Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review

Recent Progress in Semitransparent Organic Solar Cells: Photoabsorbent Materials and Design Strategies

The increasing energy demands of the global community can be met with solar energy. Solution-processed organic solar cells have seen great progress in power conversion efficiencies (PCEs). Semitransparent organic solar cells (ST-OSCs) have made enormous progress in recent years and have been considered one of the most promising solar cell technologies for applications in building-integrated windows, agricultural greenhouses, and wearable energy resources. Therefore, through the synergistic efforts of transparent electrodes, engineering in near-infrared photoabsorbent materials, and device engineering, high-performance ST-OSCs have developed, and PCE and average visible transmittance reach over 10% and 40%, respectively. In this review, we present the recent progress in photoabsorbent material engineering and strategies for enhancing the performance of ST-OSCs to help researchers gain a better understanding of structure-property-performance relationships. To conclude, new design concepts in material engineering and outlook are proposed to facilitate the further development of high-performance ST-OSCs.

Halogenated 9H-Indeno[1,2-b]Pyrazine-2,3-Dicarbonitrile End Groups Based Asymmetric Non-Fullerene Acceptors for Green Solvent-Processable, Additive-Free, and Stable Organic Solar Cells

Non-fullerene acceptors (NFAs) significantly enhance photovoltaic performance in organic solar cells (OSCs) using halogenated solvents and additives. However, these solvents are environmentally detrimental and unsuitable for industrial-scale production, and the issue of OSCs' poor long-term stability persists. This report introduces eight asymmetric NFAs (IPCnF-BBO-IC2F, IPCnF-BBO-IC2Cl, IPCnCl-BBO-IC2F, and IPCnCl-BBO-IC2Cl, where n = 1 and 2). These NFAs comprise a 12,13-bis(2-butyloctyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno-[3,2-b]indole (BBO) core. One end of the core attaches to a mono- or di-halogenated 9H-indeno[1,2-b]pyrazine-2,3-dicarbonitrile (IPC) end group (IPC1F, IPC1Cl, IPC2F, or IPC2Cl), while the other end connects to a 2-(5,6-dihalo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end group (IC2F or IC2Cl). The optical and electronic properties of these NFAs can be finely tuned by controlling the number of halogen atoms. Crucially, these NFAs demonstrate excellent compatibility with PM6 even in o-xylene, facilitating the production of additive-free OSCs. The di-halogenated IPC-based NFAs outperform their mono-halogenated counterparts in photovoltaic performance within OSCs. Remarkably, the di-halogenated IPC-based NFAs maintain 94‒98% of their initial PCEs over 2000 h in air without encapsulation, indicating superior long-term device stability. These findings imply that the integration of di-halogenated IPCs in asymmetric NFA design offers a promising route to efficient, stable OSCs manufactured through environmentally friendly processes.

Precise Methylation Yields Acceptor with Hydrogen-Bonding Network for High-Efficiency and Thermally Stable Polymer Solar Cells

Utilizing intermolecular hydrogen-bonding interactions stands for an effective approach in advancing the efficiency and stability of small-molecule acceptors (SMAs) for polymer solar cells. Herein, we synthesized three SMAs (Qo1, Qo2, and Qo3) using indeno[1,2-b]quinoxalin-11-one (Qox) as the electron-deficient group, with the incorporation of a methylation strategy. Through crystallographic analysis, it is observed that two Qox-based methylated acceptors (Qo2 and Qo3) exhibit multiple hydrogen bond-assisted 3D network transport structures, in contrast to the 2D transport structure observed in gem-dichlorinated counterpart (Qo4). Notably, Qo2 exhibits multiple and stronger hydrogen-bonding interactions compared with Qo3. Consequently, PM6 : Qo2 device realizes the highest power conversion efficiency (PCE) of 18.4 %, surpassing the efficiencies of devices based on Qo1 (15.8 %), Qo3 (16.7 %), and Qo4 (2.4 %). This remarkable PCE in PM6 : Qo2 device can be primarily ascribed to the enhanced donor-acceptor miscibility, more favorable medium structure, and more efficient charge transfer and collection behavior. Moreover, the PM6 : Qo2 device demonstrates exceptional thermal stability, retaining 82.8 % of its initial PCE after undergoing annealing at 65 °C for 250 hours. Our research showcases that precise methylation, particularly targeting the formation of intermolecular hydrogen-bonding interactions to tune crystal packing patterns, represents a promising strategy in the molecular design of efficient and stable SMAs.

