Product Introduction

Shading is one of the most common problems in real-world PV installations.

Tree shadows, utility poles, dust, bird droppings, snow, even uneven mounting angles can all cause partial shading. Shading not only cuts a module's output, it can also trigger a more serious problem: hot spots.

Lately BC solar cells have drawn a lot of attention in distributed rooftops, balcony PV, and premium modules. One big reason: BC cells usually handle shading better, and they run at lower hot-spot temperatures under shading.

At SNEC, you often see vendors shade part of a cell and then show off the shading tolerance of their BC products by watching how high a water pump can spray.

So why do BC cells have this advantage? What's the physics behind it?

Let's try to explain it in plain language.

Why does shading cause hot spots?

Cells in a PV module are usually wired in series.

Series circuits have one key trait: the current must be the same everywhere.

That means the current through the whole string is set by the series loop together. When every cell gets full light, each one generates power and they all behave pretty consistently.

But if one cell gets shaded, the photo-generated current it can produce drops. If the string still needs to push a larger current through, that shaded cell can be forced into reverse bias by the other unshaded cells. At that point it stops being a generator and turns into a power-consuming element.

For partial shading, the shaded cell isn't completely dead. The unshaded part still makes some photo-current. So what actually has to flow through the reverse breakdown path, leakage path, or bypass path isn't the full string current, but the difference between the string current and the current that cell can still produce.

We can call this difference the mismatch current:

Imismatch = Istring - Igenerate

So the hot-spot heating power can be roughly written as:

Photspot ≈ ∣Vrev∣ × Imismatch

which is:

Photspot ≈ ∣Vrev∣ × (Istring - Igenerate)

This formula points to a key issue: at the same string current, the higher the reverse voltage, the more power the shaded cell burns, and the hotter the hot spot gets.

So one key to fighting hot spots is:

how to lower the reverse voltage on the shaded cell, and spread the heat out more evenly.

This is exactly where BC cells shine.

How is a BC cell structurally different from an ordinary cell?

Ordinary crystalline silicon cells usually have a front-and-back contact structure.

Simply put:

• the front has fine gridlines and busbars, and light comes in from the front;

• the current, once generated inside the cell, is collected by the front and back electrodes.

A BC cell, meaning Back Contact, has one defining feature:

both positive and negative electrodes sit on the back of the cell, and the front has no metal gridlines.

That brings two direct benefits:

1. no gridline shading on the front, so a larger light-receiving area;

2. the back electrodes can be made interdigitated, so current collection is more uniform.

Figure 1 Schematic of BC cell structure

Source: Calcabrini, A., Procel Moya, P., Huang, B., Kambhampati, V., Manganiello, P., Muttillo, M., Zeman, M., & Isabella, O. (2022). Low-breakdown-voltage solar cells for shading-tolerant photovoltaic modules. Cell Reports Physical Science, 3(12), 101155. https://doi.org/10.1016/j.xcrp.2022.101155

The back of a BC cell has lots of interleaved p and n regions. Between these regions sit many short, heavily-doped PN junctions. From a circuit view, it no longer behaves like one big diode, but more like many small diodes in parallel. Under reverse bias, these distributed PN junctions can form a more uniform reverse conduction path.

At the same time, because these back PN junctions are short and locally heavily doped, they can enter reverse breakdown at a relatively low reverse voltage.

Of course, this depends on the specific design parameters of the BC cell.

For example, the smaller the gap between the p and n regions, the stronger the local field, and usually the easier it is to get a lower reverse breakdown voltage. But that can also bring trade-offs in leakage and shunt resistance. So a BC cell's shading tolerance isn't a fixed number, it's tightly tied to the cell structure, back pattern design, gap size, doping concentration, passivation quality, and manufacturing process.

Why do BC modules lose less power after shading?

When a module gets partially shaded, the shaded cell is pushed into reverse bias by the string current. As shading gets worse, the total voltage of that section of the string keeps dropping.

In traditional modules, a bypass diode is usually wired in parallel across a section of the string. The bypass diode isn't actively switched on by a controller. It's a passive device. Whether it conducts depends only on the voltage across it. When the total voltage of that string section becomes negative enough, the bypass diode gets forward-biased and turns on by itself.

