Fine-Tuning Semiconducting Polymer Self-Aggregation and Crystallinity Enables Optimal Morphology and High-Performance Printed All-Polymer Solar Cells

p-Toluenesulfonic Acid Modified Two-Dimensional ZrSe2 as a Hole Transport Layer for High-Performance Organic Solar Cells

Two-dimensional (2D) materials have attracted attention due to their excellent optoelectronic properties, but their applications are limited by their defects and vacancies. Surface modification is an effective method to restore their performance. Here, ZrSe2 is modified with conductive polymer p-toluenesulfonic acid (PTSA). It is found that PTSA can obtain electrons of ZrSe2 through the combination of -SO3H and ZrSe2, thus forming interfacial dipoles, which improve the work function of ZrSe2. In addition, -OH in PTSA can effectively fill the Se vacancy in ZrSe2 to form P-type doping, thereby improving its conductivity. ZrSe2 modified by the PTSA material is first used as a hole transport layer (HTL) in organic solar cells (OSCs). The efficiency of OSCs based on the PBDB-T:ITIC and PM6:L8-BO binary active layer with ZrSe2:PTSA as the novel HTL reaches 10.66 and 18.14%, which are obviously higher than the efficiency of OSCs with pure ZrSe2 as the HTL (8.48 and 15.64%). More interestingly, the stability of the device with ZrSe2:PTSA as HTL is significantly better than that of PEDOT:PSS. This study shows that the modification of the organic material can effectively improve the photoelectric performance of ZrSe2 and explores the physical mechanism of the interaction between the organic modifier and 2D materials.

Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells

All-polymer solar cells (all-PSCs) offer improved morphological and mechanical stability compared with those containing small-molecule-acceptors (SMAs). They can be processed with a broader range of conditions, making them desirable for printing techniques. In this study, we report a high-performance polymer acceptor design based on bithiazole linker (PY-BTz) that are on par with SMAs. We demonstrate that bithiazole induces a more coplanar and ordered conformation compared to bithiophene due to the synergistic effect of non-covalent backbone planarization and reduced steric encumbrances. As a result, PY-BTz shows a significantly higher efficiency of 16.4% in comparison to the polymer acceptors based on commonly used thiophene-based linkers (i.e., PY-2T, 9.8%). Detailed analyses reveal that this improvement is associated with enhanced conjugation along the backbone and closer interchain π-stacking, resulting in higher charge mobilities, suppressed charge recombination, and reduced energetic disorder. Remarkably, an efficiency of 14.7% is realized for all-PSCs that are solution-sheared in ambient conditions, which is among the highest for devices prepared under conditions relevant to scalable printing techniques. This work uncovers a strategy for promoting backbone conjugation and planarization in emerging polymer acceptors that can lead to superior all-PSCs.

Transient nucleation driven by solvent evaporation

We theoretically investigate homogeneous crystal nucleation in a solution containing a solute and a volatile solvent. The solvent evaporates from the solution, thereby continuously increasing the concentration of the solute. We view it as an idealized model for the far-out-of-equilibrium conditions present during the liquid-state manufacturing of organic electronic devices. Our model is based on classical nucleation theory, taking the solvent to be a source of the transient conditions in which the solute drops out of the solution. Other than that, the solvent is not directly involved in the nucleation process itself. We approximately solve the kinetic master equations using a combination of Laplace transforms and singular perturbation theory, providing an analytical expression for the nucleation flux. Our results predict that (i) the nucleation flux lags slightly behind a commonly used quasi-steady-state approximation. This effect is governed by two counteracting effects originating from solvent evaporation: while a faster evaporation rate results in an increasingly larger influence of the lag time on the nucleation flux, this lag time itself is found to decrease with increasing evaporation rate. Moreover, we find that (ii) the nucleation flux and the quasi-steady-state nucleation flux are never identical, except trivially in the stationary limit, and (iii) the initial induction period of the nucleation flux, which we characterize as a generalized induction time, decreases weakly with the evaporation rate. This indicates that the relevant time scale for nucleation also decreases with an increasing evaporation rate. Our analytical theory compares favorably with results from a numerical evaluation of the governing kinetic equations.

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.

