Thin-film Optoelectronic Devices Research Articles

Conjugated Polymer Nanoparticle–Graphene Oxide Charge‐Transfer Complexes

AbstractThe game‐changing role of graphene oxide (GO) in tuning the excitonic behavior of conjugated polymer nanoparticles is described for the first time. This is demonstrated by using poly(3‐hexylthiophene) (P3HT) as a benchmark conjugated polymer and employing an in situ reprecipitation approach resulting in P3HT nanoparticles (P3HTNPs) with sizes of 50–100 nm in intimate contact with GO. During the self‐assembly process, GO changes the crystalline packing of P3HT chains in the forming P3HTNPs from H to H/J aggregates exhibiting exciton coupling constants as low as 2 meV, indicating favorable charge separation along the P3HT chains. Concomitantly, π–π interface interactions between the P3HTNPs and GO sheets are established resulting in the creation of P3HTNPs–GO charge‐transfer complexes whose energy bandgaps are lowered by up to 0.5 eV. Moreover, their optoelectronic properties, preestablished in the liquid phase, are retained when processed into thin films from the stable aqueous dispersions, thus eliminating the critical dependency on external processing parameters. These results can be transferred to other types of conjugated polymers. Combined with the possibility of employing water based “green” processing technologies, charge‐transfer complexes of conjugated polymer nanoparticles and GO open new pathways for the fabrication of improved optoelectronic thin film devices.

Aperiodic Porous Metasurface-Mediated Organic Semiconductor Fluorescence

Plasmonic metasurfaces exhibit localized electromagnetic properties that are highly relevant for light management in thin-film organic optoelectronic devices, such as organic light-emitting diodes and organic solar cells. However, the effectiveness of metasurfaces for light management in these devices is complicated by molecular orientation effects. Here, we study how the fluorescence emission from organic semiconducting polymers is modified by aperiodic porous metasurfaces. In particular, we identify the role that polymer molecular orientation plays in enhancing or diminishing the light management ability of the metasurface by comparing local- and large-area fluorescence intensity and fluorescence quantum efficiency enhancements, and by varying viewing angle, for three different semiconducting polymers. We find that the porous metasurfaces improve both the local and large-area quantum efficiency of semiconducting polymer films with more out-of-plane molecular chains by up to 414% and 53%, respectively, c...

One‐Step Inkjet Printed Perovskite in Air for Efficient Light Harvesting

Solution‐processed metal‐halide perovskites have demonstrated immense potential in photovoltaic applications. Inkjet printing is a facile scalable approach to fabricate large‐area perovskite solar cells (PSCs) due to its cost‐effectiveness and near unity material utilization ratio. However, controlling crystallinity of the perovskite during the inkjet printing remains a challenge. The PSCs deposited by inkjet printing typically have much lower power conversion efficiencies (PCEs) than those by spin‐coating. Here, we show that high‐quality perovskite films could be inkjet‐printed with an innovative vacuum‐assisted thermal annealing post‐treatment and optimized solvent composition. High‐performance PSCs based on printed CH3NH3PbI3 with a PCE of 17.04% for 0.04 cm2 (13.27% for 4.0 cm2) and negligible hysteresis (lower than 1.0%) are demonstrated. These efficiencies are much higher than the previously reported ones using inkjet‐printing (≤12.3% for 0.04 cm2). The inkjet printing combined with vacuum‐assisted thermal annealing could be an effective low‐cost approach to fabricate high‐performance perovskite optoelectronic thin film devices (including solar cells, lasers, photodetectors, and light‐emitting diodes) with high‐volume production.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering.

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼104 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (Eg). Here, we present a promising FPV based on hexagonal YbFeO3 (h-YbFO) thin-film heterostructure by exploiting its narrow Eg. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces Eg substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Recent developments of truly stretchable thin film electronic and optoelectronic devices.

