Quantum Efficiency Data Research Articles

Flexible transparent conductors with a percolated Ag nanostructure and its application as efficient self-bias plasmonic photodetector

A percolated silver (Ag) nanostructured-based transparent conductor has been deposited by physical vapor deposition (PVD) technique where lateral growth of Ag has been enhanced by a pre-deposited Ag-TiO2 thin film. This Ag-TiO2 film has embedded Ag nanoparticles (NPs) within TiO2 thin film which is grown in a low temperature (100 °C) solution processed technique. This process includes Li4Ti5O12 (LTO) thin film deposition by a sol–gel method followed by ion-exchange (Li+ → Ag+) process to yield an Ag-TiO2 thin film. The percolated Ag network has appeared as soon film mass-thickness reaches close to 10 nm, resulting in an abrupt drop of electrical resistivity of the film. This percolated Ag nanostructured thin film (10 nm Ag/Ag-TiO2/plastic) has resistivity of ∼50 Ω/□ and an average visual transmittance of >70 % up to 450 nm. In higher wavelength range, transparency gradually reduces, and it reaches to ∼50 % at 600 nm which is mostly due to the plasmon absorption of this film. By utilizing its combined optical transparency and surface plasmon absorption, this film has been used to develop plasmonic hot electron photodetectors where Ag-thin film works as transparent electrode as well as plasmon induced photo-excited hot electron generation. Device has been fabricated on a heavily doped n type Si (n++-Si) with a metal–semiconductor-metal (M−S−M) device geometry. External quantum efficiency (EQE) data reveal that photocurrent of this device is mostly generated in the plasmonic absorption region with a peak detectivity of 2.84 × 1012 Jones at 510 nm under −3V external bias. Besides, the device shows fast response with a response time of ∼25 ms.

Surface Passivation by Quantum Exclusion: On the Quantum Efficiency and Stability of Delta-Doped CCDs and CMOS Image Sensors in Space.

Radiation-induced damage and instabilities in back-illuminated silicon detectors have proved to be challenging in multiple NASA and commercial applications. In this paper, we develop a model of detector quantum efficiency (QE) as a function of Si-SiO2 interface and oxide trap densities to analyze the performance of silicon detectors and explore the requirements for stable, radiation-hardened surface passivation. By analyzing QE data acquired before, during, and after, exposure to damaging UV radiation, we explore the physical and chemical mechanisms underlying UV-induced surface damage, variable surface charge, QE, and stability in ion-implanted and delta-doped detectors. Delta-doped CCD and CMOS image sensors are shown to be uniquely hardened against surface damage caused by ionizing radiation, enabling the stability and photometric accuracy required by NASA for exoplanet science and time domain astronomy.

Radiative recombination of a high internal-quantum-efficiency 268 nm ultraviolet C-band light emitting diode

After assigning a thickness d to the carrier recombination region of a light emitting diode (LED), we show that the ABC model involving Shockley–Read–Hall non-radiative, radiative, and Auger recombination coefficients, i.e., A, B, and C, respectively, can bring new insight into the radiative recombination process. In order to fit external quantum efficiency (EQE) data of ultraviolet C-band (UVC) as well as blue LEDs, the ABC model requires the product d·B to be invariant of the injection current. This can be understood that as the thickness of the recombination region increases the radiative recombination coefficient decreases due to reduced electron–hole wavefunction overlaps. For an LED with high internal quantum efficiency (IQE), its quality factor Q (Q=BAC) usually undergoes a noticeable drop as the injection current increases to pass the current of maximal EQE. This is due to an increase in the thickness of the recombination region and, hence, a reduction in the radiative recombination coefficient as the injected carriers start to drift or diffuse to involve more quantum wells for light emission. Applying this ABC model, we analyze a high-efficiency 268 nm UVC LED, which delivers ∼199 mW optical power under a direct current of 350 mA and obtains a maximal IQE of ∼86.4% and an effective light extraction efficiency of ∼15.3%.

Numerical and Experimental Study of the Front Surface Recombination Velocities and Base Widths Effect in Multi-Crystalline Silicon Solar Cell Quantum Efficiency

Photovoltaic research activities are related to material innovation that can be obtained at a comparatively low cost. Semiconductor p-type multi-crystalline Czochralskyc (CZ)-grown silicon wafers were used in this study. The effects of front surface recombination velocities and base thickness in solar cells’ quantum efficiency are theoretically calculated. The results denote that both the surface recombination velocities and the base widths significantly impact the quantum efficiency. The results are of universal technical importance in designing solar cells and their surface structures. The main goal of this paper was to confirm the validity of the above theoretical calculations; for this purpose, silicon solar cells with front-thin porous silicon and rear interdigitated contact have been produced. A good agreement was obtained between experimentally obtained solar cells’ quantum efficiency data and the theoretical results. Therefore, the quantum efficiency of the mc-Si solar cells with porous silicon and rear interdigitated contact was enhanced up to 25% at 580–1100 nm wavelength range and up to 50% at short wavelength (400–570 nm), compared to reference mc-Si solar cells. The obtained results indicate that the rear interdigitated contact maximizes the surface area of the metal contact and improves the current collection. At the same time, the porous silicon layer passivates the front surface and reduces recombination losses.

