New electronically tunable current-mode universal biquad filter using translinear current conveyors

In this study, a new electronically tunable current-mode universal filter with two inputs and two outputs employing one translinear current conveyor, one translinear current conveyor with controlled current gain and two grounded capacitors is presented. The proposed circuit offers the following attractive features: realisation of low-pass, band-pass, high-pass, band-stop and all-pass current responses from the same configuration; employment of the minimum active and passive components; no requirement of component matching conditions; independent current-control of the parameters natural frequency (ω_o) and quality factor (Q); low active and passive sensitivities; and high impedance output. The characteristics of the proposed circuit are simulated using PSPICE to confirm the theory.     

Multiple-input single-output low-input and high-output impedance current-mode biquadratic filter employing five modified CFTAs and only two grounded capacitors

This paper presents a novel current-mode biquadratic filter with three inputs and a single output. The proposed circuit employs five modified current follower transconductance amplifiers (MCFTAs) and only two grounded capacitors. It can realize all five biquadratic filter functions namely: low-pass (LP), band-pass (BP), high-pass (HP), band-stop (BS) and all-pass (AP) at the output terminal by selecting different input current signals, without requiring any parameter-matching conditions or additional circuits. The filter has an orthogonal electronic adjustment of the natural angular frequency ω₀ and the quality factor Q, and it has very low element sensitivities of ω₀ and Q. Moreover, the proposed filter has the feature of high output impedance and low input impedance, and the use of only grounded capacitors makes it convenient for integrated circuit implementation. The performances of the proposed circuit are illustrated by PSPICE simulations, and the results are in good agreement with theoretical analysis.     

New current-mode current-controlled universal filter with single input and three outputs

In this study, a new current-mode current-controlled universal filter with single input and three outputs is presented. The proposed circuit uses single-output current controlled conveyors (CCCIIs) and can simultaneously realize lowpass, bandpass and highpass filter functions all at high impedance outputs. Realization of notch and allpass responses does not require additional active elements. The circuit enjoys independent current-control of the parameters ω₀ and ω₀/Q without disturbing the gains of the lowpass, bandpass and highpass filters. Both its active and passive sensitivities are low.     

Multifunctional translinear-C current-mode filter

A multifunctional current mode filter using three current controlled conveyors and two grounded capacitors is described. The filter realizes high pass, band pass and low pass second order current transfer functions, and provides independent pole-ω ₀, and pole-Q ₀ tuning through external current control. The filter also displays low incremental parameter sensitivities. The simulation results on the filter support the theory.     

High output impedance current-mode lowpass,bandpass and highpass filters using current controlled conveyors

Three new current-mode second-order filter configurations which employ one or two current controlled conveyors (CCCIIs) and three passive elements are presented. Each proposed filter can realize one of the bandpass, lowpass or highpass responses all at high impedance outputs. The proposed filters offer current-control of pole angular frequency ω_o without disturbing the parameter ω_o /Q. No component matching is required and all the passive and active sensitivities are low. PSPICE simulation results are given to confirm the theoretical analysis.     

Tuneable filter using voltage conveyors and current active elements

In this article a new structure of the multifunction second-order frequency filter working in current mode is presented. The circuit solution employs voltage conveyors and programmable current amplifiers. The advantage of the proposed circuit is the possibility of mutually independent control of the quality factor Q and pole frequency f ₀ using the active elements, current inputs at low-impedance and ground potential, realisation of the low-, high- and band-pass response without changing the circuit topology, low passive and active sensitivities. The frequency filter has been designed using the M-C signal flow graphs and its behaviour verified by OrCAD PSpice simulations.     

CuIn1−xGaxSe2‐based thin‐film solar cells by the selenization of sequentially evaporated metallic layers

Polycrystalline CuIn_1−xGa_xSe₂ (CIGS) thin films were deposited by the non‐vacuum, near‐atmospheric‐pressure selenization of stacked metallic precursor layers. A study was carried out to investigate the influence of significant factors of the absorber on the solar cells performance. An efficiency enhancement was obtained for Cu/(In+Ga) atomic ratios between 0·93 and 0·95. The slope of the observed energy bandgap grading showed a strong influence on the V_OC and the short circuit current density J_SC. An increase of the Ga content in the active region of the absorber was achieved by the introduction of a thin Ga layer on the Mo back contact. This led to an improvement of efficiency and V_OC. Furthermore, an enhanced carrier collection was detected by quantum efficiency measurements when the absorber layer thickness was slightly decreased. Conversion efficiencies close to 10% have been obtained for these devices. Copyright © 2005 John Wiley & Sons, Ltd.     

