N-type PERT Research Articles

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.

Performance investigation of n-type PERT bifacial solar photovoltaic module installed at different elevations

In recent years, bifacial solar photovoltaic (bPV) cells have gained much interest in the solar PV market as they produce additional electrical energy at a marginally higher cost. Their ability to generate additional energy mainly depends on their rear side response. Module elevation, albedo, row spacing, and frame structure are the few factors that affect its rear side response and thus the overall efficiency. In this research work, the performance of a bPV module installed at the geographical location of latitude 9.673° N, and longitude 77.964° E is analyzed with different module elevations. Module elevation is one of the key installation parameters that causes the non-uniformity in the rear side irradiance. At low elevations, the non-uniformity is higher and starts reducing with the increase in elevation, reaching a negligible value at greater elevations. The experimental results showed that for the module elevation of above 1m, the non-uniformity is reduced to 5 % from 12 % at 0.25m. The peak power output for the elevation of above 1m is around 375W at which the bifaciality factor is about 25 %. Overall, it is observed that the bifacial solar PV module at a higher optimum elevation produces lesser non-uniformity and a higher energy yield.

A method for alleviating the effect of pinhole defects from silicon nitride film in n-type rear-junction PERT silicon solar cells

We investigated incorporation of a novel approach of phosphorous silicate glass layer thinning (PGT) process in the N-PERT process flow to minimise pinhole defects at the silicon nitride (Si3N4) surface. The thinning (PGT) process for optimum HF deposition time of 12 min resulted in excellent cell efficiency of ∼20.55% with pinhole free layer and high electrical yield (∼0% for I Rev > 1.5 A). After optimising technology, stability is also explored with and without PGT process line, which confirms advantages of this approach. This significant reverse failure reduction due to the proposed PGT process can eventually help in improving overall cell performance of the N-PERT devices. This process can be a part of strategy for reducing process cost of solar cell in any industrial mass production line with improved yield (reduction in reverse failure from 6.6 to 1.5% for one month of mass production). Thus, the PGT process with negligible electrical rejection and high yield increases the possibility of high throughput in mass production line.

Efficiency gain in plated bifacial n-type PERT cells by means of a selective emitter approach using selective epitaxy

This work evaluates the potential of selective epitaxy to mitigate the recombination losses at the p-type contact regions of plated bifacial n-type PERT cells. Following the growth of a 500 nm, 2.5 1019 cm−3 boron doped epitaxial layer at the emitter contact regions defined by laser ablation of the passivating dielectrics, both an increase in Voc (+6 mV) and FF (+1% absolute) are measured in the final cells compared to the reference with a homogeneous diffused emitter. These results come with an average efficiency gain of 0.3 and 0.5% absolute for front and rear side illumination, respectively. If the diffused, blanket emitter in the passivated regions is replaced by a thicker, lowly doped epitaxial profile (3 μm, 5 1018 cm−3) to further reduce recombination, an additional rise in implied Voc after metallization of 10 mV is estimated. This increase would be the result of a reduction in J0,pass, emitter (down to 6 fA/cm2) and J0,plated, emitter (down to 1967 fA/cm2).

High-efficiency n-type silicon PERT bifacial solar cells with selective emitters and poly-Si based passivating contacts

Bifacial crystalline silicon (c-Si) solar cells have currently attracted much attention due to the front high-efficiency and additional gain of power generation from the back side. Here, we have presented n-type passivated emitter and rear totally-diffused (n-PERT) bifacial c-Si solar cells featuring front selective emitter (SE) and polysilicon (poly-Si) based passivating contacts. The SE formation was scanned with laser doping based on front boron-diffusion p+ emitter. The poly-Si based passivating contacts consisting of nano-layer SiOx of ~1.5 nm thickness grown with cost-effective nitric acid oxidation and phosphorus-doped polysilicon exhibited excellent passivation for high open-circuit voltage. We have successfully achieved the large-area (156 × 156 mm2) n-PERT bifacial solar cells yielding top efficiency of 21.15%, together with a promising short-circuit current density of 40.40 mA/cm2. Theoretical calculation has further demonstrated that the optimal thickness of SiOx nano-layer will increase from 1.5 nm to 1.8 nm if the density of interface defect state decreases by one magnitude from 1 × 1010 cm−2/eV, and the cell efficiency can be improved up to 24.64% with open-circuit voltage over 0.720 V by optimizing the parameters of functional materials and interface layers. The present work has indicated that the commercialization of low-cost and high-efficiency n-PERT bifacial c-Si cells is possible due to the processes compatible with existing production lines.

