Visible Light Photocatalysis of Carbon–Carbon σ‑Bond Anaerobic Oxidation of Ketones with Water by Cobalt(II) Porphyrins

Co^II(por) (por = porphyrinato dianion) reacted selectively with isopropyl ketones at the carbon (CO)–carbon (α) bond at room temperature to give high yields of Co^III(por) acyls and the corresponding oxidized carbonyl compounds in up to 89% yields. Co^III(por)­OH is proposed to be the C–C bond activation (CCA) intermediate. The stoichiometric reaction is further developed into the photocatalytic CCA using both UV and visible light sources (λ 405 nm). Under ambient conditions, the photocatalytic C–C oxidation of 2,6-dimethylcyclohexanone gives 2-heptanone in up to 24 turnovers in the presence of isopropyl alcohol as the H atom donor and H₂O as the oxidant. Various isopropyl ketones successfully undergo photocatalysis

Molecular “Flower” as the High-Mobility Hole-Transport Material for Perovskite Solar Cells

To develop novel hole-transport materials (HTMs) with less synthetic steps is still a great challenge. Here, a small molecule hexakis­[4-(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamino)­phenyl]­benzene (<b>F-1</b>) was successfully synthesized by a relatively simple scenario. <b>F-1</b> exhibits a deep highest occupied molecular orbital energy level of −5.31 eV. Notably, <b>F-1</b> also features 2 times higher hole mobility of 4.98 × 10^–4 cm² V^–1 s^–1 than that of the mostly used 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-bis­(4-methoxyphenyl)­amino)-9,9′-spirobifluorene (spiro-OMeTAD). Consequently, <b>F-1</b>-based perovskite solar cells (PSCs) show markedly improved performance compared with spiro-OMeTAD-based ones. These results indicate such a material can be a promising HTM candidate to boost the overall performance of the PSC

4‑Alkyl-3,5-difluorophenyl-Substituted Benzodithiophene-Based Wide Band Gap Polymers for High-Efficiency Polymer Solar Cells

Two novel polymers <b>PTFBDT-BZS</b> and <b>PTFBDT-BZO</b> with 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as the donor unit, benzothiadiazole or benzooxadiazole as the acceptor unit, and thiophene as the spacer have been synthesized and used as donor materials for polymer solar cells (PSCs). These two polymers exhibited wide optical band gaps of about 1.8 eV. PSCs with the blend of <b>PTFBDT-BZS</b>:PC₇₁BM (1:2, by weight) as the active layer fabricated without using any processing additive and any postannealing treatment showed power conversion efficiency (PCE) of 8.24% with an open circuit voltage (<i>V</i>_oc) of 0.89 V, a short circuit current (<i>J</i>_sc) of 12.67 mA/cm², and a fill factor (<i>FF</i>) of 0.73 under AM 1.5G illumination, indicating that <b>PTFBDT-BZS</b> is a very promising donor polymer for PSCs. The blend of <b>PTFBDT-BZO</b>:PC₇₁BM showed a lower PCE of 5.67% with a <i>V</i>_oc of 0.96 V, a <i>J</i>_sc of 9.24 mA/cm², and an FF of 0.64. One reason for the lower PCE is probably due to that <b>PTFBDT-BZO</b> has a smaller LUMO offset with PC₇₁BM, which cannot provide enough driving force for charge separation. And another reason is probably due to that <b>PTFBDT-BZO</b> has a lower hole mobility in comparison with <b>PTFBDT-BZS</b>

Molecular “Flower” as the High-Mobility Hole-Transport Material for Perovskite Solar Cells

To develop novel hole-transport materials (HTMs) with less synthetic steps is still a great challenge. Here, a small molecule hexakis­[4-(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamino)­phenyl]­benzene (<b>F-1</b>) was successfully synthesized by a relatively simple scenario. <b>F-1</b> exhibits a deep highest occupied molecular orbital energy level of −5.31 eV. Notably, <b>F-1</b> also features 2 times higher hole mobility of 4.98 × 10^–4 cm² V^–1 s^–1 than that of the mostly used 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-bis­(4-methoxyphenyl)­amino)-9,9′-spirobifluorene (spiro-OMeTAD). Consequently, <b>F-1</b>-based perovskite solar cells (PSCs) show markedly improved performance compared with spiro-OMeTAD-based ones. These results indicate such a material can be a promising HTM candidate to boost the overall performance of the PSC

Effect of Non-fullerene Acceptors’ Side Chains on the Morphology and Photovoltaic Performance of Organic Solar Cells

Three indacenodithieno­[3,2-<i>b</i>]­thiophene (IT) cored small molecular acceptors (ITIC-SC6, ITIC-SC8, and ITIC-SC2C6) were synthesized, and the influence of side chains on their performances in solar cells was systematically probed. Our investigations have demonstrated the variation of side chains greatly affects the charge dissociation, charge mobility, and morphology of the donor:acceptor blend films. ITIC-SC2C6 with four branched side chains showed improved solubility, which can ensure the polymer donor to form favorable fibrous nanostructure during the drying of the blend film. Consequently, devices based on PBDB-ST:ITIC-SC2C6 demonstrated higher charge mobility, more effective exciton dissociation, and the optimal power conversion efficiency up to 9.16% with an FF of 0.63, a <i>J</i>_sc of 15.81 mA cm^–2, and a <i>V</i>_oc of 0.92 V. These results reveal that the side chain engineering is a valid way of tuning the morphology of blend films and further improving PCE in polymer solar cells

Enhancing the Performance of Polymer Solar Cells by Using Donor Polymers Carrying Discretely Distributed Side Chains

