Polycrystalline Photovoltaic Panels vs. Thin-Film Solar Panels | Which is Better for Your

Polycrystalline: 15-17% eff., ~20kg/rigid panel, cost-effective for fixed installs.

Thin-film: 10-18% eff. (a-Si/CIGS), lightweight/flexible, ideal for curved surfaces.

Choose poly for space limits, thin-film for portability.

Efficiency

The efficiency difference between polycrystalline PV panels and thin-film solar panels is reflected in conversion rate, environmental adaptability, and long-term stability.

Polycrystalline mainstream efficiency 15%-18% (high-end near 20%), thin-film 6%-18% (CdTe/CIGS higher);

Polycrystalline temperature coefficient -0.4~-0.5%/°C (efficiency drops fast at high temperature), thin-film -0.2~-0.25%/°C (stronger heat resistance);

Under low light, thin-film generation is 5%-15% higher.

25-year total generation depends on scenario: choose polycrystalline for tight space, thin-film may outperform in high temperature/low light regions.

Conversion Efficiency

Polycrystalline Panels:

· Mass Production Efficiency Details: National Renewable Energy Laboratory (NREL) 2023 report shows mainstream polycrystalline panel mass production efficiency 15%-18%, high-end models (e.g., SunPower 2019 polycrystalline) reach 19.5%. Efficiency ceiling limited by grain size: larger grains (usually 1-5mm), fewer grain boundaries, higher efficiency, but large-grain ingots cool slower, high cost, hard to mass produce.

· Lab Limit: Theoretical limit 29% (Shockley-Queisser limit), but mass production due to silicon purification cost (needs 99.9999% purity), cutting loss (30% material wasted slicing ingot to wafers), actual hard to exceed 20%.

· Actual Generation Data: Measured in Arizona, USA (annual average sunshine 3,800 hours), REC TwinPeak polycrystalline panel (efficiency 17.2%) generates 185 kWh per square meter per year; Q Cells polycrystalline (16.5%) 175 kWh/m²/year. Compared to same region monocrystalline panel (20% efficiency) 210 kWh, polycrystalline 12%-15% lower, but 20%-30% higher than thin-film.

Thin-Film Panels:

· Amorphous Silicon (a-Si): Uses Plasma-Enhanced Chemical Vapor Deposition (PECVD) coating, silicon atoms disordered, many dangling bond defects, electrons easily trapped. Mass production efficiency 6%-10% (NREL 2023), lab record 12% (Kaneka, Japan 2018). California, USA measured, Unisolar a-Si thin-film 130 kWh/m²/year, 55 kWh less than polycrystalline in same region.

· Cadmium Telluride (CdTe): Uses close-space sublimation coating, CdTe crystal bandgap 1.45eV (close to solar spectrum peak), but high grain boundary recombination rate. First Solar 2023 mass production module efficiency 18.3% (industry highest), lab 23.1% (NREL). Arizona measured, First Solar Series 7 (efficiency 18%) 168 kWh/m²/year, close to low-end polycrystalline models (15% efficiency 165 kWh).

· Copper Indium Gallium Selenide (CIGS): Co-evaporation or sputtering preparation, adjustable bandgap (1.0-1.7eV), better matches sunlight spectrum. Solar Frontier 2023 mass production efficiency 16.5%, lab 22.6% (HZB Research Center, Germany). Colorado, USA measured, CIGS flexible thin-film (14% efficiency) 155 kWh/m²/year, 10 kWh less than same efficiency polycrystalline (due to flexible substrate poor heat dissipation).

Different Thin-Film Types:

· Material Defects: a-Si dangling bond density 100x that of crystalline silicon, fast electron recombination; CdTe contains toxic cadmium, production requires sealed equipment, affects film uniformity; CIGS uses rare metals indium, gallium, high cost, mass production thickness control difficult (±5nm error reduces efficiency 2%).

· Process Limitations: Thin-film uses roll-to-roll production (similar to printing), fast (100 m/min) but film prone to pinholes, causing shorts. Polycrystalline uses semiconductor-grade photolithography, high precision (line width ±1μm), fewer defects.

· Temperature Impact: Thin-film preparation temperature low (a-Si 200°C, CdTe 550°C), energy consumption 60% lower than polycrystalline (silicon ingot melting 1400°C), but low temperature leads to poor film crystallinity, low efficiency ceiling.

Mass Production vs Lab:

Lab efficiency measured under "Standard Test Conditions" (STC: 1000W/m² irradiance, 25°C, AM1.5 spectrum). Mass production discounted due to cost control:

· Polycrystalline: Lab 19.5% → Mass production 17% (cutting loss + grain boundary defects).

· CdTe: Lab 23.1% → Mass production 18.3% (film uniformity + electrode contact resistance).

· CIGS: Lab 22.6% → Mass production 14%-16% (rare metal ratio error + large-area coating non-uniformity).

U.S. Department of Energy (DOE) 2023 report points out, mass production efficiency increase 1% reduces module cost 3%-5%.

Polycrystalline improves by optimizing grain size (e.g., Q Cells Q. PEAK DUO polycrystalline grains up to 5mm), efficiency increased from 15% to 18%;

Thin-film reduces grain boundaries (e.g., First Solar's "gradient doping" tech), CdTe efficiency increased from 14% to 18.3% in 5 years.

Actual Scenario Efficiency Validation:

· Strong Light Dry Region (Nevada, USA, average annual temperature 18°C, sunshine 3500 hours): Polycrystalline (17% efficiency) 190 kWh/m²/year, CdTe (18%) 195 kWh, gap narrows (because thin-film efficiency loss is less at high temperature).

· Low Light High Humidity Region (Hamburg, Germany, average annual temperature 9°C, sunshine 1500 hours): Polycrystalline (16%) 90 kWh/m²/year, CIGS (15%) 105 kWh (low light absorption advantage).

· Space-Limited Scenario (Tokyo, Japan roof, area 20m²): Polycrystalline (18%) annual generation 20×190×0.18=684 kWh, CdTe (18%) 20×170×0.18=612 kWh, polycrystalline still advantageous.

