Effect of fullerene doping on the electrical properties of P3HT/PCBM layers

Poly (3-hexylthiophene-2, 5-diyl) (P3HT) and its blend with Phenyl-C₆₁-Butyric acid-Methyl-Ester (PCBM) and fullerene (C₆₀) thin films were prepared and their electrical properties for memory applications were studied. Due to doping, a sharp decrease in the resistance for a P3HT:PCBM:C₆₀ device was observed at around 70 °C which makes it useful for thermal switching applications. Addition of C₆₀ to P3HT:PCBM blend gave a high value for R_RESET/R_SET in thermal switching. For bias switching, threshold voltage reduces to 1.4 V from 25 V with the addition of C₆₀ to P3HT layer.     

N,N′-diphenylperylene diimide functioning as a sensitizing light absorber based on excitation transfer for organic thin-film solar cells

We demonstrated that N,N′-diphenylperylene tetracarbonic diimide (PTCDI-Ph) could work as an n-type sensitizing layer for the C₆₀ n-type layer owing to interlayer excitation transfer (ET) when the PTCDI-Ph layer was placed between the C₆₀ layer and the aluminum anode coupled with the bathocuproine layer. Well-aligned lowest unoccupied molecular orbitals between C₆₀ and PTCDI-Ph (?4.55 eV for PTCDI-Ph and ?4.5 eV for C₆₀) and a larger bandgap for PTCDI-Ph than C₆₀ (2.04 eV for PTCDI-Ph and 2.0 eV for C₆₀) enabled this interlayer ET-based sensitization. Further, the optical interference effect could be also involved in the sensitization. It was also demonstrated that the combination of both n-type materials C₆₀ and PTCDI-Ph could successfully reduce the amount of the expensive C₆₀ used, and a thin C₆₀ layer was indispensable for efficient charge separation. PTCDI-Ph could work as a light-harvesting n-type material incorporating C₆₀-based cells to compensate for C₆₀’s weak optical absorption.     

High-performance n-type organic field-effect transistors fabricated by ink-jet printing using a C60 derivative

We report on the performance of ink-jet-printed n-type organic thin-film transistors (OTFTs) based on a C₆₀ derivative, namely, C₆₀-fused N-methyl-2-(3-hexylthiophen-2-yl)pyrrolidine (C₆₀TH-Hx). The new devices exhibit excellent n-channel performance, with a highest mobility of 2.8 × 10^?2 cm² V^?1 s^?1, an I_On/I_Off ratio of about 1 × 10⁶, and a threshold voltage of 7 V. The C₆₀TH-Hx films show large crystalline domains that result from the influence of an evaporation-induced flow, thus leading to high electron mobility in the ink-jet-printed devices.     

Dry growth of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

Dry method for monolayer deposition of n-octylphosphonic acid (C₈PA) on the surface of aluminium oxide (AlO_x) is presented. Vacuum thermal evaporation is employed to deposit initial thickness corresponding to several C₈PA monolayers, followed by a thermal desorption of the physisorbed C₈PA molecules. AlO_x functionalized with such C₈PA monolayer exhibits leakage current density of ～10^?7 A/cm² at 3 V, electric breakdown field of ～6 MV/cm, and a root-mean-square surface roughness of 0.36 nm. The performance of low-voltage pentacene thin-film transistors that implement this dry AlO_x/C₈PA gate dielectric depends on C₈PA desorption time. When the desorption time rises from 25 to 210 min, the field-effect mobility increases from ～0.02 to ～0.04 cm²/V s, threshold voltage rises from ～?1.2 to ～?1.4 V, sub-threshold slope decreases from ～120 to ～80 mV/decade, off-current decreases from ～5 × 10^?12 to ～1 × 10^?12 A, on/off current ratio rises from ～3.8 × 10⁴ to ～2.5 × 10⁵, and the transistor hysteresis decreases from 61 to 26 mV. These results collectively support a two stage model of the desorption process where the removal of the physisorbed C₈PA molecules is followed by the annealing of the defect sites in the remaining C₈PA monolayer.     

Spin injection and transport in organic spin-valves based on fullerene C60

We report the fabrication and magnetoresistance (MR) of the La_0.67Sr_0.33MnO₃/C₆₀/Co spin valves. The introduction of 1.5 nm AlO_x barrier between the C₆₀ layer and cobalt top electrode prevents effectively interfacial diffusion and Co penetration, and thus an appreciable positive MR (as large as 3.65%) at room temperature was exhibited for the devices in the thickness range (5–40 nm) of C₆₀ studied. Possible mechanisms on the MR polarity are proposed. Furthermore, based on the temperature- and thickness- dependent MR and I–V characteristics, we have obtained clear evidences that the MR of C₆₀-based spin-valves originates from the tunneling of spin-polarized electrons at the low thickness of C₆₀, however at the larger thickness (>20 nm) the electrons are injected into and subsequently hopping transport within the C₆₀ spacer.     

