Jiaju Xu

Ultrasound-Induced Organogel Formation Followed by Thin Film Fabrication via Simple Doctor Blading Technique for Field-Effect Transistor Applications

We demonstrate doctor blading technique to fabricate high performance transistors made up of printed small molecular materials. In this regard, we synthesize a new soluble phthalocyanine, tetra-n-butyl peripheral substituted copper(II) phthalocaynine (CuBuPc), that can easily undergo gel formation upon ultrasonic irradiation, leading to the formation of three-dimensional (3D) network composed of one-dimensional (1D) nanofibers structure. Finally, taking the advantage of thixotropic nature of the CuBuPc organogel, we use the doctor blade processing technique that limits the material wastage for the fabrication of transistor devices. Due to the ultrasound induced stronger π-π interaction, the transistor fabricated by doctor blading based on CuBuPc organogel exhibits significant increase in charge carrier mobility in comparison with other solution process techniques, thus paving a way for a simple and economically viable preparation of electronic circuits.

Design of three-component randomly incorporated copolymers as non-fullerene acceptors for all-polymer solar cells

A series of randomly arranged donor–acceptor-type copolymers, used as replacements for fullerene-based acceptors in organic solar cells, were synthesized by the Stille coupling copolymerization. The copolymers, PNDI–TT–TVTs, were composed of naphthalene tetracarboxylic diimide (NDI) acceptor units and randomly distributed thieno[3,2-b]thiophene (TT) and thienylene-vinylene-thienylene (TVT) donor units. The effects of the TT-to-TVT ratios on the absorption, energy levels, charge transport, morphology, and performance of the resulting fabricated solar cell devices were investigated. The polymer solar cells fabricated from blends of poly[[4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) and PNDI–TT–TVTs (containing 25% of TT and 75% of TVT) displayed a power conversion efficiency of 5.27%, which was significantly higher than those achieved by devices based on alternating arranged copolymers. The current results indicate that random copolymerization is a promising approach to develop novel polymer acceptors for application in organic photovoltaics.

A random copolymer approach to develop nonfullerene acceptors for all-polymer solar cells

We report a donor–acceptor type of random copolymer PNDI-TT-TVT as a replacement for fullerene-based acceptors in organic solar cells (OSCs). PNDI-TT-TVT is composed of naphthalene tetracarboxylic diimide (NDI) acceptor units, randomly distributed thieno[3,2-b]thiophene (TT) and thienylene-vinylene-thienylene (TVT) donor units. OSCs fabricated from a blend of PTB7-Th and PNDI-TT-TVT exhibit a power conversion efficiency of 4.86%, which is significantly higher than the PCEs of the devices based on alternating copolymers of PNDI-TT and PNDI-TVT. The results indicate that random copolymerization is a promising approach to develop novel polymer acceptors for organic photovoltaics.

Tetra-methyl substituted copper (II) phthalocyanine as a hole injection enhancer in organic light-emitting diodes

We have enhanced hole injection and lifetime in organic light-emitting diodes (OLEDs) by incorporating the isomeric metal phthalocyanine, CuMePc, as a hole injection enhancer. The OLED devices containing CuMePc as a hole injection layer (HIL) exhibited higher luminous efficiency and operational lifetime than those using a CuPc layer and without a HIL. The effect of CuMePc thickness on device performance was investigated. Atomic force microscope (AFM) studies revealed that the thin films were smooth and uniform because the mixture of CuMePc isomers depressed crystallization within the layer. This may have caused the observed enhanced hole injection, indicating that CuMePc is a promising HIL material for highly efficient OLEDs.

Enhanced lifetime of organic light-emitting diodes using soluble tetraalkyl-substituted copper phthalocyanines as anode buffer layers

This study describes two soluble tetraalkyl-substituted copper phthalocyanines (CuPcs) as hole-blocking materials that can be used in the Alq3-based organic light-emitting diodes (OLEDs). The hole-blocking characteristics of these Pc layers significantly impeded hole injection into the Alq3 emitting layer and thus decreased the production of unstable cationic Alq3 species, giving an enhanced OLED efficiency and stability compared with that of devices containing the widely used PEDOT:PSS. Moreover, the insolubility of the CuPcs in water together with their extremely high thermal and chemical stabilities led to the effective protection of the ITO anode, resulting in the increased operational stability of the OLEDs in air and the enhancement of the OLED durability.

Dopant-free hole transport materials based on alkyl-substituted indacenodithiophene for planar perovskite solar cells

Two dopant-free hole transporting materials (HTMs) comprising a planar indacenodithiophene (IDT) core with different alkyl chains (either C4 or C6) and electron-rich methoxytriphenylamine (TPA) side arms were synthesized (namely IDTC4-TPA and IDTC6-TPA, respectively) and successfully employed in CH3NH3PbI3 perovskite solar cells. These HTMs can be obtained from relatively cheap starting materials by adopting a facile preparation procedure that does not use expensive and complicated purification techniques. In the crystal lattice, each of these molecules interacted with either the CH/π or CH/O hydrogen bonds. At the same time, the IDTC6 backbone enables a tight molecular arrangement based on π–π stacking interactions (3.399 A); this causes the new material to have a higher hole mobility than the standard IDTC4-based HTM. Moreover, the photoluminescence quenching in a perovskite/HTM film structure was shown to be more effective at the perovskite/IDTC6-TPA interface than at the perovskite/IDTC4-TPA interface. Consequently, devices fabricated using IDTC6-TPA show superior photovoltaic properties (exhibiting an optimal performance of 15.43%) compared with IDTC4-TPA-containing devices.

