Recombination in polymer-fullerene bulk heterojunction solar cells

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2010-12, Vol.82 (24), Article 245207
• Main Authors: Cowan, Sarah R., Roy, Anshuman, Heeger, Alan J.
• Format: Article
• Language: English
• Subjects: solar (photovoltaic), solid state lighting, phonons, thermoelectric, bio-inspired, energy storage (including batteries and capacitors), electrodes - solar, defects, charge transport, materials and chemistry by design, optics, synthesis (novel materials), synthesis (self-assembly), synthesis (scalable processing); SOLAR ENERGY
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.82.245207

Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc ) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature-dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first-order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The “missing 0.3 V” inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open-circuit voltage at room-temperature results from the temperature dependence of the quasi-Fermi levels in the polymer and fullerene domains—a conclusion based on the fundamental statistics of fermions.