Organic Photovoltaic Cells Based On Solvent-Annealed,Textured Titanyl Phthalocyanine/C_(60) Heterojunctions

摘要

Organic photovoltaic cells (OPV) with good near-IR photoactivity are created from highly textured titanyt phthalocyanine (TiOPc)/C_(60) heterojunctions. Vacuum deposited TiOPc thin films are converted to the near-IR absorbing "Phase.II" polymorph using post-deposition solvent annealing. The Phase I - Phase II transition broadens the absorbance spectrum of the Pc film producing absorptivities (a ≈ 10~5 cm~(-1)) from 600-900 nm, along with substantial texturing of the Pc layer. Atomic force microscopy and field-emission scanning electron microscopy of the solvent annealed films show ' that the surface roughness of the Pc layers is increased by a factor of greater than 2× as a result of the phase transformation. Current-voltage (J-V) responses for white light illumination of ITO (100nm)/TiOPc (20nm)/C_(60) (40nm)/BCP (10nm)/Al (100nm) OPVs show a near doubling of the short-circuit photocurrent (J_(sc)), with only a small decrease in open-circuit photopotential (V_(oc)), and a concomitant increase in power conversion efficiency. Incident photon current efficiency (IPCE) plots confirmed the enhanced near-IR OPV activity, with maximum IPCE values of ca. 30% for devices using Phase ll-only TiOPc films. UV-photoelectron spectroscopy (UPS) of TiOPc/C_(60) heterojunctions, for both Phase I and Phase II TiOPc films, suggest that the Phase II polymorph has nearly the same HOMO energy as seen in the Phase I polymorph, and similar frontier orbital energy offsets, E_(HOMO)~(pc)-E_(LUMO)~(c60), leading to comparable open-circuit photovoltages. These studies suggest new strategies for the formation of higher efficiency OPVs using processing conditions which lead to enhance near-IR absorptivities, and extensive texturing of crystalline donor or acceptor films.