Temperature and light dependent diode current in high-efficiency solution-processed small-molecule solar cells

This paper reports on the detail analysis of the DC electrical and photoelectrical properties of the high-efficient (η = 8.01% under standard 100 mW/cm² AM1.5 illumination) small molecule bulk heterojunction (SM BHJ) solar cells p-DTS(FBTTh₂)₂/PC₇₀BM. In this SM BHJ solar cell, the dark diode current is determined by the multistep tunnel-recombination via interface states at low forward bias (V < 0.65 V) and the interface state assisted thermionic emission at high forward bias (V > 0.65 V). The effect of illumination on the diode current was also quantitatively investigated. It was observed a reduced Shockley–Read–Hall recombination via interface states at large forward bias (from the maximum power point to the open-circuit conditions). The expression of the load I–V characteristic of the illuminated high-efficient SM BHJ solar cells was derived in the presence of the light dependent series and shunt resistance.     

Field and hot electron-induced degradation in GaN-based power MIS-HEMTs

We investigate the degradation of AlGaN/GaN MIS-HEMTs submitted to gate step-stress experiments, and demonstrate the existence of field- and hot-electron induced processes. When the devices are submitted to gate-step stress with high V_DS > 50 V, four different regimes are identified: (i) for V_GS < − 10 V, no significant degradation is observed, since the devices are in the off-state; (ii) for − 10 V < V_GS < 0 V, hot electrons flow through the channel, as demonstrated by the (measurable) electroluminescence signal. These hot electrons can be trapped within device structure, inducing an increase in the threshold voltage. (iii) for V_GS > 0 V, the density of hot electrons is significantly reduced, due to the increased interface scattering and device temperature. As a consequence, EL signal drops to zero, and the electrons trapped during phase (ii) are de-trapped back to the channel, where they are attracted by the high 2DEG potential. (iv) Finally, for V_GS > 5 V, a significant increase in threshold voltage is detected. This effect is observed only for high positive voltages, i.e. when a significant leakage current flows through the gate. Such gradual degradation is ascribed to the injection of electrons from the 2DEG to the gate insulator, which is a field-driven effect. These results were obtained by combined electrical and optical characterization carried out at different voltages during the step stress.     

Deep blue/ultraviolet microcavity OLEDs based on solution-processed PVK:CBP blends

There is an increasing need to develop stable, high-intensity, efficient OLEDs in the deep blue and UV. Applications include blue pixels for displays and tunable narrow solid-state UV sources for sensing, diagnostics, and development of a wide band spectrometer-on-a-chip. With the aim of developing such OLEDs we demonstrate an array of deep blue to near UV tunable microcavity (μc) OLEDs (λ ∼373–469 nm) using, in a unique approach, a mixed emitting layer (EML) of poly(N-vinyl carbazole) (PVK) and 4,4′-bis(9-carbazolyl)-biphenyl (CBP), whose ITO-based devices show a broad electroluminescence (EL) in the wavelength range of interest. This 373–469 nm band expands the 493–640 nm range previously attained with μcOLEDs into the desired deep blue-to-near UV range. Moreover, the current work highlights interesting characteristics of the complexity of mixed EML emission in combinatorial 2-d μcOLED arrays of the structure 40 nm Ag/x  nm MoO_x/∼30 nm PVK:CBP (3:1 weight ratio)/y  nm 4,7-diphenyl-1,10-phenanthroline (BPhen)/1 nm LiF/100 nm Al, where x = 5, 10, 15, and 20 nm and y = 10, 15, 20, and 30 nm. In the short wavelength μc devices, only CBP emission was observed, while in the long wavelength μc devices the emission from both PVK and CBP was evident. To understand this behavior simulations based on the scattering matrix method, were performed. The source profile of the EML was extracted from the measured EL of ITO-based devices. The calculated μc spectra indeed indicated that in the thinner, short wavelength devices the emission is primarily from CBP; in the thicker devices both CBP and PVK contribute to the EL. This situation is due to the effect of the optical cavity length on the relative contributions of PVK and CBP EL through a change in the wavelength-dependent emission rate, which was not suggested previously. Structural analysis of the EML and the preceding MoO_x layer complemented the data analysis.     

