Benzodithiophene-Based Small-Molecule Donors for Next-Generation All-Small-Molecule Organic Photovoltaics

Abstract

All-small-molecule (ASM) organic solar cells (OSCs) have been of academic interest for decades owing to the unique superiorities of small molecules, such as well-defined structures, easy purification, and excellent batch-to-batch replicability. However, they received little attention prior to 2011 due to rather disappointing device performance. Since 2011, the photovoltaic performance of ASM OSCs has been rapidly elevated due to the advance of benzodithiophene (BDT)-based small-molecule donors, and in particular with the latest small-molecule acceptors, over 15% benchmark efficiency has been obtained in the laboratory, which introduces potential for commercialization. In this light, the ASM OSCs based on BDT small-molecule donors are being considered a promising candidate for next-generation photovoltaic technology to deliver cost-effective, versatile form-factor/application and environmentally friendly energy. Organic solar cells (OSCs) have been considered being a promising candidate for next-generation photovoltaic technology because of their low carbon footprint, short energy payback time, and facile manufacture into lightweight, flexible, and semitransparent products. In this prosperous field, there is a rising trend of developing all-small-molecule (ASM) OSCs due to the distinct merits of small molecules, such as well-defined structures, facile purification, and pre-eminent batch-to-batch replicability, making it a preferential contender for industrialization. The majority of the best-performing ASM OSCs utilize benzodithiophene (BDT) donors, and recent breakthroughs demonstrate that this system has exceeded the 15% efficiency mark in the laboratory. This review analyzes the significant study that has led to this remarkable progress and focuses on the most effective BDT small-molecule donors. The pivotal structure-property relationships, donor-acceptor matching criteria, and morphology control approaches are discussed. Lastly, we summarize the remaining challenges and offer our perspective on the future advance of ASM OSCs. Organic solar cells (OSCs) have been considered being a promising candidate for next-generation photovoltaic technology because of their low carbon footprint, short energy payback time, and facile manufacture into lightweight, flexible, and semitransparent products. In this prosperous field, there is a rising trend of developing all-small-molecule (ASM) OSCs due to the distinct merits of small molecules, such as well-defined structures, facile purification, and pre-eminent batch-to-batch replicability, making it a preferential contender for industrialization. The majority of the best-performing ASM OSCs utilize benzodithiophene (BDT) donors, and recent breakthroughs demonstrate that this system has exceeded the 15% efficiency mark in the laboratory. This review analyzes the significant study that has led to this remarkable progress and focuses on the most effective BDT small-molecule donors. The pivotal structure-property relationships, donor-acceptor matching criteria, and morphology control approaches are discussed. Lastly, we summarize the remaining challenges and offer our perspective on the future advance of ASM OSCs. Harvesting energy from the sun is a promising solution to supply environmentally friendly reproducible energy to deal with the issues consequent upon our ongoing dependence on fossil fuels.1Yan C. Barlow S. Wang Z. Yan H. Jen A.K.Y. Marder S.R. Zhan X. Non-fullerene acceptors for organic solar cells.Nat. Rev. Mater. 2018; 3: 18003Crossref Scopus (914) Google Scholar, 2Li G. Chang W.-H. Yang Y. Low-bandgap conjugated polymers enabling solution-processable tandem solar cells.Nat. Rev. Mater. 2017; 2: 17043Crossref Scopus (157) Google Scholar, 3Lu L. Kelly M.A. You W. Yu L. 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Soc. 2016; 138: 2973-2976Crossref PubMed Scopus (621) Google ScholarFigure 2OSC Device Architecture, Working Mechanism, Energy Diagram, and Energy Loss AnalysisShow full caption(A) Architecture of a BHJ OSC. In an ideal BHJ device architecture, donor and acceptor materials shape nanoscale phase separation and construct bicontinuous interpenetrating networks with large donor-acceptor interfacial areas. The blue and red areas represent the donor and acceptor domains, respectively. Interfacial layers in the device architecture are employed as charge-transport layers, charge-blocking layers, and optical spacers, which can build ohmic contacts between the electrode and active layer.(B) Schematic revealing the working principle of OSCs.(C) Orbital energy diagram for a typical donor-acceptor pairing. The optical energy gap of the blend refers to the smaller optical energy gap of the donor or acceptor. The charge-transfer state energy can be summarized as the difference between the LUMO of the acceptor and the HOMO of the donor. Eg, optical energy gap; Ect, charge-transfer state energy.(D) Energy loss for OSCs can be divided into charge generation (Eg − Ect) and charge recombination (Ect − qVOC). Charge recombination energy loss results from both radiative and non-radiative charge-transfer state decay. S0, ground state.View Large Image Figure ViewerDownload (PPT) (A) Architecture of a BHJ OSC. In an ideal BHJ device architecture, donor and acceptor materials shape nanoscale phase separation and construct bicontinuous interpenetrating networks with large donor-acceptor interfacial areas. The blue and red areas represent the donor and acceptor domains, respectively. Interfacial layers in the device architecture are employed as charge-transport layers, charge-blocking layers, and optical spacers, which can build ohmic contacts between the electrode and active layer. (B) Schematic revealing the working principle of OSCs. (C) Orbital energy diagram for a typical donor-acceptor pairing. The optical energy gap of the blend refers to the smaller optical energy gap of the donor or acceptor. The charge-transfer state energy can be summarized as the difference between the LUMO of the acceptor and the HOMO of the donor. Eg, optical energy gap; Ect, charge-transfer state energy. (D) Energy loss for OSCs can be divided into charge generation (Eg − Ect) and charge recombination (Ect − qVOC). Charge recombination energy loss results from both radiative and non-radiative charge-transfer state decay. S0, ground state. In this review, we first introduce the primary working mechanism of OSCs. Second, we summarize the key progress of the design and synthesis of BDT-based small-molecule donors and analyze their structure-property-performance relationship. We then discuss a few aspects involving device physics, morphology control approach, and ternary strategy. Finally, we summarize the remaining challenges and offer our perspective on the future advance of ASM OSCs. OSCs are devices that contain organic materials for solar radiation absorption and carrier transport to deliver electricity from sunlight via the photovoltaic effect. Generally, OSCs are based on blends of an electron-rich (“donor,” D) material and an electron-poor (“acceptor,” A) material that forms BHJs in devices.20Spanggaard H. Krebs F.C. A brief history of the development of organic and polymeric photovoltaics.Sol. Energy Mater. Sol. Cells. 2004; 83: 125-146Crossref Scopus (1037) Google Scholar Donor and acceptor possess different frontier molecular orbitals, and the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) determines the band gap (Eg) of the materials. The working principle of OSCs involves four key steps: (1) solar radiation absorption and exciton generation (the strongly bound excitons are generated on account of the low dielectric constant of organic semiconductors); (2) exciton diffusion to the donor/acceptor interface; (3) formation of a charge-transfer complex, followed by dissociation into free charge carriers at the interface; and (4) charge transport and collection. Every step is vital to delivering the PCE of OSCs. The PCE of OSCs is given byPCE = VOCJSCFF/Pin,where Pin represents the power density of the incident solar energy, VOC is the open-circuit voltage, JSC is the short-circuit current density, and FF is the fill factor (defined as Pmax/VOCJSC, where Pmax is the maximum power density). The VOC is usually dominated by the energy difference between the LUMO of the acceptor and the HOMO of the donor. 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