Recombination Strength Research Articles

Dielectronic recombination strengths and plasma rate coefficients of multiply charged ions

Context. Dielectronic recombination (DR) has been known as the dominant electron-ion recombination process in different astrophysical and laboratory plasmas, and that it determines the level population and ionization balance over a range of temperatures. Apart from a fundamental interest into the details of this process, DR plasma rate coefficients are frequently applied to estimate plasma densities and temperatures, but have been found to be notoriously difficult to calculate as they require good knowledge of the ionic resonances, which are embedded into the continuum of the next higher charges states. Aims. In this paper we explain and demonstrate how DR resonance strengths and plasma rate coefficients can be readily computed within the framework of the Jena Atomic Calculator (JAC). In contrast to other available codes, the JAC toolbox supports a much simpler handling and control of different approximations, shell structures and temperature regions, for which doubly excited resonances need to be taken into account. Methods. A multi-configuration Dirac–Hartree–Fock expansion of all atomic states is generated and applied in order to compute the transition rates (radiative and nonradiative) that contribute to the DR process. For the plasma rate coefficients, moreover, a cascade model has been developed that automatically determines and incorporates all doubly excited configurations of interest for the given plasma temperatures. Results. To demonstrate the quite flexible use of JAC, we discuss and compare the DR of initially fluorine-like Ni19+ ions with previous measurements and computations. Since it is based on Dirac’s equation, the JAC toolbox is suitable for most ions across the periodic table.

Electron-Beam-Induced Current Study of Dislocations and Leakage Sites in GaN Schottky Barrier Diodes

This work aims to clarify the electrical activities of threading dislocations and their relation with leakage sites in homoepitaxial GaN Schottky barrier diodes based on the electron-beam-induced current (EBIC) technique and transmission electron microscopy (TEM). First, the recombination activities of threading dislocations in epilayers grown on different substrates are compared by EBIC. The dislocation type is characterized based on etch pit measurements and TEM. The dislocation density and character are strongly affected by defects in the substrate. The recombination strength of dislocations is revealed to be correlated with their type. It is found that single dislocations including both edge and mixed type exhibit weak (< 5%) EBIC contrast, while dislocation clusters show strong contrast (up to 30%). Second, leakage sites in Schottky diodes are visualized by EBIC under reverse bias. There is no direct correlation between the initial leakage sites and threading dislocations; whereas, instead of dislocations, a variety of initial leakage/breakdown sites are found, including grown-in pit defects as initial breakdown sites and hillocks at the Schottky interface acting as strong leakage sites.

Local Defects in Colloidal Quantum Dot Thin Films Measured via Spatially Resolved Multi-Modal Optoelectronic Spectroscopy.

The morphology, chemical composition, and electronic uniformity of thin-film solution-processed optoelectronics are believed to greatly affect device performance. Although scanning probe microscopies can address variations on the micrometer scale, the field of view is still limited to well under the typical device area, as well as the size of extrinsic defects introduced during fabrication. Herein, a micrometer-resolution 2D characterization method with millimeter-scale field of view is demonstrated, which simultaneously collects photoluminescence spectra, photocurrent transients, and photovoltage transients. This high-resolution morphology mapping is used to quantify the distribution and strength of the local optoelectronic property variations in colloidal quantum dot solar cells due to film defects, physical damage, and contaminants across nearly the entire test device area, and the extent to which these variations account for overall performance losses. It is found that macroscopic defects have effects that are confined to their localized areas, rarely prove fatal for device performance, and are largely not responsible for device shunting. Moreover, quantitative analysis based on statistical partitioning methods of such data is used to show how defect identification can be automated while identifying variations in underlying properties such as mobilities and recombination strengths and the mechanisms by which they govern device behavior.

Investigation of the intershell higher order resonant recombination and polarization of X‐ray emission of B‐like silicon ions

K‐shell dielectronic, trielectronic, and quadruelectronic recombination and polarization of X‐ray emission have been studied for the highly charged Si9+ ions in the initial ground state 2P1/2 and the metastable state 2P3/2 using the Flexible Atomic Code. It is found that the resonant recombination cross section from the long‐lived metastable state is comparable in magnitude with that of the ground state, so it is important for plasma diagnostics. For Si9+(2P1/2), trielectronic recombination contributions of nearly 25% to the total resonant recombination strength are predicted, which is less than previous calculations. We compare the degree of linear polarization for the eleven dominant resonant recombination satellite lines from the initial parent Si9+(2P1/2) and Si9+(2P3/2) ions. For the same X‐ray lines, large variations of polarization are found between 2P1/2 and 2P3/2, which can be employed to diagnose formation mechanisms of intermediate resonance states and corresponding X‐ray lines.

