Czochralski Si Wafers 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.

High efficiency perovskite/heterojunction crystalline silicon tandem solar cells: towards industrial-sized cell and module

29.2%-conversion efficiency of a two-terminal (2T) perovskite/crystalline Si heterojunction tandem solar cell using 145 μm thick industrial Czochralski (CZ) Si wafer is obtained. The structural optimization, such as surface passivation of the perovskite layer and better light management techniques, improved power conversion efficiency (PCE). To our knowledge, this PCE is the best in 2T-tandem solar cells using CZ wafers. Towards industrialization, crucial issues with the 2T tandem solar cells with crystalline Si bottom cell are discussed. Four-terminal (4T) tandem solar cells are evaluated as an approach to avoid the crucial issues. Examining our base technologies which realize 22.2%-conversion efficiency perovskite single junction solar cell module and 26%-heterojunction back-contact solar cells, we clarified that the based technologies were ready to realize 30%-conversion efficiency 4T perovskite/heterojunction crystalline Si tandem solar cells with approximately quarter size of an industrial crystalline Si solar cell (∼64 cm2).

Ga-doped Czochralski silicon with rear p-type polysilicon passivating contact for high-efficiency p-type solar cells

The use of Ga-doped Czochralski (CZ) silicon wafers in Passivated Emitter and Rear Cells (PERC) has been confirmed to have a prominent advantage in suppressing light-induced degradation (LID), which will attract considerable attention for the application of Ga-doped wafers in more efficient photovoltaic devices. In this work, we investigate the passivation quality and address the issue of LID in Ga-doped CZ Si wafers equipped with p-type polysilicon passivating contact that consists of an ultrathin SiO x and a heavily doped polysilicon. We also present the modeling results for solar cells using this type of contact. The experiments show that samples with Ga-doped CZ Si wafers have superior anti-LID properties when compared to the samples with B-doped wafers. An excellent passivation performance with a high implied open-circuit voltage ( iV oc ) of 705 mV and a low single-sided saturation current density ( J 0,s ) of 9 fA/cm 2 was achieved. Moreover, with the help of the numerical simulations, we predict that the p-type Ga-doped CZ Si solar cells with p-type polysilicon passivating contacts have the potential to achieve a high efficiency of 23.8%, an ~0.5% absolute efficiency improvement over that of PERC solar cells. The results demonstrated in this study suggest that Ga-doped CZ Si wafers combined with p-type polysilicon passivating contacts could resolve the LID issue while maintaining good passivation properties, providing a promising alternative for p-type solar cells in the photovoltaic industry. • The p-type TOPC on featuring Ga-doped CZ Si wafers achieved an excellent passivation quality with iV oc > 705 mV. • The passivation improvement sources of Ga-doped CZ Si-based structures were attributed to the suppression of B- and O-related defects. • Ga-doped TOPCon solar cells show the LID-free performance. • Simulation results show a comparable efficiency of 23.8% for Ga-doped TOPCon solar cells.

Micro-Raman investigation of p-type B doped Si(1 0 0) revisited

The doping concentration of B doped single-crystal Czochralski Si(1 0 0) wafers (6 × 1014-5 × 1019 cm−3) has been monitored via micro-Raman spectroscopy using visible (633 and 532 nm) and near-UV (355 nm) laser excitations at low power (5 mW). Data have been analysed with unprecedented accuracy via a convoluted Fano-Gaussian model of the first-order Raman Stokes mode of Silicon. This allowed the determination of the fitting spectral parameters (peak position and width) with an accuracy of 0.01 cm−1, which enables a reliable probing of the concentration. We observed, independently on the excitation wavelength used, a widening (up to 6.5 cm−1), a frequency-softening (up to 1.5 cm−1) and an intensity reduction (down to 90%) of the Si peak with the doping concentration. The widening and frequency-softening follow a strictly linear dependence with doping concentration, allowing a calibration. A linear dependence of the reciprocal Fano asymmetry parameter (q-1) with excitation energy is verified, with the slope showing a linear behavior with the doping concentration and providing a direct estimate on the hole-phonon interaction strength. Results are reproduced with surface-sensitive near-UV Raman spectroscopy on BF2+ ion implanted and laser thermal annealed (LTA) Si, demonstrating the full portability of the Raman technique to state-of-the-art nanoelectronics.