Relating Structure to Properties in Non-Conjugated Pendant Electroactive Polymers

Non-conjugated pendant electroactive polymers (NCPEPs) are an emerging class of polymers that offer the potential of combining the desirable optoelectronic properties of conjugated polymers with the superior synthetic methodologies and stability of traditional non-conjugated polymers. Despite an increasing number of studies focused on NCPEPs, particularly on understanding fundamental structure-property relationships, no attempts have been made to provide an overview on established relationships to date. This review showcases selected reports on NCPEP homopolymers and copolymers that demonstrate how optical, electronic, and physical properties of the polymers are affected by tuning of key structural variables such as the chemical structure of the polymer backbone, molecular weight, tacticity, spacer length, the nature of the pendant group, and in the case of copolymers the ratios between different comonomers and between individual polymer blocks. Correlation of structural features with improved π-stacking and enhanced charge carrier mobility serve as the primary figures of merit in evaluating impact on NCPEP properties. While this review is not intended to serve as a comprehensive summary of all reports on tuning of structural parameters in NCPEPs, it highlights relevant established structure-property relationships that can serve as a guideline for more targeted design of novel NCPEPs in the future.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Mild Synthesis of Polychlorinated Arenes for Efficient Organic Light-emitting Diodes with Dual Thermally Activated Delayed Fluorescence

Polychlorinated (hetero)arenes have shown great promise for organic optoelectronics applications. However, the harsh synthetic routes for polychlorinated compounds and the possible luminescence quenching from the compact intermolecular π-π stacking induced by chlorine atoms limit their investigations and applications in luminescent materials. Herein, two isomeric polychlorinated polycyclic aromatic hydrocarbon (PAH) compounds JY-1-Cl and JY-2-Cl consisting of rigidified aryl ketones and amine are designed and synthesized under mild conditions through nucleophilic chlorination intermediated by an electron donor-acceptor complex. Among them, as a result of the strong π-π interactions induced by chlorine atoms, JY-2-Cl exhibits bright monomer and dimer emissions with dual thermally activated delayed fluorescence (TADF) characters. Notably, compared with the non-chlorinated compounds, a high photoluminescence quantum yield is maintained after introducing multiple chlorine atoms into JY-2-Cl. The first dual-TADF organic light-emitting diodes are also successfully fabricated with maximum external quantum efficiency as high as 29.1 % by employing JY-2-Cl as emitter. This work presents a new paradigm and synthesis of polychlorinated amine-carbonyl PAHs and demonstrates the great potential of the chlorinated materials for luminescent applications.

A 3-Fluoropyridine Manipulating the Aggregation and Fibril Network of Donor Polymers for Eco-Friendly Solution-Processed Versatile Organic Solar Cells

The development of eco-friendly solvent-processed organic solar cells (OSCs) suitable for industrial-scale production should be now considered the imperative research. Herein, asymmetric 3-fluoropyridine (FPy) unit is used to control the aggregation and fibril network of polymer blends. Notably, terpolymer PM6(FPy = 0.2) incorporating 20% FPy in a well-known donor polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PM6) can reduce the regioregularity of the polymer backbone and endow them with much-enhanced solubility in eco-friendly solvents. Accordingly, the excellent adaptability for fabricating versatile devices based on PM6(FPy = 0.2) by toluene processing is demonstrated. The resulting OSCs exhibit a high power conversion efficiency (PCE) of 16.1% (17.0% by processed with chloroform) and low batch-to-batch variation. Moreover, by controlling the donor-to-acceptor weight ratio at 0.5:1.0 and 0.25:1.0, semi-transparent OSCs (ST-OSCs) yield significant light utilization efficiencies of 3.61% and 3.67%, respectively. For large-area (1.0 cm2 ) indoor OSC (I-OSC), a high PCE of 20.6% is achieved with an appropriate energy loss of 0.61 eV under a warm white light-emitting diode (3,000 K) with the illumination of 958 lux. Finally, the long-term stability of the devices is evaluated by investigating their structure-performance-stability relationship. This work provides an effective approach to realizing eco-friendly, efficient, and stable OSCs/ST-OSCs/I-OSCs.

