Passivated Emitter And Rear Cell Solar Cells Research Articles

The Impact of Thermal Treatment on Light-Induced Degradation of Multicrystalline Silicon PERC Solar Cell

Multicrystalline silicon (mc-Si) PERC (passivated emitter and rear cell) solar cells suffer from severe light-induced degradation (LID), which mainly consists of two mechanisms, namely, BO-LID (boron–oxygen complex-related LID) and LeTID (light and elevated temperature induced degradation). The impact of thermal treatment on the LID of a mc-Si PERC solar cell is investigated in this work. The LID of mc-Si PERC solar cells could be alleviated by lowering the peak temperature of thermal treatment (namely sintering), perhaps because fewer impurities present in mc-Si tended to dissolve into interstitial atoms, which have the tendency to form LeTID-related recombination active complexes. The LID could also be effectively restrained by partially replacing the boron dopant with gallium, which is ascribed to the decreased amount of boron–oxygen (B–O) complexes. This work provides a facile way to solve the severe LID problem in mc-Si PERC solar cells in mass production.

Analysis of nanosecond and femtosecond laser ablation of rear dielectrics of silicon wafer solar cells

Laser ablation of rear dielectrics of PERC (Passivated Emitter and Rear Cell) type silicon solar cells is by far the most cost-effective method to open a variety of patterns for local contact formation. In this paper, rear dielectric ablation of p-type PERC solar cells with an implanted phosphorus emitter is investigated using two laser sources with green wavelength and with pulse durations in the nanosecond (ns) and femtosecond (fs) range. The ns laser source emits 38 ns pulses and the ablation behaviour is linear for the entire range of available pulse fluences. The fs laser source emits 480 fs pulses and has two operating regimes, gentle and strong, based on the incoming pulse fluence. To evaluate the impact of laser induced damages on PERC solar cells, four different laser conditions are considered: fs gentle regime, fs strong regime, ns low fluence and ns high fluence. The solar cells’ one-sun I–V parameters show that the cells processed in the fs strong regime have a severe reduction in the open-circuit voltage by almost 40 mV, fill factor loss by ~7% and efficiency loss by 3–4%. Fill factor loss analysis shows that a loss of almost 7% absolute comes entirely from J02 recombination, revealing a possibility of severe damage to the front side emitter. The solar cells processed with the other three laser conditions have comparable I–V characteristics. SEM analysis of the front surface of samples with laser treated rear surface shows that the fs strong regime produces severe damage to the front-surface texture resulting in lower Jsc, whereas this effect is not observed for the other laser conditions.

A Detailed Full-Cell Model of a 2018 Commercial PERC Solar Cell in Quokka3

An unprecedented detailed model of a full-size passivated emitter and rear cell (PERC) solar cell design, as manufactured at a current Trina Solar production-line during ramp-up, is presented. Combining a newly proposed multidomain approach with the multiscale skin-concept of Quokka3, the 15.6 cm × 15.6 cm three-dimensional cell geometry including the details of the emitter skins can thoroughly be solved within a single simulation. The multidomain approach uses an inner and two edge domains as irreducible symmetry elements, each containing the unequal front and rear pitch, the dashed rear contacts, as well as part of the busbars and consequently the full finger resistance. The full-cell current density is then determined by simple area-averaging, opposed to the more complicated common approach of coupling it with a distributed network model. The multiscale skin approach enables to model all emitter parts of the PERC cell in detail (accounting for dopant profiles, front surface recombination, Fermi-Dirac statistics, etc.), whereas the other skin regions can still be described by their lumped properties, i.e., Rsheet and J0/Seff. A complete set of carefully established electrical and optical input parameters as well as a detailed loss breakdown is presented, providing fellow researchers with a point of reference for modeling a state-of-the-art PERC solar cell in 2018.

