The Physics of Photovoltaic Cells

Solar panels generate electricity through the photovoltaic effect, a phenomenon first observed in 1839 and first practically harnessed in the 1950s. When photons from sunlight strike a solar cell, they knock electrons loose from atoms in a semiconductor material, typically silicon. The cell’s internal electric field pushes these freed electrons in a single direction, creating a flow of electric current.

A standard solar cell consists of two layers of silicon. The top layer is doped with phosphorus, which gives it extra electrons and a negative charge. The bottom layer is doped with boron, which creates electron vacancies called holes and gives it a positive charge. The boundary between these layers, called the p-n junction, creates the electric field that drives current flow when photons are absorbed.

From Cell to Panel to System

A single solar cell produces only about half a volt of electricity. To generate useful power, cells are connected in series and parallel within a solar panel, also called a module. A typical residential solar panel contains 60 to 72 cells and produces 350 to 450 watts at peak output. The panels produce direct current, or DC, electricity.

Since homes and the grid operate on alternating current, an inverter is required to convert DC to AC. String inverters connect multiple panels in series to a single inverter. Microinverters attach to each individual panel, optimizing output independently. Power optimizers offer a middle ground, maximizing each panel’s DC output before sending it to a central inverter.

Types of Solar Panel Technology

The solar panel market is dominated by crystalline silicon technology, which accounts for over 95% of global production. Monocrystalline panels, made from a single crystal of silicon, offer the highest efficiency, typically 20% to 23% for commercial panels. Polycrystalline panels, made from multiple silicon crystals, are slightly less efficient at 17% to 20% but historically cheaper to produce.

Thin-film solar technologies use alternative semiconductor materials deposited in extremely thin layers on glass, metal, or plastic substrates. Cadmium telluride and copper indium gallium selenide are the most common thin-film materials. These panels are less efficient than crystalline silicon but lighter, flexible, and better suited to certain applications like building-integrated photovoltaics.

Efficiency: What Limits Solar Conversion

The theoretical maximum efficiency of a single-junction silicon solar cell is about 33%, a figure known as the Shockley-Queisser limit. This limit exists because silicon can only absorb photons within a specific energy range. Photons with too little energy pass through the cell without being absorbed. Photons with too much energy are absorbed, but the excess energy is converted to heat rather than electricity.

Real-world panel efficiency is further reduced by optical losses from reflection and shading, resistive losses in the cell’s electrical contacts, and temperature effects. Silicon cells lose about 0.3% to 0.5% of their output for every degree Celsius above 25 degrees, which is why panels in hot climates produce less than their rated capacity during summer afternoons.

The Future: Perovskites and Tandem Cells

The next major advance in solar technology is likely to come from perovskite materials, a class of crystalline compounds that can be tuned to absorb different wavelengths of light. Perovskite solar cells have achieved laboratory efficiencies above 26% in just over a decade of development, a pace of improvement far faster than any previous solar technology.

The most promising near-term application is tandem cells that layer a perovskite cell on top of a silicon cell. The perovskite layer captures high-energy photons that silicon handles poorly, while the silicon layer captures lower-energy photons. Tandem cells have exceeded 33% efficiency in the laboratory, breaking through the single-junction limit. Several manufacturers are racing to commercialize perovskite-silicon tandems, with the first products expected to reach the market within the next few years.