Quantifying the Impact of Halogenation on Intermolecular Interactions and Binding Modes of Aromatic Molecules

Halogenation of aromatic molecules is frequently used to modulate intermolecular interactions with ramifications for optoelectronic and mechanical properties. In this work, we accurately quantify and understand the nature of intermolecular interactions in perhalogenated benzene (PHB) clusters. Using benchmark binding energies from the fixed-node diffusion Monte Carlo (FN-DMC) method, we show that generalized Kohn-Sham semicanonical projected random phase approximation (GKS-spRPA) plus approximate exchange kernel (AKX) provides reliable interaction energies with mean absolute error (MAE) of 0.23 kcal/mol. Using the GKS-spRPA+AXK method, we quantify the interaction energies of several binding modes of PHB clusters ((C₆X₆)_n; X = F, Cl, Br, I; n = 2, 3). For a given binding mode, the interaction energies increase 3-4 times from X = F to X = I; the X-X binding modes have energies in the range of 2-4 kcal/mol, while the π-π binding mode has interaction energies in the range of 4-12 kcal/mol. SAPT-DFT-based energy decomposition analysis is then used to show that the equilibrium geometries are dictated primarily by the dispersion and exchange interactions. Finally, we test the accuracy of several dispersion-corrected density functional approximations and show that only the r2SCAN-D4 method has a low MAE and correct long-range behavior, which makes it suitable for large-scale simulations and for developing structure-function relationships of halogenated aromatic systems.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Preparation of Efficient Organic Solar Cells Based on Terpolymer Donors via a Monomer-Ratio Insensitive Side-Chain Hybridization Strategy

Creating new donor materials is crucial for further advancing organic solar cells. Random terpolymers have been adopted to overcome shortcomings of regular alternating donor-acceptor (D-A) polymers of which the performance is often susceptible to batch-to-batch variations. In general, the properties and performance of efficient D₁ -A-D₂ -A and D-A₁ -D-A₂ terpolymers are sensitive to the D₁ /D₂ or A₁ /A₂ monomer ratios. Side-chain hybridization is a strategy to address this problem. Here, six D₁ -A-D₂ -A-type random terpolymers comprising D₁ and D₂ monomers with the same π-conjugated D unit but with different side chains were synthesized. The side chains, containing either fluorine or trialkylsilyl substituents were chosen to provide near-identical optoelectronic properties but provide a tool to create a better-optimized film morphology when blended with a non-fullerene acceptor. This strategy allows improving the device performance to over 18 %, higher than that obtained with the corresponding D₁ -A or D₂ -A bipolymers (around 17 %). Hence, side-chain hybridization is a promising strategy to design efficient D₁ -A-D₂ -A terpolymer donors that are insensitive to the D₁ /D₂ monomer ratio, which is beneficial for the scaled-up synthesis of high-performance materials.

Double Asymmetric Core Optimizes Crystal Packing to Enable Selenophene-based Acceptor with Over 18 % Efficiency in Binary Organic Solar Cells

Side-chain tailoring is a promising method to optimize the performance of organic solar cells (OSCs). However, asymmetric alkyl chain-based small molecular acceptors (SMAs) are still difficult to afford. Herein, we adopted a novel asymmetric n-nonyl/undecyl substitution strategy and synthesized two A-D₁ A'D₂ -A double asymmetric isomeric SMAs with asymmetric selenophene-based central core for OSCs. Crystallographic analysis indicates that AYT9Se11-Cl forms a more compact and order intermolecular packing compared to AYT11Se9-Cl, which contributed to higher electron mobility in neat AYT9Se11-Cl film. Moreover, the PM6 : AYT9Se11-Cl blend film shows a better morphology with appropriate phase separation and distinct face-on orientation than PM6 : AYT11Se9-Cl. The OSCs with PM6 : AYT9Se11-Cl obtain a superior PCE of 18.12 % compared to PM6 : AYT11Se9-Cl (17.52 %), which is the best efficiency for the selenium-incorporated SMAs in binary BHJ OSCs. Our findings elucidate that the promising double asymmetric strategy with isomeric alkyl chains precisely modulates the crystal packing and enhances the photovoltaic efficiency of selenophene-incorporated SMAs.

Effect of Chlorine Substituents on the Photovoltaic Properties of Monocyanated Quinoxaline-Based D-A-Type Polymers

A string of monocyanated quinoxaline (Qx)-based D-A-type polymers systematically decorated with electron-attracting chlorine (Cl) atoms was created for use in non-fullerene polymer solar cells (PSCs). First, coupling of the benzodithiophene (BDT) donor and Qx acceptor with the strong electron-attracting cyano (CN) unit at its 5-position yielded the monocyanated reference polymer PB-CNQ. Subsequently, the additional Cl atoms were separately or simultaneously incorporated into the thiophene side groups of the BDT donor and Qx acceptor to create other objective polymers, PBCl-CNQ, PB-CNQCl, and PBCl-CNQCl. The Cl substituents on the BDT donor and Qx acceptor are represented by the names of the polymers. Owing to the favorable contributions of Cl substituents, the inverted-type non-fullerene PSCs based on partially chlorinated PBCl-CNQ (12.80%) and PB-CNQCl (13.93%) exhibited better power conversion efficiencies (PCEs) than the device based on unchlorinated reference PB-CNQ (11.19%). However, a significantly reduced PCE of 9.84% was observed for the device based on PBCl-CNQCl, in which Cl atoms were loaded on both the BDT donor and Qx acceptor at the same time. Hence, these results reveal that optimization of the number and position of Cl substituents in monocyanated Qx-based polymers is essential for enhancing their photovoltaic nature through the synergistic effects between two strong electron-attracting CN and Cl substituents.

