Rear Solar Cells Research Articles

Numerical simulation of thermal silicon dioxide growth in the gap between stacked wafers

Stacking of wafers for thermal processing enables a strong increase of the throughput and thus offers a high potential for cost reduction in solar cell production. In previous studies, the high temperature stack oxidation (HiTSOx) approach showed promising solar cell results and homogeneous thermal oxide growth over the wafer surface within the stack. In this work, we investigate the thermal oxide growth in the gap between stacked wafers where the wafer surfaces touching each other. We perform finite element method (FEM) simulation to identify the mechanism for the oxygen transport inside the gap and compare the simulated layer thickness to experimental data. In this simulation, the gap height is an important parameter. Therefore, we present a gravimetric-density approach to experimentally determine the gap height. Further, passivated emitter and rear (PERC) solar cells are fabricated using the HiTSOx approach. A high open-circuit voltage of Voc = 694 mV confirms the high quality of the surface passivation achieved in the stack oxidation.

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

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

Short Drying Processes for Silicon Solar Cells

Herein, very short drying processes for the screen‐printed electrodes of passivated emitter and rear solar cells (PERC) are successfully demonstrated. The short drying processes are conducted in an in‐line heating system with vertical‐cavity surface‐emitting lasers (VCSEL) as its heat source, being a compact alternative to the conventional heating chamber. While the heating time of the conventional industrial drying process is believed to be about 20 s (to the best of the authors’ knowledge), the VCSEL system allows a significant reduction down to 0.15 s while maintaining a similar power conversion efficiency and contact adhesion quality.

Optimizing the Radius of Curvature of Black Silicon Peaks for Solar Cell Using Matlab Simulations

As part of our research on efficiency improvement of PERC (Passivated Emitter Rear Solar Cell), achieving very low reflectivity values of solar cell surface is a must. One of the most advance technologies to do so is the use of advanced texturing for the front surface of the cells. This texture, also known as Black Silicon, consists of peaks and valleys of nano metric dimensions and capable of dramatically reducing the reflectance of the front surface. A reflectance around 5% was reached ,using simulation, when using a Black-Silicon texturing with height of 50nm with peak rounding of 5nm. Even though this texturing may affect other parameters such as series resistance or surface recombination, as a starting point, simulation was used to find the optimum peak rounding, where the radius of curvature of the peak is maximum, while keeping reflectance as low as possible.

Evolution of Corrosive Potential‐Induced Degradation at the Rear Side of Bifacial Passivated Emitter and Rear Solar Cells

Detrimental corrosive potential‐induced degradation at the rear side of bifacial solar cells has been found recently. This potential‐induced degradation effect is nonreversible and shows typical surface damages in the micrometer scale. However, also areas in the cells showing no such surface damage suffer from electrical losses. Cracks in the nanometer scale underneath the surface, that is, at the interface between Si bulk and Al2O3, turn out to be the cause of this effect, leading to depassivation effects. The emergence of these cracks is assumed to originate from oxidation of the Si bulk due to potential‐induced degradation stress.

Inverted Pyramid Morphology Control by Acid Modification and Application for PERC Solar Cells.

Silicon inverted pyramid (IP) structures, with lower reflectance and increased surface recombination, are one of the best choices for light-trapping structures of high-efficiency silicon solar cells. The solution process of IP generally goes through three main steps: porous silicon etched by metal-assisted chemical etching, acid etching, and alkali anisotropic etching. In this paper, the role that acid modification plays in IP preparation and the application of our optimized texture for passivated emitter and rear solar cells (PERC) were investigated. Experimental results show that acid plays a decisive role in optimizing and modifying the morphology of porous silicon; thus, the morphology of porous silicon has no direct influence on the morphology of IP. In addition, the opening size of IP is mainly determined by the size of silicon micron holes modified by the acid process. PC1D simulation results manifest that IPs can increase the short-circuit current density (Jsc) of devices by 1.04 mA/cm2 and power conversion efficiency by 0.55%; hence, our optimized IP-based PERC achieve the highest simulative conversion efficiency of 23.21%. This is an effective and important way to manipulate the structure of IP, which points out the direction of fabrication and application of high-efficiency IP textures.

