High-Performance Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions

Recent progress and prospects of dimer and multimer acceptors for efficient and stable polymer solar cells

High power conversion efficiency (PCE) and long-term stability are essential prerequisites for the commercialization of polymer solar cells (PSCs). Small-molecule acceptors (SMAs) are core materials that have led to recent, rapid increases in the PCEs of the PSCs. However, a critical limitation of the resulting PSCs is their poor long-term stability. Blend morphology degradation from rapid diffusion of SMAs with low glass transition temperatures (Tgs) is considered the main cause of the poor long-term stability of the PSCs. The recent emergence of oligomerized SMAs (OSMAs), composed of two or more repeating SMA units (i.e., dimerized and trimerized SMAs), has shown great promise in overcoming these challenges. This innovation in material design has enabled OSMA-based PSCs to reach impressive PCEs near 19% and exceptional long-term stability. In this review, we summarize the evolution of OSMAs, including their research background and recent progress in molecular design. In particular, we discuss the mechanisms for high PCE and stability of OSMA-based PSCs and suggest useful design guidelines for high-performance OSMAs. Furthermore, we reflect on the existing hurdles and future directions for OSMA materials towards achieving commercially viable PSCs with high PCEs and operational stabilities.

Macrocyclic Encapsulation in a Non-fused Tetrathiophene Acceptor for Efficient Organic Solar Cells with High Short-Circuit Current Density

Non-fullerene acceptors have shown great promise for organic solar cells (OSCs). However, challenges in achieving high efficiency molecular system with conformational unicity and effective molecular stacking remain. In this study, we present a new design of non-fused tetrathiophene acceptor R4T-1 via employing the encapsulation of tetrathiophene with macrocyclic ring. The single crystal structure analysis reveals that cyclic alkyl side chains can perfectly encapsulate the central part of molecule and generate a conformational stable and planar molecular backbone. Whereas, the control 4T-5 without the encapsulation restriction displays cis- and twisted conformation. As a result, R4T-1 based OSCs achieved an outstanding power conversion efficiency (PCE) exceeding 15.10 % with a high short-circuit current density (Jsc ) of 25.48 mA/cm2 , which is significantly improved by ≈30 % in relative to that of the control. Our findings demonstrate that the macrocyclic encapsulation strategy could assist fully non-fused electron acceptors (FNEAs) to achieve a high photovoltaic performance and pave a new way for FNEAs design.

Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium-Substituted Acceptor for Efficient Binary Organic Solar Cells

Both the regional isomerization and selenium-substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S-CSeF, A-ISeF, and A-OSeF) via subtly manipulating the mono-selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A-OSeF presents a closer intermolecular π-π stacking and more ordered 3-dimensional network packing and efficient charge-hopping pathways. With the successive out-shift of the mono-selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A-OSeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A-OSeF-based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium-containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium-based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top-ranked selenium-containing SMAs-based OSCs.

Unidirectional Sidechain Engineering to Construct Dual-Asymmetric Acceptors for 19.23 % Efficiency Organic Solar Cells with Low Energy Loss and Efficient Charge Transfer

Achieving both high open-circuit voltage (Voc ) and short-circuit current density (Jsc ) to boost power-conversion efficiency (PCE) is a major challenge for organic solar cells (OSCs), wherein high energy loss (Eloss ) and inefficient charge transfer usually take place. Here, three new Y-series acceptors of mono-asymmetric asy-YC11 and dual-asymmetric bi-asy-YC9 and bi-asy-YC12 are developed. They share the same asymmetric D1 AD2 (D1 =thieno[3,2-b]thiophene and D2 =selenopheno[3,2-b]thiophene) fused-core but have different unidirectional sidechain on D1 side, allowing fine-tuned molecular properties, such as intermolecular interaction, packing pattern, and crystallinity. Among the binary blends, the PM6 : bi-asy-YC12 one has better morphology with appropriate phase separation and higher order packing than the PM6 : asy-YC9 and PM6 : bi-asy-YC11 ones. Therefore, the PM6 : bi-asy-YC12-based OSCs offer a higher PCE of 17.16 % with both high Voc and Jsc , due to the reduced Eloss and efficient charge transfer properties. Inspired by the high Voc and strong NIR-absorption, bi-asy-YC12 is introduced into efficient binary PM6 : L8-BO to construct ternary OSCs. Thanks to the broadened absorption, optimized morphology, and furtherly minimized Eloss , the PM6 : L8-BO : bi-asy-YC12-based OSCs achieve a champion PCE of 19.23 %, which is one of the highest efficiencies among these annealing-free devices. Our developed unidirectional sidechain engineering for constructing bi-asymmetric Y-series acceptors provides an approach to boost PCE of OSCs.

