II. New Molecular Design & Synthesis of Organic Hole Transporting
Materials for Highly Efficient Solid-state Dye-sensitized Solar Cells
Up to 13% of power conversion efficiency has been demonstrated for DSSCs, but such an efficiency is obtained by using liquid electrolytes in the cells, which may suffer from electrolyte leakage and corrosion problems, posing a challenge to the long-term stability of the cells. One way to address these issues is to replace the liquid electrolyte with solid-state hole transporting materials (HTMs). Among them, the spiro-OMeTAD as HTM has been used for achieving the best SSDSSC performance. But, at present, the power conversion efficiencies of solid-state DSSCs (ss-DSSCs) have been far below the efficiencies of their counterparts that use liquid electrolytes. The highest efficiency of such ss-DSSCs with spiro-OMeTAD is obtained when the thickness of the mesoporous TiO2 films is about 2 μm, 5 times thinner than the thickness needed for sufficient light absorption. Incomplete filling of the mesoporous TiO2 films with HTMs has been identified as a major factor that limits the performance of the ss-DSSCs when the thickness of the TiO2 films is beyond 2 μm. The ineffective filling of the pores in thick TiO2 films with spiro-OMeTAD causes low hole injection efficiency from the dye cation to spiro-OMeTAD, short recombination lifetime of charge carriers, and poor hole transport through spiro-OMeTAD. While considerable effort has been made to improve the pore filling, to effectively fill thick nanoparticle-based mesoporous TiO2 films with HTMs remains challenging. To solve such problems mentioned above, our research is focused on the design and synthesis of novel organic hole transporting materials simultaneously possessing both fluidity and charge conductivity to make the complete pore infiltration of TiO2 film for highly efficient ss-DSSCs.
The desired redox mediators between the dye and the redox electrolyte have been intensively explored to increase the VOC and improve the JSC through a cascade-type hole-hopping channel, but these have not yet been developed. The PCE is closely linked to competition between the charge transfer states and charge recombination at the TiO2/dye/electrolyte interface. For this, it is possible to develop highly efficient DSSCs with a higher dye regeneration yield through minimizing the energy difference between the HOMO levels of the sensitizing dye and the organic hole conductors (HC). Very recently, we have developed Y-shaped low molecular hole conductor (HC)-acidified coadsorbents of PTZ1, PTZ2, and HC-A1 with their tuned and up-shifted HOMO levels used as a HC coadsorbent in dye-sensitized solar cells (DSSCs) to improve cell performances through the desired cascade-type hole transfer processes, for the first time. This was due to the functions of the HC coadsorbent with the HOMO energy level well matched to that of the organic dye to induce the desired cascade-type hole transfer processes, which were 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 its electrode. As a consequence, a new class of Y-shaped low molecular organic HC coadsorbents based on phenothiazine carboxylic acid derivatives is promising candidates for developing highly efficient DSSCs.