Application of hydrosilane-free atmospheric pressure chemical vapor deposition of SiOx films in the manufacture of crystalline silicon solar cells

In this work we present SiOx films deposited in cost-effective laboratory scale three-dimensional printed atmospheric pressure chemical vapor deposition setup. As SiOx films are deposited at room temperature without complex vacuum systems, they can be a good candidate for the use in commercial c-Si solar cell production lines. The quality of the deposited films was investigated as to their integrity, conformity with various surfaces, and post-treatment resilience such as stability against etchants and annealing. Several applications of the SiOx film prepared with the atmospheric pressure chemical vapor deposition (APCVD) were discussed. In one application, the APCVD SiOx was utilized to effectively promote single-side texturing of Float Zone and Czochralski Si wafers by coating only one side with SiOx and subsequently annealing prior to texturing in an alkaline aqueous solution. Another application was to exploit the APCVD SiOx as a plating mask for silicon heterojunction solar cells. Two processing options prior to the oxide-film deposition were investigated: i) application of an Ag seed-layer, which promotes subsequent electroplating, and ii) printing of an organic grid, which, after stripping, creates openings in the SiOx that facilitate electroplating of the solar cell's electrode on the underlying transparent conducting oxide. In a different application, the APCVD SiOx films acted as protection against parasitic plating on the front side of passivated emitter and rear solar cells. The deposited films were characterized by ellipsometry, hemispherical reflectance measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy.

Delineation of point defect state in Czochralski Si by modified background haze

A novel haze technique for analyzing point-defect distribution in Czochralski Si (CZ-Si) wafers without intentional contamination by transition metals was proposed. Based on the background haze in high-temperature oxidation, a three-step oxidation process including two-step low-temperature precipitation and a single-step high-temperature wet oxidation process was designed. After the newly designed heat processing and preferential etching, surface haze was formed selectively on the interstitial-dominant region of the wafer. Through electron microscopy analysis, surface defects decorated with Ni were observed, which suggested that the surface accumulation of non-gettered Ni adsorbed from the furnace causes surface haze on the interstitial-dominated region. The widths of the hazes could be optimized by adjusting the processing times of the first two steps, which affected the Ni gettering ability in the vacancy-dominated regions of the wafers. Additionally, the second stage at 1000 °C was associated with precipitate growth, as well as with haze formation.

Mass production of industrial tunnel oxide passivated contacts (i‐TOPCon) silicon solar cells with average efficiency over 23% and modules over 345 W

AbstractWe present an industrial tunnel oxide passivated contacts (i‐TOPCon) bifacial crystalline silicon (c‐Si) solar cell based on large‐area n‐type substrate. The interfacial thin SiO2 is thermally growth and in situ capped by an intrinsic poly‐Si layer deposited by low‐pressure chemical vapor deposition (LPCVD). The intrinsic poly‐Si layer is doped in an industrial POCl3 diffusion furnace to form the n+ poly‐Si at the rear, which shows an excellent surface passivation characteristics with J0 = 2.6 fA/cm2 when passivated by a SiNx:H layer deposited by plasma‐enhanced chemical vapor deposition (PECVD). With an industrial fabrication process, the cells are manufactured with screen‐printed front and rear metallization, using large‐area 6‐in. n‐type Czochralski (Cz) Si wafers. We demonstrate an average front‐side efficiency greater than 23% and an open‐circuit voltage Voc greater than 700 mV. These results are based on more than 20 000 pieces of cells from mass production on a single day, in an old conventional multicrystalline silicon (mc‐Si) Al‐back surface field (BSF) cell workshop, which has been upgraded to i‐TOPCon process. The best cell efficiency reaches 23.57%, as independently confirmed by Fraunhofer CalLab. A median module power greater than 345 W and a best module power greater than 355 W are demonstrated with double‐glass bifacial i‐TOPCon modules consisting of 120 pieces of half‐cut 161.7 mm pseudosquare i‐TOPCon cells with nine busbars.

