Organic thin-film transistors (OTFTs) with Au electrodes were successfully used as transducers for label-free deoxyribonucleic acid (DNA) sensors. Single-strand DNA (ssDNA), perfectly-matched double-strand DNA (dsDNA) and mis-matched DNA were immobilized on the surface of the source/drain electrodes of three OTFT devices respectively. The ssDNA molecules with mercapto group (–SH) can be well immobilized on the surface of Au electrode by chemical bond between –SH and Au atom. According to the significant difference in channel current, which was attributed to the changed contact resistances by introducing different DNA molecules on Au electrode, ssDNA, matched-dsDNA and mismatched-dsDNA were differentiated successfully in the experiments. The results may provide a promising approach for detecting DNA specificity and hybridization with label-free.

•The OTFTs with TiO2/Au and Mn-doped TiO2/Au bilayer electrodes were investigated.•The carrier mobility of device with Mn-doped TiO2 is 1.13 cm2/V s and threshold voltage is −2 V.•The lower contact resistance and smoothed surface of pentacene layer are the keys to device improvement.Device performance of pentacene organic thin-film transistors (OTFTs) was significantly improved via inserting a Mn-doped TiO2 layer between pentacene semiconductor and the source–drain electrodes. In comparison with the OTFTs with only-Au electrodes, the introduction of a thin Mn-doped TiO2 layer leads to saturation current increasing from 31.9 μA to 0.22 mA, effective field-effect mobility improving from 0.24 to 1.13 cm2/V s, and threshold voltage downshifting from −11 to −2 V. These performance enhancements are ascribed to the significant reduction of contact resistance and smoothed surface of pentacene layer. This work may provide an effective approach to improve the performance of the pentacene based OTFTs by inserting a Mn-doped TiO2 layer.

•Highly ordered copper phthalocyanine (CuPc) thin films were prepared.•Dependence of p-6P thickness on the performance of CuPc transistors was studied.•High field-effect mobility of 0.18 cm2/V s was obtained by CuPc transistors.Para-sexiphenyl (p-6P) ultrathin film was utilized as the modified layer to grow copper phthalocyanine (CuPc) thin film. The order of the CuPc thin-film morphology exhibits an obvious dependence on p-6P thickness. A conversion of p-6P growth mode from layer-by-layer to island growth was observed. The highest mobility of CuPc transistor based on p-6P modified layer has been obtained by 0.18 cm2/V s with the p-6P thickness of 2.4 molecule layer. Therefore, the thickness of modified layer plays a crucial role in thin film growth of organic semiconductor that results in a remarkable influence on the electric characteristics of devices.

We report an improved label-free DNA sensor based on pentacene organic thin-film transistors (OTFTs). OTFT with top-contact structure was first fabricated by using pentacene as the active layer. Different electric biases were introduced between the source and gate contacts of OTFT during the single-stranded DNA (ssDNA) immobilization process in order to improve the immobilization efficiency of DNA molecules on the pentacene substrate. Atomic force microscopy (AFM) images show the application of positive bias can significantly improve the amount of the immobilized ssDNA. The potentiostatic effect of bias can induce more ssDNA molecules onto the pentacene surface, which leads to the improvement of sensor sensitivity reflected by the electric signals of OTFTs. Furthermore, an optimized immobilization time of 30 min was obtained at a constant bias that exhibits the highest immobilization efficiency. With this design, the optimized process conditions (+50 V bias and an immobilization of 30 min) for the ssDNA immobilization on the pentacene film were also obtained. As a result, the introduction of bias during immobilizing ssDNA molecules has been proposed to improve the immobilized efficiency, which provides an effective path to enhance the sensitivity of OTFT-based DNA sensors.Graphical abstractHighlights► A label-free DNA sensor based on pentacene transistor was fabricated. ► Different biases were introduced during DNA immobilization process. ► The immobilization of DNA molecules on pentacene was optimized. ► The sensitivity of DNA sensors has been improved.