Facile synthesis of a dopant-free hole transporting material with a phenothiazine core for planar perovskite solar cells

A novel electron-rich small-molecule, 4,4′-(10-(4-octylphenyl)-10H-phenothiazine-3,7-diyl)bis(N,N-(4-methoxyphenyl)anilene) (PTZ-TPA), containing phenothiazine as the core with triphenylamine side groups, was synthesized via a Suzuki–Miyaura cross-coupling reaction. When PTZ-TPA was incorporated into a CH3NH3PbI3 perovskite solar cell as a dopant-free hole transporting material (HTM), a short circuit photocurrent density of 21.5 mA cm−2, an open circuit voltage of 0.982 V, and a fill factor of 0.679 were obtained, giving rise to an overall power conversion efficiency of 14.3%, which is comparable to the power conversion efficiency obtained using the current state-of-the-art HTM 2,20,7,70-tetrakis(N,N′-di-p-methoxyphenylamine)-9,90-spirobifluorene with dopant (Spiro-MeOTAD, power conversion efficiency of 17.1%). PTZ-TPA is thus a promising HTM with the potential to replace the expensive Spiro-MeOTAD owing to its comparable performance and much simpler synthesis route; it also presented a better stability during a one week aging test compared with Spiro-MeOTAD.

All-polymer solar cells performance enhanced via side-chain engineering of the polymer acceptor

A series of novel donor–acceptor copolymers (PNDI-TVT-xOD) with different ratios of monomers was synthesized and used in all-polymer solar cells (all-PSCs). The copolymers were synthesized via random copolymerization between two dibromo-naphthalene tetracarboxylic diimides with different branched alkyl side chains and (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)-ethene. The effects of the changing monomer ratio on the crystallinity, charge transport, thin film, and photovoltaic properties of the copolymers were investigated. All-PSCs based on a blend of the donor polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] and random copolymer acceptors exhibited a maximum power conversion efficiency of 4.55%, which was markedly higher than that of devices containing an alternating copolymer acceptor. This study provides insight into the molecular design of conjugated polymers and the effects of their alkyl side chains on the performance of all-PSCs, leading to the fabrication of highly efficient all-PSCs.

Diketopyrrolopyrrole-based acceptors with multi-arms for organic solar cells

Three small molecules SBF-1DPPDCV, SBF-2DPPDCV and SBF-4DPPDCV consisting of a spirobifluorene (SBF) unit as the core and one, two, and four diketopyrrolopyrrole dicyanovinyl (DPPDCV) units as the arms have been designed and synthesized for solution-processed bulk-heterojunction (BHJ) solar cells. The UV-Vis absorption and cyclic voltammetry measurement of these compounds showed that all these compounds have an intense absorption band over 300–750 nm with a LUMO energy level at around −3.87 eV. When pairing with PTB7-Th as the donor, devices fabricated based on PTB7-Th : SBF-4DPPDCV blends showed a decent PCE of 3.85%, which is the highest power conversion efficiency (PCE) amongst the three DPP acceptor fabricated devices without extra treatment. Devices with SBF-1DPPDCV and SBF-2DPPDCV acceptors showed lower PCEs of 0.26% for SBF-1DPPDCV and 0.98% for SBF-2DPPDCV respectively. The three dimensional (3D) structure of SBF-4DPPDCV facilitates the formation of a 3D charge-transport network and thus enables a rational electron-transport ability (1.04 × 10−4 cm2 V−1 s−1), which further leads to a higher Jsc (10.71 mA cm−2). These findings suggest that multi-arm acceptors present better performance than one-arm or two-arm molecules for organic solar cells.

Green solvent processed tetramethyl-substituted aluminum phthalocyanine thin films as anode buffer layers in organic light-emitting diodes

This study presents the use of a green solvent processed tetramethyl-substituted aluminum phthalocyanine (AlMePc) thin film as an anode buffer layer in an organic light-emitting diode (OLED). The aim of the study was to probe the feasibility of green processing conditions for OLED fabrication. The hole-blocking properties of the AlMePc layer were determined to achieve devices with more balanced charge injection and transport processes. We achieved a significantly enhanced luminance of 11 790 cd m−2, which is twice that of a comparable diode without this anode buffer layer. Additionally, a higher luminous efficiency and power efficiency were obtained for the AlMePc-based OLED than those for a PEDOT:PSS-based diode. Furthermore, the improved charge balance in the OLED achieved by using the AlMePc layer along with its non-aqueous, organic processing methods yielded a maximum half-lifetime of 1860 min at a driving current density of 100 mA cm−2.