Quasi-steady State Photoconductance Research Articles

Improvement of charge carrier lifetime in heat exchange method multicrystalline silicon wafers by extended phosphorous gettering process

External phosphorous diffusion gettering were applied using homogenous and extended schemes on multicrystalline silicon (Mc-Si) wafers obtained from solar grade (SOG) silicon feedstock by the heat exchange method (HEM) growth technique. X-ray fluorescence (XRF) characterization shows the presence of transition metals like Chromium (Cr), manganese (Mn) and iron (Fe) in the analyzed as-cut Mc-Si substrates with a predominance of Mn and Fe elements. Before and after gettering process, we have observed that the Cr atoms concentration drop from 4.1015 at.cm-3 in the as-cut Mc-Si samples to 2.5 1015 at.cm-3 with homogeneous gettering and to 3.1014 at.cm-3 in the wafer undergoes extended gettering. The quasi-steady-state photo conductance (QSSPC) technique is used to characterize the minority charge carrier’s lifetime tn before and after gettering process. It was found that the response of Mc-Si wafers is better (form point of view of lifetime improvement) to the extended process but in some regions it is less effective and the homogenous process give the best amelioration due probably to the spatial distribution of crystalline defects and metallic precipitates densities. With tn initial values before gettering in the range of 2.5-8 usec we have measured lifetime values in the range of 15 to 37 usec with extended process and from 10 to 30 µsec with the homogenous one which ensure cell efficiencies in the range of 15 to 16%.

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.

Contactless quasi-steady-state photoconductance (QSSPC) characterization of metal halide perovskite thin films

We apply the contactless quasi-steady-state photoconductance (QSSPC) method to co-evaporated methyl ammonium lead iodide (MAPbI3) perovskite thin-films. Using an adapted calibration for ultralow photoconductances, we extract the injection-dependent carrier lifetime of the MAPbI3 layer. The lifetime is found to be limited by radiative recombination at the high injection densities applied during the QSSPC measurement, enabling the extraction of the electron and hole mobility sum in the MAPbI3 using the known coefficient of radiative recombination of MAPbI3. Combining the QSSPC measurement with transient photoluminescence measurements, performed at much lower injection densities, we obtain the injection-dependent lifetime curve over several orders of magnitude. From the resulting lifetime curve, we determine the achievable open-circuit voltage of the examined MAPbI3 layer.

Investigation of monocrystalline silicon surface texturization by image processing program

One of the main issues in the photovoltaic industry of silicon solar cells is the optical losses via reflection. To overcome this issue, texturization (chemical etching) remains the most commonly used method to produce a random pyramid structure to reduce reflection and thus increase photocurrent generation. Because of the anisotropic etching property of alkaline solution, square-based upright pyramids are formed, i.e., the difference in etching rates between (100) and (111) planes. In this paper, the following etching solutions were studied: 4/10, 6/6, and 10/4 (KOH weight percentage/IPA volume percentage).The optical, morphological, and electric characterizations using UV-Vis spectroscopy, electron scanning microscopy (SEM), and quasi-steady-state photoconductance (QSSPC) device are performed. Also, the standard weighted reflectance (SWR) was calculated. The pyramid size distribution generated using KOH-IPA solutions was investigated using an image processing program (ImageJ) via SEM images. The calculation of the height of the pyramids reveals that the typical pyramid size ranges from 3 to 9 m, with large pyramids of small proportions. Furthermore, when the pyramids are distributed in a mixture of tiny and medium pyramids in precise proportions, the reflectivity is at its lowest, and as the number of large pyramids rises, the reflectivity increases.

Selective tunnel oxide passivated contact on the emitter of large-size n-type TOPCon bifacial solar cells

This paper reports the application of novel selective tunneling oxide passivated contacts (TOPCon) on the emitter of large-Size n-type TOPCon bifacial solar cells. TOPCon technology is applied to the front surface of n-type TOPCon bifacial cells. The design of the front surface cell TOPCon layer was optimized, and only the SiO2/poly Si passivation layer was applied to metal contact of the emitter in order to avoid parasitic absorption of sunlight by the polysilicon layer. The quasi-steady state photo-conductance (QSSPC) analysis focused on the front-surface conventional n-type PERT cell structure and the front-surface selective TOPCon cell structure, resulting in a saturation current density of 1400 fA/cm2 for the front-surface metal contact recombination of the conventional n-type passivated emitter rear totally-diffused （PERT） cell structure and a saturation current density of 430 fA/cm2 for the front-surface metal contact recombination of the selective TOPCon cell structure. The maximum conversion efficiency of bifacial solar cells with a large area of 252 cm2 prepared using this technique reaches up to 23.21%. This represents an average conversion efficiency improvement of 0.21% compared to solar cells with a conventional PERT structure on the front surface and a TOPCon on the rear surface. Simulation of a cell optimized for this structure can achieve an open circuit voltage of over 720 mV, a short circuit current of 40 mA/cm2, and a cell efficiency of 24%.

Extraction of PEDOT:PSS/c‐Si Junction Properties by Modeling of Injection‐Dependent Lifetime Measurements Including Depletion Region Modulation

The carrier lifetime of n‐type silicon wafers coated with the conducting polymer PEDOT:PSS as a function of the excess carrier concentration Δp within the wafer is characterized using the quasi‐steady‐state photoconductance (QSSPC) method, and a drastic increase in the measured apparent lifetime τapp with decreasing Δp is observed. The observed increase with the depletion region modulation (DRM) effect is explained, as PEDOT:PSS‐coated p‐type silicon wafers do not show any increase in lifetime toward low injection levels. By modeling the measured τapp(Δp) curves on n‐type silicon including interface recombination as well as the DRM effect, the interface recombination velocity as well as the band bending Ψs within the silicon induced by PEDOT:PSS are able to be extracted. The impact of adding sorbitol to the PEDOT:PSS dispersion on the τapp(Δp) curves is examined, and it is demonstrated that the admixture of sorbitol improves chemical interface passivation but leaves the band bending within the silicon bulk toward the PEDOT:PSS/c‐Si interface unaffected.

Variations in Minority Carrier-Trapping Effects Caused by Hydrogen Passivation in Multicrystalline Silicon Wafer

In a multicrystalline silicon (mc-Si) wafer, trapping effects frequently occur in the carrier lifetime measurement based on the quasi-steady-state photoconductance (QSSPC) technique. This affects the accurate measurement of the carrier lifetime of an mc-Si solar cell by causing distortions at a low injection level close to the Pmax point. Therefore, it is necessary to understand this effect and effectively minimize the trapping-center density. In this study, the variations in the minority carrier-trapping effect of hydrogen at different annealing temperatures in an mc-Si were observed using QSSPC, time-of-flight secondary ion mass spectroscopy, and atom probe tomography. A trapping effect was confirmed and occurred in the grain boundary area, and the effect was reduced by hydrogen. Thus, in an mc-Si wafer, effective hydrogen passivation on the grain area and grain boundary is crucial and was experimentally proven to minimize the distortion of the carrier lifetime.

Application of Quasi-Steady-State Photoconductance Technique to Lifetime Measurements on Crystalline Germanium Substrates

Similar to other high quality crystalline absorbers, an accurate knowledge of surface passivation of crystalline Germanium (c-Ge) substrates is crucial for a straightforward improvement of photovoltaic device performance. For crystalline silicon devices, this information is typically obtained by quasisteady state photoconductance (QSS-PC) technique using Sinton WCT-120 tool. In this work, we explore the conditions to adapt this measurement technique to c-Ge substrates. Based on PC-1D simulations, we deduce that a minimum effective lifetime is needed corresponding to an effective diffusion length equal to the substrate thickness. Apart from this, an accurate estimation of the total photogeneration inside the c-Ge sample is also mandatory. This condition implies that the light intensity that impinges onto the sample must be measured with a c-Ge sensor, although the integrated c-Si sensor can be used for high flash intensities. Additionally, the optical factor used to evaluate sample reflectance must be also known, which is determined by measuring robust effective lifetime values under photoconductance decay conditions. Finally, knowledge about carrier mobility in c-Ge is also necessary to translate the measured photoconductance to the corresponding excess carrier density values. Lifetime measurements of passivated c-Ge substrates done by QSS-PC technique are validated by comparing them with the ones obtained by microwave photoconductance technique.