Defects in Oxidized p‐Type Si Wafers Observed by Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy (SPV) is widely used for semiconductor characterization in modern microelectronics. Herein, the results of a comprehensive SPV analysis of oxidized n‐ and p‐type Czochralski (Cz) Si wafers with an oxygen concentration of about 5–7 × 1017 cm−3 are reported. It is demonstrated that the oxidation of the wafers with different H2/O2 ratios can significantly influence the diffusion length of minority carriers in p‐type Si. This effect is explained by the formation of oxygen–hydrogen‐related defects which are strong recombination centers in such wafers. In addition, a significant degradation of the diffusion length of minority carriers is observed in oxidized p‐type Si wafers after annealing at 90 °C and a subsequent illumination at room temperature. This degradation is found to be independent of oxide growth conditions, and it is not observed in n‐type wafers. This degradation is correlated with the presence of low concentrations of interstitial Fe (<1011 cm−3) in some wafers and with boron–oxygen‐related defects in some other wafers.

Effect of doping profile on sheet resistance and contact resistance of monocrystalline silicon solar cells

This paper presents the preparation and characterization of phosphorous-doped silicon solar cells based on screen-printed technique with systematically changing the phosphorous doping concentration. In the work, P-type monocrystalline (100) oriented Czochralski Si wafers of the thickness of 200 μm were textured using wet alkaline solution. Phosphorous was doped on Si wafers employing the diffusion technique with a fixed flow rate of liquid phosphorus oxychloride (POCl3) at the changing diffusion time of 10 min, 15 min and 20 min, respectively. Four-point probe method was adopted to measure the sheet resistance of doped wafers that resulted in significant decrease in the sheet resistance. Moreover, the doping concentration of phosphorus atoms estimated by energy-dispersive x-ray spectroscopy (EDS) analysis as well as Hall Effect measurement found to increase with increasing doping time. Furthermore, the increasing and decreasing of the front contact and back contact resistance with increasing doping concentration was confirmed by the measured contact resistance for different doping densities of the cells using transmission line method (TLM). Thus, the study on the controllable sheet resistance and contact resistance by varying the doping concentration may assist in the commercialization of efficient crystalline Si solar cell.

Density Functional Theory Study on Defect Behavior Related to the Bulk Lifetime of Silicon Crystals for Power Device Application

Among the insulated‐gate bipolar transistors (IGBTs) and PIN junction diodes, there are devices that the recombination centers, namely lifetime‐control defects, are introduced into phosphorus (P) doped n‐type silicon (Si) crystals by electron beam irradiation. The lifetime‐control defects are considered as they form deep levels such as the vacancy–vacancy (V–V) pair and vacancy–phosphorus (V–P) pair. In the case of using Czochralski Si wafers, it is considered that some pairs of carbon (C), oxygen (O), and their complexes are believed to have a negative impact to the lifetime‐control defects. In this study, density functional theory (DFT) calculations are performed to understand the formation behavior of lifetime‐control defects and the formation behavior of complexes composed of C and O atoms believed to interact with lifetime‐control defects. On the basis of these results, DFT calculations related to the structural change of lifetime‐control defects are performed. The most important result is that instead of the interstitial carbon–substitutional carbon (Ci–Cs) pair, the interstitial Ci atom and interstitial carbon–interstitial oxygen (Ci–Oi) pair are the strong candidates to affect controllability of lifetime by interacting with V composed of lifetime‐control defects.

