Yanfang Geng

Tailoring organic heterojunction interfaces in bilayer polymer photovoltaic devices

In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices.

Interfacial modification of organic photovoltaic devices by molecular self-organization

This feature article focuses on the relationship between the interfacial structures constructed by molecular self-organization and the properties of organic photovoltaic devices. The use of self-assembled monolayers (SAMs) is reviewed for metal and metal oxide/organic interfaces, while surface-segregated monolayers (SSMs) are introduced as a new method for the modification of organic/organic interfaces. Research up to now has clearly demonstrated the effectiveness of the control of energy levels and other properties at the interfaces to enhance photovoltaic performance. The possibility of more precise control of the interfacial structures is also discussed.

Synthesis and properties of D–A copolymers based on dithienopyrrole and benzothiadiazole with various numbers of thienyl units as spacers

Three kinds of donor–acceptor (D–A) type semiconducting copolymers in which electron donating units of dithienopyrrole (DTP) and accepting units of benzothiadiazole (BT) were connected with different numbers of thienyl spacers (x = 0–2) were synthesized and used as electron donor materials in polymer solar cells (PSCs) combined with fullerene derivatives. The optical band gaps of the polymers could be tuned from 1.41 eV to 1.61 eV by changing the number of thiophene spacers. Electrochemical measurements showed that the increase of band gap is mainly due to the change of the lowest unoccupied molecular orbital (LUMO) energy level. The power conversion efficiency (PCE) of the polymer solar cells based on the three polymers and PC70BM reached 3.12% with x = 2 under the illumination of AM 1.5, 100 mW cm−2. This approach could provide a simple strategy for designing high-performance D–A type photovoltaic polymers based on the existing polymers and a large potential to improve their performance further.

Fullerene-free organic photovoltaics based on unconventional material combination: a molecular donor and polymeric acceptors

In conventional organic photovoltaic cells, the active layer consists of a polymeric donor and a molecular acceptor (PD/MA). An unconventional material combination based on molecular donor/polymeric acceptor (MD/PA) emerged in 2014 but attracted limited attention. To broaden photovoltaic material systems and understand the crucial factors related to the photovoltaic performance, in this report, we adopted a molecular donor (p-DTS(FBTTh2)2) and three polymeric acceptors based on perylenediimide (PDI). We find that the high contents (70–80%) of p-DTS(FBTTh2)2 and the better crystallinity and larger grains in the blend films induced by the addition of 1,8-diiodooctane (DIO) play an important role in constructing the continuous and effective donor phase for charge transfer and hole transport in the active layers. The highest PCE of photovoltaic cells reached 3.01% with a VOC of 0.68 V, JSC of 7.59 mA cm−2, and FF of 0.58 for the p-DTS(FBTTh2)2 : PSe-PDI active layer, although the hole and the electron mobilities are still unbalanced. Further optimization of the film morphology and improvement of the electron mobility by material design and device engineering are expected to boost the efficiency of MD/PA type fullerene-free solar cells.

Orthogonal Supramolecular Polymer Formation on Highly Oriented Pyrolytic Graphite (HOPG) Surfaces Characterized by Scanning Probe Microscopy.

Formation of an orthogonal supramolecular polymer on a highly oriented pyrolytic graphite (HOPG) surface was demonstrated for the first time by means of scanning probe microscopy (SPM). Atomic force microscopy (AFM) was employed to characterize the variation of both the thickness and the topography of the film formed from (1) monomer 1, (2) monomer 1/Zn(2+), and (3) monomer 1/Zn(2+)/cross-linker 2, respectively. Scanning tunneling microscopy (STM) was used to monitor the self-assembly behavior of monomer 1 itself, as well as 1/Zn(2+) ions binary system on graphite surface, further testifying for the formation of linear polymer via coordination interaction at the single molecule level. These results, given by the strong surface characterization tool of SPM, confirm the formation of the orthogonal polymer on the surface of graphite, which has great significance in regard to fabricating a complex superstructure on surfaces.

