Large Phase Separation Research Articles

Innovations in Silicone Hydrogels: Achieving High Oxygen Permeability with Maintained Equilibrium Water Content

Silicone hydrogels are pivotal materials used in contact lenses, biomedicine, and electronic devices, where oxygen permeability and equilibrium water content (EWC) are essential attributes. Generally, an increase in silicone content boosts oxygen permeability but diminishes EWC, presenting a notable trade‐off challenge. In this study, a solvent incorporation method that significantly elevates oxygen permeability without markedly affecting EWC is introduced. Among various solvents, isopropanol incorporation notably enhances oxygen permeability (101 barrer) while maintaining an EWC of 56.0%. This improvement is attributed to enhanced connectivity within the silicone network. Additionally, it is discovered that incorporating siloxane nanoparticles (SNPs) further increases the connectivity of the silicone network, offering a more pronounced improvement in oxygen permeability than solvent alone. However, excessive SNP addition can lead to large phase separation, resulting in decreased transmittance, lower EWC, and increased mechanical strength. Optimizing a transmittance of over 99%, SNP addition achieves a silicone hydrogel with an oxygen permeability of 125 barrer and an EWC of 49.5%, surpassing many advancements reported in the literature.

Sequentially Deposited Elastomer‐Based Ternary Active Layer for High‐Performance Stretchable Organic Solar Cells

AbstractStretchable organic solar cells (OSCs) with high power conversion efficiency and good mechanical deformation are promising as power sources for wearable electronics. However, synergistic improvement of both photovoltaic efficiency and mechanical ductility is challenging for state‐of‐the‐art polymer donor: non‐fullerene acceptor (NFA)‐based photovoltaic active layers. Here, a high‐performance stretchable OSC with a power conversion efficiency of 16.54% and a crack‐onset strain of 26.38% by synergetic optimization film microstructure of sequentially deposited ternary active layer consisting of a polymer donor 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'‐(5,8‐bis(4‐(2‐butyloctyl)thiophen‐2‐yl)dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2‐c][1,2,5]thiadiazole)] (D18), an NFA 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))dimalonitrile) (Y6), and an elastomer polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) is reported. Adding a low‐content solvent additive para‐xylene into main solvent carbon disulfide induces high‐density fibers networks with low crystallinity in bottom D18 layer, and this further suppresses the large phase separation between Y6 and SEBS in top layer. Moreover, incorporating a solid additive 1,3‐dibromo‐5‐chlorobenzene with better compatibility with Y6 can promote Y6 dispersions to form smaller ordered domains in SEBS matrix. Finally, the optimal ternary active layer shows significantly higher efficiency and stretchability, resulting in a large efficiency‐stretchability factor of 4.36%, which is among the best values for stretchable OSCs.

A review on smart strategies for active layer phase separation regulation of organic solar cells

Exciton dissociation and charge transport are important processes in the photoelectric conversion of organic solar cells (OSCs), directly affecting the performance of OSCs. In order to facilitate exciton dissociation, phase separation size should be as small as possible. However, for the sake of continuous charge transport, a large phase separation size is also necessary. To balance these two conflicting needs, it is of utmost importance to regulate the phase separation of the active layer. This Review mainly elaborated the smart tactics commonly adopted to phase separation optimization, aiming to understand how these treatments affect both the active layer phase separation and the performance of OSCs. Furthermore, we present an outlook on the new opportunities of phase separation regulation of the active layer. Finally, this Review may provide some reference for the researchers engaged in the research of OSCs.

Isomeric Effect of a π Bridge in an IDT-Based Nonfused Electron Photovoltaic Acceptor.

