A Desirable Hole‐Conducting Coadsorbent for Highly Efficient Dye‐Sensitized Solar Cells through an Organic Redox Cascade Strategy

Abstract

Over the past decade, dye-sensitized solar cells (DSSCs) based on nanocrystalline mesoporous metal oxides (typically TiO2) have been intensively studied and developed as a promising, low-cost alternative to conventional silicon photovoltaic devices. The components of such DSSCs are a dye sensitizer, a TiO2 metal oxide coated on conductive glass, a redox electrolyte couple, and a counter electrode. As illustrated in Scheme 1, the power-conversion efficiency of DSSCs is strongly dependent upon the minimization of charge recombination losses at the TiO2/dye/electrolyte interface (pathways 3 and 4). There are two recombination pathways of importance in DSSCs, in which electrons photoinjected into the TiO2 electrode can recombine with either dye cations or with redox electrolytes. Moreover, such charge recombination leads to losses in both the short-circuit photocurrent (Jsc) and the open-circuit photovoltage (Voc), resulting in a decrease in overall energy conversion efficiency (h). To reduce the possible charge recombination pathways occurring at the TiO2/dye/electrolyte interface, several kinds of additives, such as decylphosophonic acid (DPA), dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DINHOP), and chenodeoxycholic acid (CDCA) have been introduced to adsorb onto the TiO2 surface. [3] Among them, for example, cholic acid (CA) derivatives as coadsorbents in DSSCs, based on a Ru–pyridyl complex, courmarin, porphyrin, phthalocyanine, and naphthalocyanine dye, have been investigated well. Deoxycholic acid (DCA) is often used as a coadsorbent to break up organic and Ru–dye aggregates and hence to significantly improve Voc and Jsc. [5,9] Moreover, alternative approaches directed toward the minimization of recombination losses have been extensively studied by not only the insertion of a metal oxide blocking layer, but also by energetic cascades for multistep hole conductors (HC), as well as superior molecular sensitizer dyes and the insertion of a barrier layer between the sensitizer dye and the HC. To date, however, the desired redox intermediate mediators between the dye and I /I3 redox electro-