All-Polymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers

Efficient Derivatization of a Thienobenzobisthiazole-Based π-Conjugated Polymer Through Late-Stage Functionalization Towards High-Efficiency Organic Photovoltaic Cells

Derivatization is essential for optimizing organic material properties. However, because functional groups are often introduced at an early stage of the synthesis, similar intermediates have to be repeatedly synthesized to produce derivatives, which amounts to a daunting and time-consuming task. Using thienobenzobisthiazole (TBTz) as a building unit of donor polymers for organic photovoltaics (OPVs), we demonstrate an efficient derivatization of a TBTz-based π-conjugated polymer by late-stage functionalization. In the developed synthetic route, functional groups are introduced at the last step of monomer synthesis, enabling us to easily synthesize several derivatives from a common intermediate. Ester and acyl groups are introduced into the polymer instead of the alkyl group, giving rise to deep HOMO energy levels and resulting in OPV cells with high open-circuit voltage even in the absence of halogen substituents that are typically introduced into the donor polymers. Notably, the ester-functionalized TBTz-based polymer shows a small nonradiative voltage loss (ΔVnr) of 0.19 V and has one of the highest charge generation efficiencies among the halogen-free donor polymers with similar ΔVnr, improving the critical trade-off relationship between voltage loss and charge generation. Our results provide an important guideline for the efficient development of high-performance polymers for OPVs.

Design of Isoindigo-Based Small-Molecule Donors for Bulk Heterojunction Organic Solar Cell Applications in Combination with Nonfullerene Acceptors

The development of small-molecule organic solar cells with the required efficiency depends on the information obtained from molecular-level studies. In this context, 39 small-molecule donors featuring isoindigo as an acceptor moiety have been meticulously crafted for potential applications in bulk heterojunction organic solar cells. These molecules follow the D2-A-D1-A-D2 and D2-A-π-D1-π-A-D2 framework. Similar molecules considered in the previous experimental study (molecules R1 ((3E,3″E)-6,6″-(benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione)) and R2 ((3E,3″E)-6,6″-(4,8-dimethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione))) have been chosen as reference molecules. Molecules with and without π-spacers have been considered to understand the impact of the length of the π-spacer on intramolecular charge-transfer transitions and absorption properties. A detailed investigation is carried out to establish the relationship between the structure and photovoltaic parameters using density functional theory and time-dependent density functional theory methods. The newly developed molecules exhibit better electronic, excited-state, and charge transport properties than the reference molecules. Additionally, model donor-acceptor interfaces are constructed by integrating the designed donor molecules with fullerene/nonfullerene acceptors. The electronic and excited-state properties of these interfaces are rigorously evaluated. Results elucidate that the donor comprising of isoindigo-bithiophene-pyrroloindacenodithiophene (IIG-T2-PIDT) emerges as a promising candidate for bulk heterojunction solar cells based on nonfullerene acceptors. This research provides systematic design strategies for the development of small-molecule donors for organic solar cells.

High-Precision Tailored Polymer Molecular Weights for Specific Photovoltaic Applications through Ultrasound-Induced Simultaneous Physical and Chemical Events

It is highly challenging to reproducibly prepare semiconducting polymers with targeted molecular weight tailored for next-generation photovoltaic applications. Once such an easily accessible methodology is established, which can not only contribute to overcome the current limitation of the statistically determined nature of semiconducting polymers, but also facilitate rapid incorporation into the broad synthetic chemists' toolbox. Here, we describe a simple yet robust ultrasonication-assisted Stille polymerization for accessing semiconducting polymers with high-precision tailored molecular weights (from low to ultrahigh molecular weight ranges) while mitigating their interbatch variations. We propose that ultrasound-induced simultaneous physical and chemical events enable precise control of the semiconducting polymers' molecular weights with high reproducibility to satisfy all the optical/electrical and morphological demands of diverse types of high-performance semiconducting polymer-based devices; as demonstrated in in-depth experimental screenings in applications of both organic and perovskite photovoltaics. We believe that this methodology provides a fast development of new and existing semiconducting polymers with the highest-level performances possible on various photovoltaic devices.

