Minority Carrier Lifetime Measurements Research Articles

Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

In this work we have examined the effectiveness of surface passivation on ascut multicrystalline silicon (mc-Si) wafers using different techniques. The study is based on minority carrier lifetime measurements with quasi steady state photo-conductance, ‘QSSPC’. Effective minority carrier lifetime ()measured values of 12.4, 8.9, 4.9 and 3.1 are obtained respectively with four silicon surface passivation techniques: 1- Shallow phosphorous diffusion emitter (n+p), 2- Iodine-Ethanol (I-E), 3-Hydrofluoric acid (HF) emersion and 4-SiNx layer deposition. These results suggest that the shallow n+p emitter gives the eff close to the bulk lifetime due to the better surface passivation quality. Simulations made with Hornbeck-Haynes model indicate that the improvement can be correlated with the decrease of the surface recombination velocity (SRV) and the increment of bulk lifetime.

Asymmetric TMO-Metal-TMO Structure for Enhanced Efficiency and Long-Term Stability of Si-Based Heterojunction Solar Cells.

In this study, we fabricated Si-based heterojunction solar cells (HSCs) with an asymmetric TMO-metal-TMO (TMT) structure using both MoO3 and V2O5 as the hole-selective contacts. Our HSCs offer enhanced long-term stability and effective passivation for crystal defects on the Si sur-face. We analyzed the oxygen vacancy state and surface morphology of the MoO3- and V2O5-TMO thin films using X-ray photoelectron spectroscopy and atomic force microscopy to investigate their passivation characteristics for Si surface defects. From the measured minority carrier lifetime, V2O5 revealed a highly improved lifetime (590 μs) compared to that of MoO3 (122.3 μs). In addition, we evaluated the long-term stability of each TMO thin film to improve the operation stability of the HSCs. We deposited different types of TMOs as the top- and bottom-TMO layers and assessed the effect of the thickness of each TMO layer. The fabricated asymmetric TMT/Si HSCs showed noticeable improvements in efficiency (7.57%) compared to 6.29% for the conventional symmetric structure which used the same TMO material for both the top and bottom layers. Furthermore, in terms of long-term stability, the asymmetric TMT/Si HSCs demonstrated an efficiency that was 250% higher than that of symmetric TMT/Si HSCs, as determined via power conversion efficiency degradation over 2000 h which is mainly attributed by the lower oxygen vacancy of the top-TMO, V2O5. These results suggest that the asymmetric TMT structure is a promising approach for the fabrication of low-cost and high-efficiency Si-based HSCs with enhanced long-term stability.

Laser-Patterned Alumina Mask and Mask-Less Dry Etch of Si for Light Trapping with Photonic Crystal Structures

Ultra-short 230 fs laser pulses of a 515 nm wavelength were tightly focused onto 700 nm focal spots and utilised in opening ∼0.4-1 μm holes in alumina Al2O3 etch masks with a 20-50 nm thickness. Such dielectric masks simplify the fabrication of photonic crystal (PhC) light-trapping patterns for the above-Lambertian performance of high-efficiency solar cells. The conditions of the laser ablation of transparent etch masks and the effects sub-surface Si modifications were revealed by plasma etching, numerical modelling, and minority carrier lifetime measurements. Mask-less patterning of Si is proposed using fs laser direct writing for dry plasma etching of Si.

Femtosecond nonlinear optical response and minority carrier lifetime of F4TCNQ-doped bismuthene for optoelectronic and ultrafast photonic applications

Bismuthene has received a lot of interest because of its unusual electronic and photonic properties. To widen the use of bismuthene in optoelectronic and ultrafast photonic devices, its electronic and optical characteristics should be tuned. Herein, for the first time, a strong p-type molecular dopant, such as tetrafluoro-tetracyanoquinodimethane (F4TCNQ), is used to achieve efficient surface charge transfer doping (SCTD) of bismuthene. The minority carrier lifetime measurements of bismuthene shifted to 6.5 µs after doping, confirming that F4TCNQ has the opportunity to boost bismuthene's p-type conductivity due to an unusual charge transfer interaction between bismuthene and F4TCNQ molecules. The thermally generated intermolecular charge transfer influence between bismuthene and F4TCNQ resulted in an improved nonlinear optical and optical limiting response, which was estimated by femtosecond z-scan measurements. Our findings suggest that SCTD is an appropriate means for tuning the linear and nonlinear optical properties of bismuthene via modulating their band gap and minority carrier lifetime. This alternating approach expands its potential in electronic, optoelectronic, and ultrafast photonic devices.

