Azadipyrromethene-based Zn(II) complexes as nonplanar conjugated electron acceptors for organic photovoltaics

Large Non-planar Conjugated Molecule with Strong Intermolecular Interactions Achieved with Homoleptic Zn(II) Complex of Di(5-quinolylethynyl)-tetraphenylazadipyrromethene

Zinc(II) complexes of tetraphenylazadipyrromethenes are potential non-planar n-type conjugated materials. To tune the properties, we installed 5-quinolylethynyl groups at the pyrrolic positions. Compared to the complex with 1-napthylethynyl, we found evidence for stronger intermolecular interactions in the new complex, including much higher overlap integrals in crystals. X-ray analysis revealed unconventional C-H···N hydrogen bonding between two quinolyls of neighboring molecules, pointing to a new strategy for the development of non-planar molecular semiconductors with stronger intermolecular interactions.

Designing Alternative Non-Fullerene Molecular Electron Acceptors for Solution-Processable Organic Photovoltaics

Until recently, solution-processable organic photovoltaics (OPVs) mainly relied on fullerene derivatives as the n-type material, paired with a p-type conjugated polymer. However, fullerene derivatives have disadvantages that limit OPV performance, thus fueling research of non-fullerene acceptors (NFAs). Initially, NFAs showed poor performance due to difficulties in obtaining favorable blend morphologies. One example is our work with 2,6-dialkylamino core-substituted naphthalene diimides. Researchers then learned to control blend morphology by NFA molecular design. To limit miscibility with polymer while preventing excessive self-aggregation, non-planar, twisted or 3D structures were reported. An example of a 3D structure is our work with homoleptic zinc(II) complexes of azadipyrromethene. The most recent design is a planar A-D-A conjugated system where the D unit is rigid and has orthogonal side chains to control aggregation. These have propelled power conversion efficiencies (PCEs) to ∼14 %, surpassing fullerene-based OPVs. These exciting new developments prompt further investigations of NFAs and provide a bright future for OPVs.

Subnaphthalocyanines as Electron Acceptors in Polymer Solar Cells: Improving Device Performance by Modifying Peripheral and Axial Substituents

A new class of subnaphthalocyanines bearing various peripheral and axial substituents have been synthesized for use as electron acceptors in solution-processed bulk-heterojunction polymer solar cells. The resulting solar cells exhibit modest photovoltaic performance with contributions from both the polymer donor and subnaphthalocyanine acceptor to the photocurrent.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

Non-fullerene organic solar cells based on diketopyrrolopyrrole polymers as electron donors and ITIC as an electron acceptor

In this work, we provide systematic studies on the non-fullerene solar cells based on diketopyrrolopyrrole (DPP) polymers as electron donors and a well-known electron acceptor ITIC. ITIC has been widely reported in non-fullerene solar cells with high power conversion efficiencies (PCEs) above 10%, when it is combined with a wide band gap conjugated polymer, while its application in small band gap DPP polymers has never been reported. Herein, we select four DPP polymers containing different thienyl linkers, resulting in distinct absorption spectra, energy levels and crystalline properties. Non-fullerene solar cells based on DPP polymers as donors and ITIC as an acceptor show PCEs of 1.9-4.1% and energy loss of 0.55-0.82 eV. The PCEs are much lower than those of cells based on fullerene derivatives due to the poor miscibility between the DPP polymers and ITIC, as confirmed by the morphology and charge transport investigation. The results indicate that it is important to tune the miscibility between the donor and acceptor in order to realize optimized micro-phase separation, which can further enhance the performance of DPP polymer based non-fullerene solar cells.

Cyclopentannulation and cyclodehydrogenation of isomerically pure 5,11-dibromo-anthradithiophenes leading to contorted aromatics

Isomerically pure 5,11-dibromo-ADT was used to create contorted aromatics with large splay angles, low band gaps, and low LUMOs.

Efficient Organic Solar Cells with Non-Fullerene Acceptors

Fullerene-free OSCs employing n-type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non-fullerene acceptors exhibit great potential for realizing high-performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non-fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.