Red luminescence of spherical CuxZn1−xS semiconductor nanoparticles using 2-mercaptoethanol as capping agent

Cu²⁺-doped (0–2 at%) ZnS nanoparticles stabilized by 2-mercaptoethanol (2-ME) were successfully prepared using wet precipitation route in aqueous solution. The structural and optical characteristics were studied by various techniques. XRD pattern showed zinc blende cubic structure of Cu²⁺-doped ZnS with grain size of 4±0.5 nm. Spherical shape and well distribution of particles is confirmed by TEM, SEM and STM microscopy. Copper doping were identified by elemental dispersive (EDS) spectrometry. UV–vis spectroscopy revealed strong confinement effect due to blue shift in absorption shoulder peak as compared to bulk ZnS. Red luminescence band at～657 nm on Cu²⁺ doping may be arising from recombination of electrons at sulfur vacancies (V_s) and Cu(t₂) states formed at ZnS band gap. Optimum concentration of 0.25 at% (red band) of Cu²⁺ doping was selected by the observed enhanced PL emission.     

On the current conduction mechanisms of CeO2/La2O3 stacked gate dielectric

It was found that the electrical properties of CeO₂/La₂O₃ stack are much better than a single layer La₂O₃ film. A thin CeO₂ capping layer can effectively suppress the oxygen vacancy formation in the La₂O₃ film. This work further investigates the current conduction mechanisms of the CeO₂ (1 nm thick)/La₂O₃ (4 nm thick) stack. Results show that this thin stacked dielectric film still has a large leakage current density; the typical 1−V leakage can exceed 1 mA/cm² at room temperature. The large leakage current should be due to both the oxide defect centers as well as the film structure. Results show that at low electric field (<0.2 MV/cm), the thermionic emission induced current conduction in this stacked structure is quite pronounced as a result of interface barrier lowering due to the capping CeO₂ film which has a higher k value than that of the La₂O₃ film. At higher electric fields, the current conduction is governed by Poole–Frenkel (PF) emission via defect centers with an effective energy level of 0.119 eV. The temperature dependent current–voltage characteristics further indicate that the dielectric defects may be regenerated as a result of the change of the thermal equilibrium of the redox reaction in CeO₂ film at high temperature and the drift of oxygen under the applied electric field.     

Carrier transport mechanisms and photovoltaic properties of Au/p-ZnPc/Al device

The dark current density–voltage characteristic of Au/ZnPc/Al device at room temperature has been investigated. Results showed a rectification behavior. At low forward bias, the current density was found to be ohmic, while at high voltages, space charge limited the current mechanism dominated by exponential trapping levels. Junction parameters such as rectification ratio (RR), series resistance (R_s), and shunt resistance (R_sh) were found to be 9.42, 9.72 MΩ, and 0.88 × 10³ MΩ, respectively. The current density–voltage characteristics under white light illumination (100 W/m²) gives values of 0.55 V, 3 × 10⁻³ A/m², 0.18 and 5.8 × 10⁻⁴% for the open circuit voltage, V_oc, the short circuit current density (J_sc), the fill factor (FF), and conversion efficiency (η), respectively.     

One step hydrothermal synthesis of SnO2-RGO nanocomposite and its field emission studies

Field emission (FE) properties of hydrothermally synthesized, SnO₂-RGO nanocomposite have been investigated at a base pressure of 1×10⁻⁸mbar. The results reveal that the SnO₂-RGO nanocomposite emitter prevails over the pristine RGO emitter. The values of turn-on field, defined at emission current density of 1 μA/cm², are found to be 1.8 and 2.2 V/μm for the SnO₂-RGO and pristine RGO emitters, respectively. Furthermore, the SnO₂-RGO emitter delivers maximum emission current density of ~800 µA/cm² at an applied field of 5 V/μm. The observed values of applied field corresponding to emission current densities of 1 μA/cm² and 10 µA/cm² are superior to those reported for various emitters due to SnO₂ nanostructures and their composites. The emission current at the pre-set value of 1 µA is found to be very stable over a period of 3hrs. The enhanced FE behaviour of SnO₂-RGO nanocomposite emitter has been attributed to synergic effect due to its nanometric dimensions offering high aspect ratio and modulation of electronic properties via formation of heterostructure. The results obtained herein propose the SnO₂-RGO nanocomposite as a prospective candidate for FE based vacuum microelectronic devices.     