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

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

Mitigating Light and Elevated Temperature Induced Degradation in Multicrystalline Silicon Wafers and PERC Solar Cells Using Phosphorus Diffusion Gettering

Reducing light and elevated temperature induced degradation (LeTID) is important for the industrial success of PERC (passivated emitter and rear cell) solar cells fabricated on high‐performance multicrystalline silicon (multi‐Si) wafers. Although there has been a lot of progress in understanding the degradation kinetics, the defect(s) responsible for LeTID in multi‐Si wafers at elevated temperatures (≈80 °C, 1 Sun) has yet to be identified. In this study, the authors look at the possibility of using phosphorus diffusion gettering (PDG) for reducing LeTID in multi‐Si wafers and solar cells. By measuring light induced defect concentrations in multi‐Si wafers before and after LeTID, the authors observe that PDG can substantially reduce the average defect concentration. Trace element analysis using inductively coupled plasma mass spectrometry reveals that multi‐Si wafers from the edge of the ingot contain a high concentration of Cu, Ni and Ti in grains that degrade more than neighboring grains. To explore PDG for reducing LeTID in multi‐Si PERC solar cells, the authors fabricate cells with two different emitter profiles. Etching back a heavily diffused emitter to obtain a high sheet resistance is observed to improve the LeTID performance of the solar cells, an effect that is very likely related to a reduced impurity concentration within the wafer.

Implementation of Selective Emitter for Industrial-Sized PERCs Using Wet Chemical Etch-Back Process

This paper introduces a selective phosphorus emitter formed by screen-printed resist masking combined with a wet chemical etch-back process for an industrial-sized passivated emitter and rear cell (PERC). Applying the selective emitter (SE) concept is expected to decrease the recombination losses at the front surface of the PERC cell. With the SE structure, we observed an increase in the open-circuit voltage $(V_{{\rm{oc}}})$ and short-circuit current density $(J_{{\rm{sc}}})$ , but a decrease in the fill factor (FF), compared with homogeneous emitter cells. The sheet resistance $(R_{{\rm{sheet}}})$ of the lightly doped emitter (n+-emitter) by the etch-back process had a considerable impact on the $V_{{\rm{oc}}}$ and $J_{{\rm{sc}}}$ , which we attributed to the reduced emitter saturation current density $(J_{0e})$ caused by a reduction in the Auger recombination in the n+ -emitter. The diminished FF was owing to the higher $R_{{\rm{sheet}}}$ , resulting in an increased series resistance of the cell. These results suggest that an SE structure made by the wet etch-back process is a very promising technology to improve the conversion efficiency of industrial-sized PERC solar cells.

Internal resistance of rear totally diffused solar cells with line shaped contacts

We present an analytical model for the internal resistance of passivated emitter and rear totally diffused (PERT) solar cells. First, we apply the model of Saint-Cast for the spreading resistance of a passivated emitter and rear cell (PERC) structure with line-shaped contacts. To account for the additional vertical current flow through the silicon wafer and the lateral current flow through the back surface field of a PERT structure, we add a parallel current path using common analytical expressions. We compare the analytical models with two-dimensional numerical simulations based on Quokka 3 and find deviations of less than 6% for the internal resistance. In addition, we compare the analytical model of the internal resistance of PERC and PERT solar cells with experimental data of the series resistance of PERC and PERT solar cells.

A comparative life-cycle assessment of photovoltaic electricity generation in Singapore by multicrystalline silicon technologies

This paper presents a comparative life-cycle assessment of photovoltaic (PV) electricity generation in Singapore by various p-type multicrystalline silicon (multi-Si) PV technologies. We consider the entire value chain of PV from the mining of silica sand to the PV system installation. Energy payback time (EPBT) and greenhouse gas (GHG) emissions are used as indicators for evaluating the environmental impacts of PV electricity generation. Three roof-integrated PV systems using different p-type multi-Si PV technologies (cell or module) are investigated: (1) Al-BSF (aluminum back surface field) solar cells with the conventional module structure (i.e. glass/encapsulant/cell/encapsulant/backsheet); (2) PERC (passivated emitter and rear cell) devices with the conventional module structure; and (3) PERC solar cells with the frameless double-glass module structure (i.e. glass/encapsulant/cell/encapsulant/glass). The EPBTs for (1) to (3) are 1.11, 1.08 and 1.01 years, respectively, while their GHG emissions are 30.2, 29.2 and 20.9g CO2-eq/kWh, respectively. Our study shows that shifting from the conventional Al-BSF cell technology to the state-of-the-art PERC cell technology will reduce the EPBT and GHG emissions for PV electricity generation. It also illustrates that mitigating light-induced degradation is critical for the PERC technology to maintain its environmental advantages over the conventional Al-BSF technology. Finally, our study also demonstrates that long-term PV module reliability has great impacts on the environmental performance of PV technologies. The environmental benefits (in terms of EPBT and GHG emissions) of PV electricity generation can be significantly enhanced by using frameless double-glass PV module design.

