Recombination Activity Of Defects Research Articles

Formation and annihilation of bulk recombination-active defects induced by muon irradiation of crystalline silicon

Muons are part of natural cosmic radiation but can also be generated at spallation sources for material science and particle physics applications. Recently, pulsed muons have been used to characterize the density of free charge carriers in semiconductors and their recombination lifetime. Muon beam irradiation can also result in the formation of dilute levels of crystal defects in silicon. These crystal defects are only detected in high carrier lifetime silicon samples that are highly sensitive to defects due to their long recombination lifetimes. This work investigates the characteristics of these defects in terms of their formation, recombination activity, and deactivation. Charge carrier lifetime assessments and photoluminescence imaging have great sensitivity to measure the generated defects in high-quality silicon samples exposed to ∼4 MeV (anti)muons and their recombination activity despite the extremely low concentration. The defects reduce the effective charge carrier lifetime of both p- and n-type silicon and appear to be more detrimental to n-type silicon. Defects are created by transmission of muons through the wafer, and there are indications that slowed or implanted muons may create additional defects. In a post-exposure isochronal annealing study, we observe that annealing at temperatures of up to 450 °C does not by itself fully deactivate the defects. A recovery of charge carrier lifetime was observed when the annealing was combined with Al2O3 surface passivation, probably due to passivation of bulk defects from hydrogen from the dielectric film.

Effect of Gamma Irradiation on Electrophysical Parameters of Silicon Solar Cells Doped with Nickel

The results of the studies of changes in the electro-physical parameters (Voc – no-load voltage, Isc – short-circuit current density; τ – lifetime of nonequilibrium charge carriers) of photocells made on plates of monocrystalline silicon of the p-type conductivity with the specific resistance ρ 0.5 Ohm cm, doped with nickel, under irradiation with γ-quanta from 60Co source are presented. It is shown that the efficiency of the solar energy conversion in nickel-doped photovoltaic cells remains higher than in standard cells up to irradiation doses of 108 rad. It was established that with increasing diffusion temperature of nickel atoms radiation stability of electrophysical parameters of photovoltaic cells also increases. A decrease in the concentration of recombination-active radiation defects is due to the getterization by nickel atoms of technological (background) impurities and the action of nickel clusters as effluents for radiation-induced vacancies.

Measurement of local recombination activity in high diffusion length semiconductors

We present a conceptual approach for the localisation and characterisation of local sites of recombination in high diffusion length semiconductors under photovoltaic field conditions. While established imaging techniques operate in this very regime of uniform “1 sun” illumination, inevitable lateral diffusion of charge carriers veils the origin and severity of localised recombination sites. To reduce this limitation due to lateral diffusion the natural choice is using focussed charge carrier excitation and detection in combination with scanning the specimen. The resulting photoluminescence intensity maps are of high spatial resolution and may be composed of a superposition of a multitude of recombination active defects influencing each other due to the high bulk diffusion length. We demonstrate the feasibility of a self-consistent calibration of the setup quantum efficiency in such experimental condition which delivers a charge carrier density map in absolute units. A solution is presented to disentangle the superposition of local sites of recombination to isolate the actual recombination activity of every site. We demonstrate the feasibility of the approach experimentally on the high diffusion length semiconductor silicon.

Interactions of hydrogen atoms with boron and gallium in silicon crystals co-doped with phosphorus and acceptors

Reports showing that hydrogen and group-III acceptors play an important role in Light- and elevated Temperature-induced Degradation (LeTID) of Si-based solar cells highlight the need for a better understanding of interactions between these two species. In this contribution, a combination of junction spectroscopy techniques and first principles modelling has been used to study hydrogen-induced changes in electrical properties of either boron or gallium Czochralski-grown silicon co-doped with phosphorus in order to produce n-type material facilitating novel techniques to assess recombination active defects. The interactions of hydrogen with acceptor atoms have been induced via annealing of these co-doped hydrogenated samples with the application of reverse bias (RBA). These treatments have resulted in a significant increase in the net shallow donor concentration in depletion regions of both materials and in the appearance of a strong electron emission signal due to a trap with an energy level at about Ec −0.18 eV in the DLTS spectra of Si:P + B material. It is argued that this trap is related to the donor level of a BH2 complex. Calculations using density functional theory have shown that the BH2 defect has a charge-state dependent geometry, which turns out to be crucial for the proposed non-radiative recombination mechanism. The BH2 defect is therefore suggested to be the root cause of LeTID in boron-doped Si. In contrast, modelling results predict that GaH2 is a defect with shallow energy levels, without the characteristic features of a recombination centre. This is corroborated by the results of electrical measurements on hydrogenated Si:P + Ga subjected to RBA. Conventional annealing treatments were subsequently used to assess the thermal stability of acceptor-H related defects. Based on the obtained results, the peculiarities of hydrogen interactions with boron and gallium acceptors are discussed.

