Experimental External Quantum Efficiency Research Articles

Influence of the period number of superlattice electron barrier layer on the performance of AlGaN-based deep ultraviolet LED

The development of AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) is currently limited by poor external quantum efficiency (EQE) and wall-plug efficiency (WPE). Internal quantum efficiency (IQE), as an important component of EQE, plays a crucial role in improving the performance of DUV-LEDs. IQE is related to the carrier injection efficiency and the radiation recombination rate in the active region. In order to improve the IQE of AlGaN-based DUV-LEDs, this work proposes a scheme to optimize the period number of superlattice electron barrier layer (SL-EBL) to achieve better carrier injection efficiency and confinement capability. The effect of the period number of SL-EBL on the luminous efficiency, reliability and carrier recombination mechanism of AlGaN-based DUV-LEDs with an emission wavelength of 273 nm was investigated. The experimental results show that the light output power (LOP), external quantum efficiency (EQE) and wall-plug efficiency (WPE) of the DUV-LEDs tend to first increase and then decrease with the period number increase of the SL-EBL, while the leakage current decreases and the reliability is enhanced.The maximum EQE and WPE of the DUV-LED are 3.5% and 3.2%, respectively, at an injection current of 7.5 mA were achieved when the period number of SL-EBL was fixed at 7 (the thickness was 28 nm). Meanwhile, the numerical simulation results show that the electron potential barrier height is boosted with increasing the period number of SL-EBL, and the variation of the hole potential barrier height is negligible. Therefore, increasing the period number of SL-EBL is beneficial for shielding the dislocations and suppressing the leakage of electrons into the p-type layer, which improves the luminous efficiency and reliability of DUV-LEDs. However, when the period number of SL-EBL exceeds 7, the excessively thick hole potential barrier prevents the injection of holes into the activation region and degrades the radiative recombination efficiency. Therefore, EQE and WPE will show an inflection point with the variation of the period number of SL-EBL. In addition, to investigate the carrier recombination mechanism of the active region, the experimental EQE curves are fitted by the ABC model as well as the different slopes in logarithmic light output power-current (<i>L-I</i>) curves are calculated after aging. It can be found that increasing the period number of SL-EBL can effectively suppress the non-radiative combination of carriers in the active region. This investigation can provide an alternative way to enhance the photoelectric performance of DUV-LEDs.

Identifying the Quantitative Relationship Between the Molecular Structure and the Horizontal Transition Dipole Orientation of TADF Emitters

AbstractRealizing the horizontal orientation of molecular transition dipole moment (TDM) can greatly improve the out‐coupling efficiency and the resultant external quantum efficiency (EQE) of organic light‐emitting diodes (OLEDs). Herein, key parameters governing the horizontal TDM have been continuously explored. However, quantitatively identifying the key parameters from the molecular structure viewpoint is rather challenging due to the complexity of the influencing parameters. Here, by training the machine learning (ML) models using the experimental results, the quantitative relationship between the molecular structure and the horizontal TDM ratio (ϴ) of thermally activated delayed fluorescent (TADF) emitters in the host‐guest films is identified. The molecular structure is represented by either quantum chemistry‐calculated structural descriptors or topological/physical/chemical molecular descriptors. Key descriptors are ranked and can be used for guiding molecular structure design. Moreover, the accuracy of ML models is double‐verified by comparing the predicted results with experimental ϴ values and the trend of experimental EQE based on a group of materials. Using compressed sensing technology, the low‐dimension material space is also visually constructed based on key descriptors, and the results are consistent with those of the ML models.

Influence of the thickness of p-GaN ohmic contact layer on the performance of AlGaN-based deep-ultraviolet light-emitting diodes

276 nm AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) with varying p-GaN thicknesses were investigated. The simulation results demonstrate that DUV-LEDs with an ultra-thin 4 nm p-GaN layer exhibit a 96.9% increase in full-angle total radiant intensity compared to those with a 350 nm p-GaN layer. Meanwhile, the total radiant intensity of the DUV-LEDs with 4 nm p-GaN were all elevated during the degree of polarization (DOP) transition from 1 to −1. The experimental results demonstrate that the maximum external quantum efficiency (EQE) and wall-plug efficiency (WPE) values of 5.3% and 4.0%, respectively, at 4 A/cm2 for the DUV-LEDs with a 4 nm p-GaN layer. These figures represent remarkable improvements of 60.6% and 42.8% when compared to DUV-LEDs featuring a 350 nm p-GaN layer. Nevertheless, it's worth noting that the adoption of an ultra-thin p-GaN ohmic contact layer introduced some surface irregularities, potentially affecting the ohmic contact of the p-type layer and resulting in a slight voltage increase, thus tempering the WPE improvement. The light extraction efficiency (LEE) values of 10.0%, 7.4%, 6.7% and 6.3% were extracted for the four samples after fitting the experimental EQE curves by the ABC model.