A pair of isomers, IDT-BOF containing S⋅⋅⋅O/F⋅⋅⋅H noncovalently configurational locks and IDT-BFO containing F⋅⋅⋅H/O⋅⋅⋅H noncovalently configurational locks, with an acceptor-π-donor-π-acceptor (A-π-D-π-A) structure have been designed and synthesized by choosing 4,9-dihydro-s-indaceno[1,2-b : 5,6-b']dithiophene (IDT) as the D unit, an F/n-hexyloxy substituted phenyl ring as π bridge, and 3-(dicyanomethylidene)indan-1-one as the A unit. Owing to the S⋅⋅⋅O/F⋅⋅⋅H or F⋅⋅⋅H/O⋅⋅⋅H noncovalently configurational locks, both IDT-BOF and IDT-BFO have a completely planar structure. IDT-BOF exhibits a similar LUMO to IDT-BFO, but higher HOMO energy levels, leading to a smaller optical bandgap and red-shifted absorption. However, IDT-BOF-based bulk-heterojunction organic solar cells (BHJ-OSCs) coupled with PBDB-T, and PCE-10 as donor materials both exhibited a lower PCE than that of IDT-BFO (PBDB-T: 5.2 vs. 6.1 %; PCE-10: 1.7 vs. 3.2 %). Comprehensively comparing and investigating IDT-BOF : PBDB-T and IDT-BFO : PBDB-T OSCs suggested that the large phase separation and serious charge recombination of IDT-BOF-based OSCs contributed to its lower power conversion efficiency. Importantly, ternary solar cells based on PBDB-T : Y5 as control devices with an additional 10 % IDT-BFO exhibited a 5 % enhancement in the PCE compared to the control device (14.3 vs. 13.46 %).

Introducing a Phenyl End Group in the Inner Side Chains of A-DA'D-A Acceptors Enables High-Efficiency Organic Solar Cells Processed with Nonhalogenated Solvent.

Power conversion efficiency (PCE) of organic solar cells (OSCs) processed by nonhalogenated solvents is unsatisfactory due to the unfavorable morphology. Herein, two new small molecule acceptors(SMAs) Y6-Ph and L8-Ph are synthesized by introducing a phenyl end group in the inner side chains of the SMAs of Y6 and L8-BO, respectively, for overcoming the excessive aggregation of SMAs in the long-time film forming processed by nonhalogenated solvents. First, the effect of the film forming time on the aggregation property and photovoltaic performance of Y6, L8-BO, Y6-Ph, and L8-Ph is studied by using the commonly used solvents: chloroform (CF) (rapid film forming process) and chlorobenzene (CB) (slow film forming process). It isfound that Y6- and L8-BO-based OSCs exhibit a dramatic drop in PCEfrom CF- to CB-processed devices owing to the large phase separation, while the Y6-Ph and L8-Ph based OSCs show obviously increased PCEs Furthermore, L8-Ph-based OSCs processed by nonhalogenated solvent o-xylene (o-XY) achieved a high PCE of 18.40% with an FF of 80.11%. The results indicate that introducing a phenyl end group in the inner side chains is an effective strategy to modulate the morphology and improve the photovoltaic performance of the OSCs processed by nonhalogenated solvents.

Synthesis, Characterization, and Solar Cell Applications of a Non-fused-ring Electron Acceptor Based on Vinylene-bridged Difluorobenzothiadiazole

Abstract A new non-fused-ring electron acceptor (NREA), BE4T based on vinylene-bridged difluorobenzothiadiazole (FBTzE) has been synthesized and characterized. BE4T was found to be suitable for high-performance OPVs due to its strong absorption around 600–800 nm and low-lying HOMO/LUMO energy levels. However, BE4T-based cells showed a low PCE of 1.05% owing to the formation of highly ordered edge-on and large phase separation structure in the blended films.

Direct Evidence of Reversible SnO2-Li Reactions in Carbon Nanospaces.

We present herein that carbon nanospaces are the key reaction space to improve the reversibility of the reaction of SnO2 with Li-ions for lithium-ion batteries, demonstrated by both ex situ and in situ observations using high-resolution scanning transmission electron microscopy with electron energy loss spectroscopy. Conversion-type electrode materials, such as SnO2, undergo large volume changes and phase separation during the charge-discharge process, which lead to degradation in the battery performance. By confining the SnO2-Li reaction within carbon nanopores, the battery performance is improved. However, the exact phase changes of SnO2 in the nanospaces are unclear. By directly observing the electrodes during the charge-discharge process, the carbon walls are capable of preventing the expansion of SnO2 particles and minimizing the conversion-induced phase separation of Sn and Li2O on the sub-nanometer scale. Thus, nanoconfinement structures can effectively improve the reversibility performance of conversion-type electrode materials.