Rear passivated mc-Si solar cells textured by atmospheric pressure dry etching

In this contribution, atmospheric pressure dry etching (ADE) texture is integrated into a passivated emitter and rear cell (PERC) process. In order to make the texture suitable for the solar cell processing, two post-etching processes (inverted pyramid, spherical cap) are implemented and compared. The passivation of the front surface could be improved substantially by the introduction of an Al2O3 layer deposited via atomic layer deposition under the classical antireflection coating SiNX. The impact of the addition of the Al2O3 layer on the contact resistance cannot be neglected and becomes significant for thicknesses higher than 4 nm. Multicrystalline silicon (mc-Si) PERC solar cells are fabricated and measured, leading to a maximum efficiency of about 18.6% for the acidic texture, and for both posts-processed dry chemical etching (ADE) textures. No current gain for the ADE textured wafer compared to the acidic texture could be observed on cell level due to an inhomogeneous etching. However, a gain of 0.4 mA/cm2 for the ADE texture was calculated from local EQE measurements. ADE texture presents therefore 2 advantages: compatibility to diamond wire sawing and a potential current gain for PERC solar cell structure.

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.

Light‐induced degradation in quasi‐monocrystalline silicon PERC solar cells: Indications on involvement of copper

High‐efficiency solar cell designs such as the passivated emitter and rear cell (PERC) raise the quality requirements for silicon substrates, favoring monocrystalline materials. Seed‐cast quasi‐monocrystalline silicon (qm‐Si) is a promising alternative for Czochralski (Cz) Si with potential benefits of lower cost and reduced energy footprint. However, the purity and crystalline quality of qm‐Si is not on par with Cz‐Si, which can cause efficiency losses for example in the form of light‐induced degradation (LID). In this contribution, we study the LID phenomena that can be present in qm‐Si PERC solar cells, and compare them to the Cz‐Si PERC. Degradation and regeneration are analyzed especially from the viewpoint of Cu impurity, which has until very recently been omitted as a source of LID for this device type. Subsequently, differences in LID behavior between qm‐Si and Cz‐Si are investigated considering the density of dislocations in the bulk. The results imply that slurry‐based wafer slicing may introduce contamination that is capable of causing considerable LID in PERC devices fabricated of Si with inherently high defect density.

Boosting PERC Solar Cell Output

Abstract By limiting recombination, passivated emitter and rear cell (PERC) solar cells can provide an absolute efficiency boost of 1%, making their relative efficiency about 7% greater than that of other screen printing methods. This article covers the early stages of the nondestructive and destructive imaging performed to evaluate and enhance the efficiency of PERC solar cells. Electroluminescence images, acoustic images, and scanning electron micrographs were used in this research.

Light‐induced degradation and regeneration of multicrystalline silicon Al‐BSF and PERC solar cells

Light‐induced degradation (LID) is a well‐known problem faced by p‐type Czochralski (Cz) monocrystalline silicon (mono‐Si) wafer solar cells. In mono‐Si material, the physical mechanism has been traced to the formation of recombination active boron‐oxygen (B–O) complexes, which can be permanently deactivated through a regeneration process. In recent years, LID has also been identified to be a significant problem for multicrystalline silicon (multi‐Si) wafer solar cells, but the exact physical mechanism is still unknown. In this work, we study the effect of LID in two different solar cell structures, aluminium back‐surface‐field (Al‐BSF) and aluminium local back‐surface‐field (Al‐LBSF or PERC (passivated emitter and rear cell)) multi‐Si solar cells. The large‐area (156 mm × 156 mm) multi‐Si solar cells are light soaked under constant 1‐sun illumination at elevated temperatures of 90 °C. Our study shows that, in general, PERC multi‐Si solar cells degrade faster and to a greater extent than Al‐BSF multi‐Si solar cells. The total degradation and regeneration can occur within ∼320 hours for PERC cells and within ∼200 hours for Al‐BSF cells, which is much faster than the timescales previously reported for PERC cells. An important finding of this work is that Al‐BSF solar cells can also achieve almost complete regeneration, which has not been reported before. The maximum degradation in Al‐BSF cells is shown to reduce from 2% (relative) to an average of 1.5% (relative) with heavier phosphorus diffusion.

