Transient Ion Drift Research Articles

The influence of A-site dipole moment on iodine migration in perovskite films revealed by transient ion drift

Ion migration is regarded as the chief culprit for the long-term instability in perovskite solar cells (PSCs), and iodine ions are the most subjects to migration in PSCs. Strategies, such as grain size engineering, chemical bonding engineering, steric impediment effect, and interface passivation, are employed to suppress the iodine migration but unfortunately are unable to completely eliminate it. Here, the influence of the dipole moment of A-site ions on iodine migration has been comprehensively investigated for perovskite films based on different A-site cations via a transient ion drift (TID) method. TID results demonstrate that the decrease in the strength of the dipole moment of A-site ions can lead to a decrease in the activation energy (EA) of iodine ions. Therefore, this work suggests that increasing the strength of dipole moment of A-site ions can be utilized as an effective strategy for suppressing iodine migration in perovskite films providing critical insight for enhancing the long-term stability for PSCs.

Understanding the Influence of Cation and Anion Migration on Mixed‐Composition Perovskite Solar Cells via Transient Ion Drift

Ion migration has a significant influence on perovskite solar cells (PSCs), such as inducing hysteresis and long‐term instability problems. Mixed‐composition PSCs demonstrate less hysteresis and better long‐term stability. Nevertheless, the influence of cation and anion migration on reduced hysteresis and better long‐term stability in mixed‐composition PSCs is still poorly understood. Herein, the ion‐migration process in mixed‐composition PSCs is investigated by the transient ion drift (TID) method. Suppressed ion migration for both cations and anions is observed in mixed‐composition PSCs as compared with that of I‐based PSCs. Interestingly, the suppression of anion migration in mixed‐composition PSCs is most notable, that is, ten times smaller than the rate of anion migration in I‐based PSCs. Moreover, the scan‐speed‐dependent hysteresis measurement results are consistent with the migration timescale of the cations. It is inferred that the better long‐term stability of mixed‐composition PSCs predominantly originates from the remarkably suppressed anion migration, whereas the reduced hysteresis originates from the suppressed cation migration. In conclusion, this research provides new insights into the origins of better long‐term stability and reduced hysteresis in mixed‐composition PSCs.

Understanding the Stability of MAPbBr3 versus MAPbI3: Suppression of Methylammonium Migration and Reduction of Halide Migration.

Solar cells based on metal halide perovskites often show excellent efficiency but poor stability. This degradation of perovskite devices has been associated with the migration of mobile ions. MAPbBr3 perovskite materials are significantly more stable under ambient conditions than MAPbI3 perovskite materials. In this work, we use transient ion drift to quantify the key characteristics of ion migration in MAPbBr3 perovskite solar cells. We then proceed to compare them with those of MAPbI3 perovskite solar cells. We find that in MAPbBr3, bromide migration is the main process at play and that contrary to the case of MAPbI3, there is no evidence for methylammonium migration. Quantitatively, we find a reduced activation energy, a reduced diffusion coefficient, and a reduced concentration for halide ions in MAPbBr3 compared to MAPbI3. Understanding this difference in mobile ion migration is a crucial step in understanding the enhanced stability of MAPbBr3 versus MAPbI3.

Effect of low-temperature annealing on defect causing copper-related light-induced degradation in p-type silicon

Copper is a common impurity in photovoltaic silicon. While reported to precipitate instantly in n-type Si, copper causes light-induced degradation (Cu-LID) in p-type Si. Recently, partial recovery of Cu-LID was observed after only few minutes of dark annealing at 200 °C. In this contribution, we investigate the effects of the dark anneal on Cu-LID-limited minority carrier lifetime both experimentally and by simulations. Surprisingly, after initial recovery, the dark anneal results in further degradation corresponding to a many-fold increase in recombination activity compared to the degraded state after illumination. This anneal-induced degradation can potentially cause additional losses in accidentally Cu-contaminated devices when exposed to elevated temperatures, for example during recovery and regeneration treatments of solar cells. Transient ion drift measurements confirmed that the anneal-induced degradation cannot be attributed to residual interstitial Cu after illumination. After hundreds of hours of annealing, the samples showed another recovery. To analyze these experimental results, a comparison to simulations is performed at the end of the paper.