Applications of spectroscopic ellipsometry for multilayer analysis of CdTe solar cell structures incorporating Magnesium–Zinc oxide high resistivity transparent layers

An optical function parameterization of polycrystalline MgxZn1-xO (MZO) thin films as a function of bandgap Eg has been utilized in applications for the metrology of CdTe-based solar cell structures that incorporate MZO thin films as high resistivity transparent (HRT) layers. Parametric expressions are applied to facilitate mapping spectroscopic ellipsometry (M-SE) of device structures consisting of glass/SnO2:F/MZO. M-SE is shown to provide maps in the MZO effective thickness and bandgap within confidence limits of ± 1 nm and ± 0.003 eV, respectively. As a second application of these parametric expressions, an as-deposited glass/SnO2:F/MZO/CdS/CdTe device structure has been analyzed by through-the-glass spectroscopic ellipsometry (TG-SE). Such an analysis is also shown to provide the MZO effective thickness and bandgap. The outcome of the TG-SE analysis for this device structure enables simulations of the external quantum efficiency (EQE) spectrum of the resulting solar cell assuming different recombination losses within the individual layers of the structure. A comparison of these simulations with the experimental EQE spectrum reveals improved current collection from the front of the device incorporating an MZO HRT layer.

Implications of Electron Transport Layer and Back Metal Contact Variations in Tin–Lead Perovskite Solar Cells Assessed by Spectroscopic Ellipsometry and External Quantum Efficiency

The structural and optical properties of hybrid organic-inorganic metal halide perovskite solar cells are measured by spectroscopic ellipsometry to reveal an optically distinct interfacial layer among the back contact metal, charge transport, and absorber layers. Understanding how this interfacial layer impacts performance is essential for developing higher performing solar cells. This interfacial layer is modeled by Bruggeman effective medium approximations (EMAs) to contain perovskite, C60, BCP, and metal. External quantum efficiency (EQE) simulations that consider scattering, electronic losses, and the formation of nonparallel interfaces are created with input derived from ellipsometry structural-optical models and compared with experimental EQE to estimate optical losses. This nonplanar interface causes optical losses in short circuit current density (JSC) of up to 1.2 mA cm-2. A study of glass/C60/SnO2/Ag or Cu and glass/C60/BCP/Ag film stacks shows that C60 and BCP mix, but replacing BCP with SnO2 can prevent mixing between the ETLs to prevent contact between C60 and back contact metal and enable the formation of a planar interface between ETLs and back contact metals.

Electroluminescence in Unipolar-Doped In0.53Ga0.47As/AlAs Resonant-Tunneling Diodes: A Competition between Interband Tunneling and Impact Ionization

We measure and analyze the light emission from a room-temperature n-type unipolar-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}/\mathrm{Al}\mathrm{As}$ double-barrier resonant-tunneling diode (RTD) that occurs just above the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ band edge and peaks around 1631 nm. The emission is attributed to electron-hole recombination emission made possible by holes generated in the high-field region on the collector side of the device by interband tunneling and impact ionization, which contribute comparable hole densities, according to our analysis. Although the external quantum efficiency (EQE) in our experimental configuration is rather low (\ensuremath{\approx}2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ at 3.0-V bias), limited by suboptimal output coupling, the internal quantum efficiency (IQE) is much higher (\ensuremath{\approx}6% at 3.0-V bias), as derived from the experimental EQE and a radiometric analysis. To check this value and better understand the transport physics, we also carry out an independent estimate of the IQE using a combined interband-tunneling impact-ionization transport model, which yields an IQE of 10% at 3.0-V bias. The satisfactory agreement of theory with experimental data suggests that a RTD designed for better hole transport and superior optical coupling could become a useful light-emitting device, while retaining the intrinsic functionality of high-speed negative differential resistance, and all without the need for resistive p-type doping.