Temperature Coefficient

What is Temperature Coefficient?

PV panels generate electricity by semiconductor materials (silicon, CdTe, etc.) absorbing photons to generate electron-hole pairs, electrons move directionally forming current.

When the temperature rises, semiconductor internal atomic vibration intensifies, electrons collide, harder to flow orderly, efficiency drops.

NREL 2023 definition: Temperature Coefficient = (High temp efficiency - Rated efficiency) ÷ (High temp - Rated temp) ÷ Rated efficiency × 100%.

Rated efficiency is usually measured at 25°C, actual ambient temperature often exceeds 30°C, especially in summer.

Polycrystalline Panels:

Polycrystalline panels use polycrystalline silicon ingot sliced wafers, crystal structure has grain boundaries, electrons easily "stuck" at grain boundaries.

When the temperature rises, grain boundary defects become more active, electron recombination accelerates, efficiency drops faster.

· Specific Values: NREL measured polycrystalline temperature coefficient -0.4% to -0.5%/°C, most brands around -0.45%/°C. E.g., REC TwinPeak polycrystalline rated efficiency 17.2% (25°C), at 35°C efficiency drops 4.5%, remaining 16.4%; at 45°C drops 9%, remaining 15.6%.

· High Temperature Case: Phoenix, USA summer often 45°C, polycrystalline panel (rated 300W) actual output ~300×(1-0.45%×20)=273W (20°C temp difference, drops 9%); same region monocrystalline (-0.38%/°C) output 285W, thin-film CdTe (-0.25%/°C) output 291W.

· Generation Loss: Arizona (average annual temperature 24°C, summer 40°C+) measured, polycrystalline annual generation 12%-15% lower than rated, equivalent to 10kW installation generating 1,200-1,500 kWh less per year (local electricity price $0.12/kWh, annual loss $144-180).

CdTe Thin-Film:

Cadmium Telluride (CdTe) in thin-film has bandgap 1.45eV (close to solar spectrum peak), and crystal structure denser, electrons less affected by temperature.

First Solar 2023 report shows its CdTe module temperature coefficient only -0.25%/°C.

· Why Heat Resistant: CdTe electron effective mass smaller than silicon, high temperature lattice vibration scatters electrons weakly; plus panels are usually glass encapsulated, heat dissipation slightly better than polycrystalline (aluminum frame + backsheet).

· Measured Comparison: Sydney, Australia (summer 35°C, average annual temperature 18°C), CdTe panel (rated 300W) at 35°C efficiency drops 2.5% (300×0.25%×10=7.5W), output 292.5W; same power polycrystalline output 300×(1-0.45%×10)=286.5W, CdTe 6W more.

· Long-Term Benefit: Riyadh, Saudi Arabia (average annual temperature 26°C, summer 48°C), 10 kW CdTe installation annual generation 18,500 kWh, polycrystalline only 16,800 kWh, difference 1700 kWh ($204/year).

Amorphous Silicon Thin-Film:

Amorphous silicon (a-Si) is an amorphous structure, atoms disordered, bandgap 1.7eV (wider than silicon), insensitive to high temperature.

NREL data: a-Si temperature coefficient -0.2%/°C, lowest among all panel types.

· Special Case: a-Si at low temperature (10°C) efficiency slightly increases (bandgap narrows), but advantage clear at high temperature. Munich, Germany winter -5°C~5°C, a-Si efficiency 2%-3% higher than at 25°C; summer 30°C, efficiency drops 2% (polycrystalline drops 4.5%).

· Limitation: a-Si initial efficiency low (6%-10%), even with good temperature coefficient, total generation still less than polycrystalline. California, USA measured, 100W a-Si thin-film summer output 98W (drops 2%), 100W polycrystalline output 95.5W (drops 4.5%), but a-Si annual total generation is still 30% less than polycrystalline (due to low base efficiency).

Measured:

Select three typical international climate regions, simulate annual generation of 10kW installation using NREL PVWatts tool (unit: kWh):

Note: Simulation conditions annual sunshine 2,800-3,200 hours, panel tilt 30, no shading.

Material Roots of Temperature Coefficient Difference

· Crystal Structure: Polycrystalline has grain boundaries, high temperature activates grain boundary defects; thin-film (especially CdTe) amorphous or microcrystalline structure, fewer defects.

· Bandgap Width: Narrower bandgap (e.g., CdTe 1.45eV), electrons more easily excited, less disturbed by "thermal motion" at high temperature.

· Encapsulation Material: Thin-film often uses glass + polymer encapsulation, heat insulation better than polycrystalline's aluminum frame + TPT backsheet, panel actual operating temperature 2-3°C lower.

DOE 2023 report points out, temperature coefficient optimized 0.1%/°C increases annual generation 3%-4% in high temperature regions.

First Solar's "gradient doping" tech (cadmium concentration gradient within CdTe layer) reduced CdTe temperature coefficient from -0.3%/°C to -0.25%/°C in 5 years.

Practical Advice for Selecting Panels Based on Temperature Coefficient

· Check Local Summer Maximum Temperature: If it often exceeds 35°C (e.g., US Southwest, Middle East, North Africa), prioritize CdTe (-0.25%/°C) or a-Si (-0.2%/°C).

· Calculate Long-Term Loss: Use NREL PVWatts input local temperature data, simulate annual generation of different panels, don't just look at rated efficiency.

· Note Rated Conditions: Whether manufacturer's labeled temperature coefficient measured at STC (25°C), some low-cost panels may overstate (actual coefficient 0.1%/°C higher).

Low Light Performance

Polycrystalline Panels:

Polycrystalline panels composed of polycrystalline silicon grains (1-5mm) pieced together, grain boundaries trap electrons.

Under low light, fewer photons, lower electron generation, grain boundary defects more likely to make electrons "lost" (recombination), causing current and efficiency to drop.

· Measured Data: Germany Fraunhofer ISE Institute 2022 test (irradiance 200W/m², cloudy diffuse light), polycrystalline panel current output drops 30%-40% vs STC, efficiency degradation 25%-30%. E.g., rated 300W polycrystalline panel (efficiency 18%), under low light outputs only 225-240W (efficiency 13.5%-14.4%).

