Question

Figure \(13.7\) shows operational curves of a PV cell for different levels of illumination. If the illumination is low and the panel continues to operate at maximum power, \(^{110}\) which changes the most compared to full-sun operation: the voltage or the current? Why might lower light (fewer photons) directly connect to a lower current based on the physics of PV operation?

Short answer

Answer: When comparing full-sun operation to low-light operation of a photovoltaic cell, the current changes the most compared to the voltage. The main reason behind this is that lower light levels (fewer photons) directly result in the creation of fewer electron-hole pairs, which affect the current generation, while the voltage remains relatively stable and is less influenced by the illumination change.

Step-by-step solution

Recall the basic operation of a PV cell

A photovoltaic (PV) cell generates power by converting sunlight into electricity. It involves the excitation of electrons in a semiconductor material, usually silicon, by absorbing photons from the sunlight. This creates electron-hole pairs, which are then separated by an internal electric field, resulting in the flow of current.

Understand the relation between illumination and current

When the illumination is low, there will be fewer photons hitting the silicon semiconductor. Since the generation of current in the PV cell directly depends on the number of photons absorbed, fewer photons would mean fewer electron-hole pairs are created, which leads to a lower current.

Operational curves of a PV cell

The operational curves of a PV cell describe how the cell's voltage and current change with respect to changes in illumination levels. The curve at full-sun operation represents the highest power output and occurs at a specific point on the curve known as the maximum power point (MPP).

Compare voltage and current changes in different illumination levels

If we look at the operational curves, we can see that when the illumination is low and the panel operates at the maximum power, the voltage has a smaller change compared to the change in current. This is because at lower light levels, there are fewer photons creating fewer electron-hole pairs, which directly affects the current generation. However, the voltage in a PV cell is mainly determined by the internal electric field and temperature, which are not as significantly impacted by changes in illumination levels.

Conclusion

Based on the physics of PV cell operation, when the illumination is low, and the panel continues to operate at maximum power, the current changes the most compared to the voltage. The reason behind this is that lower light (fewer photons) directly results in the creation of fewer electron-hole pairs, which affect the current generation, while the voltage remains relatively stable and is less influenced by the illumination change.

Key concepts

PV cell illumination and current relationship

Photovoltaic (PV) cells transform sunlight into electricity, and this process heavily depends on the interaction between light and the semiconductor material within the cell. When discussing PV cell illumination and current relationship, it's crucial to understand that the current output of the cell is directly linked to the intensity of the light it receives.

As photons from the sunlight strike the surface of a PV cell, they impart energy to the electrons within the semiconductor, typically silicon. This energy excitation results in the creation of electron-hole pairs, the fundamental carriers of electric current in the cell. The more photons hitting the cell, the more electron-hole pairs are generated, and consequently, the higher the current that flows through an external circuit connected to the cell.

A decrease in illumination translates to fewer photons available to create electron-hole pairs, leading to a reduced current. This relationship is reflected in the operational curves of the cell, which demonstrate the variations in current as the illumination levels change. Hence, students can better understand PV cells by visualizing these operational curves, connecting the dots between light intensity, photon absorption, and current output in a tangible way.

Maximum Power Point (MPP)

Understanding the maximum power point (MPP) of a PV cell is key for optimizing its efficiency. The MPP represents the sweet spot on the cell's operation curve where the product of current (I) and voltage (V) is maximized, resulting in the greatest power output. This point is critical because operating a PV cell at or near its MPP ensures that one gets the most electrical power possible from the cell under given light conditions.

When assessing the performance of a PV cell, it's not only about generating a high current or high voltage - it's about achieving the right combination of both for optimal power. Specialized electronic devices called maximum power point trackers (MPPTs) are often used in solar installations to dynamically adjust the electrical load, keeping the operation of the solar panels close to their MPP despite fluctuations in light conditions or temperature.

Students can make better sense of the MPP by picturing a peak in a landscape of power versus operational conditions. This visualization helps in comprehending why at maximum power operation, especially under varying levels of illumination, the current typically experiences more change than voltage, as seen in the operational curves.

Electron-hole pairs in semiconductors

At the heart of a PV cell's operation lies the creation and separation of electron-hole pairs within semiconductors. These are essentially the positive and negative charges that are generated when light energy is absorbed by the semiconductor material. The concept of electron-hole pairs is central to understanding how semiconductors become conductive upon exposure to light.

In a semiconductor like silicon, electrons reside in the valence band under normal conditions. When photons with sufficient energy strike the silicon, they can excite these electrons to the conduction band, leaving behind holes in the valence band. These free electrons and the corresponding holes are then separated by an internal electric field within the PV cell, contributing to the flow of electric current.

Understanding Electron-Hole Dynamics

The movement of these electrons and holes in response to the internal electric field is what generates current. When the electrons reach the front surface of the cell and the holes reach the back surface, an external circuit can harness this flow of charge. The better students grasp the behavior of electron-hole pairs, the [more](#) proficient they become in predicting the performance of PV cells under different light conditions, such as in the scenario of reduced illumination leading to fewer electron-hole pairs and thus lesser current.