Hybrid Organic Tandem Solar Cell Comprising Small-Molecule Bottom and Polymer: Fullerene Top Subcells Fabricated by Thin-Film Transfer

Abstract

Multilayer structures involving solution-deposited polymer films are difficult to fabricate, not allowing for unrestricted designs of polymer-based optoelectronic devices required for maximizing their performance. Here, we fabricate a hybrid organic tandem solar cell whose top and bottom subcells have polymer:fullerene and small molecules active layers, respectively, by a solvent-free process based on transferring the polymer:fullerene layer from an elastomeric stamp onto a vacuum-deposited bottom subcell. The interface between small-molecule and transferred polymer:fullerene layers is void-free at the nanoscale, allowing for efficient charge transport across the interface. Consequently, the transfer-fabricated tandem cell has an open-circuit voltage (VOC) almost identical to the sum of VOC values for the single-junction devices. The short-circuit current density (JSC) of the tandem cell is maximized by current matching achieved by varying the thickness of the small-molecule active layer in the bottom subcell, which is verified by numerical simulations. The optimized transfer-fabricated tandem cell, whose active layers are composed of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]:[6,6]-Phenyl-C71-butyric acid methyl ester and Di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane:C70, has VOC = 1.46 V, JSC = 8.48 mA/cm2, a fill factor of 0.51, leading to the power-conversion efficiency of 6.26%, the highest among small molecule-polymer:fullerene hybrid tandem solar cells demonstrated so far.