quasi-Fermi Level Research Articles

Current- and temperature-dependent efficiency droops in InGaN-based blue and AlGaInP-based red light-emitting diodes

We investigate the current-dependent and temperature-dependent efficiency droops (“J-droop” and “T-droop”, respectively) in InGaN-based blue and AlGaInP-based red light-emitting diodes (LEDs). It is found that the blue and red LEDs show different droop behaviors with increasing current density and temperature. The J-droop is significant in the blue LED while the T-droop is severe in the red LED. In case of the blue LED, the carrier accumulation caused by the saturation of the radiative recombination rate is thought to increase the quasi-Fermi level rapidly, thus causing the J-droop. On the other hand, the T-droop of the red LED is influenced by redistribution of carriers due to the increased thermal energy with a small barrier height in the AlGaInP material system. The comparison of different droop characteristics of blue and red LEDs helps understand the recombination mechanisms of both LEDs and provides useful insight for improving the device performance further.

(Invited) The Role of Electronic Bulk and Surface Structures for Efficient Photo-Electro-Synthesis

For an effective conversion of solar energy to a chemical fuel a lot of different materials as well as device structures have been suggested but only very few provide technological competitive conversion efficiencies. Limitations and loss processes can be deduced from a detailed consideration of the involved photovoltaic and electrochemical elementary steps. Optimized performance can only be reached when the photovoltaic and electrolytic boundary conditions of integrated systems are comparable to those of separated devices with no extra loss due to the coupling process. Feasible solutions seem to be possible for water splitting and H2-generation, as we will show with a number of investigations performed recently combining electrochemical investigations with surface science studies. Especially photoemission results on the demands on the bulk and surface electronic structure provide clear boundary conditions on the material’s and surface properties: i) the semiconductors must provide a wide splitting of quasi Fermi levels which will be provided by multli-junctions but hardly can be expected for low band gap oxides based on localized d electron states as e. g. hematite due to polaron formation. ii) the interaction of H2O and their intermediates directly onto the light energy converting junction must be avoided because of possibly formed surface states. As a consequence a buried junction is needed. iii) Photocatalytic converters must be prepared which uses different junction properties in opposing directions (Janus type surfaces) as proven for facet engineering or hetero structure formation. In summary, we do believe that effective photoelectrochemical devices need the input of different disciplines and expertise ranging from solid state physics, surface science to electrochemistry. Based on an improved understanding of the correlation of material’s properties with the involved elementary steps promising advanced technological solutions seem possible.

Comparative operando XPS studies of quasi-Fermi level splitting and open-circuit voltage in CZTSe/CdS and CIGS/CdS junctions and device structures

This contribution describes operando x-ray photoelectron spectroscopy (opXPS) studies of Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnSe4 (CZTSe) absorber layers and device structures. X-ray-excited valence-band and core-level spectra were acquired on bare absorber surfaces and after CdS and ZnO:Al depositions in standard absorber/CdS/ZnO/ZnO:Al device stacks. OpXPS data sets were acquired at various x-ray fluxes, with and without white-light illumination. From these measurements, quasi-Fermi level splitting (ΔEF) values in the absorber/CdS junctions were measured directly as a function of excitation conditions. For both CIGS and CZTSe, results show that ΔEF proportional to the full open-circuit voltage (VOC) of the completed devices is present after the deposition of the CdS layer—i.e., ΔEF = qVOC—demonstrating that the so-called VOC deficit in CZTSe solar cells is also present at this stage of processing. The authors find that photoexcitation due to x-rays or stray visible light during XPS or similar measurements can produce measurable photovoltages in materials, absorber/CdS junctions, and finished devices. In situ current density versus voltage measurements on a typical CIGS device reveal that x-ray-induced photoexcitation associated with typical XPS measurement conditions is equivalent to ∼3 × 10−3 suns, which in this study produced VOC = 440 mV. These findings demonstrate that accounting for photoexcitation conditions during XPS or similar measurements can improve band-offset determinations and produce more reliable values for the junction built-in voltage. The implications of the study findings on the CZTSe VOC deficit and device performance are discussed.