Single Print Metal Stencils for High-efficiency PERC Solar Cells

Industrial silicon solar cells like Passivated Emitter and Rear Cells (PERC) typically apply a screen-printed (Ag) front contact with a single print process using a mesh screen. It has been shown that using stencils instead of screens improves the finger profile leading to slightly higher cell conversion efficiencies. However, so far stencil printing required an extra printing step for the busbars since conventional stencils do not support H-pattern designs. In this paper, we evaluate a novel “single print” stencil from ASM AE that allows to print busbars and fingers in a single print step. We apply the novel single print stencil to high-efficiency PERC solar cells and compare it to today's industrial screen printing processes (single print and dual print) as well as to a high performance dual-printed front side grid applying a stencil for the fingers and a screen for the busbars. The printed finger width ranges from 35.8 ± 2.3μm for 30μm stencil opening to 46.7 ± 3.4μm for 40μm screen opening which leads to an 0.5%abs increased metallized area on the front side of the screen printed fingers compared to the stencil printed fingers. The resulting average finger height is 22.3μm for the stencil groups and 11.9μm for the screen printed Ag finger which leads to a difference in the finger line resistance and an additional series resistance contribution for the screen-printed cells of 0.05Ωcm2. We achieve almost identical PERC cell efficiencies with the single print stencil and the dual print stencil process obtaining best values up to 21.1% and average values of 21.0%. In contrast, both screen printed groups achieve 0.2%abs lower average conversion efficiencies mostly due to the lower Jsc and FF. The advantage of the dual print process compared to single print is shown in the increased Voc by 1 to 2mV due to the used non-firing through Ag busbar paste. With these results we demonstrate that the single print stencil process saves two process steps compared to dual print using a stencil while obtaining the same PERC cell performance with an efficiency gain of 0.2%abs compared to today's industrial screen print processes.

Degradation and regeneration in mc‐Si after different gettering steps

AbstractLight and elevated temperature induced degradation (LeTID) affects significantly the performance of multicrystalline (mc) Si passivated emitter and rear cell (PERC) solar cells, and underlying mechanisms of LeTID are still unknown. In this work LeTID and following regeneration of an industrial mc‐Si PERC solar cell is compared to differently processed minority charge carrier lifetime samples under illumination (1 sun) and elevated temperature (75 °C). LeTID on cell level reveals the same kinetics compared to lifetime samples. Varying the processing sequence has a significant effect on LeTID of lifetime samples. Ungettered samples with fired SiNx:H surface passivation show a very strong LeTID and regeneration effect, with degradation kinetics being similar for all wafer areas irrespective of initial material quality. In contrast, regeneration sets in earlier in good quality areas. Differently gettered samples with lower contamination level than ungettered samples are less sensitive to LeTID, while overall degradation and regeneration behavior is strongly influenced by applied gettering sequences. Al‐gettered samples show a more pronounced degradation effect than P‐gettered samples, leading to the assumption that P‐gettering is more effective in the reduction of LeTID sensitive defects. If the gettering step is less effective, in lifetime samples after degradation a beginning regeneration effect could be observed. A model is presented, describing LeTID in boron as well as gallium doped mc‐Si being based on impurities that can be gettered and redistributed during high temperature steps. Using this experimental approach helps to clarify the underlying mechanisms of LeTID and leads to a better understanding of degradation and regeneration mechanisms in mc Si. Copyright © 2016 John Wiley & Sons, Ltd

Industrial Silicon Solar Cells Applying the Passivated Emitter and Rear Cell (PERC) Concept—A Review

Even though the passivated emitter and rear cell (PERC) concept was introduced as a laboratory-type solar cell in 1989, it took 25 years to transfer this concept into industrial mass production. Today, PERC-type solar cells account for 10% of the worldwide produced solar cells, and their share is expected to rapidly increase up to 35% within the next few years. Record efficiencies up to 22.1% of industrial PERC cells approach an efficiency of 22.8% of the lab-type PERC cell in 1989. This paper reviews the most important research results and technological developments of the past 25 years, which enabled the successful transfer of the lab-type PERC concept into industrial mass production. Particular attention is paid to the development of AlO x /SiN y layer stacks with excellent rear surface passivation properties and low production costs. In addition, we summarize the most important research results and technological improvements of industrially processed local aluminum rear contacts. Furthermore, we describe the most relevant process flows to manufacture industrial PERC cells and address silicon wafer material requirements regarding high and stable charge carrier lifetimes. An outlook is provided on future development opportunities, which may further increase the conversion efficiency and the energy yield of industrial PERC solar cells.