Bihalogenated Thiophene-Based Terpolymers for High-Performance Semitransparent Organic Solar Cells Processed by an Eco-Friendly Solvent and Layer-by-Layer Deposition

The development of photoactive materials simultaneously satisfying high performance, low cost, and eco-friendly processability remains challenging in organic solar cells (OSCs). Herein, a synergistic strategy is proposed to design three terpolymers (PM7(ClCl = 0.2), PM7(ClBr = 0.2), and PM7(BrBr = 0.2)) based on bihalogenated thiophenes with relatively low cost, for improving the optical and electrochemical properties, solubility in nontoxic solvents, and crystallinity and miscibility balance. In summary, a bulk-heterojunction (BHJ)-processed device based on PM7(ClCl = 0.2) with 20% dichlorinated thiophene achieves the highest power conversion efficiency (PCE) of 15.2% using toluene (best PCE ≈ 15.8% on the ternary blend). Moreover, high-performance semitransparent OSCs (ST-OSCs) were fabricated by a combination of layer-by-layer (LBL) and sequential dynamic and static spin-coating techniques according to the molecular weight of PM7(ClCl = 0.2). Using this unique LBL strategy, the PM7(ClCl = 0.2)-MW (H; high molecular weight)-processed ST-OSCs yield a high PCE of 11.5% and an average visible transmittance (AVT) of 27.1% with outstanding tolerance to device reproducibility. By optimizing ST-OSCs with tungsten trioxide as a distributed Bragg reflector, a light utilization efficiency (LUE) of 3.61% is realized with a PCE of 10.8% and an AVT of 33.4% (certified PCE ≈ 11.157%; LUE ≈ 3.73%). This study provides a novel perspective for designing and developing actual photoactive materials for OSC commercialization.

Carboxylate-Containing Wide-Bandgap Polymers for High-Voltage Non-Fullerene Organic Solar Cells

As one of the polymer modification strategies, carboxylate functionalization has proved effective in downshifting the energy levels and enhancing polymer crystallinity and aggregation. However, high-performance carboxylate-containing polymers are still limited for organic solar cells (OSCs), especially with open-circuit voltage (V_OC) above 1.0 V. Herein, we utilize two carboxylate-functionalized wide-band gap (WBG) donor polymers (TTC-F and TTC-Cl) to pair with two WBG electron acceptors (BTA5 and F-BTA5) for high-voltage OSCs. Due to the deeper molecular energy levels, chlorinated polymer TTC-Cl shows higher V_OC than fluorinated polymer TTC-F. Furthermore, because of the stronger aggregation in the film, the TTC-Cl-based devices attain suppressed energetic disorders and trap-assisted recombination, decreasing voltage loss and J_SC loss. Finally, the TTC-Cl: F-BTA5 blend achieves a higher V_OC of 1.17 V and an excellent PCE of 10.98%, one of the best results for high-voltage carboxylate-containing polymers. In addition, the TTC-Cl: BTA5 combination demonstrates the highest V_OC of 1.25 V with an ultralow nonradiative energy loss of 0.17 eV. Our results indicate that the carboxylate-containing polymer donors have significant application potential for high-voltage OSCs due to reduced energy loss and improved charge transport and dissociation. Furthermore, the matched absorption spectra with the indoor light sources and low voltage loss promote these material combinations to construct high-performance indoor photovoltaics.

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet–visible (UV–Vis) absorption bands. Bulky ligands on the metal suppress π–π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV–Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Single-Junction Organic Solar Cells with 19.17% Efficiency Enabled by Introducing One Asymmetric Guest Acceptor

The ternary strategy has been widely identified as an effective approach to obtain high-efficiency organic solar cells (OSCs). However, for most ternary OSCs, the nonradiative voltage loss lies between those of the two binary devices, which limits further efficiency improvements. Herein, an asymmetric guest acceptor BTP-2F2Cl is designed and incorporated into a PM1:L8-BO host blend. Compared with the L8-BO neat film, the L8-BO:BTP-2F2Cl blend film shows higher photoluminescence quantum yield and larger exciton diffusion length. Introducing BTP-2F2Cl into the host blend extends its absorption spectrum, improves the molecular packing of host materials, and suppresses the nonradiative charge recombination of the ternary OSCs. Consequently, the power conversion efficiency is improved up to 19.17% (certified value 18.7%), which represents the highest efficiency value reported for single-junction OSCs so far. The results show that improving the exciton behaviors is a promising approach to reducing the nonradiative voltage loss and realizing high-performance OSCs.

