Contact Solar Cells Research Articles

Progress in TOPCon solar cell technology: Investigating hafnium oxide through simulation

In the realm of solar energy technology, exploring hafnium oxide (HfO2) in Tunnel Oxide Passivated Contact (TOPCon) solar cells is pivotal. This study delineates HfO2's evolution from semiconductor applications, highlighting its crucial role in enhancing TOPCon solar cell performance. Utilizing ATLAS Silvaco software, the study anticipates a 21.3% increase in charge carrier lifetime through optimized HfO2 layers, addressing challenges in interface engineering and scalability. Innovative research integrates hafnium oxide (HfO2) into TOPCon solar cells, marking a leap in photovoltaic technology. Utilizing ATLAS Silvaco simulations, it shows that HfO2 layers can significantly enhance cell performance, increasing charge carrier lifetime by 21.3% and potentially boosting efficiency by 25%. This underscores HfO2's advantages, like a higher dielectric constant and thermal stability, in improving solar cell efficiency and durability. Future efforts target refining deposition processes, projecting a 25% boost in overall power conversion efficiency (PCE). Emphasizing HfO2's significance in solar cell technology, this research contributes to global sustainable energy initiatives. Integrating HfO2 in TOPCon solar cells signifies a key achievement in harnessing clean, renewable energy. Upcoming research focuses on experimental validation, interface engineering, optimization, stability assessments, scalability, and collaborative studies, aiming to leverage HfO2's potential for elevating solar energy conversion technologies.

Advancing Monolithic Perovskite/TOPCon Tandem Solar Cells by Customizing Industrial‐Level Micro‐Nano Structures

AbstractTexturing silicon solar cells with micro/nano‐structures is crucial for achieving outstanding optical performance in perovskite/silicon tandem solar cells (TSCs). However, ensuring excellent electrical properties remains a challenge due to reduced passivation quality of the bottom silicon sub‐cells and difficulties in perovskite formation on textured substrates. Here, an industrial‐level sub‐micron random pyramid (sMRP) structure on the front side of a tunnel oxide passivating contact (TOPCon) solar cell using a simple alkaline texturing process is presented, resulting in excellent optical and electrical properties. Through meticulous fabrication process tuning, uniform sMRP textures with a size of 0.6–0.8 µm are achieved, exhibiting low reflectance comparable to industrial‐level micron random pyramid (MRP) structures. Optimizing annealing temperatures of double‐sided TOPCon structures textured with front‐sided sMRP and rear‐sided MRP, yields high passivation quality, with a remarkable implied open‐circuit voltage (iVOC) of 713 mV. Furthermore, the sMRP‐textured surface facilitates the formation of a high‐quality perovskite film with larger grain sizes and fewer internal pinholes compared to the polished counterpart. Consequently, a proof‐of‐concept p‐i‐n typed perovskite/TOPCon TSC featuring sMRP textures obtains an outstanding efficiency of 28.67%, providing a promising approach for the commercial production of high‐efficiency perovskite/TOPCon TSCs.

Multi-build orientation effects on microstructural evolution and mechanical behavior of truly as-built selective laser melting Ti6Al4V alloys

The unique combination of lightweight, strength, corrosion resistance, high-temperature resistance, and biocompatibility make Ti6Al4V alloy an important material in metal contact of solar cells, and selective laser melting (SLM) enables the production of Ti6Al4V parts complex geometries that would be difficult or impossible to produce using traditional manufacturing methods. The build orientation of SLM process presents an essential effect on the quality of the produced parts, and thus, in this study it investigates the impact of multi-build orientation (0°, 30°, 45°, 60°, 90°) on the microstructural evolution and mechanical behavior of truly as-built Ti6Al4V alloys produced by SLM. The microstructure analysis revealed the presence of ordered subsurface pores, located at a distance of about 50 μm–150 μm from the sample surface. The microstructure consisted primarily of α′ martensite with a small amount of β phase. Massive transformation was observed in the 30° and 45° samples, with corresponding grain sizes of 5 μm and 10 μm, respectively. To be specific, the content of β phase and ultimate tensile strength (UTS) varied with the sample orientation, and the 45° build orientation exhibited the highest content of β phase with value of approximately 2.8%, but also the lowest UTS and elongation, which were 1161 MPa and 3.99%, respectively.

