Metal Complexes as Redox Shuttles in Dye-Sensitized Solar Cells Based on Electrodeposited ZnO: Tuning Recombination Kinetics and Conduction Band Energy

Abstract

In this work, a pathway to engineer both interfacial charge recombination kinetics and conduction band energy in dye-sensitized solar cells based on mesoporous electrodeposited ZnO is presented. Especially in solar cells employing metal complex redox shuttles such as Co(bpy) 3 [bpy = 2,2'-bipyridine] and Cu(tmby) 2 [tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine] these factors are crucial in order to obtain efficient devices. Controlling them is achieved by augmenting the liquid redox electrolyte with additives. The most commonly used additive 4-tert-butylpyridine (TBP) induces both an upward shift of the ZnO conduction band energy and retards recombination thus leading to higher device performance. However, the full potential of the cells cannot be exploited by TBP since high concentrations lead to unwanted side reactions. However, adding another additive such as 2,2'-bipyridine or neocuproine can circumvent these problems and opens a full range of options to tune the properties of the ZnO/electrolyte interface. Photoelectrochemical techniques, such as impedance spectroscopy, current-voltage characterization and photocurrent transient measurements are utilized to reveal the underlying mechanisms of the different additives. Using these additives, power-conversionefficiencies of 3.56% for a Co(bpy) 3 -based electrolyte and 3.85% for a Cu(tmby) 2 -based electrolyte are achieved for electrodeposited ZnO sensitized with the organic dye DN216. Dye-sensitized solar cells (DSSCs) are receiving continued interest as photovoltaic devices because they can be fabricated with low preparation expenses, 1,2 with solar-to-electrical power conversion efficiencies (PC Es) up to 14.3% under one sun illumination.3 Thereby they offer options with low energy payback times for a sustainable photovoltaic technology on a very large scale, or, at least for niche applications. In such cells, light is absorbed by dye molecules that are adsorbed to a mesoporous semiconductor. Following light absorption, the dye injects excited electrons into the conduction band of the semiconductor from where they are transported to the front contact. Then the oxidized dye molecules are regenerated by a redox mediator that, on the other hand, is regenerated by the counter electrode at the back contact. 4 Highest PC E is achieved by using nanoparticulate TiO 2 as semiconductor which has to undergo high-temperature treatments of about 400• C to ensure high electron collection efficiency. 5This treatment limits the applicability of DSSCs, since it increases the required energy input and also prevents the use of polymer foils as substrate material. In order to circumvent such high-temperature processes, ZnO has been applied as porous semiconductor because a well-connected network can be directly grown by electrodeposition from aqueous solutions at a typical process temperature of 70However, the PC E of ZnO-based DSSCs is inferior compared to TiO 2 -based cells. One of the reasons is the low quantum efficiency when ZnO is sensitized with dyes that were optimized ...

Efficient Electron Collection by Electrodeposited ZnO in Dye-Sensitized Solar Cells with TEMPO+/0 as the Redox Mediator

“…While the former consists of high-temperature sintered nanoparticles that involve grain boundaries along the path of conduction, electrodeposited ZnO is prepared at low temperatures and consists of crystalline columns perpendicular to the substrate, that is, in the transport direction of electrons, which we will confirm leading to considerably faster electron transport. , This finding fits to earlier observations that even as a bulk material, ZnO shows higher electron mobility than TiO 2 . Furthermore, we employ the sensitizer Y123 (Scheme b) that has shown to yield high efficiencies in combination with TiO 2 and kinetically fast redox couples. , Sensitization of ZnO with such organic dyes that employ only one cyanoacrylic or carboxylic acid anchoring group have shown to result in a stable power output and high EQE. − …”

“…Because both the TEMPO +/0 and the I 3 – /I – redox mediator solutions contain the same concentration of additives and the same solvent, it can be concluded that the TEMPO +/0 redox couple is responsible for this downward shift of the CB edge of ZnO. It has been observed in preliminary work that adsorbed cations induce a significant downward shift of the ZnO CB energy. ,, In TiO 2 -based DSSCs, such shifts are not observed if alkaline molecules are present in the redox electrolyte solution, such as the presently used MBI . These molecules control the amount of surface-adsorbed cations that would otherwise stabilize energy levels also in TiO 2 . , Because the surface of ZnO is alkaline by itself, the control of surface-adsorbed cations by alkaline molecules is mitigated .…”

“…A compact ZnO-underlayer (ZnO-UL) was electrodeposited from an oxygen-saturated, aqueous zinc salt solution onto cleaned aluminum-doped zinc oxide (AZO)-coated glass substrates (Kaivo, <10 Ω □ –1 ), followed by the electrodeposition of a porous ZnO film from the same solution with the addition of 50 μM eosin Y (Chemplex, >88%). The process was carried out as described in detail previously. ,, The porous film covered a circular area with diameter 0.6 cm (0.283 cm 2 ). The compact ULs and the porous films had a thickness of roughly 0.6 and 2.7 μm as determined by profilometry, respectively.…”

“…The process was carried out as described in detail previously. 37,41,42 The porous film covered a circular area with diameter 0.6 cm (0.283 cm 2 ). The compact ULs and the porous films had a thickness of roughly 0.6 and 2.7 μm as determined by profilometry, respectively.…”

Metal Coordination Complexes as Redox Mediators in Regenerative Dye-Sensitized Solar Cells

“…In addition to DSSCs with a TiO 2 substrate, Co and Cu coordination complexes can be used in devices with alternative n-semiconductor substrates, for example n-ZnO, as demonstrated by Ruess, et al (2018) [177]. In a DSSC with the organic dye DN2016, of the indoline type, a PEDOT-coated CE and the Cu(tmby) 2 mediator in acetonitrile, a PCE of 3.8% was obtained (U OP = 0.713 V, J SC = 7.5 mA•cm −2 ); with the Co(bpy) 3 mediator the PCE was 3.6% (U OP = 0.618 V, J SC = 9.3 mA•cm −2 ).…”