6 Ways Photovoltaic Cells Generate Electricity

Photovoltaic cells generate electricity via the photovoltaic effect: photons excite electrons to create current. Monocrystalline silicon hits 22-25% efficiency; perovskite cells exceed 25%. Concentrated PV uses lenses to focus light, boosting output to ~40%, while thin-film CIGS offers 15-20% for flexibility.

How Light Becomes Electric Power

The process that transforms sunlight into household electricity is a remarkable feat of engineering that happens in a fraction of a second. It all begins with the photovoltaic effect, discovered by Edmond Becquerel in 1839. Today, the global photovoltaic (PV) market is projected to exceed 1 Terawatt (TW) of installed capacity by the end of 2024, a testament to the efficiency of this fundamental principle. At the heart of every solar panel are individual cells, typically made from highly purified silicon crystals that are over 99.9999% pure. These cells operate by directly converting light particles into electrical flow, with modern residential panels achieving conversion efficiencies between 18% and 22%.

The magic starts when particles of light, known as photons, which carry varying amounts of energy depending on their wavelength, strike the surface of the silicon solar cell. The silicon is specially prepared in a semiconductor structure with two adjacent layers: one layer is doped with phosphorus to create an excess of free electrons (n-type silicon), and the other is doped with boron to create a deficit of electrons, or "holes" (p-type silicon). Where these two layers meet, a crucial internal electric field, about 0.5 volts (V) strong, is permanently established. This junction is the engine of the cell.

When a photon with sufficient energy—greater than the 1.1 electronvolt (eV) "band gap" of silicon—hits an atom in the cell, it can transfer its energy to a bound electron, liberating it and creating a free electron and a corresponding hole. This is a numbers game; a single cell receives trillions of photons every second. However, not all photons are effective. High-energy photons (like blue light) can free electrons but waste excess energy as heat, while low-energy photons (like red or infrared light) may pass through entirely if their energy is below the 1.1 eV threshold. This inherent physical limit is a primary reason even the best silicon cells have a theoretical maximum efficiency of around 29%, known as the Shockley-Queisser limit.

The critical step is the directed movement of these freed charges. The internal electric field at the p-n junction acts as a one-way gate, immediately pushing the free electrons toward the n-type side and the holes toward the p-type side. This segregation of charge creates a voltage differential. If we complete a circuit by connecting the front and back of the cell with a wire, the built-up electrons will naturally flow through the external path to get to the positive side (the p-layer), generating a direct current (DC). This current is directly proportional to the intensity of the light. For a standard residential panel with 60 cells, this process generates a DC voltage of around 30-40 volts.

As temperature increases from a standard test condition of 25°C (77°F) to a realistic operating temperature of 45°C (113°F), the voltage output of a panel typically decreases by about 0.3% to 0.5% per degree Celsius, though the current increases slightly. The net effect is a 10-15% reduction in total power output on a hot day compared to its lab-rated peak. This is why a cool, sunny day often yields the highest actual power production. Advanced cell designs, like PERC (Passivated Emitter and Rear Cell), add a dielectric layer to reflect unabsorbed light back into the silicon, boosting efficiency by capturing more photons that would otherwise be lost, pushing the practical limits of silicon performance closer to that 29% theoretical ceiling. 

Silicon's Role in Creating Current

Accounting for over 95% of the global solar market, silicon's dominance isn't accidental; it's a result of its near-perfect semiconductor properties and abundance. The journey of a silicon wafer, however, begins from humble origins—silicon is refined from quartz sand (SiO₂), which makes up about 60% of the Earth's crust. This sand is purified in an electric arc furnace at temperatures exceeding 1900°C (3452°F) to produce 99% pure metallurgical-grade silicon. For solar use, this is further refined via the Siemens process into hyper-pure polycrystalline silicon, with a typical purity of 99.9999% (often called "six nines" purity). This ultra-pure silicon block is then crystallized into ingots, which are sliced into wafers typically 160-180 micrometers (µm) thick—thinner than a human hair, which averages 70 µm.

The fundamental property that makes silicon an exceptional semiconductor is its band gap, which is precisely 1.1 electronvolts (eV) at room temperature (25°C / 77°F). This energy gap is the "Goldilocks zone" for solar conversion—it's large enough to allow a usable voltage of about 0.5 to 0.6 volts per cell to develop, but small enough for the photons in sunlight to efficiently bridge it. To make silicon conductive, we intentionally introduce tiny amounts of other atoms, a process called doping, at concentrations as low as 1 part per million (ppm). Adding phosphorus atoms, which have 5 outer electrons compared to silicon's 4, creates an excess of free electrons. This produces n-type (negative) silicon. Conversely, doping with boron atoms, which have only 3 outer electrons, creates a deficit of electrons, or "holes," which act like positive charges. This produces p-type (positive) silicon.