Current-mode active-only universal filter

A novel current-mode active-only universal filter based on and employing three DO-CCCIIs and two OAs is introduced. The circuit is fully integrable and programmable, and implements all the five generic filtering functions. The three basic filtering responses LP, BP and HP are implemented simultaneously by the circuit, while AP and notch signals can be realized simply by connecting appropriate node currents. The currents are available at high output impedances, so the synthesized filter can be cascaded without additional buffers to realize higher order filters. The filter performance factors ω ₀ and Q are electronically tunable in an orthogonal manner through separate bias currents of the CCCII. The PSPICE simulation was carried out to realize the proposed circuit.     

The Dependence of Device Dark Current on the Active‐Layer Morphology of Solution‐Processed Organic Photodetectors

Organic photodiodes are presented that utilize solution‐processed perylene diimide bulk heterojunctions as the device photoactive layer. The polymer (9,9′‐dioctylfluorene‐co‐benzothiadiazole; F8BT) is used as the electron donor and the N,N′‐bis(1‐ethylpropyl)‐3,4,9,10‐perylene tetracarboxylic diimide (PDI) derivative is used as the electron acceptor. The thickness‐dependent study of the main device parameters, namely of the external quantum efficiency (EQE), the short‐circuit current (I_SC), the open‐circuit voltage (V_OC), the fill factor (FF), and the dark current (I_D) is presented. In as‐spun F8BT:PDI devices the short‐circuit EQE reaches the maximum of 17% and the V_OC value is as high as 0.8 V. Device I_D is in the nA mm^?2 regime and it correlates with the topography of the F8BT:PDI layer. For a range of annealing temperatures I_D is monitored as the morphology of the photoactive layer changes. The changes in the morphology of the photoactive layer are monitored via atomic force microscopy. The thermally induced coalescence of the PDI domains assists the dark conductivity of the device. I_D values as low as 80 pA mm^?2 are achieved with a corresponding EQE of 9%, when an electron‐blocking layer (EB) is used in bilayer EB/F8BT:PDI devices. Electron injection from the hole‐collecting electrode to the F8BT:PDI medium is hindered by the use of the EB layer. The temperature dependence of the I_D value of the as‐spun F8BT:PDI device is studied in the range of 296–216 K. In combination with the thickness and the composition dependence of I_D, the determined activation energy E_a suggests a two‐step mechanism of I_D generation; a temperature‐independent step of electric‐field‐assisted carrier injection from the device contacts to the active‐layer medium and a thermally activated step of carrier transport across the device electrodes, via the PDI domains of the photoactive layer. Moreover, device I_D is found to be sensitive to environmental factors.     

Acceptor‐Substituted S,N‐Heteropentacenes of Different Conjugation Length: Structure–Property Relationships and Solar Cell Performance

The synthesis, optoelectronic, and photovoltaic properties of novel acceptor–donor–acceptor (A–D–A) based π‐conjugated functional molecules 1 – 3, comprising a planar S,N‐heteropentacene as central donor substituted with various terminal acceptor units, such as 1,1‐dicyanovinylene (DCV) and 1‐(1,1‐dicyanomethylene)‐cyclohex‐2‐ene (DCC), are reported. The structural variation of the end groups provides molecules 1 – 3 with gradually increased π‐conjugation due to a rising number of double bonds, which comes from the DCC unit(s). From optoelectronic investigation, structure–property relationships are deduced and the novel A–D–A heteropentacenes 1 – 3 are implemented as photoactive donor component in solution‐processed bulk heterojunction solar cells together with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester as acceptor. The structural variation in the S,N‐heteropentacenes leads to clear trends in the photovoltaic performance and power conversion efficiencies of up to 4.9% are achieved. Furthermore, due to extension of the double bonds a clear trade‐off between the open circuit voltage (V _OC) and the short circuit current density (J _SC) values is observed. The role of additives on the optimization of the nanoscale morphology and device performance is investigated. The findings presented herein demonstrate that depending on the types of materials the additive may have significantly different effects on the active layer morphology and the device performance.     

