Daobin Yang

Asymmetrical Squaraines Bearing Fluorine-Substituted Indoline Moieties for High-Performance Solution-Processed Small-Molecule Organic Solar Cells

Two novel asymmetrical squaraines based on the indoline unit, ASQ-5-F and ASQ-5-DF, with one and two fluorine substituents, have been developed to investigate the effect of fluorine substituted on small-molecule bulk-heterojunction (BHJ) organic solar cells (OSCs). In comparison with non-fluorine-substituted ASQ-5, both fluorine-substituted ASQ-5-F and ASQ-5-DF possess analogous absorption band gaps but 0.05 and 0.10 eV lowered highest occupied molecular orbital (HOMO) energy levels, respectively. Single-crystal analysis exhibits that ASQ-5-DF shows more desirable intermolecular packing patterns for the hole-carrier collection than ASQ-5 does; hence, higher hole mobility could be acquired. Therefore, solution-processed small-molecule BHJ OSCs fabricated with ASQ-5-F/PC71BM and ASQ-5-DF/PC71BM blends exhibit extremely higher power conversion efficiency (PCE; 5.0% and 6.0%, respectively) than that of ASQ-5/PC71BM (4.5%). The much improved PCE could be attributed to the simultaneously enhanced Voc, Jsc, and FF relative to those of the ASQ-5-based device. To our knowledge, this is the highest PCE (6.0%) among squaraine-based solution-processed BHJ OSCs and the highest PCE in OSCs based on the fluorinated donor segment of small molecules.

An Azulene‐Containing Low Bandgap Small Molecule for Organic Photovoltaics with High Open‐Circuit Voltage

A simple azulene-containing squaraine dye (AzUSQ) showing bandgap of 1.38 eV and hole mobility up to 1.25×10(-4)  cm(2)  V(-1)  s(-1) was synthesized. With its low bandgap, an organic photovoltaic (OPV) device based on it has been made that exhibits an impressive open-circuit voltages (Voc ) of 0.80 V. Hence, azulene might be a promising structural unit to construct OPV materials with simultaneous low bandgap, high hole mobility and high Voc .

Marked effects of indolyl vs. indolinyl substituent on solid-state structure, carrier mobility and photovoltaic efficiency of asymmetrical squaraine dyes

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

An effective π-extended squaraine for solution-processed organic solar cells with high efficiency

Among the multifarious kinds of small molecular photovoltaic materials, squaraines are quite attractive due to their facile and low-cost synthesis, intense and broad absorption in Vis-NIR spectral regions with very high molar extinction coefficients. Nevertheless, one key factor limiting the performance of squaraine-based organic solar cells (OSCs) is their low fill factor (FF), which is ascribed to the low hole mobility of the active layer. Here, we successfully reported a large π-extended bis(squaraine) electron donor, namely D-BDT-SQ, that showed one order of magnitude enhanced hole mobility in the donor–acceptor composite film, up to 10−4 cm2 V−1 s−1. As a result, an impressive power conversion efficiency (PCE) of 7.41% with a high FF of 0.60 was achieved for solution-processed bulk heterojunction OSCs when the working temperature of devices was 80 °C, which is, to the best of our knowledge, the record PCE among those reported for squaraine-based single-junction OSCs.

N,N-Diarylamino end-capping as a new strategy for simultaneously enhancing open-circuit voltage, short-circuit current density and fill factor in small molecule organic solar cells

A series of new asymmetrical squaraine derivatives bearing N,N-diarylamino substituents as end-capping groups, namely ASQAr-1–6, were designed and synthesized. In comparison with the reference compound ASQB bearing a N,N-diisobutylamino end-capper, all the six target compounds exhibit improved thermal stability, red-shifted and broadened absorption bands as well as lower HOMO and LUMO energy levels. Despite the hole mobility of most of the compounds still being lower than that of ASQB, solution-processed bulk-heterojunction small molecule organic solar cells (BHJ-SMOSCs) using ASQAr-1–6 as electron donor materials all show drastically higher power conversion efficiency (PCE, 3.08–3.69%) than that of the ASQB-based reference device (PCE = 1.54%). The much enhanced photovoltaic performance of BHJ-SMOSCs based-on ASQAr-1–6 could be attributed to the simultaneously enhanced open-circuit voltage (Voc, 0.81–0.87 V vs. 0.75 V), short-circuit current density (Jsc, 8.07–9.06 mA cm−2 vs. 5.40 mA cm−2), and fill factor (FF, 0.45–0.47 vs. 0.38) relative to those of the reference ASQB-based device.

