A dual functional asymmetric squaraine based low band gap hole transporting material for efficient perovskite solar cells.
We demonstrate for the first time an asymmetric squaraine based low band gap hole transporting material, which acted as both light harvesting and hole transporting layers in methylammonium lead triiodide perovskite solar cells. Opto electrochemical characterization revealed extremely high molar extinction coefficients of the absorption bands in the low energy region and prominent space charge delocalization due to its electronically asymmetric nature. A suitable band alignment of the squaraine HOMO level with the valence band edge of the perovskite, and the conduction band of the TiO2 with LUMO of the perovskite allowed a cascade of hole extraction and electron injection, respectively. Red shifted absorption was observed for both HTMs in thin films coated on the perovskite, and the optimized devices exhibited an impressive PCE of 14.7% under full sunlight illumination (100 mW cm( 2) AM1.5 G) The efficiency value is comparable to that of the devices using a state of the art spiro OMeTAD hole transport layer under similar conditions. Ambient stability after 300 h revealed that 88% of the initial efficiency remained for and almost no change for indicating that the devices had good long term stability thus suggesting that the asymmetric squaraines have great potential as a dual functional HTM for high performance perovskite solar cells.
Nanoscale