Huanran Feng

A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency

A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small‐Molecule Bulk‐Heterojunction Blends under Solvent Vapor Treatment

A series of solvents with different solubilities for DR3TBDTT and PC71 BM, and different boiling points, is used for solvent vapor annealing (SVA) treatment to systematically investigate the solvent-morphology-performance relationship. The presence of solvent molecules inside bulk-heterojunction (BHJ) thin films promotes the mobility of both donor and acceptor molecules, leading to crystallization and aggregation, which are important in modulating morphology.

A simple small molecule as an acceptor for fullerene-free organic solar cells with efficiency near 8%

A simple small molecule acceptor named DICTF, with fluorene as the central block and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile as the end-capping groups, has been designed for fullerene-free organic solar cells. The new molecule was synthesized from widely available and inexpensive commercial materials in only three steps with a high overall yield of ∼60%. Fullerene-free organic solar cells with DICTF as the acceptor material provide a high PCE of 7.93%.

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells.

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the Voc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

A high-performance photovoltaic small molecule developed by modifying the chemical structure and optimizing the morphology of the active layer

A small molecule (DR3TDOBDT) containing 4,8-dioctyl benzo[1,2-b:4,5-b′]dithiophene as the central block and 3-(2-ethylhexyl)-rhodanine as the end-capping groups has been designed and synthesized. A power conversion efficiency of 8.26% was achieved through the active layer morphology optimization process combining thermal annealing and solvent vapor annealing.

Evaluation of Small Molecules as Front Cell Donor Materials for High-Efficiency Tandem Solar Cells

Three small molecules as front cell donors for tandem cells are thoroughly evaluated and a high power conversion efficiency of 11.47% is achieved, which demonstrates that the oligomer-like small molecules offer a good choice for high-performance tandem solar cells.

New small-molecule acceptors based on hexacyclic naphthalene(cyclopentadithiophene) for efficient non-fullerene organic solar cells

A series of new non-fullerene small molecule acceptors (NTIC, NTIC-Me, NTIC-OMe and NTIC-F) based on the acceptor–donor–acceptor (A–D–A) architecture, using hexacyclic naphthalene-(cyclopentadithiophene) as the central unit, were designed and synthesized. The non-fullerene OSC device based on PBDB-T:NTIC showed a highest PCE of 8.63%. With a relatively high-lying LUMO level of NTIC-OMe, the PBDB-T:NTIC-OMe based device obtained a comparatively high Voc of 0.965 V and a PCE of 8.61% simultaneously. The results demonstrate that the naphthalene core is a promising building block for constructing highly efficient non-fullerene acceptors and further boosting the photovoltaic performance of the devices.

A new oligobenzodithiophene end-capped with 3-ethyl-rhodanine groups for organic solar cells with high open-circuit voltage

A new solution-processable small-molecule donor material, named DRBDT3, comprised of oligobenzo[1,2-b:4,5-b′] dithiophene as the backbone and 3-ethyl-rhodanine as the end-capped group has been designed and synthesized for application in organic photovoltaic cells. The oligobenzodithiophene derivative exhibits an absorption band from 300 to 640 nm. The film of DRBDT3 shows highly long-range ordering assembly and high mobility of 1.21×10−4 cm2 V−1 s−1. The new molecule shows a deep highest-occupied molecular orbital energy level. The device based on DRBDT3 as the donor and PC71BM as the acceptor exhibits a power conversion efficiency of 4.09% with a high open-circuit voltage of 0.99 V under AM.1.5G illumination (100 mW cm−2).

Investigation of the effect of large aromatic fusion in the small molecule backbone on the solar cell device fill factor

The structure and performance relationship in photovoltaic cells is still not fully understood, particularly in the case of controlling/optimizing the fill factor (FF). Here a pair of molecules DR2TDTCz and DR3TCz with similar backbone structures and varying conjugated central units were designed and synthesized, and their photovoltaic performance was studied and compared. The molecule DR2TDTCz, containing dithieno[3,2-b;6,7-b]carbazole (DTCz) as the central building block, with a carbazole ring in the center and two fused thiophene rings at the two sides of carbazole, exhibits improved solar light absorption and slightly narrow band gap, compared with the analogue system DR3TCz which has carbazole and two un-fused thiophene rings in the central building block. More importantly, it is found that introducing DTCz with thiophene fused 2,7-carbazole to replace 2,7-carbazole achieves a better molecular packing and favorable orientation, thus benefiting charge transport. As a result, the DR2TDTCz based device exhibits a power conversion efficiency (PCE) up to 7.03% with an impressively high FF of 75%, while the DR3TCz based device shows a PCE of 4.08% with a much lower FF of 54%. The results indicate that the FF can be tuned directly by the molecular structures and enlarged conjugation central core units could be beneficial to achieve high FF for the devices based on the acceptor–donor–acceptor (A–D–A) type small molecules.

A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary‐Blend Organic Solar Cells with Efficiency over 13%

Abstract A new acceptor–donor–acceptor (A–D–A) type nonfullerene acceptor, 3TT‐FIC, which has three fused thieno[3,2‐b]thiophene as the central core and difluoro substituted indanone as the end groups, is designed and synthesized. 3TT‐FIC exhibits broad and strong absorption with extended onset absorption to 995 nm and a low optical bandgap of 1.25 eV. The binary device based on 3TT‐FIC and the polymer PTB7‐Th exhibits a power conversion efficiency (PCE) of 12.21% with a high short circuit current density (   J sc) of 25.89 mA cm−2. To fine‐tune the morphology and make full use of the visible region sunlight, phenyl‐C71‐butyricacid‐methyl ester (PC71BM) is used as the third component to fabricate ternary devices. In contrast to the binary devices, the ternary blend organic solar cells show significantly enhanced EQE ranging from 300 to 700 nm and thus an improved  J sc with a high value of 27.73 mA cm−2. A high PCE with a value of 13.54% is achieved for the ternary devices, which is one of the highest efficiencies in single junction organic solar cells reported to date. The results provide valuable insight for the ternary devices in which the external quantum efficiency (EQE) induced by the third component is evidently observed and directly contributed to the enhancement of the device efficiency.