Bifacial N-type Solar Cells Research Articles

Corrosion effects in bifacial crystalline silicon PV modules; interactions between metallization and encapsulation

This study addresses the influence of different encapsulation materials on performance losses in bifacial PV modules after extended damp heat testing. The widely used ethylene vinyl acetate (EVA) is compared to polyolefin elastomers (POE) and thermoplastic polyolefins (TPO). We show that modules based on n-type bifacial solar cells (tunnel oxide passivated contact, TOPCON) are more prone to degradation than p-type (Passivated emitter rear cell, PERC) based laminates. More specifically, the front side metallization of these n-type cells corrodes under extended damp heat testing, yielding a decreased fill factor in the I–V characteristics and the appearance of dark spots and dark areas in electroluminescence imaging. The newer generation of encapsulant materials (POE and TPO), slowly replacing EVA in the modules commercialized today, demonstrate an improved protection of the metallization. Apart from the nature of the encapsulant, the cell type and the presence or absence of glass cover plates are also shown to influence the metal grid corrosion. To better understand the mechanisms at play, post-mortem analysis was carried out on small sized degraded PV laminates, by drilling out circular samples (“coring”) that could be analyzed in depth. The analyses revealed a weakened bond between the metallization and the silicon emitter, ultimately causing metal grid delamination. The proposed mechanism behind this delamination starts with the degradation of lead glass, which is part of the cell metallization grid and contains lead oxide (PbO). This compound is susceptible to dissolution in acidic environment.

Impact of firing temperature on fire-through metal contacts to P-doped (n) and B-doped (p) poly-Si

Screen-printed contacts to doped polysilicon (poly-Si) layers with high-temperature fire-through pastes is one of the key enablers for widespread industrial acceptance of passivating contact solar cells. In recent years, rapid progress has been made in the development of pastes for contacting doped poly-Si layers. However, there are still gaps in understanding the behavior of fire-through contacts, especially regarding damage to poly-Si layers. In this work we present a comprehensive study of metal contacts formed with fire-through pastes to both phosphorus-doped ( n + ) poly-Si and boron-doped ( p + ) poly-Si layers. The contacts are fired at different temperatures and characterized by evaluating specific contact resistivity and recombination current density under the metal contacts. The metal crystallites formed at the interface between metal fingers and poly-Si is quantitatively analyzed to determine their area fraction. It is observed that the area fraction of the crystallites tends to saturate at higher temperatures for n + poly-Si, however such behavior is not observed for p + poly-Si within the range of firing temperatures relevant for industrial solar cells. Metal crystallites appear to agglomerate at higher temperatures for contacts to both n + and p + poly-Si layers which explains increased recombination due to aggressive etching of poly-Si. For Ag contacts to n + poly-Si best results are achieved when the metal crystallites covered 50% of the surface area at the optimum firing temperature. For Ag–Al contacts to p + poly-Si the crystallites covered 30% of the area, but resulted in higher recombination under the contacts. n-type bifacial solar cells are fabricated with n + poly-Si passivating contacts on the rear side and fired at different temperatures. The solar cell results followed the trends shown by the contacts and efficiency reached maximum for an optimized firing temperature. However, the change in efficiency was dominated by the front side contact characteristics. The results obtained on solar cells validate the observations about contact formation and metal recombination. Etch pits on p + poly-Si observed under SEM at different peak firing temperatures. Different splits represent the peak firing temperatures. A) 710 B) 730 C) 750 D) 770 E) 790. • In depth analysis of fired-through contacts formed to 150 nm doped poly-Si layers at different temperatures. • Contact resistivity improved with increasing temperature, attributed to increased number of metal crystallites. • Excellent contact resistivity ≈0.5 mΩ/cm 2 and ≈4 mΩ/cm 2 achieved for Ag and Ag–Al fired through contacts respectively. • The J 0,metal increased with increasing temperature due to increasingly aggressive etching of the poly-Si layer. • An ideal metal paste should result in optimum number of crystallites that do not extensively etch the poly-Si layer.

