Copper(I) Iodide as Hole-Conductor in Planar Perovskite Solar Cells: Probing the Origin of J-V Hysteresis

Organic-inorganic lead halide perovskite solar cells are promising alternatives to silicon-based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole-conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 +/- 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI-based devices with a planar structure. In contrast to related planar 2,2', 7,7'-tetrakis-(N, N -di-4-methoxyphenylamino)9,9'-spirobifluorene (spiro-OMeTAD)-based devices, the CuI-based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open-circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro-OMeTAD. A slow response on a timescale of 10-100 s is observed for the spiro-OMeTAD-based devices. In comparison, the CuI-based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open-circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current-voltage hysteresis.