The Study On Anti-PID Performance Of High Efficiency Bifacial Cell Module

This paper mainly studied the correlation between the PID attenuation of P/N type bifacial cell and the adhesive film. At present, with the development of glass-glass module technology and bifacial cell technology, glass-glass bifacial module is an important direction of industry development. Currently P type monocrystal PERC bifacial cell and N-type PERT bifacial cell are typical representatives of the bifacial technology. However, the PID problem of bifacial cell is more serious than single side cell. How to solve the bifacial cell PID problem has become a hot topic. We mainly studied the correlation between two different adhesive film (EVA&POE), and the anti-pid attenuation ability of P/N type bifacial cell. The influence of the water vapor transmission rate and resistivity of the adhesive film on PID attenuation of bifacial cell was studied. After PID 192h test, For P type, power decay of the front side is lower than rear side, while for N type cell, power dacy of the rear side is lower than front side. Both P&N type bifacial cell module with POE are much better than with EVA. For POE, recipe adjustment affects PID performance obviously.

Optical and electrical performance of rear side epitaxial emitters for bifacial silicon solar cell application

In this work we investigated the optical and electrical performance of p-type epitaxial layers as the rear emitter of bifacial n-type PERT solar cells. In the first part of this paper, the surface morphology of epitaxial layers grown on textured surfaces is studied. Because of the epitaxial growth, a pyramids-rounding effect is observed as a result of {311} and {911} facet propagation. The growth pattern was quantified and modelled. In the second part of this paper, the optical performance of semi-device test structures is evaluated. The trend of the optical results in bifacial solar cell structures indicates that a final pyramid angle at the rear side around 20° gives the maximum light absorption in the wafer substrate. In this work we demonstrate that the epitaxial growth of the emitter on the textured rear side of these devices can already give a pyramid angle of 25° without having to introduce any additional polishing steps to modify the morphology of the textured surface. In the last part of this paper, we present the electrical results for semi-device structures created to quantify the recombination losses in the passivated and metallized regions of those p-type epitaxial emitters. These results indicate that by introducing a rear epitaxial emitter in the bifacial n-type PERT cell structure, we can increase the implied Voc up to 17 mV compared to a diffused emitter with the same sheet resistance.

Effect of Oxygen Precipitation in Silicon Wafer on Electrical Characteristics of Fully Ion-Implanted n-Type PERT Solar Cells

Fully ion-implanted n-type PERT solar cells with boron-implanted emitter and phosphorus-implanted back surface field were fabricated on the n-type silicon wafers obtained from the top part of the magnetic field-applied Czochralski ingot. It was observed that the electrical parameters are widely dispersed; among 11 solar cells, the best performing achieved 20.4% front side conversion efficiency, whereas the worst achieved 19.0%. Although the silicon wafers had low oxygen concentrations of 3–4 × 1017 atoms/cm3, the density of oxygen precipitates in the silicon wafers was on the order of 109 /cm3 as a consequence of the fully ion-implanted n-type PERT silicon solar cell processes. In addition, it was observed that the front side conversion efficiencies of the solar cells depended on the density of oxygen precipitates. Furthermore, the behavior of the oxygen precipitation during the fabrication processes of the solar cell is discussed.

Screen Printable Boron Doping Paste and Its Process for n-Type PERT Solar Cells

We have developed new boron paste materials for the process of diffusing crystalline silicon solar cells. The boron paste promotes pattern-forming, long carrier lifetime, and allows good sheet resistance uniformity. The cell efficiency achieved when applying boron paste to an n-type PERT cell was practically excellent. A high efficiency was also obtained under a codiffusion process with POCl3 gas for a simplifying process. We reported on the diffusion characteristic and process details of the boron paste in this paper.