Improved silicon surface passivation by hybrid composites formed by PEDOT:PSS with anatase TiO2 nanoparticles

In this work, hybrid composites formed by PEDOT:PSS (poly(3,4-ethylene dioxythiophene):-polystyrene sulfonate) and anatase TiO2 nanoparticles synthesized by hydrolysis were spin-coated on Si substrates in order to evaluate their potential implementation in Si-hybrid solar cells, based on their passivation behaviour. X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy, quasi-steady state photoconductance (QSS-PC) based on PL imaging, and Hall effect measurements were carried out for the structural, optical and electrical characterization. Enhanced carrier lifetime values of τ ~ 0.5 ms were obtained by adding a controlled amount of TiO2 nanoparticles (0.5% wt.) to the PEDOT:PSS matrix.

Wet Chemical Oxidation to Improve Interfacial Properties of Al2O3/Si and Interface Analysis of Al2O3/SiOx/Si Structure Using Surface Carrier Lifetime Simulation and Capacitance–Voltage Measurement

A thin silicon oxide (SiOx) layer (thickness: 1.5–2.0 nm) formed at an Al2O3/Si interface can enhance the interface properties. However, it is challenging to control the characteristics of thin SiOx layers because SiOx forms naturally during Al2O3 deposition on Si substrates. In this study, a ~1.5 nm-thick SiOx layer was inserted between Al2O3 and Si substrates by wet chemical oxidation to improve the passivation properties. The acidic solutions used for wet chemical oxidation were HCl:H2O2:H2O, H2SO4:H2O2:H2O, and HNO3. The thicknesses of SiOx layers formed in the acidic solutions were ~1.48, ~1.32, and ~1.50 nm for SiOx-HCl, SiOx-H2SO4, and SiOx-HNO3, respectively. The leakage current characteristics of SiOx-HNO3 were better than those of the oxide layers formed in the other acidic solutions. After depositing a ~10 nm-thick Al2O3 on an SiOx-acidic/Si structure, we measured the effective carrier lifetime using quasi steady-state photoconductance and examined the interfacial properties of Al2O3/SiOx-acidic/Si using surface carrier lifetime simulation and capacitance–voltage measurement. The effective carrier lifetime of Al2O3/SiOx-HNO3/Si was relatively high (~400 μs), resulting from the low surface defect density (2.35–2.88 × 1010 cm−2eV−1). The oxide layer inserted between Al2O3 and Si substrates by wet chemical oxidation helped improve the Al2O3/Si interface properties.

Hydrogen-related defects measured by infrared spectroscopy in multicrystalline silicon wafers throughout an illuminated annealing process

Hydrogen (H) is thought to be strongly involved in the light and elevated temperature-induced degradation observed predominantly in p-type silicon wafers, but the nature of the defect or defects involved in this process is currently unknown. We have used infrared (IR) spectroscopy to detect the vibrational signatures due to the H–B, H–Ga, and H2*(C) defects in thin, hydrogenated, p-type multicrystalline silicon wafers after increasing the optical path length by preparation and polishing the edges of a stack of wafers. The concentrations of the H–B and H–Ga acceptor complexes are reduced to 80% of their starting values after low intensity (5 mW/cm2) illumination at room temperature for 96 h. Subsequent high intensity illumination (70 mW/cm2) at 150 °C for 7–8 h further decreases the concentrations of these defects; to ∼40% (H–B) and ∼50% (H–Ga) of their starting values. Our results show that, with careful sample preparation, IR spectroscopy can be used in conjunction with other techniques, e.g., quasisteady-state photoconductance, to investigate the involvement of different H-related point defects on degradation in solar-grade silicon wafers.

