2, Yamagata University, Yonezawa, , Japan
3, Yamagata University, Yonezawa, , Japan
Semitransparent organic solar cells (ST-OSCs) show great promising applications in building integrated photovoltaics, solar-powered automotive and wearable electronics. However, the development of ST-OSCs is significantly lagging behind opaque OSCs, probably due to the lack of suitable photoactive materials. In this work, four different squaraines, IDPSQ, SQ-BP, D-BDT-SQ and AzUSQ are successfully used as donors in ST-OSCs (Figure 1), whose colors can be tuned form purple to blue to green to dark green, respectively. When using 10 nm Ag as the transparent electrode, the ST-OSCs fabricated with IDPSQ: PC71BM, SQ-BP: PC71BM, D-BDT-SQ: PC71BM, and AzUSQ: PC71BM yield power conversion efficiencies (PCEs) of 2.96%, 4.36%, 4.57%, and 1.63%, respectively, whose colors are purple, green, reddish brown, and light brown, respectively. Remarkably, compared with their traditional opaque OSCs (PCEs of 3.95%, 5.45%, 5.84%, and 1.91%, respectively), all of the decreases in the PCEs of ST-OSCs are less than 25%, which is very rare for ST-OSCs. Moreover, all of the ST-OSCs exhibit good average visible transmittance (AVT) of 25-30%, favorable CIE color coordinate closely to the white color point, and high color rendering index (CRI) of 72-97%. Finally, by changing the thickness of the electrode (Ag), the relationships between the PCEs and AVTs are investigated in detail, and an impressive PCE of 4.91% with an AVT of 25.1% and a CRI of 97% can be obtained in the D-BDT-SQ: PC71BM-based ST-OSCs, which is the highest value for small molecules-based ST-OSCs. This remarkable result first demonstrates squaraines should be very promising candidates for promoting the development of ST-OSCs. These colorful squaraines also offer a room for the development of tandem solar cells and solar sharing for agriculture.