High efficiency bifacial n-PERT cells with co-plated Ni/Ag contacts for multi-wire interconnection

In this paper we present bifacial n-type PERT cells (n-PERT+) with ultra-thin (< 3 μm) and narrow (< 20 μm) Ni/Ag plated contacts on both the n+ and p+ side of the cell. This cell architecture is designed for multi-wire interconnection and has the potential to deliver low cost of ownership at module level (< 0.30 $/Wp), as it can profit from bifacial gain, a low cost metallization (~ 0.07 $/cell) and reduced shadowing and recombination losses. A self-aligned, fully plated metallization sequence, able to form Ni/Ag electrodes simultaneously on both polarities of the cell with a low specific contact resistance, is demonstrated. We benchmark this novel process to a seed-and-plate sequence where the plating seed is obtained by physical vapor deposition (PVD) of Ni and we show comparable efficiency results. Efficiencies of up to 22.8% for cells featuring this fully plated Ni/Ag metallization sequence have been obtained (Cz, 239 cm2, GridTOUCH measurement). We also report 1-cell laminates assembled using multi-wire foils reaching maximum power of about 5 W (256 cm2 aperture area), which shows that the proposed metallization sequence is well suited for integration with such interconnection technology, despite small finger section and the absence of a highly conductive Cu inter-layer. The 1-cell laminates also exhibit less than 4% power degradation after 400 thermal cycles (-40 ºC to +85 ºC).

Boron Emitter Formation by Plasma Immersion Ion Implantation in n-type PERT Silicon Solar Cells

The use of plasma immersion ion implantation (PIII) is a relevant approach for the development of advanced solar cells technologies at lower cost (€/Wp). In this paper, we report on the development of homogeneous boron (B) doping of n-type crystalline silicon (cSi) substrate by the PIII technique. Using diborane (B2H6) as gas precursor, various doping profiles were identified fitting the requirements for boron-doped emitters in n-type PERT solar cells. Particularly, saturation current density (J0e) of 50 fA/cm2 were achieved on symmetrical samples for a 94 Ω/sg textured B-emitter passivated with SiO2/SiN stack. Bifacial n-type Passivated Emitter Rear Totally-diffused (n-PERT) solar cells were fabricated using the PIII technology and conversion efficiencies up to 19.8% on 239 cm2 Cz-Si wafers were obtained. As a consequence, these results indicate that PIII can compete with beamline technology but will have lower running cost. It is therefore a promising technology to create high efficiency cSi solar cells.

21.0%-efficient screen-printed n-PERT back-junction silicon solar cell with plasma-deposited boron diffusion source

The manufacturing process of Passivated Emitter and Rear Totally diffused (PERT) solar cells on n-type crystalline silicon is significantly simplified by applying multifunctional layer stacks acting as diffusion source, etching and diffusion barrier. We apply boron silicate glasses (BSG) capped with silicon nitride (SiNz) layers that are deposited by means of plasma enhanced chemical vapor deposition (PECVD). Optimum PECVD deposition parameters for the BSG layer such as the gas flow ratio of the precursor gases silane and diborane SiH4/B2H6=8% and the layer thickness of 40nm result in a boron diffusion with saturation current density J0,B below 10fA/cm2 applying an AlOx/SiNy passivation and firing. The PECVD BSG diffusion source is integrated into the n-type PERT back junction (BJ) solar cell process with screen-printed front and rear contacts. The only high temperature step is a POCl3 co-diffusion for the formation of the boron emitter from the PECVD BSG layer and for the formation of the phosphorus-doped front surface field (FSF). An independently confirmed energy conversion efficiency of 21.0% is achieved for a 156×156mm2 large n-PERT BJ cell with this simplified process flow. This is the highest efficiency reported for a large-area co-diffused n-type PERT BJ solar cell using a PECVD BSG as diffusion source. For comparison, reference n-type PERT BJ cells with separate POCl3 and BBr3 diffusions reach an efficiency of 21.2% in our lab. A synergistic efficiency gain analysis (SEGA) for the co-diffused n-PERT BJ cell shows that the main possible efficiency gain of 1.1%abs. originates from recombination in the phosphorus-diffused front surface field while the PECVD BSG boron-doped emitter accounts for only 0.1%abs. efficiency gain. We evaluate the use of the PECVD BSG/SiNz stack as a rear side passivation as a replacement of the AlOx/SiNy stack in order to further simplify the process flow. We obtain J0,B values of 40fA/cm2, an implied open-circuit voltage of 682mV and a simulated n-PERT BJ cell efficiency of 21.1%.

