Low-Temperature Cross-Linkable Hole Transport Materials for Solution-Processed Quantum Dot and Organic Light-Emitting Diodes with High Efficiency and Color Purity

Cross-linkable hole transport materials (HTMs) are ideal for improving the performance of solution-processed quantum dot light-emitting diodes (QLEDs) and phosphorescent light-emitting diodes (OLEDs). However, previously developed cross-linkable HTMs possessed poor hole transport properties, high cross-linking temperatures, and long curing times. To achieve efficient cross-linkable HTMs with high mobility, low cross-linking temperature, and short curing time, we designed and synthesized a series of low-temperature cross-linkable HTMs comprising dibenzofuran (DBF) and 4-divinyltriphenylamine (TPA) segments for highly efficient solution-processed QLEDs and OLEDs. The introduction of divinyl-functionalized TPA in various positions of the DBF core remarkably affected their chemical, physical, and electrochemical properties. In particular, cross-linked 4-(dibenzo­[b,d]­furan-3-yl)-N,N-bis­(4-vinylphenyl)­aniline (3-CDTPA) exhibited a deep highest occupied molecular orbital energy level (5.50 eV), high hole mobility (2.44 × 10–4 cm2 V–1 s–1), low cross-linking temperature (150 °C), and short curing time (30 min). Furthermore, a green QLED with 3-CDTPA as the hole transport layer (HTL) exhibited a notable maximum external quantum efficiency (EQEmax) of 18.59% with a remarkable maximum current efficiency (CEmax) of 78.48 cd A–1. In addition, solution-processed green OLEDs with 3-CDTPA showed excellent device performance with an EQEmax of 15.61%, a CEmax of 52.51 cd A–1, and outstanding CIE­(x, y) color coordinates of (0.29, 0.61). This is one of the highest reported EQEs and CEs with high color purity for green solution-processed QLEDs and OLEDs using a divinyl-functionalized cross-linked HTM as the HTL. We believe that this study provides a new strategy for designing and synthesizing practical cross-linakable HTMs with enhanced performance for highly efficient solution-processed QLEDs and OLEDs

Efficient Approach for Improving the Performance of Nonhalogenated Green Solvent-Processed Polymer Solar Cells via Ternary-Blend Strategy

The ternary-blend approach has the potential to enhance the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) by providing complementary absorption and efficient charge generation. Unfortunately, most PSCs are processed with toxic halogenated solvents, which are harmful to human health and the environment. Herein, we report the addition of a nonfullerene electron acceptor 3,9-bis­(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis­(4-hexylphenyl)-dithieno­[2,3-<i>d</i>:2′,3′-<i>d</i>′]-<i>s</i>-indaceno­[1,2-<i>b</i>:5,6-<i>b</i>′]­dithiophene (ITIC) to a binary blend (poly­[4,8-bis­(2-(4-(2-ethylhexyloxy)­3-fluorophenyl)-5-thienyl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-<i>alt</i>-1,3-bis­(4-octylthien-2-yl)-5-(2-ethylhexyl)­thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione] (P1):[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), PCE = 8.07%) to produce an efficient nonhalogenated green solvent-processed ternary PSC system with a high PCE of 10.11%. The estimated wetting coefficient value (0.086) for the ternary blend suggests that ITIC could be located at the P1:PC₇₁BM interface, resulting in efficient charge generation and charge transport. In addition, the improved current density, sustained open-circuit voltage and PCE of the optimized ternary PSCs were highly correlated with their better external quantum efficiency response and flat-band potential value obtained from the Mott–Schottky analysis. In addition, the ternary PSCs also showed excellent ambient stability over 720 h. Therefore, our results demonstrate the combination of fullerene and nonfullerene acceptors in ternary blend as an efficient approach to improve the performance of eco-friendly solvent-processed PSCs with long-term stability

Synthesis, Characterization, and Photovoltaic Properties of 4,8-Dithienylbenzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophene-Based Donor–Acceptor Polymers with New Polymerization and 2D Conjugation Extension Pathways: A Potential Donor Building Block for High Performance and Stable Inverted Organic Solar Cells

In all the previously reported 4,8-dithienylbenzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (DTBDT)-based π-conjugated polymers, the polymerization and two-dimensional (2D) conjugation extension pathways were through the thiophenes fused to the phenyl core of DTBDT and through the thiophenes linked to the benzene core of DTBDT, respectively (BDT-directed DTBDT). Herein, with the aim of discovering another potential way to introduce the DTBDT motif in the donor–acceptor alternating polymer structure, we first report the synthesis of three new π-conjugated polymers, <b>P1</b>, <b>P2</b>, and <b>P3</b>, with a modified DTBDT building block as a donor unit. This modification results in new polymerization and 2D conjugation extension pathways for the polymers through the thiophenes linked to the benzene core of DTBDT and through the thiophenes fused to the phenyl core of the DTBDT, respectively (dithienylbenzene-directed DTBDT). Although these modified polymerization pathways of DTBDT result in less delocalized conjugation along the dithienylbenzene direction, the optical and electrochemical properties reveal that the electron-donating property of dithienylbenzene-directed DTBDT was strong enough to generate strong intramolecular charge transfer (ICT) and maintain low-lying highest occupied molecular orbital (HOMO) energy levels (−5.21 to −5.28 eV) for high air stability. Inverted organic solar cells (IOSCs) were fabricated with the configuration of ITO/ZnO/polymer:PC₇₁BM/PEDOT:PSS/Ag. By systematic optimization of the performance of the IOSCs using polar solvent treatment, the IOSCs based on <b>P1</b>, <b>P2</b>, and <b>P3</b> displayed promising power conversion efficiencies (PCE) of 6.31, 5.65, and 7.10%, respectively, which compare well with the PCE of already reported BDT-directed DTBDT-based polymers. More importantly, the stability of the IOSCs was demonstrated by their retention of 83% PCE after ambient storage for 30 days. These study results revealed the promising potential of the proposed molecular design strategy for introducing new 2D conjugation extension and polymerization pathways for a DTBDT unit for high performance and stable IOSCs. This strategy can be applied to the judicious molecular design of new polymeric materials for achieving high PCE

