Non-fullerene acceptors (NFAs) based on non-fused conjugated structures have more potential to realize low-cost organic photovoltaic (OPV) cells. However, their power conversion efficiencies (PCEs) are much lower than those of the fused-ring NFAs. Herein, a new bithiophene-based non-fused core (TT-Pi) featuring good planarity as well as large steric hindrance was designed, based on which a completely non-fused NFA, A4T-16, was developed. The single-crystal result of A4T-16 reveals that a three-dimensional interpenetrating network can be formed due to the compact π–π stacking between the adjacent end-capping groups. A high PCE of 15.2% is achieved based on PBDB-TF:A4T-16, which is the highest value for the cells based on the non-fused NFAs. Notably, the device retains ~84% of its initial PCE after 1300 h under the simulated AM 1.5 G illumination (100 mW cm−2). Overall, this work provides insight into molecule design of the non-fused NFAs from the aspect of molecular geometry control. Non-fullerene acceptors based on non-fused conjugated structures have potential for realizing low-cost organic photovoltaic cells, owing to its synthetic simplicity. Here, the authors develop a non-fused molecule with a three-dimensional interpenetrating network and compact π-π stacking, which is highly suitable for PV applications.

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Completely non-fused electron acceptor with 3D-interpenetrated crystalline structure enables efficient and stable organic solar cell.

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA'D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π-π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve Jsc and FF without sacrificing Voc .

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency.

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

Near‐Infrared Materials: The Turning Point of Organic Photovoltaics

Challenges and recent advances in photodiodes-based organic photodetectors

Single-junction organic solar cells (OSCs) have made significant progress in recent years. Innovations in material design and device optimization have improved the power conversion efficiencies to over 19%. In this review, based on recent advances, we discuss molecular design strategies to tune the absorption spectrum, energy level, and intermolecular aggregation as well as highlight the role of molecular electrostatic potential in decreasing energy loss. Then, we introduce the latest progress in four types of OSCs composed of different donor:acceptor combinations: polymer donor:small-molecule acceptor, all-polymer, all-small-molecule, and small-molecule donor:polymer acceptor OSCs. Finally, the challenges of OSCs in practical applications, including material cost, stability, and multi-function integration, are discussed.

Recent Advances in Single-Junction Organic Solar Cells.

The emerging donor-acceptor-donor (A-D-A)-type nonfullerene acceptors (NFAs) featuring near-infrared (NIR) photoresponsivity have greatly boosted the development of organic photovoltaics (OPVs) and display great potential for sensitive NIR organic photodetectors (OPDs). However, NIR NFAs with absorption above 1000 nm, which is of great importance for application in NIR OPDs for bioimaging, remote communication, night surveillance, etc., are still rare due to the scarcity of strong electron-rich cores. We report herein a new dithiophene building block, namely PDT, which exhibits the strongest electron-donating ability among the widely used dithiophene building blocks. By applying PDT and PDTT as the electron-donating cores and DFIC as the electron-accepting terminals, we developed two new NIR electron acceptors, PDTIC-4F and PDTTIC-4F, with optical absorptions up to 1030 nm, surpassing that of the well-known O6T-4F acceptor. In comparison with the carbon-oxygen-bridged core COi8 in O6T-4F, the synthetic complexity of PDT and PDTT is significantly reduced. Conventional OPV devices based on PM6:PDTTIC-4F display power conversion efficiencies (PCEs) of up to 10.70% with a broad external quantum efficiency (EQE) response from the ultraviolet-visible to the infrared, leading to a high short-circuit current density (Jsc) of 25.90 mA cm-2. Encouraged by these results, we investigated inverted PM6:PDTTIC-4F-based OPD devices by suppressing the dark current via modulation of the film thickness. The optimal OPD device exhibits compelling performance metrics that can compete with those of commercial silicon photodiodes: a record responsivity of 0.55 A W-1 (900 nm) among photodiode-type OPDs and excellent shot-noise-limited specific detectivity (Dsh*) of over 1013 jones.

Carbon-Bridged 1,2-Bis(2-thienyl)ethylene: An Extremely Electron Rich Dithiophene Building Block Enabling Electron Acceptors with Absorption above 1000 nm for Highly Sensitive NIR Photodetectors

In view of the wide applications of near-infrared (NIR) light in night vision, security, medicine, sensors, telecommunications, and military applications, and the scarcity of high-efficiency NIR-emitting materials, development of alternative NIR-emitting materials is urgently required. In this review, we focus on three kinds of emerging NIR-emitting materials used in light-emitting diodes (LEDs), namely organic materials, inorganic quantum dot (QD) materials, and organic–inorganic hybrid perovskite materials; the corresponding devices are organic LEDs, QD LEDs, and perovskite LEDs. The advantages and disadvantages of the three kinds of materials are discussed, some representative works are reviewed, and a brief outlook for these materials is provided.

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Yingqi Zheng

Papers

Carbon-Bridged 1,2-Bis(2-thienyl)ethylene: An Extremely Electron Rich Dithiophene Building Block Enabling Electron Acceptors with Absorption above 1000 nm for Highly Sensitive NIR Photodetectors

Recent Progress in Emerging Near-Infrared Emitting Materials for Light-Emitting Diode Applications