Simple mono-halogenated perylene diimides as non-fullerene electron transporting materials in inverted perovskite solar cells with ZnO nanoparticle cathode buffer layers

We have synthesized and characterized three perylene diimides, X-PDI, where X = H, F, or Br. These three compounds have been tested for the substitution of [6,6]-phenyl C-61 butyric acid methyl ester (PC61BM) in inverted perovskite solar cells (PVSCs). Although the efficiency of PDI derivative-based conventional PVSCs is as high as 17.6% (J. Mater. Chem. A, 2016, 4, 8724), the corresponding performance of inverted PVSCs is still lagging behind and improvement is necessary. For electron accepting (from CH3NH3PbI3 perovskite) and electron transporting properties of the materials, UV-visible absorption spectroscopy, electrochemical cyclic voltammetry, direct current conductivity, space-charge limited current (SCLC) electron mobility, atomic force microscopy (AFM) surface morphology, and photoluminescence (PL) spectroscopy gauging charge-trapping have been studied. The HOMO/LUMO energy levels are 5.94/4.00, 5.83/3.96, 6.00/3.97, and 5.48/3.73 eV for H-PDI, F-PDI, Br-PDI, and PC61BM, respectively. Direct current conductivities of H-PDI, F-PDI, Br-PDI, and PC61BM are 8.71 x 10(-8), 1.18 x 10(-9), 2.2 x 10(-6), and 8.42 x 10(-6) S cm(-1), respectively. The SCLC electron mobility of H-PDI, F-PDI, Br-PDI, and PC61BM are 1.12 x 10(-4), 8.31 x 10(-6), 1.08 x 10(-3), and 5.00 x 10(-3) S cm(-1), respectively. PL spectroscopy of CH3NH3PbI3 perovskite provides emission at wavelength of 781 nm, which is the same as for the perovskite layer covered with a thin film of H-PDI or F-PDI. However, the emission wavelength was blue-shifted to 773-774 nm when the perovskite layer was covered with a thin film of Br-PDI or PC61BM. Using UV-visible absorption spectroscopy, the solubility (in chloroform) was determined as 1.2 x 10(-2), 8.7 x 10(-2) and > 10(-1) mol L-1 for F-PDI, H-PDI, and Br-PDI, respectively. In thin film state, UV-visible absorption spectroscopy indicated that the extent of molecular aggregation was F-PDI [H-PDI > Br-PDI, which is consistent with the AFM-estimated root-mean-square roughness of F-PDI > H-PDI > Br-PDI similar to PC61BM. Without the solution processed ZnO NP cathode buffer layer (CBL), the power conversion efficiency (PCE) of H-PDI, F-PDI, Br-PDI, and PC61BM PVSCs is similar to 1%, similar to 0%, 3.2%, and 4.1%, respectively. With the ZnO NP CBL, PCE is similar to 7.8%, similar to 0%, 10.5%, and 11.1% for H-PDI, F-PDI, Br-PDI, and PC61BM PVSCs, respectively. Through this study, we have demonstrated that the simple mono-bromine substituted perylene diimide (Br-PDI), is solution processable and has potential for use as a non-fullerene electron accepting and electron transporting material in inverted PVSCs.