Cathode Interlayer Engineering for Efficient Organic Solar Cells under Solar Illumination and Light-Emitting Diode Lamp

Organic solar cells (OSCs) have become a potential energy source for indoor light harvesting in recent years as they have witnessed a record power conversion efficiency (PCE) of over 30% under indoor lights. Among various strategies, interlayer engineering is one of the important factors in improving the performance of OSCs. Here, we reported an efficient OSC based on PM6:Y6 photoactive layer showing an excellent PCE of ~22% and ~14% under light-emitting diode (LED, 1000-lx) and 1-sun (AM1.5 G) conditions, respectively. The performance of OSCs was optimized by systematically investigating the optical, electrochemical, and morphological characteristics of three different cathode interlayers (CILs) named as: PEIE, ZnO, and ZnO/PEIE (bilayer). The high transmittance (~90%), suitable work function (~4.1 eV), and improved surface morphology (RMS: 2.61 nm) of the bilayer CIL contributes in improving the performance of OSCs. In addition, the suppressed charge recombination and improved charge carrier transport are attributed to high shunt resistance and appropriate energy levels alignment between photoactive layer and bilayer CIL. The findings in the study might provide guidelines for designing novel interlayers in the development of efficient OSCs for different illumination conditions.

Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.

Simultaneous Tuning of Alkyl Chains and End Groups in Non-fused Ring Electron Acceptors for Efficient and Stable Organic Solar Cells

Fine-tuning the alkyl chains and end groups of non-fused ring electron acceptors (NFREAs) plays vital roles in the promotion of charge transfer (CT) and power conversion efficiency (PCE). In this work, we developed a series of A-D-A'-D-A-type NFREAs, which possess the same terminals (A), the cyclopentadithiophene unit (D), and the thieno[3,4-c]pyrrole-4,6-dione (A'). Despite the subtle difference in side chains and halogenated end groups, the six acceptors exhibit a considerable difference in the efficiency and device stability of the organic solar cells (OSCs). Among the molecules, chlorinated NFREAs show a broader light absorption than the fluorinated ones do. Compared with C8C8-4F (1-octylnonyl and fluorination) and C6C4-4Cl (2-butyloctyl and chlorination), C8C8-4Cl (1-octylnonyl and chlorination) exhibits a lower highest occupied molecular orbital level, higher electron mobility, and denser molecular packing. The OSCs based on PM6:C8C8-4Cl yield the best PCE of 14.11%, which is attributed to the faster charge transport, high miscibility, and preferable morphology. Moreover, the PM6:C8C8-4Cl devices retain 91.1% of the initial PCE after being placed in air with 67% relative humidity for 50 days. This work shows that the simultaneous optimization of side chains and end groups facilitates the CT and improves the stability in the OSCs, offering a novel view into the molecular design of A-D-A'-D-A-type NFREAs.

A New Diazabenzo[k]fluoranthene-based D-A Conjugated Polymer Donor for Efficient Organic Solar Cells

The development of wide-bandgap polymer donors having complementary absorption and compatible energy levels with near-infared (NIR) absorbing nonfullerene acceptors is highly important for realizing high-performance organic solar cells (OSCs). Herein, a new thiophene-fused diazabenzo[k]fluoranthene derivative has been successfully synthesized as the electron-deficient unit to construct an efficient donor-acceptor (D-A) type alternating copolymer donor, namely PABF-Cl, using the chlorinated benzo[1,2-b:4,5-b']dithiophene as the copolymerization unit. PABF-Cl exhibits a wide optical bandgap of 1.93 eV, a deep highest occupied molecular level of -5.36 eV, and efficient hole transport. As a result, OSCs with the best power conversion efficiency of 11.8% has been successfully obtained by using PABF-Cl as the donor to blend with a NIR absorbing BTP-eC9 acceptor. Our work thus provides a new design of electron-deficient unit for constructing high performance D-A type polymer donors. This article is protected by copyright. All rights reserved.