Minimizing Light-Induced Degradation of the Al2O3 Rear Passivation Layer for Highly Efficient PERC Solar Cells

Commercializing a highly efficient passivated-emitter-and-rear-cell solar cell requires high passivation quality and stability of the cell's Al2O3 layer. This paper reports on light-induced degradation (LID) of the Al2O3 layer and the effects of post-annealing temperatures after light soaking on the passivation quality. To understand the LID phenomenon of the Al2O3 passivation layer, we used a Ga-doped Si wafer that prevented boron-oxygen LID effects. The fabrication process was carried out on large-area (156 × 156 mm2), commercially available, (100)-oriented Ga-doped Czochralski(Cz) Si wafers in the pilot line. Before and after light soaking, the effective lifetime was measured using Sinton's quasi-steady-state photoconductance as a function of annealing temperature. Chemical binding structures near the interface of the Al2O3 film and Si wafer were investigated using X-ray photoelectron spectroscopy (XPS). The passivation quality and light-induced degradation showed the best performance at an annealing temperature of 600°C. Analysis of XPS data revealed that the chemical binding structures at the interface of the Al2O3 layer and Si wafer were stabilized by optimizing the annealing condition of the Al2O3 layer. By optimizing an industrially feasible Al2O3 passivation process, an efficiency of 20.1% was achieved on large-area, commercial-grade Cz c-Si wafers.

Method of Removing Single-Side Doped Layer While Maintaining Pyramid Textured Surface of n-Type Bifacial Solar Cells

BBr3 and POCl3 thermal diffusion is a widely used technique for p- and n-type emitter formation in bifacial solar cell fabrication. However, single-side doping of BBr3 or POCl3 is difficult to achieve through gas diffusion carried out at high temperatures. Thus, removal of the unexpected doped layer on one side is undertaken in the fabrication of bifacial solar cells. We have implemented a spin-etching technique to remove the doped layer; however, the removal leads to a planar surface, and the related optical losses affect the bifaciality of the cell through a lowering of the short-circuit current. In this paper, we demonstrate a new procedure to maintain the pyramid texture on the side from which the doped layer is removed. The effect of the texture-maintaining removal process was examined using scanning electron microscopy (SEM) and reflectance measurements. SEM images show that the edges of the texture pyramids become wider. The optical losses due to the wider edges were investigated through reflection measurements. To investigate the potential of this new procedure, an n-type bifacial cell was fabricated on a 156 mm × 156 mm 180-μm-thick n-type pseudo Czochralski-Si wafer, and the current–voltage parameters were compared with the single-side planar n-type bifacial solar cell.

Ultrathin Passivation of P-type silicon Surface by Atomic Layer Deposited Gallium Oxide Thin Films

Surface passivation performances of Ga2O3 thin films deposited on p-type Czochralski Si wafers by atomic layer deposition (ALD) were investigated as a function of post-deposition annealing conditions. Minority carrier lifetimes were characterized by Semilab WT-2000PVN lifetime tester. The average effective minority carrier lifetime can reduce to 218.09 μs after annealed treatment at the temperatures of 650 °C for 3 min. It found that the surface recombination velocities of Ga2O3 thin film (3.4 nm and 7.6 nm) decrease to ∼30 cm/s and are saturated with the annealing temperature of 600 °C ∼700 °C. Such results indicate that Ga2O3 thin film show the functions of surface passivation for photovoltaic applications.