Towards 24% Efficiency for Industrial n-Type Bifacial Passivating-Contact Solar Cells with Homogeneous Emitter

Towards 24% Efficiency for Industrial n-Type Bifacial Passivating-Contact Solar Cells with Homogeneous Emitter

Fully screen-printed bifacial large area 22.6% N-type Si solar cell with lightly doped ion-implanted boron emitter and tunnel oxide passivated rear contact

We report on the fabrication of fully screen-printed bifacial large area (239 cm2) high-efficiency n-type Si solar cells with ion-implanted homogeneous boron emitter on the front side and carrier-selective tunnel oxide passivated contact (TOPCon) on the rear. Our phosphorus-doped poly-Si/SiOx passivated contact with SiNx capping layer gave excellent surface passivation with a very low recombination current density (J0_TOPCon) of ~1 fA/cm2 after a simulated firing treatment at ~770 °C without metallization. After screen-printed and fire-through metallization on n-TOPCon with ~13% metal coverage, metallized J0_TOPCon value increased to only ~5 fA/cm2. In addition, homogenous boron implanted emitter (~180 Ω/) passivated with ALD Al2O3 layer capped with PECVD SiNx/SiOx double-layer antireflection (DLAR) coating gave excellent passivation with emitter recombination current density of ~12 fA/cm2 prior to metallization. The industrial screen-printed, fire-through contacts with floating busbars on this boron emitter gave metallized emitter recombination current density of ~31 fA/cm2. This resulted in a low-cost industrial screen-printed n-type bifacial Si solar cell with 22.6% efficiency and 702 mV open-circuit voltage (Voc).

Effect of Laser Ablation on Rear Passivation Stack for N-type Bifacial Solar Cell Application

In this paper, we investigated the effect of the passivation stack with Al2O3, hydrogenated silicon nitride (SiNx:H) stack and Al2O3, silicon oxynitride (SiONx) stack in the n type bifacial solar cell on monocrystalline silicon. SiNx:H and SiONx films were deposited by plasma enhanced chemical vapor deposition on the Al2O3 thin film deposited by thermal atomic layer deposition. We focus on passivation properties of the two stack structure after laser ablation process in order to improve bifaciality of the cell. Our results showed SiNx:H with Al2O3 stack is 10 mV higher in implied open circuit voltage and 60 μs higher in minority carrier lifetime than SiONx with Al2O3 stack at Ni silicide formation temperature for 1.8% open area ratio. This can be explained by hydrogen passivation at the Al2O3/Si interface and Al2O3 layer of laser damaged area during annealing.

The impact of interstitial Fe contamination on n-type Cz-Silicon for high efficiency solar cells

In this work, we have investigated the impact of interstitial Fe contamination on the effective minority carrier lifetime of n-type Cz silicon bulk material for high efficiency solar cells. The study covers a Fe concentration in the silicon bulk from 3.5 × 1012 cm-3 to 2.7 × 1014cm-3. We have added 5 different concentrations (30, 100, 300, 1000 and 3000 ppb) of Fe intentionally to a wet chemical process tank and measured the transfer to the silicon wafer surface mimicking a possible contamination during wet chemical processing. In order to fabricate carrier lifetime test vehicles, the silicon wafer is then passivated with thermal silicon oxide from both sides. The surface contamination is driven into the bulk by mimicking a high temperature process during solar cell manufacturing. Effective minority carrier lifetime is measured at injection levels from 1 × 1013 cm-3 to 3 × 1015cm-3. We have fitted the theoretical curve for interstitial Fe derived from the SRH theory to the measured values and extracted the Fe contamination concentration. This value is comparable to the calculated value extracted from the surface contamination measurement. For low level injection (LLI), we extracted the capture cross section for interstitial Fe to be 6.45 × 10-17 cm/s ± 2.23 × 10-17 cm/s. The measured Fe contamination levels are used for the conversion efficiency fitting of a n-type bifacial silicon solar cell using QUOKKA simulations. The simulations show that very low Fe contamination concentrations of [Fe]bulk = 3.5 × 1012 cm-3 ([Fe]surf = 6 × 1010cm-2) already degrade the solar cell efficiency by 10% relative.

