Contact Solar Cells Research Articles

Band Alignment and Surface Photo-Voltage Analysis of C-Si/Siox/Poly-Silicon Passivating-Contact Stacks Through X-Ray Photoelectron Spectroscopy.

Ultrathin SiOx layers and c-Si/SiOx interfaces find application in tunnel-oxide passivated contacts (TOPcon) for high-efficiency silicon solar cells. Here, we investigate their detailed microscopic properties, with specific attention for the case of c-Si(100) substrates, capped either by p-type or n-type poly-silicon layers [c-Si/SiOx/poly-Si (p+) or c-Si/SiOx/poly-Si (n+)]. Our focus is on the effects of the substrate preparation conditions (either by a dry-plasma or wet SiOx process) and the high-temperature annealing step (as required for the poly-Si crystallization) on the SiOx stoichiometry and its microscopic structure. Through advanced photoemission techniques, we find a clearly decreased valence band offset between the c-Si and SiOx (from 4.5 eV to 4.15 eV) when comparing the dry SiOx with the wet SiOx process, independent of the SiOx film thickness, but correlating with the relative fraction of sub-stochiometric Si states. We lastly examine the magnitude of band-bending of the contact structure through controlled in-situ exposure to light of the surfaces and subsequent tracking of core and valence band levels via a surface photovoltage and a junction photo-voltage (JPV) effect. By analyzing this JPV effect qualitatively, we find it to be proportional to the expected quasi fermi level splitting within the c-Si wafer.

Ultrathin Self-Assembled Monolayer for Effective Silicon Solar Cell Passivation.

Passivation technology is crucial for reducing interface defects and impacting the performance of crystalline silicon (c-Si) solar cells. Concurrently, maintaining a thin passivation layer is essential for ensuring efficient carrier transport. With an ultrathin passivated contact structure, both Silicon Heterojunction (SHJ) cells and Tunnel Oxide Passivated Contact (TOPCon) solar cells achieve an efficiency surpassing 26%. To reduce production costs and simplify solar cell manufacturing processes, the rapid development of organic material passivation technology has emerged. However, its widespread industrial production is hindered by environmental safety concerns, such as strong acid corrosion and biological and ecological safety issues. Here, we discovered a low-cost self-assembled monolayer (SAM) hole-selective transport material known as 2PACz ([2-(9H-carbazol-9-yl) ethyl] phosphonic acid) with phosphate groups to form c-Si solar cells for the first time. The ultrathin film of 2PACz with phosphate groups can establish strong and stable P-O-Si bonds on the silicon surface. Meanwhile, like 2PACz, a uniform ultrathin film with a carbazole function group can offer electron-localizing and thus hole-selective properties, which provides ideas for studying dopant-free silicon solar cells. As a result of such interfacial passivation engineering, it plays an important role in repairing porous structures, such as pyramid-textured silicon surfaces, and cutting losses during the commercialization of c-Si solar cells. Crucially, this advancement offers insights for the development of new high-efficiency ultrathin film passivation methods in the postsilicon era.

Tunnel silicon oxynitride phase transformation for n-type polysilicon passivating contacts in crystalline silicon solar cells

The study investigates the fabrication and characterization of tunnel oxynitride (TON) layers produced using Plasma-Enhanced Chemical Vapor Deposition (PECVD) at different pressure and power variations. The aim is to explore TON layers as potential substitutes for conventional SiO2 in Tunnel Oxide Passivated Contact (TOPCon) solar cells. Foe this purpose, an effective oxide thickness (EOT) of 1.1–1.2 nm and 1.5–1.6 nm has been achieved by treating the TON samples with NH3 and N2 gas plasma, respectively. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed critical variations in the chemical bonding within TON layers, which exhibit different properties based on the deposition conditions. The presence of nitrogen-nitrogen and nitrogen–hydrogen bonds is prominently influenced by changes in the PECVD parameters, directly affecting the layer’s optical and electronic properties. Notably, the presence of hydroxyl (O–H) and amino (NH2) groups within the TON structure suggests an improvement in surface passivation, essential for optimizing solar cell performance and durability. A lifetime of 737 and 823 μs has been recorded for N2 and NH3 samples, compared to the NAOS 710 μs reference sample. NH3 plasma-treated samples show an improved iVoc of 736 mV, indicating better phase transformation and passivation quality than N2 plasma treatment and the NAOS reference sample.