High-efficiency selective boron emitter formed by wet chemical etch-back for n-type screen-printed Si solar cells

Front metal contact induced recombination and resistance are major efficiency limiting factors of large-area screen-printed n-type front junction Si solar cells with homogeneous emitter and tunnel oxide passivated back contact (TOPCON). This paper shows the development of a selective boron emitter (p+/p++) formed by a screen-printed resist masking and wet chemical etch-back process, which first grows a porous Si layer and subsequently removes it. Various wet-chemical solutions for forming porous Si layer are investigated. An industrial compatible process with sodium nitrite (NaNO2) catalyst is developed to uniformly etch-back the ∼47 Ω/◻ atmospheric pressure chemical vapor deposited heavily doped boron emitter to ∼135 Ω/◻ by growing a 320 nm porous Si layer within 3 min and subsequently removing it. After etching back, the boron emitter was subjected to a thermal oxidation to lower the surface concentration and the emitter saturation current density J0e. Various etched-back emitters were evaluated by measuring J0e on symmetric test structures with atomic layer deposited aluminum oxide (Al2O3) passivation. Very low J0e of 21, 14, and 9 fA/cm2 were obtained for the 120, 150, and 180 Ω/◻ etched-back emitters, respectively. A solar cell with a selective emitter (65/180 Ω/◻) formed by this etch-back technology and with an Al/Ag contact on the front and TOPCON on the back gave an open-circuit voltage (Voc) of 682.8 mV and efficiency of 21.04% on n-type Czochralski Si wafer. This demonstrates the potential of this technology for next generation high-efficiency industrial n-type Si solar cells.

P-Type Indium-Doped Passivated Emitter Rear Solar Cells (PERC) on Czochralski Silicon Without Light-Induced Degradation

Solar cells fabricated on boron (B)-doped Czochralski (Cz) Si wafers in the photovoltaic industry are known to suffer from light-induced degradation (LID) in efficiency. This paper reports on promising LID-free large-area indium (In)-doped Cz Si solar cells. Two different commercial-grade B-doped Cz materials were included for comparison. To study the impact of LID on the cell structure, ion-implanted large-area (239 and 242.22 cm2) screen-printed full aluminum (Al) back-surface field (BSF) baseline cells, as well as higher performance passivated emitter rear cells (PERC) with oxide passivation and local Al BSF, were fabricated. In-doped PERC cells achieved 20.3% efficiency, while the B-doped cells gave efficiencies of 20.7% and 20.5% from low- (2 Ω⋅cm) and high-resistivity (6.2 Ω⋅cm) substrates, respectively. It was found that initial efficiency of In-doped PERC cells was ∼0.2% lower due to lower bulk lifetime and higher back-surface recombination velocity. However, In-doped PERC cells showed no LID and surpassed the B-doped PERC cell efficiency by 0.3–0.5% after 0.8-sun 48-h illumination at 37 °C.

Comparison of POCl3 diffusion and phosphorus ion-implantation induced gettering in crystalline Si solar cells

Most p-type Si solar cells involves phosphorus-doped emitter by POCl3 diffusion or phosphorus ion-implantation. Although the formation of the phosphorus emitter is known to getter impurities like Fe, the difference in the impact of these two gettering techniques on cell performance is not well quantified. Therefore, this paper compares the gettering efficiency of POCl3 diffusion and phosphorus ion-implantation on Czochralski(Cz) and cast quasi-mono Si wafers. Cz-Si wafers were used to measure bulk lifetime and iron concentration before and after POCl3 diffusion and phosphorus implantation with different doses. Increase in phosphorus implantation dose improved the gettering efficiency by increasing bulk lifetime and decreasing iron concentration but remained inferior to POCl3 diffusion partly due to single side gettering as opposed to double side gettering with higher phosphorus surface concentration during POCl3 diffusion. Moreover, large-area solar cells were fabricated on cast quasi-mono and Cz-Si wafers to quantify the impact of these emitters on cell parameters. POCl3 diffused cast quasi-mono cells showed ~0.4% higher efficiency due to higher bulk lifetime compared to phosphorus implanted emitter. However, phosphorus implanted Cz cells gave ~0.3% higher efficiency due to lower emitter saturation current, resulting from the benefit of in-situ oxide surface passivation during the implant anneal.

