Simultaneous determination of carrier lifetime and electron density-of-states in P3HT:PCBM organic solar cells under illumination by impedance spectroscopy

Abstract

We report new insights into recombination kinetics in poly(3-hexylthiophene):methanofullerene (P3HT:PCBM) bulk heterojunction (BHJ) solar cells, based on simultaneous determination of the density of states (DOS), internal recombination resistance, and carrier lifetime, at different steady states, by impedance spectroscopy. A set of measurements at open circuit under illumination was performed aiming to better understand the limitations to the photovoltage, which in this class of solar cells remains far below the theoretical limit which is the difference between the LUMO level of PCBM and the HOMO of P3HT (∼1.1 eV). Recombination kinetics follows a bimolecular law, being the recombination time (lifetime) inversely proportional to the density of photogenerated charges and the recombination coefficient γ=6×10^-13 cm³ s^-1. We find that the open-circuit photovoltage is governed by the carrier ability of occupying the DOS, which results in Gaussian shape and spreads in energy σ≈125-140 meV. The energy position of the Gaussian DOS center (E_L=0.75-0.80 eV), which corresponds to half occupation of the electron DOS, approximates LUMO(PCBM)-HOMO(P3HT) difference. But the recombination rate is strongly enhanced at high illumination levels, what produces the photogenerated charge to remain in the tail of the DOS. Consequently, the electron and hole Fermi levels are unable to reach the center of the DOS, then substantially limiting the photovoltage. Detailed theoretical analysis of the lifetime dependence on photovoltage is provided. © 2009 Elsevier B.V. All rights reserved.