Emitter Doping Profiles Research Articles

Device Modeling of 4H-SiC PIN Photodiodes with Shallow Implanted Al Emitters for VUV Sensor Applications

A numerical model is presented for the simulation of ultraviolet ion-implanted 4H-SiC photodiodes with shallow p- emitter doping profiles. An existing model for SiC pin photodiodes, taken from literature, is modified with a dedicated SiO2-SiC interface layer to account for degradation of carrier mobility and lifetime at the interface. Furthermore, aluminum compensation in 4H-SiC is included and its impact on the spectral response and carrier recombination is analyzed. The simulated spectral response in the wavelength range from 200 to 400 nm is compared to experimental data. While the existing model, taken from literature, fails to predict the performance of VUV photodiodes with a shallow p- emitter, the newly designed model successfully achieves high accuracy, even with a basic modeling approach featuring an abrupt material parameter transition.

Influence of boron doping profile on emitter and metal contact recombination for n-PERT silicon solar cells

Increasing interest in n-type silicon (Si) solar cells due to the high-quality material properties of the n-Si wafers, their relatively low sensitivity to donor-like impurities like iron and their resistance to light-induced degradation brings more recent attention to the studies on the p+ emitter and its metal contact. In this work, we investigate the effect of the Boron doping profile on the quality of p+ emitter and their contacts with screen-printed Ag/Al metal fingers. The emitters within the scope of this study are created with BBr3 diffusion in an atmospheric diffusion furnace on n-type Si wafers and also applied to n-PERT Si cells to analyze their effects on device performance. Our results highlight the significance of the doping profile for solar cell performance, as it considerably affects the contact resistivity at the emitter-metal interface and the saturation current densities for emitter (J0, emitter) and Ag/Al metal contacts (J0, AgAl). Reducing the Boron concentration within the emitter leads to a decrease in carrier recombination in the non-contacted region of the emitter, which in turn decreases J0, emitter. However, this reduction increases the recombination in the contacted areas of the emitter, consequently elevating the J0, AgAl, and increasing the contact resistance values. Besides, the power loss analysis and simulation results based on experimental values obtained for our best solar cells group show how much J0, emitter and J0, AgAl values contribute to and would improve solar cell efficiency. The manuscript presents a case study of experimental emitter doping profile optimization applicable to similar n-type PERT or TOPCon cells with varying design parameters.

Towards Sic-Based VUV Pin-Photodiodes - Investigations on 4H-SiC Photodiodes with Shallow Implanted Al Emitters

4H silicon carbide (SiC) based pin photodiodes with a sensitivity in the vacuum ultraviolet spectrum (VUV) demand newly developed emitter doping profiles. This work features the first ever reported 4H-SiC pin photodiodes with an implanted p-emitter and a noticeable sensitivity at a wavelength of 200 nm. As a first step, Aluminum doping profiles produced by low energy ion implantation in 4H-SiC were characterized by secondary-ion mass spectrometry (SIMS). Photodiodes using these shallow emitters are compared to one with a deep p-emitter doping profile employing IV characteristics and the spectral response. SIMS results demonstrate the possibility of shallow Alimplantation profiles using low implantation energies with all emitter profiles featuring characteristic I-V results. For some shallow doping profiles, a meassurable signal at the upper limit of the VUV spectrum could be demonstrated, paving the way towards 4H-SiC pin photodiodes with sensitivities for wavelengths below 200 nm.

Stack Diffusion Process for Cost- and Energy-Efficient Boron Emitter Formation

The boron emitter formation for tunnel oxide passivated contact (TOPCon) solar cells faces higher costs compared to the POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> diffusion for passivated emitter and rear (PERC) solar cells due to the requirement for higher temperatures and longer process times. This work presents an alternative energy-efficient and low cost of ownership boron diffusion approach for TOPCon solar cells, enabling a highly increased throughput compared to the typically used gas phase diffusion. We use an atmospheric pressure chemical vapor deposition borosilicate glass layer as the boron dopant source and combine it with a subsequent thermal anneal in a quartz tube furnace for dopant drive-in. Here, we either use a conventional single-slot quartz boat configuration, or, for highly increased throughput, a vertical wafer stack configuration with the wafer surfaces in direct contact with each other. We show that this approach yields an emitter doping profile comparable to the state-of-the-art gas phase diffusion with sufficient uniformity across the wafer area. We further investigate the emitter dark saturation current densities <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> as well as the energy conversion efficiency of TOPCon solar cells fabricated for each configuration and compare the results to those of a BBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> reference process. These solar cells achieve energy conversion efficiencies exceeding 23% for the stack diffusion approach. Additionally, we demonstrate a potential reduction in both the cost of ownership and the specific electricity consumption of the presented approach.

