Article pubs.acs.org/cm
From Nano- to Micrometer Scale: The Role of Antisolvent Treatment on High Performance Perovskite Solar Cells S. Paek,† P. Schouwink,† E. Nefeli Athanasopoulou,‡ K. T. Cho,† G. Grancini,† Y. Lee,† Y. Zhang,† F. Stellacci,‡ Mohammad Khaja Nazeeruddin,*,† and P. Gao*,† †
Group for Molecular Engineering of Functional Materials, EPFL Valais Wallis, Sion, CH-1951 Switzerland Supramolecular Nano-Materials and Interfaces Laboratory, EPFL, Lausanne, CH 1015 Switzerland
‡
S Supporting Information *
ABSTRACT: To date, antisolvent treatment has become one of the most important means to fabricate high efficiency perovskite solar cells (PSCs); however, the few reported antisolvents have not been analyzed on a uniform platform, and there is hitherto no clear reasoning in the choice of antisolvents toward high performance PSCs. Here, we study the role of the antisolvents in the nucleation kinetics of perovskite solutions and their residual influence on perovskite crystal growth, film formation, and device performance. Through X-ray diffraction analysis on the complicated double mixed perovskite, we qualitatively evaluate the impact of thermal annealing and antisolvent treatment (A.S.T.) on the phase composition and microstructure of the films. By using miscible antisolvents with high boiling point instead of immiscible low boiling point solvents, we obtain homogeneous and almost pinhole-free perovskite films. When using trifluorotoluene (TFT) to replace toluene and chlorobenzene as a novel antisolvent, we achieve a power conversion efficiency (PCE) of 20.3% under optimized device fabrication conditions with a composite perovskite as active layer. The conclusions from this study should assist in establishing reproducible fabrication processes and finding better antisolvent candidates for perovskite solar cells. (A.S.T.),10,11 solvent vapor annealing,12,13 and vacuum treatment14 have been the most widely used post-treatment methods. Enlightened by organic photovoltaic (OPV) device engineering,15 the A.S.T. is at the origin of several reported world record efficiencies.11,16−20 Normally, the perovskite precursors are dissolved in high boiling point (BP) polar aprotic solvents such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), and γbutyrolactone (GBL). Antisolvents assisting perovskite film formation include a larger scope of solvents that cannot dissolve any precursor of the perovskite recipe and the perovskite itself. It is known that the working mechanism of an antisolvent is to speed up heterogeneous nucleation via the creation of an instantaneous local supersaturation on the
1. INTRODUCTION It has been calculated that the solar energy harvested by 0.4% of the world’s surface area would be sufficient to meet the global energy demands, assuming an average solar energy conversion efficiency of 15% can be popularized.1 Very recently, the dominating photovoltaic community based on crystalline, inorganic semiconductors was agitated by the fast development of perovskite solar cells (PSCs).2,3 The surge in the perovskite based photovoltaic technology is evidenced by the abnormally rapid update of the record efficiencies in three years.4 As a latecomer, despite the short time spent on the optimization of the perovskite devices, the huge amount of efforts dedicated by scientists from all over the world led to rapid improvements in the device engineering methods. It has been recognized that the morphology of the perovskite layer is one of the most crucial parameters that determine the final performance of a PSC device.5 Among the many different methods introduced to improve the surface morphology of perovskite film, gasblowing,6 amino halide additive,7−9 antisolvent treatment © 2017 American Chemical Society
Received: December 18, 2016 Revised: March 25, 2017 Published: March 26, 2017 3490
DOI: 10.1021/acs.chemmater.6b05353 Chem. Mater. 2017, 29, 3490−3498
Article
Chemistry of Materials spinning substrate.10 During the treatment of antisolvents, complicated interactions are happening simultaneously under the influence of several physicochemical properties of both solvents (Table S1). Xiao et al. screened 12 different kinds of solvents for the treatment of spinning methylammonium lead iodide (MAPbI3) perovskite films.10 However, some of these solvents can dissolve the ammonium halide salt and destroy the final perovskite film. Zheng et al. evaluated three different antisolvents and various interval dropping times on the final morphology of methylammonium lead bromide (MAPbBr3).21 To date, the most popular antisolvents for perovskite film formation are toluene (Tol),16 chlorobenzene (CB),18 and diethyl ether (ether).20 Interestingly, these three solvents have dramatically different physicochemical properties but still perform efficiently with different perovskite films. However, in these reports, no thorough study on the interaction between different antisolvents as shower solvent and solution solvents as bath solvent was conducted. More importantly, these popular antisolvents have not been evaluated on the same type of perovskite film to make a fair comparison and elucidate the limitations of each solvent. A complete understanding of these issues is crucially important for advancing our ability of controlling perovskite film morphology and improving solar cell performance. To shed light on this perspective, in this study, a series of six antisolvents including trifluorotoluene (TFT), toluene (Tol), chlorobenzene (CB), p-xylene (Xyl), diethyl ether (ether), and dichloromethane (DCM) with different dielectric constants (ε, solvating abilities) and other physicochemical properties are chosen to treat the spinning composite perovskite films deposited from a mixed DMSO and DMF solution (1:4) (Figure 1, Table S1). The kinetics of the treatment process was
due to differences in boiling point (BP), miscibility, and dielectric constants. Antisolvents with both low boiling point and poor miscibility with solution solvents will lead to lower efficiency and reproducibility comparing the antisolvents with higher boiling point and good miscibility with DMF/DMSO. By using high boiling point and miscible antisolvents under optimized conditions, we could always obtain compact perovskite films with full coverage on the substrate providing decent power conversion efficiencies (PCE) above 18%. When using TFT to replace Tol and CB as a novel antisolvent, we achieve a PCE of 20.3% under optimized conditions with composite perovskite as active layer, measured under one sun illumination.
2. RESULTS AND DISCUSSION 2.1. Kinetic Study of Solvent−Solvent Interaction Effect on Perovskite Phase Formation. One of the major variables during the antisolvent treatment is the interaction between the solution solvent and the antisolvent. We designed an experiment to monitor the kinetics of this process by recording the increase in absorbance at 550 nm as a function of time, when the freshly spin-coated perovskite intermediate films from DMSO/DMF (1:4) were treated with the six kinds of antisolvents (Figure 2a−f).22 Depending on the mutual miscibility and dielectric constants, the six antisolvents exhibited dramatically different kinetic behavior in terms of the increment and the time constant in the relative absorbance change. The kinetic traces can be fitted with mono-, bi-, and triexponential functions, and the corresponding time constants can be deduced (Table S2). Generally, the data shows a steep rise of the absorbance upon the addition of the antisolvents (1−42 s), which is indicative of an accelerated heterogeneous nucleation and growth process to different extents accompanying the fast solvent (mobile) extraction. This fast component (τ1) is followed by an approximate plateau that indicates the completion of nucleation and local crystal growth, when the solvent extraction reaches a state of equilibrium. The small additional increase that occurs on a slow component of about 50−1000 s (τ2 and/or τ3) contributes only little to the total change in absorbance and is attributed to the slow diffusion of remaining solution solvents (complexed) out of the film and slow growth of large crystals leading to enhanced lightscattering. Figure 2b,c,f shows that the fast increase of perovskite absorption at 550 nm is practically complete within a few seconds (
