Function Of Illumination Intensity Research Articles

Carrier Lifetime Degradation and Regeneration in Gallium- and Boron-Doped Monocrystalline Silicon Materials

In this article, carrier lifetime degradation phenomena on fired gallium-doped Czochralski-grown silicon (Cz-Si:Ga) and boron-doped float-zone silicon (FZ-Si:B) are observed. We examine lifetime degradation and regeneration as a function of illumination intensity and temperature and observe qualitatively similar degradation effects in both material classes, which are triggered by a fast-firing high-temperature step. Charge carrier injection, e.g., through illumination, is required to activate the defects responsible for degradation. The extent of degradation increases with increasing temperature, which is untypical for degradation effects reported before. Despite different degradation time constants are measured for Cz-Si:Ga and FZ-Si:B, the activation energies are for both materials in the narrow range (0.58 ±0.04) <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eV</b> . The extracted activation energy is quite different compared with other degradation effects in silicon, suggesting a novel defect formation mechanism. Since the lifetime degradation is triggered by the fast-firing of the silicon wafers during the presence of a hydrogen-rich dielectric at the surface, the involvement of hydrogen in the defect reaction is very likely. During prolonged illumination at elevated temperature (135 °C), we observe a permanent regeneration of the lifetime, whereas at temperatures close to room temperature (36 °C), the defect deactivation is only temporary.

Determination of defect states and surface photovoltage in PTB7:PC71BM based bulk heterojunction solar cells

Defects formed during the device processing acts as recombination centers and shows adverse effects on the photovoltaic performance of the organic solar cells (OSCs). In this manuscript, a study on the effect of defect states on the characteristics of bulk heterojunction (BHJ) solar cells, consisted of unannealed or thermally annealed active layer (ActL) of PTB7:PC71BM blend, have been performed by using the illumination intensity-dependent current-voltage and frequency dependent impedance spectroscopy measurements. Our results indicate that the photovoltaic performance of devices based on annealed ActL is superior than the bench dried ActL due to thermal annealing induced improvement in the thin film morphology. The measured low-frequency capacitance-voltage (C–V) characteristics under varying illumination intensities show strong dependency on the photoexcitation however, the change in the C–V characteristics is found to be quite different for unannealed and annealed devices. The C–V characteristics are analyzed using the drift-diffusion model to extract the effective built-in voltage (Vbi) and the surface photovoltage as a function of illumination intensity. The capacitance-frequency characteristics under illumination reveal that the change in the capacitance is associated with the photo-induced carrier occupation in the intermixture donor-acceptor phases, which act as the defect states in such devices. Further, it is modeled to quantify the defect states and to demonstrate the importance of thermal annealing for improving the OSCs device performance.

On thermal and optical sensor applications of chitosan molecule in the Co/Chitosan/p-Si hybrid heterojunction design

Co/Chitosan/p-Si/Al and Co/p-Si devices were made and barrier height and ideality factor values of both devices were determined at room temperature (dark conditions). After the chitosan interlayer was found to improve the device, the thermal and photosensivity properties of the Co/Chitosan/p-Si/Al device were investigated in detail. The current–voltage measurements of Co/Chitosan/p-Si/Al heterojunction were performed in temperature range of 100–460 K and illumination intensity range of 100–400 mW/cm2. It was seen that the device parameters such as barrier height, ideality factor, series resistance and interface states density were strongly dependent on temperature. Furthermore, the capacitance–voltage (C–V) characteristics of the device were investigated at various frequencies at room temperature. The responsivity (R), detectivity (D*) and rectification ratio (RR) values of the Co/Chitosan/p-Si/Al heterojunction were found to be as a function of illumination intensity. The experimental results show that the fabricated device with chitosan thin film interlayer can be used in various optoelectronic applications, especially applications in low temperature switching devices, energy generation, photodiode and photodetector.

