Organic photovoltaic cells (OPV) with good near-IR photoactivity are created from highly textured titanyl phthalocyanine (TiOPc)/C60 heterojunctions. Vacuum deposited TiOPc thin films are converted to the near-IR absorbing ldquoPhase IIrdquo polymorph using post-deposition solvent annealing. The Phase I rarr Phase II transition broadens the absorbance spectrum of the Pc film producing absorptivities (alpha ≈ 105 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 (100 nm)/TiOPc (20 nm)/C60 (40 nm)/BCP (10 nm)/Al (100 nm) OPVs show a near doubling of the short-circuit photocurrent (JSC), with only a small decrease in open-circuit photopotential (VOC), 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/C60 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, EHOMOPc-ELUMOC60, 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.
Placencia, D., W. Wang, R. C. Shallcross, K. W. Nebesny, M. Brumbach, N. R. Armstrong
Adv. Funct. Mater.