Study on efficiency improvement of multi-crystalline silicon solar cell by removing by-product and plasma induced damage generated during reactive ion etching

Silicon solar cell texturing is the process of reducing the light reflection of the solar cells by changing their surface structure. Given that multi-crystalline silicon (mc-Si) wafers have grains with different orientations, their texturing process is more difficult to realize compared with those of monocrystalline silicon (mono-Si) wafers. There are two types of texturing processes: wet and dry texturing. Acidic solution-based wet-textured mc-Si samples have higher reflectance values than mono-Si wafers. However, the reactive ion etching (RIE)-texturing of mc-Si wafers has the advantage of decreasing reflectance to values below 10%. Despite this decrease in reflectance, RIE byproducts and plasma-induced damage bring about recombination that results in a decrease the passivation properties of the solar cell. In this study, the RIE byproducts as well as the plasma-induced damage of the RIE-textured mc-Si wafers were analyzed using SEM, STEM, and EDS analysis, while quasi-steady-state photoconductance (QSSPC) was used to confirm the optical characteristics of the RIE-textured mc-Si wafers. Additionally, HF treatment resulted in byproduct removal, and a relatively high implied Voc (673 mV) was confirmed within 3 min of HF treatment. The defect removal etching (DRE) process using KOH was applied to remove the lattice defect layer, and after 40 s of the DRE treatment, the implied Voc increased to ~680 mV. Thus, the RIE-textured mc-Si solar cell showed a power conversion efficiency of 19.6%, which was ~0.6% higher than that of acid-textured mc-Si solar cells based on the increase in short-circuit current (Isc).

Influence of Si-nanoparticles on PEDOT:PSS properties for hybrid solar cells

Hole-conducting poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a material receiving increasing interest for use in photovoltaic devices. In this work, we present the results of study of the degradation of the optical and electrical properties of pure PEDOT:PSS and PEDOT:PSS doped with Si-nanoparticles. The PEDOT:PSS has been deposited on top of Si wafers by spin coating. We found that incorporation of Si nanoparticles into PEDOT:PSS enhances the surface passivation of the surface of the Si wafer by increasing the carrier lifetime measured by the photoluminesence quasi-steady state photoconductance (PL QSS-PC) method from about 320 µs to 420 µs at a minority carrier injection level of 2*1015 cm−3. Analysis showed that incorporation of the Si nanoparticles does not change the kinetics of degradation of lifetime considerably.

Correlation between in Situ Diagnostics of the Hydrogen Plasma and the Interface Passivation Quality of Hydrogen Plasma Post-Treated a-Si:H in Silicon Heterojunction Solar Cells.

Passivation of the interface defect states is crucial to mitigate the recombination losses in silicon solar cells. In this work, we have investigated the role of hydrogen plasma treatment (HPT) to passivate the interfacial defects between crystalline (c-Si) and hydrogenated amorphous silicon (a-Si:H) in silicon heterojunction (SHJ) solar cells. For the first time, we have found a correlation between the dynamic properties of hydrogen plasma and passivation quality of the films by using in situ optical emission spectroscopy and quasi-steady state photoconductance measurement. The optimum condition for saturation of the dangling bonds by HPT has been studied in detail by tuning the excited hydrogen (H) species and ion bombardment energies by controlling physical parameters like plasma current and chamber pressure. We have investigated the role of annealing after HPT to redistribute the H in the post-treated a-Si:H film and have obtained an i Voc of 755 mV, minority carrier lifetime (τeff) of 4.6 ms, and SRV of 1.5 cm/s on test structures having only an 10 nm intrinsic a-Si:H layer on textured silicon wafers. The H bond configuration at the interface of a-Si:H and c-Si has been investigated by Fourier transform infrared spectroscopy, which demonstrates improved monohydride bonding in the films after HPT derived from the analysis of microstructure parameter and H concentration values. Raman spectroscopy shows the absence of the nanocrystalline fraction after HPT and verifies reduced coordination defects due to annealing after HPT. The proof of concept has been validated by fabricated SHJ solar cells having a Voc of 729 mV and efficiency of 18.7% after HPT, with the best cell efficiency reaching 20.2% after doped layer optimization. The decrease in reverse saturation current and ideality factor after HPT verifies that the improvement in performance is from reduced recombination losses at the interface due to passivation of defects in midgap states.