Study of the open circuit voltage dependence on incident light intensity of planar heterojunction organic solar cell

Organic photovoltaic is an emerging technology employing organic molecules as absorbers has gained tremendous attention in the past two decades due to its potential utilization as a renewable energy source. Contrary to conventional solar cells, excitons are formed under illumination which diffuse towards the heterojunction interface for dissociation into the photo charge-carriers. Heterojunction in the organic photovoltaic devices are formed either by stacking the layers of donor and acceptor molecules or by blending the both. Due to weak van-der Waals interaction between the molecules, intermixed states, in addition to the pure states of donor and acceptor molecules, are formed at the heterojunction interface which affects the electronic process and the power conversion efficiency. Significant efforts have been made to understand the photovoltaic process particularly in the bulk heterojunction solar cells however, least efforts have been devoted to investigate the governing mechanisms in the planner heterojunction solar cells. The motive of this study is to characterize the planar heterojunction solar cells based on the Copper Phthalocyanine (CuPc) and Fullerene (C60) molecules under light illumination. The devices were fabricated under controlled condition in the normal device configuration, ITO (150 nm) |PEDOT:PSS(50 nm)|CuPc(20 nm)|C60(40 nm)|BCP(10 nm)| Ag(80 nm), and characterized performing the light intensity dependent measurement in the wide illumination range. The solar cell parameters such as open circuit voltage, short circuit current and the power conversion efficiency have been discussed as a function of illumination intensity in the wider illumination range.

Photothermal Transduction Efficiencies of Plasmonic Group 4 Metal Nitride Nanocrystals.

The photothermal transduction efficiencies of group 4 metal nitrides, TiN, ZrN, and HfN, at λ = 850 nm are reported, and the performance of these materials is compared to an Au nanorod benchmark. Transition metal nitride nanocrystals with an average diameter of ∼15 nm were prepared using a solid-state metathesis reaction. HfN exhibited the highest photothermal transduction efficiency of 65%, followed by ZrN (58%) and TiN (49%), which were all higher than those of the commercially purchased Au nanorods (43%). Computational studies performed using a finite element method showed HfN and Au to have the lowest and highest scattering cross section, respectively, which could be a contributing factor to the efficiency trends observed. Furthermore, the changes in temperature as a function of illumination intensity and solution concentration, as well as the cycling stability of the metal nitride solutions, were studied in detail.

Approaching new photo-electrics: CdTe nano-crystallite thin film

Cadmium telluride (CdTe) is immensely interesting narrow band gap semiconductor with high electrical conductivity having promising applications in new-generation electronics and photo-electronic devices. In the present study, CdTe thin film was fabricated by thermal evaporation technique onto pre-cleaned glass substrate. The EDAX result indicates that the prepared thin film has non-stoichiometric nature. The SEM image revealed that the film is continuous and free form defects. The optical band gap is about 1.53 eV evaluated using absorption spectra. Using photo-response characterization, the responsivity (R*), specific detectivity (D), internal quantum efficiency (IQE) and external quantum efficiency (EQE) are determined as a function of illumination intensity. The effect of cycle numbers and applied voltage on the growth and decay parameters such as dispersion parameters (α), characteristic temperature (To) and localized state distribution parameter (Eo) are determined. The trap depth parameters of CdTe thin film is explicated on the basis of defect controlled photoconductivity mechanism. These results confirm that the CdTe thin film is a capable contender for photo-detection applications.

Absolute photoluminescence intensity in thin film solar cells

A calculation is presented for the direct conversion of a measured luminescence signal to the implied open circuit voltage. The effects of re-absorption, spectral dependence, and interference with front and back interfaces are all included, so long as the optical properties of the structure are known. The method is validated through a comparison of the terminal open-circuit voltage with the photoluminescence intensity of Cu(In,Ga)Se2 solar cells, each as a function of illumination intensity.

Enhanced photodesorption from near- and mid-infrared plasmonic nanocrystal thin films

The authors show that the desorption rate of two model molecules, indole and benzoic acid, from thin films of indium tin oxide nanocrystals supporting near- and mid-infrared (0.33–0.48 eV) localized surface plasmon resonances (LSPRs) is enhanced by as much as 60% upon illumination with broadband infrared light. The desorption rate increases linearly with light intensity. No increase in the desorption rate is detected for undoped In2O3 nanocrystal thin films or when photons resonant with the LSPR are blocked. The authors study the desorption rate enhancement as a function of illumination intensity, LSPR energy, and isotopic substitution. Importantly, the authors demonstrate the accelerated desorption via in-coupling of light to LSPRs with energies well within the mid-infrared. Their work opens the door to using these low energy photons as choreographers of chemical processes and sets the stage for future mechanistic studies.

