Cyclobuteno[60]fullerenes as efficient n-type organic semiconductors

Cyclobuteno[3,4:1,2][60]fullerenes have been prepared in a straightforward manner by a simple reaction between [60]fullerene and readily available allenoates or alkynoates as organic reagents under basic and mild conditions. The chemical structure of the new modified fullerenes has been determined by standard spectroscopic techniques and confirmed by X-Ray diffraction analysis. Some of these new fullerene derivatives exhibit a remarkable intrinsic electron mobility – determined by using flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements – which surpasses that of the well-known PCBM, thus behavi ng as promising n-type organic semiconductors

Halogen‐Free πupiupi‐Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility

In π-conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a π-conjugated polymer based on a novel thiazole-fused ring, thieno[2′, 3′:5, 6]benzo[1, 2-d:4, 3-d′]bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (VOCs) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and VOCs of more than 0.84 V are observed, both of which are among the top values reported so far for “halogen-free” polymers. This study will serve as an important reference for designing π-conjugated polymers to achieve highly efficient and solution-processable OPVs

Exciton dynamics of a fused ring pi-conjugated nonfullerene molecule based on dithienonaphthobisthiadiazole

Herein, we have studied the exciton dynamics of a novel fused ring π-conjugated molecule (YS3) in solution and film states by spectroscopic measurements. This molecule incorporates dithienonaphthobisthiadiazole as a core unit that is a two-dimensionally π-extended fused ring. As a result, we found a long exciton lifetime in YS3 films originating from reduced radiative and nonradiative transitions. This is partly because radiative deactivation is effectively suppressed because of the dipole-forbidden transition in H-aggregates and partly because rotational deactivation is effectively suppressed in the crystalline film state

Dithiazolylthienothiophene Bisimide: A Novel Electron-Deficient Building Unit for N-Type Semiconducting Polymers

N-type (electron-transporting) semiconducting polymers are essential materials for the development of truly plastic electronic devices. Here, we synthesized for the first time dithiazolylthienothiophene bisimide (TzBI), as a new family for imide-based electron-deficient π-conjugated systems, and semiconducting polymers by incorporating TzBI into the π-conjugated backbone as the building unit. The TzBI-based polymers are found to have deep frontier molecular orbital energy levels and wide optical bandgaps compared to the dithienylthienothiophene bisimide (TBI) counterpart. It is also found that TzBI can promote the π–π intermolecular interactions of the polymer backbones relative to TBI most probably because the thiazole ring, which replaced the thiophene ring, at the end of the framework gives a more coplanar backbone. In fact, TzBI-based polymers function as the n-type semiconducting material in both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices. Notably, one of the TzBI-based polymers provides a power conversion efficiency of 3.3% in the all-polymer OPV device, which could be high for a low-molecular-weight polymer (<10 kDa). Interestingly, while many of the n-type semiconducting polymers utilized in OPVs have narrow bandgaps, the TzBI-based polymers have wide bandgaps. This is highly beneficial for complementing the visible to near-IR light absorption range when blended with p-type narrow bandgap polymers that have been widely developed in the last decade. The results demonstrate great promise and possibility of TzBI as the building unit for n-type semiconducting polymers

Highly Ordered n/p-Co-assembled Materials with Remarkable Charge Mobilities

Controlling self-organization and morphology of chemical architectures is an essential challenge in the search for higher energy-conversion efficiencies in a variety of optoelectronic devices. Here, we report a highly ordered donor/acceptor functional material, which has been obtained using the principle of ionic self-assembly. Initially, an electron donor π-extended tetrathiafulvalene and an electron-acceptor perylene-bisimide were self-organized separately obtaining n- and p-nanofibers at the same scale. These complementary n- and p-nanofibers are endowed with ionic groups with opposite charges on their surfaces. The synergic interactions establish periodic alignments between both nanofibers resulting in a material with alternately segregated donor/acceptor nanodomains. Photoconductivity measurements show values for these n/p-co-assembled materials up to 0.8 cm2 V–1 s–1, confirming the effectiveness in the design of these heterojunction structures. This easy methodology offers great possibilities to achieve highly ordered n/p-materials for potential applications in different areas such as optoelectonics and photovoltaics

Ordered π-conjugated polymer backbone in amorphous blend for high efficiency nonfullerene organic photovoltaics

Abstract In π-conjugated polymers, the amorphous region absent from π–π stacking order typically limits polymer functions compared to the crystalline region with high π–π stacking order. Here, we show that a benzodithiophene–thiazolothiazole copolymer containing tripropylsilyl groups (PSTz2) has a greater coplanar backbone structure when the π–π stacking order is absent, such as in solution and in a film blended with a nonfullerene acceptor, than when it is present in a neat film. The excitons and charge carriers generated in PSTz2 are more highly delocalized in the blend film than in the neat film, presumably through the backbone. The unconventional structural feature of PSTz2 shows higher photovoltaic performance in nonfullerene-based cells compared to its alkyl-functionalized counterpart. Our results show that it is possible to develop π-conjugated polymers that perform well in amorphous blends due to the ordered backbone structure

