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).

N-Annulated Perylene Bisimide-Based Double-Cable Polymers with Open-Circuit Voltage Approaching 1.20 V in Single-Component Organic Solar Cells

In this work, we have introduced single/double-sided N-annulated perylene bisimide (PBI) with deep energy levels into double-cable polymers with poly[1-(5-(4,8-bis(4-chloro-5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-5,7-bis(2-ethylhexyl)-3-(5-methylthiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c']dithiophene-4,8-dione] (PBDB-T-Cl) as a donor backbone, marking as s-PPNR and as-PPNR, according to the molecular symmetry. Both double-cable polymers displayed a high open-circuit voltage approaching 1.20 V in light of high energy level discrepancy between electron-donating and electron-withdrawing parts, which is the highest open-circuit voltage among double-cable-based single-component organic solar cell (SCOSC) devices. Additionally, the asymmetric polymer displayed improved absorption spectra, thereby promoting crystallization and phase separation. Consequently, the as-PPNR-based SCOSCs achieved a power conversion efficiency of 5.05% along with a higher short-circuit current density and fill factor than their s-PPNR-based counterparts. In this work, we have successfully incorporated N-annulated PBI into double-cable polymers and revealed the important effects on structural symmetry and phase separation of double-cable polymers for higher SCOSC performance.

Fluorinated Alcohol-Processed N-Type Organic Electrochemical Transistor with High Performance and Enhanced Stability

Tuning the film morphology and aggregated structure is a vital means to improve the performance of the mixed ionic-electronic conductors in organic electrochemical transistors (OECTs). Herein, three fluorinated alcohols (FAs), including 2,2,2-trifluoroethanol (TFE), 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and perfluoro-tert-butanol (PFTB), were employed as the alternative solvents for engineering the n-type small-molecule active layer gNR. Remarkedly, an impressive μC* of 5.12 F V^-1 cm^-1 s^-1 and a normalized transconductance of 1.216 S cm^-1 are achieved from the HFIP-fabricated gNR OECTs, which is three times higher than that of chloroform. The operational stability has been significantly enhanced by the FA-fabricated devices. Such enhancements can be ascribed to the aggregation-induced structural ordering by FAs during spin coating, which optimizes the microstructure of the films for a better mixed ion and electron transport. These results prove the huge research potential of FAs to improve OECT materials' processability, device performance, and stability, therefore promoting practical bio-applications.

Ionic Push–Pull Polythiophenes: A Further Step towards Eco-Friendly BHJ Organic Solar Cells

Four new conjugated polymers alternating benzothiadiazole units and thiophene moieties functionalized with ionic phosphonium or sulfonic acid salts in the side chains were synthesized by a postfunctionalization approach of polymeric precursors. The introduction of ionic groups makes the conjugated polymers soluble in water and/or polar solvents, allowing for the fabrication of bulk heterojunction (BHJ) solar cells using environmentally friendly conditions. All polymers were fully characterized by spectroscopic, thermal, electrochemical, X-ray diffraction, scanning electron, and atomic force techniques. BHJ solar cells were obtained from halogen-free solvents (i.e., ethanol and/or anisole) by blending the synthesized ionic push–pull polymers with a serinol-fullerene derivative or an ionic homopolymer acting as electron-acceptor (EA) or electron-donor (ED) counterparts, respectively. The device with the highest optical density and the smoothest surface of the active layer was the best-performing, showing a 4.76% photoconversion efficiency.

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.

Impact of the segment ratio on a donor-acceptor all-conjugated block copolymer in single-component organic solar cells

The development of single-component organic solar cells (SCOSCs) using only one photoactive component with a chemically bonded D/A structure has attracted increasing research attention in recent years. At represent, most relevant studies focus on comparing the performance difference between a donor-acceptor (D-A) conjugated block copolymer (CBC) and the commensurate blending systems based on the same donor and acceptor segments, and still there are no reports on the impact of the segment ratio for a certain D-A CBC on the resultant photovoltaic performance. In this study, we synthesized a D-A all-conjugated polymers based on an n-type PNDI2T block and a p-type PBDB-T donor block but with three different segment ratios (P1-P3) and demonstrate the significance of the D/A segment ratio on photovoltaic performance. Our results reveal that the n-type PNDI2T block plays a more critical role in the inter/intra-chain charge transfer. P1 with a higher content of PNDI2T delivers superior exciton dissociation and charge transfer behavior than P2 and P3, benefitting from its more balanced hole/electron mobility. In addition, a higher packing regularity associated with a more dominant face-on orientation is also observed for P1. As a result, SCOSC based on P1 exhibits the highest PCE among the synthesized CBCs. It also possesses a minimal energy loss due to the better suppressed non-radiative recombination loss. This work provides the first discussion of the impact of the segment ratio for a D-A all-conjugated block copolymer and signifies the critical role of the n-type segment in designing high-performance single-component CBCs.

