--- title: "Solar cell principle and applications" subject: "Energy Environment and Society" chapter: 3 key: "semester_7/energy_environment_and_society:solar-cell-principle-applications" source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-cell-principle-applications site: BCT Engineering --- # Solar cell principle and applications Energy Environment and Society · Chapter 3: Renewable Energy Sources - Asked in: 2072 Kartik, 8 marks; year not given - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-cell-principle-applications - Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3 ## Question Describe the principle of solar cell (PV) technology and its applications. ## Short answer A solar cell is a p-n junction converting sunlight to DC: photons create electron-hole pairs, and the built-in field separates them, giving ~0.5–0.6 V per cell. Applications include solar home systems, water pumping, grid-tied plants, telecom, street lighting, and consumer electronics. ## Answer #### Principle of solar cell (photovoltaic) technology A solar cell is a p-n junction semiconductor diode (usually silicon) that converts sunlight directly into DC electricity by the **photovoltaic effect**. 1. A silicon wafer has an n-type layer (doped with phosphorus: extra electrons) on top of a p-type layer (doped with boron: extra holes). At the junction, electrons diffuse into the p side and holes into the n side, leaving a **depletion region** with a built-in electric field (about 0.6–0.7 V for silicon). 2. When photons with energy greater than the band gap of silicon (1.1 eV) strike the cell, they create **electron–hole pairs**. 3. The built-in field sweeps electrons towards the n side and holes towards the p side, so the charges separate. 4. With metal contacts on the front (grid) and back, and an external load connected, electrons flow through the load, giving DC current. Photons with less energy than the band gap pass through, and the extra energy of higher-energy photons becomes heat. ```text Sunlight (photons) | | | | =========== front grid contact | n-type | (electrons) |----------| junction / depletion layer | p-type | (holes) =========== back contact \___[ Load ]___/ current flow ``` A typical cell gives about 0.5–0.6 V and, under full sun, about 35–40 mA/cm²; cells are connected in series and parallel to make modules. #### Types of PV technology - Monocrystalline silicon (efficiency 18–22%) - Polycrystalline silicon (15–18%) - Thin film: amorphous silicon, CdTe, CIGS (10–15%) - Emerging: perovskite and multi-junction cells (above 25–30% in labs) #### Applications 1. **Solar home systems (SHS):** lighting and phone charging in remote homes in Nepal (AEPC has installed hundreds of thousands). 2. **Water pumping:** drinking water and irrigation. 3. **Grid-connected rooftop and ground-mounted plants.** 4. **Telecommunication towers, remote health posts and schools.** 5. **Street lighting, traffic signals and signboards.** 6. **Hybrid systems with micro-hydro or diesel.** 7. **Consumer products:** calculators, watches, torches, chargers. 8. **Space satellites and transport:** solar cars, boats. #### Advantages and limits - Free fuel, silent, long life (25 years), little maintenance and modular. - Low efficiency, high initial cost, intermittent output, needs storage and area, and DC to AC conversion losses. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the photovoltaic effect do? A. Converts heat to AC electricity via steam B. Converts sunlight directly to DC electricity in a p-n junction C. Splits water by sunlight D. Stores light in a battery Answer: B. Photons create electron-hole pairs, separated by the junction field, giving DC current. (Key point: Photovoltaic effect) 2. Which doping makes the n-type and p-type layers in a silicon cell? A. Phosphorus for n-type, boron for p-type B. Boron for n-type, phosphorus for p-type C. Both with phosphorus D. Both with boron Answer: A. Phosphorus adds extra electrons (n); boron adds holes (p). (Key point: Doping) 3. What happens to photons with energy less than the band gap? A. They create double pairs B. They heat the cell entirely C. They reverse the current D. They pass through without creating pairs Answer: D. Only photons above 1.1 eV create electron-hole pairs in silicon. (Key point: Sub-gap photons) 4. Which PV technology has the highest typical commercial efficiency (18-22%)? A. Amorphous silicon thin film B. Polycrystalline silicon C. Monocrystalline silicon D. CdTe thin film Answer: C. Mono 18-22%, poly 15-18%, thin film 10-15%. (Key point: PV types) ## Related in Energy Environment and Society - [Advantages and challenges of solar PV](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Apv-advantages-disadvantages) — Energy Environment and Society, chapter 3 - [Solar radiation, solar cell and plant](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-radiation-cell-plant) — Energy Environment and Society, chapter 3 - [Working of a solar cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-working) — Energy Environment and Society, chapter 3 - [Characteristics curve of solar cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-characteristics-curve) — Energy Environment and Society, chapter 3 - [Solar intensity factors and applications](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-intensity-factors-applications) — Energy Environment and Society, chapter 3 ## Related in other subjects - [Thermal, hydro and solar PV plants compared](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-hydro-solar-pv) — Power Plant Design, chapter 1, asked 8× - [Grid-tied solar PV versus hydropower](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agrid-tied-pv-vs-hydro) — Power Plant Design, chapter 1 - [Hydropower versus solar PV in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydro-vs-solar-pv-nepal) — Power Plant Design, chapter 1 - [Solar PV versus hydro and their combination](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asolar-pv-hydro-combination) — Power Plant Design, chapter 1