A graphyne-like porous carbon-rich network synthesized via alkyne metathesis

The synthesis of graphyne has been considered challenging, especially when it comes to adopting new topologies and obtaining thinner layers. Herein, we report the synthesis and characterization of a graphyne-like porous carbon-rich network via alkyne metathesis reactions, which resulted in a sp²/sp hybridized 2D thin film structure with a layer to layer distance of 0.37 nm. This graphyne-like porous carbon-rich network is an n-type semiconductor with a low work function of 3.9 eV and a reduction potential of -0.54 V vs. SHE, which could be applied as an excellent reducing agent for metal electroless deposition. In addition, this material has a narrow pore size distribution of 2 to 4 nm, a high surface area of 675 m² g^-1 and a large pore volume of 0.795 cm³ g^-1 favoring gas adsorption. It shows the selective absorption of CO₂ over N₂ owing to the strong affinity between CO₂ and the carbon-carbon triple bond.

Photophysics and redox properties of aza-BODIPY dyes with electron-withdrawing groups

The optical and electrochemical properties are compared for aza-BODIPY dyes that differ by virtue of the substituents at 1,7- and 3,5-positions of the aza-BODIPY backbone.

Conjugated polymer acceptors based on fused perylene bisimides with a twisted backbone for non-fullerene solar cells

Electron acceptors based on fused perylene bisimide dimers with a twisted backbone were developed to show high electron mobility and isotropic crystalline properties, resulting in high power conversion efficiencies in non-fullerene solar cells.

Performance limitations in thieno[3,4-c]pyrrole-4,6-dione-based polymer:ITIC solar cells

Three thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers were applied in non-fullerene solar cells, in which the polymer PTPDBDT provided a high photovoltage but a low quantum efficiency. This was caused by the large phase separation in the bulk-heterojunction as confirmed by systematic studies.

Theory guided systematic molecular design of benzothiadiazole–phenazine based self-assembling electron-acceptors

A systematic theory-guided molecular design of electron acceptors to reveal the impact of each structural subunit on the electronic properties.

Molecular electron acceptors for efficient fullerene-free organic solar cells

Small molecule electron acceptors pairing with wide bandgap or narrow bandgap electron donors are reviewed and discussed for fullerene-free organic solar cells.

Chain conformations and phase behavior of conjugated polymers

Conjugated polymers may play an important role in various emerging optoelectronic applications because they combine the chemical versatility of organic molecules and the flexibility, stretchability and toughness of polymers with semiconducting properties. Nevertheless, in order to achieve the full potential of conjugated polymers, a clear description of how their structure, morphology, and macroscopic properties are interrelated is needed. We propose that the starting point for understanding conjugated polymers includes understanding chain conformations and phase behavior. Efforts to predict and measure the persistence length have significantly refined our intuition of the chain stiffness, and have led to predictions of nematic-to-isotropic transitions. Exploring mixing between conjugated polymers and small molecules or other polymers has demonstrated tremendous advancements in attaining the needed properties for various optoelectronic devices. Current efforts continue to refine our knowledge of chain conformations and phase behavior and the factors that influence these properties, thereby providing opportunities for the development of novel optoelectronic materials based on conjugated polymers.

Donor polymer design enables efficient non-fullerene organic solar cells

To achieve efficient organic solar cells, the design of suitable donor–acceptor couples is crucially important. State-of-the-art donor polymers used in fullerene cells may not perform well when they are combined with non-fullerene acceptors, thus new donor polymers need to be developed. Here we report non-fullerene organic solar cells with efficiencies up to 10.9%, enabled by a novel donor polymer that exhibits strong temperature-dependent aggregation but with intentionally reduced polymer crystallinity due to the introduction of a less symmetric monomer unit. Our comparative study shows that an analogue polymer with a C2 symmetric monomer unit yields highly crystalline polymer films but less efficient non-fullerene cells. Based on a monomer with a mirror symmetry, our best donor polymer exhibits reduced crystallinity, yet such a polymer matches better with small molecular acceptors. This study provides important insights to the design of donor polymers for non-fullerene organic solar cells.

In organic photovoltaics, electron acceptors are developed to replace fullerenes, and new donors need to be designed to match these acceptors. Here, the authors show that a polymer with strong temperature dependent aggregation and intentionally reduced crystallinity matches non-fullerene acceptors.