Quantum well action model for the formation of a single Shockley stacking fault in a 4H-SiC crystal under non-equilibrium conditions

The formation of single Shockley stacking faults (SSSFs) in 4H-SiC crystals under non-equilibrium conditions (e.g., the forward biasing of PiN diodes and ultraviolet light illumination) is a key phenomenon in the so-called bipolar degradation of SiC power devices. This study theoretically investigated the physical mechanism of this phenomenon based on the concept of quantum well action. As a first approximation describing the non-equilibrium state of the material, we employed quasi-Fermi level approximation. We then made improvements by considering several physical effects governing the carrier distribution near and in the SSSF. The improved model accounts well for the excitation threshold and the temperature dependence of SSSF expansion. Thus, the model provides useful insights into the driving force of SSSF expansion under non-equilibrium conditions.

Materials Selection and Mechanism of Non-linear Conduction in Chalcogenide Selector Devices

The electronic structure and conduction mechanism of chalcogenide-based Ovonic threshold switches (OTS) used as selectors in cross-point memory arrays is derived from density functional calculations and quasi-Fermi level models. The switching mechanism in OTS is primarily electronic. This uses a specific electronic structure, with a wide tail of localized states below the conduction band edge. In amorphous GeSe2−x the conduction band consists of Ge-Se σ*states with a low effective mass, and with a broad tail of localized Ge-Ge σ* states below this band edge. This leads to the OTS behavior. At high fields the electron quasi-EF moves up through these tail states, lowering the conductivity activation energy, and giving the non-linear switching process. The 4:2 coordinated GeSe2−x based alloys are the most favorable OTS material because they have the correct network connectivity to give a high electron mobility and lack of crystallization, a favorable band structure to produce the non-linear conduction, an optimum band gap, and with nitrogen or carbon alloying, a sufficiently low off-current.

Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells.

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages ( VOC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the VOC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

Photoelectrochemical Kinetics: Hydrogen Evolution on p-Type Semiconductors

The theory of intensity-modulated photocurrent spectroscopy (IMPS) is extended to consider the two-electron two-proton hydrogen evolution reaction at p-type semiconductor photocathodes. A generalized reaction scheme is described, and two limiting cases are analyzed. The first involves photoelectrochemical desorption of hydrogen by electron/proton transfer and the second chemical desorption by reaction of adsorbed hydrogen atoms. Expressions are derived for the IMPS responses in the two cases as well as for the modulated surface electron concentration, which is related to the quasi Fermi level at the interface. The surface electron concentration expressions can be used to interpret electrical, optical, or microwave experiments that detect the excess electron concentration. The IMPS responses calculated for the two limiting cases are fitted to a simplified IMPS model that gives rate constants kt nd kr for charge transfer and recombination respectively. kt and kr are then related to the rate constants describing the two limiting schemes for the light-driven hydrogen evolution reaction.

On understanding bandgap bowing and optoelectronic quality in Pb–Sn alloy hybrid perovskites

Experimental insights regarding bandgap evolution in hybrid perovskite alloys and the optimal small-bandgap absorber composition desired for next-generation perovskite tandems.

Calculating current density and quantum efficiency of p-n junction solar cell with quasi-Fermi level approximation

Current density and quantum efficiency of p-n junction GaAs solar cell is analytically investigated incorporating the effect of excess carriers due to photon incidence. Carrier transport equations for both types of carriers are solved involving both the conduction mechanisms along with generation rate. Change of dark current is reported in presence of varying operating conditions, and modulation of quantum efficiency is obtained by varying structural parameters. Results are useful for accurate estimation of figure of merit, which will play key role in photovoltaic applications.

Calculating current density and quantum efficiency of p-n junction solar cell with quasi-Fermi level approximation

Current density and quantum efficiency of p-n junction GaAs solar cell is analytically investigated incorporating the effect of excess carriers due to photon incidence. Carrier transport equations for both types of carriers are solved involving both the conduction mechanisms along with generation rate. Change of dark current is reported in presence of varying operating conditions, and modulation of quantum efficiency is obtained by varying structural parameters. Results are useful for accurate estimation of figure of merit, which will play key role in photovoltaic applications.