On the effect of surface recombination in thin film solar cells, light emitting diodes and photodetectors

Radiative and non-radiative charge carrier recombination in thin-film diodes plays a key role in determining the efficiency of electronic devices made of next generation semiconductors such as organic, perovskite and nanocrystals. In this work, we show that lowering the bulk recombination does not necessarily result in enhanced performance metrics of electronic devices. From the perspective of charge carrier extraction and injection, the radiative limit of the open-circuit voltage of solar cells, noise current of photodetectors and lasing threshold of injection lasers cannot be improved if the contacts are not perfectly selective. A numerical drift-diffusion model is used to investigate the interplay between bulk recombination and surface recombination of minority carriers at the contacts in bipolar thin diode devices based on low-mobility semiconductors. The surface recombination becomes prominent in case of reduced bulk recombination strengths when non-selective contacts, i. e. contacts that are either metallic or have imperfect charge-selective interlayer, are employed. Finally, we derive analytical approximations for the case when diffusion-limited surface recombination of minority carriers at Ohmic contacts dominates the dark current. These results indicate that having perfectly selective contacts becomes crucial in systems with suppressed bulk recombination - a challenging requirement for future state-of-the-art thin-film solar cells, light-emitting devices and photodetectors made of next generation semiconductors.

Investigating Free Charge‐Carrier Recombination in Organic LEDs Using Open‐Circuit Conditions

AbstractAlthough organic light‐emitting diodes (LEDs) are widely used in commercial applications, a comprehensive knowledge about the occurring recombination pathways is still absent. Especially access to the bandgap is needed to figure out which path the charge carriers take, key to further understand and optimize the latest generations of thin film LED technology. Here, state‐of‐the‐art organic LEDs are treated like solar cells by measuring the open‐circuit voltage under UV illumination. A variation of the illumination intensity and temperature gives access to the effective bandgap, the ideality factor and the recombination strength. A differential evaluation approach is implemented, yielding the full temperature and intensity dependence of these parameters. A radiative and a non‐radiative recombination process are identified and used to reconstruct the current density dependence of the external quantum efficiency. All recombination events are related to an effective bandgap that suggests free charge recombination between the guest and the host material of the emission layer. This approach will thus help to identify and eliminate non‐radiative recombination losses in LEDs based on other type of molecules emitters and materials, such as perovskites or quantum dots, as well. Knowing the effective bandgap will also be essential to realize LEDs with the lowest possible driving voltages.

Spatially Resolved Recombination Analysis of CuInxGa1-xSe2 Absorbers With Alkali Postdeposition Treatments

In this contribution, we probe spatial variations in charge-carrier recombination in CuInxGa1-x Se2 (CIGS) absorbers grown on soda–lime glass (SLG) and alkali-free sapphire substrates with NaF and KF postdeposition treatments (PDTs). Temperature-and illumination-dependent device measurements are used to track interface recombination and recombination in the quasi-neutral region. The analysis of these data reveals that the benefit of alkali PDTs depends on the substrate: interface recombination is reduced in devices grown on sapphire substrates, whereas recombination in the quasi-neutral regions is reduced in devices grown on SLG substrates. Cathodoluminescence (CL) spectrum imaging is used to study the spatial distribution of recombination with respect to the grain structure. The grain-boundary CL contrast is similar in films with no PDT, NaF PDT, or KF PDT. A reduced grain-boundary contrast is observed with a NaF + KF PDT; however, suggesting a reduced recombination strength at the grain boundaries (GBs) for combined NaF + KF treatment. CL spectra indicate band tailing, consistent with the fluctuating potential model. Fluctuating potentials are believed to reduce open-circuit voltage, but their spatial distribution has not been studied. Here, CL spectrum imaging data are used to generate maps of the root-mean-square value of the potential energy fluctuations—γ. These maps reveal a bimodal γ distribution for all samples: γ is generally in the range ∼15–50 meV or ∼100–180 meV. The higher γ range is more significantly affected by the PDTs; after the PDTs, it is strongly correlated with GBs. The lower γ range is correlated with higher emission intensity regions, typically grain interiors, and increases in area fraction after the PDTs. These results demonstrate how spatially resolved luminescence and device characterization measurements can be used to monitor changes in recombination in CIGS films and photovoltaic devices. Such measurements can complement empirical device optimization and help improve device performance.

