Air-stable solution-processable unipolar n-type organic semiconductors are highly desired in the field of organic thin film transistors as n-type transistors are indispensable components in low-power-consumption complementary integrated circuits. The present work seeks to address this issue and describes the synthesis of two dicyano-substituted bis(2-oxoindolin-3-ylidene)benzodifurandione (BOIBDD) derivatives with engineered alkyl side chains for n-type transistors. Two kinds of side chains are used in this study: 2-hexyldecyl (C16H33) and 3-hexylundecyl (C17H35). Surprisingly, the small structural difference between side chains induced remarkable differences in the physical properties and electronic performance of the BOIBDD derivatives. In addition, cyano substitution into the backbone of BOIBDD derivatives was shown to be effective at lowering the HOMO and LUMO energy levels of BOIBDD derivatives to obtain unipolar n-type materials. Compared with fluorine substitution, cyano substitution was readily realized by a simple and straightforward reaction using cheap CuCN as a cyanation reagent. Both atomic force microscopy (AFM) and X-ray diffraction (XRD) studies have confirmed that the BOIBDD derivative with 3-hexylundecyl side chains tended to form highly crystalline structures in the solid state. More interestingly, thermal annealing of this compound at a temperature as low as 75 degrees C, well below its melting point of 305 degrees C, can still further improve its crystallinity and thus significantly increase its electron mobility by 43 times to 0.1 cm(2) V-1 s(-1) in air. The insight gained from this study will shed some light on the design of new air-stable high-performance n-type organic semiconductors.

The synthesis of 4,4'-difluoro-2,2'-bithiophene is reported and an alternating donor-acceptor copolymer of this moiety and diketopyrrolopyrrole has been prepared. This polymer has a lower highest occupied molecular orbital than its non-fluorinated analogue polymer. Organic thin film transistors based on this polymer showed p-type charge transport behavior and a hole mobility of 0.21 cm(2) V-1 s(-1) in bottom-gate bottom-contact devices. Organic solar cells using this polymer as donor and [6,6]-Phenyl-C-71-butyric acid methyl ester as acceptor achieved a power conversion efficiency of 3.4% with a high fill factor of 69%. Our morphology analysis showed that there was a lack of long-range ordered structure in the neat polymer thin film, which could cause the inferior device performance.

Large area processing of organic photovoltaic cells and modules is a key step towards commercialization. At the same time, the scale-up of fabrication entails careful evaluation and selection of solvents, and optimization of processing parameters. It has been proven that using proper processing additives in polymer:PCBM bulk-heterojunction solution is one of the most effective strategies to improve the performance of organic photovoltaic cells. Here we report a new transition solvent strategy of using non-halogenated solvents to fabricate PDTSTPD: PC71BM based active layers. This strategy uses a combination of miscible good main solvent such as 1,2,4-trimethylbenzene and coadditives such as diphenyl ether (DPE) and 1-methylnaphtalene (MeN) for middle molecular weight PDTSTPD, or 1,8-diiodooctane (DIO) and MeN for high molecular weight PDTSTPD to tune the nano-scale morphology of the active layer during the film drying process. Inverted organic photovoltaic cells with an active area of 1cm(2) on PET substrate fabricated by a blade coating process using the formulated ink have demonstrated a power conversion efficiency up to 5 %.

The mass production of organic photovoltaics requires high throughput processes capable of producing a functional active layer which is homogenous, pinhole free and of a specified thickness. Common methods to achieve this are slot-die coating and gravure printing. We have demonstrated the fabrication of poly(N-9'heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole (PCDTBT) based photovoltaic cells using the flexographic printing technique, which enables high throughput patterned active layers to be deposited on flexible substrates at lower cost. This was achieved by optimizing the flexographic plate pattern, print speed, the solvent and the drying process. By incorporating halftone patterning, a common process in the graphics printing industry, and optimizing the printing speed, the homogeneity of the active layer print was significantly improved. Further studies of suitable solvents and drying conditions led to reduced pinhole formation and improved uniformity. The functionality of the flexographically printed active layer was demonstrated by fabrication of 1 cm(2) photovoltaic cells which showed an efficiency of up to 3.5%, which is comparable to alternative deposition techniques. These results demonstrate the suitability of flexography as a fabrication technique for bulk heterojunction organic photovoltaics.

