Understanding the Effect of Donor Layer Thickness and a MoO3 Hole Transport Layer on the Open-Circuit Voltage in Squaraine/C60 Bilayer Solar Cells

Abstract

Small molecule organic solar cells are becoming increasingly efficient through improved molecular design. However, there is still much to be understood regarding device operation. Here we study bilayer solar cells employing a 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and fullerene acceptor to probe the effect of donor layer thickness and a MoO3 electron transport layer on device performance. The thickness of SQ is seen to drastically affect the open-circuit voltage (VOC) and fill factor (FF), while the short circuit current is not altered significantly. The fact that the VOC of the bilayers with thin (6 nm) donor layers shows a strong dependence on the material and workfunction of the anode cannot be explained with a model for a perfect bilayer. Recombination of electrons from C60 at the anode contact has to be possible to understand the strong effect of the anode workfunction. Using numerical simulations and a simple two-diode model we show that the most likely interpretat...