Lead acetate film as precursor for two-step deposition of CH3NH3PbI3
Graphical abstract
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.
Section snippets
Materials and samples preparation
Fluorine-doped tin oxide coated glass substrates (FTO) with sheet resistance of ∼15 ohm/square and optical transmission >80% from 400 to 700 nm were cleaned sequentially in detergent solution, acetone and 2-propanol.
Perovskite films were synthesized by placing lead acetate films deposited on FTO substrates into a CH3NH3I solution in anhydrous 2-propanol. Lead acetate films were prepared by spin-coating a solution of 0.4173 g PbAc2 3H2O (99.99%) in a mixture of 0.5 ml dimethylformamide (DMF) and 0.5
Results and discussion
Lead acetate has a very limited solubility in common solvents except some alcohols, DMSO and water [35]. The solubility of lead acetate trihydrate in DMF is reduced. However, we obtained a slightly viscous solution, at room temperature, by dissolving 0.4173 g PbAc2 3H2O in a mixture of 0.5 ml DMF with 0.5 ml AcAc. It was shown that lead acetate trihydrate reacts with the chelating agent acetylacetone to produce a new lead precursor [36] which, in our case is soluble in the mixture of DMF/AcAc.
Conclusion
In summary, we have investigated the preparation of lead acetate film and its use as precursor for two-step solution deposition of CH3NH3PbI3 perovskite film. Transparent precursor films were prepared from a solution of lead acetate trihydrate in a mixture of DMF and AcAc. The perovskite films derived from lead acetate trihydrate consist of cuboid grains and show high surface coverage both on FTO and TiO2 substrates. The dehydration of lead acetate trihydrate film causes formation of perovskite
Acknowledgment
M. Sima gratefully acknowledges the financial support for this work under the Project SEE8/2014 (EEA-JRP-RO-NO-2013-1): Perovskites for Photovoltaic Efficient Conversion Technology – PERPHECT.
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