Rear Cell Research Articles

Investigation on the Long-Term Stability of AlOx/SiNy:H and SiNy:H Passivation Layers During Illuminated Annealing at Elevated Temperatures

Most crystalline Si based solar cells, e.g. passivated emitter and rear cells, rely on SiNy:H and AlOx/SiNy:H passivation layers. In this work, the long-term behavior of minority charge carrier lifetime in such symmetrically passivated samples during illuminated annealing at elevated temperatures is investigated by means of photoconductance decay based lifetime measurements, corona charging and capacitance voltage measurements. Thereby, AlOx layers, which are known to reduce H in-diffusion due to their barrier properties, deposited by atmospheric pressure chemical vapor deposition as well as by atomic layer deposition were considered enabling a comparison of different deposition techniques. The frequently published behavior of the bulk related degradation could be confirmed and the qualitative correlation between maximum defect density and the changing total amount of H in the Si bulk due to the barrier properties of the individual layers dielectric layers could be shown. Furthermore, for the subsequently observed degradation accelerated by a treatment at higher temperatures, literature indicates degradation to be caused by surface related degradation. Investigations on field effect passivation during degradation by means of corona charging and CV measurements showed a large drop in fixed negative charges in the passivation layer stacks.

Production of upgraded metallurgical-grade silicon for a low-cost, high-efficiency, and reliable PV technology

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations.

Current status and challenges for hole-selective poly-silicon based passivating contacts

Doped polysilicon (poly-Si) passivating contacts have emerged as a key technology for the next generation of silicon solar cells in mass production, owing to their excellent performance and high compatibility with the existing passivated emitter and rear cell technology. However, the current solar cell architecture based on a rear-side electron-selective (n+) poly-Si contact is also approaching its practical limit (∼26%) in mass production. The full potential of doped poly-Si passivating contacts can only be realized through incorporation of both electron-selective and hole-selective (p+) poly-Si contacts. While studies of both p+ and n+ poly-Si contacts commenced simultaneously, significant performance differences have arisen. Phosphorus-doped poly-Si contacts consistently outperform boron-doped counterparts, displaying typically lower recombination current density (J0) values (1–5fA/cm2 vs 7–15fA/cm2). This discrepancy can be attributed to inadequate optimization of p+ poly-Si contacts and fundamental limitations related to boron doping. The poorer passivation of p+ poly-Si contacts can be at least partly attributed to boron segregation into the interfacial oxide layers, compromising the interfacial oxide integrity and reducing the chemical passivation effectiveness. This review critically examines the progress of p+ poly-Si contacts characterized by cell efficiency and J0 values, delves into existing challenges, identifies potential solutions, and explores some potential solar cell architectures to enhance efficiency by incorporating p+ poly-Si contacts.

Potential‐Induced Electrochemical Corrosion in Crystalline Silicon Solar Cells

In this article, the electrochemical corrosion of full‐area aluminum back‐surface field (Al–BSF) and bifacial passivated emitter and rear cell (PERC) crystalline silicon (c‐Si) solar cells subjected to different potential in sodium chloride (NaCl) solution is systematically investigated. Based on different potential‐induced corrosion, the electrochemical corrosion mechanism of c‐Si solar cells is revealed for the first time. Under zero potential, an important finding is that the silicon beneath silver electrode is corroded and perforated for Al–BSF cells. This phenomenon is attributed to the interaction between hydroxide ions from galvanic corrosion and electrons from silicon corrosion. For bifacial PERC cells, the pyramidal structure of silicon surface beneath the aluminum finger is observed at zero potential, which is similar to the silicon texturing etched in alkaline environment. Additionally, it is found that silicon corrosion is inhibited when positive or negative potential is applied to the back of Al–BSF and bifacial PERC cells. Moreover, it is observed the aluminum suffers from more serious corrosion at positive potential than that at zero or negative potential, which is attributed to synthetical results of galvanic corrosion and electrolytic corrosion. In this work, a new insight on the potential‐induced electrochemical corrosion for c‐Si solar cells is provided.

Structure formation induced by non-reciprocal cell-cell interactions in a multicellular system.

