Impurity Tracking Enables Enhanced Control and Reproducibility of Hybrid Perovskite Vapor Deposition

Abstract

Metal halide perovskite semiconductors have the potential to enable lowcost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH 2 PO 3 and MAH 2 PO 2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

The Future of Perovskite Photovoltaics—Thermal Evaporation or Solution Processing?

“…[43] While several approaches have been shown to improve the control over MAI evaporation by modifying the crucible from which it is evaporated, [44,45] many groups still struggle with reproducible and controllable evaporation of MAI. [46] Another issue affecting the composition of the evaporated perovskite film is the formation of nonstoichiometric thin layers upon the evaporation of the first few nanometers on the substrate. [47] Such layers may act as blocking layer for charge extraction and lower the device performance.…”

“…[ 43 ] While several approaches have been shown to improve the control over MAI evaporation by modifying the crucible from which it is evaporated, [ 44,45 ] many groups still struggle with reproducible and controllable evaporation of MAI. [ 46 ]…”

Light Absorption and Recycling in Hybrid Metal Halide Perovskite Photovoltaic Devices

“…Alternatively, coevaporation does not require any annealing step, expanding the number of substrates upon which the film can be deposited, as well as allowing finer control over the thickness of the film . One disadvantage of coevaporation over the sequential deposition method is that controlling and optimizing the deposition rate of each precursor simultaneously is not trivial . However, once the deposition rate of each precursor is optimized, the resultant films consist of a high‐quality perovskite .…”

Efficient, Hysteresis‐Free, and Flexible Inverted Perovskite Solar Cells Using All‐Vacuum Processing

“…[26,27] However, in terms of large-area devices and scale-up, vacuum techniques have more potential as the deposition of precursors can be controlled precisely much better than a solution method even without annealing the perovskite film. [28,29] There are several approaches under the vacuum category, thermal evaporation in one step [30] or two steps, [31] layer-by-layer deposition, [32,33] and chemical vapor deposition (CVD), [34] which can provide us high-quality perovskite films conformally deposited on any substrate with an extremely large area without any pinholes. [35][36][37][38] As a PSC consists of electrodes, the electron transporting layer (ETL), hole transporting layer (HTL), and perovskite absorber layer, all the layers have to be deposited by a vacuum approach to fabricate an all-vacuum-processed device.…”