A systematic investigation of the compositional space for perovskite solar cells spanned by MAPbI3, MAPbBr3, FAPbI3and FAPbBr3.

Copper phenanthroline is an efficient molecular hole transporting material for solid-state dye-sensitized solar cells. Efficiencies of more than 8% at 1 sun were obtained using LEG4 organic dye.

During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a water-soluble lead compound. Replacing lead in the perovskite structure with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PbI2 and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal–halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the valence of Sr2+, results in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite. This is explained as a consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.

No abstract

The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy) 3 ] 2+/3+ ), were successfully replaced by bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu-(dmp) 2 ] 1+/2+ ). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3% for LEG4-sensitized TiO 2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m −2 under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady-state emission measurements and time-correlated single-photon counting (TC-SPC). Our results indicate bimolecular reductive quenching of the excited LEG4 dye by the [Cu(dmp) 2 ] 2+ complex through a dynamic mechanism. Excited-state dye molecules can readily undergo bimolecular electron transfer with a suitable donor molecule. In DSCs this process can occur when the excited dye is unable to inject electrons into the TiO 2 . With a high electrolyte concentration the excited dye can be intercepted with an electron from the electrolyte resulting in the reduced state of the dye. Quenching of the reduced dye by the electrolyte competes with electron injection and results in a lower photocurrent. Quenching of excited LEG4 by complexes of [Cu(dmp) 2 ] + , [Co(bpy) 3 ] 2+ , and [Co(bpy) 3 ] 3+ followed a static mechanism, due ground-state dye−quencher binding. Inhibition of unwanted quenching processes by structural modifications may open ways to further increase the overall efficiency.