Novel Squaraine Cosensitization System of Panchromatic Light Harvesting with Synergistic Effect for Highly Efficient Solar Cells

Cosensitization based on two or multiple dyes as "dye cocktails" is a convenient shortcut to construct panchromatic dye sensitized solar cells (DSSCs). Generally, extending absorption bands of D-pi-A featured organic sensitizers to the long-wavelength and near-infrared (NIR) region through molecular engineering always induces unmatched energy levels (LUMO and HOMO) in DSSCs. In contrast, NIR squaraine dyes commonly exhibit a strong spike like absorption peak with narrow half-bandwidth in region, along with weak absorption in the visible region, which can bring the relative matched band gap as well as the energy levels for DSSCs. It is expected that squaraine dyes are idea cosensitizers to compensate light-harvesting, which can relieve the constraint in energy levels for D-pi-A featured sensitizers. In this work, the efficient spectral-complemental couples of D-A-pi-A featured organic dye WS-1 with NIR squaraine dyes VG1-C8 and HSQ4 are well-demonstrated for cosensitized panchromatic solar cells. Interestingly, simultaneous enhancements in both photocurrent and photovoltage are obtained for the cosensitization system with respect to individual WS-1, along with the panchromatic photoresponse covering the visible and NIR regions. In comparison with individual WS-1, the optimized cosensitization devices exhibit significant improvements in overall photovoltaic efficiency of 43% and 38%, resulting in notable efficiencies up to 9.0% and 8.7% for WS-1/VG1-C8 and WS-1/HSQ4 systems, respectively, which are among the best photovoltaic results for squaraine cosensitized DSSCs. This work demonstrates that WS-1/VG1-C8 and WS-1/HSQ4 couples are synergistic and efficient cosensitization systems, especially for panchromatic light harvesting solar cells.