Improvement of amorphous silicon/crystalline silicon heterojunction solar cells by light-thermal processing

It has been known that amorphous silicon/crystalline silicon heterojunction solar cells (HJT cells) can be improved by light-thermal processes. The present work aims to acquire a further understanding of the effect of illumination intensity and temperatures in a broader range and the roles of light and heat in the improvement. Naked HJT cells without grids were used for more reliable measurement of minority carrier lifetimes. Experiments of light-thermal processes of the naked HJT cells, with lighting from red LED arrays and heating/cooling from a thermal stage/air fan, have been carried out, with the illumination intensity ranging in 1–8 kW/m2 and cell temperature ranging in 160–220 °C. The higher illumination intensity results in longer lifetimes of the cells, the optimum cell temperature appeared to be around 180 °C, with the lower temperature needing a longer processing time to reach peak lifetime increment, while the higher temperature tends to degrade the passivation of the cell. The cell temperature of 180 °C was then chosen for the light-thermal processes of commercial HJT cells of ∼23.8% efficiency on a pilot line, with 12 kW/m2 red illumination. An average efficiency gain of 0.77%abs and a maximum gain of 0.87%abs have been achieved, with a processing time of 80 s. It is encouraging that the light intensity required is much lower than the previously reported level for similar improvement, indicating much lower power consumption for industrial application of the process. Through semi-shading experimentation of the light-thermal process, it is postulated that the effect of illumination does not rely on the direct effect of the injected photons, but possibly resulted from the effect of light-generated carriers.

Stoichiometry and Morphology Analysis of Thermally Deposited V2O5-x Thin Films for Si/V2O5-x Heterojunction Solar Cell Applications.

In recent decades, dopant-free Si-based solar cells with a transition metal oxide layer have gained noticeable research interest as promising candidates for next-generation solar cells with both low manufacturing cost and high power conversion efficiency. Here, we report the effect of the substrate temperature for the deposition of vanadium oxide (V2O5−x, 0 ≤ X ≤ 5) thin films (TFs) for enhanced Si surface passivation. The effectiveness of SiOx formation at the Si/V2O5−x interface for Si surface passivation was investigated by comparing the results of minority carrier lifetime measurements, X-ray photoelectron spectroscopy, and atomic force microscopy. We successfully demonstrated that the deposition temperature of V2O5−x has a decisive effect on the surface passivation performance. The results confirmed that the aspect ratio of the V2O5−x islands that are initially deposited is a crucial factor to facilitate the transport of oxygen atoms originating from the V2O5−x being deposited to the Si surface. In addition, the stoichiometry of V2O5−x TFs can be notably altered by substrate temperature during deposition. As a result, experimentation with the fabricated Si/V2O5−x heterojunction solar cells confirmed that the power conversion efficiency is the highest at a V2O5−x deposition temperature of 75 °C.

Passivation of Ring Defects in Czochralski-Grown Silicon Using Magnesium Fluoride Films

This work investigates the potential application of magnesium fluoride (MgFx) films and compares its effectiveness to conventional silicon nitride (SiNx:H) for passivating ring defects, which sometimes occur in n-type Czochralski-grown silicon wafers subjected to high-temperature processes. We find that the passivation of ring defects in MgFx coated samples is fully activated only at 600 °C, whereas passivation in SiNx:H coated samples can be activated by 5 min anneals at 500 °C. Consistent with the passivation activation temperature, energy-dispersive X-ray shows that MgFx films significantly lose fluorine at temperature ≥ 600 °C. Further, subsequent annealing of the previously passivated samples reveals that the ring defects reappear and become recombination-active again. However, this depassivation of the ring defects occurs significantly faster in SiNx:H passivated samples than in MgFx passivated samples at temperatures ≤ 500 °C, whereas the depassivation happens at a rate similar to that at temperatures ≥ 600 °C. Furthermore, in situ monitoring of the change in band–band photoluminescence intensity during annealing also confirms that the passivation and depassivation of ring defects is faster in SiNx:H passivated samples. Based on these observations, we suggest the possibility that the dominant passivating agent in the MgFx and SiNx:H films is fluorine and hydrogen, respectively. Finally, minority carrier lifetime measurements indicate that both the passivation methods improve the bulk lifetime significantly.

