An Ortho-Bisalkyloxylated Benzene-Based Fully Non-fused Electron Acceptor for Efficient Organic Photovoltaic Cells

Design and Synthesis of Completely Nonfused Medium-Wide-Bandgap Acceptors for Efficient Organic Photovoltaic Cells

Medium-wide-bandgap (MWBG) organic photovoltaic (OPV) cells have emerged as a promising category with distinctive application possibilities, especially in environments characterized by specific light conditions, such as indoor spaces. However, there are few high-efficiency MWBG acceptors, and most of them are constructed through high-cost fused central units, which limits the industrialization of MWBG OPV cells. Here, two completely nonfused MWBG acceptors, TBT-38 and TBT-43 with different alkoxy substituent positions on the thiophene rings, are synthesized. Due to the simple synthetic route and high yield, TBT-38 achieves the lowest material-only cost among high-efficiency MWBG acceptors. When blended with high-performance donor PBQx-TF, the TBT-43-based OPV cell exhibits a power conversion efficiency (PCE) of only 8.33%. In contrast, primarily due to higher exciton dissociation efficiency, charge transport capability, and favorable morphology, the TBT-38-based OPV cell delivers a PCE of 13.5% under one sun illumination, which is one of the highest results for completely nonfused OPV cells with absorption onset below 800 nm. Besides, the PBQx-TF:TBT-38-based OPV cell exhibits a PCE of 24.1% under indoor lighting. Our work presents a practical strategy for designing cost-efficient MWBG acceptors.

Emergence of Low-Cost and High-Performance Nonfused Ring Electron Acceptors

ConspectusOrganic solar cells (OSCs) have garnered significant attention in academic and industrial circles due to their advantages such as lightweight, excellent bending performance, and the ability to be fabricated into semitransparent devices. Since the proposal of the bulk heterojunction concept by Heeger et al. in 1995, conjugated polymer/fullerene pairs have gradually emerged as the optimal choice for active layer materials in OSCs. Fullerene derivatives were preferred as electron acceptors in OSCs because of their high electron mobility. However, due to limitations such as insufficient light absorption, limited derivative potential, and poor energy level tunability, the power conversion efficiency (PCE) of OSCs based on fullerene derivatives has encountered a bottleneck of approximately 12%, despite the continuous updates in polymer donor materials over nearly two decades of development, leading to a gradual decline in their importance. By contrast, nonfullerene electron acceptors (NFAs) have gradually gained dominance in this field since first appearing in 2015, thanks to their advantages of tunable absorption spectrum, adjustable energy levels, and modifiable chemical structure. Among nonfullerene acceptors, fused-ring electron acceptors (FREAs) such as ITIC and Y6 have achieved significant progress, boosting the PCE of OSCs to 20%. This milestone achievement indicates the potential of their commercial applications. However, the synthesis process of FREA is complex and often constrained by low-yield ring-closure reactions, resulting in high costs.The molecular backbone of nonfused ring electron acceptors (NFREAs) is composed of single bonds, which enables the adoption of modular synthesis mainly via Stille (based on organotin reactant) and/or Suzuki (based on organoboron reactant) coupling or C-H activation (without prefunctionalization) and avoids low-yield ring-closing reactions, thus making them a potential alternative to fused-ring acceptors. To achieve a planar molecular backbone and minimize energy loss due to conformational rotation, our team innovatively used intramolecular noncovalent interactions as a replacement for traditional covalent bonds. Furthermore, to address the issues of poor solubility and excessive aggregation during film formation for NFREAs, we strategically introduced sterically hindered side groups, such as 2,6-bis(alkyloxy)phenyl and diphenylamino, into the molecular design, effectively mitigating these problems. These innovative design concepts have significantly advanced the development of high-performance NFREAs and have garnered increasing attention from the research community. The PCEs of OSCs based on NFREAs have significantly improved from less than 10% to close to 20% since their initial discovery. By optimizing the device fabrication process, we have achieved a PCE of over 19%, which is comparable to that of FREAs. This article will delve into the evolution and latest research progress of NFREAs, aiming to provide valuable insights and guidance for the design of cost-effective and high-performance NFREA materials.