Recombination activity of threading dislocations in GaInP influenced by growth temperature

Room-temperature non-radiative recombination is studied at single dislocations in Ga0.5In0.5P quantum wells grown on metamorphic templates using cathodoluminescence and electron channeling contrast imaging. An analysis of the light emission intensity profiles around single dislocations reveals that the average recombination strength of a dislocation decreases by a factor of four and seven as a result of decreasing growth temperature of the GaInP quantum well from 725 to 675 and 625 °C, respectively. This reduction occurs despite little change in the diffusion length, precluding the prospect of inducing carrier localization by ordering and phase separation in GaInP at lower growth temperatures. These observations are rationalized by the premise that point defects or impurities are largely responsible for the recombination activity of dislocations, and the extent of decoration of the dislocation core decreases with temperature. Preliminary evidence for the impact of the Burgers vector is also presented. The lowest growth temperature, however, negatively impacts light emission away from dislocations. Carrier recombination in the bulk and at dislocations needs to be considered together for metamorphic devices, and this work can lead to new techniques to limit non-radiative recombination.

Impact of Electrodes on Recombination in Bulk Heterojunction Organic Solar Cells.

In recent years, the efficiency of organic solar cells (OSCs) has increased to more than 13%, although different barriers are on the way for reaching higher efficiencies. One crucial barrier is the recombination of charge carriers, which can either occur as the bulk recombination of photogenerated charges or the recombination of photogenerated charges and electrodic induced charges (EICs). This work studies the impact of EICs on the recombination lifetime in OSCs. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of EICs is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach has been introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach was conducted by depositing transparent insulating layers on both sides of the OSC active layer. An examination of these approaches on OSCs with different active layer materials, thicknesses, and varying light intensities determined that the EICs can only reduce the recombination lifetime of the photogenerated charges in OSCs with very weak recombination strength. This work supports that for OSCs with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance. Therefore, the use of a proper blocking layer suppresses EIC recombination in systems with very weak recombination.

Recombination properties of dislocations in GaN

The recombination activity of threading dislocations in n-GaN with different dislocation densities and different doping levels was studied using electron beam induced current (EBIC). The recombination velocity on a dislocation, also known as the dislocation recombination strength, was calculated. The results suggest that dislocations in n-GaN giving contrast in EBIC are charged and surrounded by a space charge region, as evidenced by the observed dependence of dislocation recombination strength on dopant concentration. For moderate (below ∼108 cm−2) dislocation densities, these defects do not primarily determine the average diffusion length of nonequilibrium charge carriers, although locally, dislocations are efficient recombination sites. In general, it is observed that the effect of the growth method [standard metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth versions of MOCVD, and hydride vapor phase epitaxy] on the recombination activity of dislocations is not very pronounced, although the average diffusion lengths can widely differ for various samples. The glide of basal plane dislocations at room temperature promoted by low energy electron irradiation does not significantly change the recombination properties of dislocations.

Recombination Strength of Dislocations in High‐Performance Multicrystalline/Quasi‐Mono Hybrid Wafers During Solar Cell Processing

Wafers from a hybrid silicon ingot seeded in part for High Performance Multicrystalline, in part for a quasi‐mono structure, are studied in terms of the effect of gettering and hydrogenation on their final Internal Quantum Efficiency. The wafers are thermally processed in different groups – gettered and hydrogenated. Afterwards, a low temperature heterojunction with intrinsic thin layer cell process is applied to minimize the impact of temperature. Such procedure made it possible to study the effect of different processing steps on dislocation clusters in the material using the Light Beam Induced Current technique with a high spatial resolution. The dislocation densities are measured using automatic image recognition on polished and etched samples. The dislocation recombination strengths are obtained by a correlation of the IQE with the dislocation density according to the Donolato model. Different clusters are compared after different process steps. The results show that for the middle of the ingot, the gettering step can increase the recombination strength of dislocations by one order of magnitude. A subsequent passivation with layers containing hydrogen can lead to a decrease in the recombination strength to levels lower than in ungettered samples.

Dislocation contrast in cathodoluminescence and electron-beam induced current maps on GaN(0 0 0 1)

We theoretically analyze the contrast observed at the outcrop of a threading dislocation at the GaN(0 0 0 1) surface in cathodoluminescence and electron-beam induced current maps. We consider exciton diffusion and recombination including finite recombination velocities both at the planar surface and at the dislocation. Formulating the reciprocity theorem for this general case enables us to provide a rigorous analytical solution of this diffusion-recombination problem. The results of the calculations are applied to an experimental example to determine both the exciton diffusion length and the recombination strength of threading dislocations in a free-standing GaN layer with a dislocation density of .