Doping efficiency and energy-level scheme in C60F48-doped zinc–tetraphenylporphyrin films

High resolution synchrotron-based core level spectroscopy was used to examine the energy level alignment at the interface of zinc–tetraphenylporphyrin films doped by the surface acceptor C₆₀F₄₈. Two distinct fluorofullerene charge states were identified, corresponding to ionized and neutral molecules, and their relative concentration as a function of coverage was used to evaluate the probability of occupation of the acceptor lowest unoccupied molecular orbital (LUMO). From an initial acceptor energy of ?0.25 eV, the C₆₀F₄₈ LUMO shifts upwards with coverage due to a doping-induced interfacial dipole potential, and stabilization of the LUMO at an energy 0.45 eV above the Fermi energy was obtained. While the energy difference upon saturation is consistent with the results obtained for other donor–acceptor systems that have been interpreted as Fermi level pinning, the present work shows that the energy offset is a direct consequence of the interplay between Fermi–Dirac statistics in combination with the interfacial dipole potential.     

High performance inkjet-printed C60 fullerene thin-film transistors: Toward a low-cost and reproducible solution process

We have demonstrated high performance inkjet-printed n-channel thin-film transistors (TFTs) using C₆₀ fullerene as a channel material. Highly uniform amorphous C₆₀ thin-film patterns were fabricated on a solution-wettable polymer gate dielectric layer by inkjet-printing and vacuum drying process. Fabricated C₆₀ TFTs shows great reproducibility and high performance; field-effect mobilities of 2.2–2.4 cm² V^?1 s^?1, threshold voltages of 0.4–0.6 V, subthreshold slopes of 0.11–0.16 V dec^?1 and current on/off ratio of 10⁷–10⁸ in a driving voltage of 5 V. This is due to the efficient annealing process that extracting the solvent residue and the formation of low trap-density gate dielectric surface.     

Highly ordered C60 films on epitaxial Fe/MgO(0 0 1) surfaces for organic spintronics

Hybrid interfaces between ferromagnetic surfaces and carbon-based molecules play an important role in organic spintronics. The fabrication of devices with well defined interfaces remains challenging, however, hampering microscopic understanding of their operation mechanisms. We have studied the crystallinity and molecular ordering of C₆₀ films on epitaxial Fe/MgO(0 0 1) surfaces, using X-ray diffraction and scanning tunneling microscopy (STM). Both techniques confirm that fcc molecular C₆₀ films with a (1 1 1)-texture can be fabricated on epitaxial bcc-Fe(0 0 1) surfaces at elevated growth temperatures (100–130 °C). STM measurements show that C₆₀ monolayers deposited at 130 °C are highly ordered, exhibiting quasi-hexagonal arrangements on the Fe(0 0 1) surface oriented along the [1 0 0] and [0 1 0] directions. The mismatch between the surface lattice of the monolayer and the bulk fcc C₆₀ lattice prevents epitaxial overgrowth of multilayers.     

Evidence for causality between GaN RF HEMT degradation and the EC-0.57 eV trap in GaN

The degradation of industry-supplied GaN high electron mobility transistors (HEMTs) subjected to accelerated life testing (ALT) is directly related to increases in concentrations of two defects with trap energies of E_C-0.57 and E_C-0.75 eV. Pulsed I-V measurements and constant drain current deep level transient spectroscopy were employed to evaluate the quantitative impact of each trap. The trap concentration increases were only observed in devices that showed a 1 dB drop in output power and not the result of the ALT itself indicating that these traps and primarily the E_C-0.57 eV trap are responsible for the output power degradation. Increases from the E_C-0.57 eV level were responsible for 80% of the increased knee walkout while the E_C-0.75 eV contributed only 20%. These traps are located in the drain access region, likely in the GaN buffer, and cause increased knee walkout after the application of drain voltage.     

