Enhanced π-π Interactions of Nonfullerene Acceptors by Volatilizable Solid Additives in Efficient Polymer Solar Cells

Isomerization Engineering of Solid Additives Enables Highly Efficient Organic Solar Cells via Manipulating Molecular Stacking and Aggregation of Active Layer

Morphology control is crucial in achieving high-performance organic solar cells (OSCs) and remains a major challenge in the field of OSC. Solid additive is an effective strategy to fine-tune morphology, however, the mechanism underlying isomeric solid additives on blend morphology and OSC performance is still vague and urgently requires further investigation. Herein, two solid additives based on pyridazine or pyrimidine as core units, M1 and M2, are designed and synthesized to explore working mechanism of the isomeric solid additives in OSCs. The smaller steric hindrance and larger dipole moment facilitate better π-π stacking and aggregation in M1-based active layer. The M1-treated all-small-molecule OSCs (ASM OSCs) obtain an impressive efficiency of 17.57%, ranking among the highest values for binary ASM OSCs, with 16.70% for M2-treated counterparts. Moreover, it is imperative to investigate whether the isomerization engineering of solid additives works in state-of-the-art polymer OSCs. M1-treated D18-Cl:PM6:L8-BO-based devices achieve an exceptional efficiency of 19.70% (certified as 19.34%), among the highest values for OSCs. The work provides deep insights into the design of solid additives and clarifies the potential working mechanism for optimizing the morphology and device performance through isomerization engineering of solid additives.

Molecular-Level Insight into Impact of Additives on Film Formation and Molecular Packing in Y6-based Organic Solar Cells

Additive engineering is widely utilized to optimize film morphology in active layers of organic solar cells (OSCs). However, the role of additive in film formation and adjustment of film morphology remains unclear at the molecular level. Here, taking high-efficiency Y6-based OSC films as an example, this work thus employs all-atom molecular-dynamics simulations to investigate how introduction of additives with different π-conjugation degree thermodynamically and dynamically impacts nanoscale molecular packings. These results demonstrate that the van der Waals (vdW) interactions of the Y6 end groups with the studied additives are strongest. The larger the π-conjugation degree of the additive molecules, the stronger the vdW interactions between additive and Y6 molecules. Due to such vdW interactions, the π-conjugated additive molecules insert into the neighboring Y6 molecules, thus opening more space for relaxation of Y6 molecules to trigger more ordered packing. Increasing the interactions between the Y6 end groups and the additive molecules not only accelerates formation of the Y6 ordered packing, but also induces shorter Y6-intermolecular distances. This work reveals the fundamental molecular-level mechanism behind film formation and adjustment of film morphology via additive engineering, providing an insight into molecular design of additives toward optimizing morphologies of organic semiconductor films.

Binary All-polymer Solar Cells with a Perhalogenated-Thiophene-Based Solid Additive Surpass 18 % Efficiency

Morphological control of all-polymer blends is quintessential yet challenging in fabricating high-performance organic solar cells. Recently, solid additives (SAs) have been approved to be capable in tuning the morphology of polymer: small-molecule blends improving the performance and stability of devices. Herein, three perhalogenated thiophenes, which are 3,4-dibromo-2,5-diiodothiophene (SA-T1), 2,5-dibromo-3,4-diiodothiophene (SA-T2), and 2,3-dibromo-4,5-diiodothiophene (SA-T3), were adopted as SAs to optimize the performance of all-polymer organic solar cells (APSCs). For the blend of PM6 and PY-IT, benefitting from the intermolecular interactions between perhalogenated thiophenes and polymers, the molecular packing properties could be finely regulated after introducing these SAs. In situ UV/Vis measurement revealed that these SAs could assist morphological character evolution in the all-polymer blend, leading to their optimal morphologies. Compared to the as-cast device of PM6 : PY-IT, all SA-treated binary devices displayed enhanced power conversion efficiencies of 17.4-18.3 % with obviously elevated short-circuit current densities and fill factors. To our knowledge, the PCE of 18.3 % for SA-T1-treated binary ranks the highest among all binary APSCs to date. Meanwhile, the universality of SA-T1 in other all-polymer blends is demonstrated with unanimously improved device performance. This work provide a new pathway in realizing high-performance APSCs.