An Investigation of Internal Quantum Efficiency of Bifacial Interdigitated Back Contact (IBC) Crystalline Silicon Solar Cell

In this article, we investigated the internal quantum efficiency (IQE) properties of n-type bifacial interdigitated back contact crystalline silicon solar cells using an IQE mapping system. In the cell structure, high and low IQE values were observed above the emitter and back surface field (BSF) regions. The IQE values above the BSF busbar were drastically reduced due to electrical shading loss. Line scan profiles at different wavelengths showed the detailed distribution of IQE values. The IQE values varied greatly depending not only on the difference between emitter and BSF regions but also on the rear side structure such as the electrode width and the distance between the emitter and BSF regions. On the other hand, the IQE spectra at over 950 nm improved by increasing the light absorbance ratio from the rear side. After module formation, the IQE spectra at short wavelengths were significantly reduced. The IQE properties were obtained from the front and rear sides. The difference in the short-circuit current between front side illumination and rear side illumination was mainly due to optical shading loss and carrier recombination loss at the BSF region. For a high cell efficiency, it is necessary to improve the passivation properties of the BSF region and optimize the electrode design.

Impact of the manufacturing process on the reverse-bias characteristics of high-efficiency n-type bifacial silicon wafer solar cells

In this paper, bifacial n-type silicon wafer solar cells with a front boron-diffused emitter and a rear phosphorus-diffused back surface field are investigated. The cell structure is abbreviated as nFAB (n-type Front and Back Contact). Three different process flows are evaluated for the fabrication of nFAB cells: (1) process flow with a silicon nitride diffusion mask deposited by plasma-enhanced chemical vapour deposition (PECVD), (2) process flow with a silicon oxynitride diffusion mask also deposited by PECVD, and (3) process flow using single-sided atmospheric pressure chemical vapour deposition (APCVD), without diffusion masks. An average batch efficiency of 21.3% was measured (front side illumination only) for all three nFAB process flows. However, the reverse bias I-V characteristics are observed to be remarkably different and dependent on the process flow, especially with regards to the masking processes. For process flow 1 that uses a silicon nitride PECVD mask, the reverse bias I-V characteristics are poor with a severe breakdown occurring at − 5 V and a reverse current of 18 A at −10 V. Process flow 2 with a silicon oxynitride PECVD mask leads to significantly improved reverse characteristics with a reverse current of 1 A at −10 V. For the APCVD process flow without diffusion masks, excellent reverse bias characteristics are observed with a reverse current of only 3 mA at −10 V. Reverse-biased electroluminescence imaging is used to determine the breakdown regions of the cells for the different process flows and to analyse the impact of the diffusion masking step on the cells’ breakdown characteristics.

Screen-printed contacts with H-patterned n-type passivated emitter rear totally diffused solar cell and front-side boron selective emitter formed by wet chemical etching

This paper presents the efficiency distribution of an n-type bifacial solar cell. In this work, the solar cells were produced by our standard cell fabrication process, which is very similar to the industrial line process. The best cell efficiency was 20.4%, and the average efficiency was 20.06%. The cell efficiency ranged between 19.9 and 20.4% and showed a narrow efficiency distribution. This paper also presents the development of a boron selective emitter (p+/p++) n-type bifacial solar cell. In this work, the cells were fabricated using the standard cell fabrication process based on tube-furnace thermal diffusion employing liquid sources: BBr3 for the front-side boron emitter and POCl3 for the rear-side phosphorus back-surface field (BSF). The p+/p++ structure was formed by screen-printing resist masking and wet chemical etching technology. Both the front- and rear-side electrodes were obtained by using screen-printed contacts with H-patterns. The cell efficiency with the selective boron emitter was almost the same as that with the homogeneous emitter; however, the Voc of the selective emitter solar cell was higher than that of the homogeneous emitter solar cell by 4.4 mV. Finally, we performed the recombination analysis of the completed cell.