Dual Polymerized Y-Acceptors of Distinct-Dimensionality Create Neuron-Like Interpenetrating Hierarchical Network towards Efficient and Stable All-Polymer Solar Cells

All-polymer solar cells have garnered particular attention thanks to their superior thermal, photo, and mechanical stabilities for large-scale manufacturing, yet the performance enhancement remains largely restrained by inherent morphological challenges of the bulk-heterojunction active layer. Herein, a 3D Y-branched polymerized small-molecule acceptor named PYBF, characteristic of high molecular weight and glass transition temperature, is designed and synthesized by precisely linking C3h-symmetric benzotrifuran with Y6 acceptors. In comparison to the benchmark thiophene-bridged linear PYIT acceptor, an optical blue-shift absorption is observed for PYBF yet a slightly higher power conversion efficiency (PCE) of 15.7% (vs 15.14%) is obtained when paired with polymer donor PM6, which benefit from the more crystalline and face-on-oriented PYBF domains. However, the star-like bulky structure of PYBF results in the nucleation-growth dominant phase-separation in polymeric blends, which generates stumpy droplet-like acceptor fibrils and impairs the continuity of acceptor phases. This issue is however surprisingly resolved by incorporating a small amount of PYIT, which leads to the formation of the more interconnective neuron-like dual-acceptor domains by long-chain entanglements of linear acceptors and alleviates bimolecular recombination. Thus, the champion device realizes a respectable PCE of up to ≈17% and importantly exhibits thermal and storage stabilities superior to the linear counterpart.

Processability Considerations for Next-Generation Organic Photovoltaic Materials

The evolution of organic semiconductors for organic photovoltaics (OPVs) has resulted in unforeseen outcomes. This has provided substitute choices of photoactive layer materials, which effectively convert sunlight into electricity. Recently developed OPV materials have narrowed down the gaps in efficiency, stability, and cost in devices. Records now show power conversion efficiency in single-junction devices closing to 20%. Despite this, there is still a gap between the currently developed OPV materials and those that meet the requirements of practical applications, especially the solution processability issue widely concerned in the field of OPVs. Based on the general rule that structure determines properties, methodologies to enhance the processability of OPV materials are reviewed and explored from the perspective of material design and views on the further development of processable OPV materials are presented. Considering the current dilemma that the existing evaluation indicators cannot reflect the industrial processability of OPV materials, a more complete set of key performance indicators are proposed for their processability considerations. The purpose of this perspective is to raise awareness of the boundary conditions that exist in industrial OPV manufacturing and to provide guidance for academic research that aspires to contribute to technological advancements.

Regulating the Sequence Structure of Conjugated Block Copolymers Enables Large-Area Single-Component Organic Solar Cells with High Efficiency and Stability

Single-component organic solar cells (SCOSCs) based on conjugated block copolymers (CBCs) by covalently bonding a polymer donor and polymer acceptor become more and more appealing due to the formation of a favorable and stable morphology. Unfortunately, a deep understanding of the effect of the assembly behavior caused by the sequence structure of CBCs on the device performance is still missing. Herein, from the aspect of manipulating the sequence length and distribution regularity of CBCs, we synthesized a series of new CBCs, namely D18(20)-b-PYIT, D18(40)-b-PYIT and D18(60)-b-PYIT by two-pot polymerization, and D18(40)-b-PYIT(r) by traditional one-pot method. It is observed that precise manipulation of sequence length and distribution regularity of the polymer blocks fine-tunes the self-assembly of the CBCs, optimizes film morphology, improves optoelectronic properties, and reduces energy loss, leading to simultaneously improved efficiency and stability. Among these CBCs, the D18(40)-b-PYIT-based device achieves a high efficiency of 13.4 % with enhanced stability, which is an outstanding performance among SCOSCs. Importantly, the regular sequence distribution and suitable sequence length of the CBCs enable a facile film-forming process of the printed device. For the first time, the blade-coated large-area rigid/flexible SCOSCs are fabricated, delivering an impressive efficiency of 11.62 %/10.73 %, much higher than their corresponding binary devices.

