Local Back Surface Field Research Articles

Performance optimization of PERC solar cells based on laser ablation forming local contact on the rear

Abstract To improve the photoelectric conversion efficiency (η) of the solar cell, a green wavelength (532 nm) laser source in a nanosecond range was used to ablate the passivated emitter and rear cell (PERC) to form the contact holes. If the laser ablation hole opening process was not set properly, the diameter or the external expansion of holes would be too large, causing the decline of the PERC performance. The Gaussian distribution of the laser is regulated by the output power (P o) and the repetition frequency (f rep) of the incident pulse laser, so that the optimized morphology of holes is obtained on the back of the PERC solar cells. After the contact holes are screen printed by the aluminum paste, the local back surface field is finally formed. The experimental results showed that the outward expansion decreases obviously with the increase of laser P o. Second, the spacing of the holes decreases with the increase of the laser f rep. It was found that under the laser P o of 33.0 W and f rep of 1,400 kHz, the η of the industrial PERC solar cells was the highest. The Quokka simulations indicated that small outward expansion, small diameter, and long spacing of holes would further decrease the recombination parameter in the rear surface. With the optimized morphology of contact holes and the low contact resistance, the PERC cell’s calculated V oc and η improvements were 6.5 mV and 0.48%, respectively, which was verified with experimental findings.

22.8% efficient ion implanted PERC solar cell with a roadmap to achieve 23.5% efficiency: A process and device simulation study

Among all available photovoltaic (PV) technology options, c-Si based passivated emitter rear cell (PERC) has already proved its mettle by delivering more than 22% conversion efficiency at the industrial level. However, designing of PERC solar cell device remains a challenge owing to recombination and absorption losses, which eventually restrict the performance of the device toward achieving an Auger efficiency limit of 29.4%. The primary contributor to these losses is emitter recombination losses that limits the performance of PERC devices. The n+-emitter region in the proposed PERC devices is formed using ion implantation followed by a diffusion process. The impact of process variables such as dose (cm−2), energy (keV), diffusion time (s), and diffusion temperature (oC) on the PERC device performance is not much explored. Therefore, in an attempt to fulfill this research gap, the emitter region performance in terms of ion implantation of phosphorus, influence of local back surface field (BSF) doping concentration (1017 to 1019 cm−3), and back surface recombination velocity (SRV 101–107 cm/s) on PERC device performance followed by the loss analysis is examined in this work. An upright pyramid textured structure with constant height (5 μm) and industrial standard stacked dielectric anti-reflective coating layers, SiNX/SiO2 and Al2O3/SiNX at front and back side respectively is realized using different process statements such as deposit, etch, implantation, and diffusion with the help of a Silvaco TCAD software. Investigations reveal that the PERC device can deliver an efficiency of 22.8% with 150 μm boron-doped crystalline silicon (c-Si) wafer at an optimum phosphorous dose (5 × 1015 cm−2), BSF doping (1019 cm−3), and SRVn (107 cm/s). Short-circuit current density (JSC) of 40.8 mA/cm2, open-circuit voltage (VOC) of 0.686 V, and fill factor (FF) of 81.54% are obtained for the structure under consideration. A roadmap to achieve 23.5% conversion efficiency has also been reported by assuming a carrier selective contact at the backside with a surface recombination velocity of 10 cm/s.

Passivated, Highly Reflecting, Laser Contacted Ge Rear Side for III-V Multi-Junction Solar Cells

This article describes the successful integration of a passivated, highly reflecting Ge rear side into a III-V multijunction solar cell. The use of lowly doped Ge and the new rear side leads to the aimed increase in Ge cell current up to 1.6 mA.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , demonstrated by external quantum efficiency and I-V measurements. For the contact formation, two different types of laser processes were conducted and evaluated-the laser fired contact route and the PassDop route. In both cases, the formation of a local back surface field preserves the passivation of the contact points. A laser pitch and laser power variation leads to a good performing back contact. The passivation effect is proven experimentally and is qualitatively accessed with cell simulations.

