FULL PAPER © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 5650 wileyonlinelibrary.com of these devices, two configurations of the photoactive layer have been employed: (i) a mesoporous-structured layer in which the photoactive organo lead iodide perovskite (APbI 3 ) is infiltrated into the mesoporous material, [8] or (ii) a thinner dense planar- structured APbI 3 film, [9,10] where A is an organic cation, typically alkylammonium [5] or formamidinium, [11,12] usually deposited onto an electron-extracting layer. Both the mesoporous and planar films have produced high device efficiencies when effective electron-transporting and hole- transporting materials (HTMs) have been utilized. [13–17] Titanium dioxide has been used widely as an efficient and stable electron-extracting layer, although other materials such as ZnO [18] or [6,6]-phenyl- C 61 -butyric acid methyl ester (PCBM) [19,20] have also resulted in devices with high performance, whereas devices without an electron-extracting layer have been reported with up to 14% efficiency. [21] A different structure in which the hole-con- ductive layer is deposited directly onto the substrate followed by the perovskite layer and electron-conductive material on the top of the HTM, usu- ally referred to as “inverted” in the perovskite solar cell field, has also been reported. [20,22,23] In the first reports of perovskite solar cells, liquid-state elec- trolytes based on I - /I 3 - redox couple were employed, which resulted in inferior performance. [1,24] In contrast, the application of solid-state organic HTMs, such as 2,2′,7,7′-tetrakis-( N,N-di-4- methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD), [8,25] poly(triarylamine) (PTAA), [7,16] and poly(3-hexylthiophene) (P3HT) [26] in perovskite solar cells has resulted in outstanding device efficiencies. Perovskite-based devices largely derive their origins from dye-sensitized solar cells (DSSCs). In DSSCs, the mesoporous n-type semiconductor and adjacent HTM sepa- rate the electrons and holes generated by the light absorption of the dye. [27] In this regard, the aforementioned HTMs served as charge-separating materials as well as hole-conducting mate- rials. Perovskite-based cells are characterized by the rapid dis- sociation of excitons within the perovskite layer, [28] and free car- riers are generated within the light-absorbing material itself. [31] Due to this distinction, the design of HTMs for high efficiency perovskite-based devices differs from that for DSSCs, despite the fact that both technologies have a common history. Copper(I) Iodide as Hole-Conductor in Planar Perovskite Solar Cells: Probing the Origin of J– V Hysteresis Gaveshana A. Sepalage, Steffen Meyer, Alexander Pascoe, Andrew D. Scully, Fuzhi Huang, Udo Bach, Yi-Bing Cheng, and Leone Spiccia* Organic–inorganic lead halide perovskite solar cells are promising alterna- tives to silicon-based cells due to their low material costs and high photovol- taic 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. DOI: 10.1002/adfm.201502541 G. A. Sepalage, Dr. S. Meyer, Prof. L. Spiccia School of Chemistry Monash University Victoria 3800, Australia E-mail: leone.spiccia@monash.edu A. Pascoe, Dr. F. Huang, Prof. U. Bach, Prof. Y.-B. Cheng Department of Materials Science and Engineering Monash University Victoria 3800, Australia Dr. A. D. Scully, Prof. U. Bach CSIRO Manufacturing Flagship Bayview Avenue, Clayton, 3168 Victoria, Australia Prof. U. Bach Tech Fellow, The Melbourne Centre for Nanofabrication 151 Wellington Road, Clayton, 3168 Victoria, Australia 1. Introduction Hybrid organic–inorganic perovskite solar cells are attracting significant interest as a result of the rapid increase in record power conversion efficiencies (PCEs) from 3.6% to values cur- rently exceeding 20% within a few years. [1–7] For the majority Adv. Funct. Mater. 2015, 25, 5650–5661 www.afm-journal.de www.MaterialsViews.com