P-type Cz-Si Research Articles

Crystalline Si Surface Passivation with Nafion for Bulk Defects Detection with Electron Paramagnetic Resonance.

In monocrystalline Si (c-Si) solar cells, identification and mitigation of bulk defects are crucial to achieving a high photoconversion efficiency. To spectroscopically detect defects in the c-Si bulk, it is desirable to passivate the surface defects. Passivation of the c-Si surface with dielectrics such as Al2O3 and SiNx requires deposition at elevated temperatures, which can influence defects in the bulk. Herein, we report on the passivation of different Czochralski (Cz) Si wafer surfaces by an organic copolymer, Nafion. We test the efficacy of the surface passivation at temperatures ranging from 6 to 473 K to detect bulk defects using electron paramagnetic resonance (EPR) spectroscopy. By comparing with state-of-the-art passivation layers, including Al2O3 and liquid HF/HCl, we found that at room temperature, Nafion can provide comparable passivation of n-type Cz Si with an implied open-circuit voltage (iVoc) of 713 mV and a recombination current prefactor J0 of 5 fA/cm2. For p-type Cz Si, we obtained an iVoc of 682 mV with a J0 of 22.4 fA/cm2. Scanning electron microscopy and photoluminescence reveal that Nafion can also be used to passivate the surface of c-Si solar cell fragments scribed from a solar cell module by using a laser. Consistent with previous studies, analysis of the EPR spectroscopy data confirms that the H-terminated surface is necessary, and fixed negative charge in Nafion is responsible for the field-effect passivation. While the surface passivation quality was maintained for almost 24 h, which is sufficient for spectroscopic measurements, the passivation degraded over longer durations, which can be attributed to surface SiOx growth. These results show that Nafion is a promising room-temperature surface passivation technique to study bulk defects in c-Si.

Influence of Passivation and Solar Cell Configuration on the Electrical Parameter Degradation of Photovoltaic Modules

This article analyses and compares the influence of p-type Cz-Si solar cells produced with and without Al-BSF and silicon oxide passivation on the degradation of the electrical parameters of PV modules, after 77 months under operating conditions in a PV system. Solar cells were manufactured at a pilot plant-scale facility, and 32 PV modules with silicon oxide passivated emitter and Al-BSF solar cells were assembled. The other group was composed of 28 PV modules produced with n+pn+ solar cells. The I–V curves of the PV modules were measured under standard test conditions before and after 77 months of exposure. In both groups, the short-circuit current presented higher degradation, and the open-circuit voltage showed low reduction. However, the degradation of these electrical parameters was lower in PV modules assembled with a passivated emitter and Al-BSF solar cells. Consequently, the power degradation rate of PV modules with passivated emitter n+pp+ and n+pn+ solar cells was 0.29% and 0.66% per year, respectively. The milky pattern was observed at the edge of all solar cells and was related to titanium dioxide antireflective coating modification and laser isolation processing.

Effect of Intrinsic and Extrinsic Defects on the Structural, Thermal, and Electrical Properties in p-Type CZ-Si Wafers with Different Carrier Concentrations

Effect of Intrinsic and Extrinsic Defects on the Structural, Thermal, and Electrical Properties in p-Type CZ-Si Wafers with Different Carrier Concentrations

(Digital Presentation) SiGe Epitaxial Growth on Si Substrate Using Al-Ge Paste

Silicon-germanium (SiGe) has been considered as an important alternative material to silicon due to its high carrier mobility, low power consumption and excellent performance [1]. However, due to both material unique phase diagram with melting points separated by 376 °C, it is too difficult to realize a good bulk quality of polycrystalline-free SiGe single crystals with controlled Ge contents.In this work, crystalline thick epitaxial layers of SiGe with Ge contents exceeding 30% are fabricated by screen-printing technology on Si wafers. As illustrated in Figure 1, Aluminum-germanium (AlGe) paste with a mole ratio of 7:3 was prepared and screen-printed on p-type Cz-Si <111> substrate, dried at 100 °C for 10 minutes and then annealed in Ar ambient using IR image furnace at 900 °C for 5 minutes. The Al begins to melt at 660 °C, dissolving the silicon wafer surface and the Ge in the paste. The melted Al, Si and Ge form the liquid phase of the Al-Si-Ge, which when cooling process start, regrown epitaxially at the melt/Si interface to form the Al-doped SiGe crystalline layer.The formed SiGe was observed by scanning electron microscope (SEM), and energy dispersive X-ray spectrometry (EDX). As can be seen in the cross-sectional SEM image in Figure 2, SiGe layer of about 20mm was formed on the Si substrate surface. It is worth mentioning that the SiGe/Si interface is formed straight as the reaction stopped perfectly on façade <111> forming SiGe layer. EDX mapping of elements suggests that the Al in the Al-Si-Ge liquid phase formed at high temperature is segregated from the liquid to the Al-Ge paste layer during the cooling process and remains only as dopant in the SiGe layer due to its low solid solubility in Si and Ge. The Al-Ge paste residue layer will be etched chemically and further step of chemical mechanical polishing will be performed to improve the surface flatness of SiGe layer to be an epi-ready SiGe/Si substrate for different semiconductor devices applications.Reference[1] M. L. Lee and E. A. Fitzgerald, Strained Si/strained Ge dual-channel heterostructures on relaxed Si0.5Ge0.5 for symmetric mobility p-type and n-type metal-oxide-semiconductor field-effect transistors, Appl. Phys. Lett. 83, 4202 (2003). Figure 1

