Efficient Vacuum-Deposited Tandem Organic Solar Cells with Fill Factors Higher Than Single-Junction Subcells

Efficient vacuum-deposited tandem organic photovoltaic cells (TOPVs) composed of pristine fullerenes as the acceptors and two complementary absorbing donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile for the visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile for the near-infrared absorption, are reported. Two subcells are connected by the interconnection unit (ICU) composed of electron-transporting layer/metal/p-doped hole-transporting layer. The p-doped layer in the ICU enables increasing the short-circuit current density (J SC) of TOPVs by tuning the relative position of subcells in the tandem devices to have the maximum optical field distribution response, which is well matched with theoretical calculation. Moreover, the introduction of the doped layer benefits to the higher fill factor (FF) of the consisting subcells without losing open-circuit voltage (V OC) even with the thick active layers. As a result, power conversion efficiency of 9.2% is achieved with higher FF of 0.62 than that of single-junction subcells (0.54, 0.57), J SC of 8.7 mA cm-2, and V OC of 1.71 V using 80 nm thick active layers in both subcells. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.