Photo-generated Current Research Articles

Synthesis and characterization of TiO 2/Fe 2O 3 core–shell nanocomposition film and their photoelectrochemical property

TiO 2/Fe 2O 3 core–shell nanocomposition film has been fabricated via two-step method. TiO 2 nanorod arrays are synthesized by a facile hydrothermal method, and followed by Fe 2O 3 nanoparticles deposited on TiO 2 nanorod arrays through an ordinary chemical bath deposition. The phase structures, morphologies, particle size, chemical compositions of the composites have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and ultraviolet–visible (UV–vis) spectrophotometer. The results confirm that Fe 2O 3 nanoparticles of mean size ca. 10 nm coated on the surface of TiO2 NRs. After depositing Fe 2O 3, UV–vis absorption property is induces the shift to the visible-light range, the annealing temperature of 600 °C is the best condition for UV–vis absorption property of TiO 2/Fe 2O 3 nanocomposite film, and increasing Fe content, optical activity are enhanced one by one. The photoelectrochemical (PEC) performances of the as-prepared composite nanorods are determined by measuring the photo-generated currents under illumination of UV–vis light. The TiO 2 NRs modified by Fe 2O 3 show the photocurrent value of 1.36 mA/cm 2 at 0 V vs Ag/AgCl, which is higher than those of unmodified TiO 2 NRs.

Influence of ITO deposition and post annealing on HIT solar cell structures

Heterojunction silicon with intrinsic thin layer (HIT) solar cells that combine advanced thin-film hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) technologies are promising because of the high performance at low cost. Due to the low conductivity of a-Si:H, indium tin oxide (ITO) needs to be used as a front contact layer on top of a-Si:H in order to collect photogenerated currents. The thin a-Si:H layer requires the ITO deposition to be soft so that the passivation is maintained after deposition. Otherwise, the passivation degradation resulting from ITO deposition should be recovered by some post processing. In this contribution, we investigate how the power density and the temperature during ITO deposition as well as post annealing influence the passivation quality of HIT solar cells as characterised by the open-circuit voltage (Voc) and minority carrier lifetime. Firstly, ITO sputtering with lower power density can reduce the degradation of the passivation quality after ITO deposition. Secondly, we have investigated the simultaneous annealing during ITO deposition at elevated temperature. On one hand, simultaneous annealing can recover some of the degradation resulting from sputtering. On the other hand, there is a temperature threshold above which degradation of the passivation is observed, probably by hydrogen effusion. Thirdly, we observe that post annealing can fully recover the degradation resulting from ITO sputtering at room temperature (RT).

A fully differential transimpedance amplifier with integrated differential photodetector in standard CMOS process for optical communications and interconnects

A fully differential transimpedance amplifier (TIA) with integrated differential photodetector in standard CMOS technologies has been realized for optical receivers in optical communications and optical interconnects. And a novel, fully differential photodetector aiming to convert the incident light into a pair of fully differential photo-generated currents and ensure the differential symmetry on circuit configuration and model has also been proposed for the differential TIA to achieve the fully differential characteristic. Theoretical analysis and simulation results both indicate that the fully differential TIA reaches a higher bandwidth than a conventional one and at the same time a doubled sensitivity. Based on this new TIA, a monolithic, fully differential optoelectronic integrated receiver was designed and implemented in a Chartered 3.3 V, 0.35 μm standard CMOS process. It demonstrates 98.75 dBΩ transimpedance gain, and 0.334 μA equivalent input integrated referred noise current from 1 Hz up to −3 dB frequency. The power dissipation from a single 3.3 V supply is 100 mW for the TIA-LIA (limiting amplifier)-combination plus 138 mW for the 50 ω output buffer. At 850 nm wavelength, the optical receiver achieves a 1.1 GHz 3 dB bandwidth, and can handle bitrates up to 1.6 Gbit/s for −12.2 dBm peak-peak optical power and 231−1 PRBS (pseudorandom bit sequency) input signal.