Ultrathin atomic layer deposited niobium oxide as a passivation layer in silicon based photovoltaics

Atomic layer deposited (ALD) niobium oxide (NbOx) films were investigated for their passivation properties through minority carrier lifetime measurements and compared to a well-known passivating material, aluminum oxide. ALD alumina is known to passivate by a combination of field-effect passivation from fixed charges and chemical passivation from hydrogenation of dangling bonds. It was hypothesized that niobium oxide films passivate by varying degrees of the same mechanisms found in alumina. The effects of ALD oxygen source (water or ozone) and varying anneal temperatures were correlated to passivation quality. Lifetimes of sub-1 nanometer films were specifically investigated. It was found that water is a superior oxidant for passivation relative to O3. Thermally activated NbOx films deposited with water have near equivalent or superior lifetimes to alumina of the same thickness after equivalent annealing at temperatures up to 350 °C. Chemical analyses by x-ray photoelectron spectroscopy (XPS) were used to investigate the suspected mechanisms of passivation. It was suggested that field-effect passivation is the dominating mechanism in NbOx films based on correlations between band movement as probed by XPS and the lifetime data presented in this work. This work provides new insights into the applicability of niobium oxide as a passivating selective contact for silicon photovoltaics with the goal of reaching new record efficiencies in solar cells.

The performance of a perovskite-silicon tandem photovoltaic device coupled with the infrared-enhanced response titanium subnitride film

Two kinds of carrier’s passivating contact capping stacks have been fostered for an asymmetric silicon based solar cell, in which ITO/a-SiOx(In) is for hole selection and possesses graded distribution of In, Si, O elements as well as intrinsic- and p-type ultrathin SiOx(In) layer trait, while the SiOx-TiNy layer on rear of n-Si absorber is for electron selection and behaves as a degenerated material with higher electron concentration (1023-1024 /cm3). By using Hall effect, X-ray photoemission and minority carrier lifetime measurements and simulation analyses with AFORS-HET software of TiNy films and ITO/a-SiOx(In)/n-Si/SiOx-TiNy device, respectively, it is found that the performance of the device is strongly correlated to the N deficiency of TiNy film, in addition the simulation results demonstrate that the reduced density of interface state (Dit(E)) and the enhanced near-infrared response (EQE) lead to an increase of open-circuit voltage from 578.6 to 744.1 mV, and power conversion efficiency (PCE) has increased by 30.65%, compared with the prototype of semiconductor-quasi-insulator-semiconductor device (SQIS). Moreover, a monolithic tandem device by n-i-p type perovskite cell tunnel-connected to the modified SQIS cell has achieved a PCE of 31.39%, which is predicted through an available estimation under the maximum values of the Jsc, Voc and pFF.

High Performance Predictable Quantum Efficient Detector Based on Induced-Junction Photodiodes Passivated with SiO2/SiNx.

We performed a systematic study involving simulation and experimental techniques to develop induced-junction silicon photodetectors passivated with thermally grown SiO2 and plasma-enhanced chemical vapor deposited (PECVD) SiNx thin films that show a record high quantum efficiency. We investigated PECVD SiNx passivation and optimized the film deposition conditions to minimize the recombination losses at the silicon–dielectric interface as well as optical losses. Depositions with varied process parameters were carried out on test samples, followed by measurements of minority carrier lifetime, fixed charge density, and optical absorbance and reflectance. Subsequently, the surface recombination velocity, which is the limiting factor for internal quantum deficiency (IQD), was obtained for different film depositions via 2D simulations where the measured effective lifetime, fixed charge density, and substrate parameters were used as input. The quantum deficiency of induced-junction photodiodes that would be fabricated with a surface passivation of given characteristics was then estimated using improved 3D simulation models. A batch of induced-junction photodiodes was fabricated based on the passivation optimizations performed on test samples and predictions of simulations. Photodiodes passivated with PECVD SiNx film as well as with a stack of thermally grown SiO2 and PECVD SiNx films were fabricated. The photodiodes were assembled as light-trap detector with 7-reflections and their efficiency was tested with respect to a reference Predictable Quantum Efficient Detector (PQED) of known external quantum deficiency. The preliminary measurement results show that PQEDs based on our improved photodiodes passivated with stack of SiO2/SiNx have negligible quantum deficiencies with IQDs down to 1 ppm within 30 ppm measurement uncertainty.

OCVD Measurement of Ambipolar and Minority Carrier Lifetime in 4H-SiC Devices: Relevance of the Measurement Setup

The open-circuit voltage decay (OCVD) method is a well-known technique for conducting electrical measurements of carrier lifetime: the main advantages lie in the simple setup and the possibility of carrying out measurements in commercial devices without the need of removing the package, as for optical methods. Despite several researchers having reported carrier lifetimes measured by the OCVD method in different devices, there has been little discussion about the potential effect of the experimental setup on the obtained results. By comparing the outputs of the experimental measurements with those of numerical simulations, this study investigates the overlooked effect of the OCVD measurement setup on the former. Due to the growing importance of SiC-based devices, the analysis is applied to a 4H-SiC p-i-n diode. Two main points are addressed: 1) the effect of circuit setup on the ambipolar lifetime is discussed and a method, originally developed for improving the estimate of low-level carrier lifetime in OCVD measurements, is used to correct the measured lifetime for this influence; 2) the origin of the local minimum eventually appearing in the lifetime versus time curves is also investigated. It is found that the minimum can also be related to the time constant of the experimental setup, giving rise to doubts about the usual interpretation of this minimum as the minority carrier lifetime. A method is thus proposed to help discriminate between the two interpretations.