Effect of annealing on MoOx and interface properties for carrier-selective contact solar cells with different passivation layers

The structural, electrical and optical properties of molybdenum oxide (MoOx) thin films prepared by magnetron sputtering are studied with annealing temperature. The interface properties of silicon with the intrinsic amorphous silicon (a-Si:H(i)) and alumina (AlOx) passivation layers are investigated by the minority carrier lifetime and implied-Voc (iVoc). The AlOx shows the better passivation properties and thermal stability for the carrier-selective contact solar cells. The Ce-doped In2O3(ICO) is deposited on the surface of MoOx thin films. It is found that the optimized annealing temperature is 125 °C for the carrier selective contact solar cells with the a-Si:H(i) passivation layer. It could increase to 175 °C for the AlOx passivation layer.

Bottom Contact Engineering for Ambient Fabrication of >25% Durable Perovskite Solar Cells.

The bottom contact in perovskite solar cells (PSCs) is easy to cause deep trap states and severe instability issues, especially under maximum power point tracking (MPPT). In this study, sodium gluconate (SG) is employed to disperse tin oxide (SnO2) nanoparticles (NPs) and regulate the interface contact at the buried interface. The SG-SnO2 electron transfer layer (ETL) enabled the deposition of pinhole-free perovskite films in ambient air and improved interface contact by bridging effect. SG-SnO2 PSCs achieved an impressive power conversion efficiency (PCE) of 25.34% (certified as 25.17%) with a high open-circuit voltage (VOC) exceeding 1.19V. The VOC loss is less than 0.34V relative to the 1.53eV bandgap, and the fill factor (FF) loss is only 2.02% due to the improved contact. The SG-SnO2 PSCs retained around 90% of their initial PCEs after 1000h operation (T90 = 1000h), higher than T80 = 1000h for the control SnO2 PSC. Microstructure analysis revealed that light-induced degradation primarily occurred at the buried holes and grain boundaries and highlighted the importance of bottom-contact engineering.

Silicon heterojunction back contact solar cells by laser patterning.

Back contact silicon solar cells, valued for their aesthetic appeal by removing grid lines on the sunny side, find applications in buildings, vehicles and aircrafts, enabling self-power generation without compromising appearance1-3. Patterning techniques arrange contacts on the shaded side of the silicon wafer, offering benefits for light incidence as well. However, the patterning process complicates production and causes power loss. Here we employ lasers to streamline back contact solar cell fabrication and enhance power conversion efficiency. Our approach produces the first silicon solar cell to exceed 27% efficiency. Hydrogenated amorphous silicon layers are deposited on the wafer for surface passivation and collection of light-generated carriers. A dense passivating contact, diverging from conventional technology practice, is developed. Pulsed picosecond lasers at different wavelengths are used to create back contact patterns. The developed approach is a streamlined process for producing high-performance back contact silicon solar cells, with a total effective processing time of about one-third that of emerging mainstream technology. To meet terawatt demand, we develop rare indium-less cells at 26.5% efficiency and precious silver-free cells at 26.2% efficiency. The integration of solar solutions in buildings and transportation is poised to expand with these technological advancements.

Unveiling the origin of metal contact failures in TOPCon solar cells through accelerated damp-heat testing