Impact of Local Back-Surface-Field Thickness Variation on Performance of PERC Solar Cells

This article investigates the impact of the back-surface-field (BSF) thickness variation within a local aluminum contact on the performance of passivated emitter and rear contact solar cells. A significant difference of BSF thickness between contact endings and the center of dash-shaped contacts is verified experimentally by a comprehensive statistical analysis using scanning electron microscopy. The impact of local BSF thickness differences on the cell performance is studied with 3-D technology computer-aided design (TCAD) device simulations. Several device parameters such as BSF thicknesses, the doping concentration in the BSF profile at rear contacts, or the metallized area fraction at the cell rear side are varied. Our simulation study shows that the open-circuit voltage is mainly affected by locally reduced BSF thicknesses, resulting in an efficiency loss up to 0.14% <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">abs</sub> or 0.84% <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">abs</sub> , respectively, if an area fraction of 1% or 20% within a local contact has reduced BSF thicknesses. This effect can be minimized either by reducing the metallized area fraction at the cell rear side or by increasing the doping concentration in the BSF profile at aluminum rear contacts. In addition, we demonstrate that the 3-D simulations can be approximated with 2-D simulations by applying a single doping profile with an average BSF thickness, calculated with the harmonic mean.

Impact of Local Back-Surface-Field Thickness on Open-Circuit Voltage in PERC Solar Cells: An Experimental Study Applying ANOVA to Determine Critical Sample Size Necessary to Differentiate Mean LBSF Values With Statistical Significance

Sufficiently deep local back-surface-fields (LBSF) are crucial for achieving low contact recombination and thus high solar cell efficiencies in passivated emitter and rear contact (PERC) solar cells. In this article, we investigate spatial variations of LBSF thickness 1) across dashed contacts, 2) between lateral positions within a cell, and 3) from cell to cell. The objective of this experimental study is to proof the impact of LBSF thickness on open-circuit voltage in PERC solar cells. Our measurements and analysis of variance (ANOVA) reveal that variations in LBSF thickness across dashed contacts exceed observed variations in LBSF thickness between lateral positions within a cell as well as cell-to-cell variations. Thus, we statistically treat all LBSF measurements of a single experimental group of PERC solar cells as one population of measurements. The two investigated experimental sets of PERC cells fired using different fast-firing profiles show a difference in LBSF thickness of about 1 μm. The resulting difference in open-circuit voltages arises to 1.3 mV. ANOVA-based analysis shows for our experimental samples that at least 19 random measurements are necessary in order to resolve LBSF thickness difference of 0.5 μm with sufficient statistical significance.

Optimization of rear surface roughness and metal grid design in industrial bifacial PERC solar cells

The bifacial p-type silicon (p-Si) passivated emitter and rear cells (PERCs) are predicted to dominate the industrial bifacial solar cells. In this work, we have investigated the impact of different rear surface morphologies on the electrical performance of bifacial PERCs with both five-busbar (5BB) and nine-busbar (9BB) grid design. The passivation and optical properties with differing rear surfaces are evaluated on semi-device structures. The depth of local aluminum back surface field is hardly affected by the rear surface morphology. The calculated efficiency loss analysis indicates that the negative electrical impact with enlarged rear surface area is more serious for rear side than that of front side. The batch conversion efficiency of 9BB bifacial PERCs increases by 0.2%–0.3% comparing to 5BB ones depending on the rear surface roughness. Consequently, a highest front-side average efficiency of 22.57%, with a champion efficiency of 22.75%, has been achieved for 9BB bifacial p-Si PERCs with a nearly planar rear surface. A highest bifaciality of 78.7% is realized for both 5BB and 9BB bifacial PERCs with the roughest rear surface. We have further simulated the relative enhancement of electric generation to compare the performance of bifacial PERCs in practical application.

Direct nanoscale mapping of open circuit voltages at local back surface fields for PERC solar cells

The open circuit voltage (VOC) is a critical and common indicator of solar cell performance as well as degradation, for panel down to lab-scale photovoltaics. Detecting VOC at the nanoscale is much more challenging, however, due to experimental limitations on spatial resolution, voltage resolution, and/or measurement times. Accordingly, an approach based on Conductive Atomic Force Microscopy is implemented to directly detect the local VOC, notably for monocrystalline Passivated Emitter Rear Contact (PERC) cells which are the most common industrial-scale solar panel technology in production worldwide. This is demonstrated with cross-sectioned monocrystalline PERC cells around the entire circumference of a poly-aluminum-silicide via through the rear emitter. The VOC maps reveal a local back surface field extending ~ 2 μm into the underlying p-type Si absorber due to Al in-diffusion as designed. Such high spatial resolution methods for photovoltaic performance mapping are especially promising for directly visualizing the effects of processing parameters, as well as identifying signatures of degradation for silicon and other solar cell technologies.