The magic occurs when a p-type and an n-type layer are brought together, forming a p-n junction. This isn't a physical seam but an atomic-level boundary where a permanent electric field is established. This field, roughly 1 micrometer (µm) wide, acts as a one-way gate. When photons liberate electrons, this field forcefully pushes the free electrons toward the n-side and the holes toward the p-side, preventing them from simply recombining. The efficiency of this charge separation is critical; in a standard 21% efficient panel, about 15-20% of potential electron-hole pairs are still lost to recombination. Advanced cell designs like PERC (Passivated Emitter and Rear Cell) add a dielectric passivation layer, often 100-150 nanometers (nm) thick, to reflect charges back, reducing rear-side recombination losses and boosting efficiency by an absolute 1-2%.

Monocrystalline silicon is grown as a single, continuous crystal, resulting in a uniform structure that allows for the highest electron mobility. Cells from this material are typically the most efficient, with commercial modules reaching 22-24% efficiency, but the complex Czochralski growth process makes them 10-15% more expensive than alternatives. Polycrystalline silicon is cast from a melt containing multiple crystals. The grain boundaries between these crystals impede electron flow slightly, resulting in module efficiencies typically in the 17-20% range, but at a lower manufacturing cost per wafer. A newer contender, monocrystalline PERC, now dominates the high-end market, combining the superior crystal structure with advanced passivation to achieve efficiencies consistently above 22%.

The long-term stability of silicon is another key advantage. A high-quality silicon PV module experiences a very slow degradation in output power, averaging only 0.5% per year. This means that after a 25-year warranty period, a panel can still be expected to produce at least 85% of its original rated power. The primary reason for this degradation is the gradual deterioration of the protective encapsulant (EVA) and anti-reflective coating from constant exposure to ultraviolet (UV) light and thermal cycling, which can range daily from -10°C to 50°C (14°F to 122°F) depending on the climate. While newer materials like perovskites offer potentially higher theoretical efficiencies, their operational lifespan is often measured in months or a few years without encapsulation, compared to silicon's proven decades-long reliability, solidifying its role as the workhorse material for creating solar current.

The Electric Field Inside a Cell

This junction is incredibly thin, typically only about 1.0 to 1.5 micrometers (µm) wide, yet it is the very heart of the device. The strength of this electric field is substantial, creating a built-in voltage potential of approximately 0.5 to 0.7 volts for a standard silicon cell. This voltage is the fundamental "pressure" that will drive the current once a circuit is completed. The formation of this field is a direct result of the difference in charge carrier concentration between the p-side, which has a high concentration of holes (approximately 10^16 to 10^18 per cubic centimeter), and the n-side, with its high concentration of free electrons (also around 10^16 to 10^18 per cm³).

The creation of the electric field begins the moment the p-type and n-type silicon are joined. Electrons from the n-side diffuse across the junction to fill holes in the p-side. This movement leaves behind positively charged donor ions (from the phosphorus) on the n-side and creates negatively charged acceptor ions (from the boron) on the p-side. This region of exposed, fixed charges is called the depletion region or space charge region. It is within this 1 µm zone that the electric field is at its maximum strength, which can be as high as several thousand volts per centimeter (V/cm). This field acts with immense speed; once a photon liberates an electron-hole pair, the field separates them within picoseconds (10^-12 seconds), drastically reducing the chance of immediate recombination. The probability of an electron-hole pair recombining in the depletion region is less than 5%, compared to over 30% in the neutral regions of the silicon, highlighting the field's critical role in efficiency.

Several key parameters determine the effectiveness of this internal electric field:

l Depletion Region Width: A wider region, often achieved with lower doping concentrations (e.g., 10^16 carriers/cm³ vs. 10^18 carriers/cm³), increases the volume where light absorption leads to effective charge separation. However, if it becomes too wide, the internal resistance increases.

l Electric Field Strength: A stronger field, typically achieved with higher doping levels, more forcefully drives charges apart. This is crucial in lower-quality material with more defects, where the field must work harder to sweep electrons out before they are trapped.

l Doping Concentration Gradient: A sharper transition from p-type to n-type (a steep gradient) creates a stronger, more defined field. A gradual transition creates a weaker, more diffuse field that is less effective at separating charges.

As the cell's temperature increases from the standard test condition of 25°C (77°F) to an operating temperature of 65°C (149°F), the intrinsic carrier concentration of silicon increases exponentially. This provides more charge carriers that can "screen" the fixed charges in the depletion region, effectively weakening the built-in electric field. This thermal weakening of the field is a primary reason for the negative temperature coefficient of voltage observed in all solar panels, typically around -0.3% per degree Celsius. This means that on a hot day, the cell's maximum power point voltage can drop by over 10%, directly impacting power output. Furthermore, the electric field is not uniform across the entire cell surface. Its strength is highest directly under the thin, metallic grid fingers on the front surface, which are typically about 50-100 µm wide and have a pitch (spacing) of 2-3 millimeters. These fingers are connected to a larger busbar, which collects the current; the design goal is to collect over 99% of the generated electrons with minimal resistive losses, usually kept below 3% of the total power. The precise engineering of this field, in conjunction with the surface contacts, is what allows modern cells to achieve charge collection efficiencies exceeding 98%, meaning that nearly every electron freed within the influence of the field is successfully collected and contributes to the current flowing out of the cell.