Anthradithiophene (ADT)-Based Polymerized Non-Fullerene Acceptors for All-Polymer Solar Cells

Realizing efficient all-polymer solar cell (APSC) acceptors typically involves increased building block synthetic complexity, hence potentially unscalable syntheses and/or prohibitive costs. Here we report the synthesis, characterization, and implementation in APSCs of three new polymer acceptors P1-P3 using a scalable donor fragment, bis(2-octyldodecyl)anthra[1,2-b : 5,6-b']dithiophene-4,10-dicarboxylate (ADT) co-polymerized with the high-efficiency acceptor units, NDI, Y6, and IDIC. All three copolymers have comparable photophysics to known polymers; however, APSCs fabricated by blending P1, P2 and P3 with donor polymers PM5 and PM6 exhibit modest power conversion efficiencies (PCEs), with the champion P2-based APSC achieving PCE=5.64 %. Detailed morphological and microstructural analysis by AFM and GIWAXS reveal a non-optimal APSC active layer morphology, which suppresses charge transport. Despite the modest efficiencies, these APSCs demonstrate the feasibility of using ADT as a scalable and inexpensive electron rich/donor building block for APSCs.

Revealing the Molecular Weight Effect on Highly Efficient Polythiophene Solar Cells

Polythiophenes (PTs) are promising electron donors in organic solar cells (OSCs) due to their simple structures and excellent synthetic scalability. Benefiting from the rational molecular design, the power conversion efficiency (PCE) of PT solar cells has been greatly improved. Herein, five batches of the champion PT (P5TCN-F25) with molecular weights ranging from 30 to 87 kg mol^-1 were prepared, and the effect of the molecular weight on the blend film morphology and photovoltaic performance of PT solar cells was systematically investigated. The results showed that the PCEs of the devices improved first and then maintained a high value with the increase of molecular weight, and the highest PCE of 16.7% in binary PT solar cells was obtained. Further characterizations revealed that the promotion in photovoltaic performance mainly comes from finer phase separation structures and more compact molecular packing in the blend film. The best device stabilities were also achieved by polymers with high molecular weights. Overall, this study highlights the importance of optimizing the molecular weight for PTs and offers directions to further improve the PCE of PT solar cells.

Oxidative-Reductive Near-Infrared Electrochromic Switching Enabled by Porous Vertically Stacked Multilayer Devices

Here, we demonstrate for the first time the ability of a porous π-conjugated semiconducting polymer film to enable facile electrolyte penetration through vertically stacked redox-active polymer layers, thereby enabling electrochromic switching between p-type and/or n-type polymers. The polymers P1 and P2, with structures diketopyrrolopyrrole (DPP)-π_bridge-3,4,-ethylenedioxythiophene (EDOT)-π_bridge [π_bridge = 2,5-thienyl for P1 and π_bridge = 2,5-thiazolyl for P2] are selected as the p-type polymers and N2200 (a known naphthalenediimide-dithiophene semiconductor) as the n-type polymer. Single-layer porous and dense (control) polymer films are fabricated and extensively characterized using optical microscopy, atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. The semiconducting films are then incorporated into single and multilayer electrochromic devices (ECDs). It is found that when a p-type (P2) porous top layer is used in a multilayer ECD, it enables electrolyte penetration to the bottom layer, enabling oxidative electrochromic switching of the P1 bottom layer at low potentials (+0.4 V versus +1.2 V with dense P2). Importantly, when using a porous P1 as the top layer with an n-type N2200 bottom layer, dynamic oxidative-reductive electrochromic switching is also realized. These results offer a proof of concept for development of new types of multilayer electrochromic devices where precise control of the semiconductor film morphology and polymer electronic structure is essential.

Effects of Different Lengths of Oligo (Ethylene Glycol) Side Chains on the Electrochromic and Photovoltaic Properties of Benzothiadiazole-Based Donor-Acceptor Conjugated Polymers

