Recombination in polymer-fullerene bulk heterojunction solar cells

TL;DR

This study investigates the recombination mechanisms in polymer-fullerene bulk heterojunction solar cells, revealing voltage-dependent recombination kinetics and explaining the open circuit voltage discrepancy through temperature-dependent quasi-Fermi level behavior.

Contribution

It provides a detailed analysis of recombination processes and their evolution with voltage, offering insights for improving solar cell efficiency.

Findings

Recombination transitions from first order to bimolecular with increasing voltage.

Recombination kinetics are voltage dependent and evolve during operation.

The 'missing 0.3V' in open circuit voltage is due to temperature effects on quasi-Fermi levels.

Abstract

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 0.3V" 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 upon the fundamental statistics of Fermions.