Solar Panel Technology

How Solar Cells Work

A solar cell is a semiconductor (usually silicon) that converts sunlight directly into electricity through the photovoltaic effect:

1. Photon absorption: Sunlight hits the silicon cell and transfers energy to electrons.
2. Electron excitation: The energy knocks electrons free from their silicon atoms, creating electron-hole pairs.
3. Electric field: A P-N junction (two layers of silicon doped with different elements) creates an internal electric field that pushes electrons in one direction.
4. Current flow: Electrons flow through an external circuit (your wires) from the negative terminal to the positive terminal — this is DC electricity.

A single silicon cell produces about 0.5–0.6V. Cells are connected in series within a panel to reach useful voltages (e.g., 36 cells = ~18V Vmp for a 12V-compatible panel).

Solar Cell Technologies Compared

Technology Deep Dive

Monocrystalline (Mono-Si)

Made from a single crystal of silicon grown using the Czochralski process. The uniform crystal structure allows electrons to flow more freely, resulting in higher efficiency than polycrystalline. The cells are cut from cylindrical ingots, producing the characteristic rounded corners (though modern cells are often "pseudo-square" to minimize wasted space).

Polycrystalline (Multi-Si)

Made by melting silicon fragments together and letting them solidify into a block. The resulting crystal boundaries reduce electron flow, lowering efficiency. The speckled blue appearance comes from light reflecting off different crystal orientations. Cheaper to manufacture but being phased out as mono prices have dropped.

PERC (Passivated Emitter and Rear Cell)

An enhancement to monocrystalline cells that adds a reflective passivation layer on the back of the cell. This layer reflects unabsorbed light back through the cell for a second pass, increasing efficiency by 1–2%. PERC is now the dominant technology — most new panels labeled "monocrystalline" are actually Mono PERC.

• Better performance in low-light conditions (morning, evening, cloudy days)
• Better red-light absorption (improves morning/evening output)
• Slightly better temperature coefficient than standard mono

HJT (Heterojunction Technology)

Combines crystalline silicon with thin layers of amorphous (non-crystalline) silicon. This "sandwich" structure reduces electron recombination losses at the cell surface, achieving the highest efficiencies of any mass-produced technology. Panasonic (now partnered with Tesla) pioneered this with their HIT cells.

• Best temperature coefficient: Loses only 0.25%/°C vs 0.4%/°C for standard mono — major advantage in hot climates
• Bifacial capable: Can generate power from both sides when mounted above a reflective surface
• No LID (Light Induced Degradation): No initial power loss in the first hours of sun exposure
• Higher manufacturing cost, but prices are dropping

Thin-Film (CdTe, CIGS, a-Si)

Made by depositing extremely thin layers of photovoltaic material onto glass, metal, or plastic substrates. Much less silicon (or no silicon at all) is used. Lower efficiency but flexible, lightweight, and performs relatively well in diffuse light.

• CdTe (Cadmium Telluride): Most common thin-film. Used in utility-scale farms (First Solar). Not ideal for rooftop.
• CIGS (Copper Indium Gallium Selenide): Higher efficiency thin-film. Can be flexible.
• a-Si (Amorphous Silicon): Used in calculators and small devices. Very low efficiency.

How to Read a Solar Panel Datasheet

Every solar panel has a datasheet with standardized specs measured under STC (Standard Test Conditions): 1,000 W/m² irradiance, 25°C cell temperature, AM 1.5 spectrum.

Half-Cut Cells and Other Innovations

Half-Cut Cells

Modern panels cut each cell in half, doubling the cell count (e.g., 120 half-cut cells instead of 60 full cells). Benefits:

• Lower resistive losses: Each half-cell carries half the current, reducing I²R losses
• Better shade tolerance: The panel is split into independent upper and lower halves — shade on the bottom half doesn't affect the top half
• Higher power output: Typically 2–3% more power than equivalent full-cell panels

Shingled Cells

Cells are cut into strips and overlapped like roof shingles, eliminating the gaps between cells. This increases the active area and improves aesthetics (no visible gridlines). Used by some premium manufacturers.

Bifacial Panels

Glass-glass panels that can absorb light from both sides. When mounted above a light-colored surface (white roof, snow, sand), the backside can generate an additional 5–30% power. Most useful for ground-mount or elevated installations.

Choosing Panels for Off-Grid Use