Investigation of photovoltaic effect on electric and dielectric properties of Au/n-Si Schottky barrier diodes with nickel (Ni)–zinc (Zn) doped organic interface layer

Photovoltaic effects were tracked on both electric and dielectric properties of Au/(Ni, Zn)-doped polyvinyl alcohol/n-Si Schottky barrier diodes as function of illumination intensity by 50 W steps at 1 MHz and in the voltage interval of (− 4)–(+ 5) V. The measurements indicate that ac electrical conductivity (σ ac ), dielectric constant’s both real and imaginary parts (e′, e″), loss tangent (tanδ) and electric modulus (M′, M″) are highly relevant functions of illumination and voltage. The variations in depletion region can be ascribed to the charges at interface and its reordering and restructuring under illumination and electric field but then accumulation region variations can be ascribed to the interfacial layer and series resistance (R s ). The values of eʹʹ and tanδ show a step increase with the increasing voltage for each illumination intensity while the values of e′ show an anomalous peak (~ 1.4 V). C–V plot shows an intersection behavior at about 2.2 V due to lack of enough free charges in low illumination. The values of σ ac increase with increasing illumination and voltage due to the formation electron–hole pairs. The M″ vs V have two peaks for each illumination intensity and peak value increases with increasing illumination intensity and its positions tend to shift towards low voltage region.

Illumination intensity dependence of the performance of interdigitated back contact heterojunction silicon solar cells and the effects of front surface field

The performance of interdigitated back contact silicon heterojunction solar cells as a function of illumination intensity has been studied by two-dimensional simulation. A doped front surface field layer is effective to suppress the reduction of conversion efficiency at the illumination below 0.01 sun even with good front passivation and at 1 sun with poor passivation. The experimental devices showed the almost consistent results. For the illuminations above approximately 1 sun, the fill factor and efficiency decreased with illumination intensity, likely due to the increase in series resistance with the increase of “hole” current passing through the high potential barrier at the p-type heterojunction.

Kinetics of the permanent deactivation of the boron-oxygen complex in crystalline silicon as a function of illumination intensity

Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.

Photovoltaic properties of the 4H-pyrano[3,2-c]quinoline derivatives and their applications in organic–inorganic photodiode fabrication

‎2-Amino-6-ethyl-5-oxo-4-(3-Ph)-5, 6-dihydro-4H-pyrano[3,2-‎c]quinoline-3-carbonitrile (Ph-HPQ) and 2-Amino-4-(2-Cl)-6-ethyl-5-‎oxo-5,6-dihydro-4H-pyrano[3,2-c]quinoline-3-carbonitrile (Ch-HPQ) ‎films are deposited by using thermal evaporation technique. ‎ The absorbance of quinoline derivatives and the energy band diagrams of Ph-HPQ/p-Si and Ch-HPQ/p-Si devices are illustrated. The electrical properties of Au/Ph-HPQ/p-Si/Al and Au/Ch-HPQ/p-Si/Al heterojunction diodes are studied in terms of current–voltage characteristics. The two devices are fabricated with the same specifications. They showed a rectification behavior and photovoltaic properties in dark and under illumination conditions. The conduction mechanisms and diode parameters as a function of temperature of these devices are determined. The Schottky barrier height and the field lowering coefficients are also determined as a function of temperature. The photoconductivity sensitivity, short circuit current, open circuit voltage and the ratio between photocurrent to dark current are found to be a function of illumination intensity. The two devices satisfy the conditions to be used as photodiodes. The presence of the chlorophenyl as a substitution group improved the diode parameters.

Photovoltaic and Amplified Spontaneous Emission Studies of High‐Quality Formamidinium Lead Bromide Perovskite Films

This study demonstrates the formation of extremely smooth and uniform formamidinium lead bromide (CH(NH2)2PbBr3 = FAPbBr3) films using an optimum mixture of dimethyl sulfoxide and N,N‐dimethylformamide solvents. Surface morphology and phase purity of the FAPbBr3 films are thoroughly examined by field emission scanning electron microscopy and powder X‐ray diffraction, respectively. To unravel the photophysical properties of these films, systematic investigation based on time‐integrated and time‐dependent photoluminescence studies are carried out which, respectively, bring out relatively lower nonradiative recombination rates and long lasting photogenerated charge carriers in FAPbBr3 perovskite films. The devices based on FTO/TiO2/FAPbBr3/spiro‐OMeTAD/Au show highly reproducible open‐circuit voltage (Voc) of 1.42 V, a record for FAPbBr3‐based perovskite solar cells. Voc as a function of illumination intensity indicates that the contacts are very selective and higher Voc values are expected to be achieved when the quality of the FAPbBr3 film is further improved. Overall, the devices based on these films reveal appreciable power conversion efficiency of 7% under standard illumination conditions with negligible hysteresis. Finally, the amplified spontaneous emission (ASE) behavior explored in a cavity‐free configuration for FAPbBr3 perovskite films shows a sharp ASE threshold at a fluence of 190 μJ cm−2 with high quantum efficiency further confirming the high quality of the films.