Light management in highly-textured perovskite solar cells: From full-device ellipsometry characterization to optical modelling for quantum efficiency optimization

While perovskite solar cells (PSCs) are now reaching high power conversion efficiencies (PCEs), further performance improvement requires a fine management and an optimization of the light pathway and harvesting in the cells. These go through an accurate understanding, characterization and modelling of the optical processes occurring in these complex, often textured, multi-layered systems. In the present work, we have considered a typical methylammonium lead iodide (MAPI) solar cell built on a fluorine-doped tin oxide (FTO) electrode of high roughness (43 nm RMS). By variable-angle spectroscopic ellipsometry (VASE) of the full PSC device, we have been able to determine the optical constants of all the device layers. We have designed a one-dimensional (1D) optical model of the stacked layers where the rough texture is described as layers of effective-medium index. We have supported the model using data extracted from scanning electron microscopy, diffuse spectroscopy and photovoltaic efficiency measurements. We show that the 1D model, while insufficient to describe scattering by the FTO plate alone, gives an accurate description of the full device optical properties. By comparison with the experimental external quantum efficiency (EQE), we estimate the internal quantum efficiency (IQE) and the effect of the losses related to electron transfer. Based on this work, we finally discuss the optical losses mechanisms and the possible strategies that can be implemented to improve light management within PSC devices and further increase their performances.

Microscopic reversibility demands lower open circuit voltage in multiple exciton generation solar cells

Multiple exciton generation (MEG) increases the short circuit current of solar cells and is, therefore, often cited as a candidate scheme for surpassing the efficiency limit of single junction solar cells. Conventionally, limiting efficiencies for MEG solar cells have been calculated using quasi-equilibrium models that implicitly assume an effective separation of timescales between different processes. We show here that this separation of timescales is not possible for MEG solar cells, with Auger recombination, the inverse process to multi-exciton generation, needing to be considered explicitly in the efficiency limits of an MEG solar cell. We assess the impact of Auger recombination using a non-equilibrium model of a quantum dot solar cell that satisfies microscopic reversibility and can approximate experimental external quantum efficiency (EQE) curves of MEG solar cells. Recombination—both Auger and radiative—is treated in a quasi-equilibrium approach, which can be justified with a clear model for the separation of timescales. A key insight of this model is that the achievable voltage of the device, and hence the solar energy conversion efficiency, depends on the absolute values of the impact ionization rate and the rate at which electrons lose energy through phonon scattering. By contrast, the EQE profile at short circuit depends only on the ratio of these two rates. This shows that the potential of certain MEG solar cell approaches cannot be assessed from EQE improvements alone, which highlights the importance of considering non-equilibrium processes in models of solar energy conversion devices.

Thin Gallium Arsenide Solar Cells With Maskless Back Surface Reflectors

This work investigates the optical performance of a textured back surface reflector (BSR) produced by a maskless solution-based wet chemical etch in Al $_\text{0.3}$ Ga $_\text{0.7}$ As for a 1.1- $\mu$ m-thick GaAs solar cell. Combining the maskless texture with the 94% reflective flat mirror resulted in a high haze in reflectance near 80% across the GaAs absorbing region. The increased diffuse reflectance from the maskless BSR provided a 19.7% increase in the integrated short-circuit current density from the base region of the 1.1- $\mu$ m GaAs solar cell with the maskless BSR compared to the simulated 1.1- $\mu$ m solar cell with no light trapping structures. Based on the principle concepts for the Fabry–Perot etalon, the lifetime enhancement factor (LEF), defined as the extended photon lifetime in an optical cavity due to light trapping, was derived for the BSR devices. The LEF for the maskless BSR device was measured to be 4.3 times greater than the single-pass photon lifetime, agreeing with the high haze in reflectance and improved photoabsorption near the GaAs band edge. This value agrees with the F factor in the analytical propagation model that fits the experimental external quantum efficiency curves and justifies using the LEF to quantitatively represent the optical performance of light trapping structures in sub- $\mu$ m thick solar cells.

Extraction of the minority carrier transport properties of solar cells using the Hovel model and genetic algorithms

In this paper, a quick and accurate method for extraction of the minority carrier transport properties of p-n or n-p junction solar cells, such as diffusion lengths and surface recombination velocities, is presented. The knowledge of these parameters is essential to investigating factors that limit the performance of photovoltaic devices. The proposed method, based on genetic algorithms and the analytical Hovel model, is used to fit the external quantum efficiency (EQE) curves of solar cells with different emitter thicknesses. As a demonstrative example of application of the procedure carried out in this work, theoretical and experimental EQE curves of n-p GaAs solar cells under the standard AM1.5G spectrum have been used in order to extract the desired parameters. Errors less than 2.4% have been obtained, which shows the capability of the developed tool. An analysis of the total number of iterations is presented. The results obtained can be used to improve the design, optimization, and manufacturing process of high-efficiency photovoltaic devices.

