# Recombination and open-circuit voltage in solar cells

• I
• Mayan Fung
In summary, the ideal diode model provides a formula for calculating the open-circuit voltage (Voc) using the dark saturation current (I0) and light generated current (IL). As the recombination rate increases, the dark saturation current also increases, resulting in a smaller Voc. This can be explained by the Schottky formula, which shows that the current across a regular diode is affected by both the voltage and temperature. Additionally, when the diode is shorted and light is shined on the junction, the light current cancels out the forward current, resulting in an open-circuit voltage.
Mayan Fung
TL;DR Summary
How does recombination in solar cells reduce the open-circuit voltage?
From the ideal diode model, we can derive the open-circuit voltage (Voc) as:

$$V_{oc} = \frac{nkT}{q} ln(\frac{I_L}{I_0} + 1)$$

where ##I_0## is the dark saturation current and ##I_L## is the light generated current. From the model, if the recombination rate increases, the dark saturation current also increases, thus resulting in a smaller Voc.

I wonder if there is a more intuitive explanation on the Voc loss. As a naive picture, I am thinking that the original voltage is determined by the bandgap of the material. When recombination occurs, the hole-electron pair annihilates and release heat (or radiative recombination). It seems that in this picture, it is the current but not the voltage which is affected by recombination. Is there any intuitive explanation to address the Voc loss?

The current across a regular diode is given by the Schottky formula ##I = I_0 ( e^{\frac{eV}{nkT}})##. When you short the diode terminals and shine a light on the junstion, it will produce light current ##I_L##. Open circuit voltage means no current, that is the light current cancels exactly the forward current of the diode. That's how you get the forumula for the open circuit voltage

## 1. What is recombination in solar cells?

Recombination in solar cells refers to the process in which electrons and holes (positive charge carriers) combine and neutralize each other, resulting in a loss of current and efficiency in the cell.

## 2. How does recombination affect the efficiency of solar cells?

Recombination reduces the efficiency of solar cells by decreasing the number of charge carriers available to generate electricity. This results in a decrease in the open-circuit voltage and ultimately, a decrease in the overall power output of the cell.

## 3. What factors contribute to recombination in solar cells?

Recombination can be caused by a variety of factors, including defects in the material, impurities, and the structure of the cell. It can also be influenced by external factors such as temperature and light intensity.

## 4. What is open-circuit voltage in solar cells?

The open-circuit voltage is the maximum voltage that a solar cell can produce when there is no external load connected to it. It is a measure of the potential difference between the positive and negative terminals of the cell.

## 5. How can open-circuit voltage be increased in solar cells?

Open-circuit voltage can be increased by reducing recombination through the use of high-quality materials and minimizing defects in the cell. It can also be improved by optimizing the cell's design and operating conditions, such as temperature and light intensity.

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