In recent years, donor-acceptor (D-A)-type conjugated polymers have been widely used in the field of organic solar cells (OSCs) and electrochromism (EC). Considering the poor solubility of D-A conjugated polymers, the solvents used in material processing and related device preparation are mostly toxic halogenated solvents, which have become the biggest obstacle to the future commercial process of the OSC and EC field. Herein, we designed and synthesized three novel D-A conjugated polymers, PBDT1-DTBF, PBDT2-DTBF, and PBDT3-DTBF, by introducing polar oligo (ethylene glycol) (OEG) side chains of different lengths in the donor unit benzodithiophene (BDT) as side chain modification. Studies on solubility, optics, electrochemical, photovoltaic and electrochromic properties are conducted, and the influence of the introduction of OEG side chains on its basic properties is also discussed. Studies on solubility and electrochromic properties show unusual trends that need further research. However, since PBDT-DTBF-class polymers and acceptor IT-4F failed to form proper morphology under the low-boiling point solvent THF solvent processing, the photovoltaic performance of prepared devices is not ideal. However, films with THF as processing solvent showed relatively desirable electrochromic properties and films cast from THF display higher CE than CB as the solvent. Therefore, this class of polymers has application feasibility for green solvent processing in the OSC and EC fields. The research provides an idea for the design of green solvent-processable polymer solar cell materials in the future and a meaningful exploration of the application of green solvents in the field of electrochromism.

In Situ Optical Spectroscopy Demonstrates the Effect of Solvent Additive in the Formation of All-Polymer Solar Cells

1-Chloronaphthalene (CN) has been a common solvent additive in both fullerene- and nonfullerene-based organic solar cells. In spite of this, its working mechanism is seldom investigated, in particular, during the drying process of bulk heterojunctions composed of a donor:acceptor mixture. In this work, the role of CN in all-polymer solar cells is investigated by in situ spectroscopies and ex situ characterization of blade-coated PBDB-T:PF5-Y5 blends. Our results suggest that the added CN promotes self-aggregation of polymer donor PBDB-T during the drying process of the blend film, resulting in enhanced crystallinity and hole mobility, which contribute to the increased fill factor and improved performance of PBDB-T:PF5-Y5 solar cells. Besides, the nonradiative energy loss of the corresponding device is also reduced by the addition of CN, corresponding to a slightly increased open-circuit voltage. Overall, our observations deepen our understanding of the drying dynamics, which may guide further development of all-polymer solar cells.

Improving the Photovoltaic Performance of Dithienobenzodithiophene-Based Polymers via Addition of an Additional Eluent in the Soxhlet Extraction Process

Dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophene (DTBDT) is a kind of pentacyclic aromatic electron-donating unit with unique optoelectronic properties, but it has received less attention in the design of photovoltaic polymers. In this work, we copolymerized DTBDT with the electron-deficient unit of dithieno[3',2':3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) and obtained two polymers, PE55 and PE56, with a synergistic heteroatom substitution strategy. When blended with the classic nonfullerene acceptor Y6, PE55 and PE56 achieve power conversion efficiencies (PCEs) of 13.78% and 14.49%, respectively, which indicates that the introduction of sulfur atoms on the conjugated side chain of the D unit is a promising method to enhance the performance of DTBDT-based polymers. Besides, we utilize dichloromethane and chloroform to separate the low molecular weight (Mw) fractions in the solvent extraction process to obtain PE55-CF and PE56-CB, and the PCEs are further improved to 15.00% and 16.11%, respectively. The stronger π-π stacking, optimized blend film morphology, and higher charge mobilities contribute to the enhanced PCEs for polymers with higher Mw obtained via the multistep solvent extraction strategy. Our results not only provide a simple and effective way to improve the photovoltaic performance of conjugated polymers but also imply that some reported polymers purified from the traditional one-step solvent extraction method might be seriously underestimated.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-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-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Near-Infrared Materials: The Turning Point of Organic Photovoltaics

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by our group, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000 nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Effective Industry-Academia Collaboration Driving Polymer Innovation

Polymer science is one of the few fundamental research fields where the results can be transferred into real-life products almost immediately. Industries need collaborations with the best researchers (universities or national laboratories) to elevate the field and favor the development of new materials, which will boost the chemical and materials business economy and ensure that innovative and sustainable polymer products are constantly being brought to the market. The mechanisms to ensure a seamless and fruitful collaboration are numerous, but few approaches really manage to incorporate the full range of polymer research from a molecular understanding to a macroscopic control of properties. We review some of the main components of standard industry-academia collaborations and propose to develop polymer open centers that put the business development objective as the starting point of the collaboration and allow those to gather and focus on different scientific fields toward a common objective.