Minority carrier lifetime in HgCdTe(100) epilayers and their potential application to background radiation limited MWIR photodiodes

Measurements of minority carrier lifetime have been carried out on nonintentionally doped HgCdTe(100) layers grown by metal organic chemical vapor deposition on GaAs substrates. The studies have been carried out in order to investigate the possibility of applications of HgCdTe(100) epilayers to fully depleted P–i–N photodiodes operated at near-room temperatures and the background-radiation limit. Post-growth annealed layers show n-type conductivity with a residual donor concentration of about 1015 cm−3. The cadmium molar composition (0.262 < x < 0.336) was chosen for mid-wave infrared devices operating at 230 K to 300 K. Results show that at high temperatures, the samples show clearly Auger-limited lifetimes determined by the background doping. The measured lifetimes at 300 K range from 5 to 11 µs, and from 0.2 to 1 µs, for the cut-offs of 4 µm and 5.4 µm, respectively.

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.

Microcrystalline Silicon Tunnel Junction for Monolithic Tandem Solar Cells Using Silicon Heterojunction Technology

In this article, we developed a microcrystalline silicon tunnel junction to be used as a tunnel recombination junction between a large-gap top cell and a silicon heterojunction bottom cell, in a monolithic tandem integration. This junction is composed of a p-type layer on the top of an n-type layer, deposited by plasma-enhanced chemical vapor deposition at low temperature (200 °C). Microcrystalline phase percentage was controlled with Raman spectroscopy and ellipsometry measurements. The total stack has a thickness of 40 nm and an average conductivity around 10 S/cm. Minority carrier lifetime measurements showed an improvement of the field effect and the passivation with the addition of this junction on top of silicon heterojunction solar cells. Moreover, implementation of microcrystalline layer on top of reference rear emitter silicon heterojunction solar cells improved the fill factor and did not induce parasitic absorption above 700 nm. Simple test structures were fabricated in order to characterize the tunnel junction and optimize it. Then, we carried out dark temperature-dependent I-V measurements on those test structures and observed peaks and valleys, characteristic of the junction tunnel behavior. The developed tunnel junction shows low contact resistivity and activation energies; these are promising results for the complete integration on the tandem device.

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

Simulation and Experimental Approach to Investigate the Annealing Effect on mc-Si Ingot Grown by Directional Solidification Process for PV Application

A two-dimensional, numerical simulation using finite volume method has been made on directional solidification of multi-crystalline silicon ingot. Two identical simulations have been carried out to grow silicon ingots in which one of them was maintained at 1523K after the growth process for about an hour to study the effect of annealing process on the quality enhancement of the grown ingot. The thermal field inside the growth chamber has been investigated during and after the growth process. The thermal stress and dislocation density of as grown and annealed ingots have been investigated numerically. The simulation results show that annealing the grown ingot reduces the maximum thermal stress up to 8 MPa, and decreases dislocation density throughout the ingot. Based on the simulation results, an experiment on the growth of mc-Si ingot on G1 directional solidification furnace has been made. Different stages of directional solidification process such as melting, crystallization and annealing have been discussed. the minority carrier lifetime measurement has been carried out on mc - Si bricks as well as on wafers. The lifetime of the minority carriers is found to be higher (75µs) in the central region of the ingot. The resistivity of the wafers taken from different positions of the grown ingot has been analyzed to find the solid – liquid interface during the growth process and compared with simulated interface. The results show that, the thermal stress and the dislocation density can be significantly reduced and the minority carrier lifetime can be improved by annealing the grown ingot at 1523K. The results may be useful for the mass production of mc-Si ingots for photovoltaic application.

