Controlling Polarity of Organic Bulk Heterojunction Field-Effect Transistors via Solvent Additives

The effect of additives such as 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) on the electronic structures, charge transport and phase separation of small-molecule-based bulk heterojunction (BHJ) films was investigated. Charge transport properties of the BHJ layers significantly changed via the introduction of additives, even though the molecular energy levels remained unchanged. X-ray photoelectron microscopy (XPM) images show the distribution of each phase of the blend films upon the use of additives. The CN additive, in particular, results in a well-percolated network through the active layer

Solution-Processed Organic Solar Cells from Dye Molecules: An Investigation of Diketopyrrolopyrrole:Vinazene Heterojunctions

Although one of the most attractive aspects of organic solar cells is their low cost and ease of fabrication, the active materials incorporated into the vast majority of reported bulk heterojunction (BHJ) solar cells include a semiconducting polymer and a fullerene derivative, classes of materials which are both typically difficult and expensive to prepare. In this study, we demonstrate that effective BHJs can be fabricated from two easily synthesized dye molecules. Solar cells incorporating a diketopyrrolopyrrole (DPP)-based molecule as a donor and a dicyanoimidazole (Vinazene) acceptor function as an active layer in BHJ solar cells, producing relatively high open circuit voltages and power conversion efficiencies (PCEs) up to 1.1%. Atomic force microscope images of the films show that active layers are rough and apparently have large donor and acceptor domains on the surface, whereas photoluminescence of the blends is incompletely quenched, suggesting that higher PCEs might be obtained if the morphology could be improved to yield smaller domain sizes and a larger interfacial area between donor and acceptor phases

Stable ZnS Electron Transport Layer for High-Performance Inverted Cadmium-Free Quantum Dot Light-Emitting Diodes

We report high-efficiency and long-lifetime inverted green cadmium-free (InP-based) quantum dot light-emitting diodes (QLEDs) using a stable ZnO/ZnS cascaded electron transport layer (ETL). We have successfully developed a strategy to spin-coat stable ZnS ETLs with a relatively higher conduction band minimum (CBM) and lower electron mobility than that of ZnO, which leads to balanced carrier injection and an improved device lifetime. Analysis shows that by using the ZnO/ZnS cascaded ETL, electron injection is reduced, resulting in an improved charge balance in the QD layer and suppressed exciton quenching, which preserves the emission properties of QDs. Optimized devices with ZnO/ZnS cascaded ETLs show a maximum external quantum efficiency of 10.8% and a maximum current efficiency of 37.5 cd/A; these efficiency values are an almost 2.2-fold improvement compared to those of reference devices without ZnS. The QLED devices also showed a remarkably long lifetime (LT70) of 265 h at an initial luminance of 1000 cd/m2. The predicted half-lifetime (LT50) at 100 cd/m2 is 60,255 h, which, to our knowledge, is currently the longest lifetime yet reported for InP-based green QLEDs

Influence of Structural Variation on the Solid-State Properties of Diketopyrrolopyrrole-Based Oligophenylenethiophenes: Single-Crystal Structures, Thermal Properties, Optical Bandgaps, Energy Levels, Film Morphology, and Hole Mobility

Five new compounds, based on diketopyrrolopyrrole (DPP) and phenylene thiophene (PT) moieties, were synthesized to investigate the effect of structural variations on solid state properties, such as single-crystal structures, optical absorption, energy levels, thermal phase transitions, film morphology, and hole mobility. The molecular structures were modified by means of (i) backbone length by changing the number of thiophenes on both sides of DPP, (ii) alkyl substitution (<i>n</i>-hexyl or ethylhexyl) on DPP, and (iii) the presence of an <i>n</i>-hexyl group at the end of the molecular backbone. These DPP-based oligophenylenethiophenes were systematically characterized by UV–visible spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and hole-only diodes. Single-crystal structures were provided to probe insight into structure–property relationships at a molecule level resolution. This work demonstrates the significance of alkyl substitution as well as backbone length in tuning material’s solid-state properties

Influence of Structural Variation on the Solid-State Properties of Diketopyrrolopyrrole-Based Oligophenylenethiophenes: Single-Crystal Structures, Thermal Properties, Optical Bandgaps, Energy Levels, Film Morphology, and Hole Mobility

Five new compounds, based on diketopyrrolopyrrole (DPP) and phenylene thiophene (PT) moieties, were synthesized to investigate the effect of structural variations on solid state properties, such as single-crystal structures, optical absorption, energy levels, thermal phase transitions, film morphology, and hole mobility. The molecular structures were modified by means of (i) backbone length by changing the number of thiophenes on both sides of DPP, (ii) alkyl substitution (<i>n</i>-hexyl or ethylhexyl) on DPP, and (iii) the presence of an <i>n</i>-hexyl group at the end of the molecular backbone. These DPP-based oligophenylenethiophenes were systematically characterized by UV–visible spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and hole-only diodes. Single-crystal structures were provided to probe insight into structure–property relationships at a molecule level resolution. This work demonstrates the significance of alkyl substitution as well as backbone length in tuning material’s solid-state properties

Effects of Ionic Liquid Molecules in Hybrid PbS Quantum Dot–Organic Solar Cells

We investigated the effect of ionic liquid molecules (ILMs) in hybrid quantum dot-organic solar cells (HyQD-OSCs). The insertion of an ILM layer between PbS and phenyl-C61-butyric acid methyl ester (PCBM) can shift the band edge of PCBM closer to the vacuum level of PbS due to spontaneous dipole polarization. Because of this new architecture, improvements in device performance were achieved, including increases in open-circuit voltage (<i>V</i>_OC, from 0.41 V to 0.49 V), fill factor (FF, from 0.48 to 0.59), and power conversion efficiency (PCE, from 1.62% to 2.21%), compared to reference devices under AM 1.5G illumination at 100 mW cm^–2. We observed that treatment of the PbS layer with ILMs causes a significant increase in work function from 3.58 eV to 3.93 eV. Furthermore, the ILMs layer minimizes the contact resistance between PbS and PCBM due to the improved compatibility between the two layers, confirmed as a decrease in charge transfer resistance, as measured by electrical impedance spectroscopy