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.

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.

Tailoring Microstructure and Morphology via Sequential Fluorination to Enhance the Photovoltaic Performance of Low-Cost Polymer Donors for Organic Solar Cells.

For utilizing organic solar cells (OSCs) for commercial applications, reducing the overall cost of the photo absorbent materials is also very crucial. Herein, such a challenge is addressed by synergistically controlling the amount of fluorine (F)-substituents (n = 2, 4) on a low-cost wide-bandgap molecular design involving alternate fluorinated-thienyl benzodithiophene donor and 2,5-difluoro benzene (2FBn) or 2,3,5,6 tetrafluorobenzene (4FBn) to form two new polymer donors PBDT-2FBn and PBDT-4FBn, respectively. As expected, sequential fluorination causes a lowering of the frontier energy levels and planarization of polymer backbone via F···S and C-H···F noncovalent molecular locks, which results in more pronounced molecular packing and enhanced crystallinity from PBDT-2FBn to PBDT-4FBn. By mixing with IT-4F acceptor, PBDT-2FBn:IT-4F-based blend demonstrates favorable molecular orientation with shorter π-π stacking distance, higher carrier mobilities and desirable nanoscale morphology, hence delivering a higher power conversion efficiency (PCE) of 9.3% than PBDT-2FBn:IT-4F counterpart (8.6%). Furthermore, pairing PBDT-2FBn with BTP-BO-4Cl acceptor further improved absorption range and promoted privileged morphology for efficient exciton dissociation and charge transport, resulting in further improvement of PCE to 10.2% with remarkably low energy loss of 0.46 eV. Consequently, this study provides valuable guidelines for designing efficient and low-cost polymer donors for OSC applications.

Water-Repellent Perovskites Induced by a Blend of Organic Halide Salts for Efficient and Stable Solar Cells.

Despite tremendous progress in the power conversion efficiency (PCE) of perovskite solar cells (PeSCs), the long-term stability issue remains a significant challenge for commercialization. In this study, by blending organic halide salts, phenylethylammonium halide (PEAX, X = I, Br), with CH3NH3PbI3 (MAPbI3), we achieved remarkable enhancements in the water-repellency of perovskite films and long-term stability of PeSCs, together with enhanced PCE. The hydrophobic aromatic PEA+ group in PEAX protects the perovskite film from destruction by water. In addition, the smaller halide Br- in PEABr restructures MAPbI3 to form MAPbI3-xBrx during post-annealing, leading to lattice contraction with beneficial crystallization quality. The perovskite films modified by PEAX exhibited excellent water resistance. When the perovskite films were directly immersed in water, no obvious decompositions were observed, even after 60 s. The PEAX-decorated PeSCs exhibited considerable long-term stability. Under high-humidity conditions (60 ± 5%), the PEAX-decorated PeSCs held 80.5% for PEAI and 85.2% for PEABr of their original PCE after exposure for 100 h, whereas the pristine PeSC device lost more than 99% of its initial PCE after exposure for 60 h under the same conditions. Moreover, compared to the pristine device with a PCE of 13.28%, the PEAX-decorated PeSCs exhibited enhanced PCEs of 17.33% for the PEAI device and 17.18% for the PEABr device.

Printable and Semitransparent Nonfullerene Organic Solar Modules over 30 cm2 Introducing an Energy-Level Controllable Hole Transport Layer.

For the commercialization of organic solar cells (OSCs), the fabrication of large-area modules via a solution process is important. The fabrication of OSCs via a solution process using a nonfullerene acceptor (NFA)-based photoactive layer is limited by the energetic mismatch and carrier recombination, reducing built-in potential and effective carriers. Herein, for the fabrication of high-performance NFA-based large-area OSCs and modules via a solution process, hybrid hole transport layers (h-HTLs) incorporating WO3 and MoO3 are developed. The high bond energies and electronegativities of W and Mo atoms afford changes in the electronic properties of the h-HTLs, which can allow easy control of the energy levels. The h-HTLs show matching energy levels that are suitable for both deep and low-lying highest occupied molecular orbital energy level systems with a stoichiometrically small amount of oxygen vacancies (forming W6+ and Mo6+ from the W5+ and Mo5+), affording high conductivity and good film forming properties. With the NFA-based photoactive layer, a large-area module fabricated via the all-printing process with an active area over 30 cm2 and a high power conversion efficiency (PCE) of 8.1% is obtained. Furthermore, with the h-HTL, the fabricated semitransparent module exhibits 7.2% of PCE and 22.3% of average visible transmittance with high transparency, indicating applicable various industrial potentials.