Modeling of recombination strength at grain boundaries after phosphorus diffusion gettering and the effect of hydrogen passivation

We present a method for extracting local recombination rates from photoluminescence images of double side passivated wafers, using simulations of lateral charge carrier diffusion in two dimensions. By fitting the simulated carrier lifetime map with a calibrated photoluminescence image, the recombination profiles in bulk and at grain boundaries (GBs) can be extracted. This method can be used to quantify the GB recombination velocity even if the carrier concentration in bulk is simultaneously affected by recombination at multiple GBs. High performance multicrystalline wafers from a commercially cast ingot with low impurity concentrations were studied. The GBs were completely passivated applying a hydrogenation step simulated contact firing of a hydrogen rich silicon nitride (SiN) film. The bulk carrier recombination profile from simulations of carrier diffusion after phosphorus diffusion gettering is similar to the measured lifetime after phosphorus diffusion gettering and firing. This indicates that the main role of hydrogen passivation is deactivation of the GB recombination. No additional enhancement of the bulk lifetime is observed after hydrogen passivation.

On the recombination centers of iron-gallium pairs in Ga-doped silicon

Gallium (Ga) doped silicon (Si) is becoming a relevant player in solar cell manufacturing thanks to its demonstrated low light-induced degradation, yet little is known about Ga-related recombination centers. In this paper, we study iron (Fe)-related recombination centers in as-grown, high quality, directionally solidified, monocrystalline Ga-doped Si. While no defect states could be detected by deep level transient spectroscopy, lifetime spectroscopy analysis shows that the minority carrier lifetime in as-grown wafers is dominated by low levels of FeGa related defect complexes. FeGa pairs have earlier been shown to occur in two different structural configurations. Herein, we show that in terms of recombination strength, the orthorhombic pair-configuration is dominant over the trigonal pair-configuration for FeGa. Furthermore, the defect energy level in the band gap for the orthorhombic defect center is determined to be EV + 0.09 eV, and the capture cross-section ratio of the same defect center is determined to be 220.

Dislocation-limited performance of advanced solar cells determined by TCAD modeling

In recent years, low-cost silicon for photovoltaic applications has trended toward smaller grain sizes with lower dislocation densities. These changes, coupled with advanced device architectures like heterojunction and passivated emitter and rear cells, have enabled higher efficiencies, including new world records. To establish material requirements for further improvement, we model dislocations with different recombination strengths in Sentaurus Device using mid-gap defects spatially localized at the nanoscale in three dimensions. This approach accounts for the charging of dislocations and differentiates between dislocations and grain boundaries or metal impurities. Simulations that model dislocations as macroscopic variations in effective bulk lifetime or diode parameters do not address these issues. We validate our model by simulating the world record multicrystalline silicon solar cell. We find that three parameters influence cell efficiency: the area fraction of the cell containing dislocations, the dislocation density within defect clusters, and the concentration of recombination centers at these dislocations. We provide targets for these parameters and show how our results can be used to determine the potential gains from reducing dislocations in experimental devices and materials. We further predict that device architectures that lead to higher injection, such as silicon heterojunction cells, are more robust to the presence of dislocations.

In-depth considerations for better polyelectrolytes as interfacial materials in polymer solar cells

We perform a comprehensive study to achieve better polyelectrolytes (PEs) as electron-transport layers (ETLs) in polymer solar cells (PSCs). Three well-known PEs – PFN, PEIE, and WPF – are chosen as model systems and investigated with variations in their backbone structures and the state of the amine functionalities on their side chains. Respectively optimized PSCs using the three PEs exhibit different cell-performances, mainly owing to the diode characteristics of built-in potential and recombination strength. To identify how such deviated device-performances correlate with the structural features of PEs, the modulated interfaces of ITO/PEs and PEs/active layer are studied in detail. It is found that conjugated backbones and larger counter-anions on side chains can promote the modulation of ITO work functions (WFs) and that a large amount of protonated amines on PEs is beneficial for junction properties with a subsequent active layer. Additionally, our results indicate that interfacial dipole and electrical doping between the PE and active layer, in addition to WF modulation of the ITO cathode, are important for device efficiency. Accordingly, with the aid of the molecular features of PEIE, PEIE-PSCs exhibit excellent device efficiency and stability compared with PFN- and WPF-PSCs. In the PTB7-th:PC71BM system, a remarkable power-conversion efficiency of 9.97% is achieved with a single PEIE ETL.