Impact of Aryl End Group Engineering of Donor Polymers on the Morphology and Efficiency of Halogen-Free Solvent-Processed Nonfullerene Organic Solar Cells

End group engineering on the side chain of π-conjugated donor polymers is explored as an effective way to develop efficient photovoltaic devices. In this work, we designed and synthesized three new π-conjugated polymers (PBDT-BZ-1, PBDT-S-BZ, and PBDT-BZ-F) with terminal aryl end groups on the side chain of chlorine-substituted benzo[1,2-b:4,5b']dithiophene (BDT). End group modifications showed notable changes in energy levels, dipole moments, exciton lifetimes, energy losses, and charge transport properties. Remarkably, the three new polymers paired with IT-4F (halogen-free solvent processed/toluene:DPE) displayed high power conversion efficiencies (PCEs) compared to a polymer (PBDT-Al-5) without a terminal end group (PCE of 7.32%). Interestingly, PBDT-S-BZ:IT-4F (PCE of 13.73%) showed a higher PCE than the benchmark PM7:IT-4F. The improved performance of PBDT-S-BZ well correlates with its improved charge mobility, well-interdigitated surface morphology, and high miscibility with a low Flory-Huggins interaction parameter (1.253). Thus, we successfully established a correlation between the end group engineering and bulk properties of the new polymers for realizing the high performance of halogen-free nonfullerene organic solar cells.

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.

Increasing donor-acceptor spacing for reduced voltage loss in organic solar cells

The high voltage losses ([Formula: see text]), originating from inevitable electron-phonon coupling in organic materials, limit the power conversion efficiency of organic solar cells to lower values than that of inorganic or perovskite solar cells. In this work, we demonstrate that this [Formula: see text] can in fact be suppressed by controlling the spacing between the donor (D) and the acceptor (A) materials (DA spacing). We show that in typical organic solar cells, the DA spacing is generally too small, being the origin of the too-fast non-radiative decay of charge carriers ([Formula: see text]), and it can be increased by engineering the non-conjugated groups, i.e., alkyl chain spacers in single component DA systems and side chains in high-efficiency bulk-heterojunction systems. Increasing DA spacing allows us to realize significantly reduced [Formula: see text] and improved device voltage. This points out a new research direction for breaking the performance bottleneck of organic solar cells.

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.

Dynamics of Photoinduced Energy Transfer in Fully and Partially Conjugated Polymers Bearing π-Extended Donor and Acceptor Monomers

The photophysical properties of donor (D)-acceptor (A) polymers were studied by designing two types of polymers, (D-σ-A)_n and (D-π-A)_n, with non-conjugated alkyl (sp³) and π-conjugated (sp²) linkers using π-extended donor and acceptor monomers that exhibit planar A-D-A structures. The non-conjugated alkyl linker provides structural flexibility to the (D-σ-A)_n polymers, while the π-conjugated linker retains the rigid structure of the (D-π-A)_n polymers. Photoinduced energy transfer occurs from the large donor to acceptor units in both polymers. However, the photoinduced energy transfer dynamics are found to be dependent on the conformation of the polymers, where the difference is dictated by the types of linkers between the donor and acceptor units. In solution, intramolecular energy transfer is relatively favorable for the (D-σ-A)_n polymers with flexible linkers that allow the donor and acceptor units to be proximally located in the polymers. On the other hand, intermolecular (or interchain) energy transfer is dominant in the two polymer films because the π-extended donor and acceptor units in polymers are closely packed. The structural flexibility of the linkers between the donor and acceptor repeating units in the polymers affects the efficiency of energy transfer between the donor and acceptor units and the overall photophysical properties of the polymers.

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.

End Group Engineering on the Side Chains of Conjugated Polymers toward Efficient Non-Fullerene Organic Solar Cells

Side chains properties of conjugated polymers, such as the length, branching point, and heteroatom, have been widely studied for application in organic solar cells (OSCs), but the end groups of side chains have been rarely reported. In this work, we systematically explored a series of new conjugated polymers with distinct side-chain end groups for high performance non-fullerene OSCs. The key components for the polymers contained functionalized units as the end groups of side chains, such as Br, alkyloxy (OMe), and alkylthienyl (T) groups. We found that the new conjugated polymers have similar absorption spectra and crystallinity with the polymer without substitution, but they showed distinct photovoltaic performance in solar cells. When the polymer without functionalized units had a power conversion efficiency (PCE) of 9.94%, the modified conjugated polymers provided high PCEs of over 13% with significantly enhanced photocurrent and fill factors. In addition, they also show additive-free and highly stable characteristics. These results demonstrate that end group engineering on side chains is a promising strategy to design new conjugated polymers toward efficient OSCs.