Quick Facts
What quantum efficiency is
Quantum Efficiency (QE) is a fundamental characterisation parameter for solar cells. It describes the cell’s ability to convert photons (light particles) into electrons (charge carriers that produce current). QE is measured at each wavelength independently, giving a wavelength-resolved view of cell performance.
Two main types are commonly measured:
External Quantum Efficiency (EQE): Electrons collected per photon incident on the cell.
Internal Quantum Efficiency (IQE): Electrons collected per photon absorbed in the cell.
EQE includes all losses including reflection and parasitic absorption. IQE excludes reflection losses but includes recombination and transport losses inside the cell.
QE measurement is essential for understanding cell physics, identifying loss mechanisms, guiding design improvements, and quality control in production. Premium silicon cells achieve EQE above 90 percent across most of the useful spectrum.
EQE versus IQE
The key distinction:
EQE:
Includes reflection losses (light bouncing off the cell surface).
Includes parasitic absorption (light absorbed by encapsulant, contacts, etc.).
Includes recombination and transport losses in the cell.
Practical measure for actual cell performance.
Typically 70 to 95 percent across visible spectrum.
IQE:
Excludes reflection losses.
Excludes parasitic absorption.
Includes only recombination and transport losses.
Pure measure of cell physics.
Typically 90 to 98 percent across visible spectrum.
Both are useful. EQE shows actual cell performance. IQE shows fundamental cell physics. The difference between them indicates reflection and absorption opportunities for improvement.
QE measurement procedure
QE is measured in laboratory conditions:
Setup:
Monochromator (tunable narrowband light source).
Reference detector (calibrated photon counter).
Cell holder with bias and current measurement.
Computer-controlled wavelength stepping.
Procedure:
Set monochromator to specific wavelength.
Measure incident photon flux using reference detector.
Illuminate solar cell.
Measure cell current at short-circuit condition.
Calculate EQE = (cell current / electron charge) / (incident photon flux).
Repeat across wavelengths (typically 300 to 1200 nm).
Plot EQE versus wavelength.
For IQE measurement, additional reflection measurement is needed to correct for front-surface losses.
The result is a QE curve showing detailed cell wavelength performance.
QE curve interpretation
A typical QE curve shows:
Short wavelengths (300-400 nm, UV-blue):
EQE may drop due to front-surface absorption.
Some cells (HJT) have better short-wavelength response.
Mid wavelengths (400-800 nm, blue-red):
Peak EQE region.
Premium cells achieve 90 to 95 percent EQE here.
Long wavelengths (800-1100 nm, IR):
EQE decreases as photons penetrate deep without being absorbed.
TOPCon and BSF technologies improve long-wavelength response.
Above 1100 nm (deep IR):
EQE drops to zero (below silicon bandgap; not converted).
Each region reveals specific loss mechanisms and design opportunities.
QE differences across technologies
Different silicon technologies show characteristic QE patterns:
Mono PERC:
Peak EQE 85 to 90 percent.
Standard silicon curve.
Some IR response improvement from rear passivation.
TOPCon:
Higher EQE at long wavelengths.
Better rear-side carrier collection.
Higher cell efficiency.
HJT:
Higher EQE at short wavelengths.
Better front-side passivation.
Some loss at long wavelengths.
IBC:
Highest EQE across spectrum.
No front grid losses.
Premium efficiency.
Cell technology selection involves balancing EQE characteristics against cost and manufacturing complexity.
QE-related loss mechanisms
Multiple loss mechanisms reduce EQE:
Reflection losses:
Front surface reflection (despite ARC).
Internal reflection losses.
Improved by texturing and coatings.
Parasitic absorption:
Front grid absorption (shaded area).
Encapsulant and glass absorption (small fraction).
Internal layer absorption.
Recombination losses:
Surface recombination at front and rear.
Bulk recombination in silicon.
Recombination at metal-semiconductor interfaces.
Improved by passivation and selective contacts.
Transport losses:
Series resistance.
Carrier mobility limitations.
Particularly important at high current.
Each loss mechanism is addressed by specific cell design strategies. Quantitative QE measurement helps prioritise improvements.
QE and cell efficiency
QE relates to cell efficiency through:
Cell current: Integration of QE × incident photon flux × electron charge over wavelength.
Short-circuit current: Maximum current at zero voltage.
Open-circuit voltage: Determined by cell physics (different parameter).
Fill factor: Series and parallel resistances.
Cell efficiency: Combined effect of all parameters.
EQE measurement contributes to understanding current generation. Voltage and fill factor measurements complement EQE for full cell characterisation.
QE in cell design optimisation
Cell designers optimise QE through:
Anti-reflective coatings: Reduce reflection losses.
Surface texturing: Light trapping reduces reflection.
Front-side passivation: Reduces short-wavelength loss.
Rear-side passivation: Reduces long-wavelength loss.
Selective contacts (HJT, TOPCon): Improve carrier collection.
Reduced shading: Smaller grid lines, IBC architecture.
Light trapping: Maximises absorption in thin silicon.
Each design choice affects QE differently. Comprehensive characterisation guides optimisation.
QE and field performance
While QE is measured under laboratory conditions:
STC (Standard Test Conditions): AM 1.5G spectrum, 25 deg C, 1000 W/m².
Field conditions: Variable spectrum, temperature, irradiance.
Spectral mismatch: Field spectrum differs from STC.
Temperature: Cell temperature affects efficiency.
QE measurements at STC provide baseline. Field performance depends on actual conditions and cell characteristics.
For specific climate conditions:
Hot climates: HJT and TOPCon often perform better.
Diffuse light conditions: HJT often performs better.
Low-light conditions: Depends on technology.
Best practices for QE measurement
For laboratory measurement:
Calibrated reference detector (NIST-traceable).
Stable monochromator output.
Cell at controlled temperature (25 deg C standard).
Accurate cell holder geometry.
Reproducible procedures.
For data interpretation:
QE curve analysis identifies loss mechanisms.
Comparison across technologies guides selection.
QE alone is insufficient; voltage and fill factor also important.
Field performance integrates QE with real-world conditions.
For research:
QE measurement guides cell design improvements.
Loss mechanism quantification.
Technology comparison.
Common QE misunderstandings
Treating QE as identical across cell types. Significant variations.
Ignoring spectral integration. Total current depends on photon flux integration, not just QE peak.
Confusing EQE and IQE. Different but related metrics.
Overemphasising peak EQE. Broader response often more important.
Missing field performance correlation. STC measurements don’t capture all conditions.
Standards and references
QE measurement is standardised in IEC 60904 series (especially 60904-8) and ASTM E1021. NIST-traceable calibration ensures comparability. Cell characterisation labs (NREL, Fraunhofer ISE, CSIRO) provide reference measurements.
Related glossary terms
Key takeaways
Quantum Efficiency (QE) measures the ratio of electrons collected to photons incident on a solar cell at each wavelength. External Quantum Efficiency (EQE) includes reflection and absorption losses; Internal Quantum Efficiency (IQE) includes only recombination and transport losses. Premium silicon cells achieve EQE above 90 percent across most visible spectrum. QE measurement reveals cell physics, identifies loss mechanisms (reflection, recombination, transport), and guides design improvements. Different silicon technologies (Mono PERC, TOPCon, HJT, IBC) have characteristic QE curves reflecting their architectural advantages. QE measurement follows IEC 60904 and ASTM E1021 standards using monochromators and calibrated detectors.