A first-principles model of copper–boron interactions in Si: implications for the light-induced degradation of solar Si

The recent discovery that Cu contamination of Si combined with light exposure has a significant detrimental impact on carrier life-time has drawn much concern within the solar-Si community. The effect, known as the copper-related light-induced degradation (Cu-LID) of Si solar cells, has been connected to the release of Cu interstitials within the bulk (2016 Sol. Energy Mater. Sol. Cells 147 115–26). In this paper, we describe a comprehensive analysis of the formation/dissociation process of the CuB pair in Si by means of first-principles modelling, as well as the interaction of CuB defects with photo-excited minority carriers. We confirm that the long-range interaction between the cation and the anion has a Coulomb-like behaviour, in line with the trapping-limited diffusivity of Cu observed by transient ion drift measurements. On the other hand, the short-range interaction between the d-electrons of Cu and the excess of negative charge on produces a repulsive effect, thereby decreasing the binding energy of the pair when compared to the ideal point-charge Coulomb model. We also find that metastable CuB pairs produce acceptor states just below the conduction band minimum, which arise from the Cu level emptied by the B acceptor. Based on these results, we argue that photo-generated minority carriers trapped by the metastable pairs can switch off the Coulomb interaction that holds the pairs together, enhancing the release of Cu interstitials, and acting as a catalyst for Cu-LID.

Investigation of capacitance transients in CuInS 2 due to ionic migration

Transport of mobile ions in n-TiO 2/n-CuInS 2/p-CuInS 2 thin-film devices is studied with the transient ion-drift (TID) method. In contrast to the normal TID method, a mobile ion profile is created in the CuInS 2 layer, which can be described by the Gouy–Chapman theory. Activation energies for diffusion of 0.5 and 1.0 eV are found. We postulate that these activation energies are related to the associated defect, ( Cu In ″ In Cu ) x , which introduces a deep electronic state inside the bandgap of CuInS 2. This defect can accept or release an electron and drift out of the depletion region. This will lower the concentration of recombination centers in the depletion region, which explains the self-healing property of CuInS 2.

Quantitative copper measurement in oxidized p-type silicon wafers using microwave photoconductivity decay

We propose a method to measure trace copper contamination in p-type silicon using the microwave photoconductivity decay (μ-PCD) technique. The method is based on the precipitation of interstitial copper, activated by high-intensity light, which results in enhanced minority carrier recombination activity. We show that there is a quantitative correlation between the enhanced recombination rate and the Cu concentration by comparing μ-PCD measurements with transient ion drift and total reflection x-ray fluorescence measurements. The results indicate that the method is capable of measuring Cu concentrations down to 1010cm−3. There are no limitations to wafer storage time if corona charge is used on the oxidized wafer surfaces as the charge prevents copper outdiffusion. We briefly discuss the role of oxide precipitates both in the copper precipitation and in the charge carrier recombination processes.

Influence of light on interstitial copper in p -type silicon

In this work, the influence of light absorption on copper related defect reactions in p-type silicon is investigated by transient ion drift measurements. It is found that copper out-diffusion and formation of copper complexes are two competitive processes whose relative importance depends on light intensity. At low intensities, a reduced surface potential allows complete copper out-diffusion. Above a certain threshold value, however, copper complexes are formed which could be responsible for the earlier reported optically activated recombination activity of copper in silicon. It is shown that the observed Fermi-level dependence of the defect formation rate cannot be explained in terms of an optically induced change in the interstitial copper charge state. The high thermal stability of the formed copper complexes further points out that the formation of electrostatically bound copper-defect pairs may be ruled out.

Light Activated Copper Defects in P-Type Silicon Studied by PCD

We have studied copper defects in p-type silicon by measuring its precipitation kinetics by means of the microwave photoconductive decay (µPCD) technique. Copper atoms precipitated during high intensity light treatment at room temperature. We used the total reflection X-ray fluorescence (TXRF) and the transient ion drift (TID) techniques to determine the bulk concentration of copper. We estimated the density and the radius of the copper precipitates as well as the average capture cross-section for precipitated copper atoms from the measured copper precipitation time constant, bulk concentration of copper, and the change in the recombination rate. We also studied how the density of oxygen defect affects the copper precipitation. Our results show that copper precipitates at two different kinds of defects.

Copper Behavior in Bulk Silicon and Associated Characterization Techniques

This work reviews recent developments in the field of copper trace detection by advanced electrical techniques and presents new data on the behavior of copper in silicon. First the current understanding of copper in silicon is described. In addition, physical principles, assumptions, and limitations of electrical methods based either on minority carrier dynamics, or transient ion drift (TID) are discussed. In the second part new TID data obtained on quantitatively contaminated silicon are given. The influences of annealing temperature, cooling rate, and storage conditions on copper impurities are addressed. It is shown that in high quality silicon, copper impurities may remain on interstitial sites for several days at room temperature. Under illumination the surface photovoltage increases the out-diffusion rate considerably. In float-zone material, evidence for copper trapping by grown-in defects is given. The discussion focuses on the impact these results have on the copper trace detection methods. © 2003 The Electrochemical Society. All rights reserved.