Predictive simulation framework for boron diffused p+ layer optimization: Sensitivity analysis of boron tube diffusion process parameters of industrial n-type silicon wafer solar cells

A predictive simulation framework, combining process and device simulation, is developed in order to assist in BBr3 boron tube furnace diffusion process optimization for n-type silicon wafer solar cells. After an appropriate calibration, the influence of the tube diffusion process parameters (drive-in temperature, oxidation temperature, etc) on the final solar cell efficiency can be predicted. The key process parameters of BBr3 tube furnace diffusion are identified and their sensitivity on the final solar cell efficiency is calculated. An efficiency gain of 0.6% absolute to 20.0% is realized by optimizing only the front-side boron diffused layer of the front and back contacted cells. The efficiency is further improved to 20.9% by introduction of a local back-surface field design. A further optimization potential of up to 0.2% absolute is predicted by modifying the boron diffusion profile.

Internal quantum efficiency mapping for evaluation of rear surface of passivated emitter and rear cell

An internal quantum efficiency (IQE) mapping method that can emphasize the effects of local structures on the rear side of solar cells is demonstrated. The IQE of crystalline silicon passivated emitter and rear cells was mapped at 1000 nm and revealed periodic patterns with lower IQE in the local aluminum back surface fields on the rear surface. A larger-scale map indicated an effect that is apparently related to the spin-etching process on the rear surface. The proposed IQE mapping method can provide information that will help identify the factors that limit the efficiency.

Analysis of Laser Injection Condition and Electrical Properties in Local BSF for Laser Fired Contact c-Si Solar Cell Applications.

A crystalline silicon (c-Si) local-back-contact (LBC) solar cell for which a laser-condition-optimized surface-recombination velocity (SRV), a contact resistance (Rc), and local back surface fields (LBSFs) were utilized is reported. The effect of the laser condition on the rear-side electrical properties of the laser-fired LBC solar cell was studied. The Nd:YAG-laser (1064-nm wavelength) power and frequency were varied to obtain LBSF values with a lower contact resistance. A 10-kHz laser power of 44 mW resulted in an Rc of 0.125 ohms with an LBSF thickness of 2.09 μm and a higher open-circuit voltage (VOC) of 642 mV.

Correction to: Optimization of Controllable Factors in the Aluminum Silicon Eutectic Paste and Rear Silicon Nitride Mono-Passivation Layer of PERC Solar Cells

Passivated emitter and rear contact (PERC) is a promising technology owing to high efficiency can be achieved with p-type wafer and their easily applicable to existing lines. In case of using p-type mono wafer, 0.5–1% efficiency increase is expected with PERC technologies compared to existing Al BSF solar cells, while for multi-wafer solar cells it is 0.5–0.8%. We addressed the optimization of PERC solar cells using the Al paste. The paste was prepared from the aluminum–silicon alloy with eutectic composition to avoid the formation of voids that degrade the open-circuit voltage. The glass frit of the paste was changed to improve adhesion. Scanning electron microscopy revealed voids and local back surface field between the aluminum electrode and silicon base. We confirmed the conditions on the SiNx passivation layer for achieving higher efficiency and better adhesion for long-term stability. The cell characteristics were compared across cells containing different pastes. PERC solar cells with the Al/Si eutectic paste exhibited the efficiency of 19.6%.

Enhanced photovoltaic properties for rear passivated crystalline silicon solar cells by fabricating boron doped local back surface field

In order to enhance the p-type doping concentration in the LBSF, boron was added into the aluminum paste and boron doped local back surface field (B-LBSF) was successfully fabricated in this work. Through boron doping in the LBSF, much higher doping concentration was observed for the B-LBSF over the Al-LBSF. Higher doping concentration in the LBSF is expected to lead to better rear passivation and lower rear contact resistance. Based on one thousand pieces of solar cells for each type, it was found that the rear passivated crystalline silicon solar cells with B-LBSF showed statistical improvement in their photovoltaic properties over those with Al-LBSF.