Relating Nongeminate Recombination to Charge-Transfer States in Bulk Heterojunction Organic Photovoltaic Devices

The importance of nongeminate recombination of free photogenerated carriers through charge-transfer (CT) states compared with at the electrode interfaces in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices was investigated using impedance spectroscopy (IS) and low-energy external quantum efficiency (EQE) measurement. Thin (∼60 nm) poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-co-octylthieno[3,4-c] pyrrole-4,6-dione):[6,6]-phenyl-C70-butyric acid methyl ester (PBDT-TPD:PC71BM) OPVs processed with varying concentrations of solvent additive 1-chloronaphthalene (CN) provide a well-controlled system in which only the donor–acceptor interfacial area, hence CT concentration, is changed. We found that additive inclusion resulted in increased CT state concentration, measured by low-energy EQE spectroscopy, and higher nongeminate recombination, determined from IS. In contrast, the energetic disorder in the BHJ, as determined from the dependence of carrier density on open-circuit voltage, did not show a dependence on CN concentration, suggesting that it is not related to changes in donor–acceptor morphology. The correlated relationship between nongeminate recombination strength and CT state concentrations presents unambiguous evidence of CT states as the major channels in nongeminate recombination loss in these BHJ OPV devices.

Quantifying carrier recombination at grain boundaries in multicrystalline silicon wafers through photoluminescence imaging

We present a method based on steady state photoluminescence (PL) imaging and modelling of the PL intensity profile across a grain boundary (GB) using 2D finite element analysis, to quantify the recombination strength of a GB in terms of the effective surface recombination velocity (Seff). This quantity is a more meaningful and absolute measure of the recombination activity of a GB compared to the commonly used signal contrast, which can strongly depend on other sample parameters, such as the intra-grain bulk lifetime. The method also allows the injection dependence of the Seff of a given GB to be explicitly determined. The method is particularly useful for studying the responses of GBs to different cell processing steps, such as phosphorus gettering and hydrogenation. The method is demonstrated on double-side passivated multicrystalline wafers, both before and after gettering, and single-side passivated wafers with a strongly non-uniform carrier density profile depth-wise. Good agreement is found between the measured PL profile and the simulated PL profile for both cases. We demonstrate that single-side passivated wafers allow more recombination active grain boundaries to be analysed with less unwanted influence from nearby features. The sensitivity limits and other practical constraints of the method are also discussed.

Charge transport and recombination in PDPP5T:[70]PCBM organic solar cells: The influence of morphology

The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. Morphological parameters, such as the extent and the composition of donor- and acceptor-rich domains, influence both the charge generation and the charge transport throughout the active layer. This work focuses on a polymer:fullerene system based on a small bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (PDPP5T) mixed with [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) that is capable of efficiencies higher than 6%. By changing the processing conditions, the morphology can be varied from a coarse separated morphology, with fullerene domains (blobs) embedded in a polymer-rich matrix, to a completely mixed layer.The charge carrier transport and the strength of the bimolecular recombination in PDPP5T:[70]PCBM blends with different morphologies and fullerene concentrations are experimentally characterized. The large difference in electron and hole mobility and the electric field dependency of the electron mobility are identified as the causes that limit the performance of devices with low [70]PCBM content. These effects are not present if the concentration of [70]PCBM is increased while keeping a fine phase separation by the addition of ortho-dichlorobenzene as a cosolvent.For the phase-separated blends, a model is proposed, based on a drift–diffusion approach that combines electrical and morphological parameters; with this model the contribution of each phase to the total current is quantified. Under operating conditions, most of the current comes from the interfacial region between the phases, with holes traveling through the matrix and the electrons through the blobs. This device model consistently connects morphological features to overall device performance.

Evaluating Charge Carrier Mobility Balance in Organic Bulk Heterojunctions using Lateral Device Structures

Lateral geometry devices are useful structures to probe the charge transport properties of organic bulk heterojunction materials. In this work, we describe the lateral device as effectively having three distinct zones: two photogenerative space-charge regions next to each electrode and a middle zone that has high recombination rate. We analyze how photocurrent depends on electron and hole mobilities and recombination strength and, based on this analysis, demonstrate a steady-state method for extracting both carrier mobilities and the recombination coefficient from potentiometry and photocurrent data.