Fu-Gang Zhao

Fluorinated graphene: facile solution preparation and tailorable properties by fluorine-content tuning

Fluorinated graphene is one of the most important two-dimensional carbon nanomaterials derived from graphene, and possesses specific and outstanding properties. However, it lacks a cost-effective and large-scale preparation method. Here, we describe a novel and facile solution approach using graphene oxide (GO) and liquid diethylaminosulfur trifluoride as starting materials under mild conditions. The chemical composition and the structure of so-prepared fluorinated graphene were characterized in detail by elemental analysis, solid state F-19 NMR, XPS, FT-IR, Raman, SEM, TEM, and AFM. These studies reveal that some oxygen-containing moieties in GO are converted into C-F bonds, while some are eliminated during the reaction. More interestingly, the fluorine-loading amount can be well tuned by simply altering the reaction medium, and has a significant impact on the optical, electronic, and conductive properties of the product. Preliminary experiments on its application as an electrode material for solid-state supercapacitors were finally presented.

Influence of moiety sequence on the performance of small molecular photovoltaic materials

The purpose of this work is to study the impact of moiety sequence in the chemical structure of small molecular photovoltaic materials on their basic properties and photovoltaic performance. For this aim, two isomeric compounds, namely BDT(ThBTTh)2 and BDT(BTTh2)2, have been designed and synthesized by exchanging benzothiadiazole and thiophene positions with a structural variation. As compared with BDT(BTTh2)2, BDT(ThBTTh)2 possesses a lower melting point, a blue-shifted absorption spectrum in solution, and slightly lower-lying highest occupied and lowest unoccupied molecular orbitals. More interestingly, the hole mobility of the BDT(ThBTTh)2 neat film is 0.1 cm2 V−1 s−1, which is three-orders of magnitude larger than that of BDT(BTTh2)2. Furthermore, these two compounds display significantly different photovoltaic performance, 4.53% for BDT(ThBTTh)2versus 1.58% for BDT(BTTh2)2 in terms of their power conversion efficiency.

Dendronized graphenes: remarkable dendrimer size effect on solvent dispersity and bulk electrical conductivity

Graphene as a newly emerged carbon material has attracted considerable attention due to its outstanding properties and a wide range of fascinating applications. However, its real use is limited due to the lack of a method for mass production. The reduction from graphene oxide has been considered as one of the potential ways for mass-scalable preparation. However, it suffers from re-stacking of the final graphene sheets after reduction due to the strong intersheet interactions. To address this, we report here a strategy using three-dimensional and bulky dendritic structure to functionalize graphene sheets. We found that the treatment of the acylchlorinated graphene oxide with dendritic anilines can easily load dendritic wedges to graphene oxide sheets and simultaneously reduce graphene oxide to graphene. The afforded dendronized graphene products possess excellent dispersibility in a variety of solvents. The dispersity shows a great dependence on the size of the dendritic structure, in which the larger dendritic substituents afford a better dispersity. Surprisingly, dendronization with an appropriate size of dendritic structure does not hamper but can even greatly enhance the bulk electric conducting capability.

Long-term thermally stable organic solar cells based on cross-linkable donor–acceptor conjugated polymers

The real-life application of polymer solar cells (PSCs) requires both a high power conversion efficiency (PCE) and a long enough lifetime. In order to avoid microstructure evolution and enhance device thermal stability, various different amounts of terminal vinyl moieties have been integrated into the side chains of poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione), a previously reported high performance donor–acceptor photovoltaic polymer, to produce a series of crosslinkable polymers named PBDTTPD-Vx (where x is defined as the molar content of vinyl units). It has been found that the larger the vinyl content the polymer contains, the larger the amount of polymer remaining on the substrate after thermal crosslinking and solvent washing. However, the optimized PSC device based on such a polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) displayed a decreased efficiency. These studies have discovered that a vinyl content as small as 2.5% is enough for this family of crosslinkable polymers to achieve effective crosslinking, while at the same time maintaining their high photovoltaic performance. The optimized PBDTTPD-V0.025/PC71BM device showed a power conversion efficiency (PCE) of 6.06% after thermal crosslinking, which represents the highest recorded efficiency among PSC devices with crosslinked active layers. Furthermore, this crosslinked device successfully retained 91% of its initial PCE after thermal treatment at 150 °C for 40 h, which was much better than the non-crosslinkable PBDTTPD-V0/PC71BM cell.

High-performance flexible transparent conductive films achieved by cooperation between 1D copper nanowires and 2D graphene materials

Flexible transparent conductive films (TCFs) fabricated from indium tin oxide (ITO)-alternative materials are highly desirable for a variety of present and future (opto-)electronics. In this contribution, we report that the hybridization of a kind of two-dimensionally electro-conductive material and a kind of one-dimensionally electro-conductive material, i.e. reduced graphene oxide (rGO) and copper nanowires (CuNWs), is a good choice to meet such desire. Different combination ratios between these two kinds of materials by either adding CuNWs into rGO bulk or vice versa were tested. It was found that a significant synergistic effect in improving TCF performance takes place between two-dimensional (2D) rGO nanosheets and one-dimensional (1D) copper nanowires. That is, 1D metallic CuNWs are superior to 2D rGO nanosheets as a conducting additive to improve the performance of TCFs mainly based on the rGO material, while 2D rGO nanosheets rather than 1D CuNWs are very good additives for CuNW-based TCFs to decrease sheet resistance with a small sacrifice in film transparency. Moreover, the hybridization of CuNWs with rGO can not only significantly reduce datum fluctuation in sheet resistance, but also improve the anti-oxidation and anti-foldability properties of TCFs mainly based on CuNWs. Finally, flexible TCFs with a transmittance at 550 nm larger than 80% and a sheet resistance down to 50 Ω sq−1 have been achieved on a polyethylene terephthalate (PET) substrate.

