New insights into the alkoxy effects on auxiliary adsorption and inhibiting charge recombination in dye-sensitized solar cells with high open circuit voltage: a theoretical investigation.

Although introducing an alkoxy group is one of the most popular methods to suppress the interfacial charge recombination process of dye-sensitized solar cells, understanding of its effects is still limited and a microscopic picture of the alkoxy effects is lacking. Two ullazine dyes with distinct alkoxy chains at the donor part are used to investigate the effects of the alkoxy group on the adsorption, dye aggregation and charge recombination process in our study. Different from the usual assumption, we find that alkoxy chains can not only play a shielding role, but can also assist dye adsorption and inhibit the charge recombination process more effectively by covering the TiO2 surface. We also find that the existence of alkyl chains can well inhibit the aggregation of dyes and reduce intermolecular electron transfer. Furthermore, an important structural feature at the interface, the Ti-O interaction between the oxygen atom of the alkoxy group and the Ti atom of the surface is also found to contribute substantially to the interface stability. New insights into the effects of the alkoxy group on auxiliary adsorption and inhibiting charge recombination through reducing the recombination sites pave the way for rational design of sensitizers with high performance.

Theoretical insights on the comparison of champion dyes SM315 and C275 used for DSSCs reaching over 12% efficiency and the further optimization of C275.

Theoretical insights on the comparison between the champion dyes SM315 and C275 used for high-performance dye-sensitized solar cells (DSSCs) reaching over 12% efficiency with different electron donors only (porphyrin for SM315 and indenoperylene for C275) were explored for the first time. The intrinsic reasons for the significantly improved monochromatic photon-to-electric current conversion efficiency (IPCE) and open circuit voltage (Voc) of C275-based DSSCs over those of SM315 were revealed. According to our results, we find that the larger IPCE of C275 is attributed to its larger electronic coupling, smaller reorganization energy, reduced exciton binding energy and enhanced charge transfer character, all of which when combined lead to a larger electron injection efficiency. In addition, the larger Voc of C275 is due to a greater number of injected electrons, a smaller molecular volume and a smaller projected area, which lead to a more compact adsorption layer with a hindered charge recombination process. Thus, C275 is expected to have more potential to further optimize high-performance DSSCs. In view of the primary shortcoming of C275, which is its relatively narrow absorption spectrum, further optimization was made through structural modification using a series of heterocyclic anchoring groups. Using the same evaluation criteria, the theoretical screening of these dyes based on C275 is carried out. We find that indenoperylene dye with a barbituric acid (BA) anchoring group is a promising candidate for the experimental synthesis of high-performance DSSCs with improved Jsc, Voc and adsorption stability.

Alkyl effects on charge recombination in copper electrolyte-based dye-sensitized solar cells: Insights for targeted molecular design towards high performance.

Two quinoxaline dyes utilized in copper-electrolyte-based dye-sensitized solar cells (Cu-DSSCs) are theoretically investigated to analyze the impact of alkyl chains on dye performance. The investigation shows that ZS4, known for its record efficiency of up to 13.2 %, exhibits higher electron coupling and fewer binding sites for dye-[Cu(tmby)2]2+ interaction compared to ZS5. Contrary to common belief, alkyl chains are found to not only provide shielding but also hinder the interaction between dye and [Cu(tmby)2]2+ by influencing the optimal conformation of dyes, thereby impeding the charge recombination process. It is crucial to consider the influence of alkyl chains on dye conformation when discussing the relationship between dye structure and performance, rather than oversimplifying it as often done traditionally. Building on these findings, eight dyes are strategically designed by adjusting the position of the alkyl chain to further decrease charge recombination compared to ZS4. Theoretical evaluation of these dyes reveals that changing the alkyl chain on the nitrogen atom from 2-ethylhexyl (ZS4) to 1-hexylheptyl (D3-2) not only reduces the charge recombination rate but also enhances light harvesting ability. Therefore, D3-2 shows potential as a candidate for experimental synthesis of high-performance Cu-DSSCs with improved efficiency.