Blending Donors with Different Molecular Weights: An Efficient Strategy to Resolve the Conflict between Coherence Length and Intermixed Phase in Polymer/Nonfullerene Solar Cells

Long coherence lengths (CLs) of crystals and proper intermixed phase amount guarantee charge transport and exciton dissociate efficiently, which is crucial for organic solar cells (OSCs) to achieve high device performance. However, extending CLs usually reduces the intermixed phase, leading to an insufficient interface for exciton dissociation. Herein, a strategy using a binary polymer with different molecular weights as donor is employed, that is, poly(3-hexylthiophene-2,5-diyl) (P3HT) with high (P3HT-H) and low (P3HT-L) molecular weight are blended as donor, and (5Z,5'Z)-5,5'-(((4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) is used as acceptor. In kinetics, the entanglements of P3HT-H are relieved due to the higher molecular diffusivity of P3HT-L. In thermodynamics, the miscibility of P3HT-L/O-IDTBR, P3HT-H/O-IDTBR, and P3HT-L/P3HT-H blends increases in turn. Hence, P3HT forms a more ordered structure with longer CLs after adding P3HT-L, which also drives O-IDTBR dispersed in P3HT crystalline regions diffuse to the O-IDTBR crystalline regions to further self-organize. Consequently, the CLs of both P3HT and O-IDTBR are extended, while keeping the intermixed phase amount proper. The optimized microstructure boosts device performance from 7.03% to 7.80%, which is one of the highest values reported for P3HT/O-IDTBR blends. This is a novel way to solve the conflict mentioned above, which may provide guidance to finely regulating the morphology of the active layer.

Recent advances in non-fullerene organic photovoltaics enabled by green solvent processing

Solution-processed organic photovoltaic (OPV) as a new energy device has attracted much attention due to its huge potential in future commercial manufacturing. However, so far, most of the studies on high-performance OPV have been treated with halogenated solvents. Halogenated solvents not only pollute the environment, but are also harmful to human health, which will negatively affect the large-scale production of OPV in the future. Therefore, it is urgent to develop low-toxic or non-toxic non-halogen solvent-processable OPV. Compared with conventional fullerene OPVs, non-fullerene OPVs exist with stronger absorption, better-matched energy levels and lower energy loss. Processing photoactive layers with non-fullerenes as the acceptor material has broad potential advantages in non-halogenated solvents. This review introduces the research progress of non-fullerene OPV treated by three different kinds of green solvents as the non-halogenated and aromatic solvent, the non-halogenated and non-aromatic solvent, alcohol and water. Furthermore, the effects of different optimization strategies on the photoelectric performance and stability of non-fullerene OPV are analyzed in detail. The current optimization strategy can increase the power conversion efficiency of non-fullerene OPV processed with non-halogen solvents up to 17.33%, which is close to the performance of processing with halogen-containing solvents. Finally, the commercial potential of non-halogen solvent processing OPVs is discussed. The green solvent processing of non-fullerene-based OPVs will become a key development direction for the future of the OPV industry.

N-Type Quinoidal Polymers Based on Dipyrrolopyrazinedione for Application in All-Polymer Solar Cells

Conjugated molecules and polymers with intrinsic quinoidal structure are promising n-type organic semiconductors, which have been reported for application in field-effect transistors and thermoelectric devices. In principle, the molecular and electronic characteristics of quinoidal polymers can also enable their application in organic solar cells. Herein, two quinoidal polymers, named PzDP-T and PzDP-ffT, based on dipyrrolopyrazinedione were synthesized and used as electron acceptors in all-polymer solar cells (all-PSCs). Both PzDP-T and PzDP-ffT showed suitable energy levels and wide light absorption range that extended to the near-infrared region. When combined with the polymer donor PBDB-T, the resulting all-PSCs based on PzDP-T and PzDP-ffT exhibited a power conversion efficiency (PCE) of 1.33 and 2.37 %, respectively. This is the first report on the application of intrinsic quinoidal conjugated polymers in all-PSCs. The photovoltaic performance of the all-PSCs was revealed to be mainly limited by the relatively poor and imbalanced charge transport, considerable charge recombination. Detailed investigations on the structure-performance relationship suggested that synergistic optimization of light absorption, energy levels, and charge transport properties is needed to achieve more successful application of intrinsic quinoidal conjugated polymers in all-PSCs.