Photovoltaic poly(rod-coil) polymers based on benzodithiophene-centred A–D–A type conjugated segments and dicarboxylate-linked alkyl non-conjugated segments

Different from the well-studied photovoltaic conjugated polymers and small molecular compounds, poly(rod-coil) polymers are emerging as a new class of photovoltaic materials. Since they are composed of definite conjugated and non-conjugated segments in an alternative fashion, this kind of material is expected to merge the merits from both small molecular compounds and conjugated polymers. Based on benzodithiophene-centered acceptor–donor–acceptor (A–D–A) conjugated segments and dicarboxylate-linked alkyl non-conjugated segments, this study has newly designed and synthesized two poly(rod-coil) polymers. Together with three previously reported analogues, these polymers have been systematically investigated for their photovoltaic performances, with special attention paid to the effect of the dicarboxylate linking unit in non-conjugated segments and the alkyl side chains on rigid conjugated segments. It was found that the former factor has a small influence, while the latter has a significant impact on most film-related properties of the material, including film absorption spectrum, frontier orbital energy levels, bandgap, microstructure and morphology of pristine and photovoltaic blend films, as well as hole mobility. After optimization, bulk heterojunction organic solar cells based on this series of polymers reported power conversion efficiencies in range of 0.4-1.09%.

Improving supercapacitor performance of alkylated graphene nanosheets via partial fluorination on their alkyl chains

Redox-active graphene materials are highly desirable for the production of high performance supercapacitors. Following our previous work that found that alkylated graphene nanosheets are a new kind of such material, we report here that the simple replacement of alkyl side chains with partially fluorinated alkyl chains further improves their capacitive electrode performance. In this work, one partially fluorinated graphene material (pFAG) was prepared by the reaction of KOH-pretreated graphene oxide with 2-perfluorohexylethyl bromide in the presence of a phase transfer catalyst. Compared with the graphene material modified with octyl side chains (AG), pFAG possesses a larger amount of residual oxygen functionalities, which is favorable to endow the material with a redox-active nature and achieve a larger faradaic capacitance. Moreover, pFAG presented a special self-assembly behavior and formed continuous and large plate-like objects in the solid state. Finally, a supercapacitor electrode was fabricated with pFAG and its performance was compared with the previously reported AG-based electrode in detail. It was found that the pFAG electrode has a much better capacitive performance than that based on AG (218.3 vs. 160.0 F g−1 at a scan rate of 100 mV s−1 by cyclic voltammetry, and 187.0 vs. 118.8 F g−1 at a current density of 3 A g−1 by galvanostatic charge/discharge method). When charge/discharge was carried out at 1 A g−1, the specific capacitance of pFAG-based electrode reached 388.0 F g−1, among the highest values of reported graphene-based electrodes. Furthermore, pFAG electrodes exhibited a good cycling stability. All these demonstrate graphene nanosheets modified with partially fluorinated alkyl chains would be a good way to achieve high performance redox-active electrode materials.

Sulfanilic Acid Pending on a Graphene Scaffold: Novel, Efficient Synthesis and Much Enhanced Polymer Solar Cell Efficiency and Stability Using It as a Hole Extraction Layer

In this contribution, we describe a novel, facile, and scalable methodology for high degree functionalization toward graphene by the reaction between bulk graphite fluoride and in situ generated amine anion. Using this, the rationally designed sulfanilic acid pending on a graphene scaffold (G-SO3H), a two-dimensional (2D) π-conjugated counterpart of poly(styrenesulfonate), is available. Combined reliable characterizations demonstrate that a very large quantity of sulfanilic blocks are linked to graphene through the foreseen substitution of carbon-fluorine units and an unexpected reductive defluorination simultaneously proceeds during the one-step reaction, endowing the resultant G-SO3H with splendid dispersity in various solvents and film-forming property via the former, and with recovered 2D π-conjugation via the latter. Besides, the work function of G-SO3H lies at -4.8 eV, well matched with the P3HT donor. Awarded with these fantastic merits, G-SO3H behaves capable in hole collection and transport, indicated by the enhanced device efficiency and stability of polymer solar cells (PSCs) based on intensively studied P3HT:PCBM blends as an active layer. In particular, comparison with conventional poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) and recently rising and shining graphene oxide, G-SO3H outperforms above 17 and 24%, respectively, in efficiency. More impressively, when these three unencapsulated devices are placed in a N2-filled glovebox at around 25 °C for 7 weeks, or subject to thermal treatment at 150 °C for 6 h also in N2 atmosphere, or even rudely exposed to indoor air, G-SO3H-based PSCs exhibit the best stability. These findings enable G-SO3H to be a strongly competitive alternative of the existing hole extraction materials for PSC real-life applications.