A novel organic-inorganic hybrid perovskite CH3NH3PbX3 (X = Cl, Br, I) has emerged as a strong competitor in photovoltaic applications due to its superior characteristics including high absorption coefficient, broad absorption spectrum, tunable band gap, long carrier lifetime, high balanced hole and electron mobility, etc.[1, 2] The first embodiment of perovskite solar cells (PSCs) only demonstrated a power conversion efficiency (PCE) of 3.8%, which by now has been boosted to 22.1%.[3-4] However these PCEs have been obtained by using Spiro-OMeTAD as the hole-transporting material (HTM), which is too expensive for mass production. In this work, we designed an HTM-free PSC with the structure of ITO/Cu(mtsc)I-doped CH3NH3PbI3/PCBM/BCP/Ag. Cu(mtsc)I (mtsc: 2-methyl-3-thiosemicarbide) was first synthesized, and then doped into the precursor solution at an extremely low concentration to form a bulk heterojunction type perovskite layer ,and thus the HTM layer is not needed. After optimization, a PCE of 18.33% has been achieved on glass-substrate. Compared to the traditional p-type copper-based semiconductor such as CuI, Cu(mtsc)I has better energy level matching and is easier to form a chelate with the CH3NH3PbX3 material, resulting in much higher power conversion efficiencies.
 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 341.
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