Molecular Stacking and Aggregation Optimization of Photoactive Layer through Solid Additive Enables High-Performance Organic Solar Cells

Regulating molecular packing and aggregation of photoactive layer is a critical but challenging issue in developing high-performance organic solar cells. Herein, two structurally similar analogues of anthra[2,3-b : 6,7-b']dithiophene (ADT) and naphtho[1,2-b : 5,6-b']dithiophene (NDT) are developed as solid additive to exploit their effect in regulating the molecular aggregation and π-stacking of photoactive layer. We clarify that the perpendicular arrangements of NDT can enlarge the molecular packing space and improve the face-on stacking of Y6 during the film formation, favoring a more compact and ordered long-range π-π stacking in the out-of-plane direction after the removal of NDT under thermal annealing. The edge-to-face stacked herringbone-arrangement of ADT along with its non-volatilization under thermal annealing can induce the coexistence of face-on and edge-on stacking of blend film. As a result, the NDT treatment shows encouraging effect in improving the photovoltaic performance of devices based on various systems. Particularly, a remarkable PCE of 18.85 % is achieved in the PM6 : L8-BO-based device treated by NDT additive, which is a significant improvement with regard to the PCE of 16.41 % for the control device. This work offers a promising strategy to regulate the molecular packing and aggregation of photoactive layer towards significantly improved performance and stability of organic solar cells.

Progress in organic photovoltaics based on green solvents: from solubility enhancement to morphology optimization

Solution-processed organic photovoltaics (OPVs) is one of the most promising photovoltaic technologies in the energy field, due to their clean and renewable low-cost manufacturing potential. OPV has rapidly developed with the design and synthesis of highly efficient photovoltaic materials and the development of smart device engineering. To date, the majority of advanced OPV devices have been prepared using halogenated solvents, achieving power conversion efficiencies (PCE) exceeding 19% on a laboratory scale. However, for industrial-scale production, less toxic manufacturing processes and environmental sustainability are the key considerations. Therefore, this review summarizes recent advances in green solvent-based approaches for the preparation of OPVs, highlighting material design (including polymer donors and small molecule acceptors) and device engineering (co-solvent methods, additive strategies, post-treatment methods, and regulation of coating method), emphasizing crucial factors for achieving high performance in green solvent-processed OPV devices. This review presents potential future directions for green solvent-based OPVs, which may pave the way for future industrial development.

Octafluoronaphthalene as a thermal-annealing-free volatile solid additive enables high-performance organic solar cells

A thermal annealing-free solid additive octafluoronaphthalene was developed for high-performance organic solar cells. In an additive-modified device, an impressive power conversion efficiency of 18.59% from 17.27% was achieved with simultaneously enhanced current density from 26.86 to 27.53 mA cm-2 and fill factor from 74.34% to 78.85%.

Electrostatic force promoted intermolecular stacking of polymer donors toward 19.4% efficiency binary organic solar cells

Conjugated polymers are generally featured with low structural order due to their aromatic and irregular structural units, which limits their light absorption and charge mobility in organic solar cells. In this work, we report a conjugated molecule INMB-F that can act as a molecular bridge via electrostatic force to enhance the intermolecular stacking of BDT-based polymer donors toward efficient and stable organic solar cells. Molecular dynamics simulations and synchrotron X-ray measurements reveal that the electronegative INMB-F adsorb on the electropositive main chain of polymer donors to increase the donor-donor interactions, leading to enhanced structural order with shortened π-π stacking distance and consequently enhanced charge transport ability. Casting the non-fullerene acceptor layer on top of the INMB-F modified donor layer to fabricate solar cells via layer-by-layer deposition evidences significant power conversion efficiency boosts in a range of photovoltaic systems. A power conversion efficiency of 19.4% (certified 18.96%) is realized in PM6/L8-BO binary devices, which is one of the highest reported efficiencies of this material system. The enhanced structural order of polymer donors by INMB-F also leads to a six-fold enhancement of the operational stability of PM6/L8-BO organic solar cells.

