Retrograde Solubility of Methylammonium Lead Iodide in γ-Butyrolactone Does Not Enhance the Uniformity of Continuously Coated Films

Abstract

Halide perovskite thin films can be the centerpiece of high-performance solar cells, light-emitting diodes, and other optoelectronic devices if the films are of high uniformity and relatively free of pinholes and other defects. A common strategy to form dense films from solution has been to generate a high density of nuclei by rapidly increasing supersaturation, for example, by timely application of an antisolvent or forced convection. In this work, we examine the role of retrograde solubility, wherein solubility decreases with increasing temperature, as a means of increasing the nucleation density and film coverage of slot-die-coated methylammonium lead iodide (MAPbI₃) from γ-butyrolactone (GBL) solution. Coverage was investigated as a function of the substrate temperature and the presence and temperature of an air knife. Results were considered within the framework of the dimensionless modified Biot number, which quantifies the interplay between evaporation and horizontal diffusion. Moderate temperatures and a heated air knife improved film coverage and morphology by enhanced nucleation up to ∼80 °C. However, despite the dense nucleation enabled by retrograde solubility, slow evaporation as a result of the low vapor pressure of GBL, combined with Ostwald ripening at high temperatures, prevented the deposition of void-free, device-quality films. This work has provided a more detailed understanding of the interplay between perovskite processing, solvent parameters, and film morphology and ultimately indicates the obstacles to forming dense, uniform films from solvents with high boiling points even in the presence of rapid nucleation.

Figure 1

Solubility of MAPbI₃ in GBL and vapor pressure as a function of temperature. Solubility data from ref (27) show retrograde behavior above 60 °C. The vapor pressure was calculated from Antoine parameters from ref (28).

Figure 2

Optical microscope images of perovskite films coated without (top) and with (bottom) a room-temperature air knife at different substrate temperatures. The scale bar applies to all images. Numbers in the upper right of each panel indicate the surface coverage.

Figure 3

XRD patterns of films processed at 25 and 60 °C without an air knife. See Figure S5 of the Supporting Information for the XRD results of other films.

Figure 4

(a) Optical microscope images and (b) AFM images of perovskite films coated with a hot air knife at different temperatures. Scale bars apply to all images in the row.

Figure 5

Crystallization parameters determined from image analysis of dry films. (a) Distance between microstructures. (b) Microstructure height of perovskite films. (c) Process map of Bi and the aspect ratio for a range of processing conditions. (d) Estimated Bi* for these processing conditions. Error bars in panel a represent the range of values measured in the distribution, with outliers removed, and those in panel b represent the standard deviation of the heights. Open symbols are shown for quantities based on height measurements, where the individual domains are not well-resolved. Details on the determination of Λ, Bi, and Bi* are in the Supporting Information.