The turn-on condition can be written as:

Vsubstring ≤ -Vf

Vsubstring is the total voltage of the string section protected by the bypass diode;

Vf is the forward voltage drop of the bypass diode.

For a string section, its total voltage can be understood as:

Vsubstring = ∑Vunshaded + ∑Vshaded

where:

• unshaded cells still produce a positive voltage;

• shaded cells are reverse-biased and produce a negative voltage.

The bypass diode turn-on condition can be read as:

∣∑Vshaded∣ ≥ ∑Vunshaded + Vf

In other words:

the sum of the reverse voltages of the shaded cells has to exceed the sum of the forward voltages of the remaining unshaded cells, plus the bypass diode's turn-on drop, before the bypass diode kicks in.

The advantage of a BC module is that, before the external bypass diode even turns on, the BC cell's own back interdigitated PN junction structure already provides some distributed reverse conduction. This behaves a bit like a zener diode built into the cell.

Under reverse bias, the interdigitated PN junction structure on the back of a BC cell can form distributed reverse conduction at a lower voltage, which limits how far the reverse voltage can climb. So under partial shading, with the external bypass diode not yet triggered, a BC module can still hold a fairly high output power.

Figure 2 The module IV curve when one cell is shaded.

Source: E. Özkalay, F. Valoti, M. Caccivio, A. Virtuani, G. Friesen, and C. Ballif, "The effect of partial shading on the reliability of photovoltaic modules in the built-environment," EPJ Photovoltaics, vol. 15, p. 7, Jan. 2024, doi: 10.1051/epjpv/2024001. Available: https://doi.org/10.1051/epjpv/2024001

Better shading tolerance doesn't mean immune to shading

One common misunderstanding needs clearing up.

BC cells tolerate shading better, but that doesn't mean shading has no effect on them.

Any PV cell will produce less power once it's shaded.

If the shaded area within one substring is too large, or several cells are fully shaded, then the total reverse voltage of the shaded cells can eventually still exceed the total forward voltage of the remaining unshaded cells. At that point the external bypass diode turns on.

Once the bypass diode turns on, current routes around this whole string section. The unshaded cells in this substring get bypassed along with the shaded ones, and their contribution to the output drops noticeably. So when the shaded area is large, a BC module's generation advantage weakens too.

BC modules tend to have the upper hand when:

• a single cell or a few cells are partially shaded;

• the shaded area within each substring is small;

• the shading is diagonal, strip-like, or locally scattered;

• the external bypass diode hasn't fully turned on.

For example, a diagonal shadow from a utility pole may leave each substring with only a small shaded area. In that case, a BC module usually shows better shading-tolerant generation.

Why do BC modules run cooler at hot spots?

BC modules have lower hot-spot temperatures mainly for two reasons.

First, the reverse current is more spread out

In ordinary cells, the reverse current distribution is often uneven. Reverse breakdown tends to happen first at local weak spots, such as:

• local defect sites;

• cell edges;

• abnormal metallization areas;

• microcracks or contaminated areas;

• areas with weak local passivation.

These spots act like weak points.

Once the reverse current concentrates on these weak points, the local power density gets very high, the temperature climbs fast, and an obvious hot spot forms.

It's like heating two objects with the same amount of heat:

• a whole metal plate;

• a pinpoint-sized dot.

The latter heats up faster, no question.

So an ordinary cell's risk under shading isn't "even heating across the whole cell", it's intense local point heating.

A BC cell has many interdigitated PN junctions on the back. Reverse conduction can spread more easily across many regions instead of piling up on a few defect points.

So a BC cell's reverse current distribution is more uniform, the local power density is lower, and the hot-spot temperature is lower too.

Second, the reverse breakdown voltage is lower

You can see it from the hot-spot power formula:

Photspot ≈ ∣Vrev∣ × Imismatch

At the same mismatch current, the lower the reverse voltage, the less heating power.

That's why a low reverse breakdown voltage can actually work as a protection mechanism under shading.

Here's a simple example.

Say the module string current is 10A, and one cell is badly shaded.

If an ordinary cell reaches a reverse voltage of 15V after shading, the power it burns is about:

P = 15V × 10A = 150W

If a BC cell clamps due to its back structure and the reverse voltage is limited to around 6V, the power it burns is about:

P = 6V × 10A = 60W

The difference is striking.