Hydrogen-induced degradation dynamics in silicon heterojunction solar cells via machine learning

Among silicon-based solar cells, heterojunction cells hold the world efficiency record. However, their market acceptance is hindered by an initial 0.5% per year degradation of their open circuit voltage which doubles the overall cell degradation rate. Here, we study the performance degradation of crystalline-Si/amorphous-Si:H heterojunction stacks. First, we experimentally measure the interface defect density over a year, the primary driver of the degradation. Second, we develop SolDeg, a multiscale, hierarchical simulator to analyze this degradation by combining Machine Learning, Molecular Dynamics, Density Functional Theory, and Nudged Elastic Band methods with analytical modeling. We discover that the chemical potential for mobile hydrogen develops a gradient, forcing the hydrogen to drift from the interface, leaving behind recombination-active defects. We find quantitative correspondence between the calculated and experimentally determined defect generation dynamics. Finally, we propose a reversed Si-density gradient architecture for the amorphous-Si:H layer that promises to reduce the initial open circuit voltage degradation from 0.5% per year to 0.1% per year.

Analysis of recombination centers near an interface of a metal–SiO2–Si structure by double carrier pulse deep-level transient spectroscopy

This paper proposes a new double carrier pulse deep-level transient spectroscopy (DC-DLTS) method that is applicable for evaluating metal–insulator–semiconductor (MIS) structures and the recombination centers in carrier-selective contact solar cells. Specifically, this study evaluated recombination characteristics of defects induced in bulk Si near SiO2/Si interfaces by reactive plasma deposition (RPD). In this method, a pulse voltage was first applied to inject majority carriers. Subsequently, a second pulse voltage was applied, which allowed minority carriers to be injected into the MIS structure. With these two types of carrier injections, carriers were recombined in recombination-active defects, and the DC-DLTS spectrum changed. During the injection of minority carriers, some majority carriers were thermally emitted from the defects, resulting in a decrease in the signal intensity. The recombination activity was analyzed by considering the effect of thermal emission on the change in signal intensity. The number of induced defect types and defect properties were estimated using Bayesian optimization. According to the results, three types of electron traps were generated using the RPD process. Based on the DC-DLTS results, defects with energy level 0.57 eV below the conduction band and capture cross section of ∼10−15 cm2 act as recombination centers.

LiI doping of mixed-cation mixed-halide perovskite solar cells: Defect passivation, controlled crystallization and transient ionic response

We investigate lithium iodide (LiI) doping of mixed-cation mixed-halide (MCMH) perovskites as a method to improve film morphology and optoelectronic properties, reduce defect-related recombination losses and limit ion migration. The optimized Li-doped devices achieved 21.5% power conversion efficiency (PCE) compared to 20.4% for undoped control devices. Furthermore, optimized Li-doped devices show minimized current-voltage hysteresis across a wide range of scan rates compared to control devices, which displayed significant scan-rate dependent hysteresis. We show, using a combination of experimental analysis and numerical device simulations, that the improved device performance and transient response at optimum doping concentration are consistent with reduced non-radiative recombination in the perovskite. However, increasing the LiI dopants beyond the optimum concentration alters the ionic properties of the perovskite absorber and changes the transient response. We also find that LiI addition results in a downward shift of the valence band and a corresponding transition from a weakly n-type to p-type perovskite material. These findings reveal the beneficial role of LiI dopant at low concentrations can improve shelf-life and minimize recombination-active defects, which are favourable for designing high-performance and stable mixed-cation mixed-halide perovskite devices.