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 (Eloss ) 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 Eloss 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 Eloss values by synergistically lowering the HOMO energy levels.

High-Performance Nonfullerene Organic Photovoltaics Applicable for Both Outdoor and Indoor Environments through Directional Photon Energy Transfer.

With the advent of the smart factory and the Internet of Things (IoT) sensors, organic photovoltaics (OPVs) gained attention because of their ability to provide indoor power generation as an off-grid power supply. To satisfy these applications, OPVs must be capable of power generation in both outdoor and indoor at the same time for developing environmentally independent devices. For high performances in indoor irradiation, a strategy that maximizes photon utilization is essential. In this study, graphene quantum dots (GQDs), which have unique emitting properties, are introduced into a ZnO layer for efficient photon utilization of nonfullerene-based OPVs under indoor irradiation. GQDs exhibit high absorption properties in the 350-550 nm region and strong emission properties in the visible region due to down-conversion from lattice vibration. Using these properties, GQDs provide directional photon energy transfer to the bulk-heterojunction (BHJ) layer because the optical properties overlap. Additionally, the GQD-doped ZnO layer enhances shunt resistance (RSh) and forms good interfacial contact with the BHJ layer that results in increased carrier dissociation and transportation. Consequently, the fabricated device based on P(Cl-Cl)(BDD = 0.2) and IT-4F introduces GQDs exhibiting a maximum power conversion efficiency (PCE) of 14.0% with a superior enhanced short circuit current density (JSC) and fill factor (FF). Furthermore, the fabricated device exhibited high PCEs of 19.6 and 17.2% under 1000 and 200 lux indoor irradiation of light emitting diode (LED) lamps, respectively.

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

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

13.9%-Efficiency and Eco-Friendly Nonfullerene Polymer Solar Cells Obtained by Balancing Molecular Weight and Solubility in Chlorinated Thiophene-Based Polymer Backbones.

To industrialize nonfullerene polymer solar cells (NFPSCs), the molecular design of the donor polymers must feature low-cost materials and a high overall yield. Two chlorinated thiophene-based polymers, P(F-Cl) and P(Cl-Cl), are synthesized by introducing halogen effects like fluorine (F) and chlorine (Cl) to the previously reported P(Cl), which exhibits low complexity. However, the molecular weights of these polymers are insufficient owing to their low solubility, which in turn is caused by introducing rigid halogen atoms during the polymerization. Thus, they show relatively low power conversion efficiencies (PCEs) of 11.8% and 10.3%, respectively. To overcome these shortcomings, two new terpolymers are designed and synthesized by introducing a small amount of 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) unit into each backbone, namely, P(F-Cl)(BDD = 0.2) and P(Cl-Cl)(BDD = 0.2). As a result, both polymers remain inexpensive and show a better molecular weight-solubility balance, achieving high PCEs of 12.7% and 13.9%, respectively, in NFPSCs processed using eco-friendly solvents.

Drastic Changes in Properties of Donor-Acceptor Polymers Induced by Asymmetric Structural Isomers for Application to Polymer Solar Cells.

Appropriate design of donor-acceptor (D-A) conjugated polymers is important for enhancing their physical, optical, and electrochemical properties. The rapid development of D-A conjugated polymers based on fullerene and nonfullerene derivatives in the past decade has led to an improvement in the performance of polymer solar cells (PSCs). In this study, we designed and synthesized two donor polymers based on the DTffBT acceptor unit, with matching optical absorption range and energy levels with fullerene (PC71BM) and nonfullerene acceptors (ITIC and IDIC), by introducing asymmetric structural isomers of donor units. We demonstrated that materials design by structural modification dramatically affects the physical, optical, and electrochemical properties as well as the crystallinity and photovoltaic performance of the polymers. The results provide valuable insights into materials design for efficient PSCs.

A 3-Fluoro-4-hexylthiophene-Based Wide Bandgap Donor Polymer for 10.9% Efficiency Eco-Friendly Nonfullerene Organic Solar Cells.

