The term "low-light performance" may sound a bit technical, but it’s actually quite simple. When we buy solar panels, we often see a "power rating," which is measured under standard laboratory conditions with a standard irradiance of 1,000 watts per square meter. How intense is this 1,000-watt irradiance? It’s roughly equivalent to the sunlight intensity on a clear day at noon in summer at 35° north latitude, with air quality classified as AM 1.5.
However, in reality, sunlight rarely reaches 1,000 watts per square meter for most of the day. This is especially true in the early morning when the sun is just rising and in the late afternoon when it is about to set, when light intensity is very low. Additionally, light is also weak in winter, or on cloudy or rainy days. Low-light performance refers to a photovoltaic module’s ability to generate electricity effectively and maintain high efficiency under these conditions of relatively low light intensity.
Ideally, there should be a nearly linear relationship between module power output and irradiance. For example, a module rated at 1000 W should produce approximately 200 W of power at 200 W/m² irradiance, since the irradiance ratio is 200 to 1000. However, in reality, internal leakage currents within the module amplify under low irradiance, causing further power degradation.
In this regard, the TOPCon structure has a natural advantage over the BC structure. At a low irradiance of 200 W/m², the low-irradiance performance of the BC structure ranges from 93% to 95%, while that of JinkoSolar’s Tiger Neo 3.0 ranges from 96% to 97%, maintaining a higher power output.
This has actually become a consensus within the industry, supported by a wealth of third-party empirical data, all of which demonstrates that TOPCon’s technology delivers superior power generation performance in low-light conditions. To help everyone better understand the reasons behind this, we have conducted a detailed analysis of the underlying mechanisms:
1. The first reason lies in leakage current loss
The actual output current of a module equals the photocurrent minus the forward current flowing through the diodes and the current shunted by the parallel resistance Rsh. The magnitude of Rsh is primarily influenced by leakage current. Leakage current is mainly caused by poor p-n junction quality or impurities near the junction; these factors can lead to junction short-circuiting, particularly at the cell edges. Rsh reflects the level of leakage current in the cell. The higher the Rsh, the smaller the leakage current, resulting in a higher final output current for the module and, consequently, better output power.
For TOPCon, the electrodes are located on the front and back of the cell, providing natural isolation between the p-type region and n-type region, effectively blocking leakage current and resulting in a high Rsh. In contrast, in the BC structure, both the p-type region and n-type region are located on the back of the cell. Since the p-type region and n-type region are interdigitated, it is difficult to achieve high-quality isolation through materials or processes. As a result, leakage current is relatively high, Rsh is low, and a significant amount of current is quietly lost, leading to a loss in power generation. Under strong light, the impact of leakage current is minimal; however, under low light, the proportion of leakage current increases, thereby significantly reducing the module’s output current and power.
Let’s use an analogy. Think of electric current as water flow, and the tiny “leakage points” on the module as “small pinholes” in a water pipe.
BC modules: To achieve an aesthetically pleasing front surface, both the positive and negative electrodes are placed on the back. This process is extremely complex, and during manufacturing, it’s easy for invisible “leakage points”—or “small pinholes”—to form.
TOPCon modules: Their structure places the positive and negative electrodes on separate sides. The manufacturing process is relatively mature and stable, making it less likely to produce “pinholes.”
At noon: The sunlight is intense, much like high water pressure and a powerful flow in a pipe. At this time, even though BC modules have “pinholes” leaking water, the total water volume is so large that the tiny amount lost is barely noticeable. Therefore, everyone’s power output is roughly the same at noon.
In the morning and evening: The sunlight is weak, the water pressure drops, and the flow becomes a “trickle.” At this time, the leakage issue from the “pinholes” in BC modules becomes prominent. With such a small volume of water to begin with, the constant leakage results in a significant proportion of loss.
In contrast, TOPCon modules have virtually no “pinholes” and leak very little. Even under a “trickle” of sunlight, they reliably collect most of the water—or rather, the electrical current—and use it to generate power.
The conclusion is this: during early morning and late afternoon when sunlight is weakest, TOPCon modules suffer fewer “current leakage” losses and deliver significantly better power generation performance than BC modules.
2. The second reason is that, from a spectral perspective, TOPCon cells exhibit a stronger response to the core spectrum of low-light conditions, particularly red light.
The spectrum of sunlight at dawn and dusk, or on cloudy days, is completely different from that of midday sunlight. Due to the Rayleigh scattering effect—where scattering intensity is inversely proportional to the fourth power of wavelength—light with longer wavelength penetrates the atmosphere more easily. In the early morning and evening, when sunlight strikes the Earth at an angle, the path through the atmosphere is longer, resulting in a higher proportion of red light. In low-light conditions, the ability to better utilize red light leads to superior power generation performance. This is closely related to the proportion of heavily dopedlayers on the back of the cell and their optical properties :
Heavily doped layers generally refer to boron-diffused emitter, N-poly, P-poly, and doped amorphous-Si. These layers cause severe recombination of photogenerated carriers, preventing them from effectively contributing to current generation. Additionally, back-side layers such as N-poly, P-poly and doped amorphous-Si can cause severe parasitic absorption.
In BC cells, both the P-type and N-type regions are located on the back side, where the area of the heavily doped polycrystalline silicon layer is twice that of TOPCon cells. As a result, carriers generated by red light within those layers are recombined before they can be collected, leading to a lower infrared spectral response compared to TOPCon.
In contrast, the back side of TOPCon cells employs a localized N-poly structure, with N-poly accounting for less than 30% of the area. The heavily doped polycrystalline silicon region is smaller and contains fewer defects, resulting in a lower photogenerated carrier recombination rate and reduced parasitic absorption, leading to higher response efficiency for red light;
This is clearly evident from the EQE (external quantum efficiency) curves of both technologies: TOPCon cells demonstrate significantly higher responsiveness in the infrared spectrum than BC cells. Simply put: under low-light conditions, where red light constitutes a higher proportion of the spectrum, BC modules “cannot capture or utilize” it, whereas TOPCon modules can perfectly absorb and efficiently convert it. This is an inherent advantage determined by cell structure—one that BC technology cannot overcome no matter how much it is optimized.
Let’s use an analogy. We can compare light sources of different wavelengths to stones of varying sizes: the red light corresponds to smaller stones, while shorter-wavelength light corresponds to larger ones. The heavily doped regions within the solar cell act like a filter screen blocking the path of the current.
To achieve an aesthetically pleasing front surface, BC cells concentrate both the positive and negative electrodes on the back, resulting in a very large heavily doped area. Since these areas cannot absorb light, it’s as if a filter screen with holes and gaps has been laid down. Under low-light conditions, a large amount of red light “gravel” rushes in and passes directly through the filter, wasting a significant amount of valuable low-light energy.
In contrast, the TOPCon cells used in JinkoSolar’s Tiger Neo 3.0 feature a localized N-poly structure on the back, with the heavily doped polycrystalline silicon area accounting for less than 30% of the total. This allows for more thorough absorption of light, acting like a much tighter filter that firmly captures red light, enabling the cell to fully absorb it and efficiently convert it into electrical energy.
Simply put: since red light constitutes a larger proportion of the low-light spectrum in nature, TOPCon is better at utilizing it. Combined with TOPCon’s lower leakage current, these dual advantages allow the Tiger Neo 3. 0 to maintain a stable relative power output of 96%–97% under low-light conditions—2–3 percentage points higher than BC modules.