Tunnel oxide passivated contact (TOPCon) solar cells are expected to dominate the global photovoltaic market in the coming decade thanks to rapid advancements in power conversion efficiency (PCE). However, there are concerns about the reliability of TOPCon modules, particularly in hot and humid conditions. The current module-level fundamental analysis strategies for TOPCon solar cells provide too slow feedback for rapid process development. This study explores the degradation of metal contacts in TOPCon solar cells under accelerated testing conditions of 85 °C and 85 % relative humidity (DH85). The degradation was induced by two commonly used sodium-related salts, sodium bicarbonate (NaHCO3) and sodium chloride (NaCl), in the testing of the solar cells. When applied to the front side, NaHCO3 caused a ∼5%relPCE reduction after 100-h DH85 exposure, while NaCl leads to a more significant ∼92%relPCE reduction. The primary cause of degradation is a considerable increase in series resistance (Rs), likely due to electrochemical reactions within the Ag/Al paste. When the salts are applied to the rear of the TOPCon solar cell, the degradation becomes more complex. NaHCO3 increases recombination and results in a deterioration of the contact, resulting in a ∼16%relPCE reduction after 100-h DH85 testing. Conversely, NaCl primarily causes a decline in open-circuit voltage (Voc) and a ∼4%relPCE loss. This manuscript primarily investigates degradation mechanisms related on the rear side, with a focus on significant oxidation occurring at the interface between Ag and Si. These findings highlight the susceptibility of TOPCon solar cells to contact corrosion, emphasizing the electrochemical reactivity of metallisation as a potential risk for long-term TOPCon module operation. This study provides crucial insights into TOPCon cell degradation mechanisms, which are essential for optimising performance and enhancing the long-term reliability of TOPCon modules.

Optimizing strategy of bifacial TOPCon solar cells with front-side local passivation contact realized by numerical simulation

The tunnelling oxide passivation contact (TOPCon) solar cells have been impressive in the global photovoltaic (PV) market originating from their high efficiency and stability. However, it exhibits significant recombination losses due to its boron diffusion, laser damage and metal-semiconductor contact on front side. The bifacial TOPCon structure demonstrates massive potential in the improvement of passivation and contact performance with the premise that it can solve the parasitic absorption of polycrystalline silicon (poly-Si). In this study, the localized poly finger structure with excellent optics and passivation performance is designed in the front side of bifacial solar cells to compare with traditional TOPCon and full-area poly passivation devices. The theoretical efficiency and detailed power loss analysis in our simulation reveal that suppressing the recombination of FSF (front surface field) and the contact area is the crucial strategy to improve device performance, with optimized efficiency of 26.62 % and FF of 85.16 %. These results indicate that the route of BJ (back junction) structure containing localized selective contact and full coverage high-quality passivation holds potential in realizing both high Jsc and Voc for FBC (front and back contact) solar cells, featuring great instructive significance for future industrialization of PV production.

Boron‐Doped Zinc Oxide Electron‐Selective Contacts for Crystalline Silicon Solar Cells with Efficiency over 22.0%

The exploration of wide‐bandgap metal compound films with excellent passivation and contact properties on crystalline silicon (c‐Si) surface, as alternatives to traditional‐doped Si thin films, holds significant promise for the future enhancement of c‐Si solar cell efficiency. Herein, conductive boron‐doped zinc oxide (ZnO:B) films are deposited by atomic layer deposition (ALD) process and investigated as electron‐selective contacts, in combination with a thin SiOx passivating interlayer. This combination demonstrates a relative low contact resistivity of ≈2 mΩ cm2 and improved passivation quality. Further application of this SiOx/ZnO:B stack as a full‐area electron‐selective passivating contact in proof‐of‐concept n‐type c‐Si solar cells results in a satisfactory power conversion efficiency of over 22.0%. This electron‐selective passivating contact structure, prepared via low‐temperature, simplified, and the compositionally controlled ALD process, offers a promising pathway for the development of high‐efficiency and low‐cost c‐Si solar cells.

Enhancing Reliability and Regeneration of Single Passivated Emitter Rear Contact Solar Cell Modules through Alternating Current Power Application to Mitigate Light and Elevated Temperature‐Induced Degradation

The study explores a novel method to combat the Light and Elevated Temperature‐Induced Degradation (LeTID) in solar cell modules, which significantly reduces their efficiency and lifespan. This method involves applying alternating current (AC) of various waveforms (triangular, sinusoidal, and square) and frequencies (5 and 100 kHz) to boron‐doped p‐type passivated emitter rear contact (p‐PERC) solar cell modules. This approach effectively lowers the series resistance at the critical junction between the silver (Ag) contact and the silicon emitter layer of the PERC solar cell, thereby reducing charge recombination hindered by high resistance, especially at elevated temperatures. As a result, there is an improved flow of electrical charges, leading to decreased energy loss and increased solar cell efficiency. The study's findings indicate that a slow, smooth sinusoidal AC waveform at 100 kHz is particularly effective, restoring about 100% of the original performance of the panel. Moreover, oscillations at 5 kHz also show considerable efficacy, recovering more than 96% of the performance. The sinusoidal waveform is noted to surpass both triangular and square waveforms in recovery efficiency. This research highlights the use of high‐frequency AC electricity as a viable strategy to extend the lifespan and enhance the performance of solar panels.