Collective cellular behavior plays a crucial role in various biological processes, ranging from developmental morphogenesis to pathological processes such as cancer metastasis. Our previous research has revealed that a mutant cell of Dictyostelium discoideum exhibits collective cell migration, including chain migration and traveling band formation, driven by a unique tail-following behavior at contact sites, which we term "contact following locomotion" (CFL). Here, we uncover an imbalance of forces between the front and rear cells within cell chains, leading to an additional propulsion force in the rear cells. Drawing inspiration from this observation, we introduce a theoretical model that incorporates non-reciprocal cell-cell interactions. Our findings highlight that the non-reciprocal interaction, in conjunction with self-alignment interactions, significantly contributes to the emergence of the observed collective cell migrations. Furthermore, we present a comprehensive phase diagram, showing distinct phases at both low and intermediate cell densities. This phase diagram elucidates a specific regime that corresponds to the experimental system.

Ground-mounted or residential rooftop photovoltaic plant − European production or Chinese production: which is the most environmentally sustainable system? A case study in Italy

In recent years, decarbonization policies have pushed for a significant increase in renewable energies for power generation applications. European strategy aims to bring online over 320 GW of solar photovoltaics by 2025 and almost 600 GW by 2030. In this context, it is not only important to figure out the main hotspots of PV system but also to understand which are the best configurations from an environmental point of view. The analysis proposed in this paper aims to compare the environmental benefits of producing 1 kWh through two different system configurations: a ground-mounted PV plant (84 MW) and a residential rooftop PV plant (3 kW). In particular, the PERC (Passivated Emitter and Rear Cell) technology is investigated. In addition, in order to assess the influence of the energy mix used in the production of the PV components (mono-Si ingot, wafer, cell and module) on the LCA results, two different production sites are explored, i.e., in China and in Europe.

All-inorganic Sb2S3-based two-terminal tandem solar cells enable over 10.9% efficiency employing a concise interconnection layer

Sb2S3-based two terminal tandem solar cells (2T-TSCs) are reported for the first time with the assistance of lead sulfide quantum dot rear cells. The PCE of the champion TSC reaches 10.92%. Our work opens the door for Sb2S3 based 2T-TSCs.

Luminescence coupling in 28.4%-efficient tandem solar cell utilizing 20.8%-efficient perovskite minimodule stacked on silicon cell

The total power conversion efficiency (PCE) of 28.4% with an aperture area of 64 cm2 on a four-terminal (4T) tandem solar cell with a 20.8% efficiency-semitransparent perovskite module (SPM) with a series interconnection structure and a conventional passivated emitter and rear cell type crystalline silicon solar cell is independently confirmed. This is the independently confirmed record PCE of a 4T perovskite and crystalline silicon tandem solar cell with a structure having the same active area for both top and bottom cells. The impact of the luminescence coupling monitored by the silicon solar cell in the 4T tandem solar cell was investigated based on electrical properties of the top SPM analyzed by a simple one-diode equivalent circuit model.

Enhancement of efficiency in monocrystalline silicon solar cells

As the representative of the first generation of solar cells, crystalline silicon solar cells still dominate the photovoltaic market, including monocrystalline and polycrystalline silicon cells. With the development of silicon materials and cut-silicon wafer technologies, monocrystalline products have become more cost-effective, accelerating the replacement of polycrystalline products. In addition, the conversion efficiency of monocrystalline products increases gradually through high-efficiency cell technologies such as Passivated Emitter and Rear Cell (PERC). This paper will start with the solar cell efficiency and combine cost factor, the P-type PERC cell and additional four types of high-efficiency N-type cell technologies to improve the conversion efficiency for exploration, and will analyze and predict the future solar cell industrialization technologies. The study finally concludes that the N-type Tunnel Oxide Passivated Contact solar cell technology has a large industrial potential and could become a mainstream technology after the perc cell technology. At the same time, Interdigitated back contact cell structure and selective all-passive contact technologies are reliable paths to obtain solar cells with high conversion efficiency, integrated cost, and efficiency, depending on the advantages and compatibility of various cells. It can create conditions for the industrialization of low- cost and high-efficiency monocrystalline silicon solar cells.

Stability of industrial gallium-doped Czochralski silicon PERC cells and wafers

The carrier lifetime stability of gallium-doped silicon wafers and performance stability of industrial PERC solar cells produced from sister wafers were investigated under four different illumination conditions and temperatures. The seven investigated materials feature a resistivity variation of 0.4–1.0 Ωcm and lifetime samples were processed to create high hydrogen content (with PECVD SiNx) or low hydrogen content (with ALD Al2O3 or HfO2). Our results confirm that the material itself is prone to light and elevated temperature induced degradation (LeTID), however experiments on PERC cells produced utilising the same silicon material indicate that the production process can successfully suppress LeTID. In contrast to earlier studies, we observe only small levels of degradation at the cell level, with some showing an improvement in cell parameters under LeTID testing conditions. Our results indicate that LeTID is not necessarily a major issue for the performance of modern passivated emitter and rear cells made from gallium-doped silicon substrates.