From Direct Current to Usable Flow

The electricity generated by a single solar cell is a mere trickle: about 0.5 to 0.6 volts of Direct Current (DC), with an output of roughly 6 to 7 amps for a standard 6-inch cell under full sun. This is fundamentally incompatible with the Alternating Current (AC) systems that power our homes and the grid, which operate at 120V or 240V and 50/60 Hz. To bridge this gap, a crucial device called an inverter performs the heavy lifting of conversion. The efficiency of this conversion is paramount; while silicon cell efficiencies have plateaued around 22-24%, inverter efficiencies have soared, with modern string inverters achieving 98-99% peak efficiency and 97% weighted European efficiency. This means for every 1000 watts of DC power sent from the panels, only 20-30 watts are lost as heat during the inversion process. The global inverter market, valued at over $12 billion in 2023, is a testament to this module's critical role in making solar energy practically useful.

The core task of an inverter is to transform the DC input into a clean AC sine wave. This is done through a process called Pulse Width Modulation (PWM). Inside the inverter, solid-state switches, typically Insulated-Gate Bipolar Transistors (IGBTs), turn on and off at an incredibly high frequency, often between 10 kHz to 20 kHz. By precisely modulating the width of these DC voltage pulses, the inverter constructs an AC waveform. A critical filter then smooths this rapid switching into a near-perfect sine wave, with a Total Harmonic Distortion (THD) of less than 3% to ensure it doesn't damage sensitive electronics. The inverter must also synchronize its output frequency precisely to the grid's 60 Hz (or 50 Hz) frequency, with a deviation tolerance of less than ±0.1 Hz. Before any power is fed, the inverter performs a series of checks in under 2 seconds to ensure grid stability.

A solar panel's output voltage and current change constantly with irradiance and temperature. The MPPT algorithm, which samples conditions hundreds of times per second, continuously hunts for the specific voltage (e.g., 30V on a string of 60-cell panels) at which the array produces its maximum power. The effectiveness of this tracking is measured by its efficiency, which is typically 99.5% or higher for quality inverters. This means the inverter captures 99.5% of the maximum power theoretically available from the panels at any given moment.

l Voltage Window: The inverter's operational range, typically from 250V to 600V for residential string inverters. The array must be designed so its voltage under operating conditions stays within this window.

l Current Capacity: The inverter's maximum DC input current, often in the 10-15 amps per MPPT range, which limits how many parallel panel strings can be connected.

l Start-up Voltage: The minimum voltage, around 150V, required for the inverter to begin operation early in the morning.

This is known as self-consumption or standby loss, typically ranging from 5 to 20 watts depending on the inverter's size. This is why a system produces no power at night. Furthermore, temperature significantly impacts performance. Inverter efficiency can drop by approximately 0.1% for every degree Celsius above 25°C. Therefore, installing an inverter in a shaded, well-ventilated location can yield a 2-3% higher annual energy production compared to a hot installation spot.

For a typical US residential system, this means 240 volts AC, 60 Hz, split-phase power. The inverter continuously monitors the grid voltage; if it deviates outside a specified range (e.g., below 216V or above 264V), the inverter will automatically disconnect in less than 2 seconds for safety, a requirement known as anti-islanding. The entire process, from DC input to synchronized AC output, happens with a speed and precision that ensures over 95% of the raw solar energy captured by the panels is delivered to your home and the grid as usable, high-quality power.

Connecting Cells for More Power

A single silicon solar cell produces a modest amount of power—typically around 0.65 volts (V) and 6.5 amps (A) under standard test conditions, yielding roughly 4.2 watts (W). This is insufficient for practical applications. To generate meaningful power, cells are interconnected like links in a chain or lanes on a highway. The two fundamental ways to connect them—series and parallel—allow engineers to precisely engineer the voltage and current output of a solar panel. A standard residential panel, measuring about 1.7 meters tall by 1 meter wide, contains 60 or 72 full-size cells, which are interconnected using thin copper ribbons, typically around 0.2 mm thick and 1.5 mm wide. The specific configuration of these 120 to 144 intercell welds directly determines the panel's final electrical characteristics, its resilience to shading, and its compatibility with inverters.

