Monthly Archives: May 2014

Two-dimensional π-conjugated molecules based-on 2,6,9,10-tetra(prop-1-yn-1-yl)anthracene and their application to solution-processed photovoltaic cells

Publication date: July 2014 Source:Organic Electronics, Volume 15, Issue 7 Author(s): Jicheol Shin , Nam Su Kang , Tae Wan Lee , Min Ju Cho , Jae Min Hong , Byeong-Kwon Ju , Dong Hoon Choi Novel two-dimensional π-conjugated molecules, i.e., 5′,5″′-((9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (2,6-HBT) and 5′,5″′-((2,6-bis((4-hexylphenyl)ethynyl)anthracene-9,10-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (9,10-THB) were successfully synthesized and utilized as highly soluble p-type organic semiconductors for organic thin film transistors (TFTs) and solar cells. From the TFTs, the high hole mobility of the pristine film of 9,10-THB was measured to be 0.07cm2 V−1 s−1 (I on/off =106–107), which is mainly attributed to slip-stacked charge-transport behavior in J-aggregation-induced crystallites. Further, a solution-processed solar cell made of 9,10-THB and PC61BM exhibited very promising and reproducible power conversion efficiencies of 3.30% and 2.53% with composition 1:1 and 1:2 w/w ratio, respectively. Graphical abstract Highlights Novel two-dimensional π-conjugated anthracene-based molecules were utilized as highly soluble p-type organic semiconductors for organic thin film transistors (TFTs) and solar cells. From the TFTs, the higher hole mobility of the pristine film containing 9,10-THB was measured to be 0.07cm2 V−1 s−1 (I on/off =106–107). Further, a solution-processed solar cell made of 9,10-THB and PC61BM exhibited very promising and reproducible power conversion efficiencies of 3.30% with composition 1:1 w/w ratio.

Role of ultrathin Al2O3 layer in organic/inorganic hybrid gate dielectrics for flexibility improvement of InGaZnO thin film transistors

Publication date: July 2014 Source:Organic Electronics, Volume 15, Issue 7 Author(s): Byeong-Ung Hwang , Do-Il Kim , Sung-Won Cho , Myeong-Gu Yun , Hak Jun Kim , Youn Jea Kim , Hyung-Koun Cho , Nae-Eung Lee We investigated flexible amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) on a polyimide (PI) substrate by using organic/inorganic hybrid gate dielectrics of poly-4vinyl phenol (PVP) and ultrathin Al2O3. IGZO TFTs were fabricated with hybrid PVP/Al2O3 gate dielectrics having Al2O3 layers of different nanoscale thicknesses, which were deposited by atomic layer deposition (ALD). The electrical characteristics of the TFTs with the organic/inorganic hybrid gate dielectrics were measured after cyclic bending up to 1,00,000 cycles at the bending radius of 10mm. The ultrathin Al2O3 layer in the hybrid gate dielectrics improved the mechanical flexibility and protected the organic gate dielectric against damage during the sputter deposition of the IGZO layer. Finite elements method (FEM) simulations along with the structural characterization of the cyclically bent device showed the importance of optimizing the thickness of the Al2O3 layer in the hybrid gate dielectrics to obtain mechanically stable and flexible a-IGZO TFTs. Graphical abstract Highlights

Compressive stress profiles of chemically strengthened glass after exposure to high voltage electric fields