Investigation on temperature dependence of recent high-efficiency silicon solar modules

The temperature dependence of photovoltaic modules varies with temperature and irradiance. For recent high-efficiency solar modules such as silicon heterojunction (SHJ) solar modules and tunneling oxide passivated contact (TOPCon) solar modules, it is not clear how their temperature dependence changes with temperature and irradiance. In this study, the temperature dependence of SHJ and TOPCon solar modules was studied by measuring I–V curves with the irradiance ranging from 200 to 1000 W/m2 and temperature ranging from 25 to 75 °C. The passivated emitter and rear cell (PERC) solar modules were measured as a reference. The results indicate that the temperature dependence of short-circuit current (Isc) for the three modules is almost invariable with temperature and irradiance. The temperature dependence of open-circuit voltage (Voc) and maximum power (Pm) decreasees with the increasing module temperature, and increasees with the increasing irradiance. It can be observed that the absolute magnitude of (∂Pm/∂T)/Pm for SHJ is the smallest, which is more obvious at high irradiance. An interesting finding is that the temperature dependence of fill factor (FF) for SHJ solar module increases with increasing irradiance, which is opposite to PERC and TOPCon solar modules. This phenomenon is related to the temperature dependence of series resistance. In addition, the temperature dependence is not linear at low temperature and irradiance. The obtained results in this paper can provide useful insight into predicting performance ratio of PERC, SHJ and TOPCon solar modules.

Optimization of rear-side passivation for enhancing the performance of bifacial PERC+ solar cells

For the passivated emitter and rear cells with bifacial power generation (known as PERC+), the rear-side efficiency is very important and critical. In this work, the influence of SiNx and SiOxNy as rear-side passivation films on the performance of PERC + cells was investigated. It was found that the SiNx film is better in terms of passivation performance and resistance to aluminum paste erosion, while the SiOxNy film exhibits a better optical performance. To synthesize the benefits of SiNx and SiOxNy, the multi-layer SiNx/SiOxNy/SiNx stacks were designed with the Wafer ray tracer system. The front-side efficiency of the optimized cells with SiNx/SiOxNy/SiNx passivation stacks was increased by 0.09 % as compared with the cells passivated with a single SiNx film of 80 nm in thickness. Moreover, in contrast with the cells with a rear-side SiNx film of 150 nm in thickness and the basically same front-side efficiency, the rear-side efficiency and bifaciality were increased by 1.55 % and 6.79 %, respectively.

Finding the Best Station to Use Buildings Integrated Photovoltaics (BIPVs) in Eight Different Climates of Iran: Effect of Wind Speed and Photovoltaic Modules Type

Buildings consume a significant amount of energy worldwide. Therefore, providing a part of this energy consumption with renewable energy, and reducing energy consumption, can lead to reducing pollutants. Due to the importance of the abovementioned points, this work aims to supply part of the required electricity in the buildings by using solar cells in eight climates of Iran. According to the available photovoltaic modules in the Iranian market, the study has been performed for six different types of solar cells. Considering the effect of temperature and wind speed on the efficiency of solar cells has made the results of the present work more consistent with reality. The simulations are performed for 25 years of the project’s useful life using HOMER V2.81 software, and the studied system is connected to the grid. The conclusions displayed that the Passive Emitter Rear Cell Silicon (perc-si) has the highest efficiency (21.22%–22.8%). Marand station is economically suitable (total net present cost (NPC) of 3180$ for a polycrystalline panel), and Gonbad station is appropriate because of high solar energy production (9183 kWh/year) and low environmental pollution (12602.9 kg/year). Due to the diversity and breadth of Iran’s climates, the results of the present study can be used for other areas with similar climates.

