Dimethylformamide-free synthesis and fabrication of lead halide perovskite solar cells from electrodeposited PbS precursor films

Highlights

• A three-step dimethylformamide-free method to produce (CH₃NH₃PbI₃) perovskite films.

• Cyclic voltammetry deposition of PbS as the Pb precursor on titanium oxide substrates.

• The formation of halide perovskite cuboid crystallites with a bandgap of 1.58 eV.

• A power conversion efficiency (PCE) of up to 7.72% under the standard AM 1.5 conditions.

• Neglection of employing hazardous solvents from the routine perovskite solar cell fabrication methods.

Abstract

A multi-step dimethylformamide (DMF)-free green synthesizing method based on (i) initial electrodeposition of lead precursor, i.e. lead sulfide (PbS) on mesoporous TiO₂/fluorine-doped tin oxide (FTO) conductive glasses substrates, (ii) subsequent conversion of PbS to PbI₂ and (iii) synthesis of methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite film and their microstructural, optical and solar cell performance are described. Different electrodeposition techniques including direct current and cyclic voltammetry deposition were investigated to produce PbS films. We find that the perovskite films produced based on PbS deposited by cyclic voltammetry exhibit compact layer consisting of cuboid grains with an average size of approximately 800 nm and a bandgap of 1.58 eV whose properties are comparable to those of perovskite films generally prepared by conventional methods like spin coating. It was observed that uniform perovskite layers deposited under different conditions as the absorber layer generate a power conversion efficiency (PCE) of up to 7.72% under the standard AM 1.5 condition in the first attempt by this fabrication approach. PCEs obtained under different electrodeposition conditions were improved by eliminating of pores between the cuboid perovskite crystallites. This approach neglects employing hazardous solvents from the routine perovskite solar cell fabrication method and has potential to enhance its PCE similar to the common strategies by spin-coating methods improved over last decade by further modification of the electrodeposition process of the metal precursors and other steps towards highly efficient green perovskite solar cells.

Introduction

Organic-inorganic halide perovskites have received tremendous attention as light absorbers for use in solar cells. Perovskite based solar cells (PSCs) were firstly introduced in 2009 by Kojima [1]. So far the performance of the PSCs have experienced the fastest growth progress compared to other types of the solar cells. The most common material used as the light harvester in the PSCs, methylammonium lead triiodide (MAPbI₃), has a crystal structure defined by ABX₃ formula where A is a big organic cation (CH₃NH₃⁺), B is a smaller cation of divalent metal (Pb²⁺) and X is a halogen anion, e.g. iodine (I⁻) [2], [3]. MAPbI₃ is an inexpensive compound and easy to process [4] having a combination of exceptional photovoltaic properties that make it a promising substance for solar cell applications. Among them, the most important characteristics include high absorption coefficient [5] long electron/hole diffusion length [6], [7] small exciton binding energy [8] and direct band gap which is tunable by changing chemical composition [9], [10].

Until now, organolead halide perovskite films have generally been produced via spin coating method. The deposition of the perovskite layer by spin coating can be conducted either in one step approach, in which a single solution containing both PbI₂ and methylammonium iodide (MAI) are spin coated on a substrate [11] or two step approach which consists of sequential deposition of the precursors of final perovskite [12]. The morphological evolution of perovskite has been broadly reviewed [13], [14], [15]. Organolead halide perovskites with a dot-like morphology produced by one-step spin coating method were firstly used as sensitizer in dye-sensitized solar cells [1]. Sequential two step deposition method of organolead halide perovskite films have revealed organohalide films with bulky cuboidal morphology [16]. Later, a few approaches based on using anti-solvent [11] or inducing intermediate phase [17] were examined to optimize the morphology of perovskite toward a bulk compact layer. Despite the fact that perovskite layers realized by spin coating technique yield high performance in perovskite solar cells with the highest power conversion efficiency (PCE) [17], [18], [19], [20] it has not yet been proven for commercialization and just been used in the laboratory scale. Thermal evaporation is another method introduced for preparing the perovskite films with potential for deposition in large scales [21]. However, the need for high vacuum as well as elevated processing temperatures limits its potential for mass production purposes. Therefore, there is currently a great deal of attempts to develop scalable and economic fabrication strategies to overcome existing shortcomings.

Electrodeposition also known as electrochemical deposition is a cost-effective and flexible fabrication technique of a wide variety of materials which is extensively used for depositing large area thin films for various applications in the industrial scale. Thus, this is a way forward towards the mass production of low-cost perovskite solar cells. Although there are number of reports on the application of the electrodeposition of the halide perovskite films, it is still an immature field of research. Chen et al. [22] was the first who developed an electrochemical route for depositing perovskite film with the application in solar cells giving rise a maximum PCE of 10.19%. The process involved the iodination of electrochemically deposited PbO₂ as the metal precursor followed by the inter-diffusion reaction with CH₃NH₃I as the organic portion to form the halide perovskite material. In another report, the PbO precursor was electrodeposited on a TiO₂ scaffold which was alternatively subjected to the chemical conversion in order to sequentially achieve PbI₂ and CH₃NH₃PbI₃ layer with a record PCE of 12.5% [23]. Afterwards, the same group simplified the process via the direct conversion of electrodeposited PbO to the halide perovskite through reacting with adjacent CH₃NH₃I layer. The fabricated perovskite layer served as a harvesting layer in a planar photovoltaic device with an average PCE of 13.12% [24]. Kosta et al [25]and Li et al [26] have also reported a method of direct electrochemical deposition of PbI₂ and its subsequent chemical conversion to the perovskite films employed in solar cells with an acquired PCE of 9.0% and 6.62%, respectively.

Lead sulfide (PbS) is another non-halide compound that can be employed as an alternative for PbO₂ as a seeding or precursor layer for subsequent chemical conversion to the halide perovskite films. Since the bond dissociation energy of PbS (3.3 eV) is lower than that of PbO (4.1 eV) [27] hence it is more convenient for PbS to chemically react to produce the final perovskite films. This metal precursor has been only synthesized via vapor-assisted chemical bath deposition [28] atomic layer deposition [29] and radio-frequency (rf) sputtering [30] methods to be subsequently converted to the perovskite layer. To the best of our knowledge, the electrodeposition of PbS has not been employed towards the fabrication of the halide perovskite materials for solar cell applications. Therefore, in the present work, a sequential multi-step fabrication method of perovskite films for solar cell is introduced which includes the electrodeposition of PbS as the precursor of Pb on mp-TiO₂ substrates and chemical conversion of the lead precursor to the final perovskite layer. The solar cell fabricated based on the electrodeposited/converted film yields a reasonable PCE. Further to being a large scale production method, this process benefits from an environmentally friendly approach which eliminates using the toxic solvent such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) conventionally being used by other fabrication methods.