Passivating Contacts for Crystalline Silicon Solar Cells: An Overview of the Current Advances and Future Perspectives

AbstractSolar photovoltaics (PV) are poised to be crucial in limiting global warming by replacing traditional fossil fuel generation. Within the PV community, crystalline silicon (c‐Si) solar cells currently dominate, having made significant efficiency breakthroughs in recent years. These advancements are primarily due to innovations in solar cell technology, particularly in developing passivating contact schemes. As such, this review article comprehensively examines the evolution of high‐efficiency c‐Si solar cells, adopting a historical perspective to investigate the advancements in passivation contact techniques and materials to state‐of‐the‐art cell designs. Additionally, this work deeply studies the recent advances and critical design principles underlying each developed passivation scheme. Eventually, this work identifies existing challenges and proposes insights into future directions for c‐Si solar cells through diverse passivating contact strategies.

Low-temperature fabrication of boron-doped amorphous silicon passivating contact as a local selective emitter for high-efficiency n-type TOPCon solar cells

Tunnel oxide passivating contact (TOPCon) solar cells (SCs) currently dominate the photovoltaic industry but grapple with efficiency challenges. A primary concern is the direct contact between front-sided metal electrodes and boron emitters, resulting in substantial carrier recombination losses and limiting further efficiency improvements. Here, we introduce a low-temperature approach to deposit a local boron-doped amorphous silicon [a-Si:H(p)] between front-sided metal electrodes and boron emitters. This method, avoiding issues associated with high-temperature processes, demonstrates excellent passivation and contact properties, featuring the lowest contact resistivity (< 1 mΩ·cm2) and a low saturation current density (< 400 fA/cm2). The outstanding passivation and contact properties remain robust even with variations in diborane flow rates during a-Si:H(p) fabrication, annealing temperatures, and sheet resistances of boron emitters. We elucidate the factors contributing to the enhanced passivation observed in boron emitters with a-Si:H(p) through a combination of simulations and experiments. The a-Si:H(p) layer between boron emitters and metal electrodes acts as a protective barrier, preventing the diffusion of metal atoms and suppressing carrier recombination. A heterojunction is formed between a-Si:H(p) and the boron emitter, facilitating electric-field passivation. Consequently, the TOPCon SCs incorporating a-Si:H(p) achieve an efficiency of 24.50%, surpassing their counterparts without a-Si:H(p) (23.11%). This work utilizes low-temperature technology to achieve fully passivated contact, providing insights for the development of high-efficiency TOPCon SCs.

Silver‐lean metallization and hybrid contacts via plating on screen‐printed metal for silicon solar cells manufacturing

AbstractAs PV manufacturing heads towards the multi‐TW scale, it is required to carefully evaluate a wide range of concepts including not only efficiency and cost but also material consumption to ensure sustainable manufacturing of PV technologies. The rapid growth of PV could significantly increase the demand for several materials required in solar cells such as silver, aluminium, copper and even silicon, thereby causing dramatic price fluctuations. Furthermore, the PV manufacturing capacity would be at risk of being limited by the supply of some scarce metals, e.g. with current industrial implementations – screen printing (SP) metallization, the capacities of PERC and TOPCon could be capped at 377 GW and 227 GW with 20% of global silver supply available to the PV industry. In addition, PV systems have ~25–30 years lifespan to ensure low LCOE and emissions. Recycling alone will not provide an immediate solution to overcome the limitation of material consumption in the exponentially growing PV market. It is expected that the Ag usage needs to be reduced to no more than 5 mg/W or even 2 mg/W for all solar cell technologies to allow a multi‐TW manufacturing scale without depleting the global silver supply. Therefore, further advancements in metallization technologies are critically and urgently required to significantly reduce the silver consumption of current screen‐printed contacts in industrial silicon solar cells. This paper firstly presents a roadmap towards the 5 mg/W and 2 mg/W silver consumption targets with various metallization technologies and screen‐printing designs. Subsequently, a hybrid plating on screen‐printed metallization design was proposed to improve the performance and reduce the silver consumption of screen‐printed contacts. The experimental results have demonstrated up to 1.08%abs improvements in fill factor and 0.3%abs gains in cell efficiency. In addition, up to 40%rel reductions in finger silver consumption have been achieved without any sacrifices in the electrical conductivity of such hybrid screen‐printed and plated fingers. This work proposes not only a roadmap but also a promising approach to significantly reduce the Ag demand and benefit sustainable production of industrial screen‐printed silicon solar cells in the TW era.