The development of printable and air-stable unipolar n-type semiconductors remains a critical issue in the field of organic electronics in view of n-type transistors being indispensable components in the manufacture of low-power-consumption complementary (CMOS type) integrated circuits. In this work, we report on two dicyanomethylene-substituted diketopyrrolopyrrole derivatives with different alkyl side chains (DCM-DPP-C-13 and DCM-DPP-C-16) for n-type transistors. Compared with DCM-DPP-C-16, the side chain branching point in DCM-DPP-C-13 was moved further away from the conjugated backbone. Both materials showed promising electron mobility above 0.1 cm(2)/VS in air with remarkably low threshold voltages (-3 V -2 V) on spin-coated bottom-contact bottom-gated devices on silicon substrates. Contrary to our original expectation, the average electron mobility of DCM-DPP-C-16 (0.2 cm(2)/VS) was higher than that of DCM-DPP-C-13 (0.12 cm(2)/VS) due to the bigger crystalline domains in the thin film of the former compound. We investigated the inkjet printability of these two compounds as well, and bottom-contact top-gated transistors were successfully fabricated on flexible PET substrates. Widely used Cytop was employed as the gate dielectric. Ag gate electrodes were readily inkjet printed on the gate dielectric by using a thin layer of Nafion as an adhesion promoter. DCM-DPP-C-16 demonstrated an excellent inkjet-ability with good uniformity and reproducibility. An average electron mobility around 0.1 cm(2)/VS was achieved in air. This is an important step toward the fabrication of large-area organic CMOS logic circuits with low cost.

We report on inkjet printable gate-dielectric based on a spin-on-glass (SOG) material for applications in n-type organic field-effect transistors (OFETs). The SOG material is polymethylsilsesquioxane in alcohol mixture. After annealed at 135 degrees C in air, the SOG films are well crosslinked and have a good resistance against alcohol, which allows for the inkjet printing of Ag gate electrodes on top of the SOG dielectric. The crosslinked SOG films are very dense, and can withstand high electric field. This is very beneficial to the operation of transistors. In addition, the SOG films have very low hydroxyl content after annealing. This property is very important for n-type transistors. After ink formulation, this SOG dielectric has an excellent inkjet-ability with good uniformity and reproducibility. By using Polyera's P(NDI2OD-T2) as the semiconductor and SOG as the dielectric, bottom-contact top-gated n-type transistors were successfully fabricated on PET substrates with electron mobility above 0.1 cm(2)/V and high on/off ratio well above 10(5). These remarkable results demonstrate that this newly formulated SOG dielectric is a promising candidate for the future development of flexible electronic devices. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.

In this paper we report on the synthesis and development of vanadium oxide precursor flexographic ink for the printing of hole-transporting layers in organic solar cells. For the synthesis of vanadium oxide inks, a sol-gel methodology was utilized. By modifying the vanadium alkoxide precursor with a right type of coordinating ligands a stable and flexoprintable ink has been successfully developed. Flexo-printing afforded smooth and uniform vanadium oxide sol-gel films on top of PCDTBT:PC70BM films. The conversion of the synthesized sol-gel film into a corresponding vanadium oxide layer was followed by DSC/TGA and XPS analyses. The inks were used for the fabrication of inverted organic solar cells by flexo-printing. Power conversion efficiencies ranging between 3.5 % and 4.5 % were achieved, which are slightly lower than the reference cells using vacuum-deposited MoO3 as the hole-transporting layers.

We report the development and application of high-quality zinc oxide nanoparticles (ZnO NPs) processed in air for stable inverted bulk heterojunction solar cells as an electron extraction layer (EEL). The ZnO NPs (average size ∼11 nm) were dispersed in chloroform and stabilized by propylamine (PA). We demonstrated that the ZnO NP dispersion with 4 vol.% of PA as stabilizer can be used in air directly and remains clear up to one month after preparation. Our inverted solar cells consisted of a blade-coated poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (1: 4 by weight) active layer sandwiched between a ZnO electron extraction layer and a MoO3/Ag anode. All solar cells with ZnO films fabricated in air using PA-stabilized ZnO dispersions prepared within a time window of one month exhibited power conversion efficiencies (PCE) above 4%. In contrast, if the ZnO film was prepared in air using regular un-stabilized ZnO NP dispersion, the PCE would drop to 0.2% due to poor film quality. More interestingly, X-ray photoelectron spectroscopy and nuclear magnetic resonance measurements indicated that the PA ligands were not covalently bonded to ZnO NPs and did not exist in the deposited ZnO films. The spin-cast ZnO thin films (without any thermal treatment) are insoluble in organic solvents and can be directly used as an EEL in solar cells. This feature is beneficial for fabricating organic solar cells on flexible polymer substrates. More importantly, our non-encapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for 50 days.

We investigated the effect of solvents on the morphology, charge transport and device performance of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) based solar cells. To carry out this investigation, chloroform and 1,2-dichlorobenzene were chosen as good solvents of the two compounds. Films prepared with chloroform exhibit larger domains than those prepared with 1,2-dichlorobenzene and their size increases with the amount of PC70BM. Fine tuning of the domain size was realized by using a solvent of mixed chloroform and 1,2-dichlorobenzene. At a mixing ratio of 50%:50%, a power conversion efficiency of 6.1% was achieved on PCDTBT:PC70BM (1:3) devices with an active area of 1cm2, under air mass 1.5 global (AM 1.5G) irradiation at 100mW/cm2.

