4.1 Solar spectrum and light absorption in organic materials

The solar spectrum's characteristics are crucial for organic photovoltaics. From UV to infrared, different wavelengths impact solar cell performance. Understanding these aspects helps optimize device design and efficiency.

Light absorption in organic materials relies on electronic transitions and molecular structure. Factors like conjugation length and film morphology affect absorption properties, directly influencing solar cell performance metrics like current density and voltage.

Solar Spectrum Characteristics

Characteristics of solar spectrum

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• Solar radiation spectrum spans wavelengths 100 nm to 1 mm with peak intensity around 500 nm (visible light)
• Spectral irradiance distribution divided into UV (100-400 nm), visible (400-700 nm), and infrared (700 nm - 1 mm) regions
• Air Mass (AM) standards quantify atmospheric effects on spectrum
  • AM0 represents extraterrestrial spectrum unaffected by atmosphere
  • AM1.5 simulates terrestrial spectrum used for standardized solar cell testing
• Blackbody radiation approximates sun's emission with surface temperature ~5800 K
• Solar constant measures average irradiance at Earth's surface ~1000 W/m² varying with atmospheric conditions and location

Light Absorption in Organic Materials

Absorption in organic materials

• Electronic transitions drive light absorption in organic molecules
  • π-π* transitions serve as primary absorption mechanism in conjugated systems
  • n-π* transitions occur less frequently due to quantum mechanical restrictions
• Conjugated systems with delocalized electrons reduce bandgap enabling visible light absorption
• Absorption bands typically occur in visible and near-IR regions overlapping with solar spectrum peak
• Molar extinction coefficient quantifies absorption strength generally high for organic materials (100,000 L mol⁻¹ cm⁻¹)

Factors affecting light absorption

• Molecular structure influences absorption through conjugation length and electron-donating/withdrawing groups
• Intermolecular interactions like π-π stacking and aggregate formation (H- and J-type) alter absorption properties
• Film morphology affects light interaction through crystallinity and domain characteristics
• Environmental factors such as temperature and solvent environment modulate absorption behavior
• Optical path length determined by film thickness and light trapping techniques impacts overall absorption

Impact of absorption on performance

• Absorption coefficient (α) relates to film thickness through Beer-Lambert law $I = I_0 e^{-αx}$
• Absorption spectrum shows wavelength-dependent absorption correlating with external quantum efficiency (EQE)
• Spectral mismatch between absorption and solar spectrum impacts short-circuit current density (Jsc)
• Optical band gap determines long-wavelength absorption edge influencing open-circuit voltage (Voc)
• Absorption breadth (broad vs narrow) affects overall photocurrent generation
• Light harvesting efficiency measures absorbed to incident photon ratio correlating with device performance metrics (power conversion efficiency)