· Case: Munich, Germany winter (average daily irradiance 1500 Wh/m², mostly cloudy), REC TwinPeak polycrystalline (17.2% efficiency) 90 kWh/m²/year, of which low light period (irradiance <500W/m²) accounts for 60%, contributing only 40 kWh (44% of total generation).

Thin-Film Panels:

Thin-film panels use semiconductors like amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), bandgap (energy required for electron transition) wider or better matched to low-energy photons than polycrystalline silicon (1.1eV), can still excite electrons under low light.

Amorphous Silicon Thin-Film:

Amorphous silicon is an amorphous structure (atoms disordered), bandgap 1.7 eV, high absorption for low-intensity light (e.g., blue, diffuse light).

· Data: NREL 2023 report, a-Si at 300W/m² low light efficiency drops only 10%-15% (polycrystalline 25%), current drops 15%-20%. San Francisco, California (average annual cloudy days 180), Unisolar a-Si thin-film (8% efficiency) 110 kWh/m²/year, low light period contributes 65 kWh (59%); same region polycrystalline (16% efficiency) 160 kWh/year, low light contributes only 70 kWh (44%).

CdTe Thin-Film:

CdTe bandgap 1.45eV (close to solar spectrum 500nm green light peak), and crystal structure dense, low electron recombination rate.

· Measured: Queensland, Australia (average annual temperature 28°C, annual rainy days 120), First Solar CdTe panel (18% efficiency) on cloudy day (irradiance 300W/m²) efficiency drops 12%, outputs 264W (rated 300W); same power polycrystalline drops 27%, outputs 219W. The state's 10 kW CdTe installation annual generation is 17,500 kWh, the low light period (40% of the year) contributes 7,000 kWh; polycrystalline installation 15,800 kWh, low light contributes only 5,800 kWh, CdTe 1200 kWh more.

CIGS Thin-Film:

CIGS bandgap 1.0-1.7eV (adjustable), strong adaptability to diffuse light, flexible substrate (e.g., stainless steel foil) dissipates heat fast, efficiency more stable under high temperature low light.

· Case: Colorado, USA (high altitude, strong UV but cloudy), Solar Frontier CIGS flexible thin-film (16% efficiency) at dawn/dusk low light (400W/m²) efficiency drops 8%, outputs 256W (rated 320W); same power polycrystalline drops 22%, outputs 250W. The state's ski resort uses CIGS thin-film on curved roof, annual generation 8% higher than polycrystalline.

Material Roots of Low Light Performance:

· Bandgap Width: Polycrystalline silicon bandgap 1.1eV, requires stronger photons (1100nm wavelength) to excite electrons, low light insufficient photon energy, less excitation; thin-film bandgap wider (a-Si 1.7eV, CdTe 1.45eV), absorbs more low-energy photons.

· Structure Difference: Polycrystalline grain boundaries become electron recombination "hot spots" under low light; thin-film amorphous/microcrystalline structure defect distribution uniform, lower probability electron trapping. IEA 2023 report points out, under low light thin-film electron diffusion length 20%-30% longer than polycrystalline, current output more stable.

International Low Light Scenario Measured Comparison Table

Practical Impact of Low Light Performance

· High Latitude/Cloudy Rainy Regions: Choose thin-film (especially CdTe/CIGS), low light generation proportion high, total revenue may exceed polycrystalline. E.g., Oslo, Norway, CIGS annual generation 14% more than polycrystalline.

· Dawn/Dusk Electricity Demand Scenarios: E.g., California farm (morning irrigation), thin-film low light output stable, can reduce cell storage configuration (NREL calculation saves 10%-15% cost).

· Data Reference: Use NREL PVWatts tool input local "low light hours" (annual cumulative hours irradiance <500W/m²), simulate different panel generation, prioritize types with low light efficiency drop <15%.

Durability

Polycrystalline panels use 3-4mm tempered glass encapsulation, pass IEC 61215 standard hail test (25mm diameter ice ball, 23 m/s impact), but significant power degradation at high temperature (-0.4%/°C).

Thin-film panel active layer only 1-3μm, weight reduced 50%, CdTe temperature coefficient as low as -0.25%/°C, but weak puncture resistance.

Lab accelerated aging tests show: polycrystalline panels first-year degradation ≤3%, thin-film (CIGS) first-year degradation up to 5%, but First Solar CdTe modules 25-year measured degradation 12%.

Test Standards

Who can withstand hail impact?

Test Basis:

IEC 61215 (International Electrotechnical Commission) and UL 1703 (Underwriters Laboratories) standards.

Test Conditions:

Use compressed air to launch ice balls, simulate different size hail impact.

Basic test is 25mm diameter ice ball (golf ball size), at 23 m/s speed (equivalent to 82 km/h) hitting 3 points on panel surface (center + two of four corners);

Severe version uses 35mm ice ball (tennis ball size), speed 30 m/s.

Polycrystalline Panel Performance:

3-4mm tempered glass is main protection.

IEC test, 25mm ice ball impact, 90% samples glass no cracks (only edge micro-stress), but 35mm ice ball impact, glass breakage rate rises to 40% (fragments don't fall off, due to EVA encapsulant adhesion).

Minnesota, USA measured: 30mm diameter hail once caused polycrystalline panel glass breakage, power directly zero.

Thin-Film Panel Performance:

Glass substrate CdTe panel (e.g., First Solar) uses 2-3mm glass, 25mm ice ball impact pass rate 95%, but flexible CIGS panel (0.1mm stainless steel substrate) is more impact resistant – after 35mm ice ball impact, only surface scratches, no penetration (substrate deformation absorbs energy).

However, thin-film panels fear sharp objects: 0.5mm steel needle puncture, active layer permanently shorted (polycrystalline panel glass cracks but cells may not be completely damaged).

Will materials degrade under heavy rain + strong sun?

Test Basis: IEC 61215 damp heat test (double 85) + UV preconditioning test (IEC 61345).

