Improved photon harvesting by employing C70 in bulk heterojunction solar cells

Conference item

To achieve higher efficiencies in organic solar cells, ideally the open circuit voltage (V_OC), fill factor (FF) as well as the short current density (J_SC) have to be further improved. However, only a few suitable acceptor molecules, e.g. C₆₀, are currently available for the photoactive layer. Despite a good electron mobility on the order of 1×10⁻³ cm²/Vs the absorption of C₆₀ in the visible sun spectrum is low. From polymer based solar cells it is known that the fullerene derivative [70]PCBM used in the photoactive layer shows a significant enhancement in J compared to [60]PCBM. This work investigates the application of fullerene C₇₀ as acceptor in comparison to the well known C₆₀ in vacuum processed small molecule solar cells. C₇₀ shows a broadened and red shifted absorption (abs. maximum around 500 nm) compared to C₆₀. By fabricating p-i-i solar cells we show that the stronger absorption of C₇₀ leads to enhanced photon harvesting and increased external quantum efficiency. The bulk heterojunction p-i-i solar cell containing C₇₀ as acceptor and ZnPc as donor, co-evaporated with an optimized ratio of 2:1, and a layer thickness of 30 nm shows improved solar cell parameters: a 30% larger photocurrent of 10.1 mA/cm² is obtained. The V_OC of 0.56 V and FF of 55% remain comparable to C₆₀-containing p-i-i solar cells. Therefore, the solar cell performance is mainly improved by J_SC and leads to a mismatch corrected power conversion efficiency of 3.12%. Thus, we show that C₇₀ is an alternative fullerene to C₆₀ for solar cell applications. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Authors: Pfuetzner, S; Meiss, J; Olthof, S; Hein, M; Petrich, A

• Publication status: Published
• Peer review status: Reviewed (other)
• Publisher: Society of Photo-Optical Instrumentation Engineers
• Host title: Proceedings of SPIE - The International Society for Optical Engineering
• Volume: 7725
• Publication date: 2010-01-01
• DOI: 10.1117/12.854402
• ISSN: 0277-786X
• ISBN: 9780819481986
• Language: English
• Keywords: p-i-i; fullerene; p-i-n; C60; organic solar cell; C70; small molecule; bulk heterojunction