Zinc Phthalocyanine Conjugated Dimers as Efficient Dopant-Free Hole Transporting Materials in Perovskite Solar Cells

Four ZnPc-dimers with 2,5-thienyl (ZnPc-th-ZnPc 1), 2,7-fluorenyl (ZnPc-flu-ZnPc 2), 3,6-bisthienylldiketopyrrolopyrrole (ZnPc-DPP-ZnPc 3) and 1,4-phenyl (ZnPc-p-ZnPc 4) bridges have been studied as dopant-free hole transporting materials (HTMs) in perovskite solar cells (PSCs). The synthesis and characterization of ZnPc-th-ZnPc 1 and ZnPc-flu-ZnPc 2 dimers are reported for the first time. Steady state and time resolved photoluminescence demonstrate the good hole-extraction capability of these materials. The best efficiencies obtained for dimers 1, 2, 3 and 4 are 15.5 %, 15.6 %, 16.8 % and 15.7 %, respectively, without the addition of dopants. Besides, these derivatives demonstrated better stability both in dark storage conditions with a relative humidity 60 % for 160 h when compared to doped spiro-OMeTAD. The push-pull nature of dimer ZnPc-DPP-ZnPc 3 has led to the highest efficiency among the ZnPc derivatives under study demonstrating that donor-acceptor-donor systems can be good alternatives to commonly used materials due their energy levels, low cost and the final morphology of the hole transporting layer.