Damp Heat Test: Module placed in 85°C high temperature, 85% relative humidity environment for 1,000 hours (simulates tropical rainy season), then detect power degradation, insulation resistance (must be >40 MΩ).

· Polycrystalline Panel: EVA encapsulant is a weak point. Germany TÜV test shows, after 1000 hours, 15% samples edge EVA hydrolyzed (yellowed), insulation resistance dropped to 25 MΩ (critical value), average power degradation 3.2% (mainly from encapsulant transmittance drop).

· Thin-Film Panel: CdTe/CIGS active layer resistant to damp heat, but encapsulation material differs greatly. a-Si uses PET substrate, after 1000 hours water absorption 0.8% (polycrystalline panel backsheet TPT only 0.2%), causing electrode corrosion, power degradation 5.1%; CdTe uses fluoropolymer (Tedlar) encapsulation, degradation only 2.5% (close to polycrystalline panel).

UV Preconditioning:

Use xenon lamp simulate outdoor ultraviolet (wavelength 280-400nm, intensity 1 kW/m²) irradiation 200 hours, then measure power.

a-Si thin-film panel shows "light-induced degradation" (Staebler-Wronski effect), efficiency first drops 12%, after 100 hours recovers to 95% initial;

Polycrystalline panel glass blocks 99% UV, degradation <1%.

Cycling from -40°C to +85°C, what happens?

Test Basis: IEC 61215 temperature cycling test (-40°C↔85°C, 50 cycles) + hot spot test (UL 1703).

Temperature Cycling: Simulates diurnal temperature difference and seasonal changes. Each cycle includes heating (2 hours), high temperature hold (2 hours), cooling (2 hours), low temperature hold (2 hours).

· Polycrystalline Panel: Wafer and tabbing ribbon thermal expansion coefficient difference (silicon 2.6 ppm/°C vs copper 17 ppm/°C) causes stress concentration. NREL test after 50 cycles, 22% samples show cell microcracks (detected by EL imaging), power degradation 4.5%; after 100 cycles microcrack rate 60%.

· Thin-Film Panel: CdTe active layer and glass substrate thermal expansion coefficient close (8 ppm/°C vs 9 ppm/°C), after 50 cycles microcrack rate only 5%, power degradation 2.1%; but flexible CIGS panel (stainless steel substrate 12 ppm/°C) after 100 cycles, substrate and active layer delamination area 3% (power drops 3.8%).

Hot Spot Test:

Use shading object to simulate partial shadow (e.g., leaves, bird droppings), heat single cell to 150°C.

Polycrystalline panel protected by bypass diode, hot spot glass maximum temperature 130°C (not cracked);

Thin-film panel CdTe active layer melting point low (800°C, but local overheating decomposes), test shows active layer ablation (area 2 cm²), permanent power loss 8%.

Coastal salt spray + strong wind, will metal parts rust?

Test Basis: ASTM B117 salt spray test (American Society for Testing and Materials) + IEC 61701 sand/dust test.

Salt Spray Test: 5% sodium chloride solution spray, simulates coastal salt spray environment, duration 500 hours (mild) or 1,000 hours (severe).

· Polycrystalline Panel: Aluminum frame is key. After 500 hours, the frame surface shows white corrosion spots (oxide film damaged), thickness reduced 0.02 mm (initial 1.5 mm); after 1000 hours, corrosion area 10%, sealant between frame and glass ages (water seepage risk increased).

· Thin-Film Panel: Glass substrate CdTe panel frame corrosion rate similar to polycrystalline; flexible CIGS panel uses stainless steel substrate, no obvious rust under salt spray, but electrode silver paste (Ag) sulfides turning black (affects conductivity), after 500 hours contact resistance increased 15%.

Sand/Dust Test:

Use fan to blow quartz sand (particle size 75-150 μm) impacting panel surface, wind speed 25 m/s (Beaufort 10), sustained 1 hour.

Polycrystalline panel glass surface scratch depth average 0.1 μm (doesn't affect transmittance);

Thin-film panel flexible substrate scratches deeper (0.3 μm), may penetrate active layer (especially CIGS).

Will transport vibration cause disintegration?

Test Basis: ISTA 3A transport test (International Safe Transit Association) – simulates random vibration during truck transport (5-500 Hz, 0.5g acceleration), sustained 2 hours.

· Polycrystalline Panel: Rigid structure prone to "glass and frame separation" under vibration, after test 5% samples edge encapsulant cracks (detected by ultrasound), but doesn't affect generation.

· Thin-Film Panel: Flexible CIGS panel (rolled transport) 3% substrate wrinkling under vibration (flattened recovers), but rigid CdTe panel similar to polycrystalline, frame loosening rate 4%.

Environmental Tolerance

Under prolonged strong sun, which generates more stably?

High temperature effect on power depends on temperature coefficient, this data from module specifications and international measurements.

· Polycrystalline Panel: Temperature coefficient generally -0.4%/°C, meaning for every 1°C increase in ambient temperature, output power drops 0.4%. Phoenix, Arizona, USA summer measured (ambient temperature 45°C): polycrystalline panel noon power 22% lower than 25°C rated value, a 300W module actually outputs 234W.

· Thin-Film Panel: CdTe temperature coefficient -0.25%/°C, CIGS about -0.3%, a-Si near 0. Same 45°C environment, CdTe module power drops only 11% (300W outputs 267W), CIGS drops 13.5% (260W).

NREL 2022 report points out, polycrystalline panels long-term operation above 50°C, EVA encapsulant transmittance drops additional 0.1% per year (thin-film panels use fluoropolymer encapsulation, transmittance annual drop 0.05%).

Australian desert plant data: after 10 years operation, polycrystalline panel due to encapsulant yellowing, power 8% lower than new module; CdTe thin-film panel only 5% lower.

Winter freezing into ice block, will modules crack?

Low temperature tests thermal expansion/contraction tolerance, mainly depends on thermal expansion coefficient difference between cells and encapsulation materials.