Enhanced pure red electroluminescence intensity of Eu(o-BBA)3(phen) by red dye co-doping and inorganic semiconductor as charge carrier function layer

There is an emission peak at 494 nm in the electroluminescence (EL) of PVK [poly(n-vinylcarbazole)]: Eu(o-BBA)₃(phen) besides PVK exciton emission and Eu³⁺ characteristic emissions. Both the peaking at 494 nm emission and PVK emission influenced the color purity of red emission from Eu(o-BBA)₃(phen). In order to restrain these emissions and obtain high intensity red emission, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyljulolidy-9-enyl)-4Hpyran (DCJTB) and Eu(o-BBA)₃(phen) were co-doped in PVK solution and used as the active emission layer. The EL intensity of co-doped devices reached to 420 cd/m² at 20 V driving voltage. The chromaticity coordinates of EL was invariable (x = 0.55, y = 0.36) with the increase of driving voltage. For further improvement of EL intensity, organic–inorganic hybrid devices (ITO/active emission layer/ZnS/Al) were fabricated. The EL intensity was increased by a factor of 2.5 [(420 cd/m²)/(168 cd/m²)] when the Eu complex was doped with an efficient dye DCJTB, and by a factor of ≈4 [(650 cd/m²)/(168 cd/m²)] when in addition ZnS layer was deposited on such an emitting layer prior to evaporation of the Al cathode.     

Selective activation of failure mechanisms in packaged double-heterostructure light emitting diodes using controlled neutron energy irradiation

This paper demonstrates the feasibility of creating specific defects in double-heterostructure AlGaAsGaAs commercial light emitting diode by neutron irradiation. Using controlled neutron energy, only one failure mechanism can be activated. Defects are located in the side of the chip and increase the leakage current driven by the well-known Pool–Frenkel effect with E_c ? E_T = 130 meV electron trap energy level. The maximal amplitude of optical spectrum also reveals a drop about 20% associated to the rise of leakage current.     

Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

White OLED based on a temperature sensitive Eu3+/Tb3+ β-diketonate complex

A mixed lanthanide β-diketonate complex of molecular formula [Eu_0.45Tb_0.55(btfa)₃(4,4′-bpy)(EtOH)] (btfa^– = 4,4,4–trifluoro–1–phenyl–1,3–butanedionate; 4,4′-bpy = 4,4′-dipyridyl; EtOH = ethanol) was synthesized and its structure was elucidated by single crystal X-ray diffraction. The temperature dependence of the complex emission intensity between 11 and 298 K is illustrated by the Commission Internacionale l’Éclairage (CIE) (x, y) color coordinates change within the orange-red region, from (0.521, 0.443) to (0.658, 0.335). The existence of Tb³⁺-to-Eu³⁺ energy transfer was observed at room temperature and as the complex presents a relatively high emission quantum yield (0.34 ± 0.03) it was doped in a 4,4′-bis(carbazol-9-yl)biphenyl (CBP) organic matrix to be used as emitting layer to fabricate a white organic light-emitting diode (WOLED). Continuous electroluminescence emission was obtained varying the applied bias voltage showing a wide emission band from 400 to 700 nm. The white emission results from a combined action between the Eu³⁺ and Tb³⁺ peaks from the mixed Eu³⁺/Tb³⁺ complex and the other organic layers forming the device. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.     

Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

The effect of annealing temperature on the electronic parameters and carrier transport mechanism of Pt/n-type Ge Schottky diode

The Pt nano-film Schottky diodes on Ge substrate have been fabricated to investigate the effect of annealing temperature on the characteristics of the device. The germanide phase between Pt nano-films and Ge substrate changed and generated interface layer PtGe at 573 K and 673 K, Pt₂Ge₃ at 773 K. The current–voltage(I - V) characteristics of Pt/n-Ge Schottky diodes were measured in the temperature range of 183–303 K. Evaluation of the I - V data has revealed an increase of zero-bias barrier height Φ_B0 but the decrease of ideality factor n with the increase in temperature. Such behaviors have been successfully modeled on the basis of the thermionic emission mechanism by assuming the presence of Gaussian distributions. The variation of electronic transport properties of these Schottky diodes has been inferred to be attributed to combined effects of interfacial reaction and phase transformation during the annealing process. Therefore, the control of Schottky barrier height at metal/Ge interface is important to realize high performance Ge-based CMOS devices.     

Structural characterization of Zn1−xCdxO (0≤x≤0.20) microrods grown by spray pyrolysis

Zn_1−xCd_xO (x= 0.00, 0.05, 0.10, 0.15 and 0.20) thin films were obtained by spray pyrolysis and characterized by XRD, SEM, EDAX and optical measurements. The Zn_1−xCd_xO microrods are in the wurtzite crystallographic phase with (0 0 2) preferred orientation. A narrowing of the fundamental band gap from 3.30 to 3.10 eV was observed with the increasing nominal Cd content up to 20 at% due to the direct modulation of the band gap caused by Cd substitution. The undoped ZnO film showed two emission bands in the spectra: one sharp UV luminescence at ∼382 nm and one broad visible emission ranging from 430 to 600 nm. The sharp peak at ∼382 nm is split into two at 376 and 400 nm upon Cd doping at levels of 5 and 10 at%. However this splitting is not observed in the doped ZnO samples containing 15 at% Cd and more. It should also be mentioned that the broad peak at the range of 430–600 nm has almost disappeared in the films containing 5, 10 and 15 at% Cd.     

Electrochemical growth of tin(II) oxide films: Application in photocatalytic degradation of methylene blue

We have investigated the semiconducting and photoelectrochemical properties of SnO films grown potentiostatically on tin substrate. The oxide is characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The anodic process corresponds to the formation of SnO·nH₂O pre-passive layer that is removed upon increasing potential due to surface etching at the metal/oxide interface. SnO films deposited for long durations (>50 mn) are uniform and well adhered; they thicken up to ~50 nm by diffusion-controlled process and the growth follows a direct logarithmic law. The thickness is determined by coulometry and the X-ray diffraction indicates the tetragonal SnO phase (SG: P4/mmm) with a crystallite size of 32 nm. The Mott–Schottky plot is characteristic of n type conductivity with an electrons density of 5.72×10¹⁸ cm⁻³, a flat band potential of −0.09 V_SCE and a depletion width of ~10 nm. The valence band, located at 5.91 eV below, vacuum is made up of hybridized O²⁻:2p Sn²⁺:5s while the conduction band (4.45 eV) derives from Sn²⁺:5p orbital. The electrochemical impedance spectroscopy (EIS) measured in the range (10⁻²–10⁵ Hz) shows the contribution of the bulk and grain boundaries. The energy band diagram predicts the photodegradation of methylene blue on SnO films. 67% of the initial concentration (10 mg L⁻¹) disappears after 3 h of exposure to visible light (9 mW cm⁻²) with a quantum yield of 0.072.     

Pyrene functioned diarylfluorenes as efficient solution processable light emitting molecular glass

In this paper, we described a new category of solution processable small molecule organic light emitting materials, the pyrene functioned diarylfluorenes: 2PE-PPF and DPE-PPF. They emit blue light in solution and green light in film, and show high thermal stability with the 5% weight loss temperature (T_d) over 400 °C. The glass transition temperature (T_g) for 2PE-PPF and DPE-PPF is 102 °C and 147 °C, respectively. These molecules are interesting molecular glass and they have good film forming abilities. Smooth and uniform film could be obtained by spin-coating. This character enables them able to be used in solution processed OLEDs by spin-coating or jet-printing. Single layered device using 2PE-PPF as the active material shows a turn-on voltage of 3.2 V, brightness over 8000 cd/m² and current efficiency up to 2.55 cd/A. Double layered device by inserting TPBI as the hole-blocking electron-transporting layer increases the maximum efficiency to 5.83 cd/A.     