Publication date: 1 July 2014 Source:Journal of Non-Crystalline Solids, Volumes 394–395 Author(s): Lynn M. Thirion , Elena Streltsova , Wen-Ya Lee , Zhenan Bao , Mingqian He , John C. Mauro The ion exchange process for chemical strengthening of glass involves an inter-diffusion of alkali ions between the glass and a molten salt bath. In most commercial glasses, this comprises an exchange of Na+ ions in the glass for larger K+ ions from the salt bath. The stuffing of K+ ions into sites previously occupied by Na+ in the glass results in the formation of a compressive stress profile in the glass. In the absence of stress relaxation, this compressive stress profile largely follows the concentration profile of the K+ ions. Any subsequent migration of K+ ions within the glass would result in a change in this stress profile, which could lead to different mechanical behaviors of the glass. In particular, a reduction of the surface compressive stress could lead to a compromise in the retained strength of the glass, making it more susceptible to failure. Recent work has shown that chemically strengthened glass can be used as an effective substrate material for organic thin film transistors (TFTs), since the temperatures involved with organic TFT deposition are low enough to avoid any compromise in the compressive stress profile. This opens the possibility of fabricating high strength organic TFT displays. However, the question remains as to whether the exposure of the glass to an electric field may lead to the diffusion of the alkali ions and a corresponding alteration of the stress profile, which could compromise the strength of the organic TFT device. In this paper, we demonstrate that there is no change in the stress profile of Corning® Gorilla® Glass 3 after subjecting the glass to much higher voltage dc fields compared to the maximum field that would be exhibited in an organic TFT device. The stress profile is modified only after treating the glass at sufficiently high temperature, where alkali migration becomes thermally activated.

Fabrication of β-AgGaO2 thin films by radio frequency magnetron sputtering

Publication date: 30 May 2014 Source:Thin Solid Films, Volume 559 Author(s): Issei Suzuki , Hiraku Nagatani , Yuta Arima , Masao Kita , Takahisa Omata Thin films of β-AgGaO2, were fabricated on (0001)–Al2O3 substrates by radio frequency magnetron sputtering. β-AgGaO2 is a ternary oxide semiconductor possessing a wurtzite-derived β-NaFaO2-type structure. The effects of changing experimental parameters including sputtering atmosphere, pressure and substrate temperature were investigated. The effects of the sputtering conditions on the composition, phase, morphology and optical transmission of the films were determined. A highly crystalline β-AgGaO2 film was obtained by deposition at 200°C under a 15% O2 atmosphere at a pressure of 0.5Pa. This film was (001)-oriented, similar to wurtzite-type phases. The energy band gap of β-AgGaO2 was determined to be 2.2eV from its photocurrent spectrum.

Efficiency improvement of Cu2O-based heterojunction solar cells fabricated using thermally oxidized copper sheets

Publication date: 30 May 2014 Source:Thin Solid Films, Volume 559 Author(s): Tadatsugu Minami , Toshihiro Miyata , Yuki Nishi This paper introduces the present status and prospects for further efficiency improvement of Cu2O-based p–n heterojunction solar cells that feature an n-type oxide semiconductor thin film/p-Cu2O sheet structure. This structure was achieved by both stabilizing the surface of polycrystalline p-Cu2O sheets that had been prepared by thermal oxidization of Cu sheets and also developing low-temperature and low-damage deposition technology for applying thin films as an n-oxide semiconductor layer. The highest efficiency of 5.38% and open circuit voltage of 0.8V were obtained in an AZO/n-Ga2O3/p-Cu2O heterojunction solar cell fabricated with a non-doped Ga2O3 thin film prepared on a Cu2O sheet at room temperature with a thickness of 75nm under an O2 gas pressure of 1.7Pa by a pulsed laser deposition. It should be noted that the obtainable photovoltaic properties in AZO/n-oxide semiconductor/p-Cu2O heterojunction solar cells are considerably more affected by the surface condition of the p-Cu2O layer, i.e., the interface at the heterojunction, than the diffusion potential resulting from the difference of work functions between the p-Cu2O and n-semiconductor layers. To achieve a higher efficiency in AZO/n-oxide semiconductor/p-Cu2O heterojunction solar cells, it is necessary to improve the interface at the heterojunction as well as reduce the series resistance and increase the parallel resistance of the heterojunction solar cells.

Mobility- and temperature-dependent device model for amorphous In–Ga–Zn–O thin-film transistors

Publication date: 30 May 2014 Source:Thin Solid Films, Volume 559 Author(s): Katsumi Abe , Ayumu Sato , Kenji Takahashi , Hideya Kumomi , Toshio Kamiya , Hideo Hosono A device model for amorphous In–Ga–Zn–O thin-film transistors (a-IGZO TFTs) that explains temperature dependence is proposed. It incorporates a carrier-density dependent mobility and a density of subgap traps of a-IGZO. The model parameters were extracted from only one transfer curve of an a-IGZO TFT at a low drain voltage through a simple analytical model. Device simulation based on this model reproduced current- and mobility-gate voltage characteristics of the a-IGZO TFT well over a wide range of bias voltage and temperature (253–393K).

