fermi level splitting (QFLS) due to the localization of free charges in the smaller crystals. We have shown that the photoluminescence as well as electroluminescence can be improved by at least seven-fold due to the presence of micro-islands on a smooth background film enhancing light out-coupling. Photo-induced photoluminescence enhancement is also observed in smooth films while micro-islands show photo-degradation.

Blue-color-emitting organic semiconductors are of significance for organic light-emitting diodes (OLEDs). In this study, through Suzuki coupling polymerization, three 1,4-naphthalene-based copolymers—namely, PNP(1,4)-PT, PNP(1,4)-TF, and PNP(1,4)-ANT—were designed and synthesized. The variation of comonomers, phenothiazine (PT), triphenylamine substituted fluorene (TF), and anthanthrene (ANT), effectively tuned the emitting color and device performance of poly(9-vinyl carbazole) (PVK)-based OLEDs. Especially, the polymer PNP(1,4)-TF, bearing perpendicular aryl side groups, showed a most twisted structural geometry, which enabled an ultra-high thermal stability and a best performance with blue emitting in PVK-host-based OLEDs. Overall, in this work, we demonstrate a promising blue-color-emitting polymer through structural geometry manipulation.

Perovskite light-emitting diodes have almost reached the threshold for potential commercialization within a few years of research. However, there are still some unsolved puzzles such as large ideality factor and the presence of large negative capacitance especially at the low-frequency regime yet to be addressed. Here, we have fabricated a methylammonium lead tri-bromide perovskite n-i-p structure for light-emitting diodes from a smooth and textured emissive layer and demonstrated for the first time that these two factors are strongly dependent on the perovskite film morphology. Bias-dependent capacitance measurement also reveals the transition between negative to positive capacitance in textured films at the low-frequency regime. We have observed an anomalous capacitive behavior at the mid-frequency regime in smooth perovskite films but not in textured films. The relatively large ideality factor and anomalous capacitive behavior observed in perovskite light-emitting diodes are due to the presence of strong coupling between ions and electrons near the electrode interface. Therefore, the ideality factor and anomalous capacitance at the mid-frequency regime can be decreased by minimizing electronic-ionic coupling in textured perovskite films, while light outcoupling can be improved significantly.

Truxene (10,15-dihydro-5H-diindeno[ 1,2-a; 1', 2'-c] fluorene) derived blue emitting materials were designed and developed utilizing Suzuki and Buchwald-Hartwig type cross-coupling reactions. The target molecules T-1, T-2 and T-3 bearing N-carbazolyl, 1-pyrenyl and N-phenyl-N-(pyren-1-yl) substituents, respectively at 2,7,12-positions, were synthesized under mild conditions by palladium-catalyzed reactions in good yields. To improve the solution processability of the material, n-hexyl chains were used as substituents at 5,5'-, 10,10'-, 15,15'-positions. From UV-vis measurements, the absorption maxima were found to be at 330 nm for T-1, 356 nm for T-2 and 412 nm for T-3. The materials were found to be blue emitting with their emission maxima at 385 nm, 425 nm and 490 nm for T-1, T-2 and T-3, respectively. The excited state lifetimes were investigated using time correlated single photon counting and were found to be 10 ns, 1.4 ns and 5.4 ns for T-1, T-2 and T-3, respectively. The highest photoluminescence quantum yield was observed in the case of T-2 corresponding to a value of 0.97. The compounds T-2 and T-3 were used as active materials for the fabrication of solution processed, single layer blue light emitting diodes, with low turn-on voltage (2-3.3 V) and Commission Internationale de l'eclairage (CIE) coordinates corresponding to (0.16, 0.23) and (0.17, 0.36) for T-2 and T-3, respectively.

We demonstrate all thermal vapor-deposited CsSnBr 3 perovskite solar cells. The cell architecture consists of a planar CsSnBr 3 heterojunction where the perovskite layer is grown by thin sequential deposition of SnBr 2 and CsBr allowing for immediate reaction. While other halides of CsSnX 3 (e.g. X=Cl, I) are extremely air sensitive and change color within seconds, illumination tests performed on unencapsulated CsSnBr 3 devices in ambient air show no change over the span of hours. Further, we evaluate the impact of low-temperature annealing and incorporation of various fluoride passivating dopant profiles via vapor deposition. Our results demonstrate an exciting alternative pathway to develop all-inorganic lead-free perovskite photovoltaics.

In this chapter, we present an overview of solution-processable polymer light-emitting diodes (PLEDs) that are widely recognized as an option for large-area devices. Top-emitting devices, along with various intermediate layers of importance, are discussed along with single layer, bilayer, and blended/doped emitters layers. General device characterization techniques are explained, as well as guidelines for alternative methods to improve the efficiency of the emitting diodes. Also, a few advanced optical-electrical characterization methods are presented.