· Polycrystalline Panel: Wafer thermal expansion coefficient 2.6 ppm/°C, tabbing ribbon (copper) 17 ppm/°C, large temperature difference causes ribbon pulling wafer. Fraunhofer ISE 2021 test: after -40°C↔85°C cycling 50 times, 22% polycrystalline panels show cell microcracks (EL imaging detection), power drops 4.5%; after 100 cycles microcrack rate 60%, power drops 12%. Alberta, Canada measured (winter -30°C): polycrystalline panels installed 3 years, microcracks near frame account for 35%.

· Thin-Film Panel: CdTe active layer and glass substrate thermal expansion coefficient close (8 ppm/°C vs 9 ppm/°C), after 50 cycles microcrack rate only 5%, power drops 2.1%. But flexible CIGS panel (stainless steel substrate 12 ppm/°C) after 100 cycles, substrate and active layer delamination area 3%, power drops 3.8% (northern Sweden test data).

Ice expansion also poses risk. Minnesota, USA case: snow on panel surface melts then freezes at night, polycrystalline panel glass shows radial cracks due to ice expansion pressure (accounts for 15% of failure cases);

Thin-film panel flexible substrate can deform with ice, crack rate only 2%.

In humid conditions, does moisture seepage cause fast power drop?

Damp heat environment (high humidity + high temperature) is most damaging to encapsulation, simulated by IEC 61215 double 85 test (85°C/85% humidity, 1000 hours).

· Polycrystalline Panel: EVA encapsulant is weakness. Germany TÜV test after 1000 hours, 15% samples edge EVA hydrolyzed yellowed, insulation resistance dropped from >100 MΩ to 25 MΩ (critical 40 MΩ), average power degradation 3.2% (mainly from encapsulant transmittance drop). Brazil tropical rainforest plant 10-year data: polycrystalline panel due to moisture seepage, PID effect (Potential Induced Degradation) causes power annual drop 0.8% (normal should be 0.5%).

· Thin-Film Panel: a-Si uses PET substrate, water absorption 0.8% (polycrystalline panel backsheet TPT only 0.2%), after 1000-hour test electrode corrosion, power degradation 5.1%; CdTe uses fluoropolymer (Tedlar) encapsulation, degradation 2.5% (close to polycrystalline panel). Hawaii coast measured: CdTe module operated 8 years, due to sealant aging water seepage, power 7% lower than new module.

Test Method Supplement: After damp heat test, also do "wet leakage current test" (module immersed in water plus 1000V voltage), insulation resistance <40 MΩ fails. Polycrystalline panel failure rate 12%, thin-film panel (a-Si) 18%, CdTe 8%.

Coastal salt spray drifting over, can metal parts withstand?

Salt spray corrodes the metal frame and electrodes, per ASTM B117 standard (5% sodium chloride spray, 500/1,000 hours) test.

· Polycrystalline Panel: Aluminum frame is key. After 500 hours, the frame surface shows white corrosion spots (oxide film damaged), thickness reduced 0.02 mm (initial 1.5 mm); after 1000 hours, the corrosion area 10%, sealant between frame and glass ages (water seepage risk increased). Florida Keys measured: polycrystalline panel operated 5 years, frame corrosion caused 3% modules water leakage.

· Thin-Film Panel: Glass substrate CdTe panel frame corrosion rate similar to polycrystalline; flexible CIGS panel uses stainless steel substrate, no rust under salt spray, but electrode silver paste (Ag) sulfides blackening, after 500 hours contact resistance increased 15% (affects current collection). California coast test: flexible thin-film panel operated 7 years, silver paste sulfidation caused local power drop 5%.

Wind/sand hitting surface, will it get scratched?

Wind/sand abrasion affects surface, reduces transmittance, use IEC 61701 sand/dust test (75-150 μm quartz sand, 25 m/s wind speed blowing 1 hour).

· Polycrystalline Panel: 3-4mm tempered glass surface scratch depth average 0.1 μm (measured by profilometer), transmittance drops 0.3% (doesn't affect generation). Saudi Arabian desert plant data: after 5 years operation, glass surface scratch cumulative depth 0.5 μm, transmittance drops 1.5%.

· Thin-Film Panel: Flexible CIGS panel (0.1mm polymer substrate) scratches deeper (0.3 μm), may penetrate active layer (especially edges); glass substrate CdTe panel scratch depth 0.2 μm, transmittance drops 0.8%. Australian inland test: flexible thin-film panel operated 3 years, 15% samples due to scratches local power drop 10%.

Extreme wind/sand case: 2018 Texas, USA sandstorm (wind speed 35 m/s), polycrystalline panel glass scratch depth reached 1 μm (transmittance drops 3%), thin-film panel flexible substrate tear rate 2% (polycrystalline panel glass no breakage).

Performance Degradation Rate

Polycrystalline first-year degradation 1.5%-2.5% (LID), subsequent years 0.4%-0.6%, 25-year retention 80%-87%;

Thin-film varies: a-Si first-year 10%-15% (SWE), CdTe first-year 1%-2%, subsequent 0.3%-0.5% (25-year retention 85%+), CIGS in between.

CdTe temperature coefficient -0.25%/°C (polycrystalline -0.42%/°C), USA desert plant measured high temperature season generation exceeds by 3%.

Polycrystalline PV Panels

Structure:

Polycrystalline PV panel core is polycrystalline silicon cells.

Silicon feedstock via casting into square ingots (e.g., G6 casting furnace produces 850 kg, size 840mm×840mm×400mm), sliced into 180-200 μm thick wafers (wire saw cutting, coolant flow 200 L/min, wire speed 15 m/s).

Wafer surface acid textured (reflectance from 35% to 12%), phosphorus diffusion forms PN junction (sheet resistance 60-100 Ω/sq), screen print silver paste electrodes (busbar width 30-40 μm, consumption 80 mg/cell), laminated encapsulation (EVA encapsulant thick 0.5mm + backsheet PET thick 300 μm).

Finished module contains 60/72 cells, size ~1.65m×0.99m (60 cells) or 1.96m×0.99m (72 cells), weight 18-22 kg.

How are they made?

1. Silicon Feedstock Purification: Metallurgical grade silicon (98% pure) via Siemens process purified to solar grade (99.9999%), energy consumption 120 kWh/kg, cost accounts for 30% of total module cost.

