Organic Photovoltaic Cells Based On Solvent-Annealed, Textured Titanyl Phthalocyanine/C60 Heterojunctions

Organic photovoltaic cells (OPV) with good near-IR photoactivity are created from highly textured titanyl 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 1 -> Phase II transition broadens the absorbance spectrum of the Pc film producing absorptivities (alpha approximate to 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 x as a result of the phase transformation. Current-voltage (J-V) responses for white light illumination of ITO (100 nm)/TiOPc (20 nm)/C-60 (40 nm)/BCP (10 nm)/Al (100 nm) 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 II-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.