Ultrafast laser-annealing of hydrogenated amorphous silicon in tunnel oxide passivated contacts for high-efficiency n-type silicon solar cells

The tunnel oxide passivated contact (TOPCon) concept has been the brightest star in the field of emerging passivating contact techniques for the last few years. It has shown great potential in industrial applications due to the overwhelming advantages of high device efficiency and low cost. Here, we introduce a novel crystallization method using ultrafast laser-annealing by scanning a laser spot onto the surface of hydrogenated amorphous silicon film in TOPCon solar cells. By circumventing the high-temperature environment of the conventional annealing process, it can prevent a large number of dopant atoms from penetrating inside the crystalline silicon (c-Si) substrate, reducing the Auger recombination. Moreover, we can conduct extensive experiments to clarify the optimal conditions, including laser-annealing modes and process parameters. The hydrogenation experiments reveal that direct appropriation of the traditional hydrogenation method is not applicable. An additional ‘dehydrogenation’ step proves necessary, indicating that the differences in hydrogen content within the films due to the divergence between the principles of laser-annealing and high-temperature annealing are probably responsible for this. Consequently, the proof-of-concept devices using laser-annealing technology realize a champion efficiency of 19.91%,highlighting an alternative technical route with substantial potential to achieve high-efficiency crystalline silicon solar cells.

Polymer-acid-metal quasi-ohmic contact for stable perovskite solar cells beyond a 20,000-hour extrapolated lifetime

The development of a robust quasi-ohmic contact with minimal resistance, good stability and cost-effectiveness is crucial for perovskite solar cells. We introduce a generic approach featuring a Lewis-acid layer sandwiched between dopant-free semicrystalline polymer and metal electrode in perovskite solar cells, resulting in an ideal quasi-ohmic contact even at elevated temperature up to 85 °C. The solubility of Lewis acid in alcohol facilitates nondestructive solution processing on top of polymer, which boosts hole injection from polymer into metal by two orders of magnitude. By integrating the polymer-acid-metal structure into solar cells, devices exhibit remarkable resilience, retaining 96% ± 3%, 96% ± 2% and 75% ± 7% of their initial efficiencies after continuous operation in nitrogen at 35 °C for 2212 h, 55 °C for 1650 h and 85 °C for 937 h, respectively. Leveraging the Arrhenius relation, we project an impressive T80 lifetime of 26,126 h at 30 °C.

Theoretical limiting‐efficiency assessment on advanced crystalline silicon solar cells with Auger ideality factor and wafer thickness modifications

AbstractWith the improvement of surface passivation, bulk recombination is becoming an indispensable and decisive factor to assess the theoretical limiting efficiency () of crystalline silicon (c‐Si) solar cells. In simultaneous consideration of surface and bulk recombination, a modified model of evaluation is developed. Surface recombination is directly depicted with contact selectivity while bulk recombination is revised on the aspects of ideality factor and wafer thickness. The of the double‐side silicon heterojunction (SHJ) and double‐side tunneling‐oxide passivating contact (TOPCon) solar cells are numerically simulated using the new model as 28.99% and 29.19%, respectively. However, the of single‐side TOPCon solar cells, the more practicable scenario, is only 27.79%. Besides, the of the double‐side SHJ solar cells would exceed the double‐side TOPCon solar cells if the recombination parameter of the non‐contacted area is higher than 0.6 fA/cm2, instead of perfect passivation. Our results are instructive in accurately assessing efficiency potential and accordingly optimizing design strategies of c‐Si solar cells.