A Wide Bandgap Acceptor with Large Dielectric Constant and High Electrostatic Potential Values for Efficient Organic Photovoltaic Cells

Low-bandgap materials have achieved rapid development and promoted the enhancement of power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, the design of wide-bandgap non-fullerene acceptors (WBG-NFAs), required by indoor applications and tandem cells, has been lagging far behind the development of OPV technologies. Here, we designed and synthesized two NFAs named ITCC-Cl and TIDC-Cl by finely optimizing ITCC. In contrast with ITCC and ITCC-Cl, TIDC-Cl can maintain a wider bandgap and a higher electrostatic potential simultaneously. When blending with the donor PB2, the highest dielectric constant is also obtained in TIDC-Cl-based films, enabling efficient charge generation. Therefore, the PB2:TIDC-Cl-based cell possessed a high PCE of 13.8% with an excellent fill factor (FF) of 78.2% under the air mass 1.5G (AM 1.5G) condition. Furthermore, an exciting PCE of 27.1% can be accomplished in the PB2:TIDC-Cl system under the illumination of 500 lux (2700 K light-emitting diode). Combined with the theoretical simulation, the tandem OPV cell based on TIDC-Cl was fabricated and exhibited an excellent PCE of 20.0%.

13% Single-Component Organic Solar Cells based on Double-Cable Conjugated Polymers with Pendent Y-Series Acceptors

Double-cable conjugated polymers with pendent electron acceptors, including fullerene, rylene diimides, and nonfused acceptors, have been developed for application in single-component organic solar cells (SCOSCs) with efficiencies approaching 10%. In this work, Y-series electron acceptors have been firstly incorporated into double-cable polymers in order to further improve the efficiencies of SCOSCs. A highly crystalline Y-series acceptor based on quinoxaline core and the random copolymerized strategy are used to optimize the ambipolar charge transport and the nanophase separation of the double-cable polymers. As a result, an efficiency of 13.02% is obtained in the random double-cable polymer, representing the highest performance in SCOSCs, while the regular double-cable polymer only provides a low efficiency of 2.75%. The significantly enhanced efficiencies are attributed to higher charge carrier mobilities, better ordering conjugated backbones and Y-series acceptors in random double-cable polymers.

Double Asymmetric Core Optimizes Crystal Packing to Enable Selenophene-based Acceptor with Over 18 % Efficiency in Binary Organic Solar Cells

Side-chain tailoring is a promising method to optimize the performance of organic solar cells (OSCs). However, asymmetric alkyl chain-based small molecular acceptors (SMAs) are still difficult to afford. Herein, we adopted a novel asymmetric n-nonyl/undecyl substitution strategy and synthesized two A-D₁ A'D₂ -A double asymmetric isomeric SMAs with asymmetric selenophene-based central core for OSCs. Crystallographic analysis indicates that AYT9Se11-Cl forms a more compact and order intermolecular packing compared to AYT11Se9-Cl, which contributed to higher electron mobility in neat AYT9Se11-Cl film. Moreover, the PM6 : AYT9Se11-Cl blend film shows a better morphology with appropriate phase separation and distinct face-on orientation than PM6 : AYT11Se9-Cl. The OSCs with PM6 : AYT9Se11-Cl obtain a superior PCE of 18.12 % compared to PM6 : AYT11Se9-Cl (17.52 %), which is the best efficiency for the selenium-incorporated SMAs in binary BHJ OSCs. Our findings elucidate that the promising double asymmetric strategy with isomeric alkyl chains precisely modulates the crystal packing and enhances the photovoltaic efficiency of selenophene-incorporated SMAs.

Effect of Steric Hindrance at the Anthracene Core on the Photovoltaic Performance of Simple Nonfused Ring Electron Acceptors

Solving the contradiction between good solubility and dense packing is a challenge in designing high-performance nonfullerene acceptors. Herein, two simple nonfused ring electron acceptors (o-AT-2Cl and m-AT-2Cl) carrying ortho- or meta-substituted hexyloxy side chains can be facilely synthesized in only three steps. The two ortho-substituted phenyl side chains in o-AT-2Cl cannot freely rotate due to a big steric hindrance, which endows the acceptor with good solubility. Moreover, o-AT-2Cl displays a more ordered packing than m-AT-2Cl as revealed by the absorption measurement. When blended with polymer donor D18 for the fabrication of organic solar cells (OSCs), o-AT-2Cl-based devices exhibit a favorable morphology, more efficient exciton dissociation, and better charge transport. Consequently, the optimal OSCs based on D18:o-AT-2Cl exhibit a power conversion efficiency (PCE) of 12.8%, which is significantly higher than the moderate PCE (7.66%) for D18:m-AT-2Cl-based devices. Remarkably, o-AT-2Cl shows a higher figure-of-merit value compared with classic high-efficiency fused ring electron acceptors. As a result, our research succeeds in obtaining nonfused ring acceptors with cost-effective photovoltaic performance and provides a valuable experience for simultaneously improving solubility as well as ensuring ordered packing of acceptors through regulating the steric hindrance via changing the position of substituents.