Simulation-based Efficiency Gain Analysis of 21.2%-efficient Screen-printed PERC Solar Cells

Passivated Emitter and Rear Cells (PERC) with efficiencies well above 20% are likely to become the next mass production technology. A quantification of all power loss mechanisms of such industrial PERC cells is helpful in prioritizing future efficiency improvement measures. We report on a numerical simulation of the power losses of a 21.2%-efficient industrial PERC cell using extensive experimental input data. Our synergetic efficiency gain analysis relies on deactivating single power loss mechanisms in the simulation at a time to access the full potential power gain related to that mechanism. The complete analysis therefore explains the efficiency gap between the industrial PERC solar cell and the theoretical maximum efficiency of a crystalline Si solar cell. Based on the simulations, the largest single loss mechanism is front grid shadowing followed by recombination in the emitter and its surface. All individual resistive losses, all individual optical losses and all (avoidable) individual recombination losses sum up to efficiency gains of 0.8%, 1.6%, and 1.3%, respectively, which is 3.7% in total. The efficiency gap between real and ideal solar cell is, however, much larger with 7.3%. The discrepancy is mainly due to the non-linear behaviour of recombination-based power losses which adds synergetic efficiency enhancements.

Impact of Ag Pads on the Series Resistance of PERC Solar Cells

Screen-printed passivated emitter and rear cells (PERC) require Ag pads on the rear side to enable solderable connections for module integration. These Ag pads are separated from the silicon by a dielectric layer to avoid recombination of minority charge carriers. The drawback of this configuration is an elongated transport path for the majority charge carriers generated above the pads. This results in an increase in series resistance. The strength of this effect depends on charge carrier generation above the Ag pads that critically depends on shading of the cell's front side. Ag pads are usually wider than the busbars or the interconnector ribbons and thus are only partially shaded. We build PERC test structures with various rear side configurations of Ag and Al screen printing as well as with and without laser contact openings (LCO). Using experiments and finite element simulations we investigate the impact of shading the Ag pads by the busbars and other means. While fully shaded regions do not increase the lumped solar cell's series resistance, unshaded Ag pads lead to an increase of about 37%.

Low Cost Local Contact Opening by Using Polystyrene Spheres Spin-Coating Method for PERC Solar Cells.

The passivated emitter and rear cell (PERC) concept is one of the most promising technologies for increasing crystalline silicon solar cell efficiency. Instead of using the traditional laser ablation process, this paper demonstrates spin-coated polystyrene spheres (PS) to create local openings on the rear side of PERCs. Effects of PS concentration and post-annealing temperature on PERC performance are investigated. The experimental results show that the PS are randomly distributed on wafers and no PS are joined together at a spin rate of 2000 rpm. The PS can be removed at a temperature of 350 °C, leaving holes on the passivation layers without damaging the wafer surfaces. As compared to the laser opening technique with the same contact fraction, the PS opening technique can yield a higher minority effective lifetime, a higher implied open-circuit voltage, and a slightly higher short-circuit current. Although the fill factor of the PS opening technique is lower owing to non-optimized distribution of the openings, the conversion efficiency of the devices is comparable to that of devices prepared via the laser opening process.

Effect of laser damage etching on i-PERC solar cells

Passivated emitter and rear cells (PERC) are considered to be the next-generation silicon solar cell technology because of their significant efficiency gain originating from improved surface passivation and optical reflectivity. However, the complex fabrication process of PERC solar cells is a major hindrance to its mass production. Lasers are widely used for solar cell fabrication because of their potentially high throughput and selectivity. Laser ablation can, however, cause unintended laser-induced damage such as surface melting, heat affected zone, microcracks, point defects etc., which can negatively affect the solar cell's performance. Also, the damage can be amplified depending on the laser's operating condition. This work focuses on the effect of laser-induced damage removal by chemical etching, regardless of laser specification condition, on the silicon substrate after laser ablation. The laser ablated and etched surface were characterized by an optical microscope, electrochemical capacitance voltage (ECV) profiling and quasi-steady-state photoconductance (QSSPC). An industrial-type PERC (i-PERC) cell was fabricated after optimization of the chemical-etching process and the cell performance was evaluated. A maximum efficiency of 19.73% was obtained on a 156 mm × 156 mm crystalline-silicon solar cell. In addition, damage etching was found to have a significant effect on the increase of the shunt resistance (Rsh) and i-Voc.