COMMUNICATION www.MaterialsViews.com www.advenergymat.de © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 6) 1400355 wileyonlinelibrary.com Solution Deposition-Conversion for Planar Heterojunction Mixed Halide Perovskite Solar Cells Pablo Docampo, Fabian Hanusch, Samuel D. Stranks, Markus Döblinger, Johann M. Feckl, Martin Ehrensperger, Norma K. Minar, Michael B. Johnston, Henry J. Snaith, and Thomas Bein* Dr. P. Docampo, F. Hanusch, Dr. M. Döblinger, Dr. J. M. Feckl, M. Ehrensperger, N. K. Minar, Prof. T. Bein Department of Chemistry and Center for NanoScience (CeNS) University of Munich (LMU) Butenandtstr. 11, 81377 Munich, Germany E-mail: bein@lmu.de Dr. S. D. Stranks, Prof. M. B. Johnston, Prof. H. J. Snaith Condensed Matter Physics University of Oxford Parks Road OX1 3PU, Oxford, UK DOI: 10.1002/aenm.201400355 device performance of 15.7% is currently the highest perfor- mance achieved for perovskite solar cells, pointing towards planar heterojunction devices as a promising device architec- ture for further technological improvements. The short circuit currents demonstrated for the devices pre- pared by Liu and co-workers of 20.4 mA cm -2 , [7] while high, are still short of the maximum current of over 22 mA cm -2 reasonably achievable, taking into account other light capture losses for this material. [3a] A crucial limitation in this respect is the low diffusion length of around ≈100 nm of the photoex- cited species in the MAPbI 3 perovskite. [8] This parameter can be greatly extended to over 1 μm with the inclusion of chloride in the precursor solution. [8a,9] Furthermore, it has been recently shown that the inclusion of chloride is beneficial for charge transport in the photoactive layer. [10] It is expected that the addi- tion of chloride results in improved short circuit currents and thus overall photovoltaic performance. It is worth noting here that for devices incorporating mesoporous TiO 2 photoanodes, the neat tri-iodide perovskite functions efficiently without the need for the extended diffusion length of the photoexcited spe- cies. [11] This is a result of the interpenetrated nature of the col- lection photoanode, which exhibits pore sizes at the order of tens of nanometers, and in effect reduces the distance electrons must travel to this magnitude before being collected. In the case of planar heterojunctions, electrons must travel the entire thickness of the film, which can sometimes exceed hundreds of nanometers and thus extended diffusion lengths are a require- ment for efficient operation. Here we present planar, fully solution-processed heterojunc- tion solar cells based on the solution deposition-conversion tech- nique. We highlight that chloride is critical in MA lead halide perovskites via a controlled addition of methylammonium chloride (MACl) to the MAI immersion solution. The resulting devices exhibited power conversion efficiencies approaching 15%, and more importantly, showed short circuit currents of over 22 mA cm -2 , representing a gain of over 10% over state- of-the-art devices. [7] The parameter most influenced by the pres- ence of chloride is the photoluminescence lifetime of the pho- toexcited species in the device, which reaches values exceeding 300 ns, matching previously reported results for the solution processed mixed halide perovskite films. [8a] Additionally, a reduc- tion of series resistance from 14 to 7 Ω cm 2 was observed. The solar cells developed in this work are composed of a TiO 2 /perovskite/Spiro-MeOTAD planar heterojunction, deposited on a fluorine-doped tin oxide (FTO) electrode and capped with a gold electrode ( Figure 1). The perovskite deposi- tion was performed in two steps: firstly, an ≈200 nm PbI 2 film The alkylammonium metal trihalide perovskite absorbers first used in working photovoltaic devices were based on liquid elec- trolyte sensitized solar cells. Introduced by Kojima et al., the devices exhibited a starting point power conversion efficiency of 3.8% and, with further work, they were quickly improved to reach over 6%. [1] It was not until a solid-state configuration was employed, however, that high device efficiencies were achieved. [2] Initial results were reported at 9% for perovskite sensitized titania-based devices [2b] and further improvements were simultaneously achieved in a “meso-superstructured” configuration by replacing the mesoporous TiO 2 scaffold with an electronically inactive mesoporous Al 2 O 3 layer, exhibiting device efficiencies of over 12%. [2c,3] Some of the key advantages for this material system over other competing device concepts are that they are compatible with solution-processing tech- niques and can be fully processed at low temperatures, thus enabling their use in flexible device applications. [4] Recently, Burschka et al. have demonstrated a method whereby an initial PbI 2 film is deposited over a mesoporous TiO 2 structure, which is then fully converted into the methyl- ammonium lead triiodide (MAPbI 3 ) perovskite via a second step. [5] The lead iodide coated substrates are immersed in a methylammonium iodide (MAI) solution in isopropanol for a short time (<1 min), resulting in the conversion of PbI 2 into the perovskite phase. The resulting films were coated with the hole transporter 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9′-spirobifluorene (Spiro-MeOTAD) and a metal cathode, resulting in solar cells that approach the 15% bench- mark. [6] Recently, this fabrication method was extended by Liu et al. for planar heterojunction based devices in which a planar PbI 2 film was deposited over a ZnO blocking layer and was then converted into the MAPbI 3 perovskite in a second step. [7] This resulted in perovskite crystal sizes ranging from 100 to 1000 nm and an average thickness of ≈300 nm. The resulting Adv. Energy Mater. 2014, 1400355