Tailor-Made Hole-Conducting Coadsorbents for Highly Efficient Organic Dye-Sensitized Solar Cells

The Y-shaped, low molecular mass, hole-conductor (HC), acidic coadsorbents 4-{3,7-bis[4-(2-ethylhexyloxy)phenyl]-10H-phenothiazin-10-yl}benzoic acid (PTZ1) and 4-{3,7-bis[4-(2-ethylhexyloxy)phenyl]-10H-phenothiazin-10-yl}biphenyl-4-carboxylic acid (PTZ2) were developed. Owing to their tuned and negative-shifted HOMO levels (vs. NHE), they were used as HC coadsorbents in dye-sensitized solar cells (DSSCs) to improve cell performance through desired cascade-type hole-transfer processes. Their detailed functions as HC coadsorbents in DSSCs were investigated to obtain evidence for the desired cascade-type hole-transfer processes. They have multiple functions, such as preventing - stacking of dye molecules, harvesting light of shorter wavelengths, and faster dye regeneration. By using PTZ2 as the tailor-made HC coadsorbent on the TiO2 surface with the organic dye NKX2677, an extremely high conversion efficiency of 8.95% was achieved under 100mWcm(-2) AM 1.5G simulated light (short-circuit current J(SC)=16.56mAcm(-2), open-circuit voltage V-OC=740mV, and fill factor of 73%). Moreover, J(SC) was increased by 13%, V-OC by 27% and power-conversion efficiency by 49% in comparison to an NKX2677-based DSSC without an HC coadsorbent. This is due to the HC coadsorbent having a HOMO energy level well matched to that of the NKX-2677 dye to induce the desired cascade-type hole-transfer processes, which are associated with a slower charge recombination, fast dye regeneration, effective screening of liquid electrolytes, and an induced negative shift of the quasi-Fermi level of the electrode. Thus, this new class of Y-shaped, low molecular weight, organic, HC coadsorbents based on phenothiazine carboxylic acid derivatives hold promise for highly efficient organic DSSCs.