Preparation of Flexible Organic Solar Cells with Highly Conductive and Transparent Metal-Oxide Multilayer Electrodes Based on Silver Oxide

We report that significantly more transparent yet comparably conductive AgO_<i>x</i> films, when compared to Ag films, are synthesized by the inclusion of a remarkably small amount of oxygen (i.e., 2 or 3 atom %) in thin Ag films. An 8 nm thick AgO_<i>x</i> (O/Ag = 2.4 atom %) film embedded between 30 nm thick ITO films (ITO/AgO_<i>x</i>/ITO) achieves a transmittance improvement of 30% when compared to a conventional ITO/Ag/ITO electrode with the same configuration by retaining the sheet resistance in the range of 10–20 Ω sq^–1. The high transmittance provides an excellent opportunity to improve the power-conversion efficiency of organic solar cells (OSCs) by successfully matching the transmittance spectral range of the electrode to the optimal absorption region of low band gap photoactive polymers, which is highly limited in OSCs utilizing conventional ITO/Ag/ITO electrodes. An improvement of the power-conversion efficiency from 4.72 to 5.88% is achieved from highly flexible organic solar cells (OSCs) fabricated on poly­(ethylene terephthalate) polymer substrates by replacing the conventional ITO/Ag/ITO electrode with the ITO/AgO_<i>x</i>/ITO electrode. This novel transparent electrode can facilitate a cost-effective, high-throughput, room-temperature fabrication solution for producing large-area flexible OSCs on heat-sensitive polymer substrates with excellent power-conversion efficiencies

Alkoxyphenylthiophene Linked Benzodithiophene Based Medium Band Gap Polymers for Organic Photovoltaics: Efficiency Improvement upon Methanol Treatment Depends on the Planarity of Backbone

Two donor–acceptor (D–A) medium band gap polymers, P1 and P2, alkoxyphenylthiophene (APTh) linked benzodithiophene (BDT) as an electron-rich unit and 1,3-di­(2′-bromothien-5′-yl)-5-(2-ethylhexyl)­thieno­[3,4-c]­pyrrole-4,6-dione (TPD) (A1) or [5,6-bis­(octyloxy)-4,7-di­(thiophen-2-yl)­benzo­[c]­[1,2,5]­thiadiazole] (BT) (A2) as an electron-deficient unit, have successfully been synthesized via microwave-assisted Stille polymerization and utilized for bulk heterojunction (BHJ) polymer solar cells (PSCs). P1 shows a well-distinguished absorption shoulder between 590 and 620 nm attributed to the π–π stacking of a polymer backbone; such kind of absorption shoulder is not observed in P2, indicating that the P1 has more planar structure than that of P2. This is due to the fact that the sulfur atom of thiophene spacer and the oxygen atom of carbonyl groups in TPD have more pronounced intramolecular noncovalent interactions (INCI) in P1 than that of the sulfur atom of thiophene spacer and the oxygen atom of alkoxy groups of BT in P2. The bulk heterojunction polymer solar cells (BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer (P1 or P2):PC71BM/LiF/Al. The P1 device shows better photovoltaic performance with open-circuit voltage (Voc) of 0.91 V and the power conversion efficiency (PCE) of 4.19% than the P2 device (Voc: 0.71 V; PCE: 1.88%) in neat blend films under the illumination of AM 1.5G (100 mW/cm2). Upon treating the active layers containing P1 and P2 with methanol, the PCE of the P1 device is increased from 4.19 to 7.14%. In contrast, the PCE of the P2 device is decreased from 1.88 to 1.82%. Space charge limited current mobility, atomic force microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and impedance spectroscopy studies strongly support the enhanced PCE for the P1 device is attributed to the increased mobility, nanoscale morphology, and reduced resistance upon methanol treatment; these favorable properties for the P1 polymer are highly correlated with the planarity of the backbone

Dopant-Free Hydrogenated Amorphous Silicon Thin-Film Solar Cells Using Molybdenum Oxide and Lithium Fluoride

Toxic doping gases are usually used to produce hydrogenated amorphous silicon (a-Si:H) layers in thin-film solar cells (TFSCs). Hence, an alternative structure that avoids the use of toxic gases is desirable. In this work, we replaced both the <i>p</i>-type-a-Si:H and <i>n</i>-type-a-Si:H layers simultaneously in a normal TFSC to form a structure that is dopant-free. Molybdenum oxide (MoO₃) and lithium fluoride were used as the <i>p</i>-type and <i>n</i>-type layers, respectively. The effects of the deposition method and the thickness of the MoO₃ layer on the device performance were investigated. The power-conversion efficiency of the optimized hybrid solar cell reached a maximum of 7.08%, which is remarkable considering the novel structure of the dopant-free devices. The light stability of the devices with and without MoO₃ was also compared: the light stability of the device with MoO₃ was found to be much better than that of the device without MoO₃ and with <i>p-i-n</i> Si layers. This was ascribed to the insignificant number of defect sites generated by the nondoping elements, which led to a less contaminated, more compact, and smoother oxide surface, resulting in an increase in the electron lifetime and improved light stability. This work opens up a new direction toward the development of a truly dopant-free device that does not involve the use of toxic gases during fabrication and provides the potential for further enhancement of the efficiency of future dopant-free solar cells