Reliability Evaluation of Photovoltaic Modules Fabricated from Treated Solar Cells by Laser‐Enhanced Contact Optimization Process

According to the International Technology Roadmap for Photovoltaics, passivated emitter and rear solar cells dominate the market in 2021 of up to 80% and are forecast to remain state of the art at least for the next 5 years. Within the production process of solar cells, it is typical to have cells with lower efficiency grades due to variations in manufacturing processes or material defects. Reprocessing such solar cells could save cost due to increased production yield and simultaneously reduced cost for recycling of unusable/unsellable low‐efficiency cells. Herein, the impact of the laser‐enhanced contact optimization (LECO) process on the power output and reliability of solar modules using commercial off‐spec cells of different manufacturers is analyzed. LECO is a downstream process for optimizing metal−semiconductor contacts on finished solar cells. The treatment leads to a significant economic gain due to enhanced cell efficiency (Wp ↑, therefore manufacturing cost per Wp ↓) even of already good solar cells. Herein, the first evaluation of the impact of the LECO process on the cell output power on an industrial scale (&gt;1000 cells) and on module reliability is presented. The results for common short‐term effects like light‐induced degradation and light‐ and elevated temperature‐induced degradation are within expected limits and the durability against, for example, potential‐induced degradation is not changed due to the LECO process. The results further show that cell sorting is crucial for a reliable module and to avoid outliers in terms of unexpected degradation and recovery phenomena of individual cells.

Analyses and Excess Oxygen Investigations by Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy at AlOx/Si Interfaces in Passivated Emitter and Rear Solar Cells

Progressive optimization of the surface passivation of Si solar cells requires access to the elemental composition and chemical bonding characteristics of ultrathin layers and buried interfaces on textured or structured surfaces. The passivation of the rear side of passivated emitter and rear solar cells, for instance, can be realized through AlOx/SiNx stacks, which are known to provide an additional field effect passivation on top of the chemical surface passivation. Herein, transmission electron microscopy is used at locally prepared cross‐sections of such stacks. Electron energy loss spectroscopy is used to extract depth‐resolved information of elemental composition and chemical bonding at nanometer scale. With this, the interfacial excess O fraction, which is crucial for the field effect passivation, is successfully determined for the first time for AlOx passivation layer systems on industrially produced Si solar cells.

Proton Irradiation in Simplified PERC Silicon Solar Cells: A Simulation-Based Framework

A complete technological computer aided design (TCAD) based framework to study the proton radiation effects in silicon Passivated Emitter and Rear solar Cell (PERC) is presented. This novel approach combines the capabilities of the Radiation Effects Module (REM) in Silvaco device simulator with the vacancy information extracted from SRIM code. It is the first time that the parameters in both the tools are linked together to study the radiation effects without relying on previous experiments, which would not be possible if REM alone is used. The fluence levels used to study the radiation effects are present in the space environment but are well beyond the capabilities of laboratory settings. A simplified PERC solar cell is modeled where the average percentage difference between simulated and experimental parameters is <0.25%. The model is used for irradiation studies by the incident protons through Non-Ionizing Energy Loss (NIEL). The results agree well with experimentally studied maximum displacement damage results. It is found that the external quantum efficiency loss in the infrared region is primarily responsible for the overall efficiency degradation due to irradiation. Through this, the qualification of radiation hardness is possible, thereby opening new avenues to explore new radiation tolerant solar cell structures.

Gallium‐Doped Silicon for High‐Efficiency Commercial Passivated Emitter and Rear Solar Cells

Czochralski‐grown gallium‐doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski‐grown wafers in dependence of resistivity, finding effective lifetimes well into the millisecond region without any gettering or hydrogenation processing, thus demonstrating one advantage over boron‐doped silicon. Second, the stability of gallium‐doped PERC cells are monitored under illumination (&gt;3000 h in some cases) and anomalous behavior is detected. While some cells are stable, others exhibit a degradation then recovery, reminiscent of light and elevated temperature‐induced degradation (LeTID) observed in other silicon materials. Surprisingly, cells from one ingot exhibit LeTID‐like behavior when annealed at 300 °C but near stability when not annealed, but, for another ingot, the opposite is observed. Moreover, a stabilization process typically used to mitigate boron–oxygen degradation does not influence any cells that are studied. Secondary‐ion mass spectrometry of the PERC cells reveals significant concentrations of unintentionally incorporated boron in some cases. Nevertheless, even in the absence of mitigating light‐induced degradation, Ga‐doped silicon is still more stable than unstabilized B‐doped silicon under illumination.