Truly stretchable electronics, wherein all components themselves permit elastic deformation as the whole devices are stretched, exhibit unique advantages over other strategies, such as simple fabrication process, high integrity of entire components and intimate integration with curvilinear surfaces. In contrast to the stretchable devices using stretchable interconnectors to integrate with rigid active devices, truly stretchable devices are realized with or without intentionally employing structural engineering (e.g. buckling), and the whole device can be bent, twisted, or stretched to meet the demands for practical applications, which are beyond the capability of conventional flexible devices that can only bend or twist. Recently, great achievements have been made toward truly stretchable electronics. Here, the contribution of this review is an effort to provide a panoramic view of the latest progress concerning truly stretchable electronic devices, of which we give special emphasis to three kinds of thin film electronic and optoelectronic devices: (1) thin film transistors, (2) electroluminescent devices (including organic light-emitting diodes, light-emitting electrochemical cells and perovskite light-emitting diodes), and (3) photovoltaics (including organic photovoltaics and perovskite solar cells). We systematically discuss the device design and fabrication strategies, the origin of device stretchability and the relationship between the electrical and mechanical behaviors of the devices. We hope that this review provides a clear outlook of these attractive stretchable devices for a broad range of scientists and attracts more researchers to devote their time to this interesting research field in both industry and academia, thus encouraging more intelligent lifestyles for human beings in the coming future.

Reduced Graphene Oxide–Cadmium Zinc Sulfide Nanocomposite with Controlled Band Gap for Large-Area Thin-Film Optoelectronic Device Application

Herein, we report the one pot single step solvothermal synthesis of reduced grapheme oxide–cadmium zinc sulfide (RGO–Cd0.5Zn0.5S) composite. The reduction in graphene oxide (GO), synthesis of Cd0.5Zn0.5S (mentioned as CdZnS in the text) nanorod and decoration of CdZnS nanorods onto RGO sheet were done simultaneously. The structural, morphological and optical properties were studied thoroughly by different techniques, such as XRD, TEM, UV–Vis and PL. The PL intensity of CdZnS nanorods quenches significantly after the attachment of RGO, which confirms photoinduced charge transformation from CdZnS nanorods to RGO sheet through the interface of RGO–CdZnS. An excellent photocurrent generation in RGO–CdZnS thin-film device has been observed under simulated solar light irradiation. The photocurrent as well as photosensitivity increases linearly with the solar light intensity for all the composites. Our study establishes that the synergistic effect of RGO and CdZnS in the composite is capable of getting promising applications in the field of optoelectronic devising.

Role of vacancy defects in Al doped ZnO thin films for optoelectronic devices

We report on the electrical, optical and photoluminescence properties of industry-ready Al doped ZnO thin films grown by physical vapor deposition, and their evolution after annealing under vacuum. Doping ZnO with Al atoms increases the carrier density but also favors the formation of Zn vacancies, thereby inducing a saturation of the conductivity mechanism at high aluminum content. The electrical and optical properties of these thin layered materials are both improved by annealing process which creates oxygen vacancies that releases charge carriers thus improving the conductivity. This study underlines the effect of the formation of extrinsic and intrinsic defects in Al doped ZnO compound during the fabrication process. The quality and the optoelectronic response of the produced films are increased (up to 1.52 and 3.73 eV) and consistent with the industrial device requirements.

Solution-Phase Conformation and Dynamics of Conjugated Isoindigo-Based Donor-Acceptor Polymer Single Chains.

Conjugated polymers are the key material in thin-film organic optoelectronic devices due to the versatility of these molecules combined with their semiconducting properties. A molecular-scale understanding of conjugated polymers is important to the optimization of the thin-film morphology. We examine the solution-phase behavior of conjugated isoindigo-based donor-acceptor polymer single chains of various chain lengths using atomistic molecular dynamics simulations. Our simulations elucidate the transition from a rod-like to a coil-like conformation from an analysis of normal modes and persistence length. In addition, we find another transition based on the solvent environment, contrasting the coil-like conformation in a good solvent with a globule-like conformation in a poor solvent. Overall, our results provide valuable insights into the transition between conformational regimes for conjugated polymers as a function of both the chain length and the solvent environment, which will help to accurately parametrize higher level models.

Role of Absorbing Nanocrystal Cores in Soft Photonic Crystals: A Spectroscopy and SANS Study.

