Electron Transport Layers Based on Oligo(ethylene glycol)-Incorporated Polymers Enabling Reproducible Fabrication of High-Performance Organic Solar Cells

Highly Conductive Alcohol-Processable n-Type Conducting Polymer Enabled by Finely Tuned Electrostatic Interactions for Green Organic Electronics

Solution-processable conducting polymers open up a new era in organic electronics, fundamentally altering the processing methods of electronic devices. P-type conducting polymers, exemplified by aqueous solution-processed poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS), have been successfully commercialized. However, the performance of electron-transporting (n-type) materials remains considerably poorer. One of the primary challenges lies in striking a balance between conductivity and solvent processability. At present, most n-type conducting polymers necessitate toxic solvents for processing, which contradicts environmentally sustainable principles and impedes their potential for large-scale industrial applications. Herein, we developed an alcohol-processable high-performance n-type conducting polymer, poly(3,7-dihydrobenzo[1,2-b : 4,5-b']difuran-2,6-dione): poly(2-ethyl-2-oxazoline) (PBFDO : PEOx), which utilized electrostatic interactions between PEOx and PBFDO to simultaneously achieve high conductivity and alcohol-processability. The PBFDO : PEOx films exhibited remarkable electrical conductivity exceeding 1000 S cm-1 with outstanding stability even at temperatures up to 250 °C, establishing it as a prominent green solvent-processed n-type conducting polymer that rivals the most advanced p-type counterparts. Various applications including organic thermoelectric, electrochemical transistor, and electrochromic devices were showcased, highlighting the broad potential of PBFDO : PEOx in advancing green organic electronics.

A Polymeric Two-in-One Electron Transport Layer and Transparent Electrode for Efficient Indoor All-Organic Solar Cells

Transparent electrodes (TEs) are vital in optoelectronic devices, enabling the interaction of light and charges. While indium tin oxide (ITO) has traditionally served as a benchmark TE, its high cost prompts the exploration of alternatives to optimize electrode characteristics and improve device efficiencies. Conducting polymers, which combine polymer advantages with metal-like conductivity, emerge as a promising solution for TEs. This work introduces a two-in-one electron transport layer (ETL) and TE based on films of polyethylenimine ethoxylated (PEIE)-modified poly(benzodifurandione) (PBFDO). These PEIE-modified PBFDO layers exhibit a unique combination of properties, including low sheet resistance (130 Ω sq-1), low work function (4.2 eV), and high optical transparency (>85% in the UV-vis-NIR range). In contrast to commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the doping level of PBFDO remains unaffected by the PEIE treatment, as verified through UV-vis-NIR absorption and X-ray photoelectron spectroscopy measurements. When employed as a two-in-one ETL/TE in organic solar cells, the PEIE-modified PBFDO electrode exhibits performance comparable to conventional ITO electrodes. Moreover, this work demonstrates all-organic solar cells with record-high power conversion efficiencies of >15.1% under indoor lighting conditions. These findings hold promise for the development of fully printed, all-organic optoelectronic devices.

In situ Blending For Co-Deposition of Electron Transport and Perovskite Layers Enables Over 24% Efficiency Stable Conventional Solar Cells

Simplifying the manufacturing processes of multilayered high-performance perovskite solar cells (PSCs) is yet of vital importance for their cost-effective production. Herein, an in situ blending strategy is presented for co-deposition of electron transport layer (ETL) and perovskite absorber by incorporating (3-(7-butyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo- [lmn][3,8]phenanthrolin-2(1H)-yl)propyl)phosphonic acid (NDP) into the perovskite precursor solutions. The phosphonic acid-like anchoring group coupled with its large molecular size drives the migration of NDP toward indium tin oxide (ITO) surface to form a distinct ETL during perovskite film forming. This strategy circumvents the critical wetting issue and simultaneously improves the interfacial charge collection efficiencies. Consequently, n-i-p PSCs based on in situ blended NDP achieve a champion power conversion efficiency (PCE) of 24.01%, which is one of the highest values for PSCs using organic ETLs. This performance is notably higher than that of ETL-free (21.19%) and independently spin-coated (21.42%) counterparts. More encouragingly, the in situ blending strategy dramatically enhances the device stability under harsh conditions by retaining over 90% of initial efficiencies after 250 h in 100 °C or 65% humidity storage. Moreover, this strategy is universally adaptable to various perovskite compositions, device architectures, and electron transport materials (ETMs), showing great potential for applications in diverse optoelectronic devices.

