Improving Electron Transport in a Double-Cable Conjugated Polymer via Parallel Perylenetriimide Design

Synthesis of chitosan-based perylene dye material for photovoltaic solar-cell application

Renewable energy, such as solar energy, is infinite, readily available, and has extensive applications. Dye-sensitized solar cells (DSSCs) have been well developed; thus, they can be developed with low production costs, high efficiency, and facile manufacturing techniques. This study proposes a novel chitosan biopolymer-based perylene dye; the dye is modified by chitosan with perylene-3,4,9,10-tetracarboxylic anhydride using a one-pot acylation of nitrogen nucleophiles for DSSCs. The chitosan biopolymer-based perylene dyes were characterized using attenuated total reflection infrared spectroscopy, solid-state 13C CP-TOSS nuclear magnetic resonance spectroscopy, X-ray powder diffraction analysis, thermogravimetric analysis, X-ray photoelectron spectrometry, and high-resolution field-emission scanning electron microscopy. The ultraviolet-visible and fluorescence spectroscopy of chitosan biopolymer-based perylene dye exhibited a red-shift compared with perylene-3,4,9,10-tetracarboxylic anhydride and chitosan. The DSSC properties of chitosan biopolymer-based perylene dye were investigated, and it exhibited a 2.022 % power-conversion efficiency. Thus, this promising chitosan biopolymer-based perylene dye may have potential applications in solar-cell technology.

Desirable Uniformity and Reproducibility of Electron Transport in Single-Component Organic Solar Cells

Despite the simplified fabrication process and desirable microstructural stability, the limited charge transport properties of block copolymers and double-cable conjugated polymers hinder the overall performance of single-component photovoltaic devices. Based on the key distinction in the donor (D)-acceptor (A) bonding patterns between single-component and bulk heterojunction (BHJ) devices, rationalizing the difference between the transport mechanisms is crucial to understanding the structure-property correlation. Herein, the barrier formed between the D-A covalent bond that hinders electron transport in a series of single-component photovoltaic devices is investigated. The electron transport in block copolymer-based devices is strongly dependent on the electric field. However, these devices demonstrate exceptional advantages with respect to the charge transport properties, involving high stability to compositional variations, improved film uniformity, and device reproducibility. This work not only illustrates the specific charge transport behavior in block copolymer-based devices but also clarifies the enormous commercial viability of large-area single-component organic solar cells (SCOSCs).

Double-Cable Conjugated Polymers with Pendent Near-Infrared Electron Acceptors for Single-Component Organic Solar Cells

Double-cable conjugated polymers with near-infrared (NIR) electron acceptors are synthesized for use in single-component organic solar cells (SCOSCs). Through the development of a judicious synthetic pathway, the highly sensitive nature of the 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC)-based electron acceptors in basic and protonic solvents is overcome. In addition, an asymmetric design motif is adopted to optimize the packing of donor and acceptor segments, enhancing charge separation efficiency. As such, the new double-cable polymers are successfully applied in SCOSCs, providing an efficiency of over 10 % with a broad photo response from 300 to 850 nm and exhibiting excellent thermal/light stability. These results demonstrate the powerful design of NIR-acceptor-based double-cable polymers and will enable SCOSCs to enter a new stage.

Revealing the Side-Chain-Dependent Ordering Transition of Highly Crystalline Double-Cable Conjugated Polymers

We developed a series of highly crystalline double-cable conjugated polymers for application in single-component organic solar cells (SCOSCs). These polymers contain conjugated backbones as electron donor and pendant perylene bisimide units (PBIs) as electron acceptor. PBIs are connected to the backbone via alkyl units varying from hexyl (C₆ H₁₂ ) to eicosyl (C₂₀ H₄₀ ) as flexible linkers. For double-cable polymers with short linkers, the PBIs tend to stack in a head-to-head fashion, resulting in large d-spacings (e.g. 64 Å for the polymer P12 with C₁₂ H₂₄ linker) along the lamellar stacking direction. When the length of the linker groups is longer than a certain length, the PBIs instead adopt a more ordered packing likely via H-aggregation, resulting in short d-spacings (e.g. 50 Å for the polymer P16 with C₁₆ H₃₂ linker). This work highlights the importance of linker length on the molecular packing of the acceptor units and the influences on the photovoltaic performance of SCOSCs.

