# Effects of Adding Alkali Metals and Organic Cations to Cu-Based Perovskite Solar Cells
## 1 Introduction  
There have been many efforts to improve the conversion efficiencies and durabilities of perovskite photovoltaic devices by adding metal, halogen, and organic ions to the precursor solutions [1, 2, 3]. Other approaches have included the development of scalable processes via electrodeposition [4, 5], stable fully inorganic perovskites [6], polymer composites using perovskites [7], and solvent effects in the precursor solution on perovskite film microstructures [8]. Here, additives such as copper (Cu), sodium (Na), and ethyl ammonium (EA) were examined. Cu compounds such as copper thiocyanate (CuSCN), copper oxides, (Cu\({}_{x}\)O), and copper iodide, were mainly used as hole-transport materials that replaced organic versions [9, 10, 11, 12, 13, 14]. Although there have been many reports on the addition of alkali metals to perovskite precursor solutions [15, 16, 17, 18, 19, 20, 21, 22, 23], relatively few reported on the addition of Cu [24, 25, 26, 27]. The addition of CuBr\({}_{2}\) and NaCl to the solutions improved the solar conversion efficiency and device durability [28, 29, 30]. In those studies, lattice distortion in the perovskite crystals was reduced by small amounts of Cu, and lattice defects were suppressed with Na at the defect sites after desorption of methylammonium (MA), which improved conversion efficiency and device durability. First-principles calculations of perovskite crystals have been performed using structural models in which organic cations, metal cations, and halogen anions were replaced with other groups to examine the crystal structure stability. CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) (MAPbI\({}_{3}\)) is the standard perovskite compound used for photoelectric conversion [31]. Previously, calculations indicated that EA substitution for MA enhanced the structural stability [32], and the calculated energy gap agreed well with experimental values [33]. The addition of other organic cations and combinations of multiple organic cations have also been studied. The introduction of EA and butyl-ammonium affected the conduction-band minima,valence-band maxima, and the surface morphologies of MA-based perovskites [34]. Increased coulomb interactions between cations, attributed to the additional organic cations, enhanced the perovskite stability and device durability. There have been other reports on using larger organic cations to replace MA and improve device properties [35; 36; 37; 38; 39]. The effects of halogen additions have been examined computationally and experimentally [40; 41; 42; 43]. A Br-rich sample resulted in larger grain sizes and fewer grain boundaries because of the reduced number of nucleation sites during perovskite crystal growth [44]. Defects distributed at grain boundaries were thus reduced by fewer boundaries, which increased carrier lifetimes. Br-rich samples exhibited higher thermal and optical stabilities relative to MAPbI\({}_{3}\) devices. It is expected that perovskite film quality and stability could be improved via halogen selectivity. The replacement of I with Br or Cl changed the band gap and charge-transfer properties. Monovalent pseudo-halogens, such as thiocyanate, have properties like those of halogens, they could improve photoelectric conversion and stability, and may prevent oxidation of Sn\({}^{2+}\) to Sn\({}^{4+}\) in Sn-based perovskites [45]. The latter was expected to suppress oxidation-induced degradation. The thiocyanate ions also increased the grain size and improved the surface morphology and crystallinity, which increased photoelectric conversion and stability. Thus, the Pb, MA, and halogen sites are very important because the perovskite energy gap and crystallinity change with added materials. Here, the effects of adding Cu, Na, and EA to the perovskite precursor solution were investigated with first-principles calculations. The addition of 2% Cu and 2% Na to MAPbI\({}_{3}\) improved the photoelectric conversion efficiency [29].