Organic thin-film transistors (OTFTs) were fabricated based on the double polymeric insulators consisting of polyvinyl alcohol (PVA) and polystyrene (PS). The using of double dielectric layers exhibited an improved device performance comparing with the conventional devices with single dielectric layer. By exposing the polymer insulating layer (PS) under ultraviolet (UV) light for different times (0, 5, 10 and 15 min), the surface properties of the PS film has been further optimized for the growth of pentacene. The grain size and crystallization of pentacene film grown on PS are apparently improved as the increase of surface energy of PS. As a result, a field-effect mobility of 1.38 cm2 V−1 S−1 and an on/off current ratio of 104 had been obtained by UV treatment for 10 min. All results indicate the surface energy of dielectric layer is a crucial factor to influence the devices performance that can be well tuned by UV treatment. The polymeric materials pair consisting of PS and PVA is a promised candidate as the dielectric layers for high-performance OTFTs.Graphical abstractOrganic thin-film transistors (OTFTs) were fabricated based on the double polymeric insulators consisting of polyvinyl alcohol (PVA) and polystyrene (PS). By exposing the polymer insulating layer (PS) under ultraviolet (UV) light for different times (0, 5, 10 and 15 min), the surface properties of the PS film has been further optimized for the growth of pentacene. As a result, a field-effect mobility of 1.38 cm2 V−1 S−1 and an on/off current ratio of 104 had been obtained by UV treatment for 10 min. All results indicate the surface energy of dielectric layer is a crucial factor to influence the devices performance that can be well tuned by UV treatment.Highlights► Organic transistors based on the double insulators consisting of polyvinyl alcohol and polystyrene have been demonstrated. ► The ultraviolet light was used to treat the polymeric insulator for different times. ► A field-effect mobility of 1.38 cm2 V−1 S−1 and an on/off current ratio of 104 had been obtained from the devices with UV treatment for 10 min. ► The surface energy of insulator is a crucial factor for OTFT that can be modulated by UV light.

Organic p–p junction composed of oxotitanium phthalocyanine (TiOPc) and copper phthalocyanine (CuPc) was investigated by analyzing the corresponding field-effect transistors, which exhibited typical ambipolar transport characteristics. Electrons and holes were induced at two sides of junction interface, respectively, which formed a built-in electric potential of 0.47 V directed from CuPc to TiOPc and resulted in a high conduction deduced by diode measurements. These results implied that both electron and hole current could be accumulated and transported at junction interface determined by the difference of Fermi levels of two materials, which is independent on their conductance types.

Organic heterojunction thin-film transistors are fabricated based on copper phthalocyanine (CuPc) and hexadecafluorophtholocyaninatocopper (F16CuPc) as double active layers, which exhibit typical ambipolar conduction. Several substrate temperatures are utilized to tune film morphology, which results in a remarkable change on the electric characteristics of organic transistors. The highest balanced mobility value of 2.91 × 10−2 cm2/V s for hole and 1.04 × 10−2 cm2/V s for electron are obtained by depositing F16CuPc at 150 °C and CuPc at 200 °C, respectively, which are comparable to those conventional single-layer devices. This result demonstrates that the growth conditions of organic heterojunctions play a crucial role in ambipolar devices.

Highly oriented copper phthalocyanine (CuPc) film consisting of many crystal domains was prepared by introducing titanyl-phthalocyanine (TiOPc) as molecular template at high substrate temperature. CuPc molecules stand up on the top of TiOPc domains exhibiting herringbone structure and high-order arrangement investigated by atomic force microscopy and X-ray diffraction. Then organic transistors were fabricated that exhibited improved device performances, a 0.12 cm2/V s of mobility and a 105 of on/off ratio. Furthermore, template thickness dependence on device performance was disclosed. These results indicated that molecular template is a very effective method to prepare high-mobility organic semiconductor film.

We have investigated organic light-emitting diodes (OLEDs) with a structure of field-effect-transistor (FET) using MgAg as source and drain electrodes, respectively. These devices were found to present both a typical switchable behavior of FET and an electro-optical transfer characteristic of OLEDs. Five organic layers were employed in devices that were expected to play each role for OFET and OLED, respectively. In a result, the FET unipolar behavior has been demonstrated experimentally. Furthermore, FET characteristics were discussed based on an equivalent circuit.