9.3 Impact of processing conditions on device performance

Film deposition techniques shape organic photovoltaic performance. Spin coating, blade coating, and ink-jet printing create thin films with varying thickness control and scalability. Deposition speed affects molecular orientation and film uniformity, impacting charge transport and exciton dissociation.

Solvents and processing conditions influence heterojunction morphology. Solvent properties, temperature, and humidity affect polymer-fullerene mixing and phase separation. Post-processing techniques like thermal annealing and solvent vapor annealing optimize domain size and crystallinity, enhancing charge carrier mobility and device efficiency.

Film Deposition and Morphology

Film deposition techniques for photovoltaics

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• Spin coating rapidly rotates substrate spreading solution uniformly creating thin films through centrifugal force and fast solvent evaporation affecting crystallization (nanometer-scale thickness control)
• Blade coating moves blade to spread solution allowing slower process with more control over film thickness enabling large-area fabrication (meter-scale roll-to-roll processing)
• Ink-jet printing precisely deposits droplets enabling patterned films and multi-layer structures with droplet size and spacing impacting film uniformity (micrometer-scale resolution)
• Deposition speed influences molecular orientation and film uniformity affecting charge transport while crystallinity and domain size impact exciton dissociation (polymer alignment, fullerene aggregation)

Solvents and morphology in heterojunctions

• Solvent boiling point affects evaporation rate influencing polymer-fullerene mixing and phase separation dynamics (chlorobenzene, dichlorobenzene)
• Temperature and humidity alter film formation process and drying rate impacting phase separation dynamics (20-80℃, 30-70% RH)
• Solvent evaporation drives phase separation promoting polymer and fullerene self-organization with domain size and purity affecting charge generation and transport (10-20 nm optimal domains)

Post-Processing and Device Architecture

Post-processing for device optimization

• Thermal annealing increases polymer crystallinity promoting phase separation in bulk heterojunction and optimizing domain size for efficient charge transport (100-150℃)
• Solvent vapor annealing increases molecular mobility without high temperatures allowing reorganization of polymer and fullerene phases leading to favorable morphologies for charge separation (chloroform, toluene vapors)
• Post-processing enhances charge carrier mobility improves exciton dissociation at interfaces and creates better contact between active layer and electrodes (2-10x mobility increase)

Architecture impact on photovoltaic efficiency

• Active layer thickness affects light absorption and charge collection requiring balance between absorption and transport (optimal range 100-300 nm)
• Device architectures include:
  1. Conventional (ITO/PEDOT:PSS/Active Layer/ETL/Metal)
  2. Inverted (ITO/ETL/Active Layer/HTL/Metal)
  3. Tandem cells for improved spectral coverage
• Buffer layers enhance charge extraction (ZnO, TiOx for electrons; MoO3, V2O5 for holes)
• Short-circuit current depends on absorption and charge collection while open-circuit voltage affected by energy levels and recombination (JSC ∝ absorption, VOC ∝ HOMO-LUMO difference)
• Fill factor influenced by series and shunt resistances (ideal FF > 0.75)
• Encapsulation prevents oxygen and moisture ingress while interfacial layers improve electrode stability (glass, flexible barriers)
• Material choice impacts long-term morphological stability (fullerene-free acceptors, crosslinkable polymers)