Behavior of SiNx/SiO2 Double Layer for Surface Passivation of Compensated p-Type Czochralski Silicon Wafers

Surface passivation of Czochralski (Cz) p-type compensated silicon using a hydrogenated silicon nitride/silicon oxide (SiNx-H/SiO2) double layer (hereinafter called SiNx/SiO2) has been investigated. The characteristics of the deposited films depended strongly on surface preparation, deposition process, and annealing parameters. Fourier-transform infrared (FTIR) spectroscopy was used to analyze the chemical bonds in the Si–SiNx/SiO2 structure, and quasi-steady-state photoconductance (QSSPC) measurements were used to evaluate the minority-carrier lifetime (τeff), to check the quality of the passivation. The results showed that, after SiNx/SiO2 double-layer deposition, τeff remained rather poor (13.29 μs). This low degree of passivation is principally due to the appearance of an inversion layer after oxidation, inducing switching of the semiconductor type at the surface. This inversion layer is due to the segregation properties of both boron and phosphorus impurities. This phenomenon was highlighted by secondary-ion mass spectrometry (SIMS) characterization, revealing a substantial phosphorus concentration near the Si–SiO2 interface and penetration of boron inside the oxide, with concentration of 1 × 1018 cm−3 and 4 × 1018 cm−3, respectively. Annealing at 400°C under low pressure of 3.34 × 10−3 kPa improved the minority-carrier lifetime to τeff = 15.94 μs, leading to enhanced surface passivation related to the reduced interface state density and improved statistical distribution of different bonding environments.

Efficiency characteristics of a silicon oxide passivation layer on p-type crystalline silicon solar cell at low illumination

Silicon dioxide (SiO2) is widely used to improve the surface passivation properties of silicon solar cells. To minimize solar cell potential-induced degradation when the PV module is installed outdoors, a silicon oxide film is widely used as an insulator. However, experiments have confirmed that solar cells with a silicon oxide (SiO2) film have a lower efficiency than solar cells without a silicon oxide (SiO2) film at low illumination (<0.4 sun). Actually, the efficiency in the low illumination condition affects the average power output per day because the PV module mostly operates when the solar irradiation dose is less than 1 sun. To maximize the performance of the PV module, the output at a low light intensity level should also be considered. Shunt resistance (Rshunt) is known to cause a decrease in solar cell efficiency under low illumination conditions. PC1D simulation was used to analyze parameters, such as the series resistance, parallel resistance, and surface recombination, that affect the characteristics of the solar cell at low light intensity. In this study, we confirmed how the SiO2 layer affected the low illumination properties of solar cells, even though these cells were more efficient at 1 sun. Silicon solar cells with a SiNx/SiO2 bilayer or a SiNx single film were fabricated, and their characteristics were evaluated. Passivation characteristics were measured using the quasi-steady-state photoconductance (QSSPC) technique to evaluate the minority carrier lifetime and the implied open-circuit voltage (VOC), and capacitance-voltage measurements were used to analyze the fixed charges. The values of the shunt resistance and series resistance in solar cells with different passivation layers were compared, and the cause of the decrease in the efficiency under low illumination was also analyzed via fill factor calculation.

INFLUENCE OF THE ALKALI SURFACE TREATMENTS ON THE INTERFACE-STATES DENSITY AND MINORITY CARRIER LIFETIME IN Cz–SILICON WAFER