Effects of thin film processing on pentacene/C60 bilayer solar cell performance

Solar cells based on pentacene/C₆₀ bilayer heterojunctions have been fabricated with a structure of ITO/poly(styrenesulfonate) (PEDOT:PSS)/pentacene (40 nm)/fullerene (C₆₀)(40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/Al. The effect of pentacene crystalline domain size on performance was investigated by controlling the pentacene deposition rate. The devices show improved light-to-electricity conversion efficiencies from 0.49% to 1.12% under an AM 1.5 solar simulator (100 mW/cm²), when the pentacene evaporation rate is in a range of 5 Å/s–0.5 Å/s. Atomic force microscopy (AFM) measurements show that the pentacene films deposited by a slow evaporation rate have larger crystalline domains and a fewer amorphous domains, compared to films obtained by faster evaporation rates. Upon thermal annealing at 200 °C for 1 min, there is merging of pentacene crystalline domains. These changes in film morphology impact the charge separation at the donor/acceptor interface and the hole and electron mobilities, and hence, directly affect the device performance.     

Improved interfacial quality of GaAs metal-oxide-semiconductor device with NH3-plasma treated yittrium-oxynitride as interfacial passivation layer

The interfacial and electrical properties of GaAs metal-oxide-semiconductor capacitors with yittrium-oxynitride interfacial passivation layer treated by N₂ −/NH₃-plasma are investigated, showing that lower interface-state density (1.24 × 10¹² cm^− 2 eV^− 1 near midgap), smaller gate leakage current density (1.34 × 10^− 5 A/cm² at V_fb + 1 V), smaller capacitance equivalent thickness (1.43 nm), and larger equivalent dielectric constant (24.5) can be achieved for the sample with NH₃-plasma treatment than the samples with N₂ −/no-plasma treatment. The mechanisms lie in the fact that NH₃-plasma can provide not only N atoms, but H atoms and NH radicals to effectively passivate the high-k/GaAs interface, thus less pinning the Femi level at high-k/GaAs interface.     

Performance and lifetime improvement of polymer/fullerene blend photovoltaic cells with a C60 interlayer

Polymer solar cell based on the blend of poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with C₆₀ insertion layer were fabricated. The solar cell structure was ITO/poly(3,4-ethylenedioxythiopene) (PEDOT):poly(styrenesulfonate) (PSS)/MEH-PPV:PCBM/C₆₀/Al. It was found that the C₆₀ inserting layer could increase the device performance and lifetime. The energy conversion efficiency (ECE) of the solar cell with C₆₀ layer reached about 1.89% under Air Mass 1.5, 100 mW cm⁻² illumination, which is enhanced in comparison with that of the device without C₆₀ layer. Mechanisms of the solar cell performance and lifetime dependence on the C₆₀ layer are discussed.     

Direct MBE growth of GaN on GaAs substrates for integrated short wavelength emitters

The growth and material properties of GaN heteroepitaxial layers on vicinal (1 0 0) and exact (1 1 1)B substrates have been investigated, using molecular beam epitaxy (MBE) with N₂ RF-plasma source. We examined the approach to grow GaN directly on the oxide desorbed GaAs, without the incidence of an As beam during oxide desorption or the following stages of growth. Perfect smooth surfaces were obtained on (1 1 1)B GaAs but excellent luminescence properties were observed on vicinal (1 0 0) GaAs. Four growth temperatures (T_G) were compared for the (1 0 0) orientation and a monotonic increase of photoluminescence intensity with increasing T_G, in the range of 570–680°C, was observed. The best surface morphology of less than 10 nm rms roughness was also determined, by atomic force microscopy, for the maximum (680°C) temperature. The layers exhibited up to 10¹⁷ cm⁻³ electron concentration and it could be compensated by Mg impurities. Metallizations of Pt and Pd gave ohmic contacts on GaN/GaAs (1 0 0) but a Schottky diode contact was achieved by Ir metallization. The obtained material properties are probably sufficient for realizing efficient GaN light emitters on (1 0 0) GaAs substrates.     

High-performance C60 and picene thin film field-effect transistors with conducting polymer electrodes in bottom contact structure

C₆₀ and picene thin film field-effect transistors (FETs) in bottom contact structure have been fabricated with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes for a realization of mechanical flexible organic FETs. The C₆₀ thin film FETs showed n-channel enhancement-type characteristics with the field-effect mobility μ value of 0.41 cm² V^?1 s^?1, while the picene thin film FET showed p-channel enhancement-type characteristics with the μ of 0.61 cm² V^?1 s^?1. The μ values recorded for C₆₀ and picene thin film FETs are comparable to those for C₆₀ and picene thin film FETs with Au electrodes.     

Energy level alignment and morphology of interfaces between molecular and polymeric organic semiconductors

Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic–organic conductor–semiconductor and semiconductor–semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C₆₀ and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C₆₀ on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation.     

