Y6 Acceptor in Solar Cells: Structure, Benefits and Compatible Donors

In photovoltaics, researchers are constantly seeking better materials for organic solar cells (also known as organic photovoltaics - OPV). One standout example is Y6, a non-fullerene acceptor (NFA) that is an n-type organic semiconductor. Y6 is commonly paired with a polymer donor in bulk heterojunction solar cells. Studies have shown that Y6 forms a complex three-dimensional structure within solar cells, which enhances charge transport efficiency.

Unlike fullerenes, NFAs can be adjusted to have desirable properties for capturing light and producing electricity. Y6 is also more stable when exposed to heat and light than fullerenes. This means solar cells made with Y6 last longer and perform better over time.

Y6 Acceptor Structure

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Y6 has a unique molecular structure designed as a non-fullerene acceptor-donor-acceptor-donor-acceptor (A-DA'D-A) type small molecular acceptor (SMA). Chemically known as C₈₂H₈₆F₄N₈O₂S₅, it features a ladder-type electron-deficient core that enables a low bandgap and heightened electron affinity, crucial for efficient charge separation and transport in solar cell applications.

Y6 is also known by the name BTP-4F. BTP-4F which stands for "2,2′-((2Z,2′Z)-((12,13-bis(2-butyloctyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile," which is a chemical name that describes its molecular structure.

The key structural features of Y6 acceptor molecules are:

Electron Deficient Core: The Y6 molecule has a central benzothiadiazole (BT) based core unit. The electron affinity of the molecule is tuned through this electron poor unit, with charge mobility improved. The molecular packing of Y6 is thought to be directed by the BT core, specifically through S-N interactions. Whilst the core is electron deficient it still preserves the conjugated system with the electron rich backbone units.

Electron Rich Backbone: The fused thiophene ring provides Y6 with electron rich donor units. This section of the NFA is highly conjugated to support charge carrier mobility. The asymmetric design of the core of NFAs increases the dipole moment within the molecule and provides stronger intermolecular binding energy. This encourages intermolecular stacking and in tern the flow of charge between molecules. The LUMO is dictated by this section of the NFA

Electron Deficient End Groups: 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) end units are believed to promote intermolecular interactions and enhance optical absorption. The fluorine atoms can form non-covalent interactions with sulfur and hydrogen and this facilitates charge transport.

Side Chains: Long alkyl side chains increase the solubility of the Y6 acceptor molecule. The side chains influence the arrangement of the molecule in space. The bulky groups prevent excess aggregation of Y6 via steric hinderance whilst ensuring efficient intramolecular charge transport. It's electron-withdrawing nature improves the radiative recombination pathway, minimizing voltage loss in solar cells.

Bridging atoms: The site at which solubilizing side chains can be attached to the backbone of the NFA. Nitrogen atoms provide stronger electron-donating character and allow for charge carrier mobility which increases the HOMO energy level.  

π spacers: Used to control the extent of the π-conjugated backbone. Through broadening the conjugation length the electron donating ability of the backbone is increased.  

Y6 offers versatility across various OPV device architectures, including both conventional and inverted setups. Its compatibility with different active layer thicknesses further enhances its utility, maintaining high power conversion efficiencies (PCE) across varying conditions.

What are the Benefits of Y6 Acceptors?

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Non-fullerene acceptors (NFAs) like Y6 have gained significant attention in the field of organic solar cells due to several key advantages they offer over traditional fullerene-based acceptors. Here are some reasons why NFAs, including Y6, are preferred in organic photovoltaics:

• Tunable Energy Levels: NFAs can be chemically designed and modified to have tunable energy levels, allowing for better alignment with the donor materials in the solar cell. The energy level gap can be tuned from 1.3-1.7 eV. This tunability enhances charge transfer and separation efficiency, leading to improved device performance.
• Broad Absorption Range: NFAs often exhibit broad absorption spectra, extending into the near-infrared region. The absorption spectrum of Y6 has a maximum at around 810 nm and extends to 1100 nm. This extended absorption range enables better utilization of solar energy, increasing the overall efficiency of the solar cell. Y6 and its polymer blends have the potential to absorb light across the entire visible and near infra-red spectrum.
• High Charge Carrier Mobility: NFAs typically possess high charge carrier mobility, facilitating the efficient transport of electrons and holes within the device. This high mobility contributes to reduced charge recombination and enhanced overall device performance. Y6 has an electron mobility of 0.001 - 3 cm²/Vs in devices.
• Low Energy Losses: NFAs have been shown to exhibit low energy losses during the charge generation process, leading to higher open-circuit voltages and improved power conversion efficiencies in organic solar cells. The current record of Y6 based OPV for open-circuit voltage is ~0.9 V and for efficiency is 19%.
• Improved Stability: NFAs are known for their enhanced stability under various environmental conditions, including exposure to light, heat, and moisture. This improved stability contributes to the long-term performance and durability of organic solar cells. OPV research have reported a T₈₀ lifetime (time taken to reach 80% of the initial efficiency under standard conditions) of over 4000 hours for PM6:Y6 inverted organic solar cells.
• Versatile Molecular Design: NFAs offer a wide range of molecular design possibilities, allowing researchers to tailor the chemical structure of the acceptor material to optimize its performance in specific device configurations. This versatility enables the development of customized NFAs for different applications.
• Compatibility with Large-Area Processing: NFAs are often compatible with solution-based processing techniques, making them suitable for large-scale manufacturing of organic solar cells. This compatibility with scalable production methods is essential for commercializing organic photovoltaic technologies.