Large AuAg Alloy Nanoparticles Synthesized in Organic Media Using a One‐Pot Reaction: Their Applications for High‐Performance Bulk Heterojunction Solar Cells

A one‐pot synthesis of large size and high quality AuAg alloy nanoparticles (NPs) with well controlled compositions via hot organic media is demonstrated. Amid the synthesis, complexation between trioctylphosphine (TOP) and metal precursors is found, which slows down the rate of nucleation and leads to the growth of large‐size AuAg nanoalloys. The wavelength and relative intensities of the resulting plasmon bands are readily fine‐tuned during the synthetic process using different Au/Ag precursors molar ratios. In the polymer solar cells, a key step in achieving high efficiency is the utilization of 1% Au₁₁Ag₈₉ alloy NPs embedded in the active layer to promote the power conversion efficiency (PCE) up to 4.73%, which outperforms the reference device based on the control standard device of poly(3‐hexylthiophene) (P3HT):phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) under identical conditions. Corresponding increases in short‐circuit current density (J_sc), open‐circuit voltage (V_oc), fill factor (FF), and incident photon‐to‐current efficiency (IPCE) enable 31% PCE improvement due to the enhancement of the light‐trapping and the improvement of charge transport in the active layer. The findings advance the fundamental understanding and point to the superiority of Au₁₁Ag₈₉ nanoalloys as a promising metallic additive over Au, Ag, and Au₂₈Ag₇₂ alloy NPs to boost the solar cell performance.     

Effects of Photo‐oxidation on the Performance of Poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene]:[6,6]‐Phenyl C61‐Butyric Acid Methyl Ester Solar Cells

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, J_SC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in J_SC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.     

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (E_g = 3·6–3·8 eV) than the conventional buffer material CdS (E_g = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS₂ (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm² in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm² (total area: 22·5 mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm² substrates have been prepared and already reach an efficiency (active area: 17·2 cm²) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.     

A Versatile Universal Capacitor‐Grounded Voltage‐Mode Filter Using DVCCs

In this paper, a versatile three‐input five‐output universal capacitor‐grounded voltage‐mode filter is proposed. The circuit employs two differential voltage current conveyors as active elements together with two grounded capacitors and four resistors as passive elements. The proposed configuration can be used as either a single‐input five‐output or three‐input two‐output. Unlike the previously reported works, it can simultaneously realize five different generic filtering signals: lowpass, bandpass, highpass, bandreject, and allpass. It still maintains the following advantages: (i) the employment of all grounded capacitors, (ii) no need to employ inverting‐type input signals, (iii) no need to impose component choice, (iv) orthogonal control of the resonance angular frequency ω_o and the quality factor Q, and (v) low active and passive sensitivity performances.     

High-performance active bandpass filter using current-feedback amplifiers

An accurate current feedback amplifier based high-Q active bandpass filter whose centre frequency is independently variable over a wide frequency range is presented. The basic circuit has the important advantage that access to the Z node of the current-feedback amplifier is not required, as is the case with many existing current-feedback amplifier filter circuits. Additionally, the circuit can be implemented with operational amplifiers, though for high Q the Q? and ω₀? variabilities are 62% better with current-feedback amplifiers than with operational amplifiers. Experimental results are presented which are in good agreement with expected theory.     

Compact VDTA-based current-mode electronically tunable universal filters using grounded capacitors

This paper presents a compact current-mode three-input single-output (TISO) type universal filter. Only one voltage differencing transconductance amplifier (VDTA) and two grounded capacitors are employed in the proposed filter. The circuit can realize lowpass, bandpass, highpass, bandstop and allpass biquadratic filter outputs by connecting the appropriate inputs, and offers electronic control of the natural angular frequency (ω₀) and quality factor (Q) by means of adjusting the transconductance gain of the VDTA. In addition, by slight modification of the proposed scheme, another more useful TISO construction with orthogonal ω₀–Q tuning has been obtained. Both the discussed universal filters have been shown to have low incremental active and passive sensitivities. To demonstrate the performances of the filters and verify the theoretical analysis, computer simulations are accomplished with the PSPICE program.     

