1.3 Historical development and current state of organic solar cells

Organic photovoltaics have come a long way since 1958. From the discovery of the photovoltaic effect in organic materials to achieving over 20% efficiency in tandem devices, the field has seen remarkable progress. Key breakthroughs like bulk heterojunction architecture and non-fullerene acceptors have propelled the technology forward.

Today, organic solar cells are approaching the performance of some inorganic technologies. With improved stability and emerging applications like indoor light harvesting and flexible devices, the future looks bright. However, challenges in scaling up production and reducing costs remain hurdles to widespread adoption.

Historical Development of Organic Photovoltaics

Milestones in organic photovoltaics

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• 1958: Photovoltaic effect first observed in organic materials when Kearns and Calvin discovered it using magnesium phthalocyanine, laying foundation for future research
• 1986: Bilayer heterojunction concept introduced by Tang at Eastman Kodak achieved 1% power conversion efficiency, marking significant progress in device architecture
• 1992: Conductive polymers discovered by Heeger, MacDiarmid, and Shirakawa revolutionized organic electronics, leading to Nobel Prize in Chemistry (2000)
• 1995: Bulk heterojunction (BHJ) concept introduced by Heeger and Wudl at UCSB improved charge separation and collection, boosting device performance
• 2001: Small molecule-based organic solar cells developed by Forrest and Thompson at Princeton University expanded material options beyond polymers
• 2013: Tandem organic solar cells surpassed 10% efficiency milestone achieved by Yang Yang's group at UCLA, demonstrating potential for high-performance devices

Breakthroughs and challenges

• Breakthroughs:
  • Bulk heterojunction architecture enhanced charge separation and transport
  • Low-bandgap polymers developed to better match solar spectrum
  • Inverted device structures improved stability and compatibility with printing techniques
  • Non-fullerene acceptors boosted efficiency and tunability of absorption
  • Ternary blend systems allowed for broader spectral coverage and improved charge transport
• Challenges:
  • Improving power conversion efficiency to compete with inorganic solar cells
  • Enhancing long-term stability to match lifetimes of silicon-based panels
  • Scaling up production while maintaining performance and uniformity
  • Reducing material costs to make OPVs economically competitive
  • Addressing toxicity concerns of some materials (lead-based perovskites)

Current State and Future Prospects

State-of-the-art performance and stability

• Performance:
  • Single-junction devices reached ~18% efficiency rivaling some inorganic technologies
  • Tandem devices achieved ~20% efficiency by combining complementary absorbers
  • Module efficiencies climbed to ~10-12% demonstrating scalability potential
• Stability:
  • Operational lifetime extended to ~10 years for best-performing devices through material and interface engineering
  • Encapsulation techniques improved device longevity by protecting against moisture and oxygen
  • Burn-in effect remains a concern causing initial performance drop in first hours of operation
• Emerging trends:
  • Indoor light harvesting optimized for low-light conditions (office lighting)
  • Flexible and wearable devices integrated into clothing and portable electronics
  • Semi-transparent solar cells developed for building integration (windows)

Leading research institutions

• University of California, Los Angeles (UCLA): Yang Yang's group pioneered high-efficiency tandem cells
• University of Michigan: Stephen Forrest's group specialized in small molecule OPVs and vapor deposition techniques
• Linköping University, Sweden: Olle Inganäs' group focused on polymer solar cells and green materials
• KAUST, Saudi Arabia: Thomas Anthopoulos' group developed non-fullerene acceptors and solution-processed devices
• Chinese Academy of Sciences: Yongfang Li's group synthesized new polymer donors with improved properties
• University of Erlangen-Nuremberg, Germany: Christoph Brabec's group studied device physics and upscaling processes
• Imperial College London, UK: James Durrant's group investigated charge dynamics in OPVs using advanced spectroscopy