Improved modeling of the effect of sulfur on optical and electrical properties in a calibrated simulation model of a CIGSSe solar module

A 1D single-cell simulation model for a laminated Ga-rich cm2 n-ZnO/ZnOS/Cu(In,Ga)(S,Se)2/Mo(Se,S)2-based solar module with a power conversion efficiency of close to 19% is developed and calibrated on its current–voltage (IV), capacitance–voltage (CV), external quantum efficiency(EQE) and measurement data. A careful discussion of the simulation parameters is presented, from which three main results are found: (a) a discrepancy between the band gaps calculated from glow discharge optical emission spectroscopy (GDOES) and EQE measurement data is ascribed to the segregation behavior of sulfur in the Cu(In,Ga)(S,Se)2 crystal and can be solved by a modification of the sulfur GDOES profile in accordance with grain sizes, (b) the saturation current density and the diode factor can be calibrated by electron and hole capture cross-sections of the absorber to match the measured IV data, and (c) a modification of the illumination spectrum corresponding to reflection losses can account for missing real part values of the complex refractive index. The resulting interplay of recombination mechanisms in the simulation model is then verified by a generation-dependent V oc measurement.

Temperature-Dependent Optical Modeling of Perovskite Solar Cells

Comprehensive temperature-dependent optical modeling of perovskite solar cells (PSCs) and modules is essential to accurately predict their energy yield and quantify their energy losses under real-world operating conditions, where devices are subject to different irradiance spectra and intensities as well as operating temperatures. These models require the accurate determination of the temperature-dependent optical constants of perovskites. Here, we report on these data, empirically determined via spectroscopic ellipsometry, for triple-cation perovskites with band gaps ranging between 1.58 and 1.77 eV at temperatures between 25 and 75 °C. Using this data set, we develop a simple empirical model to obtain the temperature-dependent optical constants of perovskites of an arbitrary band gap. We validate our empirical model by comparing the measured temperature-dependent short-circuit current densities and external quantum efficiency data of single-junction PSCs with simulated results using the modeled optical constants.

Temperature and intensity dependence of the open-circuit voltage of InGaN/GaN multi-quantum well solar cells

We have analyzed the temperature and intensity dependence of the open-circuit voltage of InGaN/GaN multi-quantum well solar cells up to 725 K and more than 1000 suns. We show that the simple ABC model routinely used to analyze the measured quantum efficiency data of InGaN/GaN LEDs can accurately reproduce the temperature and intensity dependence of the measured open-circuit voltage if a temperature-dependent Shockley-Read-Hall lifetime is used and device heating is taken into account.

Fullerene-Based Triads with Controlled Alkyl Spacer Length as Photoactive Materials for Single-Component Organic Solar Cells.

Two kinds of dumbbell-shaped acceptor-donor-acceptor (A-D-A)-type triad single-component (SC) photovoltaic molecules based on a benzodithiophene-rhodanine (BDTRh) core and [6,6]-phenyl-C61 butyric acid (PC61BA) termini, BDTRh-C2-PC61BA and BDTRh-C10-PC61BA, were synthesized by modulating the alkyl (C2 and C10) spacer lengths. Both SC photovoltaic structures had similar UV-vis spectra in solution, but BDTRh-C10-PC61BA showed a significantly higher absorption coefficient as a thin film. In films, a more facile intermolecular photo-induced charge transfer was observed for BDTRh-C10-PC61BA in the broad-band transient absorption measurements. BDTRh-C10-PC61BA also exhibited a higher hole mobility (by 25 times) and less bimolecular recombination than BDTRh-C2-PC61BA. By plotting the normalized external quantum efficiency data, a higher charge-transfer state was measured for BDTRh-C10-PC61BA, reducing its voltage loss. A higher power conversion efficiency of ∼2% was obtained for BDTRh-C10-PC61BA, showing higher open-circuit voltage, short-circuit current density, and fill factor than those of BDTRh-C2-PC61BA devices. The different carrier dynamics, voltage loss, and optical and photoelectrical characteristics depending on the spacer length were interpreted in terms of the film morphology. The longer decyl spacer in BDTRh-C10-PC61BA afforded a significantly enhanced intermolecular ordering of the p-type core compared to BDTRh-C2-PC61BA, suggesting that the alkyl spacer length plays a critical role in controlling the intermolecular packing interaction.