Thermodynamic Phase Transition of Three-Dimensional Solid Additives Guiding Molecular Assembly for Efficient Organic Solar Cells

Fine-tuning the thermodynamic self-assembly of molecules via volatile solid additives has emerged to be an effective way to construct high-performance organic solar cells. Here, three-dimensional structured solid molecules have been designed and applied to facilitate the formation of organized molecular assembly in the active layer. By means of systematic theory analyses and film-morphology characterizations based on four solid candidates, we preselected the optimal one, 4-fluoro-N,N-diphenylaniline (FPA), which possesses good volatility and strong charge polarization. The three-dimensional solids can induce molecular packing in active layers via strong intermolecular interactions and subsequently provide sufficient space for the self-reassembly of active layers during the thermodynamic transition process. Benefitting from the optimized morphology with improved charge transport and reduced energy disorder in the FPA-processed devices, high efficiencies of over 19 % were achieved. The strategy of three-dimensional additives inducing ordered self-assembly structure represents a practical approach for rational morphology control in highly efficient devices, contributing to deeper insights into the structural design of efficient volatile solid additives.

Improving Cooperative Interactions Between Halogenated Aromatic Additives and Aromatic Side Chain Acceptors for Realizing 19.22% Efficiency Polymer Solar Cells

Processing additive plays an important role in the standard operation procedures for fabricating top performing polymer solar cells (PSCs) through efficient interactions with key photovoltaic materials. However, improving interaction study of acceptor materials to high performance halogenated aromatic additives such as diiodobenzene (DIB) is a widely neglected route for molecular engineering toward more efficient device performances. In this work, two novel Y-type acceptor molecules of BTP-TT and BTP-TTS with different aromatic side chains on the outer positions are designed and synthesized. The resulting aromatic side chains significantly enhanced the interactions between the acceptor molecules and DIB through an arene/halogenated arene interaction, which improved the crystallinity of the acceptor molecules and induced a polymorph with better photovoltaic performances. Thus, high power conversion efficiencies (PCEs) of 18.04% and 19.22% are achieved in binary and ternary blend devices using BTP-TTS as acceptor and DIB as additive. Aromatic side chain engineering for improving additive interactions is proved to be an effective strategy for achieving much higher performance photovoltaic materials and devices.

Intrinsic Role of Volatile Solid Additive in High-Efficiency PM6:Y6 Series Nonfullerene Solar Cells

Organic nonfullerene solar cells (ONSCs) have made unprecedented progress; however, morphology optimization of ONSCs is proven to be particularly challenging relative to classical fullerene-based devices. Here, a novel volatile solid additive (VSA), 2-hydroxy-4-methoxybenzophenone (2-HM), is reported for achieving high-efficiency ONSCs. 2-HM functions as a universal morphology-directing agent for several well-known PM6:Y6 series nonfullerene blends, viz. PM6:Y6, PM6:BTP-eC9, PM6:L8-BO, leading to a best efficiency of 18.85% at the forefront of reported binary ONSCs. VSAs have recently emerged, while the intrinsic kinetics is still unclear. Herein, a set of in situ and ex situ characterizations is employed to first illustrate the molecule-aggregate-domain transition dynamic process assisted by the VSA. More specifically, the role of 2-HM in individual donor PM6 and acceptor Y6 systems is unlocked, and the function of 2-HM in altering the PM6:Y6 bulk heterojunction blends is further revealed for enhanced photovoltaic performance. It is believed that the achievement brings not only a deep insight into emerging volatile solid additive, but also a new hope to further improve the molecular ordering, film microstructure, and relevant performance of ONSCs.

19.31% binary organic solar cell and low non-radiative recombination enabled by non-monotonic intermediate state transition

Non-fullerene acceptors based organic solar cells represent the frontier of the field, owing to both the materials and morphology manipulation innovations. Non-radiative recombination loss suppression and performance boosting are in the center of organic solar cell research. Here, we developed a non-monotonic intermediate state manipulation strategy for state-of-the-art organic solar cells by employing 1,3,5-trichlorobenzene as crystallization regulator, which optimizes the film crystallization process, regulates the self-organization of bulk-heterojunction in a non-monotonic manner, i.e., first enhancing and then relaxing the molecular aggregation. As a result, the excessive aggregation of non-fullerene acceptors is avoided and we have achieved efficient organic solar cells with reduced non-radiative recombination loss. In PM6:BTP-eC9 organic solar cell, our strategy successfully offers a record binary organic solar cell efficiency of 19.31% (18.93% certified) with very low non-radiative recombination loss of 0.190 eV. And lower non-radiative recombination loss of 0.168 eV is further achieved in PM1:BTP-eC9 organic solar cell (19.10% efficiency), giving great promise to future organic solar cell research.