Narrow-Bandgap Single-Component Polymer Solar Cells with Approaching 9% Efficiency

Two narrow-bandgap block conjugated polymers with a (D1-A1)-(D2-A2) backbone architecture, namely PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6, are designed and synthesized for single-component organic solar cells (SCOSCs). Both polymers contain same donor polymer, PBDB-T, but different polymerized nonfullerene molecule acceptors. Compared to all previously reported materials for SCOSCs, PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 exhibit narrower bandgap for better light harvesting. When incorporated into SCOSCs, the short-circuit current density (J_sc ) is significantly improved to over 15 mA cm^-2 , together with a record-high power conversion efficiency (PCE) of 8.64%. Moreover, these block copolymers exhibit low energy loss due to high charge transfer (CT) states (E_ct ) plus small non-radiative loss (0.26 eV), and improved stability under both ambient condition and continuous 80 °C thermal stresses for over 1000 h. Determination of the charge carrier dynamics and film morphology in these SCOSCs reveals increased carrier recombination, relative to binary bulk-heterojunction devices, which is mainly due to reduced ordering of both donor and acceptor fragments. The close structural relationship between block polymers and their binary counterparts also provides an excellent framework to explore further molecular features that impact the photovoltaic performance and boost the state-of-the-art efficiency of SCOSCs.

Double-Cable Conjugated Polymers with Pendant Rylene Diimides for Single-Component Organic Solar Cells

ConspectusConjugated polymers for application in organic solar cells (OSCs) have been developed from poly(phenylenevinylene) to poly(3-hexylthiophene) and then to "donor-acceptor" structures, providing power conversion efficiencies (PCEs) over 18% when blending with the electron acceptor as a two-component photoactive layer. Besides, graft-structural double-cable conjugated polymers that use an electron donor as conjugated backbones and an electron acceptor as pendant side units are one kind of conjugated polymer, in which charge carriers are generated in a single polymer. Therefore, double-cable conjugated polymers can be used as a single photoactive layer in single-component OSCs (SCOSCs). The covalently linked electron donor and acceptor enable double-cable polymers to maintain stable microstructures during long-term operation compared to two-component systems, which is very important for OSCs toward large-area applications. However, SCOSCs based on double-cable conjugated polymers provided PCEs below 3% in a long period, which is lagging far behind PCEs of two-component OSCs. The key reason for this is the limited number of chemical structures and the difficulty to tune the morphology in these polymers.In this Account, we provide an overview about our efforts on developing new double-cable conjugated polymers with rylene diimides as side units, and how to realize high PCEs in SCOSC devices. The studies start from developing a "functionalization-polymerization" method to synthesize the polymers containing rylene diimide acceptors, so that large amounts of double-cable conjugated polymers with distinct physical and electrochemical properties were obtained. Then, we will discuss how to control the nanophase separation in the crystalline region and optimize the miscibility in the amorphous region of double-cable polymers, simultaneously facilitating exciton dissociation and charge transport. With these efforts, a high PCE of 8.4% has been obtained, representing the record PCE in SCOSCs. In addition, the physical process and the stability of SCOSCs will be discussed. We hope that this account will inspire many innovative studies in this field and push the PCEs of SCOSCs to a new stage.

Synthesis and self-assembly of a penta[60]fullerene bearing benzo[ghi]perylenetriimide units

New penta(organo)fullerene bearing five benzo[ghi]perylenetriimide units has been synthesized to form self-assemblies through π–π interactions.