Unsymmetrical squaraines with new linkage manner for high-performance solution-processed small-molecule organic photovoltaic cells

Squaraines are promising donor materials because of their strong and broad absorption band in the visible and near infrared regions which is suitable for application in organic photovoltaic (OPV) cells. Two unsymmetrical squaraines (USQs), namely BIBISQ and TIBISQ, with two electron-donating aryls directly linked to the electron-withdrawing squaric acid core (Y-manner) can act as high performance donor materials for solution-processed bulk-heterojunction (BHJ) OPV cells. Both USQs show ideal low bandgaps (1.47 eV for BIBISQ and 1.39 eV for TIBISQ) with an intense and broad absorption band in the range of 500–900 nm, and relatively low HOMO levels of ∼−5.10 eV. The BHJ-OPV device based on both of them simultaneously showed excellent short current density (Jsc) (over 13 mA cm−2), open circuit voltage (Voc) (0.84 V), fill factor (FF) (0.49) and power conversion efficiency (PCE) of over 5% under a blend ratio of USQs:PC71BM = 1:3. These results indicate that the two USQs are quite promising candidates for small molecular (SM) OPV cells and the Y-manner should be a quite successful linking method for USQs.

Central dicyanomethylene-substituted unsymmetrical squaraines and their application in organic solar cells

By dicyanomethylenation on the central squaric acceptor core, three unsymmetrical squaraines (USQ), namely diCN-USQ-1–3, were synthesized, and in-depth comparative studies between them and their reference compounds (USQ-1–3) were carried out. The results indicate that the presence of dicyanomethylene groups on the central squaric ring will lead to distinctly altered properties on the entire π-conjugated system of the fluorophore, such as lowering of the optical bandgap, deepening of the HOMO energy level, increasing of the dipole moment and changing on the dipole moment direction. Consequently, solution-processed bulk-heterojunction small molecule organic solar cells (BHJ-SMOSCs) using these diCN-USQs as electron donor materials could simultaneously show much enhanced open-circuit voltage and significantly increased short-circuit current density, and hence drastically higher power conversion efficiencies (PCEs) than their corresponding USQ-based reference devices (PCE: 1.46%, 1.91%, and 4.58% for diCN-USQ-1–3, and 0.45%, 0.77%, and 3.05% for USQ-1–3 in sequence). Note that the diCN-USQ-3-based device shows a PCE of 4.58%, which is the highest value among all the reported organic solar cells based on centrally substituted squaraines.

Synthesis of 3H-Benzo[e]indoline and Its Application to Small-Molecule Organic Solar Cells

A squaraine-based small molecule (USQ-BI) bearing 3H-benzo[e]indoline was synthesized as an electron donor, and the corresponding organic solar cells show power conversion efficiency of 5.35 % with an excellent short circuit current of over 15 mA cm-2 . The hole mobility of USQ-BI was about 5 times (9.57×10-5 vs. 2.00×10-5  cm2  V-1  s-1 ) higher than that of indoline-based squaraine.

Self-assembled nanopillar arrays by simple spin coating from blending systems comprising PC61BM and conjugated polymers with special structure

Three conjugated D–A copolymers were found to form well-defined nanopillar arrays through facile spin-casting process when blended with fullerene derivatives. Research results indicate that the presence of large coplanar segments and intramolecular S⋯O attractive interactions in the polymers are both crucial factors for achieving self-assembled nanopatterned pillar arrays.

A minimal non-radiative recombination loss for efficient non-fullerene all-small-molecule organic solar cells with a low energy loss of 0.54 eV and high open-circuit voltage of 1.15 V

Organic solar cells (OSCs) are considered as a promising next-generation photovoltaic technology because of their light weight, flexibility, and the potential of roll-to-roll fabrication. However, ...