Impact of boron doping on electrical performance and efficiency of n-TOPCon solar cell

A big challenge to improve the conversion efficiency of n-type solar cell is the recombination and electrical contacting of boron (B)-doped emitters in n-TOPCon solar cells. This work investigates the emitter dark saturation current density under the passivation layer (J0e, passivated), the metallization-induced recombination current density under the metal contact (J0e, metal), and the I–V parameters, including short-circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (Eff) as functions of the peak concentration (Nmax) and junction depth of B-doped profile. We introduced the profile with the peak concentration of 1–1.5 × 1019 atoms/cm3 and a junction depth of 0.75–1.0 μm to improve the conversion efficiency of 23.4%. The results revealed that Jsc, implied Voc (iVoc) and the contact resistance (ρc) were negative correlated with the Nmax and junction depth. However, J0e, passivated had a positive correlation with the same. The results from ρc, J0e along with the I–V showed the junction depth of emitter doping profiles need to exceed the corroded depth of Ag/Al paste to get a low ρc, meanwhile, a low J0e should be ensured. The results suggested theB-selective emitters were developed with a low ρc and a low J0e. After optimizing B-selective emitters and passivation processes, we obtained industry-grade TOPCon cells with Eff, Voc, Jsc, and FF as high as 23.7%, 709 mV, 40.8 mA/cm2, and 82%, respectively. This optimized B-doped and simplified B-selective emitters processes can be commercially applied in photovoltaics.

Microstructure beneath screen-printed silver contacts and its correlation to metallization-induced recombination parameters

In this work, we present an approach to model metallization-induced recombination losses j 0,met of screen-printed fire-through metallization pastes, which are used e.g. for the contacts on the front side of passivated emitter and rear cells (PERC). Modelling is based on the local phosphorus emitter doping profile and the local microstructure of the interface between metal contact and silicon surface, which is characterized during a quantitative microstructural analysis using the scanning electron microscope (SEM). By comparing the modelled j 0,met to measured j 0,met results determined at the same position of both, the microstructural analysis and emitter doping profile, we find that in most cases recombination is dominated by the etch back of the emitter by the glass frit rather than by penetration of crystallites into the doped region. However, for crystallites protruding deep into the emitter or rather up to the junction, recombination at the crystallites becomes significant as well. Our modelled results yield indications that the etch depth into the emitter doping profile by the glass frit contained in the metallization paste, is sensitive to the emitter doping profile itself. • Metallization-induced recombination is dominated by the etch back of the emitter doping profile • Etch back of the emitter doping profile is sensitive to doping profile itself • Plating useful to enhance contrast for microstructure analysis

Exact analytical expressions for Auger saturation current in diffused Si emitters

If the densities of free minority and majority carriers have a reverse dependence on the Auger recombination enhancement factor, g, and minority-carrier diffusivity has a direct dependence on g, then the minority-carrier transport equations in non-uniformly doped Si regions in the dark can be solved analytically for the case, where lifetime is only determined by Auger recombination, and two new analytical expressions for saturation current density of emitter, J0em, can be derived, one in p-type Si and the other in n-type Si, by imposing boundary conditions including band-gap narrowing (BGN). The new expressions include the integral of emitter doping profile. We calculate curves of majority-carrier mobility versus doping by taking into account BGN and find that majority- and minority-carrier diffusivity of same carrier species can be assigned the same direct dependence on g. The integral of doping profile can be consequently expressed as a function of sheet resistance and the new expressions for J0em can be calculated without necessarily knowing the emitter doping profile. The assumption on diffusivities holds at room temperature for dopings higher than 6 × 1017 cm−3 in both p- and n-type Si. The new J0em-expressions are used to characterize actual reported n+ and p+ metal-coated and n+ passivated emitters with respect to surface doping, diffusion depth, sheet resistance, and surface recombination velocity.