Inhibiting corrosion of n-GaAs by cetylpyridinium chloride for stabilised photoelectochemical cell

The cationic surfactant cetylpyridinium chloride (CPC) provides photocorrosion inhibition to the GaAs surface when CPC is chemisorbed on the GaAs surface from aqueous acidic electrolytes at concentrations of CPC lower than its critical micelle concentration (CMC). In the presence of CPC, the anodic photogenerated current which is originally due to oxidising the n-GaAS electrode decreases, essentially, due to the adsorption of CPC on the n-GaAs surface. The presence of CPC in the electrolyte also shifts the onset potential of the photocurrent towards more negative values; this is also explained by adsorption of the surfactant molecule on GaAs surface, presumably, via its pyridine aromatic ring. In addition, CPC adsorption not only affects the electrochemical properties of the GaAs electrode but also substantially decreases the rate of photocorroion of GaAs. Photocorrosion is investigated by measuring the quantities of dissolved arsenic and gallium ions into the electrolyte by inductively coupled plasma mass spectroscopy. This dissolution results in an enriching the GaAS surface by gallium due to the referential dissolution of arsenic ions. The maximum inhibition efficiency due to the addition of CPC to the electrolyte is about 80%. Upon addition of cerous nitrate to the acidic solution of CPC the anodic photocurrent is stable and does not exhibit significant change over time though the rate of dissolving the electrode material stays low. It is concluded that in the presence of added Ce3+ the photocurrent is generated due to the oxidation of Ce+3 ions rather than the oxidation of the electrode material itself.

Comparative Study Between Various Antireflective Coatings For Solar Cells Applications

The incessant progress in thin layers technology, especially inhomogeneous materials with variable index allows the fabrication of antireflective coating (ARC) less sensitive to the thickness and the incidence angle. Accordingly, our work is devoted to study the optical behaviour of graded refractive index ARC made from: silicon oxynitrides (SiOxNy), mixture of silicon and titanium oxides (SiO2-TiO2) and porous silicon (P-Si), for solar applications. The refractive index of the studied ARC varies versus thickness according to Fermi profile variation law. The physical parameters of such coating are its: thickness, form factor, profile inflection point and silica or air (porosity) volume fraction variation at the two interfaces (air/ARC and ARC/substrate). The optimisation of the studied films performance shows that best results are obtained with P-Si ARC, which reduce weighted reflectance till to 1.86% (between 300-1100 nm) leading to an improvement in photogenerated current of 51%.

Holographically Defined TiO2 Electrodes for Dye-Sensitized Solar Cells

We describe a multibeam interference lithography for creating 3D polymeric porous structures. The coating of a TiO(2) shell and subsequent removal of the template produce holographically defined TiO(2) (h-TiO(2)) electrodes. We analyze the morphological features of the h-TiO(2) electrodes and consider their applicability to dye-sensitized solar cells (DSSCs). Specifically, the performance of the h-TiO(2) electrode was evaluated by comparison with a macroporous TiO(2) electrode produced from colloidal crystals. The h-TiO(2) structure possesses a larger specific area than the inverted colloidal crystals because of a bicontinuous air network with the TiO(2) shell. Consequently, the h-TiO(2) electrode can produce a 30% higher photogenerated electron current.

CuPc/C60 heterojunction thin film optoelectronic devices

The optoelectronic properties of heterojunction thin film devices with ITO/CuPc/C60/Al structure have been investigated by analyzing their current–voltage characteristics, optical absorption and photocurrent. In this organic photovoltaic device, CuPc acts as an optically active layer, C60 as an electron-transporting layer and ITO and Al as electrodes. It is observed that, under illumination, excitons are formed, which subsequently drift towards the interface with C60, where an internal electric field is present. The excitons that reach the interface are subsequently dissociated into free charge carriers due to the electric field present at the interface. The experimental results show that in this device the total current density is a function of injected carriers at the electrode–organic semiconductor surface, the leakage current through the organic layer and collected photogenerated current that results from the effective dissociation of excitons.

Fabrication of a novel silica PLC hybrid integrated triplexer

A new-style silica planar lightwave circuit (PLC) hybrid integrated triplexer, which can demultiplex 1490-nm download data and 1550-nm download analog signals, as well as transmit 1310-nm upload data, is presented. It combines SiO2 arrayed waveguide gratings (AWGs) with integrated photodetectors (PDs) and a high performance laser diode (LD). The SiO2 AWGs realize the three-wavelength coarse wavelength-division multiplexing (CWDM). The crosstalk is less than 40 dB between the 1490- and 1550-nm channels, and less than 45 dB between 1310- and 1490- or 1550-nm channels. For the static performances of the integrated triplexer, its upload output power is 0.4 mW, and the download output photo-generated current is 76 A. In the small-signal measurement, the upstream 3-dB bandwidth of the triplexer is 4 GHz, while the downstream 3-dB bandwidths of both the analog and digital sections reach 1.9 GHz.