High‐efficiency µc‐Si solar cells made by very high‐frequency plasma‐enhanced chemical vapor deposition

Microcrystalline silicon‐based single‐junction p–i–n solar cells have been fabricated by very high‐frequency plasma enhanced chemical vapor deposition using a showerhead cathode at high pressures and under silane depletion conditions. The i‐layers are made near the transition from amorphous to crystalline. It was found that, especially at high crystalline fractions, the open‐circuit voltage and fill factor are very sensitive to the morphology of the substrate. At an i‐layer deposition rate 0·45 nm/s, we have measured a stabilised efficiency of 10% (V_oc = 0·52 V, FF = 0·74) for a cell made on texture‐etched ZnO:Al. The performance is stable under light soaking. The defect density of the absorber layer is in the 10¹⁵ cm⁻³ range. In spite of the presence of oxygen contamination, good electrical properties and good infrared cell response are obtained. Copyright © 2006 John Wiley & Sons, Ltd.     

High voltage photovoltaic mini‐modules

This work describes the design, simulation, fabrication process, and characterization of high voltage photovoltaic mini‐modules using silicon on insulator (SOI) wafers. The mini‐modules are made of a number of small area photovoltaic cells (<1 mm²) monolithically connected in series. Isolation between cells is performed by means of anisotropic etching of the active layer of the SOI wafer. Measurements using standard sunlight (AM1·5 100 mW/cm²) confirm the viability of this technology to fabricate small area arrays showing open circuit voltages, V _oc, between 620 mV and 660 mV and photocurrent densities up to 22·3 mA/cm² for single cells of 0·225 mm² area and 10 µm active film thickness. Series connection scales up V _oc and the maximum power, P _m, from 625 mV and 21·2 µW, respectively, in a single cell to 103 V and 3·2 mW when 169 cells are connected in series in a 0·42 cm² module total area. Copyright © 2008 John Wiley & Sons, Ltd.     

Thiocyanate‐Free Ru(II) Sensitizers with a 4,4′‐Dicarboxyvinyl‐2,2′‐bipyridine Anchor for Dye‐Sensitized Solar Cells

A new class of thiocyanate‐free Ru(II) sensitizers with 4,4′‐dicarboxyvinyl‐2,2′‐bipyridine anchor and two trans‐oriented pyrid‐2‐yl pyrazolate (or triazolate) functional chromophores is synthesized, characterized, and evaluated in dye‐sensitized solar cells (DSCs). Despite their enhanced red response and absorptivity when compared to the parent sensitizer TFRS‐2 that possesses standard 4,4′‐dicarboxyl‐2,2′‐bipyridine anchor and shows the best conversion efficiency of η = 9.82%, the newly synthesized carboxyvinyl‐pyrazolate sensitizers, TFRS‐11 – TFRS‐13 , exhibit inferior performance characteristics in terms of short‐circuit current density (J_SC), open‐circuit voltage (V_OC), and power conversion efficiency (η), the latter being recorded to be in the range 5.60–7.62%. The reduction in device efficiencies is attributed to a combination of poor packing of these sensitizers on the TiO₂ surface and less positive ground‐state oxidation potentials, which, respectively, increase charge recombination with I₃^? in electrolytes and impede the regeneration of sensitizers by I^? anions. The latter obstacle can be circumvented in part by the replacement of the pyrazolates with triazolates, forming the TFRS‐14 sensitizer, which exhibits an improved J_SC, V_OC, and η of 16.4 mAcm^?2, 0.77 V, and 9.02%, respectively.     

A Near‐Infrared cis‐Configured Squaraine Co‐Sensitizer for High‐Efficiency Dye‐Sensitized Solar Cells

A cis‐configured squaraine dye (HSQ1), synthesized by incorporation of a strongly electron‐withdrawing dicyanovinyl group into the central squaric acid moiety, is employed in dye‐sensitized solar cells (DSCs). In solution, HSQ1 displays an intense absorption in the near‐infrared region with a maximum at 686 nm and when the dye is adsorbed on a TiO₂ surface, the absorption spectrum broadens in both the blue and the near‐infrared regions, which is favorable for efficient light harvesting over a broad wavelength range. A solar cell sensitized with HSQ1 shows a broader incident photon‐to‐current conversion efficiency (IPCE) spectrum (from 400 to 800 nm) and a higher IPCE in the long‐wavelength region (71% at 700 nm) than a cell sensitized with squaraine dye SQ1. Furthermore, a solar cell co‐sensitized with HSQ1 and N3 dye shows remarkably improved short‐circuit current density and open‐circuit voltage compared to those of a DSC based on N3 alone and fabricated under the same conditions. The energy‐conversion efficiency of the co‐sensitized DSC is 8.14%, which is the highest reported efficiency for a squaraine dye–based co‐sensitized DSC without using Al₂O₃ layer.