Types of PV Modules and Cells: Mono, Poly, Thin Film, Construction, Ratings and Defects
This section covers PV cell and module types: monocrystalline, polycrystalline and thin-film technologies, module construction, ratings, datasheet interpretation and defect identification. The exam tests technology selection and whether you can spot a failing module from symptoms.
Cell technologies
Monocrystalline silicon, including PERC and TOPCon, has uniform black cells and typical module efficiency around 20–23%. Polycrystalline cells are blue and speckled, usually 16–18%. Thin film (CdTe, CIGS, a-Si) uses less semiconductor, can be flexible, and has lower efficiency, so it needs more roof area. Bifacial modules gain rear-side irradiance on reflective roofs.
Construction and ratings
A typical glass-backsheet module stacks front glass, encapsulant, solar cells, rear encapsulant, backsheet or rear glass, aluminium frame and junction box with bypass diodes. Datasheet values include Pmax, Voc, Isc, Vmp, Imp, temperature coefficients, maximum system voltage (1000 V or 1500 V), series fuse rating and IEC 61215/61730 certification.
Defects
Hot spots show as scorched cells or thermal-camera hotspots, often from shading or a failed bypass diode. Microcracks appear as dark lines under electroluminescence. PID causes power loss and darkening; delamination shows bubbles or peeling; snail trails are discoloured tracks; burnt junction boxes and corroded connectors are fire risks. Physical cracks, broken glass and open frames are rejection criteria.
Worked example
A 300 W load needs module area. At 22% efficiency, area = 300 ÷ (0.22 × 1000) = 1.36 m². A 12% thin-film module needs 300 ÷ 120 = 2.5 m². For a 144 half-cell module with Voc 49.5 V, there are 72 full-cell equivalents, so about 0.69 V per cell. If one bypass diode covers 24 cells, shading that substring removes roughly one-third of voltage, not the whole module.
Traps
Identifying technology by colour alone; assuming a bypass diode protects every cell; using STC Voc without cold-temperature correction; confusing maximum system voltage with operating voltage; ignoring half-cut cell layout when diagnosing a substring fault.