Comparison of organic light emitting diode performance using the spectroradiometer and the integrating sphere measurements

In this study, we present the comparison of device performance measurements for organic light emitting diodes using a spectroradiometer through the viewing angle and integrating sphere, widely used for device measurements. The mean calculation method using these results was applied to convert the spectroradiometer (under different viewing angles) data to match with the integrating sphere measurements. The conversion of the spectroradiometer based quantum efficiency and electroluminescence data from all different angular emission patterns was similar to that of the integrating sphere data within a reasonable range of deviation. As such, it is possible to reduce the recurring costs and required time between these two measurement techniques by bypassing the integrating sphere measurement.

Temperature dependence of the Auger recombination coefficient in InGaN/GaN multiple-quantum-well light-emitting diodes.

This study investigated the temperature dependence of the Auger recombination coefficient (C) in an InGaN/GaN blue multiple-quantum-well (MQW) light-emitting diode structure at temperatures between 20 and 100°C. The temperature dependence of C was determined by fitting the measured external quantum efficiency (EQE) data using an analytical model or numerical simulation. In the analytical model, the carrier density in InGaN MQWs was assumed to be constant and independent of temperature. In contrast, the inhomogeneous carrier distribution in MQWs and its temperature-dependent redistribution were included in the numerical simulation. When the analytical model was employed to fit the EQE curve, C decreased with increasing temperature. On the other hand, when the numerical simulation was employed, C increased steadily by ∼31% as the temperature was increased from 20 to 100°C. We found that the temperature-dependent carrier distribution is important to consider when determining the temperature dependence of the Auger recombination coefficient in InGaN MQW structures.

Comparison of Back-Thinned Detector Ultraviolet Quantum Efficiency for Two Commercially Available Passivation Treatments

Back-thinned silicon detectors offer a high response over a very broad spectrum for direct detection by providing an efficient optical path into the sensing silicon avoiding front face structures manufactured from metal, polysilicon, nitrides, and oxides that may absorb the incident light before reaching the sensing silicon. We have tested two CCDs with different back-surface shallow p+ implant thicknesses (basic and enhanced) at the M4 line (wavelength between 40 and 400 nm) at Physikalisch-Technische Bundesanstalt (PTB)'s Metrology Light Source in Berlin. This characterization in the ultraviolet spectral range extends the soft X-ray quantum efficiency (QE) data set previously acquired with the exact same devices. Due to the short absorption depth and the scope for many types of interactions of the device materials with ultraviolet photons, QE measurement and stability of the device against extended exposure in the UV are of ongoing interest. Therefore, QE measurements have been carried out before and after exposures to quantify any change in behavior. To allow characterization of the passivation processes only, the devices have no antireflection coating. The measured QE of the standard back-thinned CCD is below 10% between 70 and 370 nm. An average additional 5% efficiency is achieved in the enhanced device within the same range. At the limits of the measured spectrum, toward soft X-rays or toward the visible range, the QE increases and the difference between the standard and the enhanced process is reduced as the photon absorption length increases beyond the immediate back-surface. The measured QE after long high-flux exposures at 200 nm shows remarkable improvement.

Development of an inorganic cesium carbonate-based electron transport material for a 17% power conversion efficiency perovskite solar cell device

A low-temperature solution process technique is employed to develop an inorganic cesium carbonate (Cs2CO3) as an electron transport material for inorganic–organic hybrid double cation (FAPbI3)0.85(MAPbBr3)0.15 perovskite solar cells, as an alternative to the conventional thick and meso-TiO2. A device structure of compact-TiO2/Cs2CO3 (0.2 wt. %)/perovskite/spiro-OMETAD leads to enhanced performance of the photovoltaic device, achieving a short-circuit current density (Jsc) of 22.26 mA/cm2, an open-circuit voltage (Voc) of 1054 mV, a fill factor (FF) of 71.6%, and a power conversion efficiency (PCE) of about 17% under one sun illumination, whereas the controlled device structure shows an efficiency of 16.58% without such surface modification layer. Additionally, a device structure of Cs2CO3 (6 wt. %)/perovskite/spiro-OMETAD without any TiO2 ETM has shown a Jsc of 15.40 mA/cm2, Voc of 1023 mV, FF of 51.7%, and a PCE of 8.14%. On the other hand, external quantum efficiency (EQE) data yields around 85% of incident photon to electron conversion for c-TiO2/Cs2CO3 (0.2 wt. %)/perovskite/spiro-OMETAD structure and integrated Jsc extracted from EQE data confirms that Jsc obtained from the current–voltage test is within a close agreement. The obtained results indicate that there is a possibility to further increase the performance of perovskite-based cells and reduce their processing cost by replacing the thick mesoporous TiO2 by Cs2CO3.

