4.3 Charge separation and collection

Charge separation in organic photovoltaics involves exciton formation, diffusion, and dissociation at donor-acceptor interfaces. Factors like energy level alignment, interfacial area, and molecular orientation influence this process. Understanding these mechanisms is crucial for improving device efficiency.

Charge transport relies on hopping between localized states, with mobility and recombination impacting device performance. Optimizing factors like molecular packing and active layer thickness can enhance charge collection. These processes are key to developing more efficient organic solar cells.

Charge Separation Process

Charge separation at donor-acceptor interfaces

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• Exciton formation
  • Light absorption by donor material excites electrons creating bound electron-hole pairs (excitons)
• Exciton diffusion
  • Excitons move to donor-acceptor interface constrained by diffusion length (~10-20 nm)
• Charge transfer state formation
  • Electron transfers from donor to acceptor while hole remains on donor molecule forming intermediate state
• Charge separation
  • Overcoming Coulombic attraction leads to free charge carriers (electrons and holes)

Factors for efficient charge separation

• Energy level alignment
  • HOMO-LUMO offset between donor and acceptor drives charge transfer (0.3-0.5 eV ideal)
• Interfacial area
  • Bulk heterojunction architecture increases donor-acceptor contact enhancing exciton dissociation
• Molecular orientation
  • π-π stacking facilitates charge transfer by improving orbital overlap
• Dielectric constant
  • Higher values screen Coulombic attraction promoting charge separation (ε > 3)
• Temperature
  • Thermal energy assists in overcoming exciton binding energy (0.1-0.5 eV)
• Electric field
  • Built-in field at interface aids charge separation (105-106 V/cm)

Charge Transport and Collection

Transport of charges to electrodes

• Hopping mechanism
  • Charge carriers jump between localized states influenced by energetic and positional disorder
• Charge carrier pathways
  • Continuous pathways for electrons and holes require optimized phase separation in bulk heterojunctions
• Drift and diffusion
  • Drift moves charges due to electric field while diffusion occurs from concentration gradient
• Electrode selection
  • Work function matching enables efficient charge extraction (ITO/PEDOT:PSS for holes, Al/ETL for electrons)

Impact of mobility and recombination

• Charge carrier mobility
  • Defined as drift velocity per unit electric field (typically 10^-7 to 10^-3 cm^2/Vs)
  • Higher mobility improves charge collection
  • Factors affecting mobility:
    1. Molecular packing
    2. Crystallinity
    3. Purity of materials
• Recombination mechanisms
  • Geminate recombination occurs between electron-hole pair from same exciton
  • Non-geminate recombination includes bimolecular and trap-assisted processes
• Impact on device parameters
  • Short-circuit current ($J_{sc}$) increases with higher mobility and reduced recombination
  • Fill factor (FF) depends on mobility-recombination balance
  • Open-circuit voltage ($V_{oc}$) decreases with increased recombination
• Strategies to mitigate recombination
  • Vertical phase segregation improves charge extraction
  • Interfacial layers reduce surface recombination
  • Optimizing active layer thickness balances absorption and charge collection (typically 100-200 nm)