Interface devices such as integrated planar patch‐clamp chips are being developed to study the electrophysiological activity of neuronal networks grown in vitro. The utility of such devices will be dependent upon the ability to align neurons with interface features on the chip by controlling neuronal placement and by guiding cell connectivity. In this paper, we present a strategy to accomplish this goal. Patterned chemical modification of SiN surfaces with poly‐d‐lysine transferred from PDMS stamps was used to promote adhesion and guidance of cryo‐preserved primary rat cortical neurons. We demonstrate that these neurons can be positioned and grown over microhole features which will ultimately serve as patch‐clamp interfaces on the chip. Biotechnol. Bioeng. 2010; 105: 368–373. © 2009 Wiley Periodicals, Inc.

Low-bandgap regioregular polythiophene derivatives, PTh6BTD, PTh8BTD, and PTh4TTBTD, were synthesized through Stille coupling reaction. These are alternating copolymers of an electron-deficient benzothiadiazole unit and oligothiophene units including hexathiophene, octathiophene, and thieno[3,2-b]thiophene-bridged quarterthiophene, respectively. The polymers are soluble in halogenated solvents such as o-dichlorobenzene, affording good processability in solar cell fabrication. Meanwhile, the synthesized alternating copolymers show much broader absorption than P3HT, covering the spectral region from 350 to 800 nm. DSC analysis showed that all three polymers readily crystallized, indicating highly ordered intermolecular packing, which is beneficial for achieving higher charge carrier mobility. Bulk-heterojunction solar cells using 1:1 w/w PThnBTD:PC61BM ([6,6]-phenyl C61-butyric acid methyl ester) blends as the photovoltaic active layers were fabricated and characterized. Best power conversion efficiency of ...

Extended donor-acceptor oligothiophenes exhibit a narrow optical energy gap in combination with strong light absorption and good intermolecular pi-pi stacking in the solid state for efficient charge transport. Bilayer photovoltaic devices utilizing these oligomers as p-type semiconducting photosensitizers afford high power conversion efficiencies of up to 2.7%.

Novel low optical gap, p-type semiconducting oligothiophenes asymmetrically end-capped with triarylamino and tricyanovinyl groups, PhN-OFOT(n)-TCN (n = 2, 3), have been synthesized and characterized for photovoltaic applications. With an incorporation of a tricyanovinyl accepting group to the triarylamine, the optical energy gap greatly reduces to 1.46 eV and the LUMO level lowers to 3.9 eV. The initial studies of the bilayer heterojunction PV cells based on the newly developed tricyanovinyl-substituted chromophores as a donor material and C60 as an acceptor material showed a PCE up to 1.33% with a large open-circuit voltage of 0.82 V in the annealed devices which makes this class of materials promising for further development. Our findings also suggest for the first time that the tricyanovinyl group is highly efficient to lower the LUMO level and reduce the optical energy gap of a p-type semiconducting photosensitizer. It is interesting to find that PhN-OFOT(2)-TCN showed better device performance in bilayer solar cells than PhN-OFOT(3)-TCN although the latter has a slightly narrower optical gap. Since the LUMO energy level of PhN-OFOT(3)-TCN (ca. 3.9 eV vs vacuum) is too close to that of C60 (4.0 eV), the yield of photoinduced charge carriers was low, leading to a low power conversion efficiency. Our findings highlight the importance of a large LUMO level offset between electron donors and acceptors to the achievement of high performance organic solar cells.

Bulk heterojunction organic solar cells based on conjugated polymers and soluble fullerene derivatives have been intensively studied due to their simple device structure, easy thin-film casting process, low fabrication cost, and high power conversion efficiency. To further enhance the efficiency, it is of paramount importance to develop new p-type polymeric semiconductors with high charge mobility and narrow energy band gaps. In this paper, we report on an innovative way to synthesize low band-gap conjugated polymers with high crystallinity and reasonable solubility for wet process by using electron-acceptor cored regioregular oligothiophenes as building blocks. These novel building blocks can copolymerize with coplanar fused aromatic compounds, such as indolo[3,2-b]carbazole, to form crystalline polymers with enhanced interchain interaction. Two alternating copolymers P(InCzThnBTD) (n = 2-3) synthesized in this work show much narrower optical band gaps than widely used regioregular P3HT. In addition, the first ionization energy of both synthesized polymers remains high at ~5.17 eV, which is important to achieve large open-circuit voltage (Voc) and improve air stability. Remarkably, the bulk heterojunction photovoltaic (PV) cells fabricated from P(InCzTh2BTD) and PC61BM exhibited a promising PV performance with a PCE = 3.6 %, Voc = 0.69 V, Jsc = 9.17 mA/cm2, and FF = 0.57.

A low band-gap alternating copolymer of indolocarbazole and benzothiadiazole-cored oligothiophene demonstrated balanced crystallinity and solubility; a solar cell combining this polymer with PC(61)BM in a preliminary test demonstrated power conversion efficiencies of 3.6%.