Effect of chlorine and bromine on the perovskite crystal growth in mesoscopic heterojunction photovoltaic device

Abstract

Organic-inorganic hybrid perovskite solar cells are within the emerging photovoltaic technologies. The combination of different halogen ions, in certain fill fractions, is one of the methods to improve the perovskite film properties. Herein, fabrication and characterization of perovskite cells in standard mesoscopic architecture using one-step deposition method has been done. The role of the halogen ions (Chlorine or Bromine) on crystal structure growth and photoelectric performance has been investigated. X-ray diffraction, scanning electron microscopy, atomic force microscopy and optical microscopy analysis were performed. The microstructure, composition and morphology of CH₃NH₃PbI_1.8Br_1.2 and CH₃NH₃PbI_1.8Cl_1.2 films are dissimilar, although identical fabrication method was used. Same holds for optical properties, band gap energies of 1.84 eV and 1.63 eV, respectively, being obtained. Integrated in solar cells, the maximum power conversion efficiency of the Br based devices is beyond 10%, while for those based on Cl, the efficiency drops around 5%.

Introduction

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention as a promising material for photovoltaic applications due to their photo-physical properties, high defect tolerance, a tunable band gap by compositional engineering, and their simple solution fabrication process using low-cost materials [[1], [2], [3], [4], [5], [6], [7]]. Their high-power conversion efficiency (PCE) has achieved an impressive improvement, and presently the best cells go beyond 25% [8,9].

Methylammonium lead triiodide (CH₃NH₃PbI₃) is one of the most studied perovskite materials in photovoltaics perovskite field. Solar cells based on this perovskite demonstrated relevant stable and efficient devices [[10], [11], [12]]. The heterojunction solar cells based on CH₃NH₃PbI₃ have a considerable advantage in terms of synthesis and performance, in combination with mesoporous TiO₂ as electron transport layer and spiro-OMeTAD and hole transport layer [13,14].

Improving the morphology, homogeneity and microstructure of the perovskite film are among the most important assessments to obtain devices with high efficiency. Many scientific publications state that the photovoltaic performance of PSCs is directly correlated with the perovskite deposition parameters, such as the annealing treatment [[15], [16], [17]], the molar ratio of the precursors [[18], [19], [20]] and the deposition procedure [[21], [22], [23]]. Other studies suggest that for an easy to scale-up "one-step" deposition process, the CH₃NH₃PbI₃ hybrid perovskite must contain small amounts of chloride or bromide [[24], [25], [26], [27], [28]].

Recently, it has been suggested that compositional engineering by using other halogens such as chlorine (Cl) or bromine (Br), which leads to mixed halide perovskites, is one of the most used strategies to improve the PCE in solar cells [[29], [30], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [85]]. On the other hand, the band gap of the perovskite depends on halogens concentrations [[45], [46], [47]], while the choice of organic cations impacts the stability of the devices [[48], [49], [50]].

The addition of Br and/or Cl increases the generation and diffusion length of the carriers, reducing charge recombination in the perovskite absorber film, leading to better photovoltaic performance [24]. Moreover, Cl and Br slow down the nucleation process and speed up the crystal growth rate, thus, having an impact on the microstructure (crystallinity) of the perovskite film [51,52]. Br ions have a higher activation energy of the ionic migration than I ions, respectively 0.18 eV along the grain boundary and 0.5 eV in bulk [53] and consequently, reduced movement within the perovskite film, while Cl ions improve the diffusion length of the free carriers, exceeding 1 μm [54]. Activation energy together with other parameters such as mobile ion concentration and diffusion coefficient is a good method to describe the mobility of ions resulting from the decomposition of perovskite, and for Br⁻ migration, it is about 3 times slower than the I⁻ migration [55]. Therefore, mixing the halogens in the perovskite structure is one key to obtain superior photovoltaic properties with lower hysteresis effects than devices with CH₃NH₃PbI₃ only [56,57]. Previous work has demonstrated the potential of the mixt halide perovskite with different ratios to provide more stable and more efficient devices. For example, perovskites using mixtures of I and Cl were reported by Tombe et al. [58], Po-Wei Liang et al [59] and Spalla et al [60] as active layers in planar inverted geometry devices.

This paper focuses on the mixt halogenated perovskites with either I: Cl or I: Br in the same proportion of 1.8:1.2. The study of the perovskite films with the CH₃NH₃PbI_1.8Br_1.2 composition, and how the properties of the perovskite film change when Cl is used instead of Br in the same amount, are revealed. The role of the halogen’s composition on the morphology, microstructure, and optical properties as well as on photovoltaic performance and stability in solar cells in a mesoscopic geometry is presented.