Organic photovoltaic cells based on solvent-annealed, textured titanyl phthalocyanine/c60 heterojunctions

Diogenes Placencia, Weining Wang, R. Clayton Shallcross, Kenneth W. Nebesny, Michael Brumbach, Neal R. Armstrong

Research output: Contribution to journalArticle

113 Scopus citations

Abstract

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 "Phase II" polymorph using post-deposition solvent annealing. The Phase IρPhase II transition broadens the absorbance spectrum of the Pc film producing absorptivities (α=105 cm-1) from 600-900 nm, along with substantial texturing of the Pc layer. Atomic force microscopy and fieldemission 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 shortcircuit 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.

Original languageEnglish (US)
Pages (from-to)1913-1921
Number of pages9
JournalAdvanced Functional Materials
Volume19
Issue number12
DOIs
StatePublished - Jun 23 2009

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics

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