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The science behind the light effect: How do solar cells change performance?
As the demand for renewable energy increases, solar technology continues to advance. However, many people may not realize that changes in solar cell performance when exposed to light, a phenomenon known as light soaking, can affect a cell's power output and vary depending on the cell type. This article will provide an in-depth look at the scientific basis of the light effect, the applications of commercial thin film technologies, and the potential of emerging technologies in the future.
Basics of lighting effects
The light effect refers to the change in the power output of a solar cell after illumination, which may increase or decrease depending on the situation. The cause of this phenomenon can be electrical or structural properties. Electrical effects manifest themselves differently depending on lighting, voltage, and temperature, while structural effects are changes in a material's structure that often have a permanent impact on performance.
Illumination effects play a key role in improving solar cell efficiency, but stability is critical for solar cells and the devices they connect to.
Observed effects
In solar cells, the current-voltage (I-V) characteristic curve provides information about the electrical characteristics of the device. From this relationship, we can derive the fill factor, an indicator of efficiency. In many solar cells whose efficiency is improved by illumination, the typical deformation of the I-V curve (often called an S-shape or kink shape) appears before irradiation. After exposure to light, the short-circuit current density and open-circuit voltage will increase, resulting in an increase in fill factor.
The changes in the illumination effect can be reversible, that is, the battery can return to a low-efficiency state in a dark environment or after voltage is applied.
Commercial Thin Film Technologies
Thin-film solar cells are used commercially in a variety of technologies. There are three main types of thin film modules, including cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous thin film silicon (a-Si). All types of thin film technologies will exhibit performance changes when exposed to prolonged exposure to light.
Amorphous silicon solar cell (a-Si)
The semiconductor material used in a-Si batteries is amorphous silicon. This effect on the cell when exposed to light causes a decrease in efficiency, with a decrease of approximately 10-30% occurring within the first few hundred hours of exposure. This phenomenon is called the Stabler-Roronski effect (SWE), and its root cause is the breakage of weak Si-Si bonds. Nonetheless, the microscopic mechanisms of this effect are not yet fully understood.
CIS/CIGS solar cells
Copper indium selenide (CIS) and copper indium gallium (CIGS) have shown good results in these modules. After exposure to light, this type of battery can increase efficiency by about 5%. The recovery time of the battery after light optimization ranges from 3 to 16 hours. The main mechanism is related to defects in copper selenide and changes in energy levels.
CdTe solar cell
In CdTe batteries, performance changes depend on device structure and layer composition. Experiments show that the efficiency of some CdTe modules increases by about 6-8%, while some decrease by 7-15%. This is mainly related to the copper element in the conductive layer, and the diffusion rate of copper increases significantly at high temperatures.
Emerging thin film technologies
There are several promising new solar cells, including organic solar cells, calcium-sodium perovskite solar cells, and dye-sensitized solar cells. These technologies are relatively new, and the origins of the lighting effects are not entirely clear.
Organic solar cells
The power output of organic solar cells is enhanced under light, and the kink shape in the I-V curve disappears with irradiation. This phenomenon is due to the filling of surface trap states leading to a reduction in energy barriers, allowing electrons to enter the electrode more smoothly.
Perovskite solar cells
In perovskite solar cells, a variety of effects can be observed after illumination. These effects can be either reversible or permanent, and the particle size and morphology of the film have a significant impact on performance changes. Positive changes are associated with an increase in oxygen vacancies and improved conductivity.
Dye-sensitized solar cells
After 20-30 minutes of exposure to light, the performance of these batteries will improve. The main manifestation is that the open circuit voltage remains basically stable, but the current increases. This effect is related to the rapid generation of free electrons from excited states caused by light.
In summary, the diversity and complexity of light effects reflect that there is still a lot of research space for solar cells in the future. How the scientific community can advance in this area to improve energy efficiency will determine how dependent we become on renewable energy.