Influence of Al2O3/SiNx Rear-Side Stacked Passivation on the Performance of Polycrystalline PERC Solar Cells

In recent years, polycrystalline passivated emitter and rear cell (PERC) solar cells have developed rapidly, but less research has been conducted on the preparation process of their rear side passivation layers on standard solar cell production lines. In this work, a Al2O3/SiNx rear side stacked passivation layer for polycrystalline PERC solar cells was prepared using the plasma- enhanced chemical vapor deposition (PECVD) method. The effects of different Al2O3 layer thicknesses (6.8~25.6 nm), SiNx layer thicknesses (65~150 nm) and SiNx refractive indices (2.0~2.2) on the passivation effect and electrical performance were systematically investigated, which were adjusted by TMA flow rate, conveyor belt speed and the flow ratio of SiH4 and NH3, respectively. In addition, external quantum efficiency (EQE) and elevated temperature-induced degradation experiments were also carried out to check the cell performance. The results showed that the best passivation effect was achieved at 10.8 nm Al2O3 layer, 120 nm SiNx layer and 2.2 SiNx layer refractive index. Under the optimal conditions mentioned above, the highest efficiency was 19.20%, corresponding Voc was 647 mV, Isc was 9.21 A and FF was 79.18%. Meanwhile, when the refraction index was 2.2, the EQE of the cell in the long-wavelength band (800–1000 nm) was improved. Moreover, the decrease in conversion efficiency after 45 h LeTID was around 0.55% under the different refraction indices. The above results can provide a reference for the industrial production of polycrystalline PERC solar cells.

Optical modeling and characterization of bifacial SiNx/AlOx dielectric layers for surface passivation and antireflection in PERC

AbstractIn this research, we analyzed the impact that the optical characteristics of dielectric surface passivation and antireflection coating schemes have on the performance of passivated emitter and rear cell (PERC) silicon solar cells. We employed wafer ray tracer (WRT) and automate for simulation of heterostructure (AFORS‐HET) simulations, as well as experimental characterization of fabricated thin film coatings. We investigated three distinct front surface morphologies: planar surface, upright pyramids, and inverted pyramids. Using WRT, we calculated the photogeneration current densities (JG) for PERC devices with three schemes: (i) SiNx/AlOx as antireflection coating and passivation stacks on both the front and rear sides, (ii) SiNx antireflection coating on the front side and AlOx passivation layer on the rear side, and (iii) SiNx/AlOx as antireflection coating and passivation stacks on the front side with an AlOx passivation layer on the rear side. Following simulation with optimal JG, two schemes are experimentally evaluated: PECVD SiNx (70 nm) and atomic layer deposition (ALD) AlOx (15 and 25 nm). We confirmed the growth effects and optical properties using X‐ray diffraction, Raman spectroscopy, effective lifetime, and refractive index measurements. The most favorable electrical properties were obtained with SiNx (70 nm, front) and AlOx (25 nm, front and rear), where the AlOx can be deposited via ALD bifacially on a single step, minimizing processing while maintaining passivation performance. Finally, we used AFORS‐HET to simulate the maximum performance of PERC bearing such films. The results showed a Voc = 0.688 V, Jsc = 41.42 mA/cm2, FF = 84%, and packing conversion efficiency (PCE) = 24.12% as the optimal solar cell performance values.

Theoretical analysis of backside polycrystalline silicon layer in the TOPCon solar cells

As an upgraded version of passivated emitter and rear cell (PERC) solar cells, the performance of tunnel oxide passivating contacts (TOPCon) solar cells is very dependent on the silicon oxide layer and poly-Si layer. We found that different crystallization rates or doping of germanium, carbon and other elements in poly-Si can change the band gap of poly-Si, which has an impact on the efficiency (Eff) of TOPCon solar cells. Therefore, the appropriate band gap is particularly important. At the same time, the internal defect of poly-Si also have great influence on the performance of solar cells. The simulated band diagram, charge carrier concentration and recombination rate were used to deeply explore the defect in poly-Si and the influence of poly-Si with different band gap on the performance of TOPCon solar cells.

Accelerated damp-heat testing at the cell-level of bifacial silicon HJT, PERC and TOPCon solar cells using sodium chloride

Bifacial passivated emitter and rear cells (PERC) currently have the highest share in the photovoltaic market. However, heterojunction (HJT) and tunnel oxide passivated contact (TOPCon) solar cells are expected to gain significant market share shortly. Despite technological advancements, concerns about the reliability of HJT and TOPCon technologies when deployed in the field still need to be addressed. This work investigates the impact of sodium chloride (NaCl) on damp-heat-induced degradation in bifacial HJT, PERC, and TOPCon solar cells by exposing the solar cells to NaCl before damp heat (DH) testing. It is found that among all investigated cell technologies, TOPCon solar cells degrade the most with maximum power (Pmax) loss of up to ∼75%rel, followed by HJT (Pmax drops ∼50%rel), and PERC cells (Pmax drops only ∼10%rel) after 20 h of DH testing, mainly attributed to an increase in Rs on the front side of TOPCon cells, both sides of HJT cells and the rear side of PERC cells. The front of the PERC and the rear of the TOPCon solar cell are found to be stable. The rise in Rs is attributed to the corrosion of the metal contact, which is caused by a high amount of Na+ and Cl− ions penetrating the metal contact. This corrosion leads to increased porosity, detachment of the contact from the silicon interface, and increased recombination loss in some cases. These results are crucial for all cell technologies as they highlight the potential failures that could occur in the field. Na+ and/or Cl− ions are common contaminants present in solar glass, human fingerprints, soldering flux, rainwater, soil/dust, and seawater. During field operation, these ions have the potential to penetrate and directly interact with solar cells. In our view, the preferred solution is for the solar cells to be corrosion-resistant, which can be rapidly assessed using the method presented in this work.