Miscibility-Controlled Phase Separation in Double-Cable Conjugated Polymers for Single-Component Organic Solar Cells with Efficiencies over 8

A record power conversion efficiency of 8.40 % was obtained in single-component organic solar cells (SCOSCs) based on double-cable conjugated polymers. This is realized based on exciton separation playing the same role as charge transport in SCOSCs. Two double-cable conjugated polymers were designed with almost identical conjugated backbones and electron-withdrawing side units, but extra Cl atoms had different positions on the conjugated backbones. When Cl atoms were positioned at the main chains, the polymer formed the twist backbones, enabling better miscibility with the naphthalene diimide side units. This improves the interface contact between conjugated backbones and side units, resulting in efficient conversion of excitons into free charges. These findings reveal the importance of charge generation process in SCOSCs and suggest a strategy to improve this process: controlling miscibility between conjugated backbones and aromatic side units in double-cable conjugated polymers.

Significantly Improved Morphology and Efficiency of Nonhalogenated Solvent-Processed Solar Cells Derived from a Conjugated Donor-Acceptor Block Copolymer

A highly crystalline conjugated donor (D)-acceptor (A) block copolymer (PBDT2T-b-N2200) that has good solubility in nonhalogenated solvents is successfully synthesized. PBDT2T-b-N2200 shows a broad complementary absorption behavior owing to a wide-band gap donor (PBDT2T) present as a D-block and a narrow-band gap acceptor (N2200) present as an A-block. Polymer solar cells (PSCs) with conjugated block copolymer (CBCP) are fabricated using a toluene solution and PSC created with an annealed film showing the highest power conversion efficiency of 6.43%, which is 2.4 times higher than that made with an annealed blend film of PBDT2T and N2200. Compared to the blend film, the PBDT2T-b-N2200 film exhibits a highly improved surface and internal morphology, as well as a faster photoluminescence decay lifetime, indicating a more efficient photoinduced electron transfer. In addition, the PBDT2T-b-N2200 film shows high crystallinity through an effective self-assembly of each block during thermal annealing and a predominant face-on chain orientation favorable to a vertical-type PSC. Moreover, the CBCP-based PSCs exhibit an excellent shelf-life time of over 1020 h owing to their morphological stability. From these results, a D-A block copolymer system is one of the efficient strategies to improve miscibility and morphological stability in all polymer blend systems.

Thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers for organic solar cells

Thieno[3,4-c]pyrrole-4,6-dione (TPD) based conjugated polymers as an electron donor, acceptor and single-component for application in organic solar cells in the past ten years have been intensively reviewed in this Feature Article.

Effect of Electron-Acceptor Content on the Efficiency of Regioregular Double-Cable Thiophene Copolymers in Single-Material Organic Solar Cells

Three regioregular thiophenic copolymers, characterized by a bromine atom or a C₆₀-fullerene group at different molar ratios at the end of a decamethylenic plastifying side chain, have been successfully synthesized using a straightforward postpolymerization functionalization procedure based on a Grignard coupling reaction. Owing to their good solubility in common organic solvents, the products were fully characterized using chromatographic, spectroscopic, thermal, and morphological techniques and used as single materials in the photoactive layers of organic solar cells. The photoconversion efficiencies obtained with copolymers were compared with those of a reference cell prepared using a physical blend of the precursor homopolymer and [6,6]-phenyl-C₆₁-butyric acid methyl ester. The best results were obtained with COP2, the copolymer with a 21% molar content of C₆₀-functionalized side chains. The use of the double-cable polymer made possible an enhanced control on the nanomorphology of the active blend, thus reducing phase-segregation phenomena as well as the macroscale separation between the electron-acceptor and -donor components, yielding a power conversion efficiency higher than that of the reference cell (4.05 vs 3.68%). Moreover, the presence of the halogen group was exploited for the photo-cross-linking of the active layer immediately after the thermal annealing procedure. The cross-linked samples showed an increased stability over time, leading to good efficiencies even after 120 h of accelerated aging: this was a key feature for the widespread practical applicability of the prepared devices.