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Carrier density effect on recombination in PTB7-based solar cell.

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Organic solar cells (OSCs) achieve higher power conversion efficiency (PCE) when fast carrier escape from donor/acceptor interfaces prevents recombination. Exciton-to-carrier conversion is rapid, but high carrier density accelerates recombination, limiting PCE.

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Area of Science:

  • Materials Science
  • Photovoltaics
  • Organic Electronics

Background:

  • Organic solar cells (OSCs) offer low-cost, flexible alternatives to inorganic cells.
  • Low band-gap polymers like PTB7 have improved power conversion efficiency (PCE).
  • Understanding carrier dynamics at the donor/acceptor interface is crucial for optimizing OSC performance.

Purpose of the Study:

  • Investigate the relationship between instantaneous carrier density and collected photocurrent in PTB7/C70 heterojunction devices.
  • Determine the critical carrier concentration at which recombination becomes dominant.
  • Elucidate the role of carrier escape from the interface in achieving high PCE.

Main Methods:

  • Time-resolved spectroscopy to study exciton-to-carrier conversion dynamics.
  • Combination of time-resolved and electrochemical spectroscopies to quantify carrier density.
  • Investigation of both heterojunction (HJ) and bulk heterojunction (BHJ) devices.

Main Results:

  • Exciton-to-carrier conversion occurs within ~1 ps at the PTB7/C70 donor/acceptor interface.
  • Carrier recombination dominates when carrier density exceeds a critical concentration (nc = 0.003 carriers/nm(-2)).
  • Similar behavior was observed in PTB7/PC71BM BHJ devices.

Conclusions:

  • Fast carrier escape from the donor/acceptor interface is essential for high PCE in OSCs.
  • Carrier accumulation accelerates recombination nonlinearly, thus limiting device performance.
  • Quantitative analysis reveals the importance of interface dynamics for efficient organic photovoltaics.