A monolithic, standard CMOS, fully differential optical receiver with an integrated MSM photodetector

This paper presents a realization of a silicon-based standard CMOS, fully differential optoelectronic integrated receiver based on a metal–semiconductor–metal light detector (MSM photodetector). In the optical receiver, two MSM photodetectors are integrated to convert the incident light signal into a pair of fully differential photogenerated currents. The optoelectronic integrated receiver was designed and implemented in a chartered 0.35 μm, 3.3 V standard CMOS process. For 850 nm wavelength, it achieves a 1 GHz 3 dB bandwidth due to the MSM photodetector's low capacitance and high intrinsic bandwidth. In addition, it has a transimpedance gain of 98.75 dBΩ, and an equivalent input integrated referred noise current of 283 nA from 1 Hz up to –3 dB frequency.

Fabrication, characterization and photoelectrochemical properties of Fe 2O 3 modified TiO 2 nanotube arrays

TiO 2 nanotube (NT) arrays modified by Fe 2O 3 with high sensibility in the visible spectrum were first prepared by annealing anodic titania NTs pre-loaded with Fe(OH) 3 which was uniformly clung to the titania NTs using sequential chemical bath deposition (S-CBD). The photoelectrochemical performances of the as-prepared composite nanotubes were determined by measuring the photo-generated currents and voltages under illumination of UV–vis light. The titania NTs modified by Fe 2O 3 showed higher photopotential and photocurrent values than those of unmodified titania NTs. The enhanced photoelectrochemical behaviors can be attributed to the modified Fe 2O 3 which increases the probability of charge–carrier separation and extends the range of the TiO 2 photoresponse from ultraviolet (UV) to visible region due to the low band gap of 2.2 eV of Fe 2O 3.

Detection and study of photo-generated spin currents in nonmagnetic semiconductor materials

The longitudinal current in Si-doped gallium arsenide was spin-polarized using circularly polarized light. The spin current was detected by the extraordinary Hall effect. An enhancement of Hall conductivity with increasing moderately Si-doping was found, indicating that the introduction of dopants increases the electronic spin polarization. This finding may provide an opportunity for controlling and manipulating nonmagnetic semiconductors via electron spin for operating device applications. Band energy calculations using pseudopotentials confirm the influence of Si content and electron–phonon interaction on the behaviour of the spin current and hence on the spin-dependent Hall voltage.

Photoelectrochemical oxidation behavior of methanol on highly ordered TiO2 nanotube array electrodes

The highly ordered titanium dioxide nanotube array (HOTDNA) electrodes were prepared in hydrofluoric acid solution by electrochemical anodic oxidation technique on a pure titanium sheet. The HOTDNA electrodes were characterized by X-ray diffraction, SEM microscopy, and UV–vis spectra. It has shown high density, well ordered and uniform titanium dioxide nanotube array film covered on these electrodes and the TiO2 structure depending on the heating condition, the anatase phase of TiO2 appeared when heating to 500 °C. The photoelectrochemical characteristics of methanol in 0.5 M Na2SO4 on the HOTDNA electrodes were investigated. The cyclic voltammetry, photocurrent-time and open-circuit photopotential response of methanol on the HOTDNA electrode were represented and significant photogenerated current was observed upon illumination in the UV regions with the light of 253.7 nm central wavelength. The effect of variables such as light intensity, applied potential, and methanol concentration on the photoelectrochemical response was investigated. It was found that the photocurrent was greatly influenced by these factors.

