Solution-Processed Transparent Intermediate Layer for Organic Tandem Solar Cell Using Nitrogen-Doped Graphene Quantum Dots

The solution-processed intermediate layer (IML), which physically and electrically connects two subcells, is critical in the fabrication of efficient organic tandem solar cells. Several studies have examined solution-processed electron transport materials that can be used as n-type layers in the IML. On the other hand, forming an Ohmic contact for holes is limited by the choice of an aqueous-based hole transport layer (HTL) material except for PEDOT:PSS or vacuum evaporating materials. Here, this paper reports highly transparent and solution-processable nitrogen-doped graphene quantum dots (N-GQDs) as an efficient HTL in the IML. Using an IML based on N-GQDs/PEI, the present P3HT:PCBM tandem device showed a slight improvement in the fill factor (10%) and efficiency (15%) compared to the devices using the PEDOT: PSS/PEI-based junction. The optimal tandem cells with N-GQDs showed a double open-circuit voltage of 1.16 V, short-circuit current density of 5.80 mA/cm(2), fill factor of 0.55, and power conversion efficiency of 3.69% under AM 1.5G solar irradiation. This result thus makes a start on the use of functional graphene quantum dots in organic tandem solar cells.