Periodic superstructures of plasmonic nanoparticles have attracted significant interest because they can support coupled plasmonic modes, making them interesting for plasmonic lasing, metamaterials, and as light-management structures in thin-film optoelectronic devices. We have recently shown that noble metal hydrogel core-shell colloids allow for the fabrication of highly ordered 2-dimensional plasmonic lattices that show surface lattice resonances as the result of plasmonic/diffractive coupling (Volk, K.; Fitzgerald, J. P. S.; Ruckdeschel, P.; Retsch, M.; König, T. A. F.; Karg, M. Reversible Tuning of Visible Wavelength Surface Lattice Resonances in Self-Assembled Hybrid Monolayers. Adv. Optical Mater. 2017, 5, 1600971, DOI: 10.1002/adom.201600971). In the present work, we study the photonic properties and structure of 3-dimensional crystalline superstructures of gold hydrogel core-shell colloids and their pitted counterparts without gold cores. We use far-field extinction spectroscopy to investigate the optical response of these superstructures. Narrow Bragg peaks are measured, independently of the presence or absence of the gold cores. All crystals show a significant reduction in low-wavelength scattering. This leads to a significant enhancement of the plasmonic properties of the samples prepared from gold-nanoparticle-containing core-shell colloids. Plasmonic/diffractive coupling is not evident, which we mostly attribute to the relatively small size of the gold cores limiting the effective coupling strength. Small-angle neutron scattering is applied to study the crystal structure. Bragg peaks of several orders clearly assignable to an fcc arrangement of the particles are observed for all crystalline samples in a broad range of volume fractions. Our results indicate that the nanocrystal cores do not influence the overall crystallization behavior or the crystal structure. These are important prerequisites for future studies on photonic materials built from core-shell particles, in particular, the development of new photonic materials from plasmonic nanocrystals.

Photovoltaic effect of ferroelectric Pb(Zr0.52,Ti0.48)O3 deposited on SrTiO3 buffered n-GaAs by laser molecular beam epitaxy

Ferroelectric Pb(Zr[Formula: see text],Ti[Formula: see text]O3(PZT) thin film was grown on [Formula: see text]-type GaAs (001) substrate with SrTiO3 (STO) buffer layer by laser molecular beam epitaxy (L-MBE). The epitaxial process of the STO was in situ monitored by reflection high-energy electron diffraction (RHEED). The crystallographical growth orientation relationship was revealed to be (002) [Formula: see text] PZT//(002) [Formula: see text] STO//(001) [Formula: see text] GaAs by RHEED and X-ray diffraction (XRD). It was found that a small lattice mismatch between PZT and GaAs with a 45[Formula: see text] in-plane rotation relationship can be formed by inserting of a buffer layer STO. Besides, the enhanced electrical properties of the heterostructure were obtained with the short-circuit photocurrent increased to 52[Formula: see text]mA/cm2 and the better power conversation efficiency increased by 20% under AM1.5[Formula: see text]G (100[Formula: see text]mW/cm[Formula: see text] illumination. The work could provide a way for the application of this kind of heterostructure with high photocurrent response in optoelectronic thin film devices.

Recycling indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs)

Electrodes in many thin-film optoelectronic devices are made of layers of indium tin oxide (ITO), which are suitably conductive, can be deposited by established methods, and are nearly transparent and colorless. Regrettably, indium is an uncommon element, its price continues to rise steeply, and producing ITO electrodes consumes a large fraction of the total energy needed to make thin-layer devices. Nevertheless, ITO electrodes are widely used in devices, both in research and industrial production. The growing commercial importance of these devices creates an urgent need for ways to ensure sustainability, such as by recovering ITO from devices no longer needed or even by recycling intact ITO electrodes. Recent work has shown that patterned ITO electrodes can be recovered from organic thin-film photovoltaic devices (OPVs) after simple exposure to water or dilute aqueous base, and then reused in new devices with essentially no decrease in performance. We now report that similar approaches can be used to recover and reuse ITO electrodes in organic light-emitting diodes (OLEDs), which are currently manufactured in much greater numbers than OPVs. Use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to study the surfaces of ITO electrodes recovered from OLEDs revealed minor changes in morphology, but the performance of the recycled electrodes in new OLEDs proved to be virtually unchanged, even after multiple cycles of recovery and reuse.