Visualizing the multi-level assembly structures of conjugated molecular systems with chain-length dependent behavior

It remains challenging to understand the structural evolution of conjugated polymers from single chains to solvated aggregates and film microstructures, although it underpins the performance of optoelectrical devices fabricated via the mainstream solution processing method. With several ensemble visual measurements, here we unravel the morphological evolution process of a model system of isoindigo-based conjugated molecules, including the hidden molecular assembly pathways, the mesoscale network formation, and their unorthodox chain dependence. Short chains show rigid chain conformations forming discrete aggregates in solution, which further grow to form a highly ordered film that exhibits poor electrical performance. In contrast, long chains exhibit flexible chain conformations, creating interlinked aggregates networks in solution, which are directly imprinted into films, forming interconnective solid-state microstructure with excellent electrical performance. Visualizing multi-level assembly structures of conjugated molecules provides a deep understanding of the inheritance of assemblies from solution to solid-state, accelerating the optimization of device fabrication.

Asymmetric Polymer Additive for Morphological Regulation and Thermally Stable Organic Solar Cells

High thermal stability is crucial for the commercialization of organic solar cells (OSCs). The thermal stability of OSCs has been improved using the tailoring blend morphology of bulk heterojunctions (BHJs). Herein, we demonstrated thermally stable OSCs in a ternary blended system containing low-crystalline semiconducting polymers (asy-PNDI1FTVT and PTB7-Th) and a non-fullerene acceptor (Y6). The asymmetric n-type semiconducting polymer (asy-PNDI1FTVT) differed from general symmetric semiconducting polymers as it randomly substituted fluorine atoms at the donor moiety (TVT), resulting in significantly lower crystallinity. asy-PNDI1FTVT in PTB7-Th:Y6 exhibited a well-mixed morphology at the BHJ and efficiently facilitated the charge dissociation process with an enhanced fill factor and power conversion efficiency. Furthermore, the ternary system of PTB7-Th:Y6:asy-PNDI1FTVT suppressed phase separation with negligible burn-in loss and performance degradation under thermal stress. The experiments showed that our devices without encapsulation retained over 90% of their initial efficiencies after 100 h at 65 °C. These results show significant potential for the development of thermally stable OSCs with reasonable efficiency.

Manipulating Polymer Backbone Configuration via Halogenated Asymmetric End-Groups Enables Over 18% Efficiency All-Polymer Solar Cells

High-performance all-polymer solar cells (all-PSCs) deeply rely on the joint contributions of desirable optical absorption, adaptive energy levels, and appropriate morphology. Herein, two structural analogous polymerized small-molecule acceptors (PSMAs), PYFCl-T and PYF&PYCl-T, are synthesized, and then incorporated into the PM6:PY-IT binary blends to construct ternary all-PSCs. Due to the superior compatibility of PY-IT and PYFCl-T, the ternary all-PSC based on PM6:PY-IT:PYFCl-T with 10 wt% PYFCl-T, presents higher and more balanced charge mobility, suppressed charge recombination, and faster charge-transfer kinetics, resulting in an outstanding power conversion efficiency (PCE) of 18.12% with enhanced J_sc and FF, which is much higher than that (PCE of 16.09%) of the binary all-PSCs based on PM6:PY-IT. Besides, the ternary all-PSCs also exhibit improved photostability. The conspicuous performance enhancement principally should give the credit to the miscibility-driven phase optimization of the donor and acceptor. These findings highlight the significance of polymer-backbone configuration modulation of PSMAs in morphology optimization toward boosting the device properties of all-PSCs.

Linear Regulating of Polymer Acceptor Aggregation with Short Alkyl Chain Units Enhances All-Polymer Solar Cells' Efficiency

The power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) has ascended rapidly arising from the development of polymerized small-molecule acceptor materials. However, numerous insulating long alkyl chains, which ensure the solubility of the polymer, result in inferior aggregation and charge mobility. Herein, this study proposes a facile random copolymerization strategy of two small molecule acceptor units with different lengths of alkyl side chains and synthesizes a series of polymer acceptors PYT-EHx, where x is the percentage of the short alkyl chain units. The aggregation strength and charge mobility of the acceptors rise linearly with increasing the proportion of short alkyl chain units. Thus, the PYT-EH20 reaches balanced aggregation with the star polymer donor PBDB-T, resulting in optimal morphology, fastest carrier transport, and reduced recombination and energy loss. Consequently, the PYT-EH20-based device yields a 14.8% PCE, a 16% improvement over the control PYT-EH0-based device, accompanied by an increase in open-circuit voltage (V_oc ), short-circuit current density (J_sc ), and fill factor (FF). This work demonstrates that the random copolymerization strategy with short alkyl chain insertion is an effective avenue for developing high-performance polymer acceptors, which facilitates further advances in the efficiency of all-PSCs.