Electrical and optical characterization of SiOxNy and SiO2 dielectric layers and rear surface passivation by using SiO2/SiOxNy stack layers with screen printed local Al-BSF for c-Si solar cells

In c-Si solar cells, surface recombination velocity increases as the wafer thickness decreases due to an increase in surface to volume ratio. For high efficiency, in addition to low surface recombination velocity at the rear side, a high internal reflection from the rear surface is also required. The SiOxNy film with low absorbance can act as rear surface reflector. In this study, industrially feasible SiO2/SiOxNy stack for rear surface passivation and screen printed local aluminium back surface field were used in the cell structure. A 3 nm thick oxide layer has resulted in low fixed oxide charge density of 1.58 × 1011 cm−2 without parasitic shunting. The oxide layer capped with SiOxNy layer led to surface recombination velocity of 155 cm/s after firing. Using single layer (SiO2) rear passivation, an efficiency of 18.13% has been obtained with Voc of 625 mV, Jsc of 36.4 mA/cm2 and fill factor of 78.7%. By using double layer (SiO2/SiOxNy stack) passivation at the rear side, an efficiency of 18.59% has been achieved with Voc of 632 mV, Jsc of 37.6 mA/cm2, and fill factor of 78.3%. An improved cell performance was obtained with SiO2/SiOxNy rear stack passivation and local BSF.

Effects of different particle-sized Al pastes on rear local contact formation and cell performance in passivated emitter rear cells

This work presents the effects of different Al pastes on the rear contact formation in passivated emitter rear cells (PERC) after firing as well as the impact on cell performance. In this study, the rear side of PERC cells was metalized with two different PERC Al pastes—one consisting of fine particles, and another consisting of coarse particles. Our results found that the open-circuit voltages (Voc) are significantly higher for PERC cells using the Al paste with larger particles. Scanning electron microscopy (SEM) was used to analyze the local rear contacts. When metalized with the coarse particle Al paste, the local back surface field (local–BSF) was found to be several micrometers deeper. An SEM cross-sectional image revealed a non-uniform local–BSF when metalized with the fine-particle Al paste. Furthermore, the particle size of the Al paste used may also impact the adhesion and conductivity between the Al layer and the dielectric layers on the Si substrate.

Metamodeling of numerical device simulations to rapidly create efficiency optimization roadmaps of monocrystalline silicon PERC cells

In this contribution, we present an approach to simulate the energy conversion efficiencies of monocrystalline p-type silicon passivated emitter and rear cells (PERC) using a state-of-the-art design of experiment (DoE) and metamodeling approach. We preserve the accuracy of numerical device simulations whilst reducing the time for a simulation of cell efficiency from potentially hours to milliseconds. We show that only 1000 numerical simulations arranged in a space-filling DoE and metamodeling by Gaussian process regression are sufficient to cover a 13-dimensional input space, with a small mean absolute error of only 0.056%abs determined in a 10-fold cross validation. In the second part of this work, we apply the metamodel to iteratively scan this input space for the technological improvements that allow the largest efficiency gains. Simultaneously, we consider physical and technological constraints on the input parameters and optimize technologically freely changeable parameters after each step for a fair comparison. We present a roadmap that enables the production of more than 23% efficient monocrystalline p-type silicon PERC cells. Steps on this roadmap are the permanent regeneration of the boron-oxygen defect and the introduction of a selective emitter, followed by an improved front side metallization, an improved local back surface field and an improved rear passivation.

Analysis of contact recombination at rear local back surface field via boron laser doping and screen-printed aluminum metallization on p-type PERC solar cells

The passivated emitter and rear cell (PERC) has entered the solar cell market, and it is expected that the amount of PERC production will increase yearly in the future. Improvements in the cell efficiency have a great influence on reducing the cost of power generation. In this study, we focused on the loss of the back surface field (BSF) at the rear side of a PERC, fabricated PERCs having a local BSF including boron diffused by boron laser doping (B-LD), and evaluated this effect. The results from an analysis using scanning electron microscopy (SEM) indicated that the thickness of the BSF fabricated with B-LD was thicker than that of the BSF fabricated without B-LD. The average value of the saturation current density of the metallized area (including BSF) J0, contact was 554 fA/cm2 in the PERCs fabricated with B-LD and a dedicated Al paste, and the lowest value achieved was approximately 300 fA/cm2. The number of Kirkendall voids for the PERCs fabricated with B-LD decreased in comparison with the PERCs fabricated without B-LD. The PERCs fabricated with B-LD exhibited reduced rear-side recombination.

