M5 Photovoltaics and Photodetection

Topic

A photon with sufficient energy—exceeding the bandgap of the semiconductor material defined in another model within this set—generates an electron-hole pair, in accordance with the Beer-Lambert law governing photon absorption as the photon penetrates the material. The energy of the absorbed photon excites an electron from the valence band to the conduction band, leaving behind the corresponding hole in the valence band. If the semiconductor material absorbs a larger fraction of these high-energy photons (as described by the Beer-Lambert law), electron-hole pairs are generated within it more frequently.

The internal electric field of the p-n junction—defined in another model within this set—separates the electron-hole pair before the charge carriers can recombine. This internal field drives the electron toward one side of the p-n junction and the hole toward the opposite side; this movement in opposite directions produces the semiconductor's photocurrent. If an electron-hole pair is generated closer to the p-n junction—within the range of the internal electric field—separation is more efficient and the resulting photocurrent is higher. Conversely, if the minority carrier lifetime of the semiconductor material (defined in a separate model within this set) is shorter, a larger fraction of electron-hole pairs recombine before the internal electric field can separate them, resulting in a lower photocurrent. For this reason, the model—characterized by the electron-hole pair and the photocurrent with which it was introduced—generates a photovoltage (specifically, the device's open-circuit voltage) when that photocurrent flows under open-circuit conditions, lacking any external path for effective flow. Instead, without an available external path, the photocurrent accumulates as an increasing forward bias across the p-n junction, progressively reducing the junction's built-in potential until the forward-biased diode current—described by the Shockley model for this junction—exactly cancels out the photocurrent. A longer minority-carrier lifetime allows a larger fraction of these electron-hole pairs to survive until they are separated by the internal electric field; this results in a higher photocurrent and, consequently, a higher open-circuit photovoltage.

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