Improved Molecular Ordering in a Ternary Blend Enables All-Polymer Solar Cells over 18% Efficiency

Although all-polymer solar cells (all-PSCs) show great commercialization prospects, their power conversion efficiencies (PCEs) still fall behind their small molecule acceptor-based counterparts. In all-polymer blends, the optimized morphology and high molecular ordering are difficult to achieve since there is troublesome competition between the crystallinity of the polymer donor and acceptor during the film-formation process. Therefore, it is challenging to improve the performance of all-PSCs. Herein, a ternary strategy is adopted to modulate the morphology and the molecular crystallinity of an all-polymer blend, in which PM6:PY-82 is selected as the host blend and PY-DT is employed as a guest component. Benefiting from the favorable miscibility of the two acceptors and the higher regularity of PY-DT, the ternary matrix features a well-defined fibrillar morphology and improved molecular ordering. Consequently, the champion PM6:PY-82:PY-DT device produces a record-high PCE of 18.03%, with simultaneously improved open-circuit voltage, short-circuit current and fill factor in comparison with the binary devices. High-performance large-area (1 cm² ) and thick-film (300 nm) all-PSCs are also successfully fabricated with PCEs of 16.35% and 15.70%, respectively.Moreover, 16.5 cm² organic solar module affords an encouraging PCE of 13.84% when using the non-halogenated solvent , showing the great potential of "Lab-to-Fab" transition of all-PSCs.

Molecular symmetry effect on the morphology and self-aggregation of semiconducting polymers

We investigate molecular symmetricity on the physical and photoelectric properties of semiconducting polymers by selectively incorporating different numbers of fluorine atoms at different positions in the donor unit of NDI-based copolymers.

Quasi-Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

Effects of Molecular Encapsulation on the Photophysical and Charge Transport Properties of a Naphthalene Diimide Bithiophene Copolymer

Engineering the molecular structure of conjugated polymers is key to advancing the field of organic electronics. In this work, we synthesized a molecularly encapsulated version of the naphthalene diimide bithiophene copolymer PNDIT2, which is among the most popular high charge mobility organic semiconductors in n-type field-effect transistors and non-fullerene acceptors in organic photovoltaic blends. The encapsulating macrocycles shield the bithiophene units while leaving the naphthalene diimide units available for intermolecular interactions. With respect to PNDIT2, the encapsulated counterpart displays an increased backbone planarity. Molecular encapsulation prevents preaggregation of the polymer chains in common organic solvents, while it permits π-stacking in the solid state and promotes thin film crystallinity through an intermolecular-lock mechanism. Consequently, n-type semiconducting behavior is retained in field-effect transistors, although charge mobility is lower than in PNDIT2 due to the absence of the fibrillar microstructure that originates from preaggregation in solution. Hence, molecularly encapsulating conjugated polymers represent a promising chemical strategy to tune the molecular interaction in solution and the backbone conformation and to consequently control the nanomorphology of casted films without altering the electronic structure of the core polymer.

Impact of backbone linkage positions on the molecular aggregation behavior of polymer photovoltaic materials

It is imperative to advance the structural design of conjugated materials to achieve a practical impact on the performance of photovoltaic devices. However, the effect of the linkage positions (meta-, para-) of the backbone on the molecular packing has been relatively little explored. In this study, we have synthesized two wide-bandgap polymer photovoltaic materials from identical monomers with different linkage positions, using dibenzo[c,h][2,6]-naphthyridine-5,11-(6H,12H)-dione (DBND) as the building block. This study shows that the para-connected polymer exhibits an unexpected 0.2 eV higher ionization potential and a resultant higher open-circuit voltage than the meta-connected counterpart. We found that different linkage positions result in different intermolecular binding energies and molecular aggregation conformations, leading to different HOMO energy levels and photovoltaic performances. Specifically, theoretical calculations and 2D-NMR indicate that P(p-DBND-f-2T) performs a segregated stacking of f-2T and DBND units, while P(m-DBND-f-2T) films form π-overlaps between f-2T and DBND. These results show that linkage position adjustment on the polymeric backbone exerts a profound influence on the molecular aggregation of the materials. Also, the effect of isomerism on the polymer backbone is crucial in designing polymer structures for photovoltaic applications.