Bifacial, fully screen-printed n-PERT solar cells with BF2 and B implanted emitters

For boron implants made with ion implanters designed for PV applications, a variety of ion species BFx (x=0.2) is implanted, while BF2 accounts for the lions share. We investigate the impact of the BF2 implantation dose and the annealing conditions on the resulting electrical characteristics of industrial bifacial n-type PERT solar cells. For an annealing temperature of 950°C, we observe a steep increase in the emitter saturation current density for BF2 implant doses above 1.75×1015cm−2.We compare co-annealed BF2- and B-implanted solar cells and achieve maximum energy conversion efficiencies of 20.6% with BF2 and 21.0% with B, respectively. Separate annealing processes for the p+ and n+ doped regions result in even higher energy conversion efficiencies of 20.8% with BF2 and 21.5% with B emitter implant. An optimized double-layer ARC applied on the separately annealed cells further increases the efficiency up to 20.9% for BF2 and up to 21.8% for B emitter implant. All efficiency values are independently confirmed.The solar cells have bifacial factors between 97.3% and 99.4%. The BF2-implanted solar cells with highest efficiency feature an emitter with a sheet resistance of 180 Ω/sq. contacted by a commercially available Ag/Al paste with specific contact resistance of less than 10mΩcm2.

Development of a large area n-type PERT cell with high efficiency of 22% using industrially feasible technology

A high-efficiency front junction n-type passivated emitter and a rear total diffusion (n-type PERT) solar cell with the front boron diffusion passivated by a Al2O3/SiNx stack layer deposited by plasma enhanced chemical vapor method and the rear with phosphorus total diffusion and Al evaporated local contact are presented in this paper. The main purpose of this study was to develop industrially feasible front junction n-type PERT solar cells with high-efficiency; these were realized on a large area of n-type industrial 5- and 6-in. wafers. An average of 21.85% cell efficiency was achieved on 5-in. wafers, and the highest cell convert efficiency of 21.98% was achieved with Voc of 683.8mV, Jsc of 40.13mA/cm2, and FF of 80.11%. For 6-in. cells, we get Fraunhofer independently confirmed efficiency of 21.49% with a Voc of 674.1mV, a Jsc of 39.77mA/cm2, a FF of 80.18%. Details of cell fabrication and results are presented, followed by a best cell power loss analysis on Voc, Isc and FF.

Large-Area n-Type PERT Solar Cells Featuring Rear p+ Emitter Passivated by ALD Al2 O3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and PECVD SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stack employed to passivate the boron emitter is unaffected by the rear metallization processes and can suppress the Shockley-Read-Hall surface recombination current to values below 2 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , provided that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thickness is larger than 7 nm. Efficiencies of 21.5% on 156-mm commercial-grade Cz-Si substrates are demonstrated in this study, when the rear Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivation is applied in combination with a homogeneous front-surface field (FSF). The passivation stack developed herein can sustain cell efficiencies in excess of 22% and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> above 685 mV when a selective FSF is implemented, despite the absence of passivated contacts. Finally, we demonstrate that such cells do not suffer from light-induced degradation.