A Wide Bandgap Polymer Donor Composed of Benzodithiophene and Oxime-Substituted Thiophene for High-Performance Organic Solar Cells

Oxime-substituted thiophene (TO) is used as an acceptor (A) unit to copolymerize with the benzodithiophene (BDT) donor (D) unit to form a novel D-A polymer donor, PBDTTO, which has a low-lying highest occupied molecular orbital energy level (E_HOMO) of -5.60 eV and a wide bandgap of 2.03 eV, forming complementary absorption and matching energy levels with the narrow bandgap nonfullerene acceptors. Organic solar cells using PBDTTO and Y6 as the donor and acceptor, respectively, exhibited a J_SC of 27.03 mA cm^-2, a V_OC of 0.83 V, and a fill factor of 0.59, reaching a high power conversion efficiency of 13.29%. The unencapsulated devices show good long-term stability in ambient air. Compared with the acceptor monomers used in other high-performance BDT-based D-A polymer donors, which are synthesized tediously in low yields, the TO acceptor monomer can be conveniently synthesized in only two steps with a high overall yield of 70%. These results demonstrate that TO unit can be used as a promising acceptor unit for developing BDT-based D-A polymer donors at low cost while maintaining high photovoltaic performance.

Molecular Weight Engineering in High-Performance Ambipolar Emissive Mesopolymers

Mesopolymers with high solubility, free of structural defects, and negligible batch-to-batch variation open a new avenue for organic optoelectronics. Organic light emitting transistors that combine the functions of organic light-emitting diodes and organic field-effect transistors. However, charge transport ability and light emitting strength are contradictory within one conjugated polymer. Herein, three low-molecular-weight mesopolymers with thienopyrroledione-benzothiadiazole repeating units (meso-TBTF) were obtained. The mesopolymers show strong solid-state emission and high ambipolar carrier mobility. The molecular weights of meso-TBTF can be tuned by polymerization temperature. The mesopolymers have photoluminescence quantum yields (PLQY) of about 50 % in solution and 10 % in solid state. Polymer light emitting diodes of this material are fabricated to explore its potential use in optoelectronic devices.

To Reveal the Importance of the Crystallization Sequence on Micro-Morphological Structures of All-Crystalline Polymer Blends by In Situ Investigation

In crystalline/crystalline polymer blend systems, complex competition and coupling of crystallization and morphology usually happen due to the different crystal nucleation and growth processes of polymers, making the morphology and crystallization behavior difficult to control. Herein, we probe the crystallization sequence during the film formation process (crystallize simultaneously, component A crystallizes prior to B or inverse) to illustrate the micro-morphology evolution process in poly(3-hexylthiophene) (P3HT) and poly[[N,N-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5, 5'-(2,2'-bithiophene)] (N2200) blend using in situ UV-vis absorption spectra and in situ two-dimensional grazing incidence X-ray diffraction (2D GIXRD). When P3HT and N2200 crystallize simultaneously, a large-sized morphology structure is formed. When strengthening the solution aggregation of P3HT by increasing the solvent-polymer interaction, P3HT crystallizes prior to N2200. A P3HT-based micro-morphology structure is obtained. As the molecular weight of N2200 increases to a critical value (72.0 kDa), the crystallization of N2200 dominates the film formation process. A N2200-based micro-morphology is formed guided by N2200 domains. The results confirm that the crystallization sequence is one of the most important factors to determine the micro-morphology structure in all-crystalline polymer blends.

Out-of-Plane Alignment of Conjugated Semiconducting Polymers by Horizontal Rotation in a High Magnetic Field

The effective control of film morphology and molecular packing in the out-of-plane direction of semiconductor polymers plays a critical role in governing charge carrier transport in the direction perpendicular to the substrate. In this study, a highly out-of-plane alignment of the n-type polymer P(NDI2OD-T2) film has been successfully achieved by horizontal rotation in a high magnetic field (HR-HMF). The out-of-plane alignment of the P(NDI2OD-T2) film has showed a change from 72% face-on to 98.2% face-on lamellar texture as well as a 1.6-fold increase of the π-π stacking crystalline correlation length compared with that of as-cast polymer films without HR-HMF-induced alignment. Meanwhile, the film with near-perfect face-on molecular packing exhibited more than 18-fold enhancement of electron mobility compared to the unaligned film. The excellent electrical performance achieved with the HR-HMF process indicates its application potential for fabricating high-performance sandwich-type organic electronic devices, such as solar cells and light-emitting diodes.