Recent Advances in Selenophene-Based Materials for Organic Solar Cells

Due to the low cost, light weight, semitransparency, good flexibility, and large manufacturing area of organic solar cells (OSCs), OSCs have the opportunity to become the next generation of solar cells in some specific applications. So far, the efficiency of the OSC device has been improved by more than 20%. The optical band gap between the lowest unoccupied molecular orbital (LUMO) level and the highest occupied molecular orbital (HOMO) level is an important factor affecting the performance of the device. Selenophene, a derivative of aromatic pentacyclic thiophene, is easy to polarize, its LUMO energy level is very low, and hence the optical band gap can be reduced. In addition, the selenium atoms in selenophene and other oxygen atoms or sulfur atoms can form an intermolecular interaction, so as to improve the stacking order of the active layer blend film and improve the carrier transport efficiency. This paper introduces the organic solar active layer materials containing selenium benzene in recent years, which can be simply divided into donor materials and acceptor materials. Replacing sulfur atoms with selenium atoms in these materials can effectively reduce the corresponding optical band gap of materials, improve the mutual solubility of donor recipient materials, and ultimately improve the device efficiency. Therefore, the sulfur in thiophene can be completely replaced by selenium or oxygen of the same family, which can be used in the active layer materials of organic solar cells. This article mainly describes the application of selenium instead of sulfur in OSCs.

Heteroheptacene-based acceptors with thieno[3,2-b]pyrrole yield high-performance polymer solar cells

Rationally utilizing and developing synthetic units is of particular significance for the design of high-performance non-fullerene small-molecule acceptors (SMAs). Here, a thieno[3,2-b]pyrrole synthetic unit was employed to develop a set of SMAs (ThPy1, ThPy2, ThPy3 and ThPy4) by changing the number or the position of the pyrrole ring in the central core based on a standard SMA of IT-4Cl, compared to which the four thieno[3,2-b]pyrrole-based acceptors exhibit bathochromic absorption and upshifted frontier orbital energy level due to the strong electron-donating ability of pyrrole. As a result, the polymer solar cells (PSCs) of the four thieno[3,2-b]pyrrole-based acceptors yield higher open-circuit voltage and lower energy loss relative to those of the IT-4Cl-based device. What is more, the ThPy3-based device achieves a power conversion efficiency (PCE) (15.3%) and an outstanding fill factor (FF) (0.771) that are superior to the IT-4Cl-based device (PCE = 12.6%, FF = 0.758). The ThPy4-based device realizes the lowest energy loss and the smallest optical band gap, and the ternary PSC device based on PM6:BTP-eC9:ThPy4 exhibits a PCE of 18.43% and a FF of 0.802. Overall, this work sheds light on the great potential of thieno[3,2-b]pyrrole-based SMAs in realizing low energy loss and high PCE.

Vinylene-Inserted Asymmetric Polymer Acceptor with Absorption Approaching 1000 nm for Versatile Applications in All-Polymer Solar Cells and Photomultiplication-Type Polymeric Photodetectors

The emerging polymerized small-molecule acceptors (PSMAs) with near-infrared (NIR) absorption have not only significantly boosted the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) but have also exhibited great potential for sensitive NIR polymeric photodetectors (PPDs). However, there is no report regarding PSMAs with photo-response that can approach 1000 nm, which is an important criterion for applications in NIR-responsive all-PSCs and PPDs. Herein, by unidirectionally inserting vinylene segments into a selenophene-rich polymer backbone to improve the electron-donating strength and quinoidal character, an asymmetric PSMA, namely, PY3Se-1V, was developed, which showed an extensively red-shifted absorption approaching 1000 nm. The PBDB-T:PY3Se-1V-based binary all-PSCs achieve a decent PCE of 13.2% and a record-high photocurrent density of 25.9 mA cm-2 due to the significantly broadened photo-response and efficient photon-to-electron conversion. More encouragingly, narrowband photomultiplication (PM)-type PPDs based on poly(3-hexylthiophene-2,5-diyl) (P3HT):PY3Se-1V were developed, delivering an exceptionally high external quantum efficiency of 3680% and a responsivity of 28 A W-1 at an NIR peak of 960 nm under -50 V bias, which is reported for the first time in PM-type PPDs with a response approaching 1000 nm.

Developing Efficient Small Molecule Acceptors with sp2 -Hybridized Nitrogen at Different Positions by Density Functional Theory Calculations, Molecular Dynamics Simulations and Machine Learning

Chemical structure of small molecule acceptors determines their performance in organic solar cells. Multiscale simulations are necessary to avoid trial-and-error based design, ultimately to save time and resources. In current study, the effect of sp² -hybridized nitrogen substitution at the inner or the outmost position of central core, side chain, and terminal group of small molecule acceptors is investigated using multiscale computational modelling. Quantum chemical analysis is used to study the electronic behavior. Nitrogen substitution at end-capping has significantly decreased the electron-reorganization energy. No big change is observed in transfer integral and excited state behavior. However, nitrogen substitution at terminal group position is good way to improve electron-mobility. Power conversion efficiency (PCE) of newly designed acceptors is predicted using machine learning. Molecular dynamics simulations are also performed to explore the dynamics of acceptor and their blends with PBDB-T polymer donor. Florgy-Huggins parameter is calculated to study the mixing of designed small molecule acceptors with PBDB-T. Radial distribution function has indicated that PBDB-T has a closer packing with N3 and N4. From all analysis, it is found that nitrogen substitution at end-capping group is a better strategy to design efficient small molecule acceptors.