- Photovoltaic Effect Definition: The photovoltaic effect is the direct conversion of light energy to electrical energy using semiconductor materials.
- Semiconductor Role: Semiconductors like silicon are crucial as they facilitate the movement and interaction of electron-hole pairs necessary for electricity generation.
- Charge Carrier Dynamics: The movement of electrons and holes across the semiconductor junction is essential for establishing an electric field that aids in electricity generation.
- Impact of Sunlight: Exposure to sunlight energizes electrons in silicon, leading to the creation of electron-hole pairs and subsequent electrical current.
- Efficiency Factors: Solar cell design aims to maximize the separation of electron-hole pairs to increase the efficiency of electricity generation.
The direct conversion of absorbed light into electrical voltage and current in a suitable semiconductor is called the photovoltaic effect. A solar cell uses this effect without a mechanical or thermal conversion stage. To explain the photovoltaic effect, consider a silicon crystal containing a p-n junction.
The upper region is doped with donor atoms to make n-type silicon, while the lower region is doped with acceptor atoms to make p-type silicon. The n-type region has many mobile electrons, and the p-type region has many mobile holes. Near the junction, electrons diffuse from the n-type side to the p-type side while holes diffuse in the opposite direction. These charge carriers move down their concentration gradients.
When electrons leave the n-type side near the junction, they uncover positively charged donor ions. When holes leave the p-type side, they uncover negatively charged acceptor ions.
The donor and acceptor ions are fixed in the crystal lattice. Their uncovered charge forms a region near the junction that has few mobile carriers. This region is called the depletion region.
Positive donor ions remain on the n-type side of the depletion region, and negative acceptor ions remain on the p-type side. This charge separation establishes an electric field across the junction, directed from the n-type side towards the p-type side.
The built-in field drives positive holes towards the p-type side. It drives negative electrons in the opposite direction, towards the n-type side.
Across a p-n junction, diffusion tends to move electrons and holes down their concentration gradients. The electric field drives these carriers in the opposing drift directions. At thermal equilibrium, the drift and diffusion currents of the charge carriers balance, so the net current is zero and the built-in potential remains steady. Illumination disturbs this equilibrium by generating additional carriers.
When sunlight reaches the n-type surface, the silicon absorbs photons with enough energy to cross its band gap. Each absorbed above-band-gap photon can excite an electron from the valence band into the conduction band, leaving a hole and creating an electron-hole pair. Photons below the band-gap energy do not create such a pair, and energy above the band gap is mostly lost as heat. In a practical solar cell, some generated carriers recombine before collection. Cell design therefore aims to absorb useful light, separate carriers quickly and reduce recombination.
The semiconductor (silicon) is doped so that the p-n junction lies close enough to the illuminated surface for generated carriers to reach it. Pairs created in or near the depletion region are separated by the junction’s electric field: electrons move towards the n-type region and holes towards the p-type region. Carriers generated farther away can also contribute if they diffuse to the junction before recombining. Metal contacts then collect the separated carriers, producing current through a connected external circuit.