Acceptor-oxygen defects in silicon: The electronic properties of centers formed by boron, gallium, indium, and aluminum interactions with the oxygen dimer

It is well established that boron reacts with two oxygen atoms in Czochralski-grown silicon (Cz-Si) to form a defect, which is responsible for the dominant light-induced degradation (LID) in solar cells made from Cz-Si:B material. The detrimental effect of LID has stimulated a move by solar cell manufacturers to the use of silicon with other group-III dopants, particularly with gallium. Cz-Si:Ga is immune to the BO-type LID. The information available in the literature on the interactions of oxygen with either Al, Ga, or In impurities in Si is limited. We use ab initio modeling and junction spectroscopy techniques to study a family of defects with unusual electronic properties, which have been detected in Cz-Si samples doped with different shallow acceptor species. We have carried out detailed measurements of the temperature dependencies of hole emission rate, equilibrium occupancy, and hole capture kinetics for the traps observed in differently doped p-type Cz-Si samples. It is found from the analysis of the changes in magnitude of the deep-level-transient signals with temperature that the equilibrium occupancy function of the traps is characteristic for a defect with negative-U properties in all the samples. The positions of the E(−/+) occupancy level of the defects are very close in differently doped samples, E(−/+) = Ev + (0.31 ± 0.01) eV. It is argued that the oxygen dimer interacts with group-III atoms in silicon and these interactions result in the formation of AsO2 complexes (A is either B, Al, Ga, or In atom) with very similar electronic properties.

Passivation properties of HfO2-SiO2 mixed metal oxide thin films with low reflectivity on silicon substrates for semiconductor devices

HfO2-SiO2 mixed oxide-based thin films were introduced in this work, and their passivation and anti-reflection properties were examined by using carrier lifetime, x-ray diffraction, scanning electron microscope, energy dispersive x-ray analysis, reflectance, and Fourier transform infrared measurements. The HfO2-SiO2 mixed oxide solutions were prepared with different composition ratios, and the thin films were formed on p-type CZ-Si substrates using the spin-coating technique. Annealing of the samples was carried out in ambient air at different temperatures after optimizing the solution ratio. When annealed at 600 and 700°C, the average reflectance of the samples was observed to be around 15% in the spectral range of 300-1000 nm, with a minimum reflectance point lying in the visible range. The effective carrier lifetimes increased up to 32 μs after annealing at 600°C compared to the initial lifetime of around 2 μs. Annealing at higher temperatures significantly decreased the carrier lifetimes (4.5 and 0.6 μs for 700 and 800°C, respectively). The results of this study revealed that the HfO2-SiO2 mixed oxide films can provide a good passivation effect in addition to an effective anti-reflection property depending on the annealing temperature.

Frequency-Dependent Sonochemical Processing of Silicon Surfaces in Tetrahydrofuran Studied by Surface Photovoltage Transients.

The field of chemical and physical transformations induced by ultrasonic waves has shown steady progress during the past decades. There is a solid core of established results and some topics that are not thoroughly developed. The effect of varying ultrasonic frequency is among the most beneficial issues that require advances. In this work, the effect of sonication of Si wafers in tetrahydrofuran on the photovoltage performance was studied, with the specific goal of studying the influence of the varying frequency. The applied ultrasonic transducer design approach enables the construction of the transducer operating at about 400 kHz with a sufficient sonochemical efficiency. The measurements of the surface photovoltage (SPV) transients were performed on p-type Cz-Si(111) wafers. Sonication was done in tetrahydrofuran, methanol, and in their 3:1 mixture. When using tetrahydrofuran, the enhanced SPV signal (up to ≈80%) was observed due to increasing sonication frequency to 400 kHz. In turn, the signal was decreased down to ≈75% of the initial value when the frequency is lowered to 28 kHz. The addition of methanol suppressed this significant difference. It was implied that different decay processes with hydrogen decomposed from tetrahydrofuran could be attempted to explain the mechanism behind the observed frequency-dependent behavior.