Facet effect on the photoelectrochemical performance of a WO3/BiVO4 heterojunction photoanode

Different WO3 facets have different surface energies and electronic structures, and exhibit different water oxidation abilities and photocatalytic performance as a result. Because of the material’s limited photoresponse region, loading a narrow bandgap material on WO3 is a generally known method for improving photo-harvesting. In this paper, we have synthesized WO3 films with different crystal facet ratios. After loading BiVO4 on these WO3 films, we measured the photoelectrochemical (PEC) performance to investigate the effects of WO3 facet choice on the heterojunction film electrode’s performance. We found that a high-intensity ratio of the (002) WO3 facet in X-ray diffraction (XRD) leads to a more negative onset potential and higher photocurrents in a lower potential region. The ultraviolet photoelectron spectra show a lower work function for the 002-dominant WO3 film compared to other WO3 films, which may result in a higher quasi-fermi level for the heterojunction electrode. Based on the XRD results, the high-intensity ratio of the (002) WO3 facet preferentially exposes the (020) BiVO4 facet, which may be a reason for the better charge extraction observed at low applied potential and high faradic efficiency on PEC water splitting. Together, this results in a high hole injection efficiency for 002-dominant WO3/BiVO4 films compared with WO3/BiVO4 films favoring other WO3 facet ratios.

Complexation Chemistry in N,N-Dimethylformamide-Based Molecular Inks for Chalcogenide Semiconductors and Photovoltaic Devices.

Molecular inks based on dimethyl sulfoxide, thiourea (TU), and metal salts have been used to form high optoelectronic quality semiconductors and have led to high power conversion efficiencies for solution-processed photovoltaic devices for Cu2ZnSn(S,Se)4 (CZTS), Cu2Zn(Ge,Sn)(S,Se)4 (CZGTS), CuIn(S,Se)2 (CIS), and Cu(In,Ga)(S,Se)2 (CIGS). However, several metal species of interest, including Ag(I), In(III), Ge(II), and Ge(IV), either have low solubility (requiring dilute inks) or lead to precipitation or gelation. Here, we demonstrate that the combination of N,N-dimethylformamide (DMF) and TU has the remarkable ability to form intermediate-stability acid-base complexes with a wide number of metal chloride Lewis acids (CuCl, AgCl, ZnCl2, InCl3, GaCl3, SnCl4, GeCl4, and SeCl4), to give high-concentration stable molecular inks. Using calorimetry, Raman spectroscopy, and solubility experiments, we reveal the important role of chloride transfer and TU to stabilize metal cations in DMF. Methylation of TU is used to vary the strength of the Lewis basicity and demonstrate that the strength of the TU-metal chloride complex formed after DMF evaporation is critical to prevent volatilization of metal containing species. Further, we formulated a sulfur-free molecular ink which was used to deposit crystalline CuInSe2 without selenization that sustains high quasi-Fermi level splitting under constant illumination. Finally, we demonstrate the ability of the DMF-TU molecular ink chemistry to lead to high-photovoltaic power conversion efficiencies and high-open-circuit voltages for solution-processed CIS and CZGTS with power conversion efficiencies of 13.4% and 11.0% and Voc/ Voc,SQ of 67% and 63%, respectively.

Tunable terahertz amplification based on photoexcited active graphene hyperbolic metamaterials [Invited

The efficient amplification and lasing of electromagnetic radiation at terahertz (THz) frequencies is a non-trivial task achieved mainly by quantum cascade laser configurations with limited tunability and narrowband functionality. There is a strong need of compact and efficient THz electromagnetic sources with reconfigurable operation in a broad frequency range. Photoexcited graphene can act as the gain medium to produce coherent radiation at low THz frequencies but its response is very weak due to its ultrathin thickness. In this work, we demonstrate an alternative design to achieve efficient tunable and compact THz amplifiers and lasers with broadband operation based on active THz hyperbolic metamaterials (HMM) designed by multiple stacked photoexcited graphene layers separated by thin dielectric sheets. The hyperbolic THz response of the proposed ultrathin active HMM is analytically and numerically studied and characterized. When the graphene-based HMM structure is periodically patterned, a broadband slow-wave propagation regime is identified, thanks to the hyperbolic dispersion. In this scenario, reconfigurable amplification of THz waves in a broad frequency range is obtained, which can be made tunable by varying the quasi-Fermi level of graphene. We demonstrate that the THz response of the presented tunable THz amplifiers or lasers is controlled by the incident optical pumping (photodoping) and the loaded dielectric materials in the HMM waveguide array, an interesting property that can have great potential for THz amplification, emission, and sensing applications.

