Enhanced Fill Factor of Tandem Organic Solar Cells Incorporating a Diketopyrrolopyrrole-Based Low-Bandgap Polymer and Optimized Interlayer

We demonstrate that reproducible results can be obtained from tandem solar cells based on the wide-bandgap poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2', 1', 3'-benzothiadiazole] (PCDTBT) and the diketopyrrolopyrrole (DPP)based narrow bandgap polymer (DT-PDPP2T-TT) with a decylte-tradecyl (DT) and an electron-rich 2,5-di-2-thienylthieno[3,2b] thiophene (2T-TT) group fabricated using an optimized interlayer (ZnO NPs/ph-n-PEDOT:PSS) [NPs: nanoparticles; ph-n: pH-neutral PEDOT: poly(3,4-ethylenedioxythiophene); PSS: polystyrene sulfonate]. The tandem cells are fabricated by applying a simple process without thermal annealing. The ZnO NP interlayer operates well when the ZnO NPs are dispersed in 2-methoxyethanol, as no precipitation and chemical reactions occur. In addition to the ZnO NP film, we used neutral PEDOT: PSS as a second interlayer which is not affect to the sequential deposited bulk heterojunction (BHJ) active layer of acidification. The power conversion efficiency (PCE) of a tandem device reaches 7.4% (open-circuit voltage V-OC = 1.53 V, short-circuit current density J(SC) = 7.3 mAcm(-2), and fill factor FF = 67%). Furthermore, FF is increased to up to 71% when another promising large bandgap (bandgap similar to 1.94 eV) polymer (PBnDT-FTAZ) is used. The surface of each layer with nanoscale morphology (BHJ1/ZnO NPs film/ph-n-PEDOT:PSS/BHJ2) was examined by means of AFM analysis during sequential processing. The combination of these factors, efficient DPP-based narrow bandgap material and optimized interlayer, leads to the high FF (average approaches 70%) and reproducibly operating tandem BHJ solar cells.