High-Performance All-Polymer Photodetectors Enabled by New Random Terpolymer Acceptor with Fine-Tuned Molecular Weight

Reducing the dark current density and enhancing the overall performance of the device is the focal point in research for organic photodetectors. Two novel random terpolymers (P3 and P4) with different molecular weights are synthesized and evaluated as acceptors in bulk heterojunction (BHJ) polymer photodetectors. Compared with known acceptor materials, such as N2200 (P1) and F-N2200 (P2), polymer P4 has a lower lowest unoccupied molecular orbital (LUMO) energy level, favorable morphology, and good miscibility with a donor material J71, which leads to proper phase separation of the blend film and better dissociation of excitons and transport of carriers. Therefore, a considerably low dark current density (Jd) of 1.9 × 10-10 A/cm2 and a high specific detectivity (D*) of 1.8 × 1013 cm Hz1/2/W (also "Jones") at 580 nm under a -0.1 V bias are realized for the P4-based photodetector. More importantly, the device also exhibits a fast response speed (τr/τf = 1.24/1.87 μs) and a wide linear dynamic range (LDR) of 109.2 dB. This work demonstrates that high-performance all-polymer photodetectors with ideal morphology can be realized by random polymer acceptors with a fine-tuned molecular weight.

Impact of Molecular Design on Degradation Lifetimes of Degradable Imine-Based Semiconducting Polymers

Transient electronics are a rapidly emerging field due to their potential applications in the environment and human health. Recently, a few studies have incorporated acid-labile imine bonds into polymer semiconductors to impart transience; however, understanding of the structure-degradation property relationships of these polymers is limited. In this study, we systematically design and characterize a series of fully degradable diketopyrrolopyrrole-based polymers with engineered sidechains to investigate the impact of several molecular design parameters on the degradation lifetimes of these polymers. By monitoring degradation kinetics via ultraviolet-visible spectroscopy, we reveal that polymer degradation in solution is aggregation-dependent based on the branching point and M_n, with accelerated degradation rates facilitated by decreasing aggregation. Additionally, increasing the hydrophilicity of the polymers promotes water diffusion and therefore acid hydrolysis of the imine bonds along the polymer backbone. The aggregation properties and degradation lifetimes of these polymers rely heavily on solvent, with polymers in chlorobenzene taking six times as long to degrade as in chloroform. We develop a new method for quantifying the degradation of polymers in the thin film and observe that similar factors and considerations (e.g., interchain order, crystallite size, and hydrophilicity) used for designing high-performance semiconductors impact the degradation of imine-based polymer semiconductors. We found that terpolymerization serves as an attractive approach for achieving degradable semiconductors with both good charge transport and tuned degradation properties. This study provides crucial principles for the molecular design of degradable semiconducting polymers, and we anticipate that these findings will expedite progress toward transient electronics with controlled lifetimes.

Enhancing the Performance of Small-Molecule Organic Solar Cells via Fused-Ring Design

Organic solar cells (OSCs) as the promising green energy technology have drawn much attention in the last two decades. In comparison to polymer solar cells, small-molecule organic solar cells (SMOSCs) have the advantages of precise chemical structure and molecular weight, purification feasibility, batch reproducibility, etc. Despite of the recent advances in molecular design, the efficiencies of SMOSCs are still lagging behind those of polymer-based OSCs. In this work, a new small-molecule donor (SMD) with a fused-ring-connected bridge denoted F-MD has been designed and synthesized. When F-MD was applied into SMOSCs, the F-MD:N3 blends exhibited a power conversion efficiency (PCE) of over 13%, which is much higher than that of the linear π-bridged molecule L-MD based devices (8.12%). Further studies revealed that the fused-ring design promoted the planarity of the molecular conformation and facilitated charge transport in OSCs. More importantly, this strategy also lowered the crystallinity and self-aggregation of the films, and hence optimized the microstructure and phase separation in the corresponding blends. Thereby, the F-MD-based blends have been evidenced to have better exciton dissociation and reduced charge recombination in comparison with the L-MD counterparts, explaining the enhanced PCEs. Our work demonstrates that the fused-ring π-bridge strategy in small-molecule-donor design is an effective pathway to promote the efficiency of SMOSCs as well as enhance the diversity of SMD materials.