The Coulomb interaction in van der Waals heterostructures

The giant Stark effect (GSE) in a set of van der Waals (vdW) heterostructures is studied using first-principles methods. A straightforward model based on quasi-Fermi levels is proposed to describe the influence of an external perpendicular electric field on both band gap and band edges. Although a general linear GSE is observed, which is induced by the almost linear variation of the band edges of each layer in the heterostructures, when vdW heterostructures is subjected to small electric fields the variation becomes nonlinear. This can be attributed to the band offsets-induced interlayer charge transfer and resulted intra- and inter-layer Coulomb interactions. Our work, thus offers new insight into the mechanism of the nonlinear GSE in vdW heterostructures, which is important for the applications of vdW heterostructures on nanoelectronic devices.

Potassium fluoride postdeposition treatment with etching step on both Cu-rich and Cu-poor CuInSe2 thin film solar cells

Recent progress in the power conversion efficiency of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{S}{\mathrm{e}}_{2}$ thin film solar cells has been achieved by an alkali postdeposition treatment. This treatment has been shown to change the surface composition and structure as well as the bulk properties. To investigate the relative importance of those two effects we study the impact of the treatment on Cu-rich and Cu-poor $\mathrm{CuInS}{\mathrm{e}}_{2}$, which show a different influence of interface recombination without the treatment. We develop a potassium postdeposition treatment that can be applied to Cu-rich material, where an additional etching step is necessary. The same postdeposition treatment with etching step is applied to Cu-poor material. In both cases we observe an increase of the power conversion efficiency and open circuit voltage. Comparing the increase in open circuit voltage to the increase in quasi-Fermi level splitting indicates that the improvement in Cu-poor solar cells is mostly due to changes in the bulk, whereas in Cu-rich solar cells both the bulk and the interface are improved. The improvement of the interface is corroborated by temperature dependent current-voltage characteristics, which show that the dominating recombination path in Cu-rich solar cells moves from the interface to the bulk after treatment and by admittance spectroscopy, which shows that the treatment removes a 200 meV deep defect. Photoluminescence spectroscopy shows that even in Cu-rich material the alkali treatment creates a Cu-poor surface, which in this case cannot be created by diffusion of Cu into the bulk, but is grown during the treatment.

Solution-Processed BiI3 Films with 1.1 eV Quasi-Fermi Level Splitting: The Role of Water, Temperature, and Solvent during Processing.

We present a mechanistic explanation of the BiI3 film formation process and an analysis of the critical factors in preparing high-quality solution-processed BiI3 films. We find that complexation with Lewis bases, relative humidity, and temperature are important factors during solvent vapor annealing (SVA) of films. During SVA, water vapor and higher temperatures limit the formation of the BiI3–dimethylformamide coordination complex. SVA with an optimized water content and temperature produces films with 300–500 nm grains. Films that formed solvent coordination compounds at lower temperatures showed preferential crystal orientation after solvent removal, and we elucidate its implications for carrier transport. Addition of dimethyl sulfoxide to highly concentrated tetrahydrofuran–BiI3 inks prevents film cracking after spin-coating. We have measured a quasi-Fermi level splitting of 1.1 eV and a diffusion length of 70 nm from films processed with optimal temperature and humidity. The best device produced by optimized SVA has a power conversion efficiency of 0.5%, Isc of ∼4 mA/cm2, and VOC of ∼400 mV. The low photocurrent and voltage we attribute to the low diffusion length and the unfavorable band alignment between the absorber and the adjacent transport layers. The deep understanding of the relationship between morphology/crystal structure and optoelectronic properties gained from this work paves the way for future optimization of BiI3-based solar cells.