Nonfullerene organic solar cells (NFOSCs) are attracting increasing academic and industrial interest due to their potential uses for flexible and lightweight products using low-cost roll-to-roll technology. In this work, two wide bandgap (WBG) polymers, namely P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6, are designed and synthesized using benzodithiophene (BDT) derivatives. Good oxidation stability and high solubility are achieved by simultaneously introducing fluorine and alkyl chains to a single thiophene (Th) unit. Solid P(fTh-2DBDT)-C6 films present WBG optical absorption, suitable frontier orbital levels, and strong π-π stacking effects. In addition, P(fTh-2DBDT)-C6 exhibits good solubility in both halogenated and nonhalogenated solvents, suggesting its suitability as donor polymer for NFOSCs. The P(fTh-2DBDT)-C6:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-Th) based device processed using chlorobenzene/1,8-diiodooctane (CB/DIO) exhibits a remarkably high power conversion efficiency (PCE) of 11.1%. Moreover, P(fTh-2DBDT)-C6:ITIC-Th reaches a high PCE of 10.9% when processed using eco-friendly solvents, such as o-xylene/diphenyl ether (DPE). The cell processed using CB/DIO maintains 100% efficiency after 1272 h, while that processed using o-xylene/DPE presents a 101% increase in efficiency after 768 h and excellent long-term stability. The results of this study demonstrate that simultaneous fluorination and alkylation are effective methods for designing donor polymers appropriate for high-performance NFOSCs.

Enhanced chemical and physical properties of PEDOT doped with anionic polyelectrolytes prepared from acrylic derivatives and application to nanogenerators.

Acrylic monomers, 4-hydroxybutyl acrylate (HBA) and 2-carboxyethyl acrylate (CEA), were each co-polymerized with styrene sulfonate in 10 mol% ratio to synthesize two types of anionic polyelectrolytes, P(SS-co-HBA) and P(SS-co-CEA), respectively. Through oxidative polymerization, two types of PEDOT composites (PEDOT:P(SS-co-HBA) and PEDOT:P(SS-co-CEA)) were synthesized, to which the anionic templates were applied as dopants. The composites were similar to PEDOT:PSS; however, crosslinking occurred with an increase in annealing temperature after film casting, which increased the electrical conductivity and hydrophobicity. The composites were applied as electrodes to PVDF-based piezoelectric nanogenerators (PNGs) having an electrode/PVDF/electrode structure. The output voltage, current, and maximum output power of PNG-2D(60) (PEDOT:P(SS-co-HBA)) annealed at a mild temperature (60 °C) were 4.12 V, 817.3 nA, and 847.5 nW, respectively, while those of PNG-3D(60) (PEDOT:P(SS-co-CEA)) annealed at 60 °C were 3.75 V, 756.5 nA, and 716.9 nW, respectively. Thus, the composites showed 13.4% and 11.3% improvements in the maximum output power compared with that of PNG-2D & 3D(RT) dried at room temperature, respectively. These results indicated 27.4% and 7.8% improvements, respectively, compared with PNG-1D(60) in which PEDOT:PSS without any crosslinking effect was applied. The PNGs demonstrated high potential as power sources owing to their sensitivity and excellent charging voltage performance for a 1 µF capacitor.

Effect on Electrode Work Function by Changing Molecular Geometry of Conjugated Polymer Electrolytes and Application for Hole-Transporting Layer of Organic Optoelectronic Devices.

In this study, we synthesized three conjugated polymer electrolytes (CPEs) with different conjugation lengths to control their dipole moments by varying spacers. P-type CPEs (PFT-D, PFtT-D, and PFbT-D) were generated by the facile oxidation of n-type CPEs (PFT, PFtT, and PFbT) and introduced as the hole-transporting layers (HTLs) of organic solar cells (OSCs) and polymer light-emitting diodes (PLEDs). To identify the effect on electrode work function tunability by changing the molecular conformation and arrangement, we simulated density functional theory calculations of these molecules and performed ultraviolet photoelectron spectroscopy analysis for films of indium tin oxide/CPEs. Additionally, we fabricated OSCs and PLEDs using the CPEs as the HTLs. The stability and performance were enhanced in the optimized devices with PFtT-D CPE HTLs compared to those of PEDOT:PSS HTL-based devices.

A Simple Approach to Fabricate an Efficient Inverted Polymer Solar Cell with a Novel Small Molecular Electrolyte as the Cathode Buffer Layer.

A novel small-molecule electrolyte, 1,1'-bis(4-hydroxypropyl)-[4,4'-bipyridine]-1,1'-diium bromide (V-OH), containing a mixture of PTB7:PC71BM has been designed and synthesized as a cathode buffer layer for inverted polymer solar cells (iPSCs). The molecular structure of this new compound comprises a viologen skeleton with hydroxyl group terminals. While the viologen unit is responsible for generating a favorable interface dipole, the two terminal hydroxyl groups of V-OH may generate a synergy effect in the magnitude of the interface dipole. Consequently, the devices containing the V-OH interlayer exhibited a power conversion efficiency (PCE) of 9.13% (short circuit current = 17.13 mA/cm2, open circuit voltage = 0.75 V, fill factor = 71.1%). The PCE of the devices with V-OH exhibited better long-term stability compared to that of the devices without V-OH. Thus, we found that it is possible to enhance the efficiency of PSCs by a simple approach without the need for complicated methods of device fabrication.