Reduction of Positive-Bias-Stress Effects in Bulk-Accumulation Amorphous-InGaZnO TFTs

We report an abnormal negative threshold-voltage shift (ΔVTH) in bulk-accumulation (dual-gate driven) amorphous-InGaZnO (a-IGZO) thin-film transistors (TFTs) after application of positive-bias-stress (PBS). In devices annealed at 250°C for 2 h in vacuum, the negative ΔVTH is accompanied with subthreshold swing degradation, consistent with PBS-induced defect creation. Negative-bias-stress induces negligible ΔVTH, ruling out ion migration in the gate-insulator. By varying the top-gate length, it is found that the negligible ΔVTH is a function of bulk-accumulation. However, after vacuum annealing at 250°C for 100 h, PBS induces negligible ΔVTH, verifying that the negative ΔVTH in short-time annealed devices is related to defects in the bulk a-IGZO. Therefore, good PBS stability can be achieved in bulk-accumulation dual-gate a-IGZO TFTs by long-time vacuum anneal.

Simultaneous extraction of charge density dependent mobility and variable contact resistance from thin film transistors

A model for the extraction of the charge density dependent mobility and variable contact resistance in thin film transistors is proposed by performing a full derivation of the current-voltage characteristics both in the linear and saturation regime of operation. The calculated values are validated against the ones obtained from direct experimental methods. This approach allows unambiguous determination of gate voltage dependent contact and channel resistance from the analysis of a single device. It solves the inconsistencies in the commonly accepted mobility extraction methods and provides additional possibilities for the analysis of the injection and transport processes in semiconducting materials.

Molybdenum as a contact material in zinc tin oxide thin film transistors

Amorphous oxide semiconductors are of increasing interest for a variety of thin film electronics applications. Here, the contact properties of different source/drain electrode materials to solution-processed amorphous zinc tin oxide (ZTO) thin-film transistors are studied using the transmission line method. The width-normalized contact resistance between ZTO and sputtered molybdenum is measured to be 8.7 Ω-cm, which is 10, 20, and 600 times smaller than that of gold/titanium, indium tin oxide, and evaporated molybdenum electrodes, respectively. The superior contact formed using sputtered molybdenum is due to a favorable work function lineup, an insulator-free interface, bombardment of ZTO during molybdenum sputtering, and trap-assisted tunneling. The transfer length of the sputtered molybdenum/ZTO contact is 0.34 μm, opening the door to future radio-frequency sub-micron molybdenum/ZTO thin film transistors.

Effects of thermal annealing on variations of electron traps in the channel region of amorphous In-Ga-Zn-O thin film transistor

Photoinduced transient spectroscopy (PITS) was applied to study the effects of thermal annealing in the thin-film transistor (TFT) fabrication process on the variations of the electron traps in the channel region of amorphous In-Ga-Zn-O (a-IGZO). A dominant peak with a maximum of around 130 K was observed in the PITS spectra, but the detailed features were varied depending on the annealing conditions. The six particular temperatures corresponding to the trap states were extracted at about 100, 140, 150, 210, 320, and 390 K from the differential PITS spectra, showing good correlation with the trap states observed in ZnO. The results of thermal desorption spectrometry suggested that the variation of electron traps in the a-IGZO thin films has its origin in the decomposition of O and Zn during the annealing process. The annealing after the etch-stop layer deposition was also examined. The peak at about 150 K extracted from the differential PITS spectra before and after the annealing was markedly decreased. The activation energy of the corresponding trap states was estimated to be around 0.3 eV, which was close to those known as the E3 center in ZnO. Secondary ion mass spectroscopy analysis suggested that the reduction of trap density was mainly due to a decrease in the number of defects which involve hydrogen atoms in their configuration. Considering these results, the variations in the electron traps in the a-IGZO thin films during the TFT fabrication process should be attributed to the introduction of Zn, O, and/or H-related defects into tetrahedra consisting of Zn-O bonds.