Effective Passivation with Self-Organized Molecules for Perovskite Photovoltaics

Perovskite solar cells (PSCs) have achieved power conversion efficiencies (PCEs) exceeding 25% over the past decade and effective passivation for the interface with high trap density plays a significant role in this process. Here, two organic molecules are studied as passivators, and it is demonstrated that an advantageous molecular geometry and intermolecular ordering, aside from the functional moieties, are of great significance for effective and extensive passivation. Besides, the passivation molecules spontaneously form a uniform passivation network adjacent to the bottom surface of perovskite films during a top-down crystallization via liquid medium annealing, which greatly reduces defect-assisted recombination throughout the whole perovskite/SnO₂ interface. The champion device yields an in-lab PCE of 25.05% (certified 24.39%). The investigation provides a more comprehensive understanding of passivation and a new avenue to achieve effective bottom-interface engineering for perovskite photovoltaics.

Enhanced Efficiency and Excellent Thermostability in Organic Photovoltaics via Ternary Strategy with Twisted Conjugated Compound

The thermal stability of high-efficiency organic photovoltaics (OPVs) is critical to the manufacturing of this technology. Therefore, targeted strategies and approaches shall be developed to improve efficiency and stability, simultaneously. Herein, a seleno twisted benzodiperylenediimides (TBD-PDI-Se) acceptor-doping strategy is taken advantage of to demonstrate the ternary bulk heterojunction OPVs with excellent stability, achieving outstanding power conversion efficiency of 14.43% and 17.25% based on PM6:IT-4F and PM6:Y6 ternary blend devices, respectively, which are superior to the corresponding binary devices. As evidenced by the active layer morphology, exciton dynamic study and the characterizations of the enabled device, the ternary blend device keeps nearly 90% original efficiency (t = 1000 h) under continuous constant heating at 140 °C. Furthermore, the application of acceptor as the third component in PBDB-T:ITIC, J71:ITIC, and PBDB-T:PC₇₁ BM systems is also verified, proving the good universality of acceptor-doping ternary strategy.

Multi-Functional Solid Additive Induced Favorable Vertical Phase Separation and Ordered Molecular Packing for Highly Efficient Layer-by-Layer Organic Solar Cells

Layer-by-layer (LBL) deposition strategy enabling favorable vertical phase distributions has been regarded as promising candidates for constructing high-efficient organic photovoltaic (OPV) cells. However, solid additives with the merits of good stability and reproducibility have been rarely used to fine-tune the morphology of the LBL films for improved efficiency and stability. Herein, hierarchical morphology control in LBL OPV is achieved via a dual functional solid additive. Series of LBL devices are fabricated by introducing the solid additive individually or simultaneously to the donor or acceptor layer to clarify the functions of additives. Additive in the donor layer can facilitate the formation of preferable vertical component distribution, and that in the acceptor layer will enhance the molecular crystallinity for better charge transport properties. The optimized morphology ultimately contributed to high PCEs of 16.4% and 17.4% in the binary and quaternary LBL devices. This reported method provides an alternative way to controllably manipulate the morphology of LBL OPV cells.

Achieving over 10 % Efficiency in Poly(3-hexylthiophene)-Based Organic Solar Cells via Solid Additives

The photovoltaic performance of organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) has been steadily improved by developing novel non-fullerene acceptors (NFAs) in recent years. Herein, to further improve the performance of P3HT-based OSCs, a solid additive (SA4) and a typical solvent additive [1,8-diiodooctane (DIO)] were employed to process P3HT:ZY-4Cl-based OSCs, respectively. In comparison with the DIO-processed device, the SA4-processed one exhibited a more ordered molecular packing and more favorable phase separation, leading to enhanced charge transport and reduced carrier recombination. As a result, the SA4-processed device delivered a power conversion efficiency (PCE) of 10.24 %, which was much higher than that of the DIO-processed counterpart (6.26 %). This work reported a PCE over 10 % in P3HT-based OSCs for the first time, indicating the promising development of P3HT-based OSCs by morphological modulation.