Completely Non-Fused Low-Cost Acceptor Enables Organic Photovoltaic Cells with 17 % Efficiency

To meet the industrial requirements of organic photovoltaic (OPV) cells, it is imperative to accelerate the development of cost-effective materials. Thiophene-benzene-thiophene central unit-based acceptors possess the advantage of low synthetic cost, while their power conversion efficiency (PCE) is relatively low. Here, by incorporating a para-substituted benzene unit and 1st-position branched alkoxy chains with large steric hindrance, a completely non-fused non-fullerene acceptor, TBT-26, was designed and synthesized. The narrow band gap of 1.38 eV ensures the effective utilization of sunlight. The favorable phase separation morphology of TBT-26-based blend film facilitates the efficient exciton dissociation and charge transport in corresponding OPV cell. Therefore, the TBT-26-based small-area cell achieves an impressive PCE of 17.0 %, which is the highest value of completely non-fused OPV cells. Additionally, we successfully demonstrated the scalability of this design by fabricating a 28.8 cm2 module with a high PCE of 14.3 %. Overall, our work provides a practical molecular design strategy for developing high-performance and low-cost acceptors, paving the way for industrial applications of OPV technology.

Molecular Design of Fully Nonfused Acceptors for Efficient Organic Photovoltaic Cells

The nonfused thiophene-benzene-thiophene (TBT) unit offers advantages in obtaining low-cost organic photovoltaic (OPV) materials due to its simple structure. However, OPV cells, including TBT-based acceptors, exhibit significantly lower energy conversion efficiencies. Here, we introduce a novel approach involving the design and synthesis of three TBT-based acceptors by substituting different position-branched side chains on the TBT unit. In comparison to TBT-10 and TBT-11, TBT-13, which exclusively incorporates α-position branched side chains with a large steric hindrance, demonstrates a more planar and stable conformation. When blended with the donor PBQx-TF, TBT-13-based blend film achieves favorable π-π stacking and aggregation characteristics, resulting in excellent charge transfer performance in the corresponding device. Due to the simultaneous enhancements in short-circuit current density and fill factor, the TBT-13-based OPV cell obtains an outstanding efficiency of 16.1%, marking the highest value for the cells based on fully nonfused acceptors. Our work provides a practical molecular design strategy for high-performance and low-cost OPV materials.

Morphology Control for Efficient Nonfused Acceptor-Based Organic Photovoltaic Cells

Non-fused electron acceptors have huge advantages in fabricating low-cost organic photovoltaic (OPV) cells. However, morphology control is a challenge as non-fused C─C single bonds bring more molecular conformations. Here, by selecting two typical polymer donors, PBDB-TF and PBQx-TF, the blend morphologies and its impacts on the power conversion efficiencies (PCEs) of non-fused acceptor-based OPV cells are studied. A selenium-containing non-fused acceptor named ASe-5 is designed. The results suggest that PBQx-TF has a lower miscibility with ASe-5 when compared with PBDB-TF. Additionally, the polymer networks may form earlier in the PBQx-TF:ASe-5 blend film due to stronger preaggregation performance, leading to a more obvious phase separation. The PBQx-TF:ASe-5 blend film shows faster charge transfer and suppressed charge recombination. As a result, the PBQx-TF:ASe-5-based device records a good PCE of 14.7% with a higher fill factor (FF) of 0.744, while the PBDB-TF:ASe-5-based device only obtains a moderate PCE of 12.3% with a relatively low FF of 0.662. The work demonstrates that the selection of donors plays a crucial role in controlling the blend morphology and thus improving the PCEs of non-fused acceptor-based OPV cells.