Organic photovoltaics (OPVs) represent one possible route to widespread adoption of solar energy production [1]. The most promising OPV technology to date has been the bulk heterojunction, a photoactive mixture which bridges the gap between exciton diffusion lengths (~15 nm) and typical device length scales (~ 200 nm). However, bulk heterojunctions are typically disordered, which can limit charge extraction and thereby reduce OPV performance. The ideal OPV bulk heterojunction structure has been identified as ordered interpenetrating columnar electron donor and acceptor layers [2]. The interface morphology of such structures may be fabricated with glancing angle deposition (GLAD), a well-developed, robust nanostructuring technique that has been demonstrated for numerous materials at the length scales required for high quality OPV devices.

 

Here we present a summary of organic solar cell research utilizing GLAD at the University of Alberta. GLAD’s flexibility lends itself to structuring both the transparent conductor layer as well as the donor and acceptor photoactive layers. We have demonstrated GLAD structuring of indium tin oxide (ITO), copper (II) phthalocyanine (CuPc), and fullerene (C60). ITO nanopillars fabricated by GLAD have been incorporated as three dimensional high surface area electrodes in organic photovoltaic devices [3]. The nanostructured electrodes demonstrated 30% improved performance compared to planar ITO anodes, due to increased optical absorption and high surface area. Typical OPV donor and acceptor materials, CuPc and C60, have also been successfully structured by the GLAD technique [4,5]. Advanced motion control algorithms, designed to form thinner columns, were used to optimize the material nanostructures and produce highly desirable ordered bulk heterojunctions when coupled with complementary polymers. In the case for GLAD C60 devices, the short-circuit current was double that of bulk heterojunction devices. The GLAD technique has proven to be an incredibly useful method for fabricating and tuning electrode and bulk heterojunction morphologies in OPVs.

 

[1] H. Spanggaard and F.C. Krebs, Sol. Energy Mater. Sol. Cells 83 125-146 (2004)

[2] F. Yang and S.R. Forrest, ACS Nano 2 (5), 1022-1032 (2008)

[3] D.A. Rider et al., Nanotechnology 22 (8), 085706 (2011)

[4] J.G. Van Dijken et al., J. Mater. Chem. 21 (4), 1013-1019 (2011)

[5] M. Thomas et al., ACS Appl. Mater. Interfaces (in press, 2011)