Engineering Multilayer Dyes for TiO2 Solar Cells

Solution-processable solar cells using TiO₂ as the electron transporter are promising candidates for light harvesting applications due to their low cost and compatibility with large-area, high-throughput fabrication methods. Because TiO₂ only absorbs in the UV region, a key strategy for improving the performance of TiO₂-based solar cells is to sensitize TiO₂ surfaces with a light-absorbing dye. While significant advances have been made in the design of light-absorbing dyes that anchor onto TiO₂ surfaces, poor dye loading and limited absorption of dyes in the visible region still negatively impact overall device efficiencies. Recently, we have demonstrated the ability to synthesize multilayer porphyrin arrays using metal mediators as a novel approach to address these two challenges. In our approach, multilayer dye assembly occurs through coordination between pyridyl-containing organic ligands and metallated bis-pincer complexes. To initiate assembly, Zinc 5, 10, 15, 20-tetra (4-pyridyl)-21H, 23H-porphine (ZnP) was adsorbed onto a mesoporous TiO2 sample. The UV-vis spectrum collected on the sample after a single immersion in ZnP solution displayed a peak at 480 nm, characteristic of the Soret band of ZnP. The sample was then immersed in a solution containing a palladiated bis-pincer complex (bis-Pd). Coordination between the adsorbed ZnP molecules and bis-Pd was evident in an observed red shift of 5 nm of the ZnP Soret band. The sample was then alternately immersed in solutions containing ZnP and bis-Pd. Upon each immersion in ZnP, the absorption of the Soret band increased linearly, confirming the successful multilayer deposition of ZnP on the TiO₂ surface. To further explore the ability to deposit unique layers of dyes using this modular layer-by-layer assembly approach we replaced the second layer of ZnP with two different pyridyl-terminated organic ligands,  1,2-di(4-pyridyl)ethylene (Py1) and 1,2-bis(4-pyridyl)ethane (Py2). Because the absorption bands of Py1 and Py2 lie in the ultraviolet region, we only observed increased absorbance at 480 nm after coordination of the 1^st and 3^rd ZnP layers. By incorporating other dyes that absorb in different regions of the visible light spectrum, we anticipate the assembly of supramolecular dyes that exhibit broad light absorption to improve solar cell efficiency.