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Nucleation and Crystal Growth of Organic-Inorganic Lead Halide Perovskites under Different Relative Humidity Hao Gao, Chunxiong Bao, Faming Li, Tao Yu, Jie Yang, Weidong Zhu, Xiaoxin Zhou, Gao Fu, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b00895 • Publication Date (Web): 14 Apr 2015 Downloaded from http://pubs.acs.org on April 19, 2015
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ACS Applied Materials & Interfaces
Nucleation and Crystal Growth of Organic-Inorganic Lead Halide Perovskites under Different Relative Humidity
Hao Gao,1,2 Chunxiong Bao,1 Faming Li,1 Tao Yu,1,3,4* Jie Yang,1 Weidong Zhu,1 Xiaoxin Zhou,1 Gao Fu1 and Zhigang Zou1,3,4
1. National Laboratory of Solid State Microstructures & Ecomaterials and Renewable Energy Research Center (ERERC) at Department of Physics, Nanjing University, Nanjing 210093, P.R.China 2. School of Mechanical and Electronic Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, P. R. China. 3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P.R.China 4. Jiangsu Key Laboratory for Nano Technology, Nanjing 210093, P.R.China
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Abstract Organic-inorganic lead halide perovskite compounds are very promising materials for high-efficiency perovskite solar cells. But how to fabricate high-quality perovksite films under controlled humidity conditions is still an important issue due to their sensitivity to moisture. In this study, we investigated the influence of ambient humidity on crystallization and surface morphology of one-step spin-coated perovskite films, as well as the performance of solar cells based on these perovskite films. Based on experimental analyses and thin film growth theory, we conclude that the influence of ambient humidity on nucleation at spin-coating stage is quite different from that on crystal growth at annealing stage. At spin-coating stage, high nucleation density induced by high supersaturation prefers to appear under anhydrous circumstance, resulting in layer growth and high coverage of perovskite films. But at annealing stage, the modest supersaturation benefits formation of perovskite films with good crystallinity. The films spin-coated under low relative humidity (RH) followed by annealing under high RH show an increase of crystallinity and improved performance of devices. Therefore, a mechanism of fast nucleation followed by modest crystal growth (high supersaturation at spin-coating stage and modest supersaturation at annealing stage) is suggested in formation of high-quality perovskite films.
Key words: methylammonium lead trihalide; ambient humidity; film coverage; nucleation density; crystal growth
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1. Introduction Since the pioneering work of Miyasaka et al. in 2009,1 organic-inorganic hybrid perovskite solar cells (PSCs) have attracted much attention due to their power conversion efficiencies (PCEs) improved rapidly from 3.8% to 20.1%.1-7 As light absorber in PSCs, the hybrid methylammonium lead halide perovskite materials (CH3NH3PbX3, X=halogen) combine the advantages of organic compounds and inorganic semiconductors, such as solution processability and tunable optical properties of the organic compounds, and ultrahigh light absorption, high charge mobility and small exciton binding energy of the inorganic semiconductors.8-10 These advantages make the perovskite materials become very promising for application in solar conversion devices. The solution processability of these perovskite materials is one of the most important advantages. One-step spin-coating method was adopted earlier to deposit perovskite films due to its simple and low-cost process. During the spin-coating and annealing process, solvent evaporation and the following self-assembly induce the formation of crystallized perovskite films. To obtain good performance for PSCs, perovskite films with high quality are necessary. However, the one-step spin-coating method is difficult to yield a homogeneous perovskite films, some pin-holes prefer to appear in the films and degrade the performance of PSCs. The pin-holes not only weaken the light absorption ability of perovskite films, but also deleteriously impact charge transport and recombination. The coverage of perovskite layer is influenced by many factors, such as the choice of solvent, substrate, spin-coating rate and time, precursor concentration, annealing protocol and so on. These technology parameters control the nucleation and growth of perovskite films, determine the ultimate crystallinity and
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morphology. Many endeavors have been tried to improve perovskite morphology and crystallinity by adjusting the above parameters.4,11,12 Meanwhile, some new preparation methods were developed to improve PCEs of PSCs, such as two-step sequential solution deposition, vapor deposition and derivative preparations.5,13-16 However, the vapor deposition is likely to increase the manufacturing cost. So the solution preparation methods still receive much attention. In the solution preparation methods, it has to be faced with an important factor of ambient humidity. It is well known that the perovskite CH3NH3PbI3 films are very sensitive to moisture and the long-term action of moisture could corrode CH3NH3PbI3 into PbI2.17,18 So the device fabrication should be carried out under controlled atmosphere conditions with low humidity. Many research groups fabricated CH3NH3PbI3-based PSCs with high PCEs in glovebox with RH