Graded refraction index antireflection coatings based on silicon and titanium oxides

Thin films with a graded refraction index constituted from silicon and titanium oxides were deposited by plasma enhanced chemical vapor deposition using electron cyclotron resonance. A plasma of oxygen reacted with two precursors: the tetraethoxysilane (TEOS) and the titanium isopropoxide (TIPT). The automatic regulation of the precursor flows makes it possible to modify the chemical composition, and consequently the optical index, through the deposited films. To control the thickness, the refraction index and the growth kinetics, in situ spectroscopic ellipsometer was adapted to the reactor. The analysis of ex situ ellipsometric spectra measured at the end of each deposition allow to determine a refraction index profile and optical properties of the inhomogeneous deposited films. Measurements of reflectivity carried out in the ultraviolet-visible-near infrared range show that these films could be used as antireflective coatings for silicon solar cells: 3.7 % weighted average reflectivity between 300 and 1100 nm and 48 % improvement of the photo-generated current were obtained.

Electrical and photovoltaic properties of devices based on PbPc–TiO 2 thin films

The electrical, optical and photovoltaic properties of organic–inorganic hybrid devices consisting of Al/TiO 2/PbPc/ITO and Al/PbPc/TiO 2/ITO structures have been investigated through analyzing the current–voltage characteristics, optical absorption and photocurrent action spectra of the devices. The combined presence of oxygen, light and an electric field in the photocurrent decay of Al/TiO 2/PbPc/ITO device have been studied. It is observed that under illumination, the oxygen radical anions and excitons are formed, which subsequently drift towards the interface with TiO 2, where an internal electric field is present. The excitons that reach to the interface are subsequently dissociated into free charge carriers due to the electric field present at the interface. The exciton diffusion length for PbPc calculated from the dependence of luminescence with the PbPc film is about 13 nm. We have also studied the effect of PbPc thickness and hole mobility on the device performance of organic photovoltaic device consisting of PbPc as an optically active layer, TiO 2 as the electron–transporting layer and ITO and Al used as electrodes. We have shown that the power conversion efficiency in the device is primarily limited by the short-exciton diffusion length combined with the low-hole mobility in PbPc layer. The model of charge transport in Al/TiO 2/PbPc/ITO device explained the experimental results where the total current density is a function of injected carriers at electrode–organic semiconductor surface, the leakage current through the organic layer and collected photogenerated current that results from the effective dissociation of excitons.

Unintentional calcium incorporation in Ga(Al, In, N)As

Unintentional calcium incorporation into GaInNAs causes an acceptor-type impurity, which limits the ability of ∼1eV GaInNAs-based solar cells to collect photogenerated current. Here, the authors focus on better understanding the conditions by which Ca is incorporated into GaInNAs. Various material combinations were grown including GaAs, InGaAs, GaInNAs, and Al(Ga)As. The materials were primarily grown by solid-source molecular-beam epitaxy (MBE) at ∼400 and 580–620°C, with comparisons made to metal-organic chemical vapor deposition (MOCVD)-grown materials where appropriate. Calcium incorporation was measured through secondary ion mass spectrometry. There was no measurable Ca incorporation into MBE-grown GaAs at 580°C, but Ca incorporates into GaAs at low MBE growth temperatures (∼400°C) that are comparable to those typically used for GaInNAs. This suggests that the N species is not solely responsible for the observed Ca incorporation into MBE-grown GaInNAs; but rather, defects associated with the low temperature growth may also be a factor. The effects of defect-related Ca incorporation were further studied at interfaces between Al(Ga)As and GaAs, and substrates pretreated with an O2 plasma and NH4OH etch. Ca incorporation was observed at the interface between Al0.3Ga0.7As∕GaAs, AlAs∕GaAs, and at the epilayer/substrate interface for the pretreated samples. No Ca was observed in InGaAs samples grown by either MBE or MOCVD, suggesting the Ca incorporation into GaInNAs has something to do with the addition of nitrogen. Therefore, the authors propose that the likely causes of Ca incorporation into GaInNAs are a Ca–N affinity, defects generated by low temperature growth, defects generated by the incorporation of N into GaAs, or some combination thereof.

Near-field thermophotovoltaic energy conversion

We report a quantitative model of a near-field thermophotovoltaic (TPV) device consisting in a thermal source located in the near field of a TPV cell. The enhanced radiative transfer at short distance leads to an increase of the photogeneration current. We analyze quantitatively other potential near-field effects, in particular, on the dark current. We also study the influence of the modification of the spectrum of the sources in the near field, comparing the case of a tungsten source with the case of a quasimonochromatic source. Our model leads to a quantitative evaluation of the near-field TPV device output electric power and efficiency.