Sulfur-enhanced surface passivation for hole-selective contacts in crystalline silicon solar cells

Sulfur-enhanced surface passivation for hole-selective contacts in crystalline silicon solar cells

Q.ANTUM NEO with LECO Exceeding 25.5 % cell Efficiency

In recent years, we have developed the Q.ANTUM NEO solar cell, a rear-passivated double-sided contact solar cell that achieves power conversion efficiencies exceeding 25.5 %. Our streamlined and cost-efficient process leverages the proprietary Laser Enhanced Contact Optimization (LECO) technology. The LECO process has undergone extensive optimization and has been successfully implemented in both our pilot line and mass production facilities, resulting in significant improvements in solar cell efficiency. LECO facilitates a novel formation of the contact interface between the silicon surface and metallization, leading to remarkably low j0,met values (as low as 160 fA cm−2). The contact resistivity of 20 μm wide contacts on a high sheet resistance emitter is below 1 mΩ cm2, with minimal impact on fill factor (FF) and overall cell efficiency. Rigorous reliability tests, including TC600 and DH3000, demonstrate module degradation well below 5%rel. Our champion cell conversion efficiency achieved in the pilot line using LECO technology reaches 25.6 %, accompanied by a notable open-circuit voltage exceeding 730 mV.

Hydrogenation by catalytically generated atomic hydrogen for passivating contacts in crystalline silicon solar cells

Hydrogenation employing catalytically generated atomic hydrogen (Cat-H) is implemented on stacks of n-type polycrystalline silicon (poly-Si)/Si oxide (SiOx) to enhance passivation quality. The utilization of Cat-H demonstrates a substantial enhancement in surface passivation quality on crystalline Si (c-Si), resulting in an increase of the effective minority carrier lifetime (τeff) from 2.0 to 3.9 ms. The investigation reveals a dependency of τeff on the duration of Cat-H treatment: an initial rise followed by a plateau or slight decline. Secondary ion mass spectrometry (SIMS) measurements revealed the diffusion of H atoms into poly-Si and c-Si, which accumulate at the poly-Si/c-Si interface. This accumulation terminates dangling bonds in the poly-Si layer on the c-Si surface. Furthermore, the presence of an ultra-thin thermal Si oxide layer on the poly-Si layer formed during high-temperature annealing is crucial for protecting the film against etching induced by Cat-H.

22.56% total area efficiency of n-TOPCon solar cell with screen-printed Al paste

Screen-printing Ag paste on the rear side of the Tunnel Oxide Passivated Contact solar cells (TOPCon) is still the mainstream method to form electrodes. However, the high price of precious metals increases the cost of TOPCon cells. Al is a cheap and high-performance conductor, and its work function is compatible with the rear side of TOPCon cells. In this study, we used a UV pulse laser (355 nm, 10 ps) to ablate the rear side SiNx passivation layer of TOPCon cells and then printed Al paste with different weight percentages of silicon (Si-wt.%) on the phosphorus-doped polycrystalline silicon (n+-poly-Si) layer to reduce the cost. We investigated the damage mechanism of laser on SiOx/n+-poly-Si/SiNx passivation structure and the contact mechanism between Al paste and n+-poly-Si layer. The results show that the Voc of SiOx/n+-poly-Si/SiNx passivation structure reduced about 6–7 mV when the laser energy density was 0.431 J/cm2. We got the best electrical performance of TOPCon cells printed with Al paste (25 wt%-29 wt% silicon). The open-circuit voltage (Voc) and the conversion efficiency (Eff), have reached 663.60 mV and 22.56 % respectively. Although the efficiency was 9.40 % relative lower than that of TOPCon cells printed with Ag paste, but the cost of Al paste only accounted for 10 % of Ag paste. In the future, the highest Eff of TOPCon solar cells printed with Al paste on the rear side are expected to reach the commercial TOPCon based on Ag paste, which applies laser doping selective emitter technology (SE) and bifacial poly-Si passivation structure.

