Non-fused ring acceptors for organic solar cells

Organic solar cells (OSCs) have experienced rapid development and achieved significant breakthroughs in power conversion efficiencies owing to the emergence of non-fullerene acceptors (NFAs) with ladder-type multiple fused ring structures. However, the high synthetic complexity and production cost of multiple fused ring NFAs hinder the commercial prospects of OSCs. In this context, the development of non-fused ring acceptors (NFRAs) with simple structures and facile synthesis has been proposed. In this mini review, we summarize the important progress in this field spanning from molecular design strategies to structure-performance relationships. Ultimately, with the aim of realizing the practical application of NFRAs in OSCs, we discuss the current challenges and future directions in terms of achieving high performance and low synthetic complexity simultaneously. These discussions provide valuable insights into the development of new NFRAs

Near-Infrared Electron Acceptors with Cyano-Substituted 2-(3-Oxo-2,3-dihydroinden-1-ylidene)malononitrile End-Groups for Organic Solar Cells

Near-infrared (NIR) electron acceptors are critical components for constructing organic solar cells (OSCs). Herein, we report a set of A-DA'D-A-type electron acceptors with end-groups of cyano-substituted 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (CN-IC), which possesses the strongest electron-withdrawing ability among the end-groups reported to date. An optical bandgap as low as 1.29 eV was obtained for the electron acceptors with CN-IC endgroups, which is decreased by 0.12 eV with respect to that of the reference electron acceptor. Meanwhile, deep-lying frontier molecular orbital energy levels were maintained in these electron acceptors. These advantages endow the electron acceptors with a broad light-harvesting range and the capacity to match prevalent polymer donors. Consequently, OSCs with a power conversion efficiency of 18.1% were obtained. These results suggest the huge potential of CN-IC in constructing NIR electron acceptors

Non-Fused Polymerized Small Molecular Acceptors for Efficient All-Polymer Solar Cells

The development of polymer acceptors is critical to promote the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs). Herein, two novel polymer acceptors (PBTz-TT and PFBTz-TT) derived from non-fused small molecules, which possess synthetic simplicity, narrow optical bandgap, and high absorption coefficients, are reported for the first time. The all-PSCs are fabricated by a layer-by-layer deposition technique with PBDB-T as donor, and the device performance is improved by the synergistic effect of solvent additive and thermal annealing. As a result, the all-PSCs offer PCEs of 10.14% and 6.85% for PFBTz-TT and PBTz-TT, respectively. Further morphological and electrical characterizations unveil that the higher device performance of PFBTz-TT originates from more efficient exciton separation and charge transport as a result of more ordered polymer packing in solid state. Herein, it is demonstrated that polymerizing non-fused small molecular acceptors is an effective strategy to develop polymer acceptors for high-performance all-PSCs

Sequentially Deposited Active Layer with Bulk-Heterojunction-like Morphology for Efficient Conventional and Inverted All-Polymer Solar Cells

A sequentially deposited (SD) active layer with bulk-heterojunction (BHJ) like morphology is developed by utilizing a naphthalenediimide-based polymer acceptor PTzNDI-T with a strong interchain interaction and low solubility and a well-soluble polymer donor J52-Cl. The SD active layer is prepared by first depositing PTzNDI-T solution and then depositing J52-Cl solution without any post-treatments, and a traditional blend-cast (BC) active layer is cast from the blend solution of J52-Cl:PTzNDI-T. Both the conventional and inverted all-polymer solar cells (all-PSCs) with the BC active layer present nearly no photovoltaic performance. In contrast, based on the SD active layer, not only do the inverted all-PSCs show a dramatically increased PCE of 6.08% but the conventional all-PSCs with the same deposition sequence also exhibit a similarly high PCE of 6.29%. Notably, the SD active layer shows BHJ-like morphology with well-distributed donor and acceptor phases and thus offers a similarly high photovoltaic performance in conventional and inverted all-PSCs with the same deposition sequence of polymer acceptor and donor, which is the first report of SD all-PSCs. These results provide different insight to the SD active layer for high-performance all-PSCs

Benzo[1,2-b:4,5-b' ]Difuran-Based Polymer for Organic Solar Cells with 17.5% Efficiency via Halogenation-Mediated Aggregation Control

The realization of high-efficiency organic solar cells (OSCs) from renewable sources will bring a real green energy technology. Benzo[1,2-b:4,5-b ' ]difuran (BDF) is such a building block for photovoltaic polymers as it can be built from furfural, which is available from trees and vegetables. However, the device performance of BDF-based polymers is limited by aggregation properties and unfavorable active layer morphology. Herein, two new BDF-based wide bandgap-conjugated polymers, PFCT-2F and PFCT-2Cl, are developed by copolymerizing the fluorinated or chlorinated BDF units with the 3-cyanothiophene unit for use as electron donors in OSCs. Benefitting from the more rotatable nature of the side-chain thiophene rings on the BDF unit, PFCT-2Cl exhibits more adjustable aggregation, higher pi-pi stacking ordering, and appropriate miscibility with the electron acceptor. As a result, a high power conversion efficiency (PCE) of 17.2% is offered by PFCT-2Cl, which is nearly two times higher than that of PFCT-2F (8.9%). A more remarkable PCE up to 17.5% is further achieved by PFCT-2Cl in a ternary OSC, which is the new efficiency record for BDF-based polymers. This success proves the critical role of aggregation control in BDF-based polymers and the bright future for constructing high-performance OSCs from bio-renewable sources

N-Type Quinoidal Polymers Based on Dipyrrolopyrazinedione for Application in All-Polymer Solar Cells

Conjugated molecules and polymers with intrinsic quinoidal structure are promising n-type organic semiconductors, which have been reported for application in field-effect transistors and thermoelectric devices. In principle, the molecular and electronic characteristics of quinoidal polymers can also enable their application in organic solar cells. Herein, two quinoidal polymers, named PzDP-T and PzDP-ffT, based on dipyrrolopyrazinedione were synthesized and used as electron acceptors in all-polymer solar cells (all-PSCs). Both PzDP-T and PzDP-ffT showed suitable energy levels and wide light absorption range that extended to the near-infrared region. When combined with the polymer donor PBDB-T, the resulting all-PSCs based on PzDP-T and PzDP-ffT exhibited a power conversion efficiency (PCE) of 1.33 and 2.37 %, respectively. This is the first report on the application of intrinsic quinoidal conjugated polymers in all-PSCs. The photovoltaic performance of the all-PSCs was revealed to be mainly limited by the relatively poor and imbalanced charge transport, considerable charge recombination. Detailed investigations on the structure-performance relationship suggested that synergistic optimization of light absorption, energy levels, and charge transport properties is needed to achieve more successful application of intrinsic quinoidal conjugated polymers in all-PSCs

An electron acceptor featuring a B-N covalent bond and small singlet-triplet gap for organic solar cells

BNTT2F, an electron acceptor featuring a B-N covalent bond and singlet-triplet gap as low as 0.20 eV via the multiple resonance effect, is developed for organic solar cells. The optimized device based on BNTT2F offered an efficiency of 8.3%, suggesting the great prospect of B-N covalent bond-containing π-conjugated molecules for photovoltaics