Table of Contents
- What is the Photovoltaic Effect?
- P-N Junction in a Solar Cell
- Working Principle of a Solar Panel
- Equivalent Circuit of a Solar Cell
- I-V and P-V Characteristics
- Factors Affecting Solar Cell Efficiency
- Series and Parallel Connection of Solar Cells
- Types of Solar Cells
- Applications of Solar Panels
- FAQs
Solar panels are everywhere — on rooftops, in solar farms, and even on satellites. But how does a flat panel of glass and silicon actually convert sunlight into electricity? The answer lies in the photovoltaic effect — a phenomenon where certain materials generate voltage when exposed to light.
In this article, we'll explain how solar panels work from an electrical engineering perspective — covering the p-n junction physics, equivalent circuit, I-V characteristics, and efficiency factors that every EE student should understand.
What is the Photovoltaic Effect?
The photovoltaic effect is the creation of voltage (or electric current) in a material upon exposure to light. It was first observed by French physicist Edmond Becquerel in 1839.
Here's what happens at the atomic level:
- Photons (light particles) carry energy equal to E = hf, where h is Planck's constant and f is frequency
- When a photon with sufficient energy strikes a semiconductor, it knocks an electron free from its atomic bond
- This creates an electron-hole pair — a free electron and a "hole" (missing electron) that acts as a positive charge carrier
- If an electric field exists in the material (as in a p-n junction), the electron and hole are separated before they can recombine
- This separation of charges creates a voltage — and if an external circuit is connected, current flows
The key requirement is that the photon energy must be greater than the bandgap energy of the semiconductor. For silicon (the most common solar cell material), the bandgap is approximately 1.12 eV.
P-N Junction in a Solar Cell
A solar cell is essentially a large-area p-n junction diode. Understanding the basic electrical principles helps grasp how this junction creates the built-in electric field needed for photovoltaic conversion.
The structure consists of:
- N-type layer (top): Silicon doped with phosphorus — has excess free electrons
- P-type layer (bottom): Silicon doped with boron — has excess holes
- Depletion region: The thin zone at the junction where electrons and holes have diffused across and recombined, creating an electric field
This built-in electric field (typically 0.6–0.7V for silicon) is the "engine" of the solar cell. It acts as a one-way gate — pushing photo-generated electrons toward the n-side and holes toward the p-side.
The n-type layer is made very thin (about 0.2–0.5 μm) so that sunlight can penetrate through to the depletion region where most of the charge separation occurs.
Working Principle of a Solar Panel
The complete working of a solar panel can be understood in four steps:
Step 1: Light Absorption
Sunlight hits the anti-reflective coating on the solar cell surface. Photons with energy greater than 1.12 eV (wavelength shorter than ~1100 nm) are absorbed by the silicon.
Step 2: Electron-Hole Pair Generation
Each absorbed photon creates one electron-hole pair. The excess energy (above bandgap) is lost as heat — this is one reason why solar cells can never be 100% efficient.
Step 3: Charge Separation
The built-in electric field at the p-n junction sweeps electrons to the n-side and holes to the p-side. This accumulation of charges creates a potential difference (voltage) across the cell.
Step 4: Current Flow
When an external load is connected via metal contacts on the top (grid pattern) and bottom (full metal sheet) of the cell, electrons flow through the external circuit from n-side to p-side, doing useful work. Inside the cell, the current flows from p-side to n-side (conventional current direction).
A single silicon solar cell produces approximately 0.5–0.6V open-circuit voltage regardless of its size. The current depends on the cell area — a typical 156mm × 156mm cell produces about 8–9A under standard test conditions (STC: 1000 W/m², 25°C, AM1.5).
Equivalent Circuit of a Solar Cell
The electrical behavior of a solar cell can be modeled using an equivalent circuit. This is crucial for understanding real-world performance and losses.
The equivalent circuit consists of:
- Current source (IL): Represents the photo-generated current — proportional to light intensity
- Diode (D): Represents the p-n junction — allows recombination current to flow in the opposite direction
- Series resistance (Rs): Represents resistance of metal contacts, bulk semiconductor, and connections (typically 0.5–1 Ω for a standard cell)
- Shunt resistance (Rsh): Represents leakage current paths due to manufacturing defects (ideally infinite, typically >100 Ω)
The output current equation is:
Where:
- IL = photo-generated current (proportional to irradiance)
- I0 = reverse saturation current of the diode
- n = ideality factor (1–2 for silicon)
- VT = thermal voltage (kT/q ≈ 26 mV at 25°C)
- Rs = series resistance
- Rsh = shunt (parallel) resistance
For an ideal solar cell (Rs = 0, Rsh = ∞), the equation simplifies to:
I-V and P-V Characteristics
The I-V (current-voltage) curve is the most important characteristic of a solar cell. It tells you exactly how the cell behaves under different loading conditions.