Polymer Acceptors with Flexible Spacers Afford Efficient and Mechanically Robust All-Polymer Solar Cells

High efficiency and mechanical robustness are both crucial for the practical applications of all-polymer solar cells (all-PSCs) in stretchable and wearable electronics. In this regard, a series of new polymer acceptors (P_A s) is reported by incorporating a flexible conjugation-break spacer (FCBS) to achieve highly efficient and mechanically robust all-PSCs. Incorporation of FCBS affords the effective modulation of the crystallinity and pre-aggregation of the P_A s, and achieves the optimal blend morphology with polymer donor (P_D ), increasing both the photovoltaic and mechanical properties of all-PSCs. In particular, an all-PSC based on PYTS-0.3 P_A incorporated with 30% FCBS and PBDB-T P_D demonstrates a high power conversion efficiency (PCE) of 14.68% and excellent mechanical stretchability with a crack onset strain (COS) of 21.64% and toughness of 3.86 MJ m^-3 , which is significantly superior to those of devices with the P_A without the FCBS (PYTS-0.0, PCE = 13.01%, and toughness = 2.70 MJ m^-3 ). To date, this COS is the highest value reported for PSCs with PCEs of over 8% without any insulating additives. These results reveal that the introduction of FCBS into the conjugated backbone is a highly feasible strategy to simultaneously improve the PCE and stretchability of PSCs.

Tailoring polymer acceptors by electron linkers for achieving efficient and stable all-polymer solar cells

The trade-off between efficiency and stability is a bit vague, and it can be tricky to precisely control the bulk morphology to simultaneously improve device efficiency and stability. Herein, three fused-ring conducted polymer acceptors containing furan, thiophene and selenophene as the electron linkers in their conjugated backbones, namely PY-O, PY-S and PY-Se, were designed and synthesized. The electron linker engineering affects the intermolecular interactions of relative polymer acceptors and their charge transport properties. Furthermore, excellent material compatibility was achieved when PY-Se was blended with polymer donor PBDB-T, resulting in nanoscale domains with favorable phase separation. The optimized PBDB-T : PY-Se blend not only exhibits maximum performance with a power conversion efficiency of 15.48%, which is much higher than those of PBDB-T : PY-O (9.80%) and PBDB-T : PY-S (14.16%) devices, but also shows better storage and operational stabilities, and mechanical robustness. This work demonstrates that precise modification of electron linkers can be a practical way to simultaneously actualize molecular crystallinity and phase miscibility for improving the performance of all-polymer solar cells, showing practical significance.

The subtle Structure Modulation of A2 -A1 -D-A1 -A2 type Nonfullerene Acceptors Extends the Photoelectric Response for High Voltage Organic Photovoltaic Cells