Improvement in Half-Life of Organic Solar Cells by Using a Blended Hole Extraction Layer Consisting of PEDOT:PSS and Conjugated Polymer Electrolyte.

In this study, we fabricated conventional structured organic solar cells (OSCs) by introducing a hole extraction layer (HEL) that consisted of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and conjugated polymer electrolyte (CPE) poly[9,9-bis(4'-sulfonatobutyl)fluorene-alt-thiophene] (PFT-D). PFT-D has a -SO3- functional group that acts as a conjugate base against the -SO3H of PSS. In addition, the molecular dipole of PFT-D can screen the Coulombic attraction between PEDOT chains and PSS chains. By blending PEDOT:PSS and PFT-D, the acidity of the HEL solution and changes to the surface morphology and potential of the HEL film as a function of exposure time in air were reduced. As a result, the half-life of the OSC fabricated with blended HEL was five times better at room temperature and 40% humidity without encapsulation as compared to the pristine PEDOT:PSS-based device.

Enhanced Photovoltaic Properties of Bulk Heterojunction Organic Photovoltaic Devices by an Addition of a Low Band Gap Conjugated Polymer.

In this study, we fabricated organic photovoltaics (OPVs) by introducing the polymer additive HTh6BT into the photoactive layer of a poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) system. The HTh6BT had a relatively low band gap energy of 1.65 eV and a molecular and crystalline structure similar to that of P3HT. In the photoactive layer, the HTh6BT and P3HT can both act as donors. In such parallel-type bulk heterojunctions, each donor can form excitons and generate charges while being separated from the donor/acceptor interface. Changes in the photovoltaic property of the OPV device by the addition of HTh6BT were evaluated, and the optical characteristics of the photoactive layer, as well as the surface morphology, polymer ordering, and crystallinity of the P3HT:PCBM film were analyzed. Compared to a device without HTh6BT, all short-circuit current densities, open-circuit voltages, and fill factors were enhanced, leading to the enhancement of the power conversion efficiency by 36%.

Rapid double-dye-layer coating for dye-sensitized solar cells using a new method.

Intensive research with the specific aim of developing inexpensive renewable energy sources is currently being undertaken. In dye-sensitized solar cell (DSSC) production, the most time-consuming process is coating the dye on working electrodes: absorption of ruthenium-based dyes [e.g., N719=bis(trtrabutylammonium)-cis-di(thiocyanato)-N,N'-bis(4-carboxylato-4'-carboxylic acid-2,2'-bipyridine) ruthenium(II)] on a photoanode takes a long time. We report a simple dye-coating method using a mixed solvent of ethylene glycol (EG) and glycerol (Gly). According to our experiments, dye-coating time can be reduced to 5 min from several hours. Maximum performance was obtained with an EG/Gly ratio of 1:1. This mixture of solvents gave a performance of 9.1%. Furthermore, the viscous solvent system could control coating depth; positioning dye coatings to a specific depth was rapid and facile. A cell containing two different dyes (N719+black dye) had an efficiency of 9.4%.

Synthesis and characterization of a fluorene-quinoxaline copolymer for light-emitting applications.

A conjugated copolymer based on 9,9-dioctyl-fluorene and 2,3-bis(4-(hexyloxy)phenyl) quinoxaline has been synthesized by the palladium-catalyzed Suzuki coupling reaction. The synthesized polymer was soluble in common organic solvents such as chloroform, THF, and toluene and had good film properties. The polymer was analyzed by 1H-NMR spectroscopy, UV-vis spectroscopy, GPC, TGA, DSC, and cyclic voltammetry. It had very good thermal properties with high decomposition and glass transition temperatures, 420 degrees C and 159 degrees C respectively, and a low band gap of 2.51 eV. The polymer LEDs (ITO/PEDOT:PSS/polymer/LiF/Ca/Al) showed pure green light emission with maximum peaks at 502 nm and CIE coordinates of x = 0.28 and y = 0.55. The turn-on voltage of the polymer device was 7 V and the maximum brightness was 10.16 cd/m2 at 14 V. The maximum luminescence efficiency of the polymer was 0.0011 cd/A at 11 V.