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.
Highlights
Depending on the materials used, organic tandem solar cells fall into three categories: polymer[13, 17,18,19], small-molecule[20, 21], and hybrid[22] solar cells using both polymer and small-molecule materials
Following the spin-coating, the PDMS stamp is stored in high vacuum (~10−7 Torr) for 1.5 h to remove the DIO additive, which is required to obtain a bulk heterojunction (BHJ) morphology with a high internal quantum efficiency (IQE)[31]
The immediate removal of the additive is found to be crucial for successful transfer of the PCPDTBT:PC70BM BHJ layer onto the bottom subcell, since a slower drying process, such as commonly used solvent annealing[32], causes the PDMS stamp to swell, which leads to diffusion of PCPDTBT:PC70BM into the stamp
Summary
Depending on the materials used, organic tandem solar cells fall into three categories: polymer[13, 17,18,19], small-molecule[20, 21], and hybrid[22] solar cells using both polymer and small-molecule materials. The attractive feature of hybrid organic tandem solar cells is that active materials can be chosen from a large number of candidates encompassing various polymers and small molecules Using both polymer and small-molecule materials, it is easier to realize organic tandem cells with constituent subcells having complementary absorption spectra, which minimizes the thermalization loss if a device design is such that higher (and lower) energy photons are absorbed in the subcell with a higher (and lower) open-circuit voltage (VOC). Despite this advantage, the highest PCE of hybrid tandem solar cells reported so far is 4.8%22, while tandem devices based on polymers and small molecules, respectively, have achieved PCEs as high as 10.6%15 and 12%23. Our work shows that the thin-film transfer technique is capable of overcoming the restriction present in designing small molecule–polymer tandem solar cells, that is, the polymer subcell must be at the bottom, thereby allowing for the maximal utilization of materials space that small molecules and polymers offer in combination
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