Designing High-Performance Nonfused Ring Electron Acceptors via Synergistically Adjusting Side Chains and Electron-Withdrawing End-Groups

Three nonfused ring electron acceptors, Hexyl-0F, Isopropyl-0F, and Isopropyl-2F, are designed and synthesized. Unlike Hexyl-0F, Isopropyl-0F with two sterically hindered 2,4,6-triisopropyl-phenyl groups is highly soluble, which provides a good opportunity for solution processability. Compared with Isopropyl-0F, Isopropyl-2F with fluorinated end-groups exhibits red-shifted absorption. Due to these synergistic adjustment, Isopropyl-2F-based devices displayed a high power conversion efficiency of 12.55%, higher than that of Isopropyl-0F (9.49%). The result demonstrates that the introduction of large steric substituents in the π-bridge units and electron-withdrawing end-groups plays a positive role in the construction of high-efficiency nonfused ring electron acceptors.

Effect of Electron-Withdrawing Chlorine Substituent on Morphological and Photovoltaic Properties of All Chlorinated D-A-Type Quinoxaline-Based Polymers

The choice of the chlorine (Cl) atom as an electron-withdrawing substituent in conjugated polymers leads to a higher potential in the commercialization of polymer solar cells than its fluorine counterpart because of the versatility and cost-effectiveness of the chlorination process. In addition, the population and location of Cl substituents can significantly influence the photovoltaic characteristics of polymers. In this study, three chlorinated quinoxaline-based polymers were invented to examine the numerical and positioning effects of the Cl atom on their photovoltaic characteristics. The number of Cl substituents in the reference polymer, PBCl-Qx, was adjusted to three: two Cl atoms in the benzodithiophene-type D unit and one Cl atom in the quinoxaline-type A unit. Subsequently, two more Cl atoms were selectively introduced at the 4- and 5-positions of the alkylated thiophene moieties at the 2,3-positions of the quinoxaline moiety in PBCl-Qx to obtain the additional polymers PBCl-Qx4Cl and PBCl-Qx5Cl, respectively. The conventional PBCl-Qx4Cl device exhibited a better power conversion efficiency (PCE) of 12.95% as compared to those of PBCl-Qx (12.44%) and PBCl-Qx5Cl (11.82%) devices. The highest PCE of the device with PBCl-Qx4Cl was ascribed to an enhancement in the open-circuit voltage and fill factor induced by the deeper energy level of the highest occupied molecular orbital and the favorable morphological features in its blended film with Y6.

Strengthening the Intermolecular Interaction of Prototypical Semicrystalline Conjugated Polymer Enables Improved Photocurrent Generation at the Heterojunction

The molecular packing structure of conjugated polymers are crucial in determining their optoelectronic properties. The intra and intermolecular interactions (J- and H-type aggregation) in the conjugated polymer films were found to readily facilitate the electron and hole transport, respectively. However, how those different aggregation types influence the photocurrent generation process at the heterojunction is still mysterious, especially for the newly developed semicrystalline conjugated polymers. Here, the prototypical copolymer PM6 is used as a model semicrystalline polymer to tune the relative content of aggregation types with various halogen-free processing solvents. Various measurements reveal that the toluene-processed PM6 film exhibits the increased H-aggregates and crystallinity in the π-π stacking direction compared to its o-Xylene- and trimethylbenzene (TMB)-processed counterparts. This is partly resulted from the weak steric effect and good solubility in the PM6 solution prepared with toluene bearing small molar volume, which strengthens the intermolecular interaction of adjacent polymer segments. After analyzing the photovoltaic properties of the different PM6/Y6 bilayer devices, the faster charge carrier transport, smaller charge recombination, lower energy losses and interfacial energetic disorder can be observed in the toluene-processed device, leading to the synergistically improved short-circuit current density (J_SC ) and open-circuit voltage (V_OC ). Our findings indicate that the control of the molecular packing structure in terms of aggregation types is a powerful strategy to promote the photocurrent generation process at the conjugated polymer-based heterojunction. This article is protected by copyright. All rights reserved.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Assessing Effects of Different π bridges on Properties of Random Benzodithiophene-thienothiophene Donor and Non-fullerene Acceptor Based Active Layer

This report presents the effect of insertion of four different π bridges, furan, thienothiophene, thiophene, and thiazole, into a random benzodithiophene (BDT)-fluorinated-thienothiophene (TT-F) based donor. Starting from a structure of synthesized donor (D)-acceptor (A) random copolymer with 3:1 ratio, we have designed four D-π-A systems with four different π bridges. Structural, optoelectronic, and charge transport/transfer properties of these donors and donor/NDI (NDI = poly[N,N'-bis(2-hexyldecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) blends are investigated using DFT and TD-DFT methodologies. Our results show that the thiazole based TzP1 oligomer has the deepest HOMO value resulting in the highest open circuit voltage among all systems. The maximum absorption wavelengths of π-linked systems are red-shifted compared to the parent molecule. Rates of charge transfer and charge recombination are the highest and smallest in case of the thiazole/NDI blend system. In addition, hole mobilities in thiophene, thienothiophene, and thiazole based systems are larger than in the parent system. The results indicate that the thiazole unit among the four π bridge units is the most suitable for active layer construction.

