Indium Tin Oxide-Based Fully Spray-Coated Inverted Solar Cells with Nontoxic Solvents: The Role of Buffer Layer Interface on Low-Bandgap Photoactive Layer Performance

In bulk heterojunction solar cells, the morphology of the interfaces between the photoactive layer (PAL) and charge transporting layers during the deposition process plays a key role in achieving high-efficiency devices. Herein, an inverted fully spray-coated solar cell fabricated on an indium tin oxide (ITO)-glass substrate is presented. It is demonstrated that a spray-coated double electron transporting layer composed of zinc oxide (ZnO) nanoparticles coated with polyethylenimine ethoxylated (PEIE) improves the morphology of the spray-coated active layer on top of the spray-coated cathode. Moreover, focusing on the hole transporting layer and anode, the performance obtained using a commercial poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) blend is compared with a high-conductive anhydrous PEDOT:PSS (A-PEDOT) mixed with a commercial PEDOT:PSS (CPP-105D) as transporting layer. By optimizing the spray deposition of all the layers, a fully scalable spray process is used to produce polymer solar cells with ITO/ZnO/PEIE/poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2- [(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-phenyl-C70-butyric-acid-methyl-ester (PC70BM)/CPP:A-PEDOT structure, achieving a power conversion efficiency (PCE) of 3.6%. Such result is significant if compared to a spray-coated structure with evaporated anode (MoO3-Ag). In this case (ITO/ZnO/PEIE/PTB7:PCBM/MoO3-Ag), a power conversion efficiency of 5.5% is obtained.