To realize high-voltage organic photovoltaic (OPV) for indoor application and tandem solar cells, both electron-donor and acceptor in the active layer usually adopt wide-bandgap materials. However, the consequent small energy offsets may impede the dissociation of excitons, together with the inadequate light-harvesting, usually leading to the relatively low photocurrent. In this work, we utilize molecular structural modifications to improve the short-circuit current (J_SC ) of the high-voltage OPV. With the classic non-fullerene acceptor (NFA), BTA3, as a benchmark, BTA3b contains the linear alkyl chains on the middle core, and JC14 further fuses thiophene ring on BTA unit. We deeply studied the effect of structural modification on broadening the photoelectric response and device performance by using a benzotriazole-based polymer J52-F as donor. The photovoltaic devices based o N J52-F: : BTA3b an D J52-F: : JC14 achieve wider external quantum efficiency (EQE) responses with band edges of 730 and 800 nm respectively, which are about 15 and 85 nm wider than that of the device based o N J52-F: : BTA3. The J_SC of the BTA3b and JC14 are accordingly increased to 14.08 and 15.78 mA cm^-2 respectively in comparison with BTA3 (11.56 mA cm^-2 ). The smaller Urbach energy of 28.16 meV and higher electroluminescence efficiency guarante E J52-F: : JC14 a decreased energy loss (0.528 eV) and a high V_OC of 1.07 V. Finally , J52-F: : JC14 combination achieves an increased PCE of 10.33% than that o F J52-F: : BTA3b (PCE = 9.81%) an D J52-F: : BTA3 (PCE = 9.04%). Overall, our research results indicate that subtle structure modification of non-fullerene acceptors, especially introducing fused ring, is a simple and effective strategy to extend the photoelectric response and achieve small energy loss, consequently boosting the J_SC and ensuring a high V_OC beyond 1.0 V. This article is protected by copyright. All rights reserved.

Impact of dynamic covalent chemistry and precise linker length on crystallization kinetics and morphology in ethylene vitrimers

Vitrimers, dynamic polymer networks with topology conserving exchange reactions, have emerged as a promising platform for sustainable and reprocessable materials. While prior work has documented how dynamic bonds impact stress relaxation and viscosity, their role on crystallization has not been systematically explored. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were investigated to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers ultimately taking weeks in the densest networks. An increase in melting temperatures (T_m) of 25-30 K is observed with isothermal crystallization over 30 days. Both C₁₀ and C₁₂ networks initially form hexagonal crystals, while the C₁₀ network transforms to orthorhombic over the 30 day window as observed with wide angle X-ray scattering (WAXS) and optical microscopy (OM). After 150 days of isothermal crystallization, the three linker lengths led to double diamond (C₈), orthorhombic (C₁₀), and hexagonal (C₁₂) crystals indicating the importance of precision on final morphology. Control experiments on a precise, permanent network implicate dynamic bonds as the cause of long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers. The dynamic bonds also allow the networks to dissolve in water and alcohol-based solvents to monomers, followed by repolymerization while preserving the mechanical properties and melting temperatures.

Application of reduced graphene oxide as the hole transport layer in organic solar cells synthesized from waste dry cells using the electrochemical exfoliation method

The hole transport layer (HTL) plays an important role in improving the efficiency and stability of organic solar cells (OSCs).

Rapid Determination of Phase Diagrams for Biomolecular Liquid-Liquid Phase Separation with Microfluidics

Biomolecular phase separation is currently emerging in both the medical and life science fields. Meanwhile, the application of liquid-liquid phase separation has been extended to many fields including drug discovery, fibrous material fabrication, 3D printing, and polymer design. Although more than 8600 proteins and other synthetic macromolecules are capable of phase separation as recently reported, there is still a lack of a high-throughput approach to quantitatively characterize its phase behaviors. To meet this requirement, here, we proposed fast and high-resolution acquisition of biomolecular phase diagrams using microfluidic chips. Using this platform, we demonstrated the phase behavior of polyU/RRASLRRASLRRASL in a quantitative manner. Up to 1750 concentration conditions can be generated in 140 min. The detection limitation of our device to capture the saturation concentration for phase separation is about 5 times lower than that of the traditional turbidity method. Thus, our results provide a basis for the rapid acquisition of phase diagrams with high-throughput and pave the way for its wide application.

Organoboron molecules and polymers for organic solar cell applications

Organic solar cells (OSCs) are emerging as a new photovoltaic technology with the great advantages of low cost, light-weight, flexibility and semi-transparency. They are promising for portable energy-conversion products and building-integrated photovoltaics. Organoboron chemistry offers an important toolbox to design novel organic/polymer optoelectronic materials and to tune their optoelectronic properties for OSC applications. At present, organoboron small molecules and polymers have become an important class of organic photovoltaic materials. Power conversion efficiencies (PCEs) of 16% and 14% have been realized with organoboron polymer electron donors and electron acceptors, respectively. In this review, we summarize the research progress in various kinds of organoboron photovoltaic materials for OSC applications, including organoboron small molecular electron donors, organoboron small molecular electron acceptors, organoboron polymer electron donors and organoboron polymer electron acceptors. This review also discusses how to tune their opto-electronic properties and active layer morphology for enhancing OSC device performance. We also offer our insight into the opportunities and challenges in improving the OSC device performance of organoboron photovoltaic materials.