Backbone Engineering with Asymmetric Core to Finely Tune Phase Separation for High-Performance All-Small-Molecule Organic Solar Cells

In order to obtain high-performance all-small-molecule organic solar cells (ASM-OSCs), it is crucial to exploit the available strategy for molecular design and to further understand key structure-property relationship that can rationally control the blend nanomorphology and influence the physical process. In this work, we design two small molecule donors FBD-S1 and TBD-S2 with identical electron-withdrawing units but various asymmetric central cores, which exhibit differing phase separation in Y6-based blend films. It is found that TBD-S2 with increased phase separation between donor and acceptor can lead to more favorable interpenetrating networks, effective exciton dissociation, and enhanced and more balanced charge transport. Importantly, a remarkable PCE of 13.1% is obtained for TBD-S2:Y6 based ASM-OSCs, which is an attractive photovoltaic performance for ASM-OSCs. This result demonstrates that the central core modification at the atomic level for small molecule donors can delicately control the phase separation and optimize photophysical processes, and refines device performance, which facilitate development in the ASM-OSC research field.

Impact of Chlorination Patterns of Naphthalenediimide-Based Polymers on Aggregated Structure, Crystallinity, and Device Performance of All-Polymer Solar Cells and Organic Transistors

The aggregation properties of conjugated polymers can play a crucial role in their thin film structures and performance of electronic devices. Control of these aggregated structures is particularly important in producing efficient all-polymer solar cells (all-PSCs), considering that strong demixing of the polymer donor and polymer acceptor typically occurs during film formation because of the low entropic contribution to the thermodynamics of the system. Here, three naphthalenediimide (NDI)-based polymer acceptors with different backbone chlorination patterns are developed to investigate the effect of the chlorination patterns on the aggregation tendencies of the polymer acceptors, which greatly influence their crystalline structures, electrical properties, and device performances of the resultant all-PSCs and organic field-effect transistors (OFETs). The counterparts of NDI units, dichlorinated bithiophene (Cl2T2), monochlorinated bithiophene (ClT2), and dichlorinated thienylene-vinylene-thienylene (Cl2TVT), are employed to synthesize a series of P(NDIOD-Cl2T2), P(NDIOD-ClT2), and P(NDIOD-Cl2TVT) polymers. The P(NDIOD-Cl2T2) polymer takes advantage of strong noncovalent bonding induced by its chlorine substituents, resulting in the formation of optimal face-on oriented crystalline structures which are suitable for efficient all-PSC devices. In comparison, the P(NDIOD-Cl2TVT) polymer forms bimodal crystalline structures in thin films to yield optimal performances in the resultant OFETs. When the three chlorinated polymers are applied to all-PSCs with the PBDTTTPD polymer donor, P(NDIOD-Cl2T2) achieves a maximum power conversion efficiency (PCE) of 7.22% with an appropriate blend morphology and high fill factor, outperforming P(NDIOD-ClT2) (PCE = 4.80%) and P(NDIOD-Cl2TVT) (PCE = 5.78%). Our observations highlight the effectiveness of the chlorination strategy for developing efficient polymer acceptors and demonstrate the important role of polymer aggregation in modulating the blend morphology and all-PSC performance.

A Non-Conjugated Polymer Acceptor for Efficient and Thermally Stable All-Polymer Solar Cells

A non-conjugated polymer acceptor PF1-TS4 was firstly synthesized by embedding a thioalkyl segment in the mainchain, which shows excellent photophysical properties on par with a fully conjugated polymer, with a low optical band gap of 1.58 eV and a high absorption coefficient >105  cm-1 , a high LUMO level of -3.89 eV, and suitable crystallinity. Matched with the polymer donor PM6, the PF1-TS4-based all-PSC achieved a power conversion efficiency (PCE) of 8.63 %, which is ≈45 % higher than that of a device based on the small molecule acceptor counterpart IDIC16. Moreover, the PF1-TS4-based all-PSC has good thermal stability with ≈70 % of its initial PCE retained after being stored at 85 °C for 180 h, while the IDIC16-based device only retained ≈50 % of its initial PCE when stored at 85 °C for only 18 h. Our work provides a new strategy to develop efficient polymer acceptor materials by linkage of conjugated units with non-conjugated thioalkyl segments.