Study of the properties of SiOx layers prepared by different techniques for rear side passivation in TOPCon solar cells

Passivation of n-type and p-type monocrystalline CZ Si wafers (both polished and textured) with silicon oxide layers prepared by thermal (TO), chemical (CO) and plasma (PO) techniques have been extensively investigated from the measurement of minority carrier lifetime (τ) by transient electrical photoresponse method, density of interface states (NSS) measurement by capacitance – voltage study and silicon oxidation states by X-ray photoelectron spectroscopy (XPS) study of the SiO2/Si interface. It has been observed that NSS and τ have an inverse relation but the dependence is not linear. The method (TO, CO or PO) of oxide layer development has been found to play a crucial role to control the passivation of the c-Si wafer surface. It has been observed that the thermally grown oxide layer (TO) is superior among three oxide layers for all the different c-Si surfaces. Very low density of interface states (<5 × 1011) were found in both p-type and n-type polished wafer passivated with TO layer. Highest lifetimes of 170 μs for n-type polished wafer and 102.74 μs for p-type polished wafer were obtained with TO. The amount of sub oxide formed at the interface of both n- and p-type of wafers during different oxidation process, may have some correlation with Nss which in turn determines the passivation quality of the wafers. Improvement of implied Voc for both polished and textured wafers (n-type and p-type) was found using thermally grown oxide with respect to others.

Impact of different SiO2 diffusion barrier layers on lifetime distribution in multi-crystalline silicon ingots

Abstract Three different SiO2-based barrier layers, used in industry for prevention of metallic impurity diffusion into directionally solidified silicon ingots, were investigated in detail by G1 experiments in order to find out the most relevant barrier properties responsible for the blocking behaviour. Minority carrier lifetime and interstitial iron measurements on the grown silicon ingots show significant differences between the barriers regarding the red zone extension as well as the maximum lifetime/minimum iron content in the ingot centre. Structural analysis of the barriers by optical microscopy and Raman spectroscopy reveal a clear correlation to the porosity and thickness of the barrier layers. Further, indications were found that the barriers become effective only during the process and that this should occur as early as possible to obtain an optimal impact of the barrier. Only one of the tested barriers shows an overall positive impact on the lifetime, coming close to using a high purity quartz glass SiO2 crucible instead of the standard ceramic SiO2 crucible.

Effects of damages induced by indium-tin-oxide reactive plasma deposition on minority carrier lifetime in silicon crystal

A reactive-plasma deposition (RPD) process damage is evaluated for an indium-tin-oxide (ITO)/SiO2/Si structure in terms of the recombination properties at the interface. To calculate the surface recombination velocity (SRV), the defect density at the SiO2/Si interface (Dit), the effective total charge (Qtot) which is the sum of the charges in the SiO2 and the trapped charge at the SiO2/Si interface, and the workfunction of ITO at the ITO/SiO2 interface (ϕITO), are extracted by capacitance-voltage (C-V) analysis as a function of a postdeposition annealing (PDA) temperature. The Dit and Qtot significantly increase by the RPD process to 1.4×1012 cm-2eV-1 and 1.5×1012/q cm-2, respectively, and decrease by the PDA at 200 °C in N2 ambient to 1.4×1011 cm-2eV-1 and 1.1×1011/q cm-2, respectively. The ϕITO also varies from 4.80 eV to 4.35 eV by the PDA. The correlation is examined between the SRV values calculated based on the extended SRH model using the extracted Dit, Qtot, and ϕITO values [SRV(CV)], and the SRV ones obtained from the minority carrier lifetime measurements [SRV(LT)]. It is found that multiple sets of capture cross-sections were required for SRV(CV) values to coincide with SRV(LT) ones. The effect of variation in the Qtot and ϕITO on the band bending is almost canceled out in the present PDA condition. It is thus considered that the increase in Dit is the main cause to decrease the minority carrier lifetime.

Role of Impurities in Silicon Solidification and Electrical Properties Studied by Complementary In Situ and Ex Situ Methods

All silicon (Si) ingot fabrication processes share challenges to control grain structure, defect, and impurity contamination during the solidification step to improve the material properties. The final grain structure and inherent structural defects issued from the solidification step are responsible for the photovoltaic (PV) properties for a large part, all the more as they are often associated with impurity distribution. Impurities play a major role as they not only can modify the development of the grain structure formation but can also interact with structural defects creating the regions of deleterious minority carrier lifetime recombination. Samples containing different levels of impurities and solidified with different processes are selected and analyzed as‐grown or observed by X‐ray imaging during resolidification from as‐grown seeds. The growth features and relative crystallographic orientation of neighbor grains are characterized. Moreover, minority carrier lifetime measurements are performed and correlated with the growth features. The complementarity of the different techniques improves the understanding of phenomena at stake during the formation of grains and twins, the effect of impurities, and their impact on photovoltaic properties. The results show the significant influence of light and metallic impurities, such as copper, on the grain structure and on the electrical properties.