Haijun Fan

Development of small-molecule materials for high-performance organic solar cells

With the rapid development in recent years, small-molecule organic solar cell is challenging the dominance of its counterpart, polymer solar cell. The top power conversion efficiencies of both single and tandem solar cells based on small molecules have surpassed 9%. In this mini review, achievements of small molecules with impressive photovoltaic performance especially reported in the last two years were highlighted. The relationship between molecular structure and device performance was analyzed, which draws some rules for rational molecular design. Five series of p- and n-type small molecules were selected based on the consideration of their competitiveness of power conversion efficiencies.

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.

Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4-b]thiophene Moiety for High-Performance Solar Cells.

By replacing the central thiophene of STDR, a sepithiophene terminated with two 3-ethylrhodanine moieties, with 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate, an A-D-Q-D-A-type small molecule has been developed for high-performance organic solar cells with improved photocurrent. STDR-TbT exhibits a significant bathochromic shift with a low optical bandgap of approximately 1.60 eV in the thin film. Accordingly, STDR-TbT shows broad external quantum efficiency spectral response up to 800 nm. A high short circuit current (Jsc) of 10.90 mA cm(-2) was achieved for STDR-TbT:PC71BM-based devices; this is significantly higher than that of STDR:PC71BM-based devices, Jsc: 5.61 mA cm(-2), with a power-conversion efficiency (PCE) of 5.05%. Compared with STDR-based devices, STDR-TbT-based devices show balanced charge carrier transport, better thin-film morphology, and favorable charge separation/collection.

A large-bandgap small-molecule electron acceptor utilizing a new indacenodibenzothiophene core for organic solar cells

A weak electron-donating ladder-type heteroarene, indacenodibenzothiophene (IDBT), is designed and synthesized for the development of acceptor–donor–acceptor (A–D–A)-type small-molecule acceptors featuring a large optical bandgap to match high-performance low-bandgap electron donors. Small-molecule acceptor NIDBT based on the IDBT core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile as an electron-deficient end group is designed and synthesized. NIDBT shows a wide optical bandgap of 1.84 eV with a high absorption coefficient. A low-bandgap polymer PTB7-Th is selected as the donor to construct photovoltaic devices with NIDBT as the acceptor. After a thermal-annealing treatment at 160 °C for 10 min, an optimal power conversion efficiency (PCE) of 4.45% with an open-circult voltage (Voc) of 0.831 V, a short-circuit current (Jsc) of 11.45 mA cm−2 and a fill factor (FF) of 46.73% is realized.

Dithienoindophenines: p-Type Semiconductors Designed by Quinoid Stabilization for Solar-Cell Applications.

Compared with the dominant aromatic conjugated materials, photovoltaic applications of their quinoidal counterparts featuring rigid and planar molecular structures have long been unexplored despite their narrow optical bandgaps, large absorption coefficients, and excellent charge-transport properties. The design and synthesis of dithienoindophenine derivatives (DTIPs) by stabilizing the quinoidal resonance of the parent indophenine framework is reported here. Compared with the ambipolar indophenine derivatives, DTIPs with the fixed molecular configuration are found to be p-type semiconductors exhibiting excellent unipolar hole mobilities up to 0.22 cm2  V-1  s-1 , which is one order of magnitude higher than that of the parent IP-O and is even comparable to that of QQT(CN)4-based single-crystal field-effect transistors (FET). DTIPs exhibit better photovoltaic performance than their aromatic bithieno[3,4-b]thiophene (BTT) counterparts with an optimal power-conversion efficiency (PCE) of 4.07 %.

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.

Fast construction of dianthraceno[a,e]pentalenes for OPV applications

A new method based on a palladium-catalyzed tandem reaction for the fast construction of a dianthraceno[a,e]pentalene (DAP) framework is developed, which gave a series of dianthraceno[a,e]pentalenes with good functional group tolerance, and it should be highlighted that five C–C bonds were formed in one reaction. The photophysical and electrochemical properties of DAP derivatives were examined in detail. Interestingly, cross conjugation can be observed for DAPs substituted with electron-withdrawing substituents, which results in decreased optical bandgaps and the highest occupied molecular orbital (HOMO) energy levels. Because of the good absorption features, high fluorescence quantum yield and appropriate frontier energy levels, dianthraceno[a,e]pentalene with a p-((2-hexyldecyl)-2-cyanoacetate)phenyl group (DAP-PhCA) was utilized for solution-processed bulk-heterojunction solar cells, which delivered a power conversion efficiency of 2.05% with a high open-circuit voltage (Voc) value of 0.95 V.