Enhanced efficiency in bifacial HIT solar cells by gradient doping with AFORS-HET simulation

The paper presents a systematical simulation study of the influence of gradient doping of both emitter and back surface field layer on the photovoltaic (PV) performances of TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(n+)/TCO/Ag bifacial structure heterojunction with intrinsic thin layer (HIT) solar cells using AFORS-HET software simulation package. The gradient doping instead of uniform doping for emitter can promote the PV performance due to the presence of an additional electric field coming from the gradient doping, while the gradient doping of back surface field layer has negligible effect or negative influence on the cell performance compared to uniform doping, which can be explained by the varied energy band diagrams near the back junction interface. A final efficiency of 25.75% has been realized by using gradient doping of emitter with the same total doping amount as that of uniform doping case and uniform doping of back surface field layer for n-type bifacial HIT solar cells. This study can offer guidance on how to improve HIT solar cell efficiency beyond the world record value in experiment and how to achieve higher efficiency in mass production of HIT solar cells.

Modulated Photoluminescence Lifetime Measurement of Bifacial Solar Cells

The frequency response of photoluminescence (PL) emission in a semiconductor sample is influenced by the effective carrier lifetime. As it is easy to induce and capture PL in solar cells at different stages of processing, modulated PL poses as a versatile technique to track the progression of effective lifetime in silicon wafer solar cells in the production line. This paper demonstrates the application of modulated PL to extract the effective carrier lifetime in n-type silicon bifacial solar cell samples before and after metalization. For nonmetalized samples, good agreement is found in the lifetime curves generated by modulated PL and the more established microwave photoconductance decay technique. For single and double-side metalized samples, the deduced lifetime is also consistent with the implied open-circuit voltage obtained by steady-state PL imaging.

Internal quantum efficiency mapping analysis for a >20%-efficiency n-type bifacial solar cell with front-side emitter formed by BBr3 thermal diffusion

This paper presents a large-area (239-cm2) high-efficiency n-type bifacial solar cell that is processed using tube-furnace thermal diffusion employing liquid sources BBr3 for the front-side boron emitter and POCl3 for the rear-side phosphorus back surface field (BSF). The SiNx/Al2O3 stack was applied to the front-side boron emitter as a passivation layer. Both the front and rear-side electrodes are obtained using screen-printed contacts with H-patterns. The resulting highest-efficiency solar cell has front- and rear-side efficiencies of 20.3 and 18.7%, respectively, while the corresponding bifaciality is up to 92%. Finally, the passivation quality of the SiNx/Al2O3 stack on the front-side boron emitter and rear-side phosphorus BSF is investigated and visualized by measuring the internal quantum efficiency mapping of the bifacial solar cell.

Material properties of LPCVD processed n-type polysilicon passivating contacts and its application in PERPoly industrial bifacial solar cells

We present a detailed material study of n+-type polysilicon (polySi) and its application as a carrier selective rear contact in a bifacial n-type solar cell comprising fire-through screen-printed metallization and 6" Cz wafers. The cells were manufactured with low-cost industrial process steps yielding Vocs from 676 to 683 mV and Jscs above 39.4 mA/cm2 indicating an efficiency potential of 22%. The aim of this study is to understand which material properties determine the performance of POCl3-diffused (n-type) polySi-based passivating contacts and to find routes to improve its use for industrial PERPoly (Passivated Emitter Rear PolySi) cells from the point of view of throughput, performance, and bifacial application. This paper reports on correlations between the parameters used for low pressure chemical vapour deposition (LPCVD), annealing, and doping on optical, structural, and electronic properties of the polySi-based passivating contact and the subsequent influence on the solar cell parameters.

Potential induced degradation of N-type bifacial silicon solar cells: An investigation based on electrical and optical measurements

This paper reports an extensive analysis of the potential-induced degradation (PID) of N-type bifacial solar cells. The analysis is based on combined electrical characterization, electroluminescence and external quantum efficiency measurements, carried out on solar cells submitted to high PID stresses. We investigate the impact of two different encapsulation materials: (i) polyolefin elastomer (POE) and (ii) ethylene-vinyl-acetate (EVA). We describe the degradation and recovery kinetics as a function of the temperature and the stress voltage. Moreover we demonstrate that the POE can be adopted to reduce the decrease of conversion efficiency during PID stresses. Finally we investigate the effects of PID with an optical analysis in order to find the main cause of the performance degradation.

