Improved photon harvesting by employing C[sub]70[/sub] in bulk heterojunction solar cells

Steffen Pfuetzner; Jan Meiss; Selina Olthof; Moritz P. Hein; Annette Petrich; Lothar Dunsch; Karl Leo; Moritz Riede

doi:10.1117/12.854402

18 May 2010 Improved photon harvesting by employing C₇₀ in bulk heterojunction solar cells

Steffen Pfuetzner, Jan Meiss, Selina Olthof, Moritz P. Hein, Annette Petrich, Lothar Dunsch, Karl Leo, Moritz Riede

Proceedings Volume 7725, Photonics for Solar Energy Systems III; 77250E (2010) https://doi.org/10.1117/12.854402
Event: SPIE Photonics Europe, 2010, Brussels, Belgium

Abstract

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 JSC 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 JSC 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.

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Steffen Pfuetzner, Jan Meiss, Selina Olthof, Moritz P. Hein, Annette Petrich, Lothar Dunsch, Karl Leo, and Moritz Riede "Improved photon harvesting by employing C₇₀ in bulk heterojunction solar cells", Proc. SPIE 7725, Photonics for Solar Energy Systems III, 77250E (18 May 2010); https://doi.org/10.1117/12.854402

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