Toward High Efficiency Water Processed Organic Photovoltaics: Controlling the Nanoparticle Morphology with Surface Energies

AbstractHere efficient organic photovoltaic devices fabricated from water‐based colloidal dispersions with donor:acceptor composite nanoparticles achieving up to 9.98% power conversion efficiency (PCE) are reported. This high efficiency for water processed organic solar cells is attributed to morphology control by surface energy matching between the donor and the acceptor materials. Indeed, due to a low interfacial energy between donor and the acceptor, no large phase separation occurs during the nanoparticle formation process as well as upon thermal annealing. Indeed, synchrotron‐based scanning transmission X‐ray microscopy reveals that the internal morphology of composite nanoparticles is intermixed as well as the active layer morphology after thermal treatment. The PCE of this system reaches 85% that of devices prepared from chlorinated solvent. The gap between water‐based inks and organic solvent‐based inks gets narrower, which is promising for the development of eco‐friendly processing and fabrication of organic photovoltaics.

Multimorphic Materials: Spatially Tailoring Mechanical Properties via Selective Initiation of Interpenetrating Polymer Networks.

Access to multimaterial polymers with spatially localized properties and robust interfaces is anticipated to enable new capabilities in soft robotics, such as smooth actuation for advanced medical and manufacturing technologies. Here, orthogonal initiation is used to create interpenetrating polymer networks (IPNs) with spatial control over morphology and mechanical properties. Base catalyzes the formation of a stiff and strong polyurethane, while blue LEDs initiate the formation of a soft and elastic polyacrylate. IPN morphology is controlled by when the LED is turned "on", with large phase separation occurring for short time delays (≈1-2 min) and a mixed morphology for longer time delays (>5min), which is supported by dynamic mechanical analysis, small angle X-ray scattering, and atomic force microscopy. Through tailoring morphology, tensile moduli and fracture toughness can be tuned across ≈1-2 orders of magnitude. Moreover, a simple spring model is used to explain the observed mechanical behavior. Photopatterning produces "multimorphic" materials, where morphology is spatially localized with fine precision (<100µm), while maintaining a uniform chemical composition throughout to mitigate interfacial failure. As a final demonstration, the fabrication of hinges represents a possible use case for multimorphic materials in soft robotics.

Effect of molecular weight on photoluminescence and electroluminescence properties of thermally activated delayed fluorescence conjugated polymers

Thermally activated delayed fluorescence (TADF) polymers have made remarkable progress in solution processed organic light-emitting diodes (OLEDs) due to their superior film-forming stability. However, the effect of some factors such as the molecular weight and end groups on the optoelectronic properties is never explored so far. Herein, a set of the TADF conjugated polymers with the different number-average molecular weights (Mns) of 5.4, 24.8, 41.8 and 146.1 kg/mol are synthesized and their Mn effect on the photoluminescence (PL) and electroluminescence (EL) is systematically investigated. The microstructure models of the spin-coated polymer films based on the Mn and chain length are proposed for realizing efficient non-doped OLEDs. Low Mn produces more compacted polymer chain stacking and smoother film morphology, leading to restricted non-radiative loss and more effective energy transfer. In particular, the polymers with low Mns of 5.4 and 24.8 kg/mol achieve the higher PL quantum yields and more efficient reverse intersystem cross from triplet to singlet excited state, as well as the record external quantum efficiencies of 21.2 % and 18.4 % with the EL emission peaks over 620 nm. In contrast, the PL and EL performances eventually deteriorate with further increasing Mns due to the sparser chain stacking and larger phase separation. The results show that the lower Mns enable the rigid polymers to form perfect amorphous films and render the luminescent unit more uniform distribution, similar to the classical doped systems, improving the EL behaviors of the polymers.