The chemical etching of the surface of silicon wafers is a critical step in the manufacturing process of all semiconductor devices. In this contribution, we investigate the effect of alkaline etching on minority carrier lifetime and interface-states density ([Formula: see text] of silicon wafers intended to be used as solar cell substrates. After alkali treatment, the surface morphology was analyzed using scanning electron microscopy (SEM) and UV-visible-NIR optical spectroscopy. Besides and as electrical characterizations, the minority charge carrier lifetime ([Formula: see text] was measured by the Quasi-Steady State Photoconductance technique (QSSPC), and the Electrochemical Impedance Spectroscopy was used to evaluate [Formula: see text]. These results were correlated with the surface recombination velocity (SRV) calculated by fitting the experimental data to the theory. The results of characterization showed a lower SRV and a higher apparent lifetime ([Formula: see text] obtained with 23[Formula: see text]wt.% KOH etching as compared to those obtained with 30[Formula: see text]wt.% NaOH; viz. 825[Formula: see text]cm[Formula: see text] against 1500[Formula: see text]cm.s[Formula: see text] and 32[Formula: see text][Formula: see text]s against 23[Formula: see text][Formula: see text]s, respectively. These findings were corroborated by [Formula: see text] measurements which gave [Formula: see text][Formula: see text]ev[Formula: see text]cm[Formula: see text] for KOH treatment and [Formula: see text][Formula: see text]ev[Formula: see text]cm[Formula: see text] for NaOH treatment.

Crystallization of multicrystalline silicon from reusable silicon nitride crucibles: Material properties and solar cell efficiency

In this paper experimental results on High Performance multicrystalline silicon ingots (HPmc-Si) grown from reusable silicon nitride crucibles are presented. Five ingots are grown from the same crucible and the material is analyzed for carbon and oxygen, and metallic impurities, by means of Fourier transform infrared (FTIR) spectroscopy and Neutron Activation Analysis (NAA), respectively. Microwave Photoconductance Decay (μW-PCD) and Quasi-Steady-State Photoconductance (QSSPC) is applied on samples to reveal the resulting charge carrier lifetime distribution. Full-area aluminum back surface field (BSF) solar cells are processed from 156 mm × 156 mm wafers and the solar cell efficiency is analyzed. Material properties and solar cell efficiencies are comparable to reference material grown from high purity electronic grade (EG) quartz crucibles. Obtained results are discussed with respect to results published recently.

Role of Oxygen for Boron Diffusion From Borosilicate Glass Layers Deposited by Atmospheric Pressure Chemical Vapor Deposition

We investigate the impact of different oxygen concentrations during boron diffusion at 950°C using borosilicate glass (BSG) layers deposited by atmospheric pressure chemical vapor deposition. Therefore, we vary the boron concentration in the BSG layer and the oxygen concentration in the gaseous ambient of the high-temperature step. Characterization includes sheet resistance as well as electrochemical capacitance voltage measurements for the determination of the resulting boron doping profile. In addition, we perform quasi steady-state photoconductance measurements to investigate the influence of the oxygen concentration on the recombination parameters of the boron-diffused surface. Low recombination current densities J 0 of J 0 = 38 fA/cm² at a sheet resistance of 54 Ω/sq are observed. Finally, we show that the oxygen concentration is a crucial parameter to prevent the formation of a boron-rich layer at the silicon surface.

Passivation property of ultrathin SiOx:H / a-Si:H stack layers for solar cell applications

A stacked layer of ultrathin hydrogenated silicon oxide (SiOx:H) and hydrogenated amorphous silicon (a-Si:H) has been developed to passivate the crystalline silicon (c-Si) surface (see graphical abstract). Silicon oxide has the advantage of excellent optical and passivation properties. The SiOx:H layer is deposited on the c-Si surface by atomic layer deposition (ALD), with its thickness precisely controlled below 2 nm. The a-Si:H layer is deposited on the SiOx:H layer by plasma-enhanced chemical vapor deposition (PECVD) with a specific doping property, i.e. intrinsic, n- or p-type. The samples are then annealed in the range of 100 °C to 950 °C to study the fundamental passivation properties. We find that a combination of an ultrathin SiOx:H and (p) a-Si:H layers shows a favorable passivation compared to a neat (p) a-Si:H layer. The effective minority carrier lifetime, measured by quasi-steady-state photoconductance (QSSPC), is ~ 0.5 ms after low temperature annealing at 300 °C. The passivation property is discussed in terms of hydrogen concentration, bond configurations, stoichiometry x of SiOx:H, and material microstructure, characterized by Fourier transform infra-red (FTIR) and Raman spectroscopy. It is suggested that a reorganization of both the SiOx:H, and the (p) a-Si:H layers, associated with hydrogen diffusion, plays an important role in improving the passivation.