Mechanistic Studies of Hydrogen Evolution Reaction on Donor-Acceptor Conjugated Polymer Photocatalysts

The application of donor-acceptor (D-A) conjugated polymer catalysts for hydrogen evolution reaction (HER) has shown great promise because of the tunability of such catalysts to have desired properties. Herein, we synthesized two polymer catalysts: poly[4,4′-(9-(4-aminophenyl)-9H-carbazole-3,6-diamine-alt-5-oxido-5-phenylbenzo[b]phosphindole-3,7-diyl)dibenzaldehyde] (PCzPO) and poly[N1,N1-bis(4-amino-2-fluorophenyl)-2-fluorobenzene-1,4-diamine-alt-5-oxido-5-phenylbenzo[b]phosphindole-3,7-diyl)dibenzaldehyde] (PNoFPO). The UV-vis absorption spectra showed that the less planar structure and the presence of electronegative fluorine atoms in the donor group of PNoFPO led to a higher optical gap compared to PCzPO, leading to almost five times faster HER rate using PCzPO compared to PNoFPO. However, density functional theory (DFT) calculations show that the frontier orbitals and the highest occupied molecular orbitals – lowest unoccupied molecular orbitals (HOMO-LUMO) gaps of PCzPO and PNoFPO D-A moiety models are very similar, such that, during light absorption, electrons move from donor to acceptor group where proton binding is preferred to happen thereafter. For both PCzPO and PNoFPO D-A moieties, H2 formation through an intramolecular reaction with a barrier of 0.6–0.7 eV, likely occurs at the acceptor group atoms where protons bind through electrostatic interaction. The intermolecular reaction has nearly zero activation energy but is expected to occur only when the repulsion is low between separate polymers chains. Finally, experimental and DFT results reveal the importance of extended configurations of D-A polymers on HER rate.

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Polymer Acceptors Containing B←N Units for Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron-nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.

Increasing N2200 Charge Transport Mobility to Improve Performance of All Polymer Solar Cells by Forming a Percolation Network Structure

The poor electron transport ability of the polymer acceptor is one of the factors restricting the performance of all-polymer solar cells. The percolation network of conjugated polymers can promote its charge transfer. Hence, we aim to find out the critical molecular weight (MW) of N2200 on the forming of the percolation network and to improve its charge mobility and thus photovoltaic performance of J51:N2200 blend. Detailed measurements demonstrate that when the MW of N2200 is larger than 96k, a percolation network structure is formed due to the chain tangled and multi-chain aggregations. Analysis of kinetic experiments reveals that it is the memory of the N2200 long chain conformation and the extent of aggregation in solution are carried into cast films for the formation of the percolation network. Thus, the electron mobility increases from 5.58 × 10-6 cm2V-1s-1 (N220017k) to 9.03 × 10-5 cm2V-1s-1 when the MW of N2200 is >96k. It led to a balance between hole and electron mobility. The μh/μe decrease from 16.9 to 1.53, causing a significant enhancement in the PCEs, from 5.87 to 8.28% without additives.

Synthesis of the diketopyrrolopyrrole/terpyridine substituted carbazole derivative based polythiophenes for photovoltaic cells

A series of conjugated polythiophenes (PTs) having low band gap energies (PDPP, PDPCz21, PDPCz11), with 2-ethylhexyl-functionalized 2,5-thienyl diketopyrrolopyrrole (TDPP) as the electron acceptor and terpyridine-substituted carbazole (TPCz) as the electron donor, have been synthesized and studied for their applicability in polymer-based photovoltaic cells (PVCs). The thermal stability and solvent solubility of PTs increased upon increasing the content of the TPCz derivative. PVCs were fabricated having the following architecture: indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/PT:6,6-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM)/Ca/Ag. The compatibility between the PT and PC₇₁BM improved upon increasing the TPCz content. The photovoltaic properties of the PDPCz21-based PVCs were superior to those of their PDPP- and PDPCz11-based counterparts.

A series of conjugated polythiophenes (PTs) having low band gap energies (PDPP, PDPCz21, PDPCz11) have been synthesized and studied for their applicability in polymer-based photovoltaic cells (PVCs).