Tilted bulk heterojunction organic photovoltaic cells grown by oblique angle deposition

We demonstrate small molecule bulk heterojunction organic photovoltaic cells using oblique angle vacuum deposition. Obliquely deposited donor chloroaluminum phthalocyanine (ClAlPc) films on indium tin oxide have surface feature sizes of similar to 30 nm, resulting in ClAlPc/C-60 donor-acceptor heterojunctions (HJs) with approximately twice the interface area of HJs grown at normal incidence. This results in nearly twice the external quantum efficiency in the ClAlPc absorption band compared with analogous, planar HJs. The efficiency increase is attributed to the increased surface area presented by the donor-acceptor junction to the incident illumination by ClAlPc protrusions lying obliquely to the substrate plane formed during deposition. The power conversion efficiency improves from (2.0 +/- 0.1)% to (2.8 +/- 0.1)% under 1 sun, AM 1.5G simulated solar illumination. Similarly, the power efficiency of copper phthalocyanine/C-60 organic photovoltaic cells is increased from (1.3 +/- 0.1)% to (1.7 +/- 0.1)%. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3236838]