Reduced Intramolecular Twisting Improves the Performance of 3D Molecular Acceptors in Non-Fullerene Organic Solar Cells

A small-molecular acceptor, tetraphenylpyrazine-perylenediimide tetramer (TPPz-PDI₄ ), which has a reduced extent of intramolecular twisting compared to two other small-molecular acceptors is designed. Benefiting from the lowest extent of intramolecular twisting, TPPz-PDI₄ exhibits the highest aggregation tendency and electron mobility, and therefore achieves a highest power conversion efficiency of 7.1%.

Synthesis and characterization of fluorinated azadipyrromethene complexes as acceptors for organic photovoltaics

Homoleptic zinc(II) complexes of di(phenylacetylene)azadipyrromethene (e.g., Zn(WS3)2) are potential non-fullerene electron acceptors for organic photovoltaics. To tune their properties, fluorination of Zn(WS3)2 at various positions was investigated. Three fluorinated azadipyrromethene-based ligands were synthesized with fluorine at the para-position of the proximal and distal phenyl groups, and at the pyrrolic phenylacetylene moieties. Additionally, a CF3 moiety was added to the pyrrolic phenyl positions to study the effects of a stronger electron withdrawing unit at that position. The four ligands were chelated with zinc(II) and BF2+ and the optical and electrochemical properties were studied. Fluorination had little effect on the optical properties of both the zinc(II) and BF2+ complexes, with λmax in solution around 755 nm and 785 nm, and high molar absorptivities of 100 × 103 M-1cm-1 and 50 × 103 M-1cm-1, respectively. Fluorination of Zn(WS3)2 raised the oxidation potentials by 0.04 V to 0.10 V, and the reduction potentials by 0.01 V to 0.10 V, depending on the position and type of substitution. The largest change was observed for fluorine substitution at the proximal phenyl groups and CF3 substitution at the pyrrolic phenylacetylene moieties. The later complexes are expected to be stronger electron acceptors than Zn(WS3)2, and may enable charge transfer from other conjugated polymer donors that have lower energy levels than poly(3-hexylthiophene) (P3HT).

High-Efficiency Nonfullerene Polymer Solar Cell Enabling by Integration of Film-Morphology Optimization, Donor Selection, and Interfacial Engineering

Carrier mobility is a vital factor determining the electrical performance of organic solar cells. In this paper we report that a high-efficiency nonfullerene organic solar cell (NF-OSC) with a power conversion efficiency of 6.94 ± 0.27% was obtained by optimizing the hole and electron transportations via following judicious selection of polymer donor and engineering of film-morphology and cathode interlayers: (1) a combination of solvent annealing and solvent vapor annealing optimizes the film morphology and hence both hole and electron mobilities, leading to a trade-off of fill factor and short-circuit current density (Jsc); (2) the judicious selection of polymer donor affords a higher hole and electron mobility, giving a higher Jsc; and (3) engineering the cathode interlayer affords a higher electron mobility, which leads to a significant increase in electrical current generation and ultimately the power conversion efficiency (PCE).

A High Efficiency Nonfullerene Organic Solar Cell with Optimized Crystalline Organizations

A well-organized donor-acceptor crystalline structure is examined for high -performance nonfullerene solar cells. By thermal annealing, nanoscale structures of both donor and acceptor domains are successfully modulated, followed by -significant changes in the resulting -photovoltaic characteristics. When annealed at 90 °C, a maximum power conversion efficiency of 7.64% with a -remarkable open-circuit voltage of 1.03 V is obtained.

A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells

A new 3D nonfullerene small-molecule acceptor is reported. The 3D interlocking geometry of the small-molecule acceptor enables uniform molecular conformation and strong intermolecular connectivity, facilitating favorable nanoscale phase separation and electron charge transfer. By employing both a novel polymer donor and a nonfullerene small-molecule acceptor in the solution-processed organic solar cells, a high-power conversion efficiency of close to 6% is demonstrated.

Azadipyrromethenes: from traditional dye chemistry to leading edge applications

The journey of azadipyrromethenes from accidental dye chemistry to a compound class with widely applicable near infrared photophysical properties is documented.

A nonfullerene acceptor for wide band gap polymer based organic solar cells

A small molecular acceptor was synthesized for wide band gap polymer based nonfullerene OPVs to achieve a PCE of 3.71%, which is the best value for nonfullerene OPVs with a V_oc higher than 1 V.