Thermal characterization of high power LED with ceramic particles filled thermal paste for effective heat dissipation

The next generation packaging materials are expected to possess high heat dissipation capability. Understanding the needs for betterment in the field of thermal management, the present study aims at investigating the package level analysis on a high power LED. In this study, commercially available thermal paste was heavily filled with ceramic particles of aluminium nitride (AlN) and boron nitride (BN) in order to enhance the heat dissipation of the device. Different particle sizes of AlN and BN fillers were incorporated homogenously into the thermal paste and applied as a thermal interface material (TIM) for an effective system level analysis employing thermal transient measurement. It was found that AlN TIM achieve less LED junction temperature by a difference of 2.20 °C compared to BN filled TIM. Furthermore, among D₅₀ = 1170 nm, 813 nm and 758 nm, the AlN at D₅₀ = 1170 nm was found to exhibit the lowest junction temperature of 38.49 °C and the lowest total thermal resistance of 11.33 K/W compared to the other two fillers.     

Quantification of deep hole-trap filling by molecular p-doping: Dependence on the host material purity

The p-doping effect of the fluorinated fullerene C₆₀ F₃₆ doped into organic thin films of N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) of different purification grades is systematically investigated by photoemission spectroscopy. By reducing the molar doping ratio to MR = 2.9 × 10^?4, the Fermi-level shift upon doping is resolved in particular at very low doping concentrations. In comparison to four times sublimated MeO-TPD, 5 times more C₆₀F₃₆ molecules have to be doped into unpurified MeO-TPD films to shift the Fermi-level just above its intrinsic position. This finding is discussed in terms of a statistical model, showing that narrow deep hole-trap states are additionally present in the unpurified host material which are hindering an efficient generation of free charge carriers at molar doping ratios below MR = 0.002.     

Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

Investigation of deep level defects in electron irradiated indium arsenide quantum dots embedded in a gallium arsenide matrix

Gallium arsenide diodes with and without indium arsenide quantum dots were electron irradiated to investigate radiation induced defects. Baseline and quantum dot gallium arsenide pn-junction diodes were characterized by capacitance–voltage measurements, and deep level transient spectroscopy. Carrier accumulation was observed in the gallium arsenide quantum dot sample at the designed depth for the quantum dots via capacitance–voltage measurements. Prior to irradiation, a defect 0.84 eV below the conduction band (E_C – 0.84 eV) was observed in the baseline sample which is consistent with the native EL2 defect seen in gallium arsenide. After 1 MeV electron irradiation three new defects were observed in the baseline sample, labeled as E3 (E_C – 0.25 eV), E4 (E_C – 0.55 eV), and E5 (E_C – 0.76 eV), consistent with literature reports of electron irradiated gallium arsenide. Prior to irradiation, the addition of quantum dots appeared to have introduced defect levels at E_C – 0.21, E_C – 0.38, and E_C – 0.75 eV denoted as QD–DX1, QD–DX2, and QD–EL2 respectively. In the quantum dot sample after 1 MeV electron irradiation, QD–E3 (E_C – 0.28 eV), QD–E4 (E_C – 0.49 eV), and QD–EL2 (E_C – 0.72 eV) defects, similar to the baseline sample, were observed, although the trap density was dissimilar to that of the baseline sample. The quantum dot sample showed a higher density of the QD–E4 defect and a lower density of QD–E3, while the QD–EL2 defect seemed to be unaffected by electron irradiation. These findings suggest that the quantum dot sample may be more radiation tolerant to the E3 defect as compared to the baseline sample.     

Identification of an RF degradation mechanism in GaN based HEMTs triggered by midgap traps

Quantitative defect spectroscopy was performed on low gate leakage operational S-band GaN HEMTs before and after RF accelerated life testing (ALT) to investigate and quantify potential connections between the evolution of observed traps and RF output power loss in these HEMTs after stressing. Constant drain current deep level transient spectroscopy and deep level optical spectroscopy (CI_D-DLTS and CI_D-DLOS, respectively) were used to interrogate thermally-emitting traps (CI_D-DLTS) and deeper optically-stimulated traps (CI_D-DLOS) so that the entire bandgap can be probed systematically before and after ALT. Using drain-controlled CI_D-DLTS/DLOS, with which traps in the drain access region are resolved, it is found that an increase in the concentration of a broad range of deep states between E_C–1.6 to 3.0 eV, detected by CI_D-DLOS, causes a persistent increase in on-resistance of ~ 0.22 Ω-mm, which is a likely source for the 1.2 dB reduction in RF output power that was observed after stressing. In contrast, the combined effect of the upper bandgap states at E_C–0.57 and E_C–0.72 eV, observed by CI_D-DLTS, is responsible for only ~ 10% of the on-resistance increase. These results demonstrate the importance of discriminating between traps throughout the entire bandgap with regard to the relative roles of individual traps on degradation of GaN HEMTs after ALT.