Optimisation of a combined transient-ion-drift/rapid thermal annealing process for copper detection in silicon

The transient ion drift (TID) technique has been recently proposed for copper trace detection in silicon. Cu atoms may be present either in the vicinity of the Si surface or within the volume. In the latter case they are either gathered at secondary defects or form precipitates believed to be silicides. In order to become detectable by TID Cu atoms must be put into the highly mobile interstitial state. Depending on the initial configuration of the Cu/Si system different physical mechanisms may enable Cu atoms to become ‘TID active’. In this work we study the Cu activation process using rapid thermal processing (RTP) in an attempt to minimise the thermal budget required to achieve a complete activation. Both, surface and volume contaminated samples are investigated. During RTP treatments the activation of surface Cu atoms is found to proceed significantly faster than during standard furnace anneal. We tentatively attribute this behaviour to the UV light exposure associated with the RTP, which may enhance the release of copper atoms from the surface. The dissolution kinetics of the Cu precipitates occurring during RTPs are found to be only limited by Cu diffusion. The RTP/TID process is used to study the low temperature reaction path of supersaturated Cu. If prior to the RTP process, Cu atoms are chemically removed from the surface or near surface region, TID measures only the residual bulk Cu atoms. Our results show that out-diffusion and near-surface precipitation are reducing mostly the copper supersaturation.

Analysis of Cu traces in Si using Transient Ion Drift combined with Rapid Thermal Annealing.

AbstractCopper trace analysis using Transient Ion Drift (TID) combined with a Rapid Thermal Annealing (RTA) process is investigated. A double pulse method is implemented to allow unambiguous identification of the copper-induced capacitance signal. Use of a mercury probe as sensing Schottky barrier enhances the flexibility of the method and allows mapping of the contaminant. The method is evaluated on quantitatively contaminated silicon wafers and compared to Total X-ray fluorescence (TXRF).It is shown that in Czochralski grown material, the RTA is sufficient to dissolve most copper atoms into interstitial sites independently of their initial configuration. As a result, both, the surface and bulk contamination can be monitored by RTA/TID with a bulk detection limit close to 1011cm-3.In Float Zone material mapping of the quenched interstitial copper revealed the existence of defect reactions involving presumably vacancy clusters.

Tantalum-Nitride Diffusion Barrier Studies Using the Transient-Ion-Drift Technique for Copper Detection

AbstractThe permeability of 5 nm thick TaN, Ta and TiN diffusion barriers has been studied by monitoring the bulk copper concentration in silicon after isothermal and isochronal annealing experiments with a transient-ion-drift (TID) technique. The method estimates quantitatively the bulk copper concentration in silicon from capacitance transients of a Schottky barrier which arise when copper ions drift out of the depletion region towards the quasi-neutral region. The correlation between the copper lateral distribution and the position of the copper metal is used to distinguish between background contamination and copper originating from barrier leakage. The TID detection limit is found lower than 1012at/cm3, which makes this technique particularly well adapted for quantitative diffusion barrier studies. Isothermal and isochronal annealing experiments show that TaN fails at a higher temperature than Ta barriers. The copper concentration does not exceed the solubility limit, indicating that copper precipitates nucleate rapidly at the interface. The opposite is found in TiN covered samples where a large copper supersaturation is obtained even after short annealing times.

Formation of copper precipitates in silicon

The formation of copper precipitates in silicon was studied after high-temperature intentional contamination of p- and n-type FZ and Cz-grown silicon and quench to room temperature. With the Transient Ion Drift (TID) technique on p-type silicon a critical Fermi level position at EC–0.2eV was found. Only if the Fermi level position, which is determined by the concentrations of the acceptors and the copper donors, surpasses this critical value precipitation takes place. If the Fermi level is below this level the supersaturated interstitial copper diffuses out. An electrostatic precipitation model is introduced that correlates the observed precipitation behavior with the electrical activity of the copper precipitates as detected with Deep Level Transient Spectroscopy (DLTS) on n-type and with Minority Carrier Transient Spectroscopy (MCTS) on p-type silicon.