LeTID mitigation via an adapted firing process in p-type PERC cells from gallium-doped Czochralski silicon

In this work, we analyse the light and elevated temperature induced degradation (LeTID) behaviour of gallium (Ga)-doped Czochralski (Cz) passivated emitter and rear cells (PERC). We show that these cells, based on industrial precursors, are prone to LeTID, degrading on average 8.3%rel in efficiency. The degradation and regeneration is slower than in boron (B)-doped PERC cells leading to a greater loss of energy over a 20 year lifespan of a solar module on a German rooftop, since, according to our experiments, regeneration does not start during that time frame. We show that the method of mitigating LeTID by slightly altering the temperature profile during fast firing, previously introduced on B-doped PERC cells, also works on Ga-doped PERC cells, reducing the maximum degradation to between 1%rel and 4%rel. This process does only slightly reduce the initial efficiency of the cell by up to 0.8%rel. Lastly, we show that increasing illumination and temperature from 0.15 suns eq. to 2 suns eq. and 75 °C to 140 °C, respectively, is an adequate way to accelerate LeTID testing on Ga-doped samples by more than two orders of magnitude while achieving qualitatively and quantitatively similar results to those obtained in a standard test.

Applications of vertical damping wigglers in an X-ray diffraction limited storage ring

This study presents a lattice design for an X-ray diffraction limited storage ring (DLSR) light source with a circumference of 864 m and achieving an ultralow natural emittance of 25.6 pm·rad (N-IBS). However, the ultra-low emittance and long damping times lead to significant intra-beam scattering (IBS) effects. To address this issue, the study explores the implementation of vertical damping wigglers (VDWs) to decrease damping times, reduce beam horizontal emittance, and generate vertical emittance to achieve a “round beam” in the 864mDLSR. Through theoretical analysis and accelerator toolbox (AT) simulations, the impacts of VDWs with varied parameters were investigated, and the optimal period length, peak field, and total length were determined. Additionally, the twelve quadrupole magnets in both the front and rear cells of the VDWs were employed to correct linear optics. The findings demonstrate that VDWs could significantly reduce the beam's equilibrium emittance and damping times of the 864mDLSR. Furthermore, a comparison between the spectral brightness and flux of the 864mDLSR with and without VDWs revealed that the 864mDLSR employing VDWs yielded higher brightness and flux.

Atomic layer deposition of NiO applied in a monolithic perovskite/PERC tandem cell

Monolithic perovskite/silicon tandem photovoltaics have fueled major research efforts as well as gaining rapid industrial interest. So far, most of the literature has focused on the use of currently more expensive silicon heterojunction bottom cell technology. This work demonstrates a perovskite/silicon tandem solar cell based on the industrially dominant passivated emitter and rear cell (PERC) technology. In detail, we investigate a tunnel recombination junction (TRJ) consisting of ITO/NiO/2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz) and compare it with an ITO/2PACz TRJ. Specifically, the NiO layer is deposited by atomic layer deposition (ALD). Although ITO/2PACz-based tandem devices can reach more than 24% conversion efficiency, we observe that they suffer from a large spread in photovoltaic parameters due to electrical shunts in the perovskite top cell, caused by the inhomogeneity of the 2PACz layer on ITO. Instead, when ALD NiO is sandwiched between 2PACz and ITO, the surface coverage of 2PACz improves and the yield of the devices, in terms of all device parameters, also improves, i.e., the standard deviation decreases from 4.6% with ITO/2PACz to 2.0% with ITO/NiO/2PACz. In conclusion, thanks to the presence of NiO, the TRJ consisting of ITO/NiO/2PACz leads to a 23.7% efficient tandem device with narrow device efficiency distribution.