Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives

Design, synthesis and characterization of fused bithiazole- and dithiophene-based low bandgap thienylenevinylene copolymers

The electron-deficient thiazole moiety has high planarity and effective π–π stacking, which leads to the reduction in the energy levels, exhibiting promising charge carrier mobilities.

Molecular Ordering of Dithieno[2,3-d;2',3'-d]benzo[2,1-b:3,4-b']dithiophenes for Field-Effect Transistors

Four derivatives of dithieno[2,3-d;2',3'-d']benzo[1,2-b;3,4-b']dithiophene (DTmBDT) have been synthesized to investigate the correlation between molecular structure, thin-film organization, and charge-carrier transport. Phenyl or thiophene end-capped derivatives at alpha positions of the outer thiophenes of DTmBDT present vastly different optoelectronic properties in comparison with bay-position alkyl-chain-substituted DTmBDT, which was additionally confirmed by density functional theory simulations. The film morphology of the derivatives strongly depends on alkyl substituents, aromatic end-caps, and substrate temperature. Field-effect transistors based on DTmBDT derivatives with bay-substituted alkyl chains show the best performance within this studied series with a hole mobility up to 0.75 cm²/V s. Attachment of aromatic end-caps disturbs the ordering, limiting the charge-carrier transport. Higher substrate temperature during deposition of the DTmBDT derivatives with aromatic end-caps results in larger domains and improved the transistor mobilities but not beyond the alkylated DTmBDT.

Incorporating Trialkylsilylethynyl-Substituted Head-to-Head Bithiophene Unit into Copolymers for Efficient Non-Fullerene Organic Solar Cells

Mediating the backbone coplanarity and solubility of oligothiophenes, especially the head-to-head (HH) disubstituted bithiophene, to achieve an optically and electronically advantageous building block for organic semiconductor materials is a vital yet challenging task. On the other hand, exploring polymer solar cells (PSCs) processed from nonhalogenated solvents is necessary toward their large-scale applications. In this contribution, we develop a HH-type bithiophene analogue (TIPS-T2) by strategically applying the triisopropylsilylethynyl (TIPS) scaffold as the side chain. TIPS can serve to narrow optical band gaps, lower the highest occupied molecular orbital level, reduce intrachain steric hindrance, and guarantee sufficient solubility of the involving polymers. Upon alternating with difluorobenzotriazole (FTAZ) or benzodithiophene-4,8-dione (BDD) acceptor units, two polymers named PT4Si-FTAZ and PT4Si-BDD are synthesized. Encouragingly, non-fullerene PSCs incorporating PT4Si-FTAZ yield a power conversion efficiency of 6.79% when processed from an environment-friendly solvent of trimethylbenzene because of its promoted backbone planarity, as demonstrated by density functional theory, higher hole mobility, and superior film morphology. The results indicate that TIPS-T2 is a promising building block for constructing photovoltaic polymers, and our findings offer an avenue for the ingenious use of TIPS as functional side chains.

S,N-Heteroacene-Based Copolymers for Highly Efficient Organic Field Effect Transistors and Organic Solar Cells: Critical Impact of Aromatic Subunits in the Ladder π-System

Three novel donor-acceptor alternating polymers containing ladder-type pentacyclic heteroacenes (PBo, PBi, and PT) are synthesized, characterized, and further applied to organic field effect transistors (OFETs) and polymer solar cells. Significant aspects of quinoidal characters, electrochemical properties, optical absorption, frontier orbitals, backbone coplanarity, molecular orientation, charge carrier mobilities, morphology discrepancies, and the corresponding device performances are notably different with various heteroarenes. PT exhibits a stronger quinoidal mesomeric structure, linear and coplanar conformation, smooth surface morphology, and better bimodal crystalline structures, which is beneficial to extend the π-conjugation and promotes charge transport via 3-D transport pathways and in consequence improves overall device performances. Organic photovoltaics based on the PT polymer achieve a power conversion efficiency of 6.04% along with a high short-circuit current density (J_SC) of 14.68 mA cm^-2, and a high hole mobility of 0.1 cm² V^-1 s^-1 is fulfilled in an OFET, which is superior to those of its counterparts, PBi and PBo.