Influence of Tabula Rasa on Process- and Light-Induced Degradation of Solar Cells Fabricated From Czochralski Silicon

Monocrystalline Si solar cells are fabricated from Czochralski (Cz) Si, which contains 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> oxygen atoms. Cz Si undergoes degradation during high-temperature thermal processing steps, such as dopant diffusion to form the p-n junction. This degradation in the bulk minority carrier lifetime can be related to the formation of oxygen precipitates. We found that a high-temperature annealing process known as tabula rasa (TR) not only mitigates process-induced degradation via oxygen precipitate nuclei dissolution, but also modifies subsequent light-induced degradation. We report on the bulk carrier lifetime of n- and p-type Cz Si after TR, which homogenizes the interstitial oxygen in the bulk Si to its monoatomic form in either an N2 or O2 environment. A control sample, which was not subjected to a TR processing step, experienced severe process-induced degradation during a boron emitter thermal budget as oxygen precipitates were formed in the Si bulk. These precipitates could be imaged using photoluminescence. Additionally, samples that underwent a TR processing step prior to the boron emitter thermal budget show efficient gettering of metallic impurities compared to the control sample, which showed a decline in the implied open-circuit voltage after the gettering step. Furthermore, modification of the interstitial oxygen bonding by TR had a strong effect on the light-induced degradation kinetics. Instead of a typically observed monotonic decrease, minority carrier lifetime increases first, followed by a nonmonotonic decrease over a ~20 h period. We conclude that by modifying the interstitial oxygen bonding via TR pretreatment, p-type Cz Si wafers become substantially resistant to harsh solar cell processes and strongly modified light-induced degradation, which would open alternative ways to mitigate this degradation mechanism.

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.

A High Near-Infrared Sensitivity Over 70-dB SNR CMOS Image Sensor With Lateral Overflow Integration Trench Capacitor

This article presents a 16- $\mu \text{m}$ pitch CMOS image sensor (CIS) exhibiting a high near-infrared (NIR) sensitivity and a 71.3-dB signal-to-noise ratio (SNR) with a linear response for high-precision absorption imaging. A 1.6-pF lateral overflow integration trench capacitor (LOFITreC) was introduced in each pixel to achieve a very high full well capacity (FWC), and a very low impurity concentration p-type Cz-Si substrate with a low oxygen concentration was employed for improving the NIR sensitivity. The developed CIS operated at a single exposure linear response wide dynamic range (DR) mode and a dual reset voltage mode for high SNR absorption imaging and achieved the maximum 24.3 Me− FWC, a wide spectral sensitivity from 200 to 1100 nm, and a photodiode quantum efficiency of 89.7%, 78.2%, and 26.7% at 860, 940, and 1050 nm, respectively. Both the spatial resolution and light sensitivity toward the NIR light were further improved by thinning the Si substrate and by applying a negative backside bias. Due to the LOFITreC, a record spatial efficiency of 95 ke $^{-}/\mu \text{m}^{{2}}$ with a 130-dB DR was achieved. As one of the applications of the developed CIS, the NIR absorption imaging toward a noninvasive blood glucose measurement was experimented and a diffusion of 5 mg/dl glucose was clearly visualized at 1050 nm in real time.

Optimization of phosphorus dopant profile of industrial p-type mono PERC solar cells

Passivated emitter and rear cells (PERC) on p-type Cz Si wafers are currently being migrated to mainstream production, applying the ongoing improvements in recent years. For PERC solar cells, the emitter recombination loss becomes the main loss of the entire cell. Currently, two-step diffusion consisting of low temperature phosphosilicate glass (PSG) deposition and high-temperature drive-in has been widely applied in production and can reduce saturation current density (J0) of the emitter in the passivated area. However, a low contact resistance under metal area cannot be guaranteed. Meanwhile, the determination of the saturation current density J0 in the passivated area is dependent on the injection density during the measurement, and exact calculation of J0,metal is not as convenient as J0,diffusion, which could lead to a wrong assessment in cell analysis. In this study, a three-step diffusion (low-temperature PSG deposition–high-temperature drive-in–low temperature PSG deposition) with low J0 and low Ag-Si contact resistance is investigated and combined with a selective emitter by an etch-back process. The results are compared with a conventional two-step POCl3 diffusion. The accuracy of J0 measurement and J0,metal test method is also discussed. With these improvements, the champion cell efficiency of our PERC solar cells fabricated on 156 × 156 mm2 wafers using screen printing technology and industrial-type process has reached 22.61% with Voc of 684.4 mV and a fill factor of 81.49%, as confirmed by Fraunhofer ISE CalLab PV cells. By implementing the diffusion process to mass production, the average cell-efficiency gain is approximately 0.2% with a median efficiency of 21.7%.