By varying the concentration of a solvent additive, we demonstrate the modulation of intermolecular (donor/acceptor (D/A) interface) and intramolecular (bulk) disorder in blends of the low-band gap polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopental[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Using the solvent additive concentration of 1,8-diiodooctane (DIO) in the host-processing solvent, the disorder in the bulk and at the interface is studied in terms of Urbach energy, electroluminescence (EL) broadening, and EL quantum efficiency (ELQE). The Urbach energy varies from 80 to 39 meV for bulk and 39 to 51 meV for D/A interface. An interesting feature is that changes in the Urbach energy of the D/A interface are opposite to those of the Urbach energy of bulk; i.e., the disorder at the D/A interface increases as the disorder in the bulk decreases with increase in DIO concentration. Our study evidently suggested a...

A new method was developed for doping and fabricating hysteresis-free hybrid perovskite-based photovoltaic devices by using alkali metal halide salts as interface layer additives. Such salt layers introduced at the perovskite interface can provide excessive halide ions to fill vacancies formed during the deposition and annealing process. A range of solution-processed halide salts were investigated. The highest performance of methylammonium lead mixed-halide perovskite device was achieved with a NaI interlayer and showed a power conversion efficiency of 12.6% and a hysteresis of less than 2%. This represents a 90% improvement compared to control devices without this salt layer. Through depth-resolved mass spectrometry, optical modeling, and photoluminescence spectroscopy, this enhancement is attributed to the reduction of iodide vacancies, passivation of grain boundaries, and improved hole extraction. Our approach ultimately provides an alternative and facile route to high-performance and hysteresis-free perovskite solar cells.

We report a blue-emitting random copolymer (termed modified Aryl-F8) consisting of three repeat units of polydioctylfluorene (F8), Aryl-polydioctylfluorene (Aryl-F8), and an aromatic amine comonomer unit, poly(bis-N,Ν′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) chemically linked to get an improved charge carrier balance without compromising on the photoluminescence (PL) quantum yield with respect to the Aryl-F8 homo-polymer. The measured photoluminescence quantum efficiency (∼70%) of the blue-emitting polymer is comparable to or greater than the individual monomer units. The time resolved PL spectra from the modified Aryl-F8 are similar to those of Arylated-poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB) even at a time scale of 100–250 ps, indicating an ultrafast energy transfer from the (Aryl-F8 or F8):Arylated-PFB interface to Arylated-PFB, i.e., endothermic transfer of non-radiative exciplex to a radiative molecular exciton. Furthermore, the presence of non-radiative exciplex is confirmed by the photoluminescence decay profile and temperature dependent PL spectra. The luminance efficiency achieved for the modified Aryl-F8 polymer light-emitting diodes is ∼11 cd A−1 with an external quantum efficiency (EQE) of ∼4.5%, whereas it is 0.05 cd/A with an EQE of ∼0.025% for Aryl-F8. Almost two orders of higher efficiency is achieved due to the improved charge carrier balance from the random copolymer without compromising on the photoluminescence yield.We report a blue-emitting random copolymer (termed modified Aryl-F8) consisting of three repeat units of polydioctylfluorene (F8), Aryl-polydioctylfluorene (Aryl-F8), and an aromatic amine comonomer unit, poly(bis-N,Ν′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) chemically linked to get an improved charge carrier balance without compromising on the photoluminescence (PL) quantum yield with respect to the Aryl-F8 homo-polymer. The measured photoluminescence quantum efficiency (∼70%) of the blue-emitting polymer is comparable to or greater than the individual monomer units. The time resolved PL spectra from the modified Aryl-F8 are similar to those of Arylated-poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB) even at a time scale of 100–250 ps, indicating an ultrafast energy transfer from the (Aryl-F8 or F8):Arylated-PFB interface to Arylated-PFB, i.e., endothermic transfer of non-radiative exciplex to a radiative molecular exciton. Furthermore, the ...

The role of high pressure on a low molecular weight nematic liquid crystalline organic semiconductor, ethyl-hexyl substituted polyfluorene (PF2/6) is investigated using photoluminescence (PL), Raman scattering, and X-ray scattering studies at pressures from 1 to 8 GPa. The PL and the Raman data under pressure are consistent with each other with no abrupt changes in the pressure coefficients of PL or Raman peaks. The PL energies redshift and broaden, consistent with both enhanced intra- and interchain interactions. The Raman peak positions yield pressure coefficients similar to other phenyl based π-conjugated polymers. The broadening of a doublet peak in the 1135 cm−1 region indicates a more planar backbone conformation with increasing pressure. X-ray scattering indicates that the torsion angle between adjacent repeats reduces with increasing pressure and reverts back with decompression. The intermolecular structure is weakly ordered (frozen nematic) and essentially maintained with increasing pressure, in contrast to a high molecular weight PF2/6. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1014–1023

