Effect of non-stoichiometric solution chemistry on improving the performance of wide-bandgap perovskite solar cells Mengjin Yang a, 1 , Dong Hoe Kim a, 1 , Yue Yu b , Zhen Li a , Obadiah G. Reid a , Zhaoning Song b , Dewei Zhao b , Changlei Wang b , Liwei Li c, d , Yuan Meng c, d , Ted Guo c, d , Yanfa Yan b , Kai Zhu a, * a Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401, United States b Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, United States c ENN Energy Research Institute, Langfang 065001, China d ENN Solar Energy Co., Ltd., Langfang 065001, China article info Article history: Received 7 September 2017 Received in revised form 2 October 2017 Accepted 2 October 2017 Available online xxx Keywords: Wide bandgap perovskite Tandem solar cell Non-stoichiometric chemistry abstract A high-efficiency wide-bandgap (WBG) perovskite solar cell is critical for developing perovskite-related (e.g., all-perovskite, perovskite/Si, or perovskite/Cu(In,Ga)Se 2 ) tandem devices. Here, we demonstrate the use of non-stoichiometric precursor chemistry with excess methylammonium halides (MAX; X ¼ I, Br, or Cl) for preparing high-quality ~1.75-eV FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 perovskite solar cells. Among various methylammonium halides, using excess MABr in the non-stoichiometric precursor exhibits the strongest effect on improving perovskite crystallographic properties and device characteristics without affecting the perovskite composition. In contrast, using excess MAI significantly reduces the bandgap of perovskite due to the replacement of Br with I. Using 40% excess MABr, we demonstrate a single-junction WBG perovskite solar cell with stabilized efficiency of 16.4%. We further demonstrate a 20.3%-efficient 4- terminal tandem device by using a 14.7%-efficient semi-transparent WBG perovskite top cell and an 18.6%-efficient unfiltered (5.6%-efficient filtered) Si bottom cell. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Organic-inorganic metal halide perovskites represent a new class of multifunctional semiconductors that have led to a wide range of breakthroughs in the field of electronic and optoelectronic applications such as photovoltaics (PV) [1e4], light-emitting diodes [5], photodetectors [6], X-ray detectors [7] and gamma-ray de- tectors [8]. The most impressive single-junction perovskite solar cell (PSC) has reached a certified power conversion efficiency (PCE) of 22.1% [4]. In addition, the bandgap tunability from about 1.2 eV to 2.3 eV has enabled various perovskite-related tandem PV devices such as perovskite/perovskite [9e12], perovskite/Si [13e15], and perovskite/CIGS tandem solar cells [16]. At present, the perovskite/ perovskite tandem has reached 21.2% efficiency for a 4-terminal (4- T) device [12] and 18.5% for a 2-T structure [11], whereas the perovskite/Si tandem has reached 26.4% efficiency for 4-T [14] and 23.6% for 2-T configurations [15]. A key effort in developing these tandem devices is to improve the quality of wide-bandgap (WBG) perovskites with bandgaps in the range of about 1.7 eVe1.9 eV [17]. Increasing the Br portion at the halide anion position of perov- skites is the common approach to developing WBG devices with various perovskite compositions based on methylammonium (MA), formamidinium (FA), cesium (Cs), or their mixtures on the A site of perovskites. In general, as the amount of substitution of I to Br reaches a certain range, the crystallographic, optoelectronic, and chemical properties of perovskites exhibit undesired changes such as phase segregation, higher energetic disorder, and inferior light absorption [18,19]. Thus, most studies on WBG perovskites have focused on compositional engineering to obtain phase-stable WBG perovskite with a high level of Br incorporation. It is desirable to develop a process for forming high-quality WBG perovskite thin films. Using an additive in the precursor is a useful approach to control perovskite film morphology. Yan and coworkers have recently reported highly efficient WBG PSCs by using Pb(SCN) 2 as the additive to increase the perovskite grain size. However, the use * Corresponding author. E-mail address: Kai.Zhu@nrel.gov (K. Zhu). 1 These two authors contributed equally to this work. Contents lists available at ScienceDirect Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/ https://doi.org/10.1016/j.mtener.2017.10.001 2468-6069/© 2017 Elsevier Ltd. All rights reserved. Materials Today Energy xxx (2017) 1e7 Please cite this article in press as: M. Yang, et al., Effect of non-stoichiometric solution chemistry on improving the performance of wide- bandgap perovskite solar cells, Materials Today Energy (2017), https://doi.org/10.1016/j.mtener.2017.10.001