Electrical characterization of copper related defect reactions in silicon

Defect reactions involving interstitial copper impurities (Cu i) in silicon are reviewed. The influence of the Coulomb interaction between positively charged copper and negatively charged defects, such as acceptor states of transition metals and lattice defects, on the complex formation rate is discussed in detail. The diffusivity of interstitial copper and the dissociation kinetics of copper–acceptor pairs are studied using the recently introduced transient ion drift (TID) method. TID results reveal that most interstitial copper impurities remain dissolved immediately after the quench and form pairs with shallow acceptors. It is shown that in moderately and heavily doped silicon the diffusivity of copper is trap limited, while in low B-doped silicon the interstitial copper–acceptor pairing is weak enough to allow the assessment of the copper intrinsic diffusion coefficient. The intrinsic diffusion barrier is estimated to be 0.18±0.01 eV. It is concluded that the Coulomb potential used in previous publications underestimated considerably the acceptor–copper interaction. In light of these results, a general discussion on Cu related defect reactions is given.

Intrinsic Diffusion Coefficient of Interstitial Copper in Silicon

Transient ion drift experiments designed to obtain reliable values for the intrinsic copper diffusivity in silicon are reported. From these measurements, the diffusion barrier of Cu in Si is determined to be $0.18\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$. It is shown that the commonly used expression of Hall and Racette [J. Appl. Phys. 35, 379 (1964)] actually gives an effective diffusion coefficient for heavily boron-doped silicon and can neither be used for other doping levels nor extrapolated to lower temperatures. A model is developed which predicts the effective diffusion coefficient as a function of temperature, doping level, and the type of dopant.

Space charge effects on dopant diffusion coefficient measurements in semiconductors

Systematic errors are likely to affect the results of indirect methods used for measuring dopant diffusion in semiconductors, which, for this purpose should be considered as mixed electronic-ionic conductors. The highest contribution to these errors is introduced by the presence of an internal electric field, i.e., by space charge effects. The electric field can be the result either of a dopant concentration gradient or of external bias, applied during the measurement. We consider here three methods in detail, viz. measurement of p-n junction motion, of current or potential decay, and of the time dependence of capacitance (transient ion drift). We show that space charge effects can lead to overestimating diffusion coefficients by a few orders of magnitude. We use the results of our analyses to review and compare the experimental data obtained by different direct and indirect methods, for Cu diffusion in CuInSe2, an issue of considerable current interest for solar cells.

Study of Ta(N,O) diffusion barrier stability: analytical and electrical characterization of low level Cu contamination in Si

The diffusion barrier behaviour of 20 to 200 nm thin Ta(N,O) films in the Cu/Si contact system was evaluated by materials and transient ion drift analysis. Using high resolution transmission electron microscopy, grazing incidence X-ray diffractometry, Auger electron spectroscopy and secondary ion mass spectrometry depth profiling it was revealed that the as-deposited amorphous-like Ta(N,O) diffusion barriers tend to partially recrystallize after annealing at 1 h/500°C without any detectable diffusion and/or reaction of Cu and Si up to at least 1 h/600°C. By making use of the newly developed transient ion drift technique for trace element analysis down to 5 × 10 11 cm −3 of interstitially dissolved Cu in Si, an exponential decrease of the Cu concentration on the Ta(N,O) barrier thickness is observed.

Transient ion drift detection of low level copper contamination in silicon

The transient ion drift (TID) method was used to measure quenched interstitial copper concentrations in both copper plated and copper implanted silicon. Comparison with existing literature data allows one to conclude that, contrary to the general expectation, it is possible to quench in most of the Cu dissolved at temperatures of 600 °C and below. This result suggests that the TID technique could be an excellent means to detect copper contamination in p-type silicon. The expected detection limit, on the order of 1011 cm−3, makes the method a potentially interesting tool to use in gettering or in-diffusion barrier studies.

Room-temperature detection of mobile impurities in compound semiconductors by transient ion drift

We show that the transient ion drift (TID) method, which is based on recording junction capacitance under constant reverse bias [A. Zamouche, T. Heiser, and A. Mesli, Appl. Phys. Lett. 66, 631 (1995)], can be used not only for measurements of the diffusion coefficient of mobile impurities, but also to estimate the concentration of mobile species as part of the total dopant density. This is illustrated for CdTe, contaminated by Cu, and intentionally doped by Li or Ag and for CuInSe2. We show also that, with some restrictions, the TID method can be used if the mobile ions are major dopants. This is demonstrated using Schottky barriers on CdTe, and p–n junction devices in (Hg,Cd)Te, and CuInSe2. The values that we obtain for the diffusion coefficients (for Li, Ag, and Cu in CdTe and for Cu in CuInSe2) agree well with measured or extrapolated values, obtained by other methods, as reported in the literature. Furthermore, we could distinguish between diffusion and chemical reactions of dopants, as demonstrated for the case of Cu in CdTe and Ag-doped (Hg,Cd)Te. In the former case this allows us to separate copper-free from contaminated CdTe samples.