Control of dislocation clusters by artificially-introduced micro-twins in cast-mono silicon

Cast-mono silicon (CM-Si) is a promising method to fabricate cost-effective photovoltaic silicon wafers, and dislocation clusters are currently one of the most detrimental defects in CM-Si. Once formed, they propagate rapidly and unlimitedly in the single crystalline lattice and then serve as strong recombination centers and shunts in solar cells, which severely influence the crystal quality and cell performance. This work innovatively introduces a method to restrain and modulate the propagation of dislocation clusters during CM-Si growth. The constitutional supercooling effect was induced in CM-Si by performing boron-indium co-doping to generate harmless micro-twins. Related characterizations are mainly focused on the microstructure as well as electrical properties of wafers. In addition, an interaction model that suggests micro-twins’ formation and the interaction with dislocations was proposed. The results show that these micro-twins grow in preferred 〈112〉 orientations so they can partially block the growth and propagation of dislocation clusters within the 〈110〉 directions, and the recombination-active defects at the upper crystal were remarkably redistributed. The density and area of the detrimental defects were decreased. We think these conclusions could offer strategies for silicon wafer manufacturers, and we hope the physical mechanism might intrigue the insights for other crystalline growth.

Light-induced performance of SHJ solar modules under 2000 h illumination

The light-induced degradation (LID) of solar module leads to severe loss in generated power due to the formation of recombination active defects. The silicon heterojunction (SHJ) solar cells based on n-type wafers are less affected by the LID test. In this paper, the glass/back sheet structure (GBS) modules with different encapsulant materials (TPO, POE, EVA) were laminated to evaluate their performance changes under 2000 h light soaking stress for the first time. The current-voltage (light I–V, dark I–V) parameters of the modules were measured, as well as the external quantum efficiency (EQE) and electroluminescence (EL), to verify the effect of light irradiation on SHJ solar modules. The SHJ modules with different encapsulant materials show excellent light-induced reliability. There is no degradation after 2000 h light irradiation, and even exhibits light-induced performance increase. The gain in output power (Pmax) is up to 1.41%, which is primarily due to an improved open-circuit Voltage (Voc) and fill factor (FF) as the result of a reduced density of recombination-active interface states. The front sheet/back sheet (FBS) modules also verified the accumulated light-induced stress could result in deteriorating interconnection between SHJ solar cell and ribbon, which suffers from significant output power degradation. The LID-free SHJ solar cells show great potential for lower levelized cost of energy (LCOE) of photovoltaic power generation.

Carrier Lifetime Limitation of Industrial Ga-Doped Cz-Grown Silicon After Different Solar Cell Process Flows

Gallium-doped silicon material has been rapidly gaining importance in the photovoltaic industry as a boron-oxygen defect-free material with promising minority carrier lifetime. We investigate the influence of different cell process flows [passivated emitter and rear cell tunneling-oxide-passivating contact, and a “hot oxidation” process] on the bulk material quality of an industrial Ga-doped Cz-grown silicon material, as well as its light- and elevated temperature induced degradation degradation behavior under light at elevated temperature. We measure a generally high carrier lifetime level, which remains limited by an unknown recombination-active defect after most processes. Hydrogenation seems to passivate this unknown defect. In addition, we demonstrate that such high-quality p-type material can suffer noticeably by iron even for extremely low concentrations below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> at/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

Growth of multicrystalline silicon assisted by electrophoretic deposited quartz

Cast multicrystalline silicon (mc-Si) requires high quality at low cost, while fully-melted seed-assisted growth has a reasonable utilization rate but urges better initial grain control. An electrophoretic deposited quartz nucleation layer is introduced to obtain a uniform and oriented nucleation layer. Carrier recombination characterization, including photoluminescence and carrier lifetime mapping, shows that the density of recombination-active structural defects is reduced due to better grain distribution. The average conversion efficiency values of silicon particle, sprayed quartz, and deposited quartz assisted wafers are 20.52%, 20.48%, and 20.68%, respectively Moreover, deposited quartz assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Investigation on the light and elevated temperature induced degradation of gallium-doped Cz-Si

In this study, we performed light and elevated temperature induced degradation (LeTID) experiments in gallium-doped Czochralski silicon (Cz-Si) wafers and investigated the impact of temperature on reaction kinetics. The degradation was found to have a fast and a slow process, and their reaction rates were extracted by using an exponential function. We further determined the activation energies of each degradation as well as the regeneration according to Arrhenius plots. Moreover, we found that the capture cross-section ratio of the generated defect remained in the range of 27 < k < 35 throughout the degradation. Based on these findings, we suggested that both fast and slow degradation might be attributed to the generation of a same recombination-active defect from different precursors, and consequently have different reaction rates.

