Synergistic improvements in stability and performance of lead iodide perovskite solar cells incorporating salt additives† Karunakara Moorthy Boopathi, abc Ramesh Mohan, c Tzu-Yen Huang, cd Widhya Budiawan, abc Ming-Yi Lin, c Chih-Hao Lee, a Kuo-Chuan Ho d and Chih-Wei Chu * c The main issues in planar perovskite solar cells are the coverage and crystallinity of the perovskite film on the PEDOT:PSS layer. To enhance these features, we introduced alkali metal halides (salts) as additives into the perovskite precursor solution used in a two-step preparation method. These alkali metal halides chelate with Pb 2+ ions and enhance the crystal growth of PbI 2 films, resulting in nanostructured morphologies. The nanostructured PbI 2 films promote homogeneous nucleation and larger crystallite sizes, thereby enhancing the morphology and crystallinity of the perovskite films. The alkali metal halides recrystallize the small grains and passivate the grain bound- aries and interface states, allowing effective charge generation and dissociation in perovskite films. Photoluminescence measurements indicated that perovskite films prepared with salt additives featured fewer charge traps and defects. The power conversion efficiency of the device incorporating a small amount of a salt additive increased by approximately 33%—from 11.4 to 15.08%. This device was more stable than a corresponding device prepared without the additive, with only 16.5% degradation occurring over a period of 50 days. Introduction Organometal halide perovskites are emerging materials in photovoltaics (PVs), 1–8 light emitting diodes, 9–12 and lasers 13,14 because of their high absorption coefficients, long carrier life times, micrometer diffusion lengths, high uorescence yields, and wavelength tunability. 2,11,13,15–17 Indeed, an exceptionally high solar cell performance of 20.2% has been achieved when using a solution-processed lead halide perovskite. 18 Two kinds of device architectures have been used in perovskite solar devices: mesoporous 2,3 and planar heterojunctions. 6,19–21 High temperature annealing is required to make crystalline TiO 2 layers in mesoporous-type solar cells, whereas planar hetero- junction-based solar cells can be fabricated at low tempera- ture—more suitable for roll-to-roll manufacturing of large-area, exible solar cells. 8,22 Unlike mesoporous-type devices, in which the perovskite material can inltrate within the porous matrices, perovskite lms with pinholes and non-uniform coverage are usually present in planar-type perovskite solar cells; these factors are mainly responsible for their poor device performance. Controlling the morphology and crystallization of perovskite thin lms in planar architectures is a main challenge affecting our ability to develop high-performance devices. 23–25 These features are determined by their thermodynamics and growth kinetics, and can be controlled by varying the thin lm fabri- cation process, the choice of solvent, the annealing tempera- ture, and the levels of moisture and processing additives. Several thin lm fabrication processes, including spin coating, dip coating, spray coating, blade coating, and thermal evapo- ration, have been investigated to control perovskite lm formation. 4,24,26–29 Two-step perovskite thin lm preparation has been superior to single-step formation when attempting to obtain continuous and pinhole-free lms. Recently, incorpora- tion of additives in the perovskite solution process has become a simple way to improve the lm coverage and crystallinity. Several additives, including NH 4 Cl, 1,8-diiodooctane, 1-chlor- onaphthalene, polyvinylpyrrolidone, hydroiodic acid (HI), and hydrochloric acid, can be used to form smooth, continuous, and uniform lms with awless perovskite nanocrystals, signi- cantly improving the device performance. 30–37 In this study, we investigated the inuence of alkali metal halides as additives on the performance of perovskite solar cells. We incorporated alkali metal halides into the lead iodide (PbI 2 ) precursor solution to tune the morphology of the result- ing PbI 2 lms. These halogenated additives chelate with Pb 2+ a Department of Engineering and Systems Science, National Tsing Hua University, Hsinchu 30013, Taiwan b Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Tsing Hua University, Taiwan c Research Center for Applied Science, Academia Sinica, Taipei 115, Taiwan. E-mail: gchu@gate.sinica.edu.tw d Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan † Electronic supplementary information (ESI) available: Experimental details, device fabrication, solar testing, salt concentration optimization, comparison table and XPS data. See DOI: 10.1039/c5ta10288j Cite this: J. Mater. Chem. A, 2016, 4, 1591 Received 16th December 2015 Accepted 7th January 2016 DOI: 10.1039/c5ta10288j www.rsc.org/MaterialsA This journal is © The Royal Society of Chemistry 2016 J. Mater. Chem. A, 2016, 4, 1591–1597 | 1591 Journal of Materials Chemistry A COMMUNICATION Published on 07 January 2016. Downloaded by Academia Sinica - Taipei on 16/03/2016 06:30:36. View Article Online View Journal | View Issue