Efficient quantum dot-sensitized solar cells through sulfur-rich carbon nitride modified electrolytes

For quantum dot sensitized solar cells (QDSSCs), modifying conservative polysulfide electrolytes with polymer additives has been proven as an effective method to control charge recombination processes at the TiO₂/QDs/electrolyte interface and to accomplish efficient cell devices. In this respect, the polysulfide electrolyte is modified with polymeric and sulfur-rich graphitic carbon nitride (SGCN) to enhance the photovoltaic performance of QDSSCs. For the first time, SGCN is used to passivate surface trap states and act as the steric hindrance between TiO₂/QDs/electrolyte interfaces. The QDSSCs fabricated with GCN and SGCN additives exhibited higher efficiencies, especially improved short-circuit current (J_SC) and fill factors (FFs) than those of the liquid electrolyte. Cu-In-S sensitized QDSSCs constructed with GCN and SGCN additives exhibited efficiencies of 6.73% and 7.13%, respectively, whereas the liquid electrolytes delivered an efficiency of 6.16%. Additionally, the applicability of SGCN additives in various Cu-based QDSSCs to enhance their photovoltaic performance is further verified using Cu-In-Se QDSSCs. An increase in the conversion efficiencies of QDSSCs with SGCN additives is possibly due to (1) their electron-rich surface which can act as an obstacle for electron-hole recombination, thereby suppressing the back-transfer of photo-induced electrons to the QD/electrolyte interface; (2) SGCN facilitates the reduction of S_n^2- to S^2- redox couple, thus providing holes towards the QDs/electrolyte more efficiently. Overall, this work provides an innovative and economic additive to modify polysulfide electrolytes, thereby controlling the TiO₂/QDs/electrolyte interfaces of QDSSCs.