High-efficiency TOPCon solar cell with superior P + and P++ layer via one-step processing

The boron diffusion process in the front field of N-type tunnel oxide passivated contact (TOPCon) solar cells is crucial for PN junction formation and the creation of a selective emitter. This study presents a theoretical model of boron diffusion in silicon using molecular dynamics. The research examines the mean square displacement and diffusion coefficient of boron atoms at varying temperatures, confirming their diffusion behavior. The simulations indicate predominant boron diffusion in the z-direction within the silicon matrix, with the diffusion depth being temperature dependent. The optimal temperature range for boron diffusion in silicon is identified as 950 °C to 1050 °C. Using boron-doped silicon paste and boron trichloride as dopants, thermal diffusion experiments were conducted to fabricate the front-field PN junction (p+ layer) and selective emitter (p++ layer) by one step. Subsequent processing and performance evaluation were performed on a production line. Experimental findings reveal a decrease in boron diffusion at higher temperatures, reduced sheet resistance, increased doping concentration, and deeper junction formation. The ideal boron concentration in the p+ layer is 8.68 × 1018 atom/cm3 with a depth of 0.53 μm, while the p++ layer is 2.35 × 1019 atom/cm3 and 0.82 μm. The efficiency of the optimized TOPCon + cell production line reaches up to 25.17 %, marking an improvement of 0.23 % over the standard cell production line. This research contributes to elucidating the mechanism of boron diffusion and offers insights for enhancing the efficiency of TOPCon solar cells.

Hydrogen Sulfide Passivation for p-Type Passivated Emitter and Rear Contact Solar Cells

Hydrogen Sulfide Passivation for p-Type Passivated Emitter and Rear Contact Solar Cells

Extraction of contact resistivity for a partial contact solar cell

Partial contacts are used in solar cells and other electronic devices. The contact resistivity and contact resistance of these partial contacts play a significant role in the performance optimization of these devices. In this work, we propose a strategy for accurate extraction of the partial contact resistivity (ρPC) of a solar cell with Ohmic (or linear) and Schottky (or non-linear) contacts. We demonstrate how the Cox–Strack method can accurately estimate the ρPC of a solar cell using Sentaurus TCAD-based simulations. Our simulation predicts that for linear contact solar cells, ρPC is approximately constant when the contact fraction is varied, whereas for non-linear contact, ρPC decreases with contact fraction as opposed to the previously reported works. The experimentally reported data is also shown to support the claim for the linear contacts further. Despite the larger full-contact resistivity of non-linear contacts, the contact resistivity and fill factor (FF) of both linear and non-linear partial contact solar cell is almost similar to ∼3.5−5mΩcm−2 and ∼ 75% respectively at a 0.3% contact fraction. This is attributed to the reduction in contact resistivity for non-linear contacts with contact fraction. We finally show the generality of this study for any contact material system. This work can be applied to any device with partial contacts.

Implementation of nickel and copper as cost‐effective alternative contacts in silicon solar cells

AbstractEfficient metal contact formation is pivotal for the production of cost‐effective, high‐performance crystalline silicon (Si) solar cells. Traditionally, screen‐printed silver (Ag) contacts on the front surface have dominated the industry owing to their simplicity, high throughput, and significant electrical benefits. However, the high cost associated with using over 13–20 mg/Wp of Ag can impede the development of truly cost‐effective solar cells. Therefore, there is an urgent need to explore alternative, economically viable metals compatible with silicon substrates. This study reports on the application of a contact stack consisting of Ag, nickel (Ni), and copper (Cu) in Si solar cells. To prevent Schottky contact formation, Ag is implemented as a seed layer, whereas Ni and Cu form the metal bulk layer. The fabricated bi‐layer stack without selective emitter exhibits a maximum efficiency of ~21.5%, a fill factor of 81.5%, and an average contact resistance of 5.88 mΩ·cm2 for a monofacial PERC cell. Microstructure analysis demonstrates that the metals within the contacts remain distinct, and Cu diffusion into the silicon during the firing process is absent. Consequently, printed bi‐layer contacts emerge as a promising alternative to Ag contacts, reducing the Ag consumption to below 2.5 mg/Wp per cell without compromising overall efficiency.