An asymmetric binuclear zinc(ii) complex with mixed iminodiacetate and phenanthroline ligands: synthesis, characterization, structural conversion and anticancer properties

A water-soluble asymmetric binuclear zinc(ii) complex with mixed iminodiacetate and 1,10-phenanthroline ligands exhibited promising anticancer activity and low toxicity, suggesting potential as a chemotherapeutic agent.

Beyond Fullerenes: Designing Alternative Molecular Electron Acceptors for Solution-Processable Bulk Heterojunction Organic Photovoltaics

Organic photovoltaics (OPVs) are promising candidates for providing a low cost, widespread energy source by converting sunlight into electricity. Solution-processable active layers have predominantly consisted of a conjugated polymer donor blended with a fullerene derivative as the acceptor. Although fullerene derivatives have been the acceptor of choice, they have drawbacks such as weak visible light absorption and poor energy tuning that limit overall efficiencies. This has recently fueled new research to explore alternative acceptors that would overcome those limitations. During this exploration, one question arises: what are the important design principles for developing nonfullerene acceptors? It is generally accepted that acceptors should have high electron affinity, electron mobility, and absorption coefficient in the visible and near-IR region of the spectra. In this Perspective, we argue that alternative molecular acceptors, when blended with a conjugated polymer donor, should also have large nonplanar structures to promote nanoscale phase separation, charge separation and charge transport in blend films. Additionally, new material design should address the low dielectric constant of organic semiconductors that have so far limited their widespread application.

Fine-Tuning the 3D Structure of Nonfullerene Electron Acceptors Toward High-Performance Polymer Solar Cells

Arylene linkers in a series of new tetraaza-benzodifluoranthene diimide dimers enable tuning of the 3D molecular structure of nonfullerene electron acceptors, facilitating observation of dramatic variation of the power conversion efficiency from 2.6% to 6.4% as the twist angle between the monomeric building blocks in the dimer is varied.

Roll-Coated Fabrication of Fullerene-Free Organic Solar Cells with Improved Stability

Large area, fullerene-free organic solar cells with improved stability and efficiency of up to 1% are fabricated by the roll-coating process on indium tin oxide free and flexible substrates, under ambient conditions.

A Tetraperylene Diimides Based 3D Nonfullerene Acceptor for Efficient Organic Photovoltaics

A nonfullerene acceptor based on a 3D tetraperylene diimide is developed for bulk heterojunction organic photovoltaics. The disruption of perylene diimide planarity with a 3D framework suppresses the self-aggregation of perylene diimide and inhibits excimer formation. From planar monoperylene diimide to 3D tetraperylene diimide, a significant improvement of power conversion efficiency from 0.63% to 3.54% can be achieved.

A tetraphenylethylene core-based 3D structure small molecular acceptor enabling efficient non-fullerene organic solar cells

A tetraphenylethylene core-based small molecular acceptor with a unique 3D molecular structure is developed. Bulk-heterojunction blend films with a small feature size (≈20 nm) are obtained, which lead to non-fullerene organic solar cells (OSCs) with 5.5% power conversion efficiency. The work provides a new molecular design approach to efficient non-fullerene OSCs based on 3D-structured small-molecule acceptors.

New advances in non-fullerene acceptor based organic solar cells

Non-fullerene organic molecules are alternative and competitive acceptor materials for high-efficiency organic solar cells.

Diindeno[1,2-g:1′,2′-s]rubicene: all-carbon non-fullerene electron acceptor for efficient bulk-heterojunction organic solar cells with high open-circuit voltage

An all carbon non-fullerene electron acceptor material based on diindeno[1,2-g:1′,2′-s]rubicene (DIR) was readily synthesized and processed for bulk-heterojunction organic solar cells.

Synthesis and photovoltaic properties of A–D–A type non-fullerene acceptors containing isoindigo terminal units

Four solution-processable acceptor–donor–acceptor structured organic molecules with isoindigo as terminal acceptor units and different aromatic rigid planar cores as donor units were designed and synthesized as the acceptor materials in organic solar cells (OSCs).

Pyrene terminal functionalized perylene diimide as non-fullerene acceptors for bulk heterojunction solar cells

Two perylene diimide based small molecules with different terminal groups of pyrene andtert-butyl pyrene were developed as acceptor materials in organic solar cells.