Impact of the firing step on Al2O3 passivation on p-type Czochralski Si wafers: Electrical and chemical approaches

The development of an efficient surface passivation is a key feature of silicon solar cells towards the improvement of €/W ratio. An Al2O3 layer coated by plasma-enhanced atomic layer deposition has proven its efficiency to increase the minority carrier lifetime on p-type silicon. However, the firing step, which is a common part of the manufacturing process that includes metallic pastes for screen-printed contacts, ruins this passivation effect. On the basis of photoelectric, electric, and chemical experimental studies, a correlation is provided in this paper between the different microscopic and macroscopic behaviors that govern the passivation process. To show this correlation, photoconductance decay measurements have been carried out to determine minority carrier lifetime. Following which, the capacitance–voltage measurement results are used to extract electrical parameters, namely, the densities of interface defects and effective charges. In addition, complementing secondary ion mass spectrometry (SIMS) experiments revealed the different chemical species that can be relevant for the explanation of passivation quality and macroscopic electrical measurements.

Laser doping of boron-doped Si paste for high-efficiency silicon solar cells

Boron laser doping (LD) is a promising technology for high-efficiency solar cells such as p-type passivated locally diffused solar cells and n-type Si-wafer-based solar cells. We produced a printable phosphorus- or boron-doped Si paste (NanoGram® Si paste/ink) for use as a diffuser in the LD process. We used the boron LD process to fabricate high-efficiency passivated emitter and rear locally diffused (PERL) solar cells. PERL solar cells on Czochralski Si (Cz-Si) wafers yielded a maximum efficiency of 19.7%, whereas the efficiency of a reference cell was 18.5%. Fill factors above 79% and open circuit voltages above 655 mV were measured. We found that the boron-doped area effectively performs as a local boron back surface field (BSF). The characteristics of the solar cell formed using NanoGram® Si paste/ink were better than those of the reference cell.

Dielectric Charge Tailoring in PECVD SiO<inline-formula> <tex-math>${}_x$</tex-math> </inline-formula>/SiN <inline-formula> <tex-math>${}_x$</tex-math> </inline-formula> Stacks and Application at the Rear of Al Local Back Surface Field Si Wafer Solar Cells

State-of-the-art surface passivation results are obtained on undiffused p-type commercial-grade Czochralski Si wafers with effective surface recombination velocity S eff values of ∼8 cm/s and implied open-circuit voltage iV oc values of up to 715 mV with an industrially fired dielectric stack of silicon oxide and silicon nitride (SiO ${}_x$ /SiN ${}_x$ ) deposited in an industrial inline plasma-enhanced chemical vapor deposition reactor. We are able to controllably vary the total positive charge density Q total in the stack by more than one order of magnitude (1011–10 12 cm−2) with no impact on midgap interface state density D it,midgap (5 × 1011 eV−1·cm−2) by altering the deposition temperature of the SiO ${}_x$ layer in the stack. We show experimentally that, for inversion conditions, S eff scales with the inverse square of the charge density $1/Q_{{\rm total}}^2$ , which is in good agreement with theory. Based on the measured injection-level–dependent minority carrier lifetimes and the total positive charge densities, it is shown that films with higher positive charge density have higher 1-sun V oc and fill factor ( FF ) potential. Large-area alloyed aluminum local back surface field solar cells confirmed this by showing higher conversion efficiency by 0.17% absolute due to improved cell V oc and FF of the solar cells featuring a SiO ${}_x$ /SiN ${}_x$ stack with a higher Q total.

Advantage in solar cell efficiency of high-quality seed cast mono Si ingot

We have grown 50 cm2 mono Si ingots by the seed cast technique. The carbon and oxygen concentrations of the ingots have been significantly reduced by improving the gas flow condition and coating. The dislocation density was also reduced by eliminating the extra dislocation generation sources. Owing to these developments, the lifetime of wafers has reached 465 µs. Finally, the efficiency of 18.7% has been achieved, which is comparable to 18.9% of the reference Czochralski (CZ) Si wafer.