Lead acetate film as precursor for two-step deposition of CH₃NH₃PbI₃

Highlights

• Lead acetate transparent film was prepared by spin coating on different substrates.

• Lead acetate trihydrate film was used as precursor for perovskite film deposition.

• Significant changes appear in perovskite structure after lead acetate dehydration.

Abstract

This paper relates to a method for the preparation of lead acetate film and its use as precursor for two-step solution deposition of CH₃NH₃PbI₃ perovskite film. For the preparation of perovskite film, lead acetate was used both in the form of trihydrate and dehydrated. The effects of lead acetate trihydrate dehydration on the structure of the resulted perovskite films were studied. The prepared perovskite films which consist of cuboid grains, show high surface coverage both on FTO and TiO₂ substrates. XRD measurements have not revealed the presence of the PbI₂ contaminant in perovskite film. Solvent vapor annealing method was employed to prepare perovskite film that contains lead iodide. Using impedance spectroscopy measurements, we further evaluated the influence of the prepared perovskite films on the performance of the photovoltaic devices.

Introduction

Organic-inorganic halide perovskite solar cells are considered one of the competitive next generation power sources due to achieved record efficiencies [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and their facile fabrication. Responsible for this performance are the properties of the organic-inorganic perovskites including favorable direct band gap, large absorption coefficient, long carrier-diffusion lengths, high carrier mobilities [11]. The development of the perovskite solar cells is mainly based on the improvement in the perovskite film quality (film coverage, film thickness, grain size, surface roughness), device structure and interfaces engineering [12].

The preparation of a perovskite (CH₃NH₃PbI₃) film has been achieved by several deposition techniques, including vapor deposition [7], vapor assisted solution process [13] and solution processed via one-step [4], [14] and two-step deposition [8], [15].

The two-step deposition process consists of first spincoating lead iodide (PbI₂) on the substrate surface from a solution (preferably 1.0 M [16], [17]) in dimethylformamide (DMF) under appropriate conditions (solution concentration, spincoating speed) followed by PbI₂ film dipping in a methylammonium iodide (CH₃NH₃I) solution in 2-propanol solvent. The two step deposition process permitted a better infiltration of perovskite film into mesoporous substrate [18]. A good surface coverage was obtained even in absence of a mesoporous scaffold when perovskite film was deposited in two steps [19].

Photovoltaic performance is closely related to perovskite crystallinity, morphology and composition [20], [21], [22]. It was found that the morphology of PbI₂ film influences the morphology of the perovskite film [23]. Spincoating deposition of the PbI₂/DMF solution produces a crystalline film and the use of dimethyl sulfoxide (DMSO), another coordinative solvent produces an amorphous PbI₂ film. Highly crystalline, more homogeneous and smoother PbI₂ films have been prepared by introduction of controlled amounts of water into PbI₂/DMF solutions [24]. On the other hand, the size and the shape of the perovskite cuboids prepared from crystalline PbI₂ film depends on iodine concentration [25] that determines the kinetics of the chemical reaction [26]. Some authors observed that increasing the dipping time increases the size of perovskite crystals [27]. A perovskite film with increased grain size and crystallinity was prepared using solvent annealing technique, where solvent vapor is introduced during crystallization of perovskite thin film [28]. The size of the perovskite grain was also increased by dripping a solution of CH₃NH₃I/CH₃NH₃Cl onto PbI₂ film during spinning of substrate, for multi-cycle coating and annealing [29].

Recently, lead acetate (PbAc₂) was used instead of lead iodide or chloride to the preparation of perovskite films by one step solution coating [30], [31], [32]. It was reported accelerated perovskite crystal growth due to facile removal of N-methylammonium acetate (CH₃NH₃Ac) [33] and synthesis of perovskite films more uniform (both in thickness and coverage [34]) than those prepared with lead halides.

Herein we report the preparation lead acetate film and its use as precursor for two-step solution deposition of CH₃NH₃PbI₃ perovskite film. This approach offers the advantage of synthesis of perovskite film with controlled PbI₂ impurities from solutions prepared at room temperature. The thickness of the perovskite film can be easily controlled by changing the spin speed in spin-coating process of the lead acetate solution.