Synergistic Effects of Selenophene and Extended Ladder-Type Donor Units for Efficient Polymer Solar Cells

Two pairs of polymer donor materials based on indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) as the donor units are synthesized. Thiophene or selenophene is introduced as the π-bridge units and electron-deficient fluorine-substituted quinoxaline is used as acceptor unit. Selenophene-containing polymers PIDT-DFQ-Se and PIDTT-DFQ-Se show redshifted absorption and narrower bandgaps. Combined with IDTT donor unit, PIDTT-DFQ-Se shows the highest absorption coefficient. Both the IDTT unit and selenophene unit have positive effects on the hole mobilities, making PIDTT-DFQ-Se the highest one. The best power conversion efficiency of 7.4% is obtained from devices based on PIDTT-DFQ-Se:[6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁ BM) with a J_sc of 12.6 mA cm^-2 , a V_oc of 0.89 V, and a fill factor (FF) of 0.66.

Effects of including electron-withdrawing atoms on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene-based donor–acceptor polymers: towards an acceptor design for efficient polymer solar cells

Three new D–A polymers using indacenodithieno[3,2-b]thiophene (IDTT) as an electron-rich unit and benzoxadiazole (BO), benzodiathiazole (BT) or difluorobenzothiadiazole (FBT) as an electron-deficient unit were synthesized and applied in solar cells.

Head-to-Head Linkage Containing Bithiophene-Based Polymeric Semiconductors for Highly Efficient Polymer Solar Cells

Narrow bandgap (1.37-1.46 eV) polymers incorporating a head-to-head linkage containing 3-alkoxy-3'-alkyl-2,2'-bithiophene are synthesized. The head-to-head linkage enables polymers with sufficient solubility and the noncovalent sulfur-oxygen interaction affords polymers with high degree of backbone planarity and film ordering. When integrated into polymer solar cells, the polymers show a promising power conversion efficiency approaching 10%.

A femtosecond transient absorption study of charge photogeneration and recombination dynamics in photovoltaic polymers with different side-chain linkages

A pair of 9-arylidene-9H-fluorene and benzothiadiazole based, low-bandgap copolymers differing merely in the para or meta substitution of alkoxy groups to the arylidene linkages, i.e. p-PAFDTBT and m-PAFDTBT respectively, were comparatively investigated by using morphological characterization, ultrafast spectroscopy and quantum chemical calculations. Despite the subtle difference in the alkoxy substitution patterns, p-PAFDTBT molecules in photoactive films were shown to have a higher degree of crystallinity owing to the relatively less rotational torsion of the arylidene linkages. As a result, in either neat or fullerene-blended films, p-PAFDTBT compared to m-PAFDTBT gave rise to a substantially higher charge yield and much slower charge recombination. This work demonstrates that the alkoxy substitution pattern and the arylidene linkage are highly influencing on the morphology of the photoactive layers and thereby on the photovoltaic performance of the semiconducting copolymers.

Dependence of Excited-State Properties of a Low-Bandgap Photovoltaic Copolymer on Side-Chain Substitution and Solvent

The excited-state properties and chain conformations of a new low-bandgap copolymer based on benzo[1,2-b:4,5-b']dithiophene (BDT) and thieno[3,4-b]thiophene with meta-alkoxyphenyl-substituted side chains in solution were investigated comprehensively. Time-resolved spectroscopy suggested that the excited-state properties were sensitive to the conformations of the copolymer in solution. In addition, excited-state dynamics analyses revealed the photogeneration of triplet excited states by intersystem crossing (ISC) at a rate constant of ∼0.4×10(9)  s(-1) as a result of direct meta-alkoxyphenyl connection to the donor unit BDT irrespective to the macromolecular conformations. According to El-Sayed's rule, the fast ISC herein is correlated with the change of orbital types between singlet and triplet excited states as also shown by quantum chemical calculations. Our studies may shed light on the structure-property relationships of photovoltaic materials.