Conjugated polymers with three components, P1-1 and P1-2, were prepared by one-pot Stille polymerization. The two-component polymer P1-0 is only composed of a 5-fluoro-6-alkyloxybenzothiadiazole (AFBT) acceptor unit and a thiophene donor unit, while the three-component polymers P1-1 and P1-2 contain 10% and 20% 5,6-difluorobenzothiadiazole (DFBT), respectively, as the third component. The incorporation of the third component, 5,6-difluorobenzothiadiazole, makes the side chains discretely distributed in the polymer backbones, which can enhance the π–π stacking of polymers in film, markedly increase the hole mobility of active layers, and improve the power-conversion efficiency (PCE) of devices. Influence of the third component on the morphology of active layer was also studied by X-ray diffraction (XRD), resonant soft X-ray scattering (R-SoXS), and transmission electron microscopy (TEM) experiments. P1-1/PC₇₁BM-based PSCs gave a high PCE up to 7.25%, whereas similarly fabricated devices for P1-0/PC₇₁BM only showed a PCE of 3.46%. The PCE of P1-1/PC₇₁BM-based device was further enhanced to 8.79% after the use of 1,8-diiodooctane (DIO) as the solvent additive. Most importantly, after the incorporation of 10% 5,6-difluorobenzothiadiazole unit, P1-1 exhibited a marked tolerance to the blend film thickness. Devices with a thickness of 265 nm still showed a PCE above 8%, indicating that P1-1 is promising for future applications

Enhancing the Efficiency of Polymer Solar Cells by Incorporation of 2,5-Difluorobenzene Units into the Polymer Backbone via Random Copolymerization

A series of conjugated polymers <b>P0</b>, <b>P5</b>, and <b>P7</b> containing 0, 5, and 7 mol % 2,5-difluorobenzene units, respectively, were prepared and utilized as electron donors in polymer solar cells. Incorporation of a small amount of 2,5-difluorobenzene unit into the backbone of donor polymers can significantly increase their planarity and crystallinity as well as decrease their solubility. The improved molecular conformation can markedly affect the morphology of polymer:PC₇₁BM blend films. After incorporation of 5 mol % 2,5-difluorobenzene unit into the backbone of donor polymers, the domain size of blend films became smaller and the hole mobility increased. Increasing the content of 2,5-difluorobenzene to 7 mol % can further decrease the solubility of resulting polymers and resulted in poor solution processability. As a result, <b>P5</b>-based devices achieved a power conversion efficiency (PCE) of 8.5%, whereas <b>P0</b> based devices gave a PCE of 7.8%

Exploiting Noncovalently Conformational Locking as a Design Strategy for High Performance Fused-Ring Electron Acceptor Used in Polymer Solar Cells

We have developed a kind of novel fused-ring small molecular acceptor, whose planar conformation can be locked by intramolecular noncovalent interaction. The formation of planar supramolecular fused-ring structure by conformation locking can effectively broaden its absorption spectrum, enhance the electron mobility, and reduce the nonradiative energy loss. Polymer solar cells (PSCs) based on this acceptor afforded a power conversion efficiency (PCE) of 9.6%. In contrast, PSCs based on similar acceptor, which cannot form a flat conformation, only gave a PCE of 2.3%. Such design strategy, which can make the synthesis of small molecular acceptor much easier, will be promising in developing a new acceptor for high efficiency polymer solar cells

Enhancing the Performance of Organic Solar Cells by Hierarchically Supramolecular Self-Assembly of Fused-Ring Electron Acceptors

Three novel non-fullerene small molecular acceptors <b>ITOIC</b>, <b>ITOIC-F</b>, and <b>ITOIC-2F</b> were designed and synthesized with easy chemistry. The concept of supramolecular chemistry was successfully used in the molecular design, which includes noncovalently conformational locking (via intrasupramolecular interaction) to enhance the planarity of backbone and electrostatic interaction (intersupramolecular interaction) to enhance the π–π stacking of terminal groups. Fluorination can further strengthen the intersupramolecular electrostatic interaction of terminal groups. As expected, the designed acceptors exhibited excellent device performance when blended with polymer donor PBDB-T. In comparison with the parent acceptor molecule DC-IDT2T reported in the literature with a power conversion efficiency (PCE) of 3.93%, <b>ITOIC</b> with a planar structure exhibited a PCE of 8.87% and <b>ITOIC-2F</b> with a planar structure and enhanced electrostatic interaction showed a quite impressive PCE of 12.17%. Our result demonstrates the importance of comprehensive design in the development of high-performance non-fullerene small molecular acceptors

Nonfullerene Acceptors with Enhanced Solubility and Ordered Packing for High-Efficiency Polymer Solar Cells

The performance of polymer solar cells (PSCs) is commonly improved using additives or annealing treatment. However, these processes are accompanied by disadvantages, including poor reproducibility and stability. Herein, a molecular design strategy is proposed to obtain additive- and annealing-free PSCs. <b>IDTOT2F</b> containing two alkoxyl side chains at the central unit of the nonfullerene acceptor <b>IDTT2F</b> was developed. This molecular design results in excellent solubility in solutions, ordered molecular packing in films, slightly elevated energy levels, and a higher film absorption coefficient. Compared with its counterpart <b>IDTT2F</b>, its improved solubility provides an active layer with better morphology, its ordered molecular packing enhances the charge mobility in blend films, and its slightly elevated energy level furnishes a higher open-circuit voltage of devices. As a result, <b>IDTOT2F</b>-based devices display a maximum power conversion efficiency of 12.79%, which is one of the highest values reported for a PSC fabricated without any extra treatment