MBB vs. 0BB Solar Cell Technologies | Shading Tolerance, Silver Paste Consumption, Microcrack Prevention

Multi-Busbar (MBB) and Zero-Busbar (0BB) are two important metallization and interconnection routes in crystalline silicon photovoltaic cell and module manufacturing. They are often discussed together because both routes aim to reduce optical loss, lower series resistance, cut silver consumption, and improve module reliability.

MBB increases the number of busbars from older 3BB or 5BB structures to 9BB, 12BB, 16BB, 18BB or even higher busbar counts. By distributing current collection across more conductive paths, MBB shortens the lateral current transport distance in the fingers and allows each busbar or ribbon to be narrower.

0BB removes the printed busbars from the cell surface. Instead, the current is collected through dense wires, conductive adhesive, SmartWire-type interconnection, welding-plus-adhesive reinforcement, or other busbarless interconnection designs. This can reduce busbar-related shading and silver paste usage, but the final benefit depends on the cell technology, interconnection route, process yield, adhesive or wire cost, and long-term reliability performance.

The key point is that MBB and 0BB are not simple one-to-one replacements. MBB is a mature, stable, and widely deployed production route. 0BB has stronger long-term potential in busbar-related shading reduction and silver reduction, but it also introduces tighter requirements for wire alignment, adhesive control, contact resistance, lamination process, and reliability validation.

The International Technology Roadmap for Photovoltaics describes crystalline silicon PV trends across wafering, cell processing, module manufacturing, and system-level technology development. Recent roadmap discussions identify silver reduction and busbarless interconnection as important industry trends, especially as PV production scales to very large volumes.

Shading Tolerance

Busbar and Contact-Area Shading

In a conventional busbar cell, part of the incoming light is blocked by front-side metal. This optical loss comes from busbars, fine fingers, solder ribbons, wires, and other interconnection structures. Older 3BB and 5BB designs used wider busbars, which increased local shading and current-collection non-uniformity.

MBB reduces this problem by increasing the number of busbars while reducing the width of each busbar or ribbon. The current-collection path becomes shorter, so finger resistance decreases. At the same time, the metal grid must still be optimized because excessive metal coverage increases optical shading.

Front grid optimization studies show that busbar width, finger spacing, and finger geometry strongly affect the balance between optical shading loss and electrical resistance loss. Therefore, an MBB design should not be judged only by the number of busbars; it must be evaluated through total front-grid loss, including both optical and electrical components.

0BB removes printed busbars from the cell surface. This can significantly reduce busbar-related optical blocking, especially when compared with older wide-busbar designs. However, 0BB does not eliminate total metallization shading. Fine fingers, wires, conductive adhesive points, encapsulation geometry, and contact structures still create optical and electrical losses.

For this reason, it is more accurate to say that 0BB reduces busbar-related shading rather than claiming 0BB has "zero shading loss." The final optical gain depends on wire diameter, wire number, finger width, finger pitch, adhesive pattern, glass and encapsulant optical behavior, and cell-to-module integration.

Busbarless cells also require careful measurement. Research on contacting busbarless cells shows that the contacting scheme can influence measured fill factor and series-resistance-related parameters. A fair comparison between MBB and 0BB should therefore use consistent I–V measurement methods and suitable contacting layouts.

Finger Shadow Effect

Fine fingers are necessary because they collect current from the emitter or passivating contact region and deliver it to the busbar, wire, or conductive contact. However, the fingers themselves block part of the incident light. This creates a design trade-off: wider and taller fingers reduce electrical resistance but increase silver use and optical shading; narrower fingers reduce shading and silver use but can increase resistance and printing defects.

In MBB and SMBB designs, the effective length of each finger section is shortened because current is collected more frequently. This makes narrower finger designs possible compared with older low-busbar structures. However, the grid must still maintain sufficient conductivity, adhesion, and continuity under firing, soldering, lamination, and outdoor aging conditions.

0BB can support even more frequent current collection because wires or contact points can be distributed densely across the cell. This can make narrow-finger optimization more attractive. However, if the finger becomes too narrow or discontinuous, the finger-to-wire or finger-to-adhesive contact may become unstable, causing higher contact resistance or local current-collection non-uniformity.

The practical target is not the minimum finger width, but the lowest combined loss. A good grid design must balance finger shading, finger resistance, contact resistance, paste transfer, printability, yield, and long-term reliability.

Optical and Electrical Loss Pathways

Beyond direct busbar and finger shading, metallization and interconnection create several additional loss pathways:

• Optical blocking from busbars, ribbons, wires, conductive adhesive areas, and fine fingers.

• Series resistance loss in fingers, busbars, wires, solder joints, and interconnection paths.

• Contact resistance between the printed finger and the solder, wire, or conductive adhesive.

• Cell-to-module loss caused by encapsulation, glass, spacing, current mismatch, and interconnection geometry.

• Reliability-related electrical non-uniformity caused by broken fingers, cracked cells, weak bonds, or aged contacts.