Formation of C═C Bond via Knoevenagel Reaction between Aromatic Aldehyde and Barbituric Acid at Liquid/HOPG and Vapor/HOPG Interfaces

Controlling chemical reactions on surface is of great importance to constructing self-assembled covalent nanostructures. Herein, Knoevenagel reaction between aromatic aldehyde compound 2,5-di(5-aldehyde-2-thienyl)-1,4-dioctyloxybenzene (PT2) and barbituric acid (BA) has been successfully performed for the first time at liquid/HOPG interface and vapor/HOPG interface. The resulting surface nanostructures and the formation of C═C bond are recorded through scanning tunneling microscopy (STM), and confirmed by attenuated total reflectance Fourier-transform infrared (ATR/FT-IR) spectrometer and UV-vis absorption. The obtained results reveal that Knoevenagel condensation reaction can efficiently occur at both interfaces. This surface reaction would be an important step toward further reaction to produce innovative conjugated nanomaterial on the surface.

Effects of a side chain sequence on surface segregation of regioregular poly(3-alkylthiophene) and interfacial modification of bilayer organic photovoltaic devices

A regioregular poly(3-alkylthiophene) with a statistical sequence of alkyl/semifluoroalkyl side chains (stat-P3DDFT) was synthesized through the copolymerization of mixed monomers. The surface segregation behavior was compared with the corresponding alternating copolymer (alt-P3DDFT) that forms highly ordered dipole layers by surface segregation. X-ray and ultraviolet photoelectron spectroscopies of the polymer films revealed that alt-P3DDFT formed a much more ordered surface segregated monolayer than did stat-P3DDFT, indicating the importance of the alternating sequence of the side chains. When stat-P3DDFT was inserted into the donor/acceptor interfaces in bilayer organic photovoltaic devices, the short circuit current and the open circuit voltages of the devices were related to the disordered interface structures of stat-P3DDFT.

Peptide recognition by functional supramolecular nanopores with complementary size and binding sites

The precise control of the conformations of biomolecules adsorbed on a surface at the single-molecule level is significant. However, it remains a huge challenge because of the complex structure and conformation diversity of biomolecules. Herein, a “nanopore-confined recognition” strategy is proposed to manipulate the adsorption of individual valinomycin molecules at room temperature through precise design of functionalized conjugated macrocycle (CPN8) supramolecular nanopores with complementary architectures and binding sites. We revealed that CPN8 prefers to selectively recognizing valinomycin with complementary architecture because of the strong synergistic interactions between the isopropyl groups of valinomycin and the amino groups of CPN8, with valinomycinhighly oriented pyrolytic graphite (HOPG) interactions. Our perspectives at the single-molecule level will provide valuable insights to improve the design of supramolecular nanopores for conformation-selective recognition of non-conjugated molecules.

Pyridine-induced interfacial structural transformation of tetraphenylethylene derivatives investigated by scanning tunneling microscopy

The two-dimensional self-assembly behaviors of tetraphenylethylene (TPE) molecules are significant for further applications, but reports are rare. The self-assembled structures of two C2-symmetry TPE derivatives (H4TCPE and H4ETTC) possessing propeller structures and their stimulus responses to the addition of vinylpyridine derivatives were thoroughly studied with the assistance of scanning tunneling microscopy (STM) technique in combination with density functional theory (DFT) calculations. Although their chemical structures were similar, the H4TCPE and H4ETTC molecules self-assembled into closely packed lamellar and quadrilateral structures, respectively, at the 1-heptanoic acid/HOPG interface. After the addition of pyridine derivatives (DPE, PEBP-C4, and PEBP-C8), H4TCPE and H4ETTC showed different responsiveness resulting in different co-assembly structures. The results indicated that the structures of pyridine derivatives—including backbones and substituents—affected the intermolecular interactions of both H4TCPE/pyridine and H4ETTC/pyridine systems. The modification of the self-assembly behaviors of propeller-shaped H4TCPE and H4ETTC would contribute to the construction of more complex multilevel nanostructures.

Specific distribution of orientated C70-fullerene triggered by solvent-tuned macrocycle adlayer

The precise localization of organic molecules in controllable positions is an important step towards constructing functional nanostructures via the bottom-up strategy. Herein, supramolecularly organized C70-fullerene assemblies on macrocycle-modified surfaces were investigated using scanning tunneling microscopy (STM) in combination with theoretical calculations. The results revealed that an up-assembly of C70-fullerene adlayers was successfully formed on top of the bottom macrocycle arrays. Density functional theory (DFT) calculations confirmed that the macrocycle networks along with the co-adsorbed solvent 1-phenyloctane served as a selective template for trapping C70-fullerene molecules in the spectral sites and acted as a support for the C70-fullerene molecules. The periodical distribution of the C70-fullerene molecules should facilitate understanding of the strong dependence of the arrangement of C70-fullerene upon the specific interactions (apart from spatial recognition) derived from modification of the sub-monolayers.