Control of molecular orientation and morphology in organic bilayer solar cells: Copper phthalocyanine on gold nanodots

• Organic bilayer photovoltaic cells were constructed with inserted Au nanodot layer.
• Face-on orientation of copper phthalocyanine was achieved on Au nanodots.
• Photoabsorption of copper phthalocyanine layer was enhanced.
• Donor/acceptor interface area was largely increased.
• Inserting the Au nanodot layer enhanced power conversion efficiency.

Molecular orientation, morphology of donor (D)/acceptor (A) interface and photoabsorptivity in organic bilayer solar cells were controlled using Au nanodots with an ~ 20 nm diameter inserted between the bottom electrode and the organic layer. Copper phthalocyanine (CuPc) molecules deposited onto the Au nanodot-coated electrode were mostly oriented face-on with large surface roughness, which is beneficial for photoabsorption, charge separation and transport. Furthermore, Au nanodots exhibit blue-shifted plasmon bands so that CuPc absorbs light more efficiently than that on thin Au layer. Bilayer C60/CuPc solar cells containing Au nanodots exhibited 1.4 times higher photoelectric conversion efficiency than those without Au nanodots. Factors for the enhanced efficiency are (i) improvement of the optical absorption characteristics by face-on orientation of CuPc and (ii) increase of the D/A heterointerface area. In addition, the shift of the plasmon absorption band of Au by the formation of nanodots makes absorption of the CuPc layer much more efficiently, resulting in better photovoltaic output.