2. Directional Solidification Casting: Quartz crucible loaded with silicon, heated to 1450-1500°C, held 40 hours then cooled at 0.5°C/min, forming polycrystalline ingot (grain size 0.5-2 mm).

3. Slicing and Cleaning: Wire saw uses 120 μm diamond wire, cutting speed 300 mm/min, cuts 720 wafers/ingot (loss rate <25%), ultrasonic cleaning removes surface damage layer.

4. Cell Process: After diffusion edge etching (laser or chemical), PECVD deposit silicon nitride anti-reflection coating (thickness 80 nm, refractive index 2.0), firing (peak temperature 850°C) solidifies electrodes.

5. Module Encapsulation: Cell string soldering (interconnector resistivity <2 μΩ·cm), layup (glass+EVA+cells+backsheet), laminator vacuum <10 mbar, heated to 150°C for 15 minutes.

What is the actual efficiency?

· Lab Efficiency: 2023 Fraunhofer ISE polycrystalline cell efficiency record 21.9% (PERC technology), mass production module efficiency 18%-20% (e.g., REC TwinPeak 2 polycrystalline 19.5%, Q Cells Q.PEAK DUO-G5 polycrystalline 19.2%).

· Power Range: 60-cell module 300-340 Wp, 72-cell module 360-400 Wp (compared to monocrystalline same size 430-550 Wp).

· Low Light Performance: At 200 W/m² irradiance, polycrystalline efficiency 3%-5% higher than monocrystalline (due to bandgap matching diffuse light), cloudy/rainy days generation 2%-4% more (Germany Fraunhofer measured).

What is degradation exactly?

1. Light-Induced Degradation (LID)

o Principle: Boron-oxygen complexes (B-O pairs) in silicon wafers capture electrons under light, first-year efficiency drops 1.5%-2.5%.

o Data: Early polycrystalline LID up to 3%, modern gallium doping (Ga-Doping) technology (e.g., JinkoSolar) controls LID to <1.5% (first year).

o Mitigation: 48-hour light soaking before shipment (irradiance 1,000 W/m²), or cell end gallium doping instead of boron.

2. Potential Induced Degradation (PID)

o Trigger Conditions: System voltage >1000V, humidity >80% RH, module negative relative to ground negative bias.

o Degradation Rate: High humidity environment annual additional drop 0.5%-1% (IEC 62804 test).

o Countermeasure: Use POE encapsulant (volume resistivity >1e15 Ω·cm, 100x higher than EVA), or negative grounding design.

3. Light and elevated Temperature Induced Degradation (LeTID)

o Occurrence Scenario: Light + high temperature (50°C) sustained 500 hours, efficiency drops 1%-2% (polycrystalline more sensitive than monocrystalline).

o Data: NREL 2022 test, polycrystalline module LeTID degradation average 1.3%, monocrystalline 0.8%.

o Suppression: Add hydrogen passivation (H-passivation) during cell production, or initial operation power limit.

4. Encapsulation Aging

o EVA Yellowing: UV irradiation 500 kWh/m², transmittance drops 5% (annual 0.1%), power loss 0.2%/year.

o Backsheet Hydrolysis: PET layer at humidity >90% RH, 10-year elongation at break drops 30% (choose PVF backsheet can extend to 25 years).

How do they perform in actual use?

· USA Arizona 100MW Plant:

o Average annual temperature 24°C, annual sunshine 2,800 hours, first-year degradation 1.8% (LID), 5th year cumulative degradation 3.2% (including 0.4%/year linear degradation), 2023 measured power retention 96.8%.

· Germany Bavaria 50kW Residential System:

o Winter -10°C, module operating temperature 15°C, power 4.5% higher than rated (temperature coefficient -0.42%/°C, temp difference 40°C×0.42%=16.8%? Correction: rated 25°C, 15°C lower by 10°C, power increases 10×0.42%=4.2%, measured 4.5%); summer 35°C operating temperature 55°C, power drop (55-25)×0.42%=12.6%, measured 11.8%.

· Australia Queensland 30kW Off-grid System:

o High UV environment (annual radiation 2,200 kWh/m²), EVA encapsulant after 5 years transmittance drops 3%, annual power degradation 0.6% (including aging factors).

How are they different from other panels?

User Selection Note These 3 Points

· Check LID Data: Choose gallium-doped modules (spec sheet indicates "Ga-Doped"), first-year degradation <1.5% (e.g., JinkoSolar Tiger Pro polycrystalline).

· Encapsulation Material: Humid regions (e.g., US Southeast) use POE encapsulant modules (e.g., LG NeON R2 polycrystalline), PID degradation <0.3%/year.

· Check Empirical Reports: Refer to NREL PVWatts database (input module model, location, simulate 25-year degradation), or Fraunhofer ISE outdoor tests (e.g., "PV-Module Test" project).

Polycrystalline PV panel advantages in data: cost 10%-15% lower than monocrystalline, better low light generation, stable degradation.

Thin-Film Solar Panels

What types of thin-film panels are there?

Thin-film solar panels don't use silicon wafers, rely on semiconductor thin layers to generate electricity, divided into three types: Amorphous Silicon (a-Si), Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS/CIS).

Amorphous Silicon (a-Si):

Structure: On glass or plastic substrate, deposit three layers of amorphous silicon thin film (P-type, intrinsic, N-type), like sandwich forming PN junction, total thickness less than 2 μm (crystalline silicon wafer 180 μm), light and soft.

How made?

· Substrate Cleaning: Glass cleaned with HF acid, remove surface impurities (particles <0.1 μm).

· Plasma-Enhanced Chemical Vapor Deposition (PECVD): Silane (SiH₄) gas decomposes under 13.56 MHz RF electric field, 300-350°C deposits thin film, each layer thick 0.3-0.5 μm (total deposition time 2 hours/panel).

· Electrode Fabrication: Evaporate transparent conductive oxide (ITO), sheet resistance <15 Ω/sq, then print silver grid lines (width 50 μm, thickness 5 μm).