A Comparison Study of n-type PERT and IBC Cell Concepts with Screen Printed Contacts

In this work we conducted a comparison study of our n-type PERT (BiSoN) and IBC (ZEBRA) cell technologies. We investigated the effect of material quality and base resistivity on the performance and efficiency potential of the two device architectures. We observed only a slight efficiency variation (less than 3%relative) for wafer bulk lifetimes ranging from 1.2-6.5ms and base resistivities from 2-9 Ωcm, with no significant difference between the n-PERT andthe IBC concept. We achieved cell efficiencies, as high as 20.4% for BiSoN concept and 21.5% for ZEBRA on 15.6×15.6 cm2 n-type Cz Si wafers. Additionally, we demonstrated that both cell concepts have a high bifacial performance, both at cell and module level.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Influence of the boron emitter profile on V OC and J SC losses in fully ion implanted n-type PERT solar cells

In this work, we report on fully ion implanted 156 × 156 mm2 n-type PERT solar cells fabricated in an industry-capable process. The implant damage of phosphorous and boron was co-annealed in a single furnace annealing step. The cells feature a screen-printed front side metallization and an evaporated rear side metallization. The influence of boron emitter profile on the open-circuit voltage VOC and the short-circuit current density JSC was studied by comparing two boron doses (2.5 × 1015 cm−2, 3 × 1015 cm−2) and two annealing durations (20 min, 80 min). The solar cells reach tightly distributed efficiencies above 20% (20.3% maximum). Cells featuring an emitter implanted with 2.5 × 1015 cm−2 boron dose annealed for 80 min show the highest open-circuit voltages up to 668 mV. As compared to implied open-circuit voltages measured on non-metallized cell precursors, this corresponds to a metallization-induced voltage loss of 14 mV. For the shorter annealing time (corresponding to a shallower profile) but same boron dose, the VOC loss is 18 mV. The fact that the cells with the higher boron dose of 3 × 1015 cm−2 showed lower VOC values indicates that the recombination at the Ag/Al-p+ Si contacts is not totally dominating. We determine the recombination current densities of the metallized emitter regions to 900–1900 fA cm−2. Contrary to the dependence of VOC on the emitter profile, JSC is lower for deeper emitters. The loss in JSC is visible in the internal quantum efficiencies IQE at short wavelengths. Strategies for an optimization of both quantities, JSC and VOC, are discussed.

20.5% Efficiency on Large Area N-type PERT Cells by Ion Implantation

We developed a high efficiency N-type PERT (Passivated Rear Totally Diffused) bifacial structure based on B and P ion implantation doping, SiO2 passivation and conventional screen-printing metallization. Two process flows were compared: a “co-anneal” process and a process using separated anneals for B and P activation. We highlight the impact of the variations of the B- emitter and P- BSF profiles on the solar cells performance. The impact of the boron implantation dose was studied allowing to optimize this parameter. Concerning the BSF, two temperature ranges were studied for the P activation leading to very different BSF profiles. A shallower profile enables to reach high implied Voc while keeping low contact resistivity. The overall optimization was integrated into a simplified and industrial process flow on large area Cz-Si solar cells (239cm2). An average efficiency of 19.7% was reached using the “co-annealing” process. The efficiency in this case was limited by a low PFF. This limitation was solved using the “separated anneal” process where an average efficiency of 20.2% was obtained on a 15 cells batch with a 20.5% champion cell.

Influence of laser power on POCl3 diffused back surface field on n-type PERT silicon solar cells with local back contact

We report n-type passivated emitter rear totally diffused (PERT) silicon solar cells with local back contacts (LBC) formed by laser process. With passivated back surface field (BSF), the PERT cell design shows an improved open circuit voltage (Voc) with reduced recombination at the rear due to improved optical confinement. The rear side was diffused by POCl3 diffusion with low sheet resistance (Rs) BSF and passivated using SiNx. Laser ablation was used to open the SiNx on the rear for LBC. The Nd:YAG laser power (mW) parameters and POCl3 doping temperature were varied to obtain the BSF with lower sheet resistance. Laser power of 44 mW with 10 kHz resulted in 30 Ω/sq BSF layer with effective lifetime (τeff) of 90 μs and a higher Voc of 646 mV. With the optimized laser parameters the best electrical results yielded a short circuit current density (Jsc) of 36 mA/cm2 and efficiency of 18.54%.