Impact of Standard Cleaning on Electrical and Optical Properties of Phosphorus-Doped Black Silicon

Black silicon (b-Si) has been estimated to considerably grow its market share as a front texture of high-efficiency silicon solar cells. In addition to excellent optical properties, high-efficiency cell process requires extreme cleanliness of the bulk material, and thus cleaning of b-Si surfaces is often a critical process step. While standard clean (SC) 1 solution efficiently removes possible contamination from wafer surfaces, we show here that it may cause challenges in b-Si solar cells. First, the silicon etch rate in SC1 solution is shown to depend on the phosphorous concentration and as high rate as ∼1.4 nm/min is observed on planar emitter surfaces. When extending the study to b-Si, which has much larger surface area in contact with the cleaning solution, even higher volumetric Si consumption occurs. This is observed in significant changes in emitter doping profiles, for instance, a 10 and 30-min cleaning increases the sheet resistance from 47 to 57 Ω/□ and 127 Ω/□, respectively. Furthermore, the SC1 solution alters substantially the nanostructure morphology, which impacts the optics by nearly doubling and more than tripling the surface reflectance after a 30 and 60-min immersion, respectively. Thus, uncontrolled cleaning times may impair both the electrical and optical properties of b-Si solar cells.

Effects of diffusion process on potential induced degradation of silicon solar cells.

Potential induced degradation (PID) has recently been identified as one of the most important degradation mechanisms for silicon solar cells. It is widely considered that PID is closely related with the manufacture and application period of solar modules. In this study, the effects of diffusion sheet resistance on PID were verified and explained by testing the emitter doping profile, the minority carrier lifetime, the emitter saturation current, the electrical performance of different cells, and the PID process. With increasing sheet resistance of cells, the depth and saturation current density of the emitter both decreased, and the cell efficiency increased, whereas the PID phenomenon became serious. It was found that higher sheet resistance or thinner P-N junction could lead to higher PID sensitivity. Therefore, more attention should be paid to PID phenomenon as the photovoltaic industry develops in the direction of high sheet resistance.

Challenges for lowly-doped phosphorus emitters in silicon solar cells with screen-printed silver contacts

This work points out that the application of phosphorus emitters with low surface concentration Nsurf of a few 1019 cm-3 in combination with state-of-the-art screen-printed and fired silver contacts is no more limited by the specific contact resistance ρC but by the dark saturation current densities underneath the metal contacts j0,met. Eight emitter doping profiles have been designed with different Nsurf ranging between 3.3·1019 cm-3 and 1.2·1020 cm-3 which feature very similar junction depths of about 350 nm. The measured ρC values for these emitters are determined to be between 2 mΩcm2 and 5 mΩcm2 for about 50 µm-wide contact fingers. Their emitter dark saturation current density (alkaline textured and passivated surface) range from 40 fA/cm² to 90 fA/cm². From these parameters and the current-voltage data of large-area Czochralski-grown silicon solar cells featuring an aluminum back surface field, we estimate j0,met to be between 1200 fA/cm² and 3900 fA/cm². Our results show that for decreasing Nsurf, the recombination underneath the metal contacts increases significantly which mainly limits the cell’s open-circuit voltage. We thus conclude that the major challenge for emitters with low Nsurf consists in achieving reasonable low j0,met values in order to benefit on the cell level from the lower recombination activity in the passivated, non-metallized surface area.

Lifetime Analysis for Comparing POCl3 Diffused Emitter Doping Characteristics in p-Type Silicon Solar Cells Using QSSPC

Analysis of the emitter property of solar cells is important because the emitter doping characteristics can affect the surface recombination velocity, contact resistance, emitter saturation current density, and cell efficiency. To analyze the emitter quality, we used the following methods: the four-point probe method, quasi-steady-state photoconductance (QSSPC), and secondary ion mass spectroscopy (SIMS). The four-point probe method is used to measure the doping dose in the emitter. Using QSSPC, we can characterize the emitter quality, including the lifetime of the emitter, and using SIMS, we can measure the concentration of dopants as a function of depth in the emitter. However, SIMS measurement is destructive and limited to the measurement of planar surface wafers. To solve this problem, we investigated the relationship between the minority carrier lifetime and the emitter doping profile using the QSSPC. The relationship between the lifetime and emitter doping profile showed that the lifetime of the emitter decreases as the emitter doping concentration increases. From this result, we performed a lifetime analysis for differently doped POCl3-diffused emitters. The results obtained using the theoretical model for the lifetime agreed with experimental SIMS measurement results, indicating that the model can be used as a quantitative model for comparing emitter doping characteristics.