Electroluminescence (EL) imaging is useful for detecting and interpreting defects such as cracks, inactive regions, broken cells, interrupted fingers, and interconnection problems. However, EL images should not be treated as direct optical-shading measurements. Optical loss should be evaluated through optical modeling, reflectance or current measurements, and controlled module performance testing.

0BB can reduce several busbar-related optical loss pathways, but it increases the importance of contact uniformity. Wire alignment, adhesive thickness, lamination temperature, finger continuity, and bond strength can all affect current collection. If these parameters are not well controlled, the busbarless design may lose part of its theoretical advantage.

Published research on busbarless multi-wire interconnection shows that the number of wires, wire diameter, and contact condition between the wire and the finger electrode must be optimized together. Increasing wire number can reduce finger resistance, but it can also increase shading. Increasing wire diameter can reduce series resistance, but it may also increase optical blocking. This confirms that 0BB performance depends on the full interconnection design rather than on busbar removal alone.

Silver Paste Consumption

Busbar Silver Usage

Silver paste is one of the most important non-silicon cost items in crystalline silicon solar cells. It is used to form conductive front and rear electrodes, depending on the cell technology. Because silver is expensive and supply-sensitive, reducing silver consumption has become a major direction for MBB, SMBB, 0BB, copper plating, silver-coated copper paste, and other metallization technologies.

In older busbar structures, a meaningful part of front-side silver paste was consumed in the busbar area. As the industry moved from 3BB and 5BB to MBB and SMBB, each busbar became narrower and current collection became more distributed. This helped reduce busbar-related paste use while improving electrical performance.

Modern silver consumption should be discussed by cell technology rather than by a single universal "mg/cell" number. PERC, TOPCon, HJT, and XBC use different metallization materials and different front/rear electrode designs. Wafer size, cell power, finger width, finger height, screen design, paste formulation, and interconnection route all affect the final silver consumption.

According to the China Photovoltaic Industry Association's 2024–2025 roadmap, in 2024 p-type cells mainly used 10BB or above, including 11BB and 16BB designs, with front-side silver consumption reduced to about 52 mg/cell and rear-side silver consumption about 22 mg/cell. The same roadmap reports that n-type TOPCon cells used about 86 mg/cell of double-sided silver paste on average, HJT cells used about 75 mg/cell of double-sided low-temperature silver paste, and XBC cells used about 135 mg/cell.

0BB removes the printed busbar, so it can reduce the busbar-related portion of silver paste. This advantage is clearer when the baseline process uses relatively high busbar silver laydown. In routes where the MBB or SMBB baseline has already reduced busbar paste significantly, the additional silver saving from 0BB may be smaller.

0BB is especially important in HJT cost-reduction roadmaps because HJT historically relied on low-temperature silver paste. Combining 0BB with silver-coated copper paste or lower-silver-content paste can reduce silver cost more effectively than simply changing busbar count. In TOPCon, the cost benefit is still meaningful, but it is more sensitive to adhesive cost, equipment depreciation, yield, and process maturity.

Finger Silver Paste Usage

Finger silver paste normally accounts for the major share of front-side metallization consumption. Reducing finger width and height can lower silver use, but the finger still needs enough cross-sectional area to carry current with acceptable resistance.

In MBB and SMBB cells, the current path within each finger becomes shorter. This allows finger narrowing compared with older low-busbar designs. However, narrow fingers require tighter control of screen printing, paste rheology, firing, contact formation, and finger continuity.

In 0BB cells, the absence of printed busbars can further support fine-line metallization because current can be collected by more frequent wire or adhesive contact points. However, the finger-to-wire or finger-to-adhesive interface becomes more critical. If the finger is too thin, broken, or poorly contacted, the 0BB design may suffer from increased contact resistance or local current mismatch.

Therefore, it is not accurate to state that 0BB always reduces total silver consumption by a fixed percentage. The reduction depends on the original busbar design, cell technology, paste system, finger geometry, interconnection route, and yield. In mature TOPCon baselines, the incremental reduction may be modest. In HJT routes using silver-coated copper and 0BB interconnection, the reduction potential is usually more attractive.

Per-Cell and Per-Watt Cost

A correct cost comparison should include both cell-side and module-side effects. Removing busbars reduces silver paste use, but 0BB may add cost through conductive adhesive, wires, embedded films, lamination requirements, equipment changes, extra inspection, and yield ramp-up.

At a silver paste price of 6–8 yuan/g, every 10 mg/cell reduction in silver-bearing paste corresponds to about 0.06–0.08 yuan/cell before other process costs are considered. This is only a rough material-saving estimate. It should not be treated as net cost reduction.

Industry estimates often show that 0BB can reduce silver paste cost more clearly in HJT than in TOPCon. However, the final net saving depends on whether the extra adhesive, film, equipment, and yield costs remain below the gross silver-saving benefit.

For this reason, the most reliable way to compare MBB and 0BB is to use a full bill-of-material and process-cost model. The comparison should include silver paste, adhesive, wire, encapsulation film, equipment depreciation, throughput, yield, reliability screening, module power gain, and warranty risk.