Key Points on the I-V Curve
- Short-circuit current (Isc): Maximum current when V = 0 (terminals shorted). For a typical 6-inch monocrystalline cell: ~9–10A
- Open-circuit voltage (Voc): Maximum voltage when I = 0 (no load). For silicon: ~0.6–0.7V per cell
- Maximum Power Point (MPP): The point on the curve where P = V × I is maximum
- Fill Factor (FF): Ratio of actual maximum power to the theoretical maximum (Isc × Voc)
A good silicon solar cell has a fill factor of 0.75–0.85. Higher fill factor means the I-V curve is more "square" — indicating lower resistive losses.
P-V Curve
The P-V (power-voltage) curve is derived from the I-V curve by multiplying current and voltage at each point. It shows a clear peak — the Maximum Power Point (MPP). This is the operating point that solar inverters with MPPT algorithms continuously track to extract maximum energy.
Where G = irradiance (W/m²) and A = cell area (m²).
Factors Affecting Solar Cell Efficiency
The theoretical maximum efficiency of a single-junction silicon solar cell is about 33.7% (Shockley-Queisser limit). In practice, commercial cells achieve 20–24%. Here's why:
Effect of Temperature
Solar cell performance degrades with increasing temperature:
- Voc decreases by approximately −2.2 mV/°C per cell
- Isc increases slightly (+0.05%/°C) — but not enough to compensate
- Net effect: power output drops by about 0.4–0.5%/°C above 25°C (STC)
This is why solar panels in hot climates (like India) produce less power at midday despite maximum sunlight — the panel temperature can reach 60–70°C.
Effect of Irradiance
- Isc is directly proportional to irradiance (double the light = double the current)
- Voc increases logarithmically with irradiance (small increase)
- At low light (cloudy days), both current and voltage drop — power output can fall to 10–25% of rated capacity
Series and Parallel Connection of Solar Cells
A single solar cell produces only ~0.5V — far too low for practical use. Solar panels combine multiple cells:
Series Connection
- Voltages add up: 60 cells in series → 60 × 0.5V = 30V (typical for a 60-cell panel)
- Current remains the same as a single cell
- Problem: If one cell is shaded, it limits the current of the entire string (bypass diodes are used to mitigate this)
Parallel Connection
- Currents add up
- Voltage remains the same as a single string
- Used when higher current is needed at the same voltage
Panel → Array Hierarchy
The DC output from the array is then fed to a solar inverter which converts it to AC for home or grid use.
Types of Solar Cells
In India, monocrystalline panels dominate the rooftop market due to space constraints and falling prices (₹18–25 per watt as of 2025).
Applications of Solar Panels
- Rooftop solar systems: 3–10 kW systems for homes (PM Surya Ghar scheme provides subsidy for up to 3 kW)
- Solar farms: Utility-scale power generation (100 MW–GW scale)
- Solar water pumping: Agricultural irrigation in rural India
- Street lighting: Off-grid LED lighting with battery storage
- Satellites and space: Multi-junction cells with 30%+ efficiency
- Portable chargers: Small panels for phones and camping equipment
India targets 500 GW of renewable energy capacity by 2030, with solar contributing the largest share. Understanding how solar panels work is now essential for every electrical engineering student.
FAQs
What is the photovoltaic effect in simple words?
The photovoltaic effect is the generation of voltage and current when light falls on a semiconductor material like silicon. Photons knock electrons free, and the built-in electric field of a p-n junction separates them to produce electricity.
Why can't a solar cell be 100% efficient?
Because not all sunlight is usable. Photons below the bandgap energy pass through without being absorbed, and photons above the bandgap waste their excess energy as heat. The theoretical maximum for a single-junction cell is 33.7% (Shockley-Queisser limit).
How much voltage does one solar cell produce?
A single silicon solar cell produces approximately 0.5–0.6V open-circuit voltage. This is why panels connect 60–72 cells in series to achieve usable voltages of 30–40V.
Does a solar panel work on cloudy days?
Yes, but at reduced output. On cloudy days, diffuse radiation still reaches the panel, producing about 10–25% of rated power. The current drops proportionally with irradiance, while voltage drops only slightly.
What is the difference between a solar cell, module, and array?
A solar cell is a single p-n junction unit (~0.5V). A module (panel) is 60–72 cells connected in series and encapsulated (~30–40V, 300–400W). An array is multiple modules connected together to form a complete power system (kW to MW scale).
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