Advancing nickel seed layer electroplating for enhanced contact and passivation performance in TOPCon solar cells

Electroplating copper technology offers advantages such as low cost, good conductivity, and a simple process, making it a promising solution in the photovoltaics (PV) field. However, despite its effectiveness in reducing silver consumption, a significant challenge arises from suboptimal electrical contact between metal electrodes and the polycrystalline silicon films, limiting the application of electroplating techniques in tunnel oxide passivated contact (TOPCon) solar cells (SCs). In this study, we propose a simple method to enhance the quality of electrodeposited metals and improve contact performance with polycrystalline silicon films by lowering the electrodepositing temperature for nickel seed layer. Additionally, we optimize contact properties by modulating nickel deposition time and annealing temperature. This process results in excellent physical and electrical contact performance, achieving the lowest contact resistivity of 0.19 mΩ cm2 and the improved adhesion strength. Ultimately, TOPCon SCs using low-temperature plated seed nickel and copper metal electrodes achieve an efficiency of 23.90%, which is attributed to advance the application of optimal nickel seed layer electroplating.

Synthesis and simulation study of titanium dioxide-based nanomaterial for electron carrier selective contact solar cell

This research introduces a novel approach to synthesizing titanium dioxide (TiO2) nanomaterials using the sol–gel method, specifically aimed at enhancing the performance of Silicon Heterojunction (SHJ) solar cells. The innovation lies in utilizing Iso-Propanol Alcohol and Titanium Tetra Isopropoxides for synthesis, leveraging TiO2’s wide bandgap and low work function to replace the conventional n-doped amorphous silicon layer. This approach is expected to overcome the limitations of traditional materials and significantly improve electron transport efficiency. Advanced characterization techniques were employed to assess the synthesized TiO2 nanomaterials. Atomic Force Microscopy (AFM) measured the roughness and 3D profile, while UV-V spectrophotometry evaluated the absorption properties. The structure and surface morphology were meticulously analyzed using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). These comprehensive analyses ensured a detailed understanding of the nanomaterials’ physical properties. Simulations conducted with the AFORS-HET simulator revealed that TiO2, with a refractive index compatible with a-Si:H(n) and a work function of 4.6 eV, significantly enhances SHJ solar cell performance. The results demonstrated a remarkable 22.57 % improvement in efficiency, achieving an open-circuit voltage (Voc) of 716.8 mV, a short-circuit current density (Jsc) of 37.71 mA/cm2 and a fill factor (FF) of 83.49 %. These findings underscore the potential of TiO2 nanomaterials to revolutionize SHJ solar cell technology by providing a more efficient and effective electron transport layer. Future research will focus on refining the nanostructure of TiO2 further to enhance its photovoltaic applications, aiming to push the boundaries of current solar cell efficiencies even further. This study marks a significant advancement in the field of photovoltaic materials, presenting a novel and promising solution to improving solar energy conversion.

Fabrication of Thermally Evaporated CuIx Thin Films and Their Characteristics for Solar Cell Applications

Carrier-selective contacts (CSCs) for high-efficiency heterojunction solar cells have been widely studied due to their advantages of processing at relatively low temperatures and simple fabrication processes. Transition metal oxide (TMO) (e.g., molybdenum oxide, vanadium oxide, and tungsten oxide) thin films are widely used as hole-selective contacts (HSCs, required work function for Si solar cells > 5.0 eV). However, when TMO thin films are used, difficulties are faced in uniform deposition. In this study, we fabricated a copper (I) iodide (CuI) thin film (work function > 5.0 eV) that remained relatively stable during atmospheric exposure compared with TMO thin films and employed it as an HSC layer in an n-type Si solar cell. To facilitate efficient hole collection, we conducted iodine annealing at temperatures of 100–180 °C to enhance the film’s electrical characteristics (carrier density and carrier mobility). Subsequently, we fabricated CSC Si solar cells using the annealed CuIx layer, which achieved an efficiency of 6.42%.