Simultaneously Enhanced Efficiency and Operational Stability of Nonfullerene Organic Solar Cells via Solid-Additive-Mediated Aggregation Control

The additive strategy is widely used in optimizing the morphology of organic solar cells (OSCs). The majority of additives are liquid with high boiling points, which will be trapped within device and consequently deteriorate performance during operation. In this work, solid but volatile additives 2-(4-fluorobenzylidene)-1H-indene-1,3(2H)-dione (INB-F) and 2-(4-chlorobenzylidene)-1H-indene-1,3(2H)-dione (INB-Cl) are designed to replace the common 1,8-diiodooctane (DIO) in nonfullerene OSCs. These additives present during solution casting but evaporate after moderate heating. Molecular dynamics simulations show that they can reduce the adsorption energy to improve π-π stacking among nonfullerene acceptor (NFA) molecules, an effect that enhances light absorption and electron mobility. Both INB-F and INB-Cl enhance efficiency, with INB-F achieving a maximum efficiency of 16.7% from 15.1% of the reference PBDB-T-2F (PM6):BTP-BO-4F (Y6-BO) cell, and outperforming DIO. Remarkably, they can simultaneously enhance the operational stability, with the INB-F-treated OSC maintaining over 60% of the initial efficiency after 1000 h operation, demonstrating a T₈₀ lifetime of 523 h, which is a significant improvement over T₈₀ values of 66.2 h for the reference and 6.6 h for DIO-treated OSC. The simultaneously enhanced efficiency and operational lifetime are also effective in PM6:BTP-BO-4Cl (Y7-BO) OSCs, demonstrating a universal strategy to improve the performance of OSCs.

A Simple Dithieno[3,2-b:2',3'-d]pyrrol-Rhodanine Molecular Third Component Enables Over 16.7% Efficiency and Stable Organic Solar Cells

Organic solar cells (OSCs) can achieve greatly improved power conversion efficiency (PCE) by incorporating suitable additives in active layers. Their structure design often faces the challenge of operation generality for more binary blends. Herein, a simple dithieno[3,2-b:2',3'-d]pyrrole-rhodanine molecule (DR8) featuring high compatibility with polymer donor PM6 is developed as a cost-effective third component. By employing classic ITIC-like ITC6-4Cl and Y6 as model nonfullerene acceptors (NFAs) in PM6-based binary blends, DR8 added PM6:ITC6-4Cl blends exhibit significantly promoted energy transfer and exciton dissociation. The PM6:ITC6-4Cl:DR8 (1:1:0.1, weight ratio) OSCs contribute an exciting PCE of 14.94% in comparison to host binary devices (13.52%), while PM6:Y6:DR8 (1:1.2:0.1) blends enable 16.73% PCE with all simultaneously improved photovoltaic parameters. To the best of the knowledge, this performance is among the best for ternary OSCs with simple small molecular third components in the literature. More importantly, DR8-added ternary OSCs exhibit much improved device stability against thermal aging and light soaking over binary ones. This work provides new insight on the design of efficient third components for OSCs.

Subtle Morphology Control with Binary Additives for High-Efficiency Non-Fullerene Acceptor Organic Solar Cells

Adding an additive is one of the effective strategies to fine-tune active layer morphology and improve performance of organic solar cells. In this work, a binary additive 1,8-diiodooctane (DIO) and 2,6-dimethoxynaphthalene (DMON) to optimize the morphology of PBDB-T:TTC8-O1-4F-based devices is reported. With the binary additive, a power conversion efficiency (PCE) of 13.22% was achieved, which is higher than those of devices using DIO (12.05%) or DMON (11.19%) individually. Comparison studies demonstrate that DIO can induce the acceptor TTC8-O1-4F to form ordered packing, while DMON can inhibit excessive aggregation of the donor and acceptor. With the synergistic effect of these two additives, the PBDB-T:TTC8-O1-4F blend film with DIO and DMON exhibits a suitable phase separation and crystallite size, leading to a high short-circuit current density (J_sc) of 23.04 mA·cm^-2 and a fill factor of 0.703 and thus improved PCE.