On the Correlation between Light-Induced Degradation and Minority Carrier Traps in Boron-Doped Czochralski Silicon.

Boron-doped Czochralski-grown silicon wafers dominate the photovoltaic market. Light-induced degradation of these wafers is one of the most significant roadblocks for high-efficiency solar cells. Despite a very large number of publications on this topic, only a few studies have directly investigated the precursor of the defect responsible for this degradation. In this study, using the photoconductance decay measurement method, we identify the precursor of the defect responsible for light-induced degradation. By comparing the photoconductance decay of samples in the different states, we observe the presence of a minority carrier trap in the annealed state, which is not present after degradation. Trap annihilation shows a clear anticorrelation with the generation of the recombination-active boron-oxygen defect, as determined from minority carrier lifetime measurements. Furthermore, it is concluded that a model based on a single-level trap cannot explain the doping-dependent measurements, meaning that the detected trap has two or more energy levels.

Self-aligned local contact opening and n+ diffusion by single-step laser doping from POx/Al2O3 passivation stacks

Laser doping is a promising route to realise industrially compatible processing of local contacts for high-efficiency solar cells, especially when the same film acts as both dopant source and passivation layer. In this work we demonstrate simultaneous local contact opening and n+ laser doping of silicon from positively charged POx/Al2O3 thin-film stacks, which also provide outstanding passivation of n-type silicon surfaces. Local n+ doped regions with sheet resistance ranging from 35 to ~540 Ω/□ are formed using single nanosecond laser pulses with varying fluence. ECV profiling shows net n-type doping in all cases, confirmed by SIMS profiling to be due to phosphorus from the POx layer. J0 of metallised laser-doped regions is consistent with values achieved for state-of-the-art furnace diffusions with similar sheet resistance, confirming that laser-induced recombination-active defects are avoided. A minimum J0 of 540 fA cm−2 is obtained for metallised laser-doped regions formed from POx/Al2O3 passivation stacks having J0 of 2.5 fA cm−2. The combination of outstanding passivation of uncontacted n-type regions offered by POx/Al2O3, with self-aligned formation of locally-diffused contact openings via single-step laser processing, opens up exciting possibilities for simplified fabrication of high-efficiency cell structures.

Atom Probe Tomography Study of Gettering in High-Performance Multicrystalline Silicon

During the production of high-performance multicrystalline silicon (HPMC-Si) solar cells, gettering occurs inherently during the formation of an emitter. The material benefits with an increase in minority carrier lifetime from the external gettering of impurities into the diffused layer. However, depending on the thermal budget and parameters of the emitter diffusion, as well as the specific material properties, the process can also be detrimental in terms of increased recombination activity of specific crystallographic defects. Thus, it is important to understand the root causes behind the change in recombination activity of defects following gettering. Here, we present a correlative atom probe tomography study of grain boundaries in both p- and n-type HPMC-Si before and after gettering. The presence of nitrogen was found to directly correlate with the increase in recombination activity at grain boundaries. Additionally, an estimation of the atom probe tomography detection limit for transition metals in silicon is made and found to be greater than known impurity levels in commercial HPMC-Si.

What is a deep defect? Combining Shockley-Read-Hall statistics with multiphonon recombination theory

Slow nonradiative recombination is a key factor in achieving high open-circuit voltages or high luminescence yields in any optoelectronic material. Whether a defect is contributing substantially to nonradiative recombination is often estimated by defect statistics based on the model by Shockley, Read, and Hall. However, defect statistics are agnostic to the origin of the capture coefficients and therefore conclude that essentially every defect between the two quasi-Fermi levels is equally likely to be a recombination-active defect. Here, we combine Shockley-Read-Hall statistics with microscopic models for defect-assisted recombination to study how the microscopic properties of a material affect how recombination active a defect is depending on its energy level. We then use material parameters representative of typical photovoltaic absorber materials ($\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, Si, and GaAs) to illustrate the relevance, but also the limitations of our model.