Effects of Acyloxy Groups in Anthrabisthiadiazole-Based Semiconducting Polymers on Electronic Properties, Thin-Film Structure, and Solar Cell Performance

Abstract Donor-acceptor (D-A) polymers with the anthra[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (ATz)-based acceptor unit bearing acyloxy groups in the 6,12-positions were synthesized. By incorporating electron-withdrawing acyloxy groups, the synthesized monomers 5a and 5b showed a down-shifted HOMO while maintaining LUMO energy level compared to the alkoxy-substituted ATz monomer ATz2T-o6OD, which we have previously reported. The DFT calculations revealed that the LUMO of the ATz core at 6,12-positions is a nodal plane with negligible changes in LUMO energy levels. In contrast, despite the presence of the acyloxy groups, the polymer PATz4T-a12R (a12R = a12OD and a12DT) synthesized in this study was found to have higher HOMO energy levels than the previously reported alkoxy-substituted polymer PATz4T-o6OD. Such elevation of the HOMO energy levels may be attributed to the unique electronic effects of the acyloxy groups, where the electronic effects of the functional groups are weakened by the lengthening of the π-electron system in the polymer and the electron-donating mesomeric effects may be dominant. PATz4T-a12R formed unsuitable edge-on orientation and large phase separation in the blended films, resulting in solar cells using it exhibiting a lower power conversion efficiency (PCE) of 3.47% than that using PATz4T-o6OD.

Preparation, thermal properties and charging/discharging characteristics of sodium acetate/stearic acid/octadecyl alcohol composite as phase change materials

In order to improve the thermal conductivity of organic phase change materials and overcome the shortcomings of large supercooling degree and phase separation of inorganic phase change materials, the composites of sodium acetate, stearic acid and octadecyl alcohol as phase change materials were prepared by melt blending method, where disodium hydrogen phosphate dodecahydrate was used as nucleating agent and carboxymethyl cellulose as thickener in this work. XRD, DSC and T-history techniques were used to analyze the composite phase change materials. Sodium acetate, stearic acid and octadecyl alcohol are physically blended in the ternary composite phase change materials. Sodium acetate improves the thermal conductivity and latent heat of the eutectic mixtures of stearic acid and octadecyl alcohol, where the thermal conductivity and latent heat of the eutectic mixtures of stearic acid and octadecyl alcohol is increased by 384 % and 28%, respectively. Heat conduction and natural convection act together, but natural convection plays an important role in the melting process. While heat conduction plays an important part in the discharging process. Due to natural convection, the charging time of the composite phase change materials is less than the discharging time.

Efficient recovery and enrichment of rare earth elements by a continuous flow micro-extraction system

The excessive exploitation of rare earth elements (REEs) has caused major losses of non-renewable resources and damage to the ecosystem. The processes of mining and smelting produce massive amounts of wastewater with low concentrations of REEs. Consequently, the enrichment and recovery of low-concentration REEs from wastewater has significant economic and environmental value. For this purpose, operation under large phase ratios (the flow rate ratio between the aqueous phase and extractant) is more desirable and economically viable. However, the traditional REE extraction process suffers from the uneven dispersion of the extractant and the difficulty of phase separation, which leads to long extraction times and large consumption of extractants. Hence, there is an urgent need to develop a green and efficient technique to extract low concentrations of REEs from wastewater. In this work, a droplet-based microfluidic technique was used to continuously extract and recover low-concentration REEs at large phase ratios. Snowman-shaped magnetic Janus nanoparticles were added to the continuous phase as emulsifiers to facilitate uniform extractant dispersion and rapid phase separation. Several key factors affecting the extraction efficiency, including pH, residence time, and the amount of added Janus nanoparticles, were systematically investigated. Compared to batch extraction, droplet-based microfluidic extraction with the addition of Janus nanoparticles showed the advantages of a large specific surface area and fast phase separation during extraction. Meanwhile, the Janus nanoparticles exhibited good emulsification performance after three extraction cycles. In summary, the Janus nanoparticle-stabilized droplet generated by microfluidic methods provides a feasible path for the efficient enrichment and recovery of low-concentration REEs.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Achieving Balanced Crystallization Kinetics of Donor and Acceptor by Sequential‐Blade Coated Double Bulk Heterojunction Organic Solar Cells