Illumination and Voltage Dependence of Electrical Characteristics of Au/0.03 Graphene-Doped PVA/n-Si Structures via Capacitance/Conductance-Voltage Measurements

Au/n-Si (MS) structures with a high dielectric interlayer (0.03 graphene-doped PVA) are fabricated to investigate the illumination and voltage effects on electrical and dielectric properties by using capacitance-voltage (C-V) and conductance-voltage (G/ω-V) measurements at room temperature and at 1 MHz. Some of the main electrical parameters such as concentration of doping atoms (ND), barrier height (ϕB(C - V)), depletion layer width (WD) and series resistance (Rs) show fairly large illumination dispersion. The voltage-dependent profile of surface states (Nss) and resistance of the structure (Ri) are also obtained by using the dark-illumination capacitance (Cdark-Cill) and Nicollian-Brews methods, respectively. For a clear observation of changes in electrical parameters with illumination, the values of ND, WD, ϕB(C - V) and Rs are drawn as a function of illumination intensity. The values of ND and WD change almost linearly with illumination intensity. On the other hand, Rs decreases almost exponentially with increasing illumination intensity whereas ϕB(C - V) increases. The experimental results suggest that the use of a high dielectric interlayer (0.03 graphene-doped PVA) considerably passivates or reduces the magnitude of the surface states. The large change or dispersion in main electrical parameters can be attributed to generation of electron-hole pairs in the junction under illumination and to a good light absorption. All of these experimental results confirm that the fabricated Au/0.03 graphene-doped PVA/n-Si structure can be used as a photodiode or a capacitor in optoelectronic applications.

Raman Vibrations, Domain Structures, and Photovoltaic Effects in A‐Site La‐Modified BiFeO3Multiferroic Ceramics

Micro‐Raman spectroscopy, X‐ray diffraction, high‐resolution transmission electron microscopy (TEM), oxygen vacancies, synchrotron X‐ray absorption spectroscopy, magnetizations, optical band gaps, and photovoltaic (PV) effects have been studied in (Bi1−xLax)FeO3(BFO100xL) ceramics forx= 0.0, 0.05, 0.10, and 0.15.XRD, Raman spectra, andTEMconfirm a rhombohedralR3csymmetry with the tilted FeO6oxygen octahedra in all compounds. The low‐frequency Raman vibrations become broader and shift toward higher frequency as La3+increases. Fe K‐edge synchrotron X‐ray absorptions reveal that Fe3+valence and Fe–O–Fe bond angle are not modified by the La3+substitution. All compounds exhibit a linear antiferromagnetic feature. Optical transmission reveals band gaps in the range of 2.22–2.24 eV. The heterostructures of indium tin oxide (ITO) film/(Bi1−xLax)FeO3ceramics/Au film show ap–njunction‐likeI–Vcharacteristic behavior. The maximalPVpower conversion efficiency can reach 0.19% inITO/BFO15L/Au under illumination of λ = 405 nm. A junction‐like theoretical model can reasonably describe open‐circuit voltage and short‐circuit current as a function of illumination intensity.

Raman vibrations and photovoltaic conversion in rare earth doped (Bi0.93RE0.07)FeO3 (RE=Dy, Gd, Eu, Sm) ceramics