Single-Junction Organic Photovoltaic Cell with 19% Efficiency

Improving power conversion efficiency (PCE) is important for broadening the applications of organic photovoltaic (OPV) cells. Here, a maximum PCE of 19.0% (certified value of 18.7%) is achieved in single-junction OPV cells by combining material design with a ternary blending strategy. An active layer comprising a new wide-bandgap polymer donor named PBQx-TF and a new low-bandgap non-fullerene acceptor (NFA) named eC9-2Cl is rationally designed. With optimized light utilization, the resulting binary cell exhibits a good PCE of 17.7%. An NFA F-BTA3 is then added to the active layer as a third component to simultaneously improve the photovoltaic parameters. The improved light unitization, cascaded energy level alignment, and enhanced intermolecular packing result in open-circuit voltage of 0.879 V, short-circuit current density of 26.7 mA cm^-2 , and fill factor of 0.809. This study demonstrates that further improvement of PCEs of high-performance OPV cells requires fine tuning of the electronic structures and morphologies of the active layers.

Nongeminate Recombination in All-Polymer Solar Cells with Different Crystallinities

One of the most challenging issues facing the organic photovoltaic community is to realize a high fill factor (FF) even with thick active layers. This is because the thick active layer is beneficial for photon absorption but makes charge collection difficult, which is primarily restricted by nongeminate recombination in solar cells. In this work, we have studied nongeminate recombination in four kinds of polymer solar cells based on blends of donor-conjugated polymers with different crystallinities and acceptor-conjugated polymers with a naphthalene diimide unit by using transient photovoltage and photocurrent techniques. As a result, we find that nongeminate recombination is considerably suppressed with an increasing degree of crystallinity of donor polymers, leading to a high FF of more than 0.6 even with an active layer thickness of 300 nm. The origin of such a phenomenon is further discussed in terms of variations in the states of mixed phases with a cascaded energy structure between crystalline domains and amorphous domains evaluated by conductive atomic force microscopy.

Light intensity dependence of organic solar cell operation and dominance switching between Shockley-Read-Hall and bimolecular recombination losses

We investigated the variation of current density-voltage (J-V) characteristics of an organic solar cell (OSC) in the dark and at 9 different light intensities ranging from 0.01 to 1 sun of the AM1.5G spectrum. All three conventional parameters, short-circuit currents (Jsc), open-circuit voltage (Voc), and Fill factor (FF), representing OSC performance evolved systematically in response to light intensity increase. Unlike Jsc that showed quasi-linear monotonic increase, Voc and FF showed distinctive non-monotonic variations. To elucidate the origin of such variations, we performed extensive simulation studies including Shockley-Read-Hall (SRH) recombination losses. Simulation results were sensitive to defect densities, and simultaneous agreement to 10 measured J-V curves was possible only with the defect density of [Formula: see text]. Based on analyses of simulation results, we were able to separate current losses into SRH- and bimolecular-recombination components and, moreover, identify that the competition between SRH- and bimolecular-loss currents were responsible for the aforementioned variations in Jsc, Voc, and FF. In particular, we verified that apparent demarcation in Voc, and FF variations, which seemed to appear at different light intensities, originated from the same mechanism of dominance switching between recombination losses.

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA'D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π-π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve J_sc and FF without sacrificing V_oc .

Recent advances in the application of isoindigo derivatives in materials chemistry

In this review, the data on the application of isoindigo derivatives in the chemistry of functional materials are analyzed and summarized. These bisheterocycles can be used in the creation of organic solar cells, sensors, lithium ion batteries as well as in OFET and OLED technologies. The potentials of the use of polymer structures based on isoindigo as photoactive component in the photoelectrochemical reduction of water, as matrix for MALDI spectrometry and in photothermal cancer therapy are also shown. Data published over the past 5 years, including works published at the beginning of 2021, are given.

A comprehensive survey upon diverse and prolific applications of chitosan-based catalytic systems in one-pot multi-component synthesis of heterocyclic rings

Heterocyclic compounds are among the most prestigious and valuable chemical molecules with diverse and magnificent applications in various sciences. Due to the remarkable and numerous properties of the heterocyclic frameworks, the development of efficient and convenient synthetic methods for the preparation of such outstanding compounds is of great importance. Undoubtedly, catalysis has a conspicuous role in modern chemical synthesis and green chemistry. Therefore, when designing a chemical reaction, choosing and or preparing powerful and environmentally benign simple catalysts or complicated catalytic systems for an acceleration of the chemical reaction is a pivotal part of work for synthetic chemists. Chitosan, as a biocompatible and biodegradable pseudo-natural polysaccharide is one of the excellent choices for the preparation of suitable catalytic systems due to its unique properties. In this review paper, every effort has been made to cover all research articles in the field of one-pot synthesis of heterocyclic frameworks in the presence of chitosan-based catalytic systems, which were published roughly by the first quarter of 2020. It is hoped that this review paper can be a little help to synthetic scientists, methodologists, and catalyst designers, both on the laboratory and industrial scales.