Influence of Foliar Kaolin Application and Irrigation on Photosynthetic Activity of Grape Berries

Climate changes may cause severe impacts both on grapevine and berry development. Foliar application of kaolin has been suggested as a mitigation strategy to cope with stress caused by excessive heat/radiation absorbed by leaves and grape berry clusters. However, its effect on the light micro-environment inside the canopy and clusters, as well as on the acclimation status and physiological responses of the grape berries, is unclear. The main objective of this work was to evaluate the effect of foliar kaolin application on the photosynthetic activity of the exocarp and seeds, which are the main photosynthetically active berry tissues. For this purpose, berries from high light (HL) and low light (LL) microclimates in the canopy, from kaolin-treated and non-treated, irrigated and non-irrigated plants, were collected at three developmental stages. Photochemical and non-photochemical efficiencies of both tissues were obtained by a pulse amplitude modulated chlorophyll fluorescence imaging analysis. The maximum quantum efficiency (Fv/Fm) data for green HL-grown berries suggest that kaolin application can protect the berry exocarp from light stress. At the mature stage, exocarps of LL grapes from irrigated plants treated with kaolin presented higher Fv/Fm and relative electron transport rates (rETR200) than those without kaolin. However, for the seeds, a negative interaction between kaolin and irrigation were observed especially in HL grapes. These results highlight the impact of foliar kaolin application on the photosynthetic performance of grape berries growing under different light microclimates and irrigation regimes, throughout the season. This provides insights for a more case-oriented application of this mitigation strategy on grapevines.

Exploration of CdMnTe thin film solar cells

In today's world, the energy demand is growing but primary energy sources are gradually depleted and humanity will face more severe energy and environmental crisis. The efficiency of thin film solar cells has stagnated in the last few years and new viable and affordable alternative energy resources are needed. Therefore, an exploration of CdMnTe (CMT) solar cells is carried out in this work and an attempt has been made to enhance the efficiency by post-annealing treatment in vacuum at a different temperature. The structure, surface morphology and optical properties of polycrystalline CMT layers were investigated, and then CMT solar cells were produced by vapor evaporation. The device post-treated at 300 °C has maximum efficiency (3.36%) and then starts decreasing at a higher temperature. The capacitance-voltage measurements are undertaken to determine the depletion layer width and dopant density. The absolute quantum efficiency data are taken into account to validate the short circuit current and the device performance parameters as well. This study divulges that the CMT solar cells might be a potential candidate as an affordable alternative energy resource and further investigation is required to enhance the performance.

An investigation on the relationship between open circuit voltage and grain size for CZTSSe thin film solar cells fabricated by selenization of sputtered precursors

The low open circuit voltage (Voc) of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells limits their efficiency. CZTSSe absorbers were fabricated by sputtering Cu2ZnSnS4 (CZTS) target and subsequent selenization treatment and then incorporated into solar cells. The influence of selenization temperature on the growth of the CZTSSe grains and device performance was examined. The absorber films were composed of a CZTSSe phase with high Se/(Se+S) ratios. As the selenization temperature was increased from 460 °C to 500 °C, the grains grew from the top to the bottom of the CZTSSe absorbers, and the average Voc of the CZTSSe solar cells increased from 284 mV to 371 mV. The band gaps (Eg), derived from external quantum efficiency (EQE) data, were approximately 1.11 eV. Activation energies (Ea) was extracted from temperature-dependent current density-voltage (J-V) measurements and used to evaluate the interface recombination level. The Ea increased from 0.82 eV to 0.89 eV as the selenization temperature was increased, which approached the Eg of CZTSSe. This was likely caused by reduced interface recombination because the grain boundaries decreased as the grains grew larger. An approximately linear relationship between the grain size and Voc was observed. The increase of grain size was achieved by optimizing the selenization temperature, which reduced interface recombination and resulted in an increased Voc. CZTSSe solar cells were fabricated with a maximum efficiency of 8.97%.

