Solution-processed nickel compound as hole collection layer for efficient polymer solar cells

We demonstrated efficient bulk heterojunction polymer solar cells (PSCs) by inserting a solution-processable hole collection layer (HCL) between the indium tin oxide (ITO) electrode and photoactive layer. The HCL was prepared by spin-coating nickel acetylacetonate (Ni(acac) (2)) isopropanol solution on ITO, and then baking in air at 180 degrees C for 10 min followed by UV ozone treatment, which was marked as a-Ni(acac)(2). The a-Ni(acac)(2) HCL shows suitable energy levels, high hole mobility of 4.09 x 10(-3) cm(2) V-1.s(-1), and high transparency with light transmittance better than poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) in the wavelength range 550-800 nm. The PSCs with a-Ni(acac)(2) HCL showed improved performance compared with the PSCs without or with traditional PEDOT: PSS HCL. The power conversion efficiency of the PSC based on PBDTTT-C-T: PC70BM with a-Ni(acac)(2) HCL reached 7.84% under the illumination of AM 1.5 G, 100 mW cm(-2).