Recent Advances of Chlorination in Organic Solar Cells

Chlorination has been used in the fields of organic solar cells (OSCs) for a long time, and impressive progress has been made over the years. Recently developed chlorinated OSCs have achieved an efficiency of over 18%. For better understanding and application of chlorination in the fields of OSCs, we will briefly introduce the general properties of chlorine and recent advances in its introduction and applications in OSCs in this cluster article. Finally, we also provide a short discussion of current questions regarding chlorination in OSCs and future developments in this area.

Hexaarylbenzene based high-performance p-channel molecules for electronic applications

Hexaarylbenzene-based molecules find potential applications in organic electronics due to wider energy gap, high HOMO level, higher photoconductivity, electron-rich nature, and high hole-transporting property. Due to the unique propeller structure, these molecules show low susceptibility towards self-aggregation. This property can be tailored by proper molecular engineering by the incorporation of appropriate groups. Therefore, hexaarylbenzene chromophores are widely used as the materials for high-efficiency light-emitting materials, charge transport materials, host materials, redox materials, photochemical switches, and molecular receptors. This review highlights the diverse structural modification techniques used for the synthesis of symmetrical and unsymmetrical structures. Also, the potential applications of these molecules in organic light-emitting diodes, organic field-effect transistors, organic photovoltaics, organic memory devices, and logic circuits are discussed.

Understanding the Critical Role of Sequential Fluorination of Phenylene Units on the Properties of Dicarboxylate Bithiophene-Based Wide-Bandgap Polymer Donors for Non-Fullerene Organic Solar Cells

Design and development of wide bandgap (WBG) polymer donors with low-lying highest occupied molecular orbitals (HOMOs) are increasingly gaining attention in non-fullerene organic photovoltaics since such donors can synergistically enhance power conversion efficiency (PCE) by simultaneously minimizing photon energy loss (E_loss ) and enhancing the spectral response. In this contribution, three new WBG polymer donors, P1, P2, and P3, are prepared by adding phenylene cores with a different number of fluorine (F) substituents (n = 0, 2, and 4, respectively) to dicarboxylate bithiophene-based acceptor units. As predicted, fluorination effectively aides in the lowering of HOMO energy levels, tailoring of the coplanarity and molecular ordering in the polymers. Thus, fluorinated P2 and P3 polymers show higher coplanarity and more intense interchain aggregation than P1, leading to higher charge carrier mobilities and superior phase-separated morphology in the optimized blend films with IT-4F. As a result, both P2:IT-4F and P3:IT-4F realize the best PCEs of 6.89% and 7.03% (vs 0.16% for P1:IT-4F) with lower E_loss values of 0.65 and 0.55 eV, respectively. These results signify the importance of using phenylene units with sequential fluorination in polymer backbone for modifying the optoelectronic properties and realizing low E_loss values by synergistically lowering the HOMO energy levels.

Chlorinated Benzo[1,2-b:4,5-c']dithiophene-4,8-dione Polymer Donor: A Small Atom Makes a Big Difference

The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D-A conjugated polymers PBBD-Cl-α and PBBD-Cl-β are synthesized based on two new building blocks (TTO-Cl-α and TTO-Cl-β) respectively by introducing the chlorine atom into α or β position of the upper thiophene of the highly electron-deficient benzo[1,2-b:4,5-c']dithiophene-4,8-dione moiety. Single-crystal analysis demonstrates that the chlorine-free TTO shows a π-π stacking distance (d π-π) of 3.55 Å. When H atom at the α position of thiophene of TTO is replaced by Cl, both π-π stacking distance (d π-π = 3.48 Å) and Cl···S distance (d Cl-S = 4.4 Å) are simultaneously reduced for TTO-Cl-α compared with TTO. TTO-Cl-β then showed the Cl···S non-covalent interaction can further shorten the intermolecular π-π stacking separation to 3.23 Å, much smaller than that of TTO-Cl-α and TTO. After blending with BTP-eC9, PBBD-Cl-β:BTP-eC9-based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP-eC9 (10.06%) and PBBD-Cl-α:BTP-eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.