Of course, the real hot-spot temperature depends on shaded area, ambient temperature, wind speed, module encapsulation, glass size, cell design, and test method, so you can't judge it by a single fixed number.

Still, in some real tests and field experience, BC modules usually run cooler at hot spots than conventional ones. For instance, some BC modules can keep hot-spot temperature below about 120 °C, while other module types may reach 160 °C or even higher.

Some specially designed BC cells achieve something like a "built-in bypass diode", getting hot-spot temperature down to around 90 °C while a reference module sits near 190 °C, which shows this distributed reverse conduction design can cut hot-spot temperature a lot.

Is a lower reverse breakdown voltage always better?

Not necessarily.

A low reverse breakdown voltage helps lower hot-spot temperature under shading, but it can bring design trade-offs too.

If the reverse conduction path is poorly designed, it may increase leakage and lower shunt resistance, which hurts the cell's normal generation performance.

So a high-efficiency BC cell usually has to balance two goals:

1. during normal operation, keep high efficiency, low leakage, and high shunt resistance;

2. under reverse bias from shading, form safe, uniform reverse conduction at a low voltage.

That's also why different BC cells vary in shading performance.

Some BC cells lean toward efficiency, so they may isolate more strongly and end up with a higher reverse breakdown voltage. Others lean toward shading tolerance, so they may design lower, more uniform reverse breakdown paths.

So you can't just say "all BC cells tolerate shading the same". A more accurate statement is:

a well-designed BC cell can achieve lower, more uniform reverse breakdown through its back interdigitated PN junction structure, which improves shading and hot-spot tolerance.

Summary of BC cell advantages

Putting it together, a BC cell's advantages under shading mainly include:

• less module power loss under small-area shading, before the external bypass diode turns on;

• lower local power density;

• lower hot-spot temperature;

• higher module safety margin.

What does this mean for module applications?

In practice, shading often can't be fully avoided.

Especially in distributed scenarios, such as:

• residential rooftops;

• commercial and industrial rooftops;

• balcony PV;

• BIPV;

• multi-orientation mounting;

• sites with complex surrounding buildings.

In these applications, modules may often get partially shaded.

If a cell tolerates shading better and runs cooler at hot spots, that means:

• Better module safety: lower hot-spot temperature reduces encapsulant aging, backsheet damage, local glass stress, and electrical risk.

• Better long-term reliability: local high temperature speeds up material aging. The weaker the hot spot, the more stable the module stays over time.

• More controllable generation loss: when partial shading is unavoidable, a BC module can ease some of the power loss.

• Friendlier system design

BC modules adapt better to complex roofs, distributed mounting environments, and multi-shading scenarios.

Wrapping up

BC cells tolerate shading better and run cooler at hot spots, mainly not because they "aren't affected by shading", but because they have advantages in structure and reverse-bias behavior.

With an ordinary cell under shading, reverse breakdown may concentrate on local defect points, driving high local power density and high hot-spot temperature.

The back interdigitated PN junction structure of a BC cell acts like a distributed, built-in reverse clamp. Under shading, it can form reverse conduction at a lower reverse voltage and spread the reverse current more evenly, which lowers hot-spot power and hot-spot temperature.

But keep in mind, BC cells aren't fully shading-proof. When the shaded area is too large, several cells are fully shaded, and the substring voltage goes negative enough, the external bypass diode still turns on. At that point the bypassed substring's output drops noticeably.

So more precisely:

A BC cell's advantage isn't eliminating shading effects, it's making them more controllable. Under small-area shading it can cut power loss; under heavy shading it can lower hot-spot risk.

That's the fundamental reason BC cells do better in complex shading environments.

Ooitech's View

What really strikes us here is that BC's shading edge lives at the back-contact metallization step, not in some magic material, which means the module line has to hit tight tolerances on the interdigitated pattern to actually get that low, even reverse breakdown. On a production line we've seen the same physics play out in EL and hot-spot testing, where uneven back patterning shows up as scattered breakdown points long before the module ever sees a shadow. If you like this kind of teardown of what happens between the cell and a finished module, our YouTube channel at www.youtube.com/ooitech has more from inside real solar factories.