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Organic Photovoltaics

9.4 Structure-function relationships in donor-acceptor systems

2 min readLast Updated on July 25, 2024

Donor-acceptor systems are the backbone of organic photovoltaics. These two-component systems enable efficient charge separation, with donors providing electrons and acceptors creating an electron-deficient region. This setup promotes hole transfer and enhances photocurrent generation in organic solar cells.

HOMO-LUMO energy levels are crucial for solar cell efficiency. They determine open-circuit voltage and charge transfer efficiency. Tuning these levels allows optimization of both voltage and charge transfer, while proper orbital overlap enhances charge separation and overall device performance.

Fundamentals of Donor-Acceptor Systems

Donor-acceptor systems in photovoltaics

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  • Two-component system in organic photovoltaics enables efficient charge separation and facilitates electron transfer from donor to acceptor
  • Donor materials (conjugated polymers or small molecules) provide electron-rich environment while acceptor materials (fullerene derivatives or non-fullerene acceptors) create electron-deficient region
  • Promotes hole transfer from acceptor to donor enhancing photocurrent generation in organic solar cells
  • System architecture optimizes charge separation and transport improving overall device efficiency

HOMO-LUMO levels for solar efficiency

  • HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels determine key performance parameters
  • Open-circuit voltage (VOCV_{OC}) correlates with energy difference between donor HOMO and acceptor LUMO (VOCV_{OC} ∝ (HOMOdonor_{donor} - LUMOacceptor_{acceptor}))
  • Charge transfer efficiency depends on energy offset between donor LUMO and acceptor LUMO with optimal LUMO-LUMO offset of 0.3-0.5 eV
  • Cascading energy levels (Donor HOMO > Donor LUMO > Acceptor LUMO > Acceptor HOMO) ensure efficient charge separation and transport
  • Tuning HOMO-LUMO levels allows optimization of both VOCV_{OC} and charge transfer efficiency

Orbital overlap for charge separation

  • Molecular orbital overlap determines electronic coupling strength between donor and acceptor influencing charge transfer rate
  • Charge transfer states form during electron transfer with binding energy affecting separation efficiency
  • Delocalization of charge transfer states energetic disorder at donor-acceptor interface and local electric fields impact charge separation
  • Marcus theory of electron transfer describes charge transfer rate dependence on reorganization energy and electronic coupling
  • Optimizing orbital overlap through molecular design enhances charge separation and overall device performance

Structural modifications of donor-acceptor cells

  • Non-fullerene acceptors (NFAs) offer advantages over fullerene acceptors:
    • Tunable energy levels enhance light absorption and reduce voltage losses
    • High-performance NFA examples include ITIC derivatives and Y-series acceptors
  • Ternary blends (donor + primary acceptor + secondary component) provide:
    • Broadened absorption spectrum improved charge transport and enhanced morphological stability
  • Side chain engineering influences solubility and molecular packing
  • Backbone modifications affect energy levels and absorption properties
  • Interfacial engineering improves charge extraction and reduces recombination
  • Morphology control techniques (solvent additives thermal annealing solvent vapor annealing) optimize donor-acceptor phase separation
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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