Control of unwanted low-energy emission originating due to trap states is crucial for improving the emission efficiency and for maintaining the color purity in blue light-emitting diodes. Here, a truxene-based emitter (T-pyrene) that is a good candidate for the blue emitter is investigated using photophysical studies. Steady-state absorption of the emitter reveals the presence of a charge-transfer state, and the photoluminescence spectrum suggests an occurrence of a strong interchain/exciplex state. Time-resolved photoluminescence (TRPL) and fluence-dependent measurements of the material in the dilute solution show the presence of multiple excitonic processes contributing to the delayed fluorescence. Similar studies on the pristine film suggest that the delayed fluorescence is dominated by the presence of the traps. By blending the oligomer with a higher bandgap polymer matrix, poly(9-vinyl carbazole) (PVK), a type I heterojunction at the T-pyrene:PVK interface is obtained that confines the generated excitons in T-pyrene. With this architecture, device efficiencies with T-pyrene (20 wt. %):PVK show significantly enhanced efficiency than those of the pristine device. We elucidate the relevance of the matrix:T-pyrene system using TRPL and fluence-dependent studies on T-pyrene (20 wt. %):PVK. Time-resolved emission spectra on the T-pyrene (20 wt. %):PVK films do not show red-shifted emission with time delays, which suggests the passivation of the quenching sites due to the dispersion of T-pyrene in PVK to prevent the intermolecular interactions.

Effects of substitutions of rare earth (RE) elements (Y, La, Ce, and Nd) to the Zr-based AB2 multi-phase metal hydride (MH) alloys on the structure, gaseous phase hydrogen storage (H-storage), and electrochemical properties were studied and compared. Solubilities of the RE atoms in the main Laves phases (C14 and C15) are very low, and therefore the main contributions of the RE additives are through the formation of the RENi phase and change in TiNi phase abundance. Both the RENi and TiNi phases are found to facilitate the bulk diffusion of hydrogen but impede the surface reaction. The former is very effective in improving the activation behaviors. −40 °C performances of the Ce-doped alloys are slightly better than the Nd-doped alloys but not as good as those of the La-doped alloys, which gained the improvement through a different mechanism. While the improvement in ultra-low-temperature performance of the Ce-containing alloys can be associated with a larger amount of metallic Ni-clusters embedded in the surface oxide, the improvement in the La-containing alloys originates from the clean alloy/oxide interface as shown in an earlier transmission electron microscopy study. Overall, the substitution of 1 at% Ce to partially replace Zr gives the best electrochemical performances (capacity, rate, and activation) and is recommended for all the AB2 MH alloys for electrochemical applications.

The exciton diffusion length in organic semiconductors strongly impacts the efficiency of energy harvesting organic optoelectronics. We demonstrate an experimental technique to estimate the diffusion length of triplet excitons in a ladder-type conjugated organic semiconductor from quenching of the triplet-triplet absorption in the presence of acceptor molecules. The diffusion lengths are consistent with a phenomenological random-walk model. Experiments on two different molecular weights of the polymer sample show that the diffusion length of triplet excitons is in the few micrometers range and depends upon the size of the $\ensuremath{\pi}$-conjugated system.

Diketopyrrolopyrrole (DPP) containing copolymers have gained a lot of interest in organic optoelectronics with great potential in organic photovoltaics. In this work, DPP based statistical copolymers, with slightly different bandgap energies and a varying fraction of donor-acceptor ratio are investigated using monochromatic photocurrent spectroscopy and Fourier-transform photocurrent spectroscopy (FTPS). The statistical copolymer with a lower DPP fraction, when blended with a fullerene derivative, shows the signature of an inter charge transfer complex state in photocurrent spectroscopy. Furthermore, the absorption spectrum of the blended sample with a lower DPP fraction is seen to change as a function of an external bias, qualitatively similar to the quantum confined Stark effect, from where we estimate the exciton binding energy. The statistical copolymer with a higher DPP fraction shows no signal of the inter charge transfer states and yields a higher external quantum efficiency in a photovoltaic structure. In order to gain insight into the origin of the observed charge transfer transitions, we present theoretical studies using density-functional theory and time-dependent density-functional theory for the two pristine DPP based statistical monomers.

We present optical studies of both singlet and triplet states of a ladder-type conjugated polymer as a function of hydrostatic pressure. The pressure coefficient of the triplet-triplet absorption is smaller compared to the pressure coefficient of the singlet excitation, highlighting the more localized nature of triplet excitons. The photoluminescence and phosphorescence energies red-shift at similar rates with increasing pressure, thus giving experimental evidence for the first time that the singlet-triplet splitting remains almost a constant under high pressure until 4 GPa. The diffusion length of the triplet excitons decreases to a few hundred nm at high pressures, as compared with a few micrometers at atmospheric pressure.