AP Syllabus focus:
‘Photovoltaic cells capture sunlight and convert it directly into electrical energy; their output is limited by available sunlight.’
Photovoltaic (PV) solar cells are a major solar-electric technology that converts sunlight directly into usable electricity. Understanding how PV works and why its power varies with sunlight helps explain performance, siting decisions, and variability in solar generation.
Core idea: direct conversion of light to electricity
What a photovoltaic cell is
Photovoltaic (PV) cell: A device that converts light energy (photons) directly into electrical energy (electric current and voltage) using the photovoltaic effect.
PV differs from technologies that first make heat; it produces electricity directly when illuminated.
The photovoltaic effect (conceptual steps)
Sunlight delivers photons to the PV cell surface.
Photons transfer energy to electrons in a semiconductor, freeing some electrons to move.
An internal electric field in the cell drives electrons in a preferred direction.
Movement of electrons through an external circuit produces direct current (DC) electricity.
Labeled photovoltaic-cell schematic illustrating how incident light generates charge carriers that are separated by an internal electric field, driving current through an external circuit. This reinforces the conceptual steps of the photovoltaic effect and why PV outputs DC electricity before inversion to AC. Source
Semiconductor: A material (commonly silicon) whose electrical conductivity can be controlled, enabling light to free electrons and produce a usable electric current.
PV cell structure and system components
Cell and module basics
A typical silicon PV cell is engineered to create an internal electric field using two layers with different electrical properties (often described as a p–n junction).
Energy-level diagram(s) for p-type and n-type semiconductors and the p–n junction, showing band positions and how an internal potential forms across the junction. This is a more physics-forward view of the “internal electric field” that enables charge separation in photovoltaic devices. Source
Many cells are wired together and sealed into a module (panel) to increase voltage and power. Panels can be combined into arrays to meet larger electrical loads.
Balance of system (what makes PV usable)
Inverter: Converts PV-produced DC into alternating current (AC) for buildings and the grid.
Mounting/racking: Positions panels at a chosen tilt and orientation to maximize sunlight capture.
Wiring and protection devices: Reduce hazards from faults and lightning, and manage safe disconnection.
Metering and controls: Track production; in grid-tied systems, enable exporting electricity when generation exceeds use.
Output is limited by available sunlight (and why)
Sunlight availability is the primary limiter
PV output depends strongly on the amount of solar radiation reaching the panels:
Time of day: Lower output in early morning/late afternoon due to lower sun angle.
Season and latitude: Shorter winter days and lower sun angles reduce insolation in many locations.
Weather and clouds: Clouds scatter and reduce sunlight; heavy overcast can sharply cut output.
Shading: Even small shadows from trees, chimneys, or nearby buildings can reduce output significantly.
Soiling: Dust, pollen, and snow reduce light reaching the cells until cleaned or melted.
A compact way to express PV power
PV power scales with sunlight intensity, panel size, and conversion efficiency.
= electrical power output (W)
= solar irradiance on the panel surface (W/m)
= panel area receiving sunlight (m)
= conversion efficiency (unitless; often reported as a percent)
Because varies constantly with sun conditions, PV generation is inherently variable over short (minutes) and long (seasonal) timescales.
Efficiency and real-world performance factors (PV-specific)
Efficiency (what it means for PV)
Efficiency is the fraction of incoming sunlight converted to electrical energy. Not all sunlight becomes electricity because:
Some photons lack enough energy to free electrons.
Some energy becomes heat within the cell.
Electrical resistance and internal losses reduce delivered power.
Temperature effects
PV modules often produce less power when hot; high cell temperatures can reduce voltage and overall output. This matters on sunny days, when panels may heat up even as sunlight is strong.
Orientation and tracking
Fixed systems aim for an orientation and tilt that increases yearly sunlight capture.
Tracking systems can follow the sun to increase daily energy production, but add cost and maintenance.
DC electricity and integration
PV produces DC, so most applications require an inverter for standard AC use. Inverter performance and system design affect how much of the panel’s potential output becomes usable electricity.
FAQ
Irradiance is instantaneous power from sunlight per unit area (W/m$^2$).
Insolation is the total sunlight energy received over time per unit area (e.g. kWh/m$^2$/day), useful for estimating energy yield.
Cells in a panel are electrically connected; a shaded cell can limit current through a whole string.
Bypass diodes help, but shading can still cause disproportionate losses depending on where and how much shading occurs.
Capacity factor compares actual energy produced over time to what would be produced if the system ran at rated power continuously.
For PV, it is typically well below 100% due to night-time, weather, and seasonal sun changes.
Monocrystalline panels are made from single-crystal silicon and typically have higher efficiency per area.
Polycrystalline panels use many crystals, often slightly lower efficiency but sometimes lower cost.
Recycling typically separates the aluminium frame and glass first, then processes the encapsulated cell layers.
Recovered materials can include glass, aluminium, copper, and small amounts of silicon and silver, depending on the recycling method and local facilities.
Practice Questions
State one reason why the electrical output of a photovoltaic (PV) panel changes throughout the day. (2 marks)
Any one valid factor linked to sunlight availability, e.g. sun angle/time of day, cloud cover, shading, or day length (1)
Clear link to reduced/greater irradiance reaching the panel causing lower/higher output (1)
Describe how a photovoltaic (PV) cell converts sunlight into electrical energy and explain two factors that limit its power output in real conditions. (6 marks)
Photons provide energy that frees electrons in a semiconductor (1)
Internal electric field/junction drives electron flow, producing a current in an external circuit (1)
Produces direct current (DC) electricity (1)
Factor 1 limitation (e.g. cloud cover/season/sun angle/shading/soiling) described (1)
Explanation that the factor reduces irradiance reaching the cell, lowering power output (1)
Factor 2 limitation (different from factor 1) described with correct link to reduced incident light or reduced delivered power (e.g. high temperature reducing output) (1)