Pronounced Backbone Coplanarization by π-Extension in a Sterically Hindered Conjugated Polymer System Leads to Higher Photovoltaic Performance in Non-Fullerene Solar Cells

Achieving both the backbone order and solubility of π-conjugated polymers, which are often in a trade-off relationship, is imperative for maximizing the performance of organic solar cells. Here, we studied three different π-conjugated polymers based on thiazolothiazole (PSTz₁ and POTz₁) and benzobisthiazole (PNBTz₁) that were combined with a benzodithiophene unit in the backbone, where PNBTz₁ was newly synthesized. Because of the steric hindrance between the side chains located on neighboring heteroaromatic rings, POTz₁ had a much less coplanar backbone than PSTz₁ in which such a steric hindrance is absent. However, POTz₁ showed higher photovoltaic performance in solar cells that used Y6 as the acceptor material. This was likely due to the significantly higher solubility of POTz₁ than PSTz₁, resulting in a better morphology. Interestingly, PNBTz₁ was found to have markedly higher backbone coplanarity than POTz1, despite having similar steric hindrance between the side chains, most likely owing to the more extended π-electron system, whereas PNBTz₁ had good solubility comparable to POTz₁. As a result, PNBTz₁ exhibited higher photovoltaic performance than POTz₁ in the Y6-based cells: specifically, the fill factor was significantly enhanced. Our results indicate that the backbone order and solubility can be achieved by the careful molecular design, which indeed leads to higher photovoltaic performance

Hetero Bis-Addition of Spiro-Acetalized or Cyclohexanone Ring to 58π Fullerene Impacts Solubility and Mobility Balance in Polymer Solar Cells

Fullerene bis-adducts are increasingly being studied to gain a high open circuit voltage (<i>V</i>_oc) in bulk heterojunction organic photovoltaics (OPVs). We designed and synthesized homo and hetero bis-adduct [60]­fullerenes by combining fused cyclohexanone or a five-membered spiro-acetalized unit (SAF₅) with 1,2-dihydromethano (CH₂), indene, or [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). These new eight 56π fullerenes showed a rational rise of the lowest unoccupied molecular orbital (LUMO). We perform a systematic study on the electrochemical property, solubility, morphology, and space-charge-limited current (SCLC) mobility. The best power conversion efficiency (PCE) of 4.43% (average, 4.36%) with the <i>V</i>_oc of 0.80 V was obtained for poly­(3-hexylthiophene) (P3HT) blended with SAF₅/indene hetero bis-adduct, which is a marked advancement in PCE compared to the 0.9% of SAF₅ monoadduct. More importantly, we elucidate an important role of mobility balance between hole and electron that correlates with the device PCEs. Besides, an empirical equation to extrapolate the solubilities of hetero bis-adducts is proposed on the basis of those of counter monoadducts. Our work offers a guide to mitigate barriers for exploring a large number of hetero bis-adduct fullerenes for efficient OPVs

Facile and Exclusive Formation of Aziridinofullerenes by Acid-catalyzed Denitrogenation of Triazolinofullerenes

Variously substituted [6,6]closed aziridinofullerenes were exclusively obtained from acid-catalyzed denitrogenation of triazolinofullerenes without formation of relevant [5,6]open azafulleroids, which are the major products on noncatalyzed denitrogenation. The mechanistic consideration by DFT calculations suggested a reaction sequence involving initial pre-equilibrium protonation of the triazoline N₁ atom, generation of aminofullerenyl cation by nitrogen-extrusion, and final aziridination

Stereochemistry of Spiro-Acetalized [60]Fullerenes: How the <i>Exo</i> and <i>Endo</i> Stereoisomers Influence Organic Solar Cell Performance

Exploiting bis-addition products of fullerenes is a rational way to improve the efficiency of bulk heterojunction-type organic photovoltaic cells (OPV); however, this design inherently produces regio- and stereoisomers that may impair the ultimate performance and fabrication reproducibility. Here, we report unprecedented <i>exo</i> and <i>endo</i> stereoisomers of the spiro-acetalized [60]­fullerene monoadduct with methyl- or phenyl-substituted 1,3-dioxane (<b>SAF</b>_<b>6</b>). Although there is no chiral carbon in either the reagent or the fullerene, equatorial (<i>eq</i>) rather than axial (<i>ax</i>) isomers are selectively produced at an <i>exo-eq</i>:<i>endo</i>-<i>eq</i> ratio of approximately 1:1 and can be easily separated using silica gel column chromatography. Nuclear Overhauser effect measurements identified the conformations of the straight <i>exo</i> isomer and bent <i>endo</i> isomer. We discuss the origin of stereoselectivity, the anomeric effect, intermolecular ordering in the film state, and the performance of poly­(3-hexylthiophene):substituted <b>SAF</b>_<b>6</b> OPV devices. Despite their identical optical and electrochemical properties, their solubilities and space-charge limited current mobilities are largely influenced by the stereoisomers, which leads to variation in the OPV efficiency. This study emphasizes the importance of fullerene stereochemistry for understanding the relationship between stereochemical structures and device output