Minority lifetime and efficiency improvement for CZTS solar cells via Cd ion soaking and post treatment

Short minority carrier lifetime and low open circuit voltage (Voc) are key issues limiting the current efficiency development for Cu2ZnSnS4 (CZTS) thin film solar cells. This work presents the use of Cd ion soaking and post-heat treatment to increase the lifetime and Voc of CZTS. The time-resolved photoluminescence (TRPL) results confirmed that Cd ion soaking and post heat treatment is effective in enhancing the minority lifetime, from ∼10 ns up to ∼28 ns. With analyses of the current density- voltage (J-V) curves and external quantum efficiency (EQE) data, Cd ion soaking and subsequent post heat treatment is found to greatly reduce the recombination and improve the junction quality, contributing to a ∼70 mV Voc enhancement and resulting in the increased device efficiency of 7.6%, from 5.9%.

Improved quantum efficiency models of CZTSe: GE nanolayer solar cells with a linear electric field.

We fabricated and characterized CZTSe:Ge nanolayer (<10 nm) thin film solar cells to quantitatively demonstrate an exact analytical model of quantum efficiency for Ge doped CZTSe devices. The linear electric field model is developed with the incomplete gamma function of the quantum efficiency as compared to the empirical data at forward bias conditions. This model is characterized with a consistent set of parameters from a series of measurements and the literature. Using the analytical modelling method, the carrier collection profile in the absorber is calculated and closely fitted by the developed mathematical expressions to identify the carrier dynamics during the quantum efficiency measurement of the device. The analytical calculation is compared with the measured quantum efficiency data at various bias conditions.

Relative External Quantum Efficiency of Crystalline Silicon Wafers From Photoluminescence

The external quantum efficiency is routinely measured to determine the carrier collection probability of solar cells as a function of illumination wavelength. The common method used to perform these measurements requires a fully metalized device such that the short circuit current can be measured under monochromatic illumination. This paper demonstrates the extraction of relative external quantum efficiency data from open circuit photoluminescence measurements in a contactless fashion. Good agreement is observed between the photoluminescence-based technique and traditional external quantum efficiency data for illumination wavelengths in the range 400 to 625 nm. Deviations are observed for longer illumination wavelengths and their origin is discussed.

Analytical model for simulating thin-film/wafer-based tandem junction solar cells

Replacing the present-day commercial single junction silicon (Si) solar cells with low cost, high efficiency solar cells is imperative, in order to compete with other existing energy technologies. Many research groups have looked into using III–V materials, tandem junction solar cells and thin-film technologies to reach higher efficiencies. However, many of these techniques involve expensive materials or costly manufacturing processes. In this research, we focus on a tandem junction solar cell design that is based on a thin-film perovskite top cell and wafer-based Si bottom cell. In order to analyze the performance of the tandem cell, an analytical model is needed to compute the quantum efficiency and characteristic solar cell data. The highly versatile Matlab-based analytical model presented in this work is capable of modeling different kinds of tandem cells based on a variety of solar absorber combinations. The model allows user to adjust input parameters, such as reflectivity, material thickness, donor and acceptor densities, and carrier lifetimes in order to optimize the quantum efficiency, maximum power output, open circuit voltage, and short circuit current quantities of the cell. Using this analytical model, we were able to design a perovskite and black Silicon (bSi) tandem cell, which reached an efficiency of greater than 30%.

Earth-Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu2 BaSn(S,Se)4 Absorber.

In recent years, Cu2 ZnSn(S,Se)4 (CZTSSe) materials have enabled important progress in associated thin-film photovoltaic (PV) technology, while avoiding scarce and/or toxic metals; however, cationic disorder and associated band tailing fundamentally limit device performance. Cu2 BaSnS4 (CBTS) has recently been proposed as a prospective alternative large bandgap (~2 eV), environmentally friendly PV material, with ~2% power conversion efficiency (PCE) already demonstrated in corresponding devices. In this study, a two-step process (i.e., precursor sputter deposition followed by successive sulfurization/selenization) yields high-quality nominally pinhole-free films with large (>1 µm) grains of selenium-incorporated (x = 3) Cu2 BaSnS4-x Sex (CBTSSe) for high-efficiency PV devices. By incorporating Se in the sulfide film, absorber layers with 1.55 eV bandgap, ideal for single-junction PV, have been achieved within the CBTSSe trigonal structural family. The abrupt transition in quantum efficiency data for wavelengths above the absorption edge, coupled with a strong sharp photoluminescence feature, confirms the relative absence of band tailing in CBTSSe compared to CZTSSe. For the first time, by combining bandgap tuning with an air-annealing step, a CBTSSe-based PV device with 5.2% PCE (total area 0.425 cm2 ) is reported, >2.5× better than the previous champion pure sulfide device. These results suggest substantial promise for the emerging Se-rich Cu2 BaSnS4-x Sex family for high-efficiency and earth-abundant PV.