© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION Understanding the Impact of Bromide on the Photovoltaic Performance of CH 3 NH 3 PbI 3 Solar Cells M. Ibrahim Dar,* Mojtaba Abdi-Jalebi, Neha Arora, Thomas Moehl, Michael Grätzel, and Mohammad Khaja Nazeeruddin* Dr. M. Ibrahim Dar, Dr. N. Arora, Prof. M. K. Nazeeruddin Group for Molecular Engineering of Functional Materials Institute of Chemical Sciences and Engineering École Polytechnique Fédérale de Lausanne CH-1015-Lausanne, Switzerland E-mail: ibrahim.dar@epfl.ch; mdkhaja.nazzeruddin@epfl.ch M. Abdi-Jalebi Cavendish Laboratory JJ Thomson Avenue, Cambridge CB3 0HE, UK Dr. T. Moehl, Prof. M. Grätzel Laboratory of Photonics and Interfaces Institute of Chemical Sciences and Engineering École Polytechnique Fédérale de Lausanne CH-1015-Lausanne, Switzerland DOI: 10.1002/adma.201503124 CH 3 NH 3 Br, and a maximum efficiency of 13.1% was achieved after passivizing the surface of the CH 3 NH 3 PbI 3-x Br x layer. [12] Hao et al. optimized the band gap by chemical substitution of iodide with bromide in lead-free solar cells and reported a power conversion efficiency of 5.73% under simulated full sunlight. [13] Furthermore, fabrication of 10% efficient perovs- kite solar cells with a planar architecture, from nanosheets of CH 3 NH 3 PbI 2 Br with a band gap of 1.8 eV, has been reported. [14] A certified efficiency of 16.2% has been reported for the per- ovskite solar cells fabricated by employing a technique based on solvent engineering. [2] In the report, authors demonstrated the preparation of a highly uniform and dense overlayer of perovskite composed of a mixture of MAPbI 3 and MAPbBr 3 (MA=(CH 3 NH 3 + )). Further enhancement in power conver- sion efficiency to >18% (certified = 17.9) was achieved by com- bining formamidinium lead iodide (FAPbI 3 ), (FA=CH(NH 2 ) 2 + ) with methylammonium lead bromide (MAPbBr 3 ) to obtain (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 , which was used as the light-har- nessing material in a bilayer solar-cell architecture. [1] In this work, we have studied the effect of bromide con- centration, supplied in the form of PbBr 2 on the proper- ties of CH 3 NH 3 PbI 3 and on the performance of the devices based on it, fabricated using a modified sequential deposition approach. An understanding of the transformation of PbBr 2 into CH 3 NH 3 PbI 3-x Br x was also gained through detailed struc- tural, spectroscopic, and morphological characterization of the PbBr 2 samples in which the time allotted for the conver- sion reaction and halide exchange (dealloying) was systemati- cally varied. Although the presence of bromide influenced the growth of perovskite structures, the band gap of the resulting CH 3 NH 3 PbI 3, i.e., CH 3 NH 3 PbI 3-x Br x (where x ≈ 0) remained unchanged which could be explained by invoking the mecha- nism of halide exchange or dealloying, occurring simulta- neously during the conversion reaction. After unraveling the impact of bromide on the properties, a significant enhance- ment in the PCE of the CH 3 NH 3 PbI 3-x Br x (where x ≈ 0) devices to 16.9% was realized under simulated full sunlight while using a judicious amount of lead bromide. It is to be noted that a reference device fabricated under similar conditions showed a PCE of 14.7%. 2. Results and Discussion It has been found that the kinetics of the conversion reaction, i.e., transformation of the films of PbX 2 (X = Cl, Br, I) into CH 3 NH 3 PbX 3 by dipping them in a solution of CH 3 NH 3 X (X = Cl, Br, I) in the two step methodology, plays a critical role 1. Introduction The upsurge in the development of efficient perovskite solar cells commenced with pure CH 3 NH 3 PbI 3 but record high effi- ciencies have been realized in devices fabricated using mixed organic–inorganic lead halide perovskites. [1,2] Kojima et al. reported for the first time a power conversion efficiency (PCE) of 3.8% while using CH 3 NH 3 PbI 3 as light-harnessing mate- rial in a liquid electrolyte based sensitized solar cell. [3] Later on, Park and co-workers documented an improved efficiency of 6.5% for CH 3 NH 3 PbI 3 perovskite device. [4] Subsequently, a higher efficiency of 9.7% was reported by Park and co-workers while retaining the architecture of solid state dye-sensitized solar cells. [5] One of the remarkable features associated with organic–inorganic lead halide perovskites is the formation of solid solutions or alloying over a wide range of composi- tions. [6] In principle, these perovskite materials, with the gen- eral formula ABX 3 , offer a great flexibility of changing all of its three components, namely, the organic cation (A), the cen- tral metal (B), and the halide anion (X), collectively or indi- vidually. [7,8] By meticulous optimization of stoichiometry, one can tailor the properties of the perovskite material, in which way as to achieve the development of high-efficiency solar cells. [1,2,9,10,13] In this direction, a monotonic tuning of band gap between 1.5 and 2.3 eV was achieved by substituting iodide in CH 3 NH 3 PbI 3 with various proportions of bromide using a single step approach. [11] To attain a band gap of 1.72 eV, incorporation of bromide into CH 3 NH 3 PbI 3 perovskite was controlled by using an optimum ratio of CH 3 NH 3 I and Adv. Mater. 2015, DOI: 10.1002/adma.201503124 www.advmat.de www.MaterialsViews.com