AbstractSequential deposition has great potential to achieve high performance in organic solar cells due to the resulting well‐controlled vertical phase separation. In this work, double bulk heterojunction organic solar cells are fabricated by sequential‐blade cast in ambient conditions. Probed by the in situ grazing incidence X‐ray diffraction and in situ UV–vis absorption measurements, the seq‐blade system exhibits a different tendency from each of the binary films during the film formation process. Due to the extensive aggregation of FOIC, the binary PBDB‐T:FOIC film displays a strong and large phase separation, resulting in low current density (Jsc) and unsatisfactory power conversion efficiency. In the seq‐blade cast system, the bottom layer PBDB‐T:IT‐M produces many crystal nuclei for the top layer PBDB‐T:FOIC, so the PBDB‐T molecules are able to crystallize easily and quickly. Balanced crystallization kinetics between polymer and small molecule and an ideal percolation network in the film are observed. In addition, the balanced crystallization kinetics are favorable toward realizing lower recombination loss through charge transport processes.

Phase morphology and tribological properties of PI/UHMWPE blend composites

Polyimide/ultrahigh molecular weight polyethylene (PI/UHMWPE) blends with different mass ratio were produced. A ratio-image method was adopted by plotting the ratio of Raman peak intensity of a typical band of PI with that of UHMWPE as a function of position, Raman Mapping images with a relatively high accuracy and enhanced visualization were obtained. Two-phase distribution morphologies of the blends were studied using Raman Mapping. Effects of blend phase morphology and phase distribution on the tribological properties of blend composites were investigated. The results indicated that both the phase structures and their distribution are obviously different for the PI/UHMWPE blends with different mass ratios, which significantly influence the tribological properties of the blends. PI phase disperses in an island form in the UHMWPE matrix phase when the mass ratio of PI/UHMWPE is relatively small (10/90). The scattered PI dispersed phase effectively reduces the friction coefficient of the blend and improves the wear resistance. For the PI/UHMWPE blend composite with the mass ratio of 30/70, layered PI phase forms and distributes in the UHMWPE matrix phase, improving bearing capacity and wear resistance of the blends. Large continuous area structure and phase separation occurs in the blend when the mass ratio of PI/UHMWPE increases to 50/50, resulting in a significant reduction in the wear resistance of the blend composite.

Synthesis of multi-functional porous superhydrophobic trioxybenzene cross-linked silica aerogels with improved textural properties

The development of organically modified cross-linked silica aerogels with an ordered porous structure and a high surface area is of utmost importance for practical applications. However, practical applications have been limited, as the organic cross-linkers lead to decrease the textural properties due to large phase separation and large size difference between organic and inorganics. Also cross-linkers are not commercially available or are prohibitively expensive. To overcome these, we proposed to use small size organic cross-linker i. e aryl group with three cross-linking sites to minimize phase separation and to obtain ordered silica network. Initially, three cross-linkers are synthesized starting from trihydroxybenzene and tetraethoxysilane using acid catalyst. Further, using these cross-linkers, trioxybenzene cross-linked silica aerogels were prepared by a simple cost-effective sol-gel process. Three different structural isomers of trihydroxybenzene precursors were used to analyze its suitability to obtain cost-effective silica aerogels with high porosity and hydrophobicity. Among these cross-linked silica aerogels, 1,3,5-trihydroxybenzene cross-linked aerogels possess a high surface area (1268 m2/g) and uniform porous morphology with a highly interconnected structure due to symmetry and lower phase separation. It also exhibits low density (0.02431 g/cm3) and low shrinkage, leading to excellent thermal insulation performance with low thermal conductivity (0.061 W/m·K). Also, the cross-linked aerogels are hydrophobic without using silylating reagents. This is helpful for large-scale production of aerogels for thermal applications.