High-resolution Raman spectra, X-ray diffraction, oxygen vacancies, synchrotron X-ray absorption spectroscopy, magnetization, optical band gap, and photovoltaic (PV) conversion have been studied in BiFeO3 (BFO) and (Bi0.93RE0.07)FeO3 (RE=Dy, Gd, Eu and Sm) multiferroic ceramics (7%Dy–BFO, 7%Gd–BFO, 7%Eu–BFO, and 7%Sm–BFO). 7%Dy–BFO exhibits a weak ferromagnetic behavior instead of the linear antiferromagnetic responses found in the other compounds. Optical transmissions reveal band gaps of 2.20–2.21eV, which are slightly smaller than 2.24eV in pure BFO. The current vs. voltage (I–V) characteristic curves of indium tin oxide (ITO)/(Bi0.93RE0.07)FeO3 ceramics/Au heterostructures suggest a p–n-junction-like behavior. The maximal PV power-conversion efficiencies under illumination of λ=405nm in ITO/7%Dy–BFO/Au, ITO/7%Gd–BFO/Au, ITO/7%Eu–BFO/Au, and ITO/7%Sm–BFO/Au respectively reach 0.22%, 0.35%, 0.27%, and 0.24%, which are much larger than 0.017% in ITO/BFO/Au. The PV open-circuit voltage and short-circuit current can be reasonably described by a junction model as a function of illumination intensity.

A-site strontium doping effects on structure, magnetic, and photovoltaic properties of (Bi1−xSrx)FeO3−δ multiferroic ceramics

Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi1−xSrx)FeO3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr2+ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of FeOFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi1−xSrx)FeO3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi1−xSrx)FeO3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.

Optical properties and device characteristics of 2-(antipyrin-4-ylhydrazono)-2-(4-nitrophenyl)acetonitrile thin films for photodiode applications

2-(Antipyrin-4-ylhydrazono)-2-(4-nitrophenyl)acetonitrile (AHNA) films were deposited via thermal evaporation technique. The optical properties of AHNA films and electrical characteristics of Au/AHNA/n-Si/Au heterojunction diode have been reported. The optical properties of AHNA films were investigated using the spectrophotometric measurements of optical transmittance and reflectance over spectral range 190–2500nm. The films have indirect allowed optical band gap of 3.6eV. The refractive index of the films was calculated and the dispersion parameters of the films were determined on the light of the single oscillator model. The electrical properties of Au/AHNA/n-Si/Au heterojunction diode were studied in terms of current–voltage characteristics. The device showed rectification behaviour with a rectification ratio of 100 at ±1V. The conduction mechanisms and diode parameters such as ideality factor, barrier height and series resistance of the device were determined. The device under illumination showed photovoltaic properties. The short circuit current and open circuit voltage were found to be function of illumination intensity. The device satisfies the conditions to be used as photodiode.

NIR Detector Nonlinearity and Quantum Efficiency

A study was performed to investigate the experimental conditions and systematic uncertainties that need to be considered in order to precisely characterize quantum efficiency (QE). Measurements were performed on a HAWAII-2RG1.7 μm detector but the methodology of characterization is applicable to other detectors as well and may be useful in characterization of detectors used in future ground and space based surveys. For this study the detector QE as a function of illumination intensity, total integrated signal, and temperature was measured. A 3% relative systematic uncertainty on the measured QE value was achieved at wavelengths longer than 800 nm but the total uncertainty in the determination of absolute QE is dominated by the uncertainty in the conversion gain, which adds an additional 3.4% scale uncertainty. It was found that the measured detector QE depends on illumination intensity and that temperature dependence of QE can, at least in part, be attributed to reciprocity failure. Well-chosen detector bias voltages can reduce integrated signal nonlinearity.

Calcium-doping effects on photovoltaic response and structure in multiferroic BiFeO3 ceramics

Photovoltaic (PV) effects, power-conversion efficiencies, and structures have been systematically measured in (Bi1−xCax)FeO3−δ ceramics for x = 0.05, 0.10, and 0.15. The heterostructures of indium tin oxide (ITO) film/(Bi1−xCax)FeO3−δ ceramics/Au film exhibit significant PV effects under illumination of λ = 405 nm. The maximum power-conversion efficiency in the ITO/(Bi0.90Ca0.10)FeO2.95 (BFO10C)/Au can reach 0.0072%, which is larger than 0.0025% observed in the graphene/polycrystalline BFO/Pt films [Zang et al., Appl. Phys. Lett. 99, 132904 (2011)]. A theoretical model based on optically excited current in the depletion region between ITO film and Ca-doped BFO ceramics is used to describe the I-V characteristic, open-circuit voltage, and short-circuit current density as a function of illumination intensity. This work suggests that the Ca-substitution can reduce the rhombohedral distortion and stabilize the single-phase structure.