Marriage of heavy main group elements with π-conjugated materials for optoelectronic applications

This review article summarizes recent progress in the synthesis and optoelectronic properties of conjugated materials containing heavy main group elements from Group 13-16 as integral components. As will be discussed, the introduction of these elements can promote novel phosphorescent behavior and support desirable molecular and polymeric properties such as low optical band gaps and high charge mobilities for photovoltaic and thin film transistor applications.

A heptacyclic acceptor unit developed for D–A copolymers used in polymer solar cells

A heptacyclic acceptor unit TTP and a D–A copolymer PBDTTTP based on it were synthesized. PBDTTTP:PC₇₁BM solar cells gave a PCE of 5.53%, and the maximum V_oc reached 1 V.

Polymers and the p-block elements

A survey of the state-of-the-art in the development of synthetic methods to incorporate p-block elements into polymers is given. The incorporation of main group elements (groups 13-16) into long chains provides access to materials with fascinating chemical and physical properties imparted by the presence of inorganic groups. Perhaps the greatest impedance to the widespread academic and commercial use of p-block element-containing macromolecules is the synthetic challenge associated with linking inorganic elements into long chains. In recent years, creative methodologies have been developed to incorporate heteroatoms into polymeric structures, with perhaps the greatest advances occurring with hybrid organic-inorganic polymers composed of boron, silicon, phosphorus and sulfur. With these developments, materials are currently being realized that possess exciting chemical, photophysical and thermal properties that are not possible for conventional organic polymers. This review focuses on highlighting the most significant recent advances whilst giving an appropriate background for the general reader. Of particular focus will be advances made over the last two decades, with emphasis on the novel synthetic methodologies employed.

Enhancing mechanical properties of highly efficient polymer solar cells using size-tuned polymer nanoparticles

The low mechanical durability of polymer solar cells (PSCs) has been considered as one of the critical hurdles for their commercialization. We described a facile and powerful strategy for enhancing the mechanical properties of PSCs while maintaining their high power conversion efficiency (PCE) by using monodispersed polystyrene nanoparticles (PS NPs). We prepared highly monodispersed, size-controlled PS NPs (60, 80, and 100 nm), and used them to modify the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) anode buffer layer (ABL). The PS NPs played two important roles; i.e., they served as (1) binders in the PEDOT:PSS films, and (2) interfacial modifiers between ABL and the active layer, resulting in remarkable improvement of the mechanical integrity of the PSCs. The addition of PS NPs enhanced the inherent mechanical toughness of the PEDOT:PSS ABL due to their elastic properties, allowing the modified ABL to tolerate higher mechanical deformations. In addition, the adhesion energy (Gc) between the active layer and the modified PEDOT:PSS layer was enhanced significantly, i.e., by a factor of more than 1.5. The Gc value has a strong relationship with the sizes of the PS NP, showing the greatest enhancement when the largest size PS NPs (100 nm) were used. In addition, PS NPs significantly improve the air-stability of the PSCs by suppressing moisture adsorption and corrosion of the electrodes. Thus, the modification of ABL with PS NPs effectively enhances both the mechanical and the long-term stabilities of the PSCs without sacrificing their PCE values, demonstrating their great potential as applications in flexible organic electronics.

2,1,3-Benzothiadiazole-5,6-dicarboxylic imide--a versatile building block for additive- and annealing-free processing of organic solar cells with efficiencies exceeding 8%

A new photoactive polymer comprising benzo[1,2-b:3,4-b':5,6-d']trithiophene and 2,1,3-benzothiadiazole-5,6-dicarboxylic imide is reported. The synthetic design allows for alkyl chains to be introduced on both electron-rich and electron-deficient components, which in turn allows for rapid optimization of the alkyl chain substitution pattern. Consequently, the optimized polymer shows a maximum efficiency of 8.3% in organic photovoltaic devices processed in a commercially viable fashion without solvent additives, annealing, or device engineering.

Highly efficient small molecule solar cells fabricated with non-halogenated solvents

Fabricated with non-halogenated solvents toluene and o-xylene, the bulk-heterojunction organic solar cells based on a porphyrin small molecule show high power conversion efficiencies up to 5.46% and 5.85%, respectively.

