Secondary crystallization strategy for highly efficient inorganic CsPbI2Br perovskite solar cells with efficiency approaching 17%
Graphical abstract
A strategy of secondary crystallization for CsPbI2Br film is demonstrated at low-temperature, which can regulate the grown grains, passivate defects and lower the Fermi level. Finally, the champion cell shows a satisfactory PCE of 16.97%.
Introduction
In last ten years, organic–inorganic hybrid halide perovskite solar cells (PSCs) have developed rapidly to an incredible certified power conversion efficiency (PCE) of 25.5% from the first PCE of 3.8% [1], [2], [3], [4], [5], [6]. In addition, the organic–inorganic hybrid perovskite has shown broad applications in LEDs, photodetectors, transistors and electronic devices [7], [8], [9], [10], [11]. Nevertheless, the operational stability of PSCs is a major obstacle to commercialization. The unavoidable organic groups containing perovskites, such as formamidinium (FA) and methylammonium (MA) cations, always exhibit poor durability because of the inherent nature of volatility and hygroscopicity. At the standard operating temperature of 85 °C or higher temperature, the traditional perovskite is easily decomposed into PbI2 and MAI/FAI, negatively affecting the performance of perovskite solar cells and hindering the commercial application [12], [13], [14].
The all-inorganic perovskite CsPbX3 (X = I, Br) greatly improves the thermal compositional stability by replacing organic molecules with Cs atoms and inorganic PSCs have made great progress in the past few years [15], [16], [17], [18]. Very recently, all-inorganic PSCs based on β-CsPbI3 which has a favorable bandgap from 1.68 to 1.70 eV have successfully achieved the PCE of 18.4% and 20.37% by Wang and Yoon et al, respectively [19], [20]. However, the Cs atom (1.81 Å) is smaller than MA (2.70 Å), which will cause the lattice distortion of perovskite [21], while the Goldschmidt tolerance factor of CsPbI3 will reduce to 0.81 [22], easily resulting in the CsPbI3 perovskite phase transition from photosensitive phase (α, β, γ) to non-photosensitive phase (δ) at room temperature with trace moisture [23], [24], [25]. On the contrary, CsPbBr3 possesses better phase stability, while the photocurrent is limited by its large bandgap (~2.3 eV), leading to the low cell efficiency. In order to balance the efficiency and stability, the doping strategy of halogen-Br doped most stable phase α-CsPbI3 was proposed. The Br dopant cannot only compensate the lattice distortion caused by the small size of Cs and improve phase stability, but also accelerate the nucleation process and crystal growth rate, and reduce the phase transition temperature [26]. Among them, PSCs based on CsPbI2Br which has a ~1.92 eV bandgap have obtained a high PCE (17%) with good stability and the Voc also broke through 1.4 V with low Eloss (Eloss = Eg − eVoc, <0.5 eV), making it become a good candidate for the top cell of tandem cells [27].
Despite the rapid development of perovskite solar cells, their efficiency is still limited by the existence of significant non-radiative recombination [28]. In addition to the perovskite itself and grain boundary defects which have been studied extensively, the interface of ETL/perovskite or perovskite/HTL is another important source of non-radiative recombination. Interface engineering is a common way to passivate the defects of perovskite and induce a band alignment [18], [29]. Zhao Huan et al. introduced an interface passivation for CsPbI2Br PSCs with DPP-DTT. Due to the DPP-DTT passivation, the perovskite film quality is improved, the Fermi level is lowered, and both the performance and stability are enhanced [30]. Guo Xing et al. recently reported that FDC-2-5Cl molecular could cause p-doping effect on the perovskite surface, induce the band bending and promote the extraction of the hole to Spiro-OMeTAD, suppressing the charge recombination between perovskite/hole transport layer [31]. In general, the passivated molecular cannot only change the surface energy of perovskite but also play a synergistic role in the perovskite surface defect passivation. Therefore, we proposed a strategy of secondary crystallization (SC) for CsPbI2Br film with molecular guanidinium bromide (GABr), forming the Br-rich region, reducing the defects and accelerating the efficiency of hole extraction and collection. Besides, Br-region could also regulate the Fermi level of perovskite film to achieve better band matching with HTL. Consequently, the champion device based on CsPbI2Br perovskite exhibits a PCE of 16.97% with a high Voc of 1.31 V and better stability.
Section snippets
Materials
Lead iodide (PbI2, 99.999%), lead bromide (PbBr2, 99.999%) Spiro-OMeTAD (99.5%), tris[2-(1H-pyrazol-1-yl)-4-tertbutylpyridine] cobalt (III) tris [bis-(turfluoromethylsuflonyl)imide] (FK209), bis-(trifluoromethance) solfonimide lithium salt (LiTFSI) and guanidinium bromine (GABr) all were purchased from Xi’an Polymer Light Technology Corp. Other materials are the same as previously reported [29], [32]. All materials were used as received without further purification.
Sample preparation
The indium tin oxide (ITO)
Results and discussion
A simple secondary crystallization strategy was used to regulate the all-inorganic CsPbI2Br perovskite film with GABr, and the specific experimental process is illustrated in Fig. 1a. In this study, the CsPbI2Br perovskite films were deposited on the ZnO-SnO2 bilayer ETL with sequential graded thermal annealing process [29], [32]. Four concentrations (1, 2, 5, 10 mg/mL) of GABr precursor were adopted to optimize the device performance. The morphologies of the pristine film and the SC-CsPbI2Br
Conclusions
In summary, we have demonstrated that the reduction of defects at grain boundaries and optimization of the band alignment between CsPbI2Br perovskite and HTL were successfully realized by the secondary crystallization strategy with GABr, accelerating hole collection and transfer, and suppressing the interface carrier recombination. Meanwhile, a better band alignment at the interface of perovskite/HTL was obtained. As a result, the SC-CsPbI2Br device delivers a high Voc of 1.31 V, an excellent
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (61704131, 61804111), the National Key Research and Development Program of China (Grant 2018YFB2202900), the Key Research and Development Program of Shaanxi Province (Grant 2020GY-310), the Joint Research Funds of Department of Science & Technology of Shaanxi Province and Northwestern Polytechnical University (2020GXLH-Z-018), the Fundamental Research Funds for the Central Universities and the Innovation
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