• Energy Research

Passivation and interlayer engineering are important approaches to increase the efficiency and stability of perovskite solar cells. Thin insulating dielectric films at the interface between the perovskite and the charge carrier transport layers have been suggested to passivate surface defects. Here, we analyze the effect of depositing poly(methyl methacrylate) (PMMA) from a very low-concentration solution. Spatial- and time-resolved photoluminescence and atomic force microscopy analyses of samples with diverse morphologies demonstrate the preferential deposition of PMMA in topographic depressions of the perovskite layer, such as grain and domain boundaries. This treatment results in an increase in the fill factor of more than 4% and an absolute efficiency boost exceeding 1%, with a maximum efficiency of 20.4%. Based on these results, we propose a physical isolation mechanism rather than a chemical passivation of perovskite defects, which explains not only the data of this study but also most results found in earlier works. This work was partially funded by a Swiss Government Excellence Scholarship (2017.1080) and by the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie project PerSisTanCe with grant agreement No. 841005 (E.O.M.). It also received financial support from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is cofinanced by the Participating States and by the European Union’s Horizon 2020 research and innovation programme. E.O.M., R.D.O., P.F., and U.S. acknowledge financial support by the Adolphe Merkle Foundation. E.O.M.would like to thank Dr. Silver Hamill Turren-Cruz from Helmholtz-Zentrum Berlin for fruitful discussions and Dr. Jovana Milic from the University of Fribourg and Brian Carlsen from the Laboratory of Photomolecular Science (EPFL) for facilitating and carrying out stability measurements.

AbstractA metal‐free organic sensitizer, suitable for the application in dye‐sensitized solar cells (DSSCs), has been designed, synthesized and characterized both experimentally and theoretically. The structure of the novel donor–acceptor–π‐bridge–acceptor (D–A–π–A) dye incorporates a triphenylamine (TPA) segment and 4‐(benzo[c][1,2,5]thiadiazol‐4‐ylethynyl)benzoic acid (BTEBA). The triphenylamine unit is widely used as an electron donor for photosensitizers, owing to its nonplanar molecular configuration and excellent electron‐donating capability, whereas 4‐(benzo[c][1,2,5]thiadiazol‐4‐ylethynyl)benzoic acid is used as an electron acceptor unit. The influences of I3−/I−, [Co(bpy)3]3+/2+ and [Cu(tmby)2]2+/+ (tmby=4,4′,6,6′‐tetramethyl‐2,2′‐bipyridine) as redox electrolytes on the DSSC device performance were also investigated. The maximal monochromatic incident photon‐to‐current conversion efficiency (IPCE) reached 81 % and the solar light to electrical energy conversion efficiency of devices with [Cu(tmby)2]2+/+ reached 7.15 %. The devices with [Co(bpy)3]3+/2+ and I3−/I− electrolytes gave efficiencies of 5.22 % and 6.14 %, respectively. The lowest device performance with a [Co(bpy)3]3+/2+‐based electrolyte is attributed to increased charge recombination.

Abstract Novel facile synthetic protocol is developed to prepare electrochemically and optically clean Cu(tmby)2TFSI and Cu(tmby)2TFSI2 in a mixture (tmby = 4,4,6,6-tetramethyl-2,2-bipyridine; TFSI = trifluoromethylsufonylimide). This pure Cu(II/I) redox mediator exhibits improved charge-transfer rate at the counterelectrode (PEDOT) and faster diffusion transport in the solution. Four pyridine derivatives: 4-tert-butylpyridine, 2,6-bis-tert-butylpyridine, 4-methoxypyridine and 4-(5-nonyl)pyridine are evaluated as electrolyte additives. Base-specific electrochemical properties of the redox mediator are found for Cu(tmby)22+/+, but not for Co(bpy)33+/2+ which is used as control system. Due to steric hindrance, 2,6-bis-tert-butylpyridine has the smallest influence on the mediator's electrochemistry, but is also ineffective for the VOC enhancement through TiO2 conduction band upshift. Charge-transfer rates at PEDOT surface and diffusion resistances correlate with the basicity (pKa) of the used pyridine derivatives. The dye (Y123)-sensitized solar cells are evaluated by solar conversion performance in addition to electron lifetime, charge extraction and long-term stability tests. The optimization of pyridine bases for the Cu-mediated solar cells represents interplay of basicity and coordination ability. In turn, this allows for tuning of the charge transfer rate at counterelectrode and the mass transport in the electrolyte solution. The 4-(5-nonyl)pyridine is outperforming all the remaining bases in performance metrics of the corresponding solar cells.