In short, 0BB should not be promoted solely as "removing busbar silver." Its value comes from a combined effect: lower busbar-related shading, possible silver reduction, better current collection density, compatibility with low-silver paste, and potential improvement in thin-wafer stress control. These benefits must be weighed against process complexity and reliability risk.

Microcrack Prevention

Stress Hot Spots

Microcracks are a major reliability concern in crystalline silicon PV modules. They can originate from wafer cutting, cell handling, soldering, stringing, lamination, transport, installation, mechanical loading, or long-term thermal cycling. Once a crack electrically isolates part of a cell, it can reduce power output and increase hot-spot risk.

In MBB and SMBB modules, soldered ribbons or wires connect the cell electrodes. During soldering and cooling, thermal expansion mismatch between silicon, copper, solder, metallization paste, and encapsulant can create localized thermal-mechanical stress. This stress is especially important for thinner wafers, half-cut cells, large-format cells, and modules exposed to strong mechanical load.

However, it is not correct to describe all MBB modules as having high microcrack risk. MBB and SMBB are mature processes, and well-optimized soldering profiles, ribbon geometry, cell handling, and lamination conditions can keep crack risk under control. The main concern is localized stress concentration around soldered contact regions and cell edges.

0BB can reduce some soldering-related stress when implemented with conductive adhesive, SmartWire-type interconnection, or low-temperature bonding. This can be helpful for ultra-thin wafers and flexible-module designs. But this advantage depends on the specific 0BB route. Some 0BB routes still include welding or soldering steps, so 0BB should not be described as universally "low temperature" or "solder-free."

Current 0BB approaches include SmartWire, dispensing-lamination, and welding-inspection-dispensing routes. Each route has different advantages and risks. SmartWire can distribute current collection more uniformly but may increase material and process complexity. Dispensing-lamination can simplify the busbarless structure but requires stable adhesive control. Welding-plus-dispensing can improve contact strength but may still introduce thermal stress.

Soldering and Bonding Damage Risk

For MBB, the main process risks are soldering temperature, heating rate, cooling rate, ribbon stiffness, ribbon thickness, solder wetting, cell edge quality, and residual wafer stress. Excessive local stress can lead to cracks, broken fingers, weak solder joints, or long-term fatigue under thermal cycling.

For 0BB, the main process risk shifts from busbar soldering to contact quality. Adhesive curing, wire-to-finger contact, lamination pressure, lamination temperature, adhesive aging, peel strength, and contact resistance become the key control points.

Research on busbarless multi-wire interconnection with embedded electrode sheets shows that lamination temperature can significantly influence the contact condition between wire and finger electrode. The study reported that contact performance and module efficiency depended on the process temperature, confirming that busbarless interconnection must be optimized as a complete contact system rather than as a simple busbar-removal step.

This also means that 0BB reliability cannot be judged only by the absence of printed busbars. A stable 0BB process must maintain low contact resistance, sufficient bond strength, uniform adhesive or wire contact, good damp-heat resistance, and acceptable mechanical-load performance.

EL imaging is useful for identifying cracks, broken cells, interrupted fingers, inactive cell areas, and interconnection defects. IEC TS 60904-13 specifies methods for capturing electroluminescence images of PV modules, processing image metrics, and providing qualitative interpretation guidance. Therefore, EL is appropriate for defect detection and reliability screening, but it should not be used as direct evidence of optical shading loss.

Mechanical Load Impact

PV modules experience static and dynamic mechanical loads during transportation, installation, and outdoor operation. Wind pressure, snow load, vibration, frame deformation, clamp position, and glass-glass or glass-backsheet structure all affect how stress is transferred to cells and interconnects.

MBB and SMBB modules benefit from mature soldered interconnection and established process control. However, soldered ribbons can create local stress concentration, especially when the module bends under mechanical load. The risk becomes more important as wafers become thinner and cell formats become larger.

0BB modules can distribute current collection across many wires or contact points, which may reduce some local stress concentration. At the same time, the wire-finger or adhesive-finger interface becomes a new reliability-critical region. If the contact area is too small, the adhesive ages poorly, or the wire alignment is unstable, the module may experience contact fatigue or local resistance increase under cyclic load.

IEC TS 62782 provides a test method for cyclic dynamic mechanical load testing of PV modules. The test applies alternating positive and negative uniform load normal to the module surface, and the specification describes 1,000 cycles at a maximum pressure of ±1,000 Pa with a tolerance of ±100 Pa and a rate between 3 and 7 cycles per minute.

Static mechanical load testing is also used to evaluate PV modules under wind and snow load conditions. IEC 61215-type design qualification is intended to evaluate whether modules are suitable for long-term outdoor operation, but passing qualification tests should not be interpreted as a direct prediction of exact field lifetime.

For 0BB, increasing adhesive coverage or improving wire contact can enhance mechanical reliability, but it may also increase material cost and optical blocking. For high-snow-load, high-wind, or transport-sensitive applications, a wider or stronger contact design may be justified. For standard rooftop applications, a narrower contact design may be sufficient if it passes thermal cycling, damp heat, dynamic mechanical load, and EL inspection requirements.