BharatNotes What is Photovoltaic Effect?
The photovoltaic (PV) effect is the generation of a voltage and electric current in a material upon exposure to light. When photons with sufficient energy strike a semiconductor, they excite electrons from the valence band into the conduction band, creating mobile electron-hole pairs. An internal electric field, typically established at a p-n junction, separates these charges, producing a usable direct current. First observed by Alexandre-Edmond Becquerel in 1839, it is the working principle of every solar cell.
Photovoltaic vs Photoelectric Effect
These two are frequently confused but are physically distinct.
| Feature | Photoelectric Effect | Photovoltaic Effect |
|---|---|---|
| Electron fate | Electrons ejected out of the material into space | Electrons stay within the material |
| Typical medium | Metal surface in vacuum | Semiconductor p-n junction |
| Output | Emitted electrons (current in vacuum tube) | Voltage and current across a junction |
| Application | Photocells, light sensors | Solar panels, PV modules |
In short, the photoelectric effect emits electrons; the photovoltaic effect keeps them inside and separates the charges by diffusion across a junction.
How a Solar Cell Works
A standard cell sandwiches an n-type layer (excess electrons) over a p-type layer (excess holes). The junction creates a built-in field. Incoming sunlight generates electron-hole pairs; the field pushes electrons to the n-side and holes to the p-side, creating a potential difference that drives an external circuit. Crystalline silicon, the dominant material, has a band gap of about 1.1 eV. A single-junction cell's efficiency is capped by the Shockley-Queisser limit of roughly 33.7% (optimal band gap near 1.34 eV under the AM1.5G spectrum, calculated by Shockley and Queisser in 1961). Multi-junction and tandem cells aim to exceed this.
Significance for India
Solar power is the cornerstone of India's clean-energy push and a recurring theme in policy and exams. Key verified data points:
| Indicator | Figure |
|---|---|
| Installed solar capacity | ~132.85 GW (MNRE, as on 30 Nov 2025) |
| Total renewable capacity | ~258 GW (MNRE, December 2025) |
| Solar share of RE | ~53% (largest contributor) |
| 2030 non-fossil target | 500 GW |
| Leading state | Rajasthan (~36 GW, ~27%) |
The PM Surya Ghar: Muft Bijli Yojana, launched on 13 February 2024 with an outlay of about Rs 75,021 crore, promotes rooftop PV for one crore households, providing up to 300 free units monthly. It crossed 10 lakh installations by March 2025.
UPSC Angle
The PV effect links physics fundamentals to live policy. Prelims may test the photovoltaic-versus-photoelectric distinction, the semiconductor junction mechanism, or scheme details. Mains (GS3) ties it to energy security, the Panchamrit commitments, Net Zero by 2070, and self-reliance in solar manufacturing (PLI scheme, ALMM). It is a foundational concept that reliably underpins questions on renewable energy and emerging technologies.
