- 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 effect due to which light energy is converted to electric energy in certain semiconductor materials is known as photovoltaic effect. This directly converts light energy to electricity without any intermediate process. For demonstrating the photovoltaic effect let us assume a block of silicon crystal.
The block’s upper portion has donor impurities (n-type), while the lower part has acceptor impurities (p-type). As a result, the n-type region has a high concentration of free electrons, and the p-type region has many holes. Electrons from the n-type region naturally diffuse toward the less dense p-type area, and holes behave similarly, moving to the n-type. This movement results from charge carriers migrating from areas of high to low concentration.
Each free electron in the n-type region originates from a neutral donor atom. Likewise, as a hole moves from the p-type to the n-type region, it leaves behind a negative acceptor ion in the p-type region.
Since each hole is contributed by one acceptor atom in p-type region. Both of these ions i.e. donor ions and acceptor ions are immobile and fixed at their position in crystal structure. It is needless to say that those free electrons of n-type region which are nearest to the p-type region first diffuse in the p-type region consequently create a layer of positive immobile donor ions in the n-type region adjacent to the junction.
Similarly, the closest free holes in the p-type region diffuse into the n-type region, forming a layer of negative immobile acceptor ions near the junction. This accumulation of positive and negative ions forms an electric field across the junction, flowing from the n-type to the p-type side.
The electric field forces charge carriers to drift along its direction. Positively charged holes in the n-type region drift towards the p-side, following the electric field’s flow.
On the other hand, negatively charged electrons in p-type region (if any) drift to n-region as negative charge always drift opposite to the direction of electric field. Across a p-n junction diffusion and drift of charge carriers continues. Diffusion of charge carriers creates and increases the thickness of the potential barrier across the junction and drift of the charge carriers reduces the thickness of the barrier. In normal thermal equilibrium condition and in absence of any external force, the diffusion of charge carrier is equal and opposite of drift of charge carriers hence the thickness of potential barrier remains fixed.
Now the n-type surface of the silicon crystal block is exposed to the sunlight. Some of the photons are absorbed by the silicon block. Some of the absorbed photon will have energy greater than the energy gap between valence and conduction band of valence electrons of the silicon atoms. Hence, some of the valence electrons in the covalent bond will be excited and jump out from the bond leaving behind a hole in the bond. In this way electron-hole pairs are generated in the crystal due to incident light. The holes of these light generated electron-hole pairs in the n-type side have enough probability of recombination with enormous electrons (majority carriers). Hence, solar cell is so designed, that the light- generated electrons or holes will not get enough chances to recombine with majority carriers.
The semiconductor (silicon) is so doped that the p-n junction forms in very close vicinity of exposed surface of the cell. If an electron hole pair is created within one minority carrier diffusion length, of the junction, the electrons of electron-hole pair will drift toward n-type region and hole of the pair will swept to p region due to in influence of electric field of the junction and hence on the average, it will contribute to current flow in an external circuit.