A Detailed Study on Light-Induced Degradation of Cz-Si PERC-Type Solar Cells: Evidence of Rear Surface-Related Degradation

A light-induced degradation phenomenon of unknown origin in p-type Cz-Si passivated emitter and rear cell (PERC)-type solar cells is thoroughly investigated by collating results from different measurement techniques and predictions from various simulations. The observed degradation manifests in slight losses in j sc , strong losses in V oc, and devastating losses in FF, and thus massively impacts efficiency. It is found that the series resistance degrades significantly due to a degradation of front contact resistance. This, however, does not explain losses in V oc and j sc , which are attributed to the degradation of a different cell component. Neither a degradation by defect formation in the space charge region, nor the emitter, nor the bulk is found to consistently explain the observations. Only a rear surface-related degradation mechanism explains consistently all experimental findings.

Building Blocks for Industrial, Screen-Printed Double-Side Contacted POLO Cells With Highly Transparent ZnO:Al Layers

We report on an industrial large area, screen-printed, double-side contacted cell with polysilicon on oxide (POLO) junctions on both sides and an energy conversion efficiency of 22.3% ( A = 244.15 cm 2, V oc = 714 mV, FF = 81.1%, J sc = 38.5 mA/cm2, measured in-house). This cell shows an extraordinarily low series resistance below 0.05 Ω cm2. This confirms the low specific junction resistance observed recently for POLO junctions. The present cell suffers from 1) low short-circuit current due to parasitic absorption in the rather thick poly-Si (30 nm), as well as in the indium tin oxide, 2) deterioration of the recombination behavior upon sputter deposition of a transparent conductive oxide (TCO), and 3) shunts near the edge due to nonadapted TCO edge exclusion. We address all of these limitations experimentally. In particular, we developed a plasma-enhanced chemical vapor deposition process for ZnO:Al, which does not compromise the passivation of the POLO junctions underneath. An estimation of the efficiency potential (based on the two-diode model and the assumption that all these building blocks can be successfully combined on a cell level) shows that 25.3% can be achieved with this cell concept. We also look into potential cost advantages of the POLO junction scheme for this cell structure, such as the usage of p-type Cz-Si material and the omission of Ag fingers.

Anneal induced transformations of defects in hadron irradiated Si wafers and Schottky diodes

In this research, the anneal induced transformations of radiation defects have been studied in n-type and p-type CZ and FZ Si samples, irradiated with relativistic protons (24GeV/c) and pions (300MeV/c) using particle fluences up to 3 × 1016cm−2. The temperature dependent carrier trapping lifetime (TDTL) spectroscopy method was combined with measurements of current deep level transient spectroscopy (DLTS) to trace the evolution of the prevailing radiation defects. The contactless TDTL technique has been shown to be preferential when the radiation induced trap density approaches or exceeds the dopant concentration and when it is necessary to avoid modification of a detector structure due to annealing processes at elevated temperatures. The deep level spectra were complementarily examined by using DLTS spectroscopy on Schottky diodes made of irradiated Si wafer fragments. The dominant radiation defects and their transform paths under isothermal and isochronal anneals have been revealed. A good agreement between the DLTS and TDTL spectra has been obtained.

Mass production of p-type Cz silicon solar cells approaching average stable conversion efficiencies of 22 %