Introduction of Siloxane-Terminated Side Chains into Semiconducting Polymers To Tune Phase Separation with Nonfullerene Acceptor for Polymer Solar Cells.

In this work, five PTB7-Th-based conjugated polymers (PTB7-Th, PTBSi20, PTBSi25, PTBSi33, and PTBSi100) with different contents of siloxane-terminated pentyl side chain were synthesized, and properties of corresponding blend films with narrow band gap nonfullerene IEICO-4F acceptor were extensively investigated. According to the contact angle testing, the PTB7-Th with 100% alkyl side chain and PTBSi100 100% siloxane-terminated side chain on the benzodithiophene unit showed surface energy values of 40.04 and 34.52 mJ/m2, respectively. The results demonstrate that relative to alkyl side chain in PTB7-Th, the siloxane-terminated side chain could effectively reduce the surface energy of a resulting polymer. Based on Flory-Huggins interaction parameter estimations, the miscibility between the polymer and IEICO-4F would vary in an order of PTB7-Th > PTBSi20 > PTBSi25 > PTBSi33 > PTBSi100, suggesting that siloxane-terminated side chain would afford a tunable driving force for phase separation. Transmission electron microscopy and Raman mapping could confirm large bulk domains inside the PTBSi100:IEICO-4F blend film. In polymer solar cells, the blend film of the PTBSi100 with the lowest miscibility to IEICO-4F showed an undesirable power conversion efficiency (PCE) of 8.52%, which was significantly lower than that of 11.23% for PTB7-Th, suggesting that too large phase separation driving force is not beneficial for the device performance. Side-chain random copolymers PTBSi20, PTBSi25, and PTBSi33 for fine tuning could display PCEs of 11.94, 12.61, and 11.80%, respectively, all higher than that of PTB7-Th. Our results not only reveal the big surface energy difference between the siloxane-terminated side chain and the common alkyl side chain but also provide a guideline for side chain engineering of conjugated polymer donors with tunable morphology and optimal matching with a nonfullerene acceptor.

Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

AbstractA new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (Voc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (Eloss) of 0.521 eV. Such small Eloss is the best record for polymer solar cells with PCEs over 16% to date.

Chlorination Strategy‐Induced Abnormal Nanomorphology Tuning in High‐Efficiency Organic Solar Cells: A Study of Phenyl‐Substituted Benzodithiophene‐Based Nonfullerene Acceptors

A new heptacyclic core based on phenyl‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDT) is designed and paired with 1,1‐dicyano methylene‐3‐indanone (INCN) end group to construct a nonfullerene acceptor, BPIC. The strong aggregation and large phase separation in the poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)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))]) (PBDB‐T):BPIC blend cause inefficient exciton dissociation and ineffective charge transport, resulting in a low 11.12% power conversion efficiency (PCE) with low short‐circuit current density (JSC) and fill factor (FF). To finely control the active‐layer nanomorphology, the chlorine atom is introduced into the INCN termini, and di‐chlorinated BPIC‐2Cl and tetra‐chlorinated BPIC‐4Cl are synthesized. It is an interesting phenomenon that, unlike other literature reports, while the di‐chlorination reduces crystallinity and phase‐separation scale, further chlorination increases crystallinity and phase separation. The PBDB‐T:BPIC‐2Cl device exhibits suitable molecular packing and nearly ideal nanoscale phase separation, which facilitates exciton dissociation and charge transport and thus yields the higher PCE of 12.63% with significantly improved JSC and FF. PBDB‐T:BPIC‐4Cl device, however, exhibits strong stacking intensity and excessively large phase separation, leading to the clearly reduced JSC, FF, and PCE of only 8.23%. This work demonstrates that novel phenyl‐substituted BDT core and delicated chlorination strategy provides powerful tools for high‐performance nonfullerene acceptors in organic solar cells.