Organic field-effect transistor and its photoresponse using a benzo[1,2-b:4,5-b′]difuran-based donor–acceptor conjugated polymer

Organic field-effect transistors (OFETs) were fabricated through a solution process with a donor–acceptor (D–A) conjugated polymer poly{4,8-bis(2′-ethylhexylthiophene)benzo [1,2-b;3,4-b′]difuran-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)} (PBDFTDTBT) as the active layer, which is a highly efficient D–A conjugated polymer as a donor in polymer solar cells with a power conversion efficiency (PCE) over 6.0%. The OFET devices showed a hole mobility of 0.05 cm²/Vs and an on/off ratio of 4.6 × 10⁵. Those are one of the best performance parameters for OFETs based on D–A conjugated polymers including benzo[1,2-b:4,5-b′]dithiophene (BDT) or benzo[1,2-b:4,5-b′]difuran (BDF) unit. The photoresponse of OFETs was investigated by modulating light with various intensities. The devices produced a photosensitivity (I_light/I_dark) of 1.2 × 10⁵ and a photoresponsivity of 360 mA W⁻¹ under white light illumination. The drain current in saturation region increases gradually with increasing illumination intensity. The threshold voltage exhibited a positive shift from −15.6 V in darkness to 27.8 V under illumination, which can be attributed to the well-known photovoltaic effect resulting from the transport of photogenerated holes and trapping of photogenerated electrons near the source electrode in organic phototransistors. Meanwhile, the devices showed good stability and with no obvious degeneration for 3 months in air. The study suggests that D–A conjugated polymers including BDF unit can be potentially applied in OFETs and organic phototransistors in addition to highly efficient polymer solar cells.     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Multiple exciton generation regions in phosphorescent white organic light emitting devices

We demonstrate a white electrophosphorescent organic light emitting device (WOLED) with a three-section emission layer (EML) where excitons are formed in the multiple emission regions. The EML consists of a stepped progression of highest occupied and lowest unoccupied molecular orbital energies of the ambipolar hosts. Analysis shows that (36 ± 6)% of the excitons form in the blue emitting region, while (64 ± 6)% form in the green emitting region at 100 mA/cm². The doping of the red, green and blue phosphors, each in its own host, allows for efficient utilization of excitons formed in these multiple regions. Based on this architecture, the WOLED has an internal quantum efficiency close to unity. The WOLED has total external quantum and power efficiencies of η_ext,t = (26 ± 1)% and η_p,t = (63 ± 3) lm/W at 12 cd/m², decreasing to η_ext,t = (23 ± 1)% and η_p,t = (37 ± 2) lm/W at 500 cd/m². When an undoped electron transport layer is used, the peak efficiency is η_ext,t = (28 ± 1)%. Due to the distributed exciton formation in the EML, the WOLED exhibits higher total efficiency than monochromatic devices employing the same red, green and blue dopant–host combinations.     

Low temperature fabrication of a ZnO nanoparticle thin-film transistor suitable for flexible electronics

A bottom-gate/top-drain/source contact ZnO nanoparticle thin-film transistor was fabricated using a low temperature annealing process (150 °C) suitable for flexible electronics. Additionally, a high-k resin filled with TiO₂ nanoparticles was used as gate dielectric. After fabrication, the transistors presented almost no hysteresis in the I–V curve, a threshold voltage (V_T) of 2.2 V, a field-effect mobility on the order of 0.1 cm²/V s and an I_ON/I_OFF ratio of about 10⁴. However, the transistor is sensitive to aging effects due to interactions with the ambient air, resulting in current level reduction caused by trapped oxygen at the nanoparticle surface, and an anti-clockwise hysteresis in the transfer curve. It was demonstrated, conjointly, the possible desorption of oxygen by voltage stress and UV light exposure.