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non‐radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide‐based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non‐encapsulated analogue P(NDI‐2OD‐T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π‐interchain stacking distance. This suggests that decreasing π‐stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

In Situ Optical Studies on Morphology Formation in Organic Photovoltaic Blends

The efficiency of bulk heterojunction (BHJ) based organic solar cells is highly dependent on the morphology of the blend film, which is a result of a fine interplay between donor, acceptor, and solvent during the film drying. In this work, a versatile set-up of in situ spectroscopies is used to follow the morphology evolution during blade coating of three iconic BHJ systems, including polymer:fullerene, polymer:nonfullerene small molecule, and polymer:polymer. the drying and photoluminescence quenching dynamics are systematically study during the film formation of both pristine and BHJ films, which indicate that the component with higher molecular weight dominates the blend film formation and the final morphology. Furthermore, Time-resolved photoluminescence, which is employed for the first time as an in situ method for such drying studies, allows to quantitatively determine the extent of dynamic and static quenching, as well as the relative change of quantum yield during film formation. This work contributes to a fundamental understanding of microstructure formation during the processing of different blend films. The presented setup is considered to be an important tool for the future development of blend inks for solution-cast organic or hybrid electronics.

Polymerized small molecular acceptor based all-polymer solar cells with an efficiency of 16.16% via tuning polymer blend morphology by molecular design

All-polymer solar cells (all-PSCs) based on polymerized small molecular acceptors (PSMAs) have made significant progress recently. Here, we synthesize two A-DA’D-A small molecule acceptor based PSMAs of PS-Se with benzo[c][1,2,5]thiadiazole A’-core and PN-Se with benzotriazole A’-core, for the studies of the effect of molecular structure on the photovoltaic performance of the PSMAs. The two PSMAs possess broad absorption with PN-Se showing more red-shifted absorption than PS-Se and suitable electronic energy levels for the application as polymer acceptors in the all-PSCs with PBDB-T as polymer donor. Cryogenic transmission electron microscopy visualizes the aggregation behavior of the PBDB-T donor and the PSMA in their solutions. In addition, a bicontinuous-interpenetrating network in the PBDB-T:PN-Se blend film with aggregation size of 10~20 nm is clearly observed by the photoinduced force microscopy. The desirable morphology of the PBDB-T:PN-Se active layer leads its all-PSC showing higher power conversion efficiency of 16.16%.

Through development of non-fullerene acceptors, OPVs have reached efficiencies of 18%, yet the inadequate operational lifetime still poses a challenge for the commercialisation. Here, the authors investigate the origin of instability of NFA solar cells, and propose some strategies to mitigate this issue.

A Design Strategy for Intrinsically Stretchable High-Performance Polymer Semiconductors: Incorporating Conjugated Rigid Fused-Rings with Bulky Side Groups

Strategies to improve stretchability of polymer semiconductors, such as introducing flexible conjugation-breakers or adding flexible blocks, usually result in degraded electrical properties. In this work, we propose a concept to address this limitation, by introducing conjugated rigid fused-rings with optimized bulky side groups and maintaining a conjugated polymer backbone. Specifically, we investigated two classes of rigid fused-ring systems, namely, benzene-substituted dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene (Ph-DBTTT) and indacenodithiophene (IDT) systems, and identified molecules displaying optimized electrical and mechanical properties. In the IDT system, the polymer PIDT-3T-OC12-10% showed promising electrical and mechanical properties. In fully stretchable transistors, the polymer PIDT-3T-OC12-10% showed a mobility of 0.27 cm² V^-1 s^-1 at 75% strain and maintained its mobility after being subjected to hundreds of stretching-releasing cycles at 25% strain. Our results underscore the intimate correlation between chemical structures, mechanical properties, and charge carrier mobility for polymer semiconductors. Our described molecular design approach will help to expedite the next generation of intrinsically stretchable high-performance polymer semiconductors.

