Shengjie Xu

An electron-rich 2-alkylthieno[3,4-b]thiophene building block with excellent electronic and morphological tunability for high-performance small-molecule solar cells

Bulk-heterojunction organic solar cells have been attracting much attention because of the potential for producing low-cost, large-area, and flexible PV panels. Generally, the ideal donor materials should have an appropriate electronic structure to absorb more sunlight and drive charge separation and to form an optimized morphology that can efficiently split excitons and collect charges. 2-Alkylthieno[3,4-b]thiophene (T34bT) as an electron-rich moiety possesses three features that are highly desirable for donor materials: (i) to modulate electronic structure; (ii) to manipulate the blend morphology of the active layer; (iii) to function as the π-bridge to link donor and acceptor moieties. We report herein a new category of small molecules STB-n based on the electron-rich T34bT moiety. STB-n featuring T34bT and rhodanine components showed intense absorption, reduced bandgap, and proper alignment of frontier orbital energy levels. Because of the optimized charge-transport properties and weak charge recombination, STB-3-based solar cells exhibit a power conversion efficiency of 9.26%, which is among the best reported for small-molecule-based solar cells. The relationships among molecular structure, thin-film morphology, and device performance were further investigated systematically.

Design of a New Fused‐Ring Electron Acceptor with Excellent Compatibility to Wide‐Bandgap Polymer Donors for High‐Performance Organic Photovoltaics

Fused-ring electron acceptors (FREAs) have recently received intensive attention. Besides the continuing development of new FREAs, the demand for FREAs featuring good compatibility to donor materials is becoming more and more urgent, which is highly desirable for screening donor materials and achieving new breakthroughs. In this work, a new FREA is developed, ZITI, featuring an octacyclic dithienocyclopentaindenoindene central core. The core is designed by linking 2,7-dithienyl substituents and indenoindene with small methylene groups, in which the indeno[1,2-b]thiophene-2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile part provides a large and unoccupied π-surface. Most notably, ZITI possesses an excellent compatibility with commercially available polymer donors, delivering very high power conversion efficiencies of over 13%.

Diaceno[a,e]pentalenes: An Excellent Molecular Platform for High‐Performance Organic Semiconductors

Three diaceno[a,e]pentalene analogues with pendant sterically bulky di-tert-butylphenyl groups have been designed and synthesized. With the extension of the conjugated molecular framework, the molecular arrangement is apparently tuned by the balance between the π-extended surface and pendant alkyl or aryl substituents. Theoretical calculations of the morphologies were in good agreement with the experimental results. Ambient-stable field-effect transistors based on dianthraceno[a,e]pentalene (DAP) have been fabricated, which exhibited excellent hole mobilities (up to 6.55 cm(2) V(-1) s(-1)). Thus, this study has shown that diaceno[a,e]pentalenes are stable even with an extraordinarily large π-surface area, and may thus serve as excellent molecular platforms for further exploring high-performance semiconducting materials.

Applying the heteroatom effect of chalcogen for high-performance small-molecule solar cells

Small molecules with defined chemical structures and low quality variation are important for organic solar cells (OSCs). Three thieno[3,4-b]thiophene small-molecule donor materials, STB-C, STB-O and STB-S, with different side-chain substitutions of alkyl, alkoxy and alkylthio were synthesized and applied to investigate the heteroatom effects on the OSC performance. Optimized devices based on STB-C, STB-O and STB-S delivered power conversion efficiencies (PCEs) of 7.84%, 8.68% and 4.05%, respectively, revealing the distinct influence of heteroatoms. Systematic structure–property relationships were further investigated by using incident X-ray diffraction, transmission electron microscopy, atomic force microscopy, and resonant soft X-ray scattering. Compared with those of STB-C and STB-S, the highest efficiency of STB-O can be attributed to the excellent charge transport properties, originating from the stronger π–π stacking, a multi-length scaled phase separation and a slightly elevated LUMO energy level. It is noteworthy that STB-O delivered the highest PCE among small-molecule donors based on alkoxy-substituted BDTs.

Stereocontrolled Synthesis of Benzo[k]fluoranthenes—An Unexpected Isomerization Mediated by Rhodacyclopentadiene

Herein, a Rh(III) -catalyzed stereocontrolled synthesis of benzo[k]fluoranthenes is reported. It was found that the unexpected E/Z isomerization was highly sensitive to the electronic effects of the substituents on the aryl groups. Theoretical calculations revealed that this controllable stereochemistry originates from the mediation of rhodacyclopentadiene intermediates during the isomerization. The fact that similar stereochemistry was observed when using an Ir(III) catalyst further suggests a certain generality of this discovery toward some other transition metals.

A thieno[3,4-b]thiophene linker enables a low-bandgap fluorene-cored molecular acceptor for efficient non-fullerene solar cells

A low-bandgap small-molecule acceptor NFTI with a fluorene central core, 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene) propane-dinitrile end groups and thieno[3,4-b]thiophene linkers was designed and synthesized for bulk-heterojunction organic solar cell applications. NFTI exhibits broad absorption with a low optical bandgap of approximately 1.57 eV in the thin film state. An optimized power conversion efficiency (PCE) of 9.02% with a high short-circuit current of 17.8 mA cm−2 was achieved with diphenyl ether (DPE) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) binary-processing additives. According to the detailed morphological investigation, we found that binary-processing additives helped to reduce the size of nanocrystals and enhance the intermolecular interaction, which led to improved charge separation and transport in the BHJ thin film, and thus achieved high device performance.

A thieno[3,4-b]thiophene-based small-molecule donor with a π-extended dithienobenzodithiophene core for efficient solution-processed organic solar cells

A small molecule donor STB-4 with dithieno[2,3-d′:2′,3′-d′]benzo[1,2-b:4′,5′-b′]dithiophene (DTBDT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. An optimized power conversion efficiency of 8.17% with an open-circuit voltage of 0.904 V, a short-circuit current of 13.33 mA cm−2, and a fill factor FF of 0.67 was achieved after solvent annealing (SVA). According to the detailed morphology investigations, we found that SVA refined the phase-separated morphologies of the blends, allowing the domains to become well defined with anappropriate size that is beneficial for device performance.

Design and synthesis of medium-bandgap small-molecule electron acceptors for efficient tandem solar cells

Medium-bandgap small-molecule acceptors including YITI-0F, YITI-2F, and YITI-4F with a thiophene π-bridge are designed and synthesized. The PBDB-T:YITI-2F-based device shows the best photovoltaic performance with a PCE of up to 10.05%, a Voc of 0.93 V, a Jsc of 15.54 mA cm−2 and a FF of 69%. Tandem devices are fabricated by applying it as a front cell and the PTB7-Th:ATT-2-based device as a rear cell, giving a high PCE up to 11.86%.