Origin of external quantum efficiency roll-off in 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz)-based inverted organic light emitting diode under high pulsed electrical excitation

Laser diodes based on organic semiconductor materials have high threshold current densities that require the suppression of various inherent loss processes. One way to study such loss processes is to analyze the external quantum efficiency (EQE) roll-off in organic light-emitting diodes (OLEDs). In this work, we used electrical simulations to analyze the origin of the experimental EQE roll-off of an OLED based on 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) under extremely high current injection (∼1 kA/cm2). We considered various singlet exciton annihilations and quenching processes (i.e., singlet-singlet annihilation, singlet-triplet annihilation, singlet-polaron annihilation, singlet-heat quenching, and electric field quenching of singlet excitons). These results showed that the EQE roll-off can be attributed to Joule heating and/or singlet-triplet annihilation and/or the dissociation of singlet excitons under a high applied electric field. The electric field quenching of singlet excitons was confirmed by a field-induced photoluminescence (PL) quenching experiment. By applying an electric field-induced charge dissociation model to both the EQE and field-induced PL quenching, we estimated the singlet exciton binding energy of a BSBCz film to be in the range of 0.64–0.71 eV.

N-i-p Nanocrystalline Hydrogenated Silicon Solar Cells with RF-Magnetron Sputtered Absorbers

Nanocrystalline hydrogenated silicon (nc-Si:H) substrate configuration n-i-p solar cells have been fabricated on soda lime glass substrates with active absorber layers prepared by plasma enhanced chemical vapor deposition (PECVD) and radio frequency magnetron sputtering. The cells with nanocrystalline PECVD absorbers and an untextured back reflector serve as a baseline for comparison and have power conversion efficiency near 6%. By comparison, cells with sputtered absorbers achieved efficiencies of about 1%. Simulations of external quantum efficiency (EQE) are compared to experimental EQE to determine a carrier collection probability gradient with depth for the device with the sputtered i-layer absorber. This incomplete collection of carriers generated in the absorber is most pronounced in material near the n/i interface and is attributed to breaking vacuum between deposition of layers for the sputtered absorbers, possible low electronic quality of the nc-Si:H sputtered absorber, and damage at the n/i interface by over-deposition of the sputtered i-layer during device fabrication.

Bandgap of thin film solar cell absorbers: A comparison of various determination methods

Knowledge of the absorber bandgap is often needed for assessing the junction quality of a thin film solar cell, for example when computing the open-circuit voltage deficit. The bandgap is typically estimated from the routine measurement of the external quantum efficiency (EQE) of finished devices. However the extraction becomes ambiguous in the case of very thin absorbers, or in presence of bandgap gradients or collection issues of charge carriers. This work reviews several methods for the determination of the bandgap from EQE measurements and discusses their validity conditions. The numerical results are compared based on experimental EQE of several different thin film solar cells such as CuInGaSe2, CuInSe2, CdTe, CuZnSnSe and perovskite, and systematic trends are identified. Numerical simulations are also performed to illustrate the behavior of the different bandgap extraction methods in presence of a bandgap gradient and different magnitudes of the exponential tail states.

The Effect of Base Thickness on Photoconversion Efficiency in Textured Silicon-Based Solar Cells

A theory is developed, which describes the experimental external quantum efficiency, EQE(λ) as a function of photon wavelength for structured Si-based solar cells. Short-circuit current density as a function of base thickness, d, is calculated for the high-efficiency solar cells with the photoconversion efficiency η ≥ 25% The procedure allows one to carry out a complete optimization of such solar cells, in particular, to find the optimal base thickness, dopt.

Влияние толщины базы на эффективность фотопреобразования текстурированных солнечных элементов на основе кремния

AbstractA theory is developed, which describes the experimental external quantum efficiency, EQE (λ) as a function of photon wavelength for structured Si-based solar cells. Short-circuit current density as a function of base thickness, d , is calculated for the high-efficiency solar cells with the photoconversion efficiency η ≥ 25% The procedure allows one to carry out a complete optimization of such solar cells, in particular, to find the optimal base thickness, d _opt.

Fast determination of the current loss mechanisms in textured crystalline Si-based solar cells

A quite general device analysis method that allows the direct evaluation of optical and recombination losses in crystalline silicon (c-Si)-based solar cells has been developed. By applying this technique, the current loss mechanisms of the state-of-the-art solar cells with ∼20% efficiencies have been revealed. In the established method, the optical and electrical losses are characterized from the analysis of an experimental external quantum efficiency (EQE) spectrum with very low computational cost. In particular, we have performed the EQE analyses of textured c-Si solar cells by employing the experimental reflectance spectra obtained directly from the actual devices while using flat optical models without any fitting parameters. We find that the developed method provides almost perfect fitting to EQE spectra reported for various textured c-Si solar cells, including c-Si heterojunction solar cells, a dopant-free c-Si solar cell with a MoOx layer, and an n-type passivated emitter with rear locally diffused solar cell. The modeling of the recombination loss further allows the extraction of the minority carrier diffusion length and surface recombination velocity from the EQE analysis. Based on the EQE analysis results, the current loss mechanisms in different types of c-Si solar cells are discussed.