Ultra-thin Al2O3 capped with SiNx enabling implied open-circuit voltage reaching 720 mV on industrial p-type Cz c-Si wafers for passivated emitter and rear solar cells

In this study, we report on the passivation quality of atomic layer deposition grown ultra-thin Al2O3 and Al2O3 capped with plasma-enhanced chemical vapor deposition deposited SiNx on Cz p-type wafers for the rear side of a passivated emitter and rear cell (PERC). Different activation recipes using N2, forming gas (FG), and two-step annealing for different durations are investigated before SiNx deposition. The effect of different Al2O3 thicknesses and corresponding activation processes on the Al2O3/SiNx passivation performance, after a high temperature firing step, is studied to reach a new optimization toward higher efficiency and lower cost. A record high iVoc of 720 mV is obtained after firing step from Al2O3/SiNx stacks with Al2O3 thickness as thin as ∼2 nm with FG annealing. Our results demonstrate that, under well-optimized process conditions, ultra-thin Al2O3 thicknesses provide superior passivation quality as compared to the larger thicknesses which are commonly applied in the PERC industrial line and the potential for further improvement of industrial PERC solar cells in terms of cost reduction and efficiency.

Application of hydrosilane-free atmospheric pressure chemical vapor deposition of SiOx films in the manufacture of crystalline silicon solar cells

In this work we present SiOx films deposited in cost-effective laboratory scale three-dimensional printed atmospheric pressure chemical vapor deposition setup. As SiOx films are deposited at room temperature without complex vacuum systems, they can be a good candidate for the use in commercial c-Si solar cell production lines. The quality of the deposited films was investigated as to their integrity, conformity with various surfaces, and post-treatment resilience such as stability against etchants and annealing. Several applications of the SiOx film prepared with the atmospheric pressure chemical vapor deposition (APCVD) were discussed. In one application, the APCVD SiOx was utilized to effectively promote single-side texturing of Float Zone and Czochralski Si wafers by coating only one side with SiOx and subsequently annealing prior to texturing in an alkaline aqueous solution. Another application was to exploit the APCVD SiOx as a plating mask for silicon heterojunction solar cells. Two processing options prior to the oxide-film deposition were investigated: i) application of an Ag seed-layer, which promotes subsequent electroplating, and ii) printing of an organic grid, which, after stripping, creates openings in the SiOx that facilitate electroplating of the solar cell's electrode on the underlying transparent conducting oxide. In a different application, the APCVD SiOx films acted as protection against parasitic plating on the front side of passivated emitter and rear solar cells. The deposited films were characterized by ellipsometry, hemispherical reflectance measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy.

The importance of total hemispherical emittance in evaluating performance of building-integrated silicon and perovskite solar cells in insulated glazings

Temperature control in solar cells is important as elevated temperature adversely affects performance and lifetime. In building-integrated photovoltaics (BIPV), the overall energy management of an installation must include not only the electrical output from the photovoltaic component but also the net light and heat flows as well as the temperature distributions. As the light reflectance and emittance of solar cells are strongly angle-dependent, total thermal hemispherical emittance should be used instead of normal spectral emittance for accurate calculation of radiative heat transfers and hence solar cell operating temperature. Here we report the analysis of solar cell and internal glass temperature as a function of the measured total hemispherical emittance for the first time. We present a comprehensive model using total hemispherical emittance for determining solar cell and internal glass surface temperatures for insulating and laminated glazing units incorporating an operating photovoltaic cell. In warm weather (30 °C outdoors), solar cell and internal glass temperatures are 45–55 °C in laminated glass while in an insulated glazing the solar cell temperature is 60–75 °C and the internal glass temperature is maintained close to ambient temperature (20 °C indoors). We show that the solar cell front and rear emittance, location, and encapsulation method as well as the type of glazing system impact on the solar cell performance and internal glass temperatures. This study provides recommendations for designing BIPV glazing systems that minimize power loss from the solar cells while optimizing transmitted heat and shows the importance of engineering the correct front and rear solar cell emittances.