The collection probability and spectral response in isotype heterolayers of tandem solar cells

An analytical model for the position-dependent collection probability in uniformly doped one-dimensional layers with abrupt compositional and bandgap changes is presented. The collection probability is derived from coupled system of diffusion equations for low-level injection of photo-generated minority carriers in a stack of isotype heterolayers. Collection probability maintained continuity across isotype heterojunctions despite discontinuities of excess minority carrier concentration. An estimate for window and BSF passivation showed that the effective surface recombination velocity exponentially depended on the energygap difference, and linearly depended on increased doping, reduced mobility and increased thickness of sub-diffusion length passivation layers. Analytical expressions for the photo-generated current, and the internal quantum efficiency from each heterolayer were developed, and applied to the analysis of reported spectral response of a dual junction GaInP/GaAs tandem solar cell. Calculated internal quantum efficiencies closely matched reported experimental results, with the exception of sub-band absorption due to sub-bandgap deficiencies in the optical models and photon recycling. Calculated spectral response showed that upper AlInP2 window, quasi-neutral emitter, and SCR layers dominated collection of photo-generated carriers in the top GaInP2 cell, whereas, the base dominated collection of photo-generated carriers in the bottom GaAs cell. Results show that augmenting Hovel’s three layers (emitter, SCR, and base) analysis with the response from the top window layer should be sufficient to capture the spectral response of solar cells with thin passivation layers.

Ultraviolet photoconductive detector based on Al doped ZnO films prepared by sol–gel method

We report a study on the fabrication and characterization of ultraviolet photodetectors based on ZnO:Al films. Using sol–gel technique, highly c-axis oriented ZnO films with 5mol% Al doping were deposited on Si(111) substrates. The photoconductive UV detectors based on ZnO:Al thin films, having a metal–semiconductor–metal (MSM) structure with interdigital (IDT) configuration, were fabricated by using Au as a contact metal. The characteristics of dark and photocurrent of the UV detector and the UV photoresponse of the detector were investigated. The linear current–voltage (I–V) characteristics under both forward and reverse bias exhibit ohmic metal–semiconductor contacts. Under illumination using monochromatic light with a wavelength of 350nm, photo-generated current was measured at 58.05μA at a bias of 6V. The detector exhibits an evident wide-range spectral responsivity and shows a trend similar to that in transmittance and photoluminescence spectrum.

A hybrid drift-diffusion-TLM analysis of traveling-wave photodetectors

An efficient hybrid method for numerical analysis of traveling-wave photodetectors (TWPDs) is presented. The method is a combination of two-dimensional semiconductor and three-dimensional full-wave electromagnetic (EM) simulators. The time-domain drift-diffusion method is used to determine the photogenerated currents at the cross section of the device. Those currents are then defined as sources in the EM analysis to calculate the microwave bandwidth and output current of the device. In the analysis, the effects of the carrier velocity and lifetime, optical power, bias voltage, velocity mismatch, and microwave loss are taken into account. The method is tested in case of GaAs and low-temperature-grown GaAs-based TWPDs. A very good agreement between the theoretical and experimental data is observed. It was also possible to predict the saturation behavior of the TWPD under high optical illumination due to the field screening effect.

Investigation of charge transport, photo generated electron transfer and photovoltaic response of iron phthalocyanine (FePc):TiO2 thin films

Abstract We have investigated the blend of semiconducting materials (iron(II) phthalocyanine (FePc)) with TiO 2 nano crystals (NC) for photovoltaic application. The photo-induced electron transfer in composite of TiO 2 with FePc is studied by means of photoluminescence (PL) of the blend as well as photo-action spectra of the device. The quenching of the PL and occurrence of broad absorption have been interpreted as an electron transfer between an organic semiconductor and TiO 2 (NC) in excited state. Also, the effects of hole mobility and light intensity on the device properties have been analyzed. The observation of an increase in photocurrent of the device (consisting of TiO 2 ) is interpreted by the formation of an interconnecting network of FePc with TiO 2 nano particle and formation of bulk hetero junction between aluminum (Al) and indium tin oxide (ITO) electrodes. The electron transfer between FePc and TiO 2 is induced by photo excitation. The experimental results were explained with the help of a model that describes total current density as a function of injected carriers as donor/acceptor interface and contributed photo-generated current is due to the efficient dissociation of excitons.