Simultaneous enhanced efficiency and thermal stability in organic solar cells from a polymer acceptor additive

The thermal stability of organic solar cells is critical for practical applications of this emerging technology. Thus, effective approaches and strategies need to be found to alleviate their inherent thermal instability. Here, we show a polymer acceptor-doping general strategy and report a thermally stable bulk heterojunction photovoltaic system, which exhibits an improved power conversion efficiency of 15.10%. Supported by statistical analyses of device degradation data, and morphological characteristics and physical mechanisms study, this polymer-doping blend shows a longer lifetime, nearly keeping its efficiency (t = 800 h) under accelerated aging tests at 150 ^oC. Further analysis of the degradation behaviors indicates a bright future of this system in outer space applications. Notably, the use of polymer acceptor as a dual function additive in the other four photovoltaic systems was also confirmed, demonstrating the good generality of this polymer-doping strategy.

Thermal instability is a critical bottleneck for bulk heterojunction organic solar cells. Here Yang et al. use barely 1 wt% of a polymer acceptor as an additive to simultaneously improve the device efficiency and thermal stability of several state-of-the-art organic photovoltaic systems at high temperatures.

A Benzo[1,2-b:4,5-c']Dithiophene-4,8-Dione-Based Polymer Donor Achieving an Efficiency Over 16

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk-heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT-F, based on a strongly electron-deficient core (5,7-dibromo-2,3-bis(2-ethylhexyl)benzo[1,2-b:4,5-c']dithiophene-4,8-dione, TTDO), is developed through the design of cyclohexane-1,4-dione embedded into a thieno[3,4-b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT-F-based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick-film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT-F-based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT-F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene-free acceptor materials for the research community.

Graphdiyne Derivative as Multifunctional Solid Additive in Binary Organic Solar Cells with 17.3% Efficiency and High Reproductivity

Morphology tuning of the blend film in organic solar cells (OSCs) is a key approach to improve device efficiencies. Among various strategies, solid additive is proposed as a simple and new way to enable morphology tuning. However, there exist few solid additives reported to meet such expectations. Herein, chlorine-functionalized graphdiyne (GCl) is successfully applied as a multifunctional solid additive to fine-tune the morphology and improve device efficiency as well as reproductivity for the first time. Compared with 15.6% efficiency for control devices, a record high efficiency of 17.3% with the certified one of 17.1% is obtained along with the simultaneous increase of short-circuit current (J_sc ) and fill factor (FF), displaying the state-of-the-art binary organic solar cells at present. The redshift of the film absorption, enhanced crystallinity, prominent phase separation, improved mobility, and decreased charge recombination synergistically account for the increase of J_sc and FF after introducing GCl into the blend film. Moreover, the addition of GCl dramatically reduces batch-to-batch variations benefiting mass production owing to the nonvolatile property of GCl. All these results confirm the efficacy of GCl to enhance device performance, demonstrating a promising application of GCl as a multifunctional solid additive in the field of OSCs.

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

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

Regulation of Molecular Packing and Blend Morphology by Finely Tuning Molecular Conformation for High-Performance Nonfullerene Polymer Solar Cells

The asymmetric thienobenzodithiophene (TBD) structure is first systematically compared with the benzo[1,2-b:4,5-b']dithiophene (BDT) and dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) units in donor-acceptor (D-A) copolymers and applied as the central core in small molecule acceptors (SMAs). Specific polymers including PBDT-BZ, PTBD-BZ, and PDTBDT-BZ with different macromolecular conformations are synthesized and then matched with four elaborately designed acceptor-donor-acceptor (A-D-A) SMAs with structures comparable to their donor counterparts. The resulting polymer solar cell performance trends are dramatically different from each other and highly material-dependent, and the active layer morphology is largely governed by polymer conformation. Because of its more linear backbone, the PTBD-BZ film has higher crystallinity and more ordered and denser π-π stacking than those of the PBDT-BZ and PDTBDT-BZ films. Thus, PTBD-BZ shows excellent compatibility with and strong independence on the SMAs with varied structures, and PTBD-BZ-based cells deliver high power conversion efficiency (PCE) of 10-12.5%, whereas low PCE is obtained by cells based on PDTBDT-BZ because of its zigzag conformation. Overall, this study reveals control of molecular conformation as a useful approach to modulate the photovoltaic properties of conjugated polymers.