Q.What is the function of a solar cell ? Briefly explain its working and draw its I-V characteristic curve.

Concept: Photovoltaic effect – direct conversion of light energy into electrical energy through generation and separation of charge carriers in a semiconductor junction.

A solar cell (photovoltaic cell) converts sunlight into electricity. When photons strike the p-n junction, they create electron-hole pairs in the depletion region and nearby neutral regions. The built-in electric field sweeps electrons toward the n-side and holes toward the p-side, generating a photocurrent $I_L$ (light current). Under illumination with an external load, the cell delivers current; the terminal voltage opposes the junction's forward bias, so the net current is:

where $I_0$ is the reverse saturation current. In the fourth quadrant (positive voltage, negative current flowing out), the cell supplies power. Key parameters: open-circuit voltage $V_{oc}$ (at $I=0$), short-circuit current $I_{sc} = I_L$, and maximum power point $(V_m, I_m)$. …

Function of a Solar Cell

A solar cell is a semiconductor device that converts light energy (photons) directly into electrical energy through the photovoltaic effect. It acts as an energy transducer, transforming electromagnetic radiation into usable electric power without any moving parts or chemical reactions.

The fundamental principle relies on the behavior of a p-n junction when exposed to light. Unlike a regular diode that operates in darkness, a solar cell is designed to absorb photons and use their energy to create charge carriers that can drive an external current.

Working Principle

The operation hinges on three key processes:

1. Photon Absorption and Carrier Generation

When light strikes the solar cell, photons with energy $E = h\nu$ greater than the band gap $E_g$ of the semiconductor are absorbed. Each absorbed photon excites an electron from the valence band to the conduction band, creating an electron-hole pair. Silicon solar cells typically use $E_g \approx 1.1 \text{ eV}$, which matches well with the solar spectrum.

2. Charge Separation by the Built-in Field

The p-n junction creates a depletion region with a built-in electric field pointing from the n-side to the p-side. This field is crucial: when electron-hole pairs are generated in or near the depletion region, the field sweeps electrons toward the n-side and holes toward the p-side before they can recombine. This spatial separation of charges creates a potential difference across the junction.

3. Current Collection

The separated charges accumulate on opposite sides of the junction, with the n-side becoming negative and the p-side positive. When an external circuit is connected, electrons flow from the n-side through the load to the p-side, delivering electrical power. The illuminated junction thus acts as a current source, with the light-generated current $I_L$ flowing opposite to the normal diode current.

Current-Voltage Characteristic

The I-V relationship of an illuminated solar cell is given by:

where $I_L$ is the light-generated current, $I_0$ is the reverse saturation current, $q$ is electron charge, $k$ is Boltzmann's constant, and $T$ is temperature.

Key points on the characteristic:

• Short-circuit current ($I_{sc}$): When $V = 0$ (terminals shorted), $I = I_{sc} \approx I_L$. This is the maximum current the cell can deliver.

• Open-circuit voltage ($V_{oc}$): When $I = 0$ (open terminals), solving gives $V_{oc} = \frac{kT}{q} \ln\left(\frac{I_L}{I_0} + 1\right)$. This is the maximum voltage across the cell. …