Bi-layer in-situ phosphorus doped poly-Si films by PECVD for blistering-free high-efficiency industrial TOPCon solar cells

The passivating contact concept stands out as one of the most promising and industrially viable photovoltaic (PV) technologies. Further improving the quality of physical contact has become a focus of ongoing research. The film blistering issue has been identified as one of the major bottlenecks for the polysilicon (poly-Si) films deposited by the PECVD approach. In this study, we investigated how the in-situ phosphorus (P) doping level within the poly-Si films contributes to the occurrence of blistering. Our investigations into the film blistering mechanisms reveal that a high in-situ P-doping suppresses hydrogen release levels and reduces the accumulation of residual stress during annealing, which leads to the blistering-free appearance, especially observed in heavily P-doped poly-Si films. However, as excessive P-doping could weaken the interfacial passivation quality, we propose a bi-layer structure of P-doped poly-Si films which allows the doping profile to be tailored and maintain good quality passivating contacts. Based on the bi-layer structure, we fabricated industrial-sized tunnel oxide passivated contact (TOPCon) solar cells, which attained an average efficiency of 23.84%. Our work not only presents a promising strategy for improving the performance of passivating contacts via the PECVD approach but also underscores the significant potential for its widespread implementation in industrial TOPCon solar cell manufacturing.

Tracking Hydrogen During Poly-Si/SiOx Contact Fabrication: An Infrared Spectroscopy Analysis of Si–H Bonds Configurations

The hydrogenation step contributing to the high efficiencies (&gt;25%) reached with poly-Si/SiOx passivated contacts solar cells is still poorly understood. In this study, Fourier transform infrared spectroscopy (FTIR) is used to follow the different bonding configurations of H during the fabrication process. The carrier lifetime degradation upon annealing is correlated to an important loss of Si–H bonds, from both the a­‑Si:H film and the SiOx interfaces. The subsequent hydrogenation step results in the formation of a small number of Si–H bonds near the crystalline silicon c-Si/SiOx interface, associated with the low stretching mode (LSM) and correlated to a significant lifetime improvement. These bonds feature a preferential orientation, as shown by polarized measurements.

Aluminum Halide-Based Electron-Selective Passivating Contacts for Crystalline Silicon Solar Cells.

Extensive research has focused on developing wide-bandgap metal compound-based passivating contacts as alternatives to conventional doped-silicon-layer-based passivating contacts to mitigate parasitic absorption losses in crystalline silicon (c-Si) solar cells. Herein, thermally-evaporated aluminum halides (AlX)-based electron-selective passivating contacts for c-Si solar cells are investigated. A low contact resistivity of 60.5 and 38.4 mΩ cm2 is obtained on the AlClx/n-type c-Si (n-Si) and AlFx/n-Si heterocontacts, respectively, thanks to the low work function of AlX. Power conversion efficiencies (PCEs) of 19.1% and 19.6% are achieved on proof-of-concept n-Si solar cells featuring a full-area AlClx/Al and AlFx/Al passivating contact, respectively. By further implementing an ultrathin SiO2 passivation interlayer and a pre-annealing treatment, the electron selectivity (especially the surface passivation) of AlX is significantly enhanced. Accordingly, a remarkable PCE of 21% is achieved on n-Si solar cells featuring a full-area SiO2/AlFx/Al rear contact. AlFx-based electron-selective passivating contacts exhibit good thermal stability up to ≈400 °C and better long-term environmental stability. This work demonstrates the potential of AlFx-based electron-selective passivating contact for solar cells.