Optimization of BBr₃-Based Co-Diffusion Processes for Bifacial N-Type Solar Cells.

We report on the co-diffused bifacial N-type solar cells based on N-type Si wafers using the process of spin on doping (SOD, phosphorous source) and boron tribromide (BBr₃) diffusion by atmospheric pressure chemical vapor deposition (APCVD). For bifacial co-diffusion, a phosphorous layer was deposited by SOD on the rear side of N-type Si wafer and a BBr₃ as boron dopant source deposited by APCVD. Co-diffusion process was controlled by changing the flowrate of carrier N₂ gas and drive-in temperatures. It was found that the fabricated bifacial co-diffused N-type solar cell with 2% H₃PO₄ doping, the flowrate of N₂ carrier gas of 15 slm and drive-in temperature at 930°C exhibited the highest conversion efficiency of 15.8% with high open circuit voltage (V(oc)) of 593 mV. As compared to high H₃PO₄ concentrations (5% and 9%), the low H₃PO₄ concentration of SOD showed the higher sheet resistance and decreased in the thickness of N + emitter layer, resulting in the high V(oc), shunt resistance, fill factor and conversion efficiency of solar cells.

Effects of thermal budget in n-type bifacial solar cell fabrication processes on effective lifetime of crystalline silicon

The effects of residual C on cell properties are investigated from the view point of thermal budget in the n-type bifacial cell processes. Implied Voc obtained from wafers with same Oi concentration depend on the thermal budgets decreases as the Cs concentration increases. The Voc values vary depending on the wafer with different growth cooling rate. To analyze the effect of thermal budget correspond to solar cell fabrication process, CZ wafers with almost the same Oi concentrations are prepared. One of the wafers with relatively high residual Cs concentration shows the longer lifetime than the initial value after the 950 oC annealing step. On the other hand, the lifetime of a wafer with relatively low Cs concentration dramatically decreased by the same process due to the O segregation. These results suggest that it is important to choose appropriate wafer specification, starting with feedstock material, for increasing the solar cell efficiency.

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

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

Fast Co-Diffusion Process for Bifacial n-Type Solar Cells

We present a new simplified and fast process sequence for the fabrication of large area bifacial n-type silicon solar cells. A co-diffusion process simultaneously forms the boron-doped emitter and the phosphorus-doped back surface field in one single high temperature step. We use borosilicate glasses (BSG) deposited by atmospheric pressure chemical vapor deposition (APCVD) and a POCl3 gas ambient as doping sources. Tailoring of the diffusion process yields a reduced co-diffusion process time of 114 min, well suited for production. The presented co-diffusion process allows a lean fabrication process. A proof of concept experiment leads to 20.1% bifacial n-type solar cells.

Voltage Loss Analysis for Bifacial Silicon Solar Cells: Case for Two-Dimensional Large-Area Modeling

While it is relatively easy to analyze the optical and recombination losses contributing to a solar cell's short-circuit current, there are many nuanced effects that lead to the solar cell's voltage at the maximum power point and at open circuit. This paper analyzes the impact of some commonly considered factors, such as series resistance, as well as some hard-to-discern factors, like localized recombination at the wafer periphery and metal busbars, on the solar cell's open-circuit voltage, maximum power point, and fill factor. A series of samples related to an n-type bifacial solar cell are chosen as an example.

The Glass-glass Module Using n-type Bifacial Solar Cell with PERT Structure and its Performance

In this work, the industrial glass-glass module was developed using bifacial n-type solar cell. The passivation emitter and rear total diffusion cells (PERT) structure solar cell combined boron spin-on with POCl3 diffusion and double sides H-pattern screen printing metallization. With the assistance of the spin-on single side doping method, an average efficiency of 20% with 90% bifaciality was obtained in our laboratory, 6*10 cells bifacial glass-glass modules were fabricated in industrial line. The PID about 3.5% was obtained after 600hrs under 85°C-85% humidity with a bias of -1000V, while the LID was about 0.19% under the IEC standard of 60 kW·h/m2 illumination. As expected, power output gains of 15% on sand and 30% on snow were recorded for the glass-glass bifacial modules compared with mono-facial modules in our outdoor experiments.