Characterization of Mono-Crystalline and Multi-Crystalline Silicon by the Extended Lateral Photovoltage Scanning and Scanning Photoluminescence

Lateral Photovoltage Scanning (LPS) and Scanning Photoluminescence (SPL) methods are used simultaneously to reveal spatial distribution of electrically active defects in crystalline silicon. Defect information can be gained fast by measuring the phase shift between laser modulation and the LPS/SPL signals detected as the real and imaginary parts of the measurement signals. The laser power was varied from 10 μW to 100 mW at laser wavelengths of 660 nm and 830 nm. Simultaneously detected LPS and SPL signals from one and the same excited spot were used to determine minority charge carrier lifetimes. The signal gathering by two different methods shows a huge benefit to investigate the nature of recombination active defects. The minority carrier lifetime determined by SPL depends on the injection level while the minority lifetime analysed by LPS does not show a similar behaviour. Defect-related charge minority carrier lifetime maps of SPL scans show high similarity to those measured by MDP but have a higher local resolution. To investigate the smoothing effect, LPS measurements were made on mc-Si and FZ-Si samples.

Direct observation of hydrogen at defects in multicrystalline silicon

AbstractHydrogen passivation is a key industrial technique used to reduce the recombination activity of defects in multicrystalline silicon (mc‐Si). However, not all dislocations and grain boundaries respond well to traditional hydrogen passivation techniques. In order to understand the reasons for these different behaviours, and how superior passivation might be achieved, a method is required for the direct observation of hydrogen at these defects. Here, we present a novel characterisation technique based on a combination of transmission Kikuchi diffraction (TKD), atom probe tomography (APT), and isotopic substitution that enables unambiguous detection and quantification of hydrogen atoms present at crystallographic defects in mc‐Si.

Degradation and regeneration of radiation-induced defects in silicon: A study of vacancy-hydrogen interactions

Silicon lattice vacancies exist in all silicon wafers, will increase in concentration due to high temperature processing, can form recombination active defects and have been proposed as a possible candidate for LeTID. Despite this, there have been relatively few studies in the solar field dedicated to investigating vacancies, their recombination properties or their interaction with hydrogen. In this work, we use high energy electron radiation to create large numbers of vacancies in silicon and study the lifetime response to subsequent thermal processes in the presence or absence of bulk hydrogen. The radiation results in the formation of radiation-induced defects which we interpret as being related to the creation of vacancies in the silicon bulk. Modelling of injection dependent minority carrier lifetime finds that the defects formed via this radiation process have different Shockley-Read-Hall properties to LeTID and are as such, unlikely to be related. Furthermore, this defect can be removed during low temperature annealing but only if hydrogen is present in the bulk of the silicon wafers. This is in contrast to samples with no bulk hydrogen which do not recover during low temperature annealing. The study thus finds no evidence for any link between LeTID and vacancies in silicon, but it does demonstrate the ability of hydrogen to repair radiation damage.

Evaluating the Impact of SiNx Thickness on Lifetime Degradation in Silicon

There has been continuous effort to understand the cause of light- and elevated-temperature-induced degradation (LeTID) in silicon solar cells; however, the actual origin of the defect is still under investigation. Multiple reports in the literature suggest the involvement of hydrogen in activating the recombination-active defect that is responsible for this degradation. In this paper, we investigate the influence of the amount of in-diffused hydrogen in the bulk on the degradation in silicon lifetime test structures. We examine this by varying the thickness of hydrogenated silicon nitride (SiNx:H) before high-temperature firing. Fourier transform infrared spectroscopy is performed to confirm that the hydrogen content in SiNx:H film scales with its thickness. We observe that an increase in the thickness of hydrogen-rich SiNx:H leads to an almost proportional increase in the extent of defect concentration in multicrystalline silicon wafers. We attribute this increase to the higher amount of hydrogen released from thicker SiNx:H layers into the bulk during firing. This paper provides further evidence for the involvement of hydrogen in the formation of the LeTID defect in silicon.