Transparent Hole‐Selective Molybdenum Oxide Passivating Contact with Chlorine‐Based Interlayer Enabling 22.5% Efficient Silicon Solar Cells

The need to increase transparency in existing passivating contacts for crystalline silicon solar cells has motivated the development of transparent contacts based on transition metal oxides (TMOs). Among hole‐selective materials, molybdenum oxide (MoOx) has achieved the greatest success so far. However, despite providing low contact resistivity, MoOx relies on an intrinsic hydrogenated amorphous silicon (a‐Si:H(i)) interlayer to achieve high levels of surface passivation and thus high open‐circuit voltage at a device level, partially defeating the objective of improved transparency. Herein, we report unprecedented performance for a‐Si:H‐free MoOx‐based contacts by employing an alternative passivating interlayer based on a well‐engineered chlorine‐containing Al‐alloyed titanium oxide/titanium dioxide (AlyTiOx/TiO2 )stack. The resulting AlyTiOx/TiO2/MoOx stack achieved record levels of passivation, reaching J0 values as low as 16 fA cm−2, closer to values reported for a‐Si:H‐based contacts, while maintaining lower contact resistivity, well below 100 mΩ cm−2. Additionally, the stack presents improved transparency compared to a‐Si:H‐based contacts, with gains in short‐circuit current density of at least 0.8 mA cm−2. The work pushes the performance of hole‐selective passivating contacts based on TMOs to new levels, enabling a record efficiency of 22.53% for cells with fully transparent hole‐selective passivating contacts. This work serves as an important stepping stone toward low‐thermal‐budget, simple manufacturing of high‐efficiency solar cells.

Optimization strategies for metallization in n-type crystalline silicon TOPCon solar cells: Pathways to elevated fill factor and enhanced efficiency

In advancing photovoltaic technology, optimizing the metallization process is crucial for balancing electrical conductivity and optical performance in solar cell fabrication. This process directly impacts the efficiency and quality of solar cells, traditionally measured by the fill factor (FF). Historically, efforts have focused on evolving metal contacts to reduce optical shading and series resistance, which degrade solar cell efficiency. Our study enhances n-type Tunnel Oxide Passivated Contact (n-TOPCon) solar cells by optimizing screen-printing metallization, particularly by examining the effects of squeegee speeds. Employing a mix of experimental and analytical methodologies, we aimed to identify optimal conditions that improve electrical and optical performance, thereby elevating cell efficiency. Our findings indicate that a squeegee speed of 170 mm/s substantially boosts solar cell performance, evidenced by a current density (Jsc) of 38.96 mA/cm2, open-circuit voltage (Voc) of 684.29 mV, fill factor (FF) of 78.77 %, and a power conversion efficiency (PCE) of 21.00 %. Further, dark I–V measurements confirmed a shunt resistance (Rsh) of 6.25 × 106 Ω and a reduced series resistance (Rs) of 6.48 Ω, underscoring the significance of precise metallization in reducing resistive losses and enhancing efficiency. Future research will explore innovative materials and cutting-edge printing techniques beyond squeegee speed adjustments. The potential incorporation of nanomaterials and conducting polymers aims to refine the metallization process further, promising to push the boundaries of efficiency and cost-effectiveness. This progression is essential for advancing n-TOPCon solar cell development, setting new industry standards, and propelling the sustainable energy movement.

Continuous distribution of interface states in n-type double-side poly-Si/SiOx passivating contact solar cells

Advanced passivation contact in high-efficiency silicon solar cells plays an important role for the sake of minimizing recombination losses. A stack of heavily doped polycrystalline silicon (poly-Si) and tunnel SiOx contact has attracted much attention, benefitting from its excellent characteristics of carrier selectivity and passivation, and which has been successfully applied in double-side poly-Si/SiOx passivating contact solar cells. Note that characteristics analysis of defects at tunnel SiOx/c-Si (crystalline silicon) interface remains an issue of concern. Herein, we observe capacitance transient spectroscopy arise from both electron and hole traps in the passivating contact structure of poly-Si(p+)/tunnel SiOx/c-Si(n). Subsequently, we propose a skillful procedure of deep-level transient spectroscopy (DLTS) measurement to confirm that the observed electron and hole traps are located at the tunnel SiOx/c-Si interface but not in the bulk of the c-Si substrate. Finally, we show a clear physical picture of continuous energy distribution for interface states.