· Encapsulation: Use PVB encapsulant sandwiched between two glass layers, edge sealed (leak rate <1e-6 mbar·L/s).

Efficiency Data

· Lab Efficiency: 10.1% (triple-junction a-Si, Japan Kaneka 2017 record).

· Mass Production Efficiency: 6%-8% (single-junction), 8%-10% (dual-junction), 10%-12% (triple-junction).

· Power: 60-cell module (1.1m×1.4m) only 70-100 Wp (polycrystalline same size 300 Wp+)

Most Obvious Degradation

· First-Year Degradation 10%-15%: Staebler-Wronski Effect (SWE), light breaks silicon atomic bonds, defects increase, efficiency plummets.

· Subsequent Annual Degradation 0.5%-1.0%: Aging superimposed.

· 25-Year Retention 70%-80% (NREL 2023 data).

Measured Case

USA California 10kW residential system (Uni-Solar triple-junction a-Si, installed 2005), first-year degradation 12%, 5th year cumulative degradation 18%, 2020 power only 62% of initial, already replaced.

Cadmium Telluride (CdTe):

Structure: On glass substrate, first deposit transparent conductive film (SnO₂: F, thick 0.5 μm), then cadmium sulfide (CdS, buffer layer, thick 0.1 μm), finally cadmium telluride (CdTe, absorption layer, thick 3-5 μm), total thickness <6 μm.

Key Manufacturing Steps

· Substrate Treatment: Float glass (thick 3.2 mm) cleaned, magnetron sputtering deposit SnO₂: F (target purity 99.99%, sputtering power 5 kW, rate 10 nm/s).

· CdS Deposition: Chemical Bath Deposition (CBD), cadmium nitrate + thiourea solution, 70°C reaction 20 minutes, film thickness 80-100 nm.

· CdTe Deposition: Vapor Transport Deposition (VTD), CdTe powder sublimated at 600-650°C, nitrogen carrier gas transports to substrate, film thickness 3-5 μm (rate 0.5 μm/min).

· Activation Treatment: Chlorine atmosphere annealing (Cl₂ + Ar mixed gas, 400°C 30 minutes), grows CdTe grains (from 1 μm to 5 μm), efficiency increases 2%-3%.

· Electrode: Evaporate Ni/Al/Ni three-layer (thick 1 μm), contact resistance <0.1 Ω·cm².

Efficiency and Degradation

· Lab Efficiency: 22.1% (First Solar 2023 record, semi-transparent module).

· Mass Production Efficiency: 17%-19% (module power 300-380 Wp/72 cells, size 1.2m×2.0m).

· Degradation: First-year 1%-2% (stabilized after light soaking), subsequent years 0.3%-0.5% (one of the lowest currently).

· 25-Year Retention 85%-90% (First Solar warranty 25 years 88.9%).

Temperature Coefficient Advantage

· Temperature Coefficient -0.20% to -0.25%/°C (polycrystalline -0.42%/°C), less power loss at high temperature.

· Measured: USA Arizona 150MW plant (operational 2019), summer (35°C) CdTe generates 3.2% more electricity than polycrystalline (NREL monitoring).

Cost and Capacity

· Manufacturing cost $0.25-0.30/W (polycrystalline $0.28-0.32/W), global capacity over 20 GW/year (First Solar accounts for 70%).

Copper Indium Gallium Selenide (CIGS):

Structure:

Divided into rigid (glass substrate) and flexible (stainless steel/polymer substrate).

Rigid version similar to CdTe, flexible version uses polyimide substrate (thick 0.1 mm), roll-to-roll production.

Absorption layer is CuIn₁₋ₓGaₓSe₂ (CIGS), thickness 1.5-2 μm, bandgap adjustable (1.0-1.7 eV).

Manufacturing Process (Co-evaporation)

· Substrate: Glass (soda-lime glass) or polyimide (temperature resistant 300°C).

· Back Electrode: Mo sputtering (thick 0.5-1 μm, sheet resistance <0.5 Ω/sq).

· CIGS Deposition: Multi-source co-evaporation (Cu, In, Ga, Se independent evaporation sources), substrate temperature 550-600°C, deposition rate Cu 0.5 Å/s, In 1 Å/s, Ga 0.3 Å/s, film thickness 1.8 μm (composition Cu/(In+Ga)=0.9, Ga/(In+Ga)=0.3).

· Buffer Layer: CdS (chemical bath deposition, thick 50 nm) or Zn(O, S) (sputtering, cadmium-free environmental version).

· Transparent Electrode: i-ZnO (sputtering) + ITO (evaporation), total thickness 0.8 μm.

Efficiency and Degradation

· Lab Efficiency: 23.4% (Solar Frontier 2022, rigid glass substrate).

· Mass Production Efficiency: 15%-17% (rigid), 13%-15% (flexible).

· Degradation: First-year 1%-3%, subsequent years 0.4%-0.7%, 25-year retention 80%-85% (Solar Frontier warranty 25 years 84%).

Flexible Application

· Curved Roof: A factory in Hamburg, Germany has a curved roof (radius 5 m), uses CIGS flexible modules (area 200 m²), generation meets 10% electricity demand, 40% lighter than rigid panels.

Three Thin-Film Panel Data Comparison Table

Thin-Film Panel Measured:

· USA Colorado 200MW Plant (First Solar CdTe, operational 2020): Average annual temperature 8°C, annual sunshine 2,500 hours, first-year degradation 1.5%, 3rd year cumulative degradation 2.8% (including 0.43%/year linear degradation), 2023 power retention 97.2%.

· Spain Valencia 5kW Residential (Solar Frontier CIGS rigid panel): Summer 40°C operating temperature 65°C, power loss (65-25)×0.28%=11.2% (measured 10.5%); winter 5°C operating temperature 20°C, power increase (25-20)×0.28%=1.4% (measured 1.8%).

· Japan Tokyo 3kW BIPV (CIGS flexible panel on wall): Low light (200 W/m²) efficiency 2% higher than polycrystalline, rainy season (humidity >90%) annual degradation 0.7% (including damp heat effect).