Impacts of light illumination on monocrystalline silicon surfaces passivated by atomic layer deposited Al2O3 capped with plasma-enhanced chemical vapor deposited SiNx

In this work, we investigate the impact of light illumination on crystalline silicon surfaces passivated with inline atomic layer deposited aluminum oxide capped with plasma-enhanced chemical vapor deposited silicon nitride. It is found that, for dedicated n-type lifetime samples under illumination, there is no light induced degradation (LID) but enhanced passivation. The lifetime increase happened with a much faster speed compared to the lifetime decay during dark storage, resulting in the overall lifetime enhancement for actual field application scenarios (sunshine during the day and darkness during the night). In addition, it was found that the lifetime enhancement is spectrally dependent and mainly associated with the visible part of the solar spectrum. Hence, it has negligible impact for such interfaces applied on the rear of the solar cells, for example p-type aluminum local back surface field (Al-LBSF) cells.

알루미늄-실리콘 공융 조성 합금 페이스트를 이용한 국부 후면 전계 태양전지 특성 분석

Characteristic of aluminum-silicon alloy paste which is applied on the rear side of PERC cell was investigated. The paste was made by aluminum-silicon alloy with eutectic composition to avoid the formation of void which is responsible for the degradation of the open-circuit voltage. Also, the glass frit component of the paste was changed to improve the adhesion of aluminum-silicon paste. We observed the formation of void and local back surface field between aluminum electrode and silicon base by SEM. The light IV, quantum efficiency and reflectance of the solar cells were characterized and compared for each paste.

20.8% industrial PERC solar cell: ALD Al2O3 rear surface passivation, efficiency loss mechanisms analysis and roadmap to 24%

PERC cell is currently entering the industrial crystalline silicon solar cell production lines. While there has been many reports focusing on research PERCs, this paper aims to present a cost-efficient PERC roadmap at fully industrial level, i.e. beyond the research and pilot lines. We present a systematic experimental study on the most important material and cell parameters for PERC, for instance, the key processes of ozone based ALD Al2O3 rear surface passivation and screen printed aluminum local back surface field are discussed in detail, especially highlighting the importance of the process integration. Industrial PERC cells using this roadmap have demonstrated average efficiency of 20.5% and champion efficiency of 20.8% with open circuit voltage of 660–666mV. Light-induced degradation analysis shows that the PERC cells are subject to bulk degradation and not to surface degradation. An anti-LID treatment processed by simultaneous applying forward voltages and anneal can drastically decrease LID. The cell efficiency loss mechanisms are analyzed based on the quantum efficiency measurement, suns-Voc tests and series resistance loss calculations. These are combined with PC1D and PC2D simulations to analyze recombination loss mechanisms present in the cells in order to promote viable solutions to extend the current industrial PERC cell efficiency to 24%.

Influence of aluminum workfunction and capping dielectric thickness on the performance of local back surface field solar cell using numerical simulation

High efficiency partial rear contact solar cells forms a Metal-Insulator-Semiconductor (MIS) stack in the rear side. Aluminum is used as a local back surface field (localized heavily doped pregion) and a hole contact. The workfunction difference between the aluminum contact and the c-Si(p) causes inversion in the silicon surface of the MIS structure. The surface inversion affects the quality of passivation and hence, the performance of the solar cell which requires further understanding. In this work, we have analysed the influence of aluminum on surface passivation in two passivation stacks namely (i)AlOx/SiNx and (ii) SiOx/SiNx using Sentaurus TCAD simulation.Our simulation result shows that in case of positive dielectric stack (SiOx/SiNx), aluminum workfunction enhances the passivation (effective surface recombination velocity <15cm/s) for the injection condition of 2×1013cm-3 which is common at the rear side of AlLBSF solar cell. But, in negative dielectric stack (AlOx/SiNx), aluminum workfunction influence is negligible since it is surpassed by high negative fixed charge density. In addition, we have varied the capping layer (SiNx) thickness in the dielectric stack to alter the aluminum influence on the crystalline silicon surface. It shows up to 1.0% absolute improvement in the efficiency for reduced SiNx thickness in the SiOx/SiNx stack for the fixed charge density of 2×1011cm-2, interface trap density of 5×1011cm-2, assumed electron and hole capture cross section of 10-14cm2 and 4×10-16cm2 respectively. This is solely attributed to the aluminum workfunction influence on the rear passivation.