Molecular Organization of 2,1,3-Benzothiadiazoles in the Solid State.

Derivatives of 2,1,3-benzothiadiazole (1) are widely used in many areas of science and are particularly valuable as components of active layers in various thin-film optoelectronic devices. Even more effective benzothiadiazoles are likely to result if a deeper understanding of their preferred patterns of molecular association can be acquired. To provide new insight, we have analyzed the structures of compounds in which multiple benzothiadiazole units are attached to well-defined planar and nonplanar molecular cores. Our results show that molecular organization can be controlled in complex structures by using directional S···N bonding of benzothiadiazole units and other characteristic interactions. Moreover, the observed structures are distinctly different from those of analogous arenes. Replacing benzene rings in arenes by thiadiazoles thereby provides a strategy for making new compounds with extended systems of π-conjugation and unique patterns of molecular organization, including the ability to co-crystallize with the fullerenes C60 and C70.

Effect of sputtering power on crystallinity, intrinsic defects, and optical and electrical properties of Al-doped ZnO transparent conducting thin films for optoelectronic devices

The crystallinity and intrinsic defects of transparent conducting oxide (TCO) films have a high impact on their optical and electrical properties and therefore on the performance of devices incorporating such films, including flat panel displays, electro-optical devices, and solar cells. The optical and electrical properties of TCO films can be modified by tailoring their deposition parameters, which makes proper understanding of these parameters crucial. Magnetron sputtering is the most adaptable method for preparing TCO films used in industrial applications. In this study, we investigate the direct and inter-property correlation effects of sputtering power (PW) on the crystallinity, intrinsic defects, and optical and electrical properties of Al-doped ZnO (AZO) TCO films. All of the films were preferentially c-axis-oriented with a wurtzite structure and had an average transmittance of over 80% in the visible wavelength region. Scanning electron microscopy images revealed significantly increased AZO film grain sizes for PW ≥ 150 W, which may lead to increased conductivity, carrier concentration, and optical band gaps but decreased carrier mobility and in-plane compressive stress in AZO films. Photoluminescence results showed that, with increasing PW, the near band edge emission gradually dominates the defect-related emissions in which zinc interstitial (Zni), oxygen vacancy (VO), and oxygen interstitial (Oi) are possibly responsible for emissions at 3.08, 2.8, and 2.0 eV, respectively. The presence of Zni- and Oi-related emissions at PW ≥ 150 W indicates a slight increase in the presence of Al atoms substituted at Zn sites (AlZn). The presence of Oi at PW ≥ 150 W was also confirmed by X-ray photoelectron spectroscopy results. These results clearly show that the crystallinity and intrinsic-defect type of AZO films, which dominate their optical and electrical properties, may be controlled by PW. This understanding may facilitate the development of TCO-based optoelectronic devices for industrial production.

Highly Efficient and Stable Perovskite Solar Cells by Interfacial Engineering Using Solution-Processed Polymer Layer

Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.

Quantitative measurement of transport properties: Ag-doped nanocrystalline CdS thin films

This work highlights the transport properties of undoped and Ag doped nc-CdS thin films for optoelectronic devices.

Structural and Optical Properties of Nanostructured Cu2O Thin Films for Optoelectronic Devices

In the present work nanostructured cuprous oxide thin films were deposited on glass substrates by DC reactive magnetron sputtering by varying substrate temperature from 100 to 400 °C. X-ray diffraction patterns of Cu2O films deposited at different substrate temperatures show (1 1 1) orientation. The surface topography of the films was studied by atomic force microscopy (AFM). Energy dispersive spectroscopy (EDS) is carried out to determine the compositional analysis of thin films. The optical transmittance of Cu2O films increased with the increase of substrate temperature from 100 to 400 °C. Optical study is performed to calculate refractive index (n), absorption coefficient (α), extinction coefficient (k), optical band gap (Eg) and urbach energy (EU) using the transmission spectra in the wavelength range of 300 - 800 nm.