Influence of SiOx film thickness on electrical performance and efficiency of TOPCon solar cells

The tunnel oxide passivated contact solar cells (TOPCon) on n-type Cz Si wafers instead of passivated emitter and rear solar cells are currently migrated to mainstream production, with ongoing improvements in recent years. In this study, we investigated and characterized one recent batch of TOPCon cells fabricated on 156.75 × 156.75 × 0.18mm3 wafers with fully screen-printed technology by an industrial-type process. TOPCon cells with an efficiency as high as 22.43%, a Voc value as high as 689.4 mV, and a fill factor as high as 81.35% were obtained. The P dopant diffusion in the poly Si layer as well as the thickness of the SiOx layer were optimized in order to obtain good electrical contact between screen-printed Ag and Si. The thickness of the SiOx layer should be over 1.5 nm, which is beneficial for obtaining a good passivation for the cells. The surface P dopant concentration in the polycrystalline Si was higher than 1 × 1020 atom/cm3, resulted in a low series resistance and a high fill factor. The characterization and simulation results show that both metal shading loss and rear surface recombination were dominant among all of the losses. Application to solar cells with a Voc of 697 mV and a fill factor of 82.45% could lead to an independently confirmed cell efficiency of over 23% for n-type cells with poly Si the herein developed passivated rear contacts, and the front metal contacts by addressing the issue of reducing metal recombination and implementing a selective emitter.

Characterizing Point Contacting by Localized Dielectric Breakdown and Its Use in Silicon Solar Cell Applications

The point contacting by localized dielectric breakdown (PLDB) method utilizes a dielectric breakdown above a locally doped region to form ohmic contacts for a solar cell rear surface. This article describes the design and fabrication of the PLDB solar cells and the contact properties characterization. A complete solar cell fabrication process applying PLDB as a rear contact design was developed, with the demonstration of an 18.0% proof-of-concept PERC structure PLDB solar cell. Two major loss mechanisms in the fabricated solar cell were characterized to be a high series resistance and nonideal recombination. By modulating the local boron doping profile with acidic etching, the impact of the surface doping concentration on local contact recombination and contact resistivity was investigated, resulting in an estimated contact resistivity of approximately 5 mΩ·cm2 and contact recombination of approximately 600 fA/cm2. With these metrics, modeling suggests that by optimizing the contact pitch of our solar cells, efficiencies of up to 24% are well achievable for this low-temperature contacting method.

Controlling Light- and Elevated-Temperature-Induced Degradation With Thin Film Barrier Layers

In this article, we investigate the extent of lifetime degradation attributed to light- and elevated-temperature-induced degradation (LeTID) in p- type multicrystalline silicon wafers passivated with different configurations of hydrogenated silicon nitride (SiNx:H) and aluminum oxide (AlOx:H). We also demonstrate a significant difference between AlOx:H layers grown by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) with respect to the extent of LeTID. When ALD AlOx:H is placed underneath a PECVD SiNx:H layer, as used in a passivated emitter and rear solar cell, a lower extent of LeTID is observed compared with the case when a single PECVD SiNx:H layer is used. On the other hand, the LeTID extent is significantly increased when an ALD AlOx:H is grown on top of the PECVD SiNx:H film. Remarkably, when a PECVD AlOx:H is used underneath the PECVD SiNx:H film, an increase in the LeTID extent is observed. Building on our current understanding of LeTID, we explain these results with the role of ALD AlOx:H in impeding the hydrogen diffusion from the dielectric stack into the c-Si bulk, while PECVD AlOx:H seems to act as an additional hydrogen source. These observations support the hypothesis that hydrogen is playing a key role in LeTID and provide solar cell manufacturers with a new method to reduce LeTID in their solar cells.

Micro cracks distribution and power degradation of polycrystalline solar cells wafer: Observations constructed from the analysis of 4000 samples

In this paper, the impact of Photovoltaic (PV) micro cracks is assessed through the analysis of 4000 polycrystalline silicon solar cells. The inspection of the cracks has been carried out using an electron microscopy, which facilitate the detection of the cracks though the acquisition of both Everhart-Thornley Detector (ETD) and the Back Scatted Electron Diffraction (BSED) image, where it was found that the size micro cracks are ranging from 50 μm to a maximum of 4 mm. Micro cracks have been categorized into two main categories, including cracks in the solar cell front or rear contact. Several remarkable observations have been found, including but not limited to, (i) the output power loss due to micro cracks varies from 0.9% to 42.8%, subject to micro crack type and size, (ii) cracks in solar cells fingers reduce the finger width, resulting an increase in the output power loss by at least 1.7%, and (iii) there is a substantial correlation between PV hot-spots and the presence of micro cracks, while minimum increase in the cell temperature is observed at 7.6 °C.