Evaluating the Practical Efficiency Limit of Silicon Heterojunction–Interdigitated Back Contact Solar Cells by Creating Digital Twins of Silicon Heterojunction Solar Cells with Amorphous Silicon and Nanocrystalline Silicon Hole Contact Layers

Improving power conversion efficiency (PCE) in photovoltaics has driven innovative approaches in solar cell design and technology. Silicon heterojunction (SHJ) solar cells exhibit advantages in PCE due to their effective passivating contact structures. SHJ–interdigitated back contact (SHJ–IBC) solar cells have the potential to surpass traditional SHJ cells, attributed to their advantage in short‐circuit current (JSC). Herein, Silvaco Atlas technology computer‐aided design is used to create digital twins of high‐efficiency SHJ solar cells with amorphous silicon and nanocrystalline silicon hole selective contact (HSC) layers. Using parameters from digital twins of SHJ solar cells, the practical efficiency limit of SHJ–IBC solar cells is assessed. The results show that SHJ–IBC cells can achieve potential efficiencies of 27.01% with amorphous HSC and 27.38% with nanocrystalline HSC. Further efficiency augmentation to 27.51% can be achieved by narrowing the gap from 80 to 20 μm. This study not only advances comprehension of SHJ–IBC solar cells but also provides insights into optimizing geometrical configurations for improved performance. The utilization of digital twins provides a valuable tool for predicting and evaluating the performance of SHJ–IBC solar cells, contributing substantively to the ongoing development of high‐efficiency photovoltaic technology.

Combined Fabrication and Performance Evaluation of TOPCon Back‐Contact Solar Cells with Lateral Power Metal‐Oxide‐Semiconductor Field‐Effect Transistors on a Single Substrate

Nowadays, an increasing share of photovoltaic (PV) systems makes use of module‐ or submodule‐level power electronics (PE). Furthermore, PE is used in stand‐alone devices powered by PV‐storage solutions. One way to facilitate further implementation of PE in PV applications is to integrate PE components into crystalline silicon PV cells. Herein, the COSMOS device is introduced, denoting COmbined Solar cell and metal‐oxide‐semiconductor field‐effect transistor (MOSFET). Specifically, the combined manufacturing of lateral power MOSFETs and interdigitated back contact solar cells with tunnel‐oxide passivated contacts (TOPCon) on a single wafer is reported. Many steps of the proposed process flow are used for the fabrication of both devices, enabling cost‐effective integration of the MOSFET. Both n‐type solar cells with integrated p‐channel MOSFETs (PMOS) and p‐type solar cells with integrated n‐channel MOSFETs (NMOS) are successfully manufactured. NMOS devices perform better in achieving low on‐resistance, while PMOS devices exhibit lower leakage currents. Furthermore, the study reveals integration challenges where off‐state leakage currents of the MOSFET can increase due to illumination and specific configurations of monolithic interconnections between the MOSFET and the solar cell. Nevertheless, for both n‐type and p‐type solar cells, efficiencies exceeding 20% are achieved, highlighting the potential of the proposed process for COSMOS devices.

First-principles investigation on the impact of copper concentration on zinc telluride as the back contact for cadmium telluride solar cells

First-principles investigation on the impact of copper concentration on zinc telluride as the back contact for cadmium telluride solar cells

Sn‐Doped Nb2O5 Films as Effective Hole‐Selective Passivating Contacts for Crystalline Silicon Solar Cells

Carrier‐selective contacts (CSCs) in crystalline silicon (c‐Si) solar cells have attracted great attention due to suppressing contact region recombination and achieving higher conversion efficiency. Among transition metal oxides for CSCs, niobium oxide (Nb2O5) is considered as an attractive candidate for electron‐selective contact due to its excellent passivation properties and small conduction band offsets with c‐Si. Nevertheless, the performance of c‐Si solar cells employing Nb2O5 contact layer has not been explored yet. Herein, the carrier selectivity of solution‐processed Nb2O5 films is investigated for c‐Si. Interestingly, Nb2O5 exhibits high electron‐blocking performance and low contact resistivities with p‐Si. The ultra‐thin SiOx interlayer formed by UV–O3 pretreatment further reduces the contact resistivities and increases minority carrier lifetime due to the improved contact interface. The Sn4+ doping improves the work function of Nb2O5 to induce larger upward band bending at c‐Si surface, thus enhancing the hole selectivity. As a result, p‐type c‐Si solar cells with solution‐processed Nb2O5 hole‐selective contact layer have achieved the highest power conversion efficiency of 18.4%, with a high‐thermal stability superior to the typical hole transport layers. This work first demonstrates the exceptional hole selectivity of Nb2O5, which shows very promising applications in high‐efficiency c‐Si solar cells.