Photonics Essentials

Cheatsheet Content

Solar Cell: Construction & Working Solar cells convert light energy into electrical energy. They are made of semiconductors like Silicon, GaAs, CdSe. Construction p–n junction: Narrow, heavily doped. Top layer: Very thin n-type layer to allow sunlight to pass. Bottom layer: p-type material. Cathode: Connected to the top n-layer. Anode: Connected to the bottom p-layer. Anti-reflection coating: Applied on top to reduce light reflection loss. Working Principle Sunlight enters through the thin n-layer and reaches the p-n junction. Light energy creates many electron-hole pairs in the junction region. The p-n junction has an in-built electric field (barrier potential). This electric field: Pushes electrons to the n-side. Pushes holes to the p-side. Separation of charges creates a potential difference across the cell. When connected to a load, current flows from the p-terminal to the n-terminal in the external circuit. Output of a Single Solar Cell A single solar cell produces $\approx 0.6V$ and $\approx 60mA$. To increase output voltage and current, multiple cells are connected to form a solar panel. IV Characteristics of a Solar Cell A circuit is set up with a solar cell, rheostat (variable resistor), voltmeter, and ammeter. The solar cell is illuminated by a light source. Adjust the rheostat to get different values of current and voltage. Plot an I-V graph using recorded readings. Short Circuit Current ($I_{sc}$) Rheostat is shorted ($R = 0 \Omega$). Current is maximum; voltage is zero. The current obtained is the short-circuit current $I_{sc}$. Open Circuit Voltage ($V_{oc}$) Rheostat is removed ($R \rightarrow \infty$). Current is zero; voltage is maximum. The voltage obtained is the open-circuit voltage $V_{oc}$. Power of a Solar Cell Ideal Power: $P_{ideal} = I_{sc} \times V_{oc}$ Maximum Useful Power ($P_{max}$): The actual power delivered to a load is $P_{max} = V_m \times I_m$. On the IV curve, $P_{max}$ is at the maximum power point (MPP), which is the largest rectangle under the IV curve. Fill Factor (FF) $FF = \frac{P_{max}}{P_{ideal}}$ Typical FF for solar cells ranges from $0.3$ to $0.7$. Efficiency of Solar Cell ($\eta$) $\eta = \frac{P_{max}}{\text{light intensity} \times \text{area of solar cell}} \times 100\%$ Efficiency typically varies from $10\%$ to $20\%$. Stringing of Solar Cells Stringing involves connecting multiple solar panels together using series or parallel connections to achieve the required voltage and current for a solar power system. Series Connection (Stringing) When strings are connected in series, the voltage is additive, while the current remains constant. Used to increase the overall voltage to a level suitable for the inverter. Purpose of String Configuration To efficiently manage and optimize the energy produced by the solar panels before conversion to AC and feeding into the grid or storage system. Parallel Strings When strings are connected in parallel, the current is additive, while the voltage remains constant. Careful balancing is required to ensure uniform current distribution. Advantages of String Configuration Scalability Flexibility Fault isolation Photodiode: Construction & Working A photodiode is a PN-junction device that produces current when light falls on it, working mainly as a photo detector/light sensor and operating in reverse bias. Construction Formed by lightly doped P-region diffused into a heavily doped N-region. Active area is coated with anti-reflection coating. Inactive area is covered with $SiO_2$ insulation. A large depletion region is created to absorb maximum photons. Working Principle Reverse Bias Condition: A small reverse current (dark current) flows even in darkness due to thermally generated minority carriers. When Light Falls on Photodiode: Light generates electron-hole pairs in the depletion region. Due to reverse bias: Electrons move to the N-side. Holes move to the P-side. This movement increases the reverse current, called photocurrent . Total output current: $I = I_0 + I_s$, where $I_0$ is dark current and $I_s$ is photocurrent. Important Point: Only carriers generated in the depletion region contribute to photocurrent, so the depletion width must be large. Applications Smoke detectors CD/DVD players TV remote control sensors Light detection systems Optical communication systems Light-activated switches Security alarm systems PIN Photodiode Definition A PIN photodiode is a photodiode with an intrinsic (undoped) region placed between the P and N regions. The intrinsic (i) region increases sensitivity and speed. Construction Three layers: P-layer $\rightarrow$ Intrinsic (i) layer $\rightarrow$ N-layer. The intrinsic region is wide compared to the depletion region of a normal PN photodiode. Under reverse bias, the depletion region extends fully across the i-region. Working Device is operated in reverse bias. The wide intrinsic region creates: A large depletion region. A strong electric field. When light (photons) enters the device: Electron-hole pairs are generated in the i-region. Due to strong electric field: Electrons move quickly to the N-side. Holes move quickly to the P-side. This fast movement increases: Photocurrent Speed of response Efficiency & sensitivity Advantages of PIN Photodiode Higher photocurrent Faster response (high-speed operation) Better sensitivity Can detect weak light signals Suitable for high-frequency applications Applications High-voltage rectifiers High-speed switching devices X-ray and Gamma-ray detection Light sensing systems Optical fibre communication Light Emitting Diode (LED) An LED is a p-n junction diode that emits light when forward biased. Movement of Charge Carriers when Forward Biased Electrons from the N-region move towards the junction. Holes from the P-region move towards the junction. Both meet and recombine near the p-n junction. When an Electron Recombines with a Hole The electron loses energy. This excess energy comes out as light. The process of emitting light during recombination is called electroluminescence . This is the principle behind LED operation.