Which Donor Materials are Compatible with Y6 Acceptors?

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In organic solar cells, the compatibility between the donor and acceptor materials is crucial for achieving efficient charge generation, separation, and transport within the device. When considering compatible donors to pair with non-fullerene acceptors like Y6, several factors such as energy levels, morphology, and charge transport properties need to be taken into account to optimize device performance. Here are some examples of compatible donor materials that have been studied in conjunction with Y6 as an acceptor:

• PM6 (Polymer Donor): PM6, also known as PBDB-T-2F, is a widely used polymer donor material that has shown excellent compatibility with Y6 as an acceptor in organic solar cells. The PM6:Y6 blend has demonstrated high power conversion efficiencies and good morphological stability, making it a successful donor-acceptor combination in efficient photovoltaic devices. The long exciton lifetime of Y6, coupled with its unique molecular arrangement, enhances the photovoltaic action of the PM6:Y6 blend, leading to impressive device efficiencies exceeding 19% and even reaching 23.1% in hybrid tandem devices.

• DRTB-T-C4 (Small Molecule Donor): DRTB-T-C4 is a small molecule donor material that has been investigated for its compatibility with the PM6:Y6 system. When incorporated as a third component in ternary organic solar cells, DRTB-T-C4 has shown to promote exciton dissociation, improve charge transfer, and enhance device performance by optimizing the morphology and charge transport properties of the active layer. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor of 0.813 and yielding an impressive efficiency of 17.13%.
• S3 (Polymer Donor): S3 is a polymer donor material that shares a similar chemical structure to PM6. When combined with PM6 and Y6 in a ternary blend, S3 can create an alloy-like state in the bulk heterojunction film, leading to improved charge generation and transport properties. S3 was synthesized and incorporated into a PM6:Y6 system to fabricate ternary OSCs with an efficiency of 17.53%. The compatibility between S3, PM6, and Y6 highlights the importance of selecting donors that complement the acceptor material for efficient charge extraction and collection.
• Other Small Molecule Donors: Apart from DRTB-T-C4, various other small molecule donors with suitable energy levels, molecular structures, and charge transport properties can be compatible with Y6 as an acceptor. By carefully selecting small molecule donors that can form well-organized donor-acceptor interfaces and facilitate efficient charge separation, researchers can optimize the performance of organic solar cells based on the PM6:Y6 system.
• Polymer Donors with Complementary Properties: In addition to PM6 and S3, other polymer donors with complementary properties to Y6 can also be considered for creating efficient donor-acceptor blends in organic solar cells. By tailoring the chemical structure, molecular weight, and processing conditions of polymer donors, researchers can enhance the compatibility and performance of the donor-acceptor system in achieving high-efficiency photovoltaic devices.

Y6 Acceptor Applications

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Non-fullerene acceptors (NFAs) like Y6 are primarily used in organic solar cells due to their compatibility with organic semiconductors and solution-based processing techniques commonly employed in organic photovoltaics. However, the concept of non-fullerene acceptors is not limited to organic solar cells alone. NFAs have also found applications in other emerging technologies and fields, showcasing their versatility and potential beyond organics:

1. Perovskite Solar Cells: NFAs have been explored as electron acceptors (electron transport layers) in hybrid perovskite solar cells, where they can be combined with perovskite absorber materials to enhance device performance. The use of NFAs in perovskite solar cells aims to improve stability, efficiency, and scalability of these promising photovoltaic devices.
2. Organic Light-Emitting Diodes (OLEDs): NFAs have been investigated for their potential use in organic light-emitting diodes, where they can serve as electron transport materials or as components in charge generation layers. By incorporating NFAs in OLEDs, researchers aim to improve device efficiency, color purity, and operational stability.
3. Organic Field-Effect Transistors (OFETs): NFAs have been studied for their application in organic field-effect transistors, where they can function as electron acceptors in the active semiconductor layer. By utilizing NFAs in OFETs, researchers seek to enhance charge transport properties, increase device switching speeds, and optimize device performance.
4. Sensing and Photodetection Devices: NFAs have shown promise in sensing and photodetection applications, where their unique electronic properties can be leveraged for detecting specific analytes or capturing light signals. By incorporating NFAs into sensor and photodetector devices, researchers aim to improve sensitivity, selectivity, and response times.
5. Energy Storage Devices: NFAs have been explored for use in energy storage devices such as organic batteries and supercapacitors. By integrating NFAs into electrode materials or electrolytes, researchers aim to enhance energy storage capacity, cycling stability, and overall performance of organic-based energy storage systems.

What are the Competitors to Y6 Acceptors?

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Y6 faces competition from a variety of other acceptor materials that are also being actively researched and developed. These competing NFAs offer different advantages and characteristics, contributing to the diversity and ongoing innovation in the field of organic photovoltaics. Some notable competitors to Y6 include:

• BTP-eC9: BTP-eC9 typically features a combination of benzothiadiazole (BT) and thienothiophene (TT) units. These components are known for their electron-accepting and electron-donating properties, respectively, which are advantageous for optimizing light absorption and charge separation in OPVs. Due to better charge extraction properties, BTP-eC9 based organic solar cells have exceeded 19% efficiency and Y6 based acceptors face tough competition from these.
• Y18: Y18 is another A-DA’D-A type small molecule acceptor that has shown promising results in organic solar cells. Y18 offers a unique molecular structure and favorable electronic properties, making it a strong competitor to Y6 in terms of efficiency and stability. Currently Y18 based organic solar cells have reached 16.52% efficiency.
• Y11: Y11 is an emerging NFA with a similar A-DA’D-A structure to Y6 but with distinct molecular characteristics. Y11 has shown potential for achieving high power conversion efficiencies and improved charge transport properties in organic solar cells, positioning it as a competitive alternative to Y6. The organic solar cell devices based on Y11 offers an efficiency of 16.54%.
• Non-Fullerene Polymer Acceptors: In addition to small molecule NFAs like Y6, non-fullerene polymer acceptors have gained attention for their potential to combine the advantages of both polymers and NFAs. Polymer acceptors offer tunable energy levels, improved processability, and enhanced mechanical properties, presenting a competitive option to small molecule NFAs in organic solar cell applications.
• Fullerene-Based Acceptors: While NFAs have gained prominence in recent years, fullerene-based acceptors such as PCBM (phenyl-C61-butyric acid methyl ester) continue to be used in organic solar cells. Fullerene acceptors have well-established performance characteristics and are still considered in the competitive landscape of acceptor materials for organic photovoltaics (OPV) and electron transport materials for perovskite photovoltaics. PCBM based OPV has reached 16.67% efficiency and PCBM based perovskite solar cells have exceeded 20% efficiency.
• Perovskite Solar Cell Electron Transport Materials: With the rapid development of perovskite solar cells, many organic and inorganic electron transport layers have emerged as competitors to NFAs in the field of perovskite photovoltaics.

Challenges and Outlook

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Despite the remarkable progress enabled by Y6, challenges and controversies persist in the field. Ongoing debates surround the D/A (donor/acceptor) energy offset, mechanisms of free charge generation, and charge recombination dynamics within photovoltaic devices utilizing Y6. Addressing these unresolved issues is crucial for maximizing the fill factor and open-circuit voltage, optimizing carrier lifetime, and enhancing device efficiency. Furthermore, challenges related to material stability, batch-to-batch variation, and module efficiency must be overcome to realize the full commercial potential of Y6-based organic solar cells.

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Further Reading

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• Engineering non-fullerene acceptors as a mechanism to control film morphology and energy loss in organic solar cells, Mola, G.T. et al., Energy & Fuels (2022)
• Recent progress of Y-series electron acceptors for organic solar cells, Lu, B. et al., Nano Select (2021)
• What we have learnt from PM6:Y6, Shoaee, S. et al., Advanced Materials (2023)
• High-performance PM6:Y6-based ternary solar cells with enhanced open circuit voltage and balanced mobilities via doping a wide-band-gap amorphous acceptor, Liu, S. et al., ACS Applied Materials & Interfaces (2024)
• Ternary organic solar cells based on non-fullerene acceptors: A review, Chang, L. et al., Organic Electronics (2021)