Analytical Modeling of Pinning Process in Pinned Photodiodes

In this paper, the pinning process of pinned photodiodes (PPDs) is described by a new analytical model, assuming the PPD to be composed of inner and junction regions. There are two definitions of the pinning potential: maximum conduction band potential variation ( $\Delta \varphi _{M}$ ) and maximum electron quasi-fermi level potential variation ( $\Delta $ FP $_{M}$ ). The output of the previous pinning potential models is only an approximation of $\Delta \varphi _{M}$ . In this paper, a comprehensive model is proposed in which both aforementioned definitions of pinning potential can be achieved analytically. The proposed model is a system of equations relating PPD’s main characteristics such as $\Delta \varphi _{M}$ and equilibrium full-well capacity to other main PPD parameters such as PPD spatial dimensions, impurity dopant concentrations, maximum inner region potential, and remnant carrier population at $\Delta \varphi _{M}$ . The proposed model is verified by two previously reported experiment-based data.

Multibandgap quantum dot ensembles for solar-matched infrared energy harvesting

As crystalline silicon solar cells approach in efficiency their theoretical limit, strategies are being developed to achieve efficient infrared energy harvesting to augment silicon using solar photons from beyond its 1100 nm absorption edge. Herein we report a strategy that uses multi-bandgap lead sulfide colloidal quantum dot (CQD) ensembles to maximize short-circuit current and open-circuit voltage simultaneously. We engineer the density of states to achieve simultaneously a large quasi-Fermi level splitting and a tailored optical response that matches the infrared solar spectrum. We shape the density of states by selectively introducing larger-bandgap CQDs within a smaller-bandgap CQD population, achieving a 40 meV increase in open-circuit voltage. The near-unity internal quantum efficiency in the optimized multi-bandgap CQD ensemble yielded a maximized photocurrent of 3.7 ± 0.2 mA cm−2. This provides a record for silicon-filtered power conversion efficiency equal to one power point, a 25% (relative) improvement compared to the best previously-reported results.

Multiphysics Modeling and Simulation of Carrier Dynamics and Thermal Transport in Monolayer MoS2/WSe2 Heterojunction

Computational study of atomically thin monolayer MoS2/WSe2 heterojunction with focus on carrier dynamics and thermal transport is performed by using finite-difference method to solve carrier transport equations and heat conduction equation. Carrier transport in the horizontal direction is in the micrometer scale, while in the vertical direction it is on the subnanometer scale. Carrier transport is modeled as diffusive transport in the horizontal direction with tunneling-assisted carrier recombination between two monolayers in the vertical direction as in the previous study. Band profile, quasi-Fermi level splitting, carrier distribution, and heat generation are investigated in detail. The heterojunction with high-doping concentration has larger current compared to that with low-doping concentration. Heat generation has two components which are due to the Joule heating and inelastic carrier recombination, respectively. The former is provided by the applied voltage, while the latter is from surrounding environment which acts as thermal reservoir. Since the heterojunction works very differently from the conventional p-n diode, it is compared to typical field-effect transistors due to similar device structures. The heat generation of the heterojunction is eight orders lower than that of typical nanoscale field-effect transistors because of its low-current density. Temperature increase in the heterojunction is also several orders of magnitude lower compared to typical nanoscale field-effect transistors.

Exploring deep defect state impact on open circuit voltage of conventional and inverted organic solar cells

In this manuscript, conventional and inverted organic solar cells based on P3HT:PC[60]BM have been explored to understand the effect of deep defect states on the open circuit voltage. The enhancement in the open circuit voltage in the inverted structure compared to the conventional structure has been comprehensively discussed in terms of density of defect states. To comply with the investigation, DC and AC measurements (impedance spectroscopy) at various temperatures have been performed extensively. Enhancement in open circuit voltage at low temperature is observed which is described by the shifting of hole and electron quasi-Fermi levels. The important observation from the defect density of states profile is that the center of Gaussian distribution is shifted to high energy as the temperature is increased which is an indication of the creation of shallow traps in polymers. In the inverted device, the disorder parameter (σ) is 33 meV, whereas in the conventional device it becomes 75 meV. This implies that the energetic disorder is reduced in an inverted device which helps in the improvement of open circuit voltage.