Comparison of Boron diffused emitters from BN, BSoD and H3BO3 dopants

In this work, we are comparing different limited boron dopant sources for the emitter formation in n-type c-Si solar cells. High purity boric acid solution, commercially available boron spin on dopant and boron nitride solid source are used for comparison of emitter doping profiles for the same time and temperature conditions of diffusion. The characterizations done for the similar sheet resistance values for all the dopant sources show different surface morphologies and different device parameters. The measured emitter saturation current densities (Joe) are more than 20 fA cm−2 for all the dopant sources. The bulk carrier lifetimes measured for different diffusion conditions and different solar cell parameters for the similar sheet resistance values show the best result for boric acid diffusion and the least for BN solid source. So, different dopant sources result in different emitter and cell performances.

POCl3 diffusion for industrial Si solar cell emitter formation

POCl3 diffusion is currently the de facto standard method for industrial n-type emitter fabrication. In this study, we present the impact of the following processing parameters on emitter formation and electrical performance: deposition gas flow ratio, drive-in temperature and duration, drive-in O2 flow rate, and thermal oxidation temperature. By showing their influence on the emitter doping profile and recombination activity, we provide an overall strategy for improving industrial POCl3 tube diffused emitters.

Emitter doping profiles optimization and correlation with metal contact of multi-crystalline silicon solar cells

A systematic investigation of Phosphorus Oxychloride (POCl3) based diffusion optimization for the formation of homogeneous emitters and correlation with metal contact in p-type multi-crystalline silicon (multi-Si) solar cell is presented. The deposition temperature (810, 804, 798, 792 and 786°C) was varied to achieve different sheet resistance (85, 90, 95, 100, and 105ohm/square) measured by four points probes. The correlation between emitter quality, metal contact resistance (Rc) and cell performance was investigated. The results showed that the sheet resistance is higher, phosphorus surface concentration (Cs) is lower and emitter saturation current density (Joe) is lower. The cell data showed that the higher Rsheet result in higher Voc and Isc due to better short wavelength response from the External Quantum Efficiency (EQE) measurement, but lower fill factor (FF) due to higher emitter sheet resistance and contact resistance with Ag paste. The optimal Rsheet for this type of Ag paste (Samsung SDI 8630A) in this test is 90ohm/square with the highest cell efficiency of 18.325%.

Boron Oxygen Pair Effect in p+ Emitter and Nanosized Boron Rich Layer by Fold Coordination Analysis for Crystalline Silicon Solar Cell Applications.

N-type substrates possess better material characteristics than p-type substrates for high efficiency mass producible Si solar cells with HIT, IBC structures. The major drawbacks of these structures are a complicated fabrication process and an expensive unit cost. In this paper, the boron emitter doping profile of a nanosized boron rich layer (BRL), for which the boron and oxygen concentrations are correlated, is optimized to fabricate high efficiency solar cells on an n-type substrate. Boron doping was carried out using a BBr3 furnace with varying oxygen gas ratios and the surface was treated with acid etching. The effect of the oxygen on the nanosized BRL was analyzed using both FTIR spectroscopy and XPS, where by conductivity and the Si-B bond were observed for the three-fold and four-fold coordinated borons, respectively. The results showed that the oxygen quantities in the boron doped emitter and the nanosized BRL affected the characteristics of the solar cell. Regarding the solar cells that were fabricated using the boron emitter and shallow emitter (90 ohm/sq) processes, the open-circuit voltage increased by 54 mV and the short circuit current (J(sc)) increased by 3.7 mA/cm2. The J(sc) increase was due to an increased quantum efficiency in the short wavelength range. The shallow emitter etch back process minimized the boron-oxygen defects in the doping profile.

Impact of the phosphorus emitter doping profile on metal contact recombination of silicon wafer solar cells

Metallisation of phosphorus-doped silicon (Si) surfaces using screen-printed silver (Ag) pastes is a well-established process and also a key process in the production of Si wafer solar cells. The metal-Si interface in a solar cell is a highly recombination-active region that impacts the device voltage. In this work, a facile test metallisation pattern with regions of varying front metal contact fractions is screen printed to create test cells on different phosphorus emitter doping profiles, all on commercially available multicrystalline Si wafers. On these and in conjunction with H-pattern cells and cells without front metallisation, intensity-dependent photoluminescence imaging is performed to extract both the metal-induced recombination saturation current densities and parameters related to wafer edge and peripheral recombination. It is observed that as the phosphorus emitter profile becomes lightly doped and shallower, the metallisation-induced recombination losses increase. The extracted metal recombination saturation current densities (J01-metal and J02-metal) on a 50Ω/sq phosphorus emitter surface were 1800fA/cm2 and 10nA/cm2 respectively when compared to values of 2500fA/cm2 and 120nA/cm2 respectively on a 130Ω/sq phosphorus emitter surface. As a consequence of high metal recombination current density values for the lightly doped emitters of this study (130Ω/sq), the solar cells fabricated from this group have a significant drop of 12mV in open-circuit voltage values after front grid metallisation.