A Highly Conductive n-Type Conjugated Polymer Synthesized in Water

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a benchmark hole-transporting (p-type) polymer that finds applications in diverse electronic devices. Most of its success is due to its facile synthesis in water, exceptional processability from aqueous solutions, and outstanding electrical performance in ambient. Applications in fields like (opto-)electronics, bioelectronics, and energy harvesting/storage devices often necessitate the complementary use of both p-type and n-type (electron-transporting) materials. However, the availability of n-type materials amenable to water-based polymerization and processing remains limited. Herein, we present a novel synthesis method enabling direct polymerization in water, yielding a highly conductive, water-processable n-type conjugated polymer, namely, poly[(2,2'-(2,5-dihydroxy-1,4-phenylene)diacetic acid)-stat-3,7-dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione] (PDADF), with remarkable electrical conductivity as high as 66 S cm-1, ranking among the highest for n-type polymers processed using green solvents. The new n-type polymer PDADF also exhibits outstanding stability, maintaining 90% of its initial conductivity after 146 days of storage in air. Our synthetic approach, along with the novel polymer it yields, promises significant advancements for the sustainable development of organic electronic materials and devices.

Interface Engineering for Highly Efficient Organic Solar Cells

Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

High-Performance Organic Electrochemical Transistors Achieved by Optimizing Structural and Energetic Ordering of Diketopyrrolopyrrole-Based Polymers

For optimizing steady-state performance in organic electrochemical transistors (OECTs), both molecular design and structural alignment approaches must work in tandem to minimize energetic and microstructural disorders in polymeric mixed ionic-electronic conductor films. Herein, a series of poly(diketopyrrolopyrrole)s bearing various lengths of aliphatic-glycol hybrid side chains (PDPP-mEG; m = 2-5) is developed to achieve high-performance p-type OECTs. PDPP-4EG polymer with the optimized length of side chains exhibits excellent crystallinity owing to enhanced lamellar and backbone interactions. Furthermore, the improved structural ordering in PDPP-4EG films significantly decreases trap state density and energetic disorder. Consequently, PDPP-4EG-based OECT devices produce a mobility-volumetric capacitance product ([µC*]) of 702 F V-1 cm-1 s-1 and a hole mobility of 6.49 ± 0.60 cm2 V-1 s-1 . Finally, for achieving the optimal structural ordering along the OECT channel direction, a floating film transfer method is employed to reinforce the unidirectional orientation of polymer chains, leading to a substantially increased figure-of-merit [µC*] to over 800 F V-1 cm-1 s-1 . The research demonstrates the importance of side chain engineering of polymeric mixed ionic-electronic conductors in conjunction with their anisotropic microstructural optimization to maximize OECT characteristics.

Polymerized naphthalimide derivatives as remarkable electron-transport layers for inverted organic solar cells

Two polymerized naphthalimide derivatives, named as N-TBHOB and N-DBH, have been prepared by quaternization, which exhibit excellent performance as electron-transport layers (ETLs) in inverted organic solar cells (i-OSCs). The results indicate N-TBHOB with a reticulated structure owns a superior performance on electron extraction, electron transport, thickness tolerance and less carrier recombination compared with N-DBH with linear structure. The i-OSCs based on N-TBHOB with PTB7-Th:PC₇₁ BM as the active layer achieve PCEs of 10.72% and 10.03% under the thickness of 11 and 48 nm respectively, which indicates N-TBHOB possesses better thickness tolerance than most of organic ETLs in i-OSCs. N-TBHOB also shows more competent performance than N-DBH and ZnO in non-fullerene i-OSCs for comprehensively improved J_sc , V_oc and FF values. Its i-OSC with PM6:Y6 blend presents a high PCE of 16.78%. The study provides an efficient strategy to prepare ETLs by combining conjugated and nonconjugated backbone with a reticulated structure for high-performance i-OSCs. This article is protected by copyright. All rights reserved.

A novel naphthalene diimide-based conjugated polymer as an electron transport material for non-fullerene organic solar cells

The only aliphatic amino side chains at the N-position of naphthalene diimide endow novel electron transport materials with good film-forming and interface modification properties, which improves the device efficiency and stability.

Precision Synthesis of Conjugated Polymers Using the Kumada Methodology

Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers.In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer-catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.

Boosting the Efficiency of Non-fullerene Organic Solar Cells via a Simple Cathode Modification Method

This work demonstrates a simple yet effective method to significantly improve the power conversion efficiency (PCE) of highly efficient non-fullerene organic solar cells by mixing two electron transport materials. The new electron transport layer shows an energy level better aligned with the active layer and an improved morphology that could reduce the active layer-electrode contact. These improvements lead to enhanced charge extraction, better charge selectivity, suppressed exciton recombination, and finally a boosted PCE in the PM6:Y6-based solar cells. When applied in conjunction with the non-halogenated solvent-processed PM6:PY-IT-based active layer, the mixed ETL also gives rise to a leading result for binary all-polymer solar cells (PCE of >16%) with a concurrent increase in V_OC, J_SC, and FF.