A thin film of naphthalenediimide-based metal-organic framework with electrochromic properties

A metal-organic framework (MOF) thin film constructed from Zn nodes and naphthalenediimide (NDI) linkers was grown in-situ uniformly on a transparent conducting glass substrate. This transparent thin film exhibits intriguingly high-contrast electrochromic (EC) switching between canary yellow and dark brown by means of a one-electron redox reaction at its NDI linkers. The findings provide a basic comprehension of the relations between redox state and electrochromism and enrich the application of MOF in the field of optoelectronic materials.

Multi-Selenophene-Containing Narrow Bandgap Polymer Acceptors for All-Polymer Solar Cells with over 15 % Efficiency and High Reproducibility

All-polymer solar cells (all-PSCs) progressed tremendously due to recent advances in polymerized small molecule acceptors (PSMAs), and their power conversion efficiencies (PCEs) have exceeded 15 %. However, the practical applications of all-PSCs are still restricted by a lack of PSMAs with a broad absorption, high electron mobility, low energy loss, and good batch-to-batch reproducibility. A multi-selenophene-containing PSMA, PFY-3Se, was developed based on a selenophene-fused SMA framework and a selenophene π-spacer. Compared to its thiophene analogue PFY-0Se, PFY-3Se shows a ≈30 nm red-shifted absorption, increased electron mobility, and improved intermolecular interaction. In all-PSCs, PFY-3Se achieved an impressive PCE of 15.1 % with both high short-circuit current density of 23.6 mA cm^-2 and high fill factor of 0.737, and a low energy loss, which are among the best values in all-PSCs reported to date and much better than PFY-0Se (PCE=13.0 %). Notably, PFY-3Se maintains similarly good batch-to-batch properties for realizing reproducible device performance, which is the first reported and also very rare for the PSMAs. Moreover, the PFY-3Se-based all-PSCs show low dependence of PCE on device area (0.045-1.0 cm² ) and active layer thickness (110-250 nm), indicating the great potential toward practical applications.

Nonconjugated Terpolymer Acceptors with Two Different Fused-Ring Electron-Deficient Building Blocks for Efficient All-Polymer Solar Cells

The ternary polymerization strategy of incorporating different donor and acceptor units forming terpolymers as photovoltaic materials has been proven advantageous in improving power conversion efficiencies (PCEs) of polymer solar cells (PSCs). Herein, a series of low band gap nonconjugated terpolymer acceptors based on two different fused-ring electron-deficient building blocks (IDIC16 and ITIC) with adjustable photoelectric properties were developed. As the third component, ITIC building blocks with a larger π-conjugation structure, shorter solubilizing side chains, and red-shifted absorption spectrum were incorporated into an IDIC16-based nonconjugated copolymer acceptor PF1-TS4, which built up the terpolymers with two conjugated building blocks linked by flexible thioalkyl chain-thiophene segments. With the increasing ITIC content, terpolymers show gradually broadened absorption spectra and slightly down-shifted lowest unoccupied molecular orbital levels. The active layer based on terpolymer PF1-TS4-60 with a 60% ITIC unit presents more balanced hole and electron mobilities, higher photoluminescence quenching efficiency, and improved morphology compared to those based on PF1-TS4. In all-polymer solar cells (all-PSCs), PF1-TS4-60, matched with a wide band gap polymer donor PM6, achieved a similar open-circuit voltage (Voc) of 0.99 V, a dramatically increased short-circuit current density (Jsc) of 15.30 mA cm-2, and fill factor (FF) of 61.4% compared to PF1-TS4 (Voc = 0.99 V, Jsc = 11.21 mA cm-2, and FF = 55.6%). As a result, the PF1-TS4-60-based all-PSCs achieved a PCE of 9.31%, which is ∼50% higher than the PF1-TS4-based ones (6.17%). The results demonstrate a promising approach to develop high-performance nonconjugated terpolymer acceptors for efficient all-PSCs by means of ternary polymerization using two different A-D-A-structured fused-ring electron-deficient building blocks.

Systematically investigating the effect of the aggregation behaviors in solution on the charge transport properties of BDOPV-based polymers with conjugation-break spacers

Conjugation-break spacers in conjugated polymers significantly affect the aggregation behavior in solution and in the solid state, which further influences the charge transport properties and doping efficiency.