Energy Level Alignment at the Anode of Poly(3-hexylthiophene)/Fullerene-Based Solar Cells

We have used ultraviolet photoelectron spectroscopy to investigate the energy-level and band alignment near the anode for poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT/PCBM)-based organic solar cells. Analysis of various batches of indium-tin oxide (ITO) revealed that the photoresist residues had a strong effect, reducing the work functions of ITO (Phi(ITO)) by as much as 0.61 eV. The energy-level alignment of poly(3,4-ethylenedioxythiophene)/ITO (Phi(PEDOT/ITO)) interfaces obey the Mott-Schottky rule at values of Phi(ITO) of less than 3.92 eV. In contrast, we observed Fermi-level pinning for the blend/PEDOT interfaces at values of Phi(PEDOT/ITO) greater than 4.26 eV; this finding is consistent with a previous report that the positive polaronic energy of P3HT is equal to 4.0 eV. Consequently, we suspect that the similar efficiency levels and open-circuit voltages of devices prepared from various ITO samples were due mainly to the constant interfacial energy barrier at the blend/PEDOT interface with Fermi-level pinning.