Efficient silicon nitride SiNx:H antireflective and passivation layers deposited by atmospheric pressure PECVD for silicon solar cells

AbstractThis work demonstrates the efficient optical and passivation properties provided by hydrogenated silicon nitride (SiNx:H) layers deposited in a lab‐scale atmospheric pressure plasma enhanced chemical vapor deposition (AP‐PECVD) reactor. By applying modulated low‐frequency plasma (200 kHz), homogeneous SiNx:H layers, with small variances in thickness w and refractive index n (Δw ≤ 2 nm; Δn ≤ 0.02), were achieved on a surface area of 45 × 55 mm2. The use of voltage amplitude modulation enabled discharge optimization and led to greatly enhanced SiNx:H film homogeneity and conformity in comparison with continuous plasma discharge conditions. Additionally, AP‐PECVD SiNx:H showed good thermal stability (Δw ≤ 1 nm; Δn ≤ −0.02) with low absorption coefficients (k ≤ 0.1 at 275 nm), demonstrating that such layers could act as efficient antireflective coatings. Furthermore, outstanding surface passivation properties were achieved after firing, both on n‐type FZ c‐Si substrates of standard 2.8 Ω.cm doping (τeff = 1.45 ms) and on highly doped 85 Ω/sq n+ emitters (j0e = 74 ± 2 fA.cm−2). Finally, AP‐PECVD SiNx:H thin films were tested on industrial passivated emitter and rear solar cell (PERC) architectures, where the potential of applying these layers both as efficient rear‐side capping layer and front‐side antireflective coating was demonstrated. The first lab‐scale 40 × 40 mm2 PERC solar cells featuring AP‐PECVD SiNx:H layers led to conversion efficiencies of up to 20.6%. These results pave the way for upscaling the dielectric barrier discharge lab‐scale reactor in an industrial in‐line process, which could provide low‐cost and high‐throughput SiNx:H capping and antireflective layers.

Impact of Light Soaking on p‐Type Boron‐ and Indium‐Doped Passivated Emitter and Rear Solar Cells on Czochralski‐Grown Silicon

Solar cells fabricated on boron‐doped Czochralski‐grown silicon (Cz‐Si) wafers are known to suffer from light‐induced degradation (LID). In this work, the effect of light soaking on passivated emitter and rear (PERC) solar cells fabricated on both indium (In)‐doped Cz‐Si and boron (B)‐doped Cz‐Si wafers are investigated. Efficiencies up to 20.7% and 21.2% are obtained on In‐doped and B‐doped Si wafers, respectively. Though the initial conversion efficiency is lower with In‐doped Si wafers, these cells show no LID after light soaking (1.0 Sun illumination at 25 °C for 85 h). In contrast, cell efficiencies dropped significantly for B‐doped Si wafers, by 0.8%.

Performance Analysis on Conversion Efficiency of PERC Solar Cell by PC2D Simulation

PERC technology or Passivated Emitter Rear solar Cell on the p-type mono-crystalline silicon wafers is one of the new widely known solar cell concepts that are currently introduced in mass production by various manufacturers. Research is under way in the field of laboratories and in industry to develop this technology. In this study the optimization parameters of the PERC solar cell are defined using simulation program. The Influences of the three parameters: the thickness of the wafer, the lifetime of the minority carriers and the resistivity of the p-substrate has been investigated using two-dimensional solar cell simulation software PC2D. In this study, the maximum efficiency obtained are 21.47%, with an optimization of 170 μm, 3000 μs and 1 Ω.cm of wafer thickness, minority carrier lifetime and resistivity respectively. The results found using PC2D numerical simulations show that this type of PERC solar cells can reach a theoretical efficiency of 21% higher when the parameters of the substrate are well chosen.