The most important outcome is that the voltages of each cell add together. For example, connecting 36 cells in series, each producing 0.65V, results in a total module voltage of about 23.4V. However, the current remains the same as that of a single cell, approximately 6.5A. This higher voltage is crucial for system efficiency. It reduces resistive power losses during transmission through the DC cables to the inverter, as power loss is proportional to the square of the current (P_loss = I²R). By raising the voltage to over 30V (for a typical 60-cell panel, which actually has 3 groups of 20 series-connected cells linked in parallel), the current is kept lower, minimizing these losses. The downside of a series connection is that the entire circuit becomes vulnerable to a "weakest link" problem. If one cell in the string is heavily shaded, reducing its current output by even 50%, it can limit the current of the entire series string, potentially causing a power loss disproportionate to the shaded area. This is why panels incorporate bypass diodes, usually 3 diodes in a 60-cell panel, which create alternative current paths around underperforming sections, mitigating losses by up to 70%.

In this configuration, the voltage remains constant at the level of a single cell (~0.65V), but the currents add together. Connecting two cells in parallel would still yield 0.65V, but the current would double to about 13A. While pure parallel connections of individual cells are rare within a standard panel, the principle is applied at a module level. Connecting multiple panels in parallel is essential for increasing the system's total current output. The combined current of multiple parallel strings must be carefully managed to ensure it does not exceed the maximum DC input current rating of the inverter, which is often in the range of 10 to 25 amps per Maximum Power Point Tracker (MPPT). The critical consideration for parallel connections is the need for fusing. If one panel in a parallel string develops an internal short circuit, it can force all the other panels to dump their current into the faulty one, creating a fire risk. To prevent this, fuses or breakers with a specific amperage rating, typically 15A, are installed on each parallel string.

A standard 60-cell panel is actually composed of three separate groups of 20 cells wired in series. These three series strings are then connected in parallel within the panel's junction box. This configuration offers a compromise, providing a high enough voltage (around 20 cells * 0.65V/cell = 13V per string, but optimized to around 30-40V at the maximum power point) while increasing the current and providing some resilience. The fill factor (FF), a key metric of cell quality that represents the squareness of the current-voltage curve, is critical here. A high fill factor, typically above 80%, indicates low internal electrical resistance, ensuring that the power gained by connecting cells is not lost as heat within the panel itself. The temperature coefficient of power, approximately -0.4% per degree Celsius, also impacts the interconnected cells; as temperature rises from 25°C to 65°C, the panel's output power can decrease by about 15%, a factor that must be accounted for when designing the entire string configuration to ensure the system's voltage remains within the inverter's operational window (e.g., 250V to 600V) throughout the year.

Sending Solar Power to the Grid

Over 95% of residential solar installations are grid-tied, creating a symbiotic relationship between the home and the utility grid. This process is governed by a sophisticated inverter that does much more than just convert DC to AC; it acts as an intelligent gateway, ensuring the power it produces is perfectly synchronized and safe. The inverter must match the grid's exact frequency of 60.00 Hz in North America (or 50.00 Hz elsewhere) with a tolerance of less than ±0.1 Hz, and its voltage must be within a tight window, typically 1-2% higher than the grid voltage at the point of interconnection, which is usually around 240 volts. 

This is a non-negotiable requirement defined by standards like UL 1741 and IEEE 1547. If the grid power goes down for any reason—such as a fallen tree on a power line—the inverter must detect the outage and automatically shut down within 2.0 seconds. This prevents it from sending power onto what should be a de-energized line, protecting utility workers who might be making repairs. The inverter continuously monitors grid voltage and frequency at a rate of hundreds of times per second. If the voltage strays outside a mandated range, for example, dropping below 216 volts or surging above 264 volts for a 60 Hz system, the inverter will disconnect. Similarly, if the frequency deviates by more than ±0.5 Hz from the standard 60 Hz, it will cease operation.

The National Electrical Code (NEC), particularly Article 690 for Solar Photovoltaic Systems and Article 705 for Interconnected Electrical Power Production Sources, provides the legal framework for safe interconnection, specifying everything from wire sizing and disconnect switches to labeling requirements.

For example, if your home is using 1.5 kilowatts (kW) of power and your solar system is producing 3.0 kW, the 1.5 kW surplus is automatically exported to the grid. This export is measured by a bi-directional utility meter, which spins backwards or digitally decrements, effectively banking the credit for you. The financial benefit comes through net metering policies, which vary by location but often credit the homeowner at the full retail electricity rate, which can be 0.12to0.25 per kilowatt-hour (kWh). The inverter also ensures power quality by maintaining a Total Harmonic Distortion (THD) of less than 3% and operates at a power factor very close to 1.0 (often above 0.99), meaning the power it sends is efficient and doesn't waste capacity on the grid. The entire process, from electron movement in the panel to power export, happens with a total delay of less than 50 milliseconds, ensuring that the clean energy generated on your roof is utilized almost instantaneously, either in your home or by your neighbors.