The electronic character of PTCDA thin films in comparison to other perylene-based organic semi-conductors: ab initio-, TD-DFT and semi-empirical computations of the opto-electronic properties of large aggregates.

Perylene-based compounds are promising materials for opto-electronic thin film devices but despite intense investigations, important details of their electronic structure are still under debate. For perylene-3,4,9,10-tetracarboxylic dianhydrid (PTCDA), the theoretical models predict a different relative energetic order of Frenkel and Charge Transfer (CT) states. Additionally, while one model indicates strong differences between PTCDA on one hand and other perylene-based compounds on the other, recent ab initio computations indicate electronic properties of all perylene-based compounds to resemble each other. Finally, the models disagree about the amount of mixing between CT and Frenkel states. Definitive answers to these questions are difficult because the approaches use various approximations. Up to date, the ab initio based methods employ rather small model systems and neglect environmental effects. In the present work, we improve our former approach by analyzing the effects of the various simplifications. In more detail, we increase the size of the model systems, include environmental effects and investigate the influence of exciton-phonon couplings on the absorption spectrum. The computations for larger aggregates were performed with the ZINDO/S approach, because benchmark computations show that it provides accurate vertical excitation energies for Frenkel, as well as CT states.

Formation of Perovskite Heterostructures by Ion Exchange

Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organolead-halide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI3) on methylammonium lead bromide (MAPbBr3) single crystals. The formation of the MAPbI3 layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI3 layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices.

Efficient semi-transparent planar perovskite solar cells using a ‘molecular glue’

Abstract Semitransparent perovskite solar cells are promising for their use in building integrated photovoltaic applications. One critical requirement for this application is the ability to thin down the perovskite absorber layer without sacrificing much of its optimum device efficiency. However, the production of high quality and defect-free very thin films of perovskites in an all low temperature solution processed p-i-n type device structure has been elusive due to the sensitivity of the materials crystallization dynamics and the variability of surface specific interactions. Here, we show that a unique surface passivation method based on thiourea vapor treatment can effectively produce high quality CH3NH3PbI3 thin-films up to 110 nm of thickness on top of a poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) surface. The thiourea vapor is believed to act as a ‘molecular glue’ by introducing favorable surface interactions in the PEDOT: PSS film with the soft, polarizable inorganic species (i.e. Pb and I) present in the thin-film precursors, and helps to achieve continuous, pinhole-free perovskite films. Using these solution-processed thin and high quality CH3NH3PbI3 films, semitransparent p-i-n planar perovskite-PCBM solar cells are fabricated with an evaporated transparent Ag top electrode, and the solar cells show a high average visible transparency of 34% at a power conversion efficiency level of 8.2%, which are among the highest reported performance values for semitransparent perovskite solar cells. This demonstration provides an excellent example of how surface passivation can be an effective approach to grow high quality all solution-processed semitransparent perovskite films, and will lead to more efficient thin-film optoelectronic devices in the future.

Electronic structures and enhanced optical properties of blue phosphorene/transition metal dichalcogenides van der Waals heterostructures

As a fast emerging topic, van der Waals (vdW) heterostructures have been proposed to modify two-dimensional layered materials with desired properties, thus greatly extending the applications of these materials. In this work, the stacking characteristics, electronic structures, band edge alignments, charge density distributions and optical properties of blue phosphorene/transition metal dichalcogenides (BlueP/TMDs) vdW heterostructures were systematically studied based on vdW corrected density functional theory. Interestingly, the valence band maximum and conduction band minimum are located in different parts of BlueP/MoSe2, BlueP/WS2 and BlueP/WSe2 heterostructures. The MoSe2, WS2 or WSe2 layer can be used as the electron donor and the BlueP layer can be used as the electron acceptor. We further found that the optical properties under visible-light irradiation of BlueP/TMDs vdW heterostructures are significantly improved. In particular, the predicted upper limit energy conversion efficiencies of BlueP/MoS2 and BlueP/MoSe2 heterostructures reach as large as 1.16% and 0.98%, respectively, suggesting their potential applications in efficient thin-film solar cells and optoelectronic devices.