Effect of fluorine substitution on the photovoltaic performance of poly(thiophene-quinoxaline) copolymers

Three new conjugated polymers with or without fluorine substituents on quinoxaline ring were investigated in detail to assess the role of F in OPV performance.

Enhanced fill factor of tandem organic solar cells incorporating a diketopyrrolopyrrole-based low-bandgap polymer and optimized interlayer

We demonstrate that reproducible results can be obtained from tandem solar cells based on the wide-bandgap poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole] (PCDTBT) and the diketopyrrolopyrrole (DPP)-based narrow bandgap polymer (DT-PDPP2T-TT) with a decyltetradecyl (DT) and an electron-rich 2,5-di-2-thienylthieno[3,2-b]thiophene (2T-TT) group fabricated using an optimized interlayer (ZnO NPs/ph-n-PEDOT:PSS) [NPs: nanoparticles; ph-n: pH-neutral PEDOT: poly(3,4-ethylenedioxythiophene); PSS: polystyrene sulfonate]. The tandem cells are fabricated by applying a simple process without thermal annealing. The ZnO NP interlayer operates well when the ZnO NPs are dispersed in 2-methoxyethanol, as no precipitation and chemical reactions occur. In addition to the ZnO NP film, we used neutral PEDOT:PSS as a second interlayer which is not affect to the sequential deposited bulk heterojunction (BHJ) active layer of acidification. The power conversion efficiency (PCE) of a tandem device reaches 7.4 % (open-circuit voltage VOC =1.53 V, short-circuit current density JSC =7.3 mA cm(-2) , and fill factor FF=67 %). Furthermore, FF is increased to up to 71 % when another promising large bandgap (bandgap ∼1.94 eV) polymer (PBnDT-FTAZ) is used. The surface of each layer with nanoscale morphology (BHJ1/ZnO NPs film/ph-n-PEDOT:PSS/BHJ2) was examined by means of AFM analysis during sequential processing. The combination of these factors, efficient DPP-based narrow bandgap material and optimized interlayer, leads to the high FF (average approaches 70 %) and reproducibly operating tandem BHJ solar cells.

Donor–acceptor conjugated polymers based on multifused ladder-type arenes for organic solar cells

In this review, we summarize the recent development of the multifused ladder-type conjugated building blocks for making donor–acceptor conjugated copolymers.

Design, synthesis and electronic properties of push–pull–push type dye

Alkyne conjugated push–pull–push type dye where alkyne π-spacer and push units monitor the electronic properties of the dye.

Influences of the backbone randomness on the properties, morphology and performances of the fluorinated benzoselenadiazole–benzothiadiazole based random copolymers

A series of FBT–FBSe ternary random copolymers were synthesized to identify the influences of backbone randomness on the morphology and performances.

The influence of binary processing additives on the performance of polymer solar cells

In this study, we report the investigation of the influence of binary processing additives, 1,8-octanedithiol (ODT) and 1-chloronaphthalene (CN) on the performance of polymer solar cells (PSCs). It was found that the power conversion efficiency (PCE) can be enhanced to 8.55% from the PSCs processed with binary processing additives as compared with ∼6.50% from the PSCs processed with either ODT or CN processing additives. With binary processing additives, the crystallinity of the electron donor polymer, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2 ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]], was elevated, which in turn facilitated charge transport within the bulk heterojunction (BHJ) layer, resulting in a high short-circuit current and large fill factor. By photophysical studies, we further found that the high PCE is majorly attributed to the minimized nongeminate recombination by controlling the kinetic film morphologies of the BHJ composite by binary solvent processing additives.

High-Performance All-Polymer Solar Cells Based on Face-On Stacked Polymer Blends with Low Interfacial Tension

We report highly efficient all-polymer solar cells with power conversion efficiencies of over 4.5% by highly intermixed blends of PTB7-Th donor and P(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on π-π stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from the active layer to each electrode. In addition, the incorporation of 1,8-diiodooctane additives was able to tune the degree of crystallinity and orientation of P(NDI2OD-T2) acceptors, resulting in remarkable enhancement of electron mobility, external quantum efficiency, and J_SC values.