Within this work, both the performance and reliability of industrial p-type monocrystalline solar cells with dielectrically passivated rear side and corresponding modules are investigated. Results of the mass production of Q.ANTUM solar cells at Hanwha Q CELLS on boron-doped p-type Czochralski-grown silicon (Cz-Si) substrates are presented, exceeding 21.5 % average conversion efficiency. Without power-enhancing measures such as the use of half cells, multi-wire approaches or light-capturing ribbons, essentially all currently (as of March 2017) produced Cz-Si Q.ANTUM solar modules exhibit output powers of > 300 Wp with 60 full 4-busbar cells. In terms of reliability, light-induced degradation (LID) is investigated in detail, with conditions relevant for the activation of, both, the boron-oxygen (BO) defect, and, so-called “Light and Elevated Temperature Induced Degradation” (LeTID). While the formation of the BO defect has been considered the most prominent LID mechanism in boron-doped p-type Cz-Si, LeTID has so far been discussed mainly as a potential issue for passivated emitter and rear cells (PERC) on multicrystalline silicon (mc-Si) substrates. This work shows that, if not adequately suppressed, LeTID can also occur in p-type Cz-Si PERC with a degradation in output power of up to > 6 %, which cannot be suppressed in a straightforward manner by conventional processing steps to permanently deactivate the BO defect. In contrast to conventional PERC, Hanwha Q CELLS’ Q.ANTUM technology is shown to reliably suppress, both, LID due to BO defect formation, and, LeTID in modules manufactured from, both, p-type mc-Si and Cz-Si substrates.

Minority carrier lifetimes in Cz-Si wafers with intentional V-I transitions

A p-type Cz-Si crystal has been pulled with varying pulling speed in order to produce wafers containing two distinct regions; A region with silicon self-interstitial defects, and a vacancy dominated region. Band-to-band photoluminescence imaging has been used to study the minority charge carrier lifetimes in these wafers after different processing steps. Despite the different defects found in the different regions of the wafers carrier lifetimes appear to be uniform across the entire wafers, both for ungettered and gettered samples. Only after an oxidation process at 1100 °C oxygen related ring patterns become visible. It is, however, difficult to identify the band structure of the transition area between the regions among all the striations in the crystal.

The microstructure of Si surface layers after plasma-immersion He+ ion implantation and subsequent thermal annealing.

The structural changes in the surface layer of p-type Cz-Si(001) samples after high-dose low-energy (2 keV) He+ plasma-immersion ion implantation and subsequent thermal annealing were studied using a set of complementary methods: high-resolution X-ray reflectometry, high-resolution X-ray diffraction, transmission electron microscopy and atomic force microscopy. The formation of a three-layer structure was observed (an amorphous a-SiO x layer at the surface, an amorphous a-Si layer and a heavily damaged tensile-strained crystalline c-Si layer), which remained after annealing. Helium-filled bubbles were observed in the as-implanted sample. The influence of annealing on the evolution of the three-layer structure and the bubbles is considered. The bubbles are shown to grow after annealing. Their characteristic size is determined to be in the range of 5-20 nm. Large helium-filled bubbles are located in the amorphous a-Si layer. Small bubbles form inside the damaged crystalline Si layer. These bubbles are a major source of tensile strain in the c-Si layer.

The effect of small pyramid texturing on the enhanced passivation and efficiency of single c-Si solar cells

In this work, small random pyramid texturing (0.5–2 μm size) was generated with chemical nano-masking, to enhance the surface passivation of commercial p-type Cz-Si wafers.

Industrial Si Solar Cells With Cu-Based Plated Contacts

Until today, most industrial c-Si solar cells have been limited by front emitter and front metal contact properties. This study demonstrates that laser ablation and inline plating of nickel and copper followed by inline thermal annealing results in improved performance and reduced cost. Stable efficiencies exceeding 20.8% on p-type PERC CZ-Si solar cells have been independently confirmed by FhG-ISE CalLab. Average fill factors up to 80.8% have been demonstrated on large-area solar cells with a homogeneous emitter P surface concentration below 4 × 1019 P/cm3. Reliable module performance according to the IEC61215 standard is reported.

Low Surface Recombination Velocity on P-Type Cz-Si Surface by Sol-Gel Deposition of Al2O3 Films for Solar Cell Applications.

High quality surface passivation has gained a significant importance in photovoltaic industry for fabricating low cost and high efficiency solar cells using thinner and lower cost wafers. The passivation property of spin coated Al2O3 films with a thickness of about 50 nm on p-type Cz-Si wafers has been investigated as a function of annealing temperatures. An effective surface recombination velocity of 55 cm/s was obtained for the films annealed at 500 °C. The chemical and field effect passivation was analyzed by C-V measurements. A high density of negative fixed charges (Qf) in the order of 9 x 10(11) cm(-2) was detected in Al2O3 films and its impact on the level of surface passivation was demonstrated experimentally. The C-V curves show density of the interface state (Dit) of 1 x 10(12) eV(-1)cm(-2) at annealing temperature of 500 °C. During annealing, a thin interfacial SiOx is formed, and this interfacial layer is supposed to play a vital role in the origin of negative QF and Dit. The homogeneous SiOx interlayer result in higher passivation performance due to both the increase of negative Qf and the decrease of Dit.