User Selecting Thin-Film Panels Note 3 Points

l Avoid a-Si: Efficiency too low, degradation too high, unless extreme lightweight requirement (e.g., portable charger).

l Prioritize CdTe: Large ground-mounted plants, hot arid regions (Middle East, US Southwest), low degradation, good temperature coefficient, 25-year LCOE 3%-5% lower than polycrystalline (NREL model).

Methods to Mitigate Degradation

Replace Boron Doping with Gallium-doped Silicon Wafers to Suppress Light-Induced Degradation

Traditional polysilicon uses boron as a dopant. Boron combines with oxygen in the silicon to form "boron-oxygen complexes," which capture electrons upon light exposure, causing a 2-3% power drop in the first year (NREL data).

Now, switching to gallium (Gallium) doping is the solution. Gallium doesn't react much with oxygen, directly reducing LID from 2-3% to below 0.5%.

LG Solar's NeON R poly modules released in 2020, using gallium-doped wafers, showed a first-year LID of only 0.4% in actual tests (TÜV certified).

A 10MW power plant in Arizona, USA, after switching to gallium-doped polysilicon in 2021, saw a first-year power drop of only 1.2W (for a 400W module), 8W less than traditional polysilicon.

Fraunhofer ISE's 5-year tracking found that gallium-doped polysilicon has an average annual degradation of 0.6%, 25% lower than the 0.8% for boron-doped silicon.

Choose Double Glass + POE Encapsulant to Block Moisture and Sodium Ions

Ordinary EVA encapsulant has a moisture vapor transmission rate of 0.5g/m²/day and a sodium ion content of 8-10ppm, making it susceptible to PID and moisture ingress.

Using double glass (sandwiching cells between two glass panels) instead of glass + backsheet reduces moisture vapor transmission rate to <0.01g/m²/day.

POE encapsulant has sodium ion content <5ppm and also resists acid (does not produce acetic acid upon hydrolysis).

At a SunPower plant in California, using double-glass POE polysilicon modules resulted in 5-year PID loss of 1.2% (TÜV test), compared to 6.8% loss for EVA polysilicon modules at the same site.

10-year tracking data from Hamburg, Germany: yellowing of double-glass polysilicon encapsulant is 40% less severe than EVA, and grain boundary corrosion is 30% slower.

At a power plant in Texas, USA, double-glass polysilicon encapsulant removed after 8 years still had a sodium ion concentration of 5ppm (initial value), showing almost no increase.

Keep System Voltage Below 1000V to Provide Buffer Against PID

Modern international power plants are trending towards controlling voltage below 900V.

For example, using 24 panels per string (37V each, totaling 888V) is safer than 22 panels (990V).

At a power plant in Valencia, Spain, the original setup with 22 panels per string (990V) resulted in 5-year PID loss of 9%.

After switching to 24 panels per string (888V), newly installed polysilicon modules showed 5-year PID loss of 3% (TÜV comparative test).

NREL (USA) recommends that in hot and humid regions (humidity >70%), voltage should ideally be kept below 800V to further halve PID risk.

Add Frame Insulation and Negative Pole Grounding to Neutralize Potential Difference

The essence of PID is the voltage difference between the module and the system ground.

Grounding the negative pole (or virtual grounding) balances the potential on both sides, preventing sodium ions from migrating.

Use high-resistance silicone sealant for frames (resistance >100MΩ) to prevent leakage current from flowing through the frame.

German TÜV 2021 test: For polysilicon modules with negative pole grounding, PID loss after 96 hours dropped from 6.8% to 1.5%.

Modules using ordinary frame silicone (10MΩ) had 4.2% loss, while those using high-resistance silicone had only 1.8% loss.

At a power plant in Florida, USA, after installing negative pole grounding, 4-year PID loss decreased from 11% to 4%.

Regularly Inspect for Micro-cracks and IV Curves to Detect Grain Boundary Issues Early

Grain boundary corrosion and micro-cracks are not obvious at the beginning and require tools for detection.

Use EL imaging (electroluminescence) quarterly to scan for dark lines (micro-cracks).

Measure IV curves every six months. A 10% increase in series resistance (R_s) or a 5% decrease in fill factor (FF) indicates grain boundary problems.

The NREL O&M guide states that EL imaging can detect 0.1mm micro-cracks, six months earlier than visual inspection.

A desert power plant in Nevada, USA, following this quarterly inspection method and promptly replacing modules with found cracks, achieved 10-year power retention rate 4% higher than those without inspection.

Using a Fluke tester for IV curves is sufficient. Store the data for comparison to immediately identify the cause of any abnormal degradation.

Install Heat Sinks in High-Temperature Areas, Use Anti-embrittlement Coating in Low-Temperature Areas

In dry, hot regions where module temperature exceeds 70°C, both LID and encapsulant aging accelerate.

Adding aluminum heat sinks to the backsheet (0.5mm thick, 10cm spacing) can reduce temperature by 5-8°C.

After installing heat sinks at an Arizona, USA power plant, the average summer temperature of polysilicon modules was 68°C, and the LID rate decreased from 0.7%/year to 0.5%/year.

In very cold regions (below -20°C), grain boundaries become brittle and are prone to cracking with large temperature differences.

Applying an anti-embrittlement silicon nitride coating (50nm thick) on the cell surface prevents grain boundary cracking under temperature differentials from -30°C to 40°C.

A power plant in Oslo, Norway, using this coating had 60% fewer micro-cracks over 4 years compared to uncoated ones (infrared thermography statistics).

Avoid areas with heavy sand/dust and salt spray, choose the correct installation orientation

In sandy/dusty areas (e.g., deserts), increase the south-facing tilt angle to 30° (from the typical 25°) to allow sand/dust to slide off.

In salt spray areas (coastal), use anodized aluminum frames (resistant to chloride ion corrosion) instead of ordinary aluminum alloy.

At an inland power plant in Australia, changing the tilt angle from 25° to 30° reduced sand accumulation by 40% and micro-cracks by 30%.

At a coastal power plant in Florida, USA, after switching to anodized aluminum frames, the frame corrosion area after 8 years was 80% smaller than with ordinary frames (visual inspection records).