Quantum Efficiency Measurements and Modeling as Tools to Monitor Air Annealing of Cu2SnS 3 Solar Cells

Air annealing of chalcopyrite solar cells at either 200 °C or higher is often known to increase their power conversion efficiency. In this paper, we investigate the nature of this effect for Cu2SnS3 (CTS) solar cells by modeling the experimental external quantum efficiency. We find that the cell efficiency increase stems from increased diffusion length and depletion width and decreased interface recombination at the p-n junction. The increased diffusion length is also reproduced when only the absorber layer is air annealed. When solar cells are annealed in N2, no increase in diffusion length is measured. Hence, we attribute the increase in diffusion length to passivation of the grain boundaries in the bulk by oxygen. The larger depletion width on air and N2 annealing in the devices is independent of the CdS buffer layer thickness and occurs in its absence. We ascribe it to copper diffusion from the absorber layer to the n-type buffer and window layers. Interface recombination positively correlates with increasing buffer layer thickness. Based on our modeling, we conclude that the CTS absorber layer is still too highly doped to obtain large depletion widths and is highly recombination active at the p–n interface.

Mechanism of hole injection enhancement in light-emitting diodes by inserting multiple hole-reservoir layers in electron blocking layer

In this study, gallium nitride (GaN) based light-emitting diodes (LEDs) with single and multiple hole-reservoir layers (HRLs) inserted in the electron-blocking layer (EBL) have been investigated numerically and experimentally. According to simulation results, a better electron confinement and a higher hole injection level can be achieved by the multiple HRLs inserted in the EBL region. To further reveal the underlying mechanism of hole injection enhancement experimentally, the active regions were intentionally designed to emit photons with three different wavelengths of 440 nm, 460 nm, and 480 nm, respectively. Based on the experimental results of photoluminescence (PL) and time-resolved PL (TRPL) measurements conducted at 298 K, the remarkable enhancement (148%) of PL intensities and significant increase in the decay times of the quantum wells close to p-GaN can be obtained. Therefore, the mechanism is proposed that carriers are able to reserve in the EBL region with multiple HRLs for a much longer time. Meanwhile, carriers could diffuse into the active region by tunnelling and/or thermo-electronic effect and then recombine efficiently, leading to the better carrier reservoir effect and higher hole injection in LEDs. As a result, by inserting multiple HRLs in the EBL region instead of single HRL, the experimental external quantum efficiency is enhanced by 19.8%, while the serious droop ratio is markedly suppressed from 37.0% to 27.6% at the high current injection of 100 A/cm2.

How tilting and cavity-mode-resonant absorption contribute to light harvesting in 3D radial junction solar cells.

Radial junction (RJ) architecture has proven beneficial in boosting light harvesting and fast carrier separation in thin film solar cells. While a comprehensive understanding of the detailed absorption distribution and light incoupling mechanism within such a 3D RJ configuration remains largely unexplored. Taking hydrogenated amorphous Si (a-Si:H) RJ solar cells as an example, we here address in both experimental and theoretical manners the impacts of tilting and spacing configuration on the light absorption and external quantum efficiency (EQE) responses. A nice agreement between the calculated and experimental EQE responses indicates that the light harvesting realized within RJ thin film solar cells is quite robust against geometric variations and shadowing effects. Following the concepts of optical fiber injection, we have been able to single out the contribution arising solely from a resonant-mode-incoupling into the RJ cavities against a sidewall scattering incidence scenario. These results provide insightful viewpoints as well as practical guides in developing a new generation of high performance RJ thin film solar cells.

Improvement in the light outcoupling efficiency of organic light-emitting diodes using a hemispherical lens and a multipatterned one-dimensional photonic crystal fabricated by autocloning

The outcoupling efficiency of organic light-emitting diodes was improved using a photonic crystal fabricated by autocloning, which has potential advantages for mass production. By using layer thicknesses optimized by optical simulations, the experimental external quantum efficiency of a device with the photonic crystal and a hemispherical lens was 43.4%, which is 2.13 times that of a device on a flat substrate without the lens. Comparisons with the simulated distributions of the light modes in the devices show that the photonic crystal reduces the light loss from the waveguide and evanescent modes. The emission pattern was independent of substrate rotation.