Towards understanding the characteristics of Ag–Al spiking on boron-doped silicon for solar cells

With this work, we introduce numeric three-dimensional simulation of metal spiking into highly boron-doped surfaces of n-type silicon solar cells, which is moreover performed with a simulation of the quasi-steady-state photoconductance technique. This setup serves as a virtual experiment to simulate the dark saturation current density j0,met of metallized boron-doped emitters with respect to metal spikes originating from the silver–aluminum (Ag–Al) contact. With the results obtained from this simulation model we approach quality and quantity of increased j0,met and give detailed insight to which degree a solar cell’s performance is possibly harmed by this effect. We show that metal spikes penetrating into boron-doped emitters are of harmless nature concerning j0,met until their tips reach depths where boron doping concentration is lower than approximately 1018cm−3. Deeper spikes then lead to an exponential increase in j0,met as more and more carriers from emitter and also the base are able to diffuse to its tip and recombine there. With the help of j0-results obtained experimentally in combination with the simulation results, we discuss the influence of spikes on emitter recombination, the benefits that can be achieved with deeper emitter doping profiles, and suggestions for the further development of pastes to contact boron-doped surfaces.

Shallow B-implanted Emitters with Laser Overdoping from AlOx Passivating Layers

A simple process for the fabrication of selective emitter structures on n-PERT cells is investigated, using shallow Boron emitters obtained by ion implantation. By tuning the emitter doping process parameters, J0e values as low as 10 fA.cm-2 have been obtained with highly resistive profiles. Laser overdoping processes from AlOx passivating layers are tested on these profiles to locally increase the emitter conductivity and allow better contact properties. Through this process the emitter sheet resistance and doping profile may be locally controlled with a limited impact on the J0e values.

Reverse Bias Behavior of Diffused and Screen-Printed n-Type Cz-Si Solar Cells

In this study, we investigate current flow in reverse bias mode and its impact on conversion efficiency for large-area n-type Cz-Si H-pattern and n-type Cz-Si metal wrap through (MWT) solar cells. Shunting is studied as a function of the boron emitter doping profile and by comparing MWT cells with two different phosphorus-doped back surface field (BSF) structures. Less shunting is observed for cells with deeper boron-doped emitters (depth d ≈ 700 nm) compared with cells with shallower emitters (d ≈ 500 nm). Cells with a deeper doping profile have initial shunt resistances of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> > 30 kΩ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (without prior reverse load), while cells with shallower emitters exhibit initial values of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> ≈ 9 kΩ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , irrespective of the cell type. Furthermore, cells with deeper boron doping profiles show significantly lower current flows under reverse bias. We observe a halving of the RP-values after reverse biasing the H-pattern and the MWT cells with structured BSF where, on the other hand, the conversion efficiencies are hardly affected. MWT cells featuring a BSF below the external p-type contacts show a drop in conversion efficiency of 0.3%abs. This is due to degradation of the electrical insulation between via paste and BSF after reverse bias stress.

Micro Characterization and Imaging of Spikes in Nickel Plated Solar Cells

Annealing induced silicidation of plated nickel contacts can severly lower the solar cell performance due to deep nickel silicide spikes penetrating the space charge region. This work summarizes several attempts to characterize performance limiting deep silicide structures and determines the influence of different passivation layer structuring technologies on the silicide growth. Reverse biased electroluminescence measurements revealed that the deep nickel silicides occur after an anneal and arelocated along the structured and plated passivation layer openings. Cross-section studies of the plated contacts demonstrated that deep silicide growth is present independently of the applied passivation layer structuring technology. While for laser-ablation the critical silicide structures could be mostly attributed to accelerated silicide growth at laser induced defects, there are also deep silicide structures that appear without any obvious correlation even in the case of defect free wet chemical passivation structuring of the contact openings. By varying the emitter doping profile and the annealing temperature after plating the distance of the space charge region and the surface was found to influence the annealing-induced solar cell performance decrease. Considering the presented results there are three ways to overcome annealing induced degradation of Ni plated solar cells. Either optimized process designs with sufficient contact adhesion and contact resistivity without the need of thermal silicide formation, improved silicide depth control (e.g. Ni source limited growth) or the implementation of a selective emitter design with pn-junction depths well above 1 μm within the contacted area.