Perovskite Solar Cells DOI: 10.1002/anie.201309361 Mixed-Organic-Cation Perovskite Photovoltaics for Enhanced Solar- Light Harvesting** Norman Pellet, Peng Gao, Giuliano Gregori, Tae-Youl Yang, MohammadK. Nazeeruddin, Joachim Maier, and Michael Grätzel* Abstract: Hybrid organic–inorganic lead halide perovskite APbX 3 pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid-state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite-based photo- voltaics is to extend their optical-absorption onset further into the red to enhance solar-light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN = CHNH 3 + , FA) and methylammonium (CH 3 NH 3 + , MA) cations in the A position of the APbI 3 perovskite structure. This combination leads to an enhanced short-circuit current and thus superior devices to those based on only CH 3 NH 3 + . This concept has not been applied previously in perovskite-based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelec- tronic properties of the light-harvesting materials. The large family of inorganic halometallate perovskites has attracted a lot of attention owing to its wide range of outstanding properties, such as antiferromagnetism, [1–3] pho- toconductivity, [4, 5] ionic conductivity, [6] and bipolar semicon- ductivity. [7] Within this family, the fully inorganic cesium– metal–trihalide perovskites (CsAX 3 ,X = Cl, Br, I) have been the subject of intense study for many years. [8–17] In particular, organic–inorganic iodoplumbate and iodostannate perov- skites, pioneered by Mitzi et al., [18] have been recognized for their excellent semiconducting properties. [19] However, the extraordinary photovoltaic performance of similar hybrid perovskites only became evident after the demonstration of MAPbI 3 nanoparticles as potent light harvesters in a liquid- electrolyte-based dye-sensitized solar-cell configuration by Miyasaka and co-workers, [20] who observed a power-conver- sion efficiency (PCE) of 3.9 %. A drawback of this system is its poor stability, as the perovskite rapidly degrades owing to its high solubility in the liquid electrolyte. This problem was overcome by replacing the electrolyte with a solid organic hole conductor. [21–27] Recently, we reported a new record of 15% PCE for a FTO/TiO 2 /MAPbI 3 /spiro-MeOTAD/Au device in which the perovskite was deposited by a novel sequential deposition technique. [23] By using our two-step deposition technique, we witnessed a significant increase in the open-circuit voltage (V oc ) and fill factor (FF) of our devices as compared to devices prepared by the commonly used one-step deposition method from g-butyrolac- tone. [20, 24, 26, 27] However, the short-circuit photocurrent density (J sc ) was limited to an average value of 17 mA cm 2 . In theory, a semiconductor with a band gap of 1.5 eV can deliver photocurrents up to 27 mA cm 2 under standard AM 1.5 G illumination. The large difference arises mainly from the lack of light absorption in the 550–800 nm range by the infiltrated perovskite and the parasitic absorption of the conductive oxide glass. Hybrid organic–inorganic perovskites are synthesized with a variety of organic cations. [26, 28–34] It has been demon- strated that the size of the organic ammonium cation influences the optical band gap of the perovskite by affecting the M-I-M (M = Sn, Pb) angle [35] or promoting the formation of insulating barriers between semiconducting PbI 4 layers. [30] In the latter case, bigger cations usually lead to two-dimen- sional perovskite, in which the PbI 6 4 octahedrons are edge- sharing. Two-dimensional (2D) iodoplumbate and iodostan- nate perovskites usually show wider band gaps, [36, 37] which make them unsuitable for panchromatic absorption of the visible solar spectrum. A variety of organic cations have been shown to affect the band gap by as much as 1 eV in iodostannate perovskites. [36] Theoretically, methylammonium (CH 3 NH 3 , MA + ) and formamidinium (HN = CHNH 3 + , FA + ) are sufficiently small cations to form the 3D perovskite, whereas ethylammonium (CH 3 CH 2 NH 3 + ) is known to already form a 2D perovskite. [26] FASnI 3 has been described by Mitzi and co-workers as early as 1995, [31] whereas its Pb analogue was only recently investigated by Kanatzidis and co-work- ers, [38] who reported a significant red shift of the optical absorption as compared to that of MAPbI 3 . We reasoned that formamidinium offers the potential to lower the band gap of [*] N. Pellet, Dr. P. Gao, Dr. M. K. Nazeeruddin, Prof. M. Grätzel Laboratory of Photonics and Interfaces, Department of Chemistry and Chemical Engineering, Swiss Federal Institute of Technology Station 6, 1015 Lausanne (Switzerland) E-mail: michael.graetzel@epfl.ch N. Pellet, Dr. G. Gregori, Dr. T.-Y. Yang, Prof. J. Maier, Prof. M. Grätzel Max-Planck-Institute for Solid-State Research Heisenbergstrasse 1, 70569 Stuttgart (Germany) [**] We thank K. Schenk for the XRD characterization, P. Labouchre for the SEM micrographs, and R. Humphry-Baker for fruitful discus- sions. We acknowledge financial support from Aisin Cosmos R&D Co., Ltd (Japan); the European Union Seventh Framework Program (FP7/2007–2013) under grant agreement “ENERGY-261920, ESCORT”; NANOMATCELL, grant agreement no. 308997; and the CCEM-CH in the 5th call proposal (DURSOL). M.G. thanks the Max Planck Society for a Max Planck Fellowship at the MPI for Solid State Research in Stuttgart (Germany); the King Abdulaziz University, Jeddah and the Nanyang Technolocal University, Singapore for Adjunct Professor appointments; and the European Research Council for an Advanced Research Grant (ARG 247404) funded under the “Mesolight” project. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201309361. Angewandte Chemie 3151 Angew. Chem. Int. Ed. 2014, 53, 3151 –3157 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim