Morphology control of perovskite film for efficient CsPbIBr2 based inorganic perovskite solar cells
Introduction
In the past decade, metal halide perovskite materials have been emerging as a potential candidate for highly efficient solar cells due to their excellent properties, such as high absorption coefficient, tunable bandgap (Eg), high tolerance towards defects and long charge diffusion length and lifetimes [[1], [2], [3]]. The power conversion efficiency (PCE) of organic-inorganic perovskite solar cells (PSCs) has reached a certified value of 25.2% in 2019 [4], which is comparable to silicon solar cells. However, for commercialization, solar cells have to show robust operational stability towards environment, where organic-inorganic hybrid perovskites are vulnerable to high temperature due to the existence of organic component [5]. The replacement of organic part by inorganic components should fundamentally solve such thermal stability problem. Therefore, the metal halide inorganic perovskite materials for photovoltaics have been flourishing in the recent years in the research community [[6], [7], [8], [9], [10], [11]].
For a typical perovskite, its formula can be described as ABX3, where A is methylammonium (MA+), formamidinium (FA+), cesium, B is lead, tin, X is halide elements, such as Cl−, Br− and I−. To obtain inorganic perovskites, A site should be occupied by elements such as cesium. The widely studied inorganic perovskites is CsPbI3 with a narrow bandgap of 1.73 eV, which is perfect for constructing tandem solar cells. However, CsPbI3 suffers from phase instability seriously at ambient condition. The high temperature α-phase can be easily transformed into low-temperature δ-phase upon cooling to room temperature [12]. This δ-phase is also known as yellow phase, which is photo inactive and detrimental to device performance. Engineering X site by mixing smaller ion Br with I is a useful strategy to stabilize α-phase at room temperature while maintaining a reasonable bandgap for efficient light harvesting [13,14]. For this reason, mixed-halide CsPbIBr2 (Eg = 2.09 eV) presents a balance between bandgap and ambient phase stability among different inorganic metal halide perovskite materials and is a good choice for stable and efficient PSCs.
Perovskite films featuring dense packing and large crystal size with low defect density are considered as high quality that can produce excellent device efficiency and stability. Therefore, a number of high-performance PSCs based on CsPbIBr2 have been reported by developing new film deposition techniques [15,16], compositional engineering [17,18], surface modification [19,20], and interfacial engineering [21,22] to obtain high quality perovskite films. Hao and coworkers used intermolecular exchange method by spin-coating a methanol solution of CsI on CsPbIBr2 precursor film, which yields full‐coverage and pure‐phase CsPbIBr2 films featured with large grain size, less grain boundaries and high crystallinity [19]. Jin et al. reported that by incorporating Sn2+ into CsPbIBr2, the films show continuous surface without obvious pinholes [17]. However, compared with its analogues such as CsPbI2Br and CsPbI3, PCEs of CsPbIBr2-based PSCs are still far lagging behind [11,23]. This dilemma is mainly attributed to poor film morphology that are characteristic of incomplete coverage over the substrate, high trap density and crystal defects that exist both at the grain boundaries and on the film surface, which results in severe nonradiative charge recombination within the perovskite bulk layer and at the interface [24]. Therefore, the exploration of new strategies to enhance the CsPbIBr2 film quality is still urgent. Previous studies [25] have shown the advantages of using preheated substrate in modulating the inorganic perovskite morphology. Que et al. [26] prepared high-quality CsPbIBr2 perovskite films through a one-step spin-coating method assisted by a preheating process, resulting in the complete coverage and enhanced crystallinity of the films and superior device PCE of 9.86%. Wang et al. [27] prepare the high-quality CsPbIBr2 perovskite film by optimizing both the substrate pre-heating temperature and the post-annealing temperature to improve the coverage and crystallinity. FTO/TiO2/CsPbIBr2/carbon exhibits a gratifying conversion efficiency of 8.10% with a high open-circuit voltage (VOC) of 1.27 V.
Since the engineering at X site is important in achieving PSCs with high PCEs and stability, apart from halegon ions, pseudohalogen ions, such as SCN− [28,29], BF4− [30], PF6− [31] have also been reported to be incorporated into organic-inorganic hybrid perovskites with improved film quality that exhibit higher PCEs and better moisture stability. Park et al. [31] report on interface engineering via ion exchange reaction, where iodide ion in FA0.88Cs0·12PbI3 is partially exchanged with PF6− to form thin FA0.88Cs0·12PbI3−x (PF6)x layer between perovskite and 2,2′,7,7′-tetrakis [N,N-di (4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD) hole transport layer, which greatly enhances the performance of solar cells. However, rare studies have been reported so far reagrding the addition of pseuduhalide molecular ion, such as BF4−, PF6− in stable and efficient inorganic metal halide PSCs.
In this work, we demonstrated a facile two-step strategy to improve the inorganic perovskite film quality, which is then applied in the fabrication of highly efficient CsPbIBr2-based PSCs. Firstly, we employed substrate preheating treatment (SPT) method during perovskite film deposition, which aims to realize full coverage of film over substrate without pinholes. Secondly, by using NH4PF6 as precursor additive, the perovskite grain size is further enlarged. With this strategy, we fabricated CsPbIBr2-based PSCs with a champion PCE of 10.1%. Moreover, the corresponding devices without encapsulation demonstrate superior ambient stability over 30 days with negligible drop in PCEs.
Section snippets
Materials
Fluorine-doped tin oxide (FTO) glass was purchased from Nippon Corporation Japan. PbBr2 and PbI2 were from TCI. Spiro-OMeTAD was obtained from Merck. Other chemicals were purchased from Sigma-Aldrich and were used as received without purification.
Solution preparation
The perovskite precursor solution was prepared by dissolving 259.8 mg CsI and 367.0 mg PbBr2 in 1 ml anhydrous dimethylsulfoxide (DMSO). Different amounts of NH4PF6 were added into precursor solution when necessary. Titania precursor solution was
Results and discussion
Fig. 1a shows the schematic illustrations of SPT method for preparing CsPbIBr2 films and the details have been described in Experimental section. It is interesting to note that the precursor films gradually turned from colorless to light orange during SPT process probably due to the partial transition into perovskite phase. Fig. 1b is the XRD pattern of CsPbIBr2 perovskite films without and with SPT at 90 °C. It can be observed that sample without SPT shows not only perovskite characteristic
Conclusions
In summary, in this work we reported efficient and stable inorganic PSCs based on CsPbIBr2 photoactive layer, whose morphology is critical for device performance. Two steps were adopted for obtaining high-quality perovskite films: 1) SPT method was utilized to obtain dense and uniform perovskite layer; and 2) NH4PF6 was adopted as precursor additive to further enlarge the perovskite grain size. With these strategies, we achieved a champion efficiency of 10.1%. Charge recombination studies
CRediT authorship contribution statement
Junye Pan: conducted the SPT temperature optimization, SEM, PL, TRPL. Xing Zhang: contributed to the data processing, manuscript writing and revising. Yong Zheng: conducted SEM, XRD, UV–vis spectra, stability and optimization of the device PV performance. Wanchun Xiang: conceived the idea of the work, wrote and revised the manuscript.
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.
Acknowledgement
This work is supported by National Science Foundation of China (No. 51972255, 21707047).
References (44)
- et al.
A holistic approach to interface stabilization for efficient perovskite solar modules with over 2,000-hour operational stability
Nat. Energy
(2020) - et al.
Ultrahydrophobic 3D/2D fluoroarene bilayer-based water-resistant perovskite solar cells with efficiencies exceeding 22%
Sci. Adv.
(2019) - et al.
Universal approach toward high-efficiency two-dimensional perovskite solar cells via vertical-rotation process
Energy Environ. Sci.
(2020) - et al.
Photodecomposition and thermal decomposition in methylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability
J. Mater. Chem.
(2018) - et al.
Review on recent progress of all-inorganic metal halide perovskites and solar cells
Adv. Mater.
(2019) - et al.
Intermediate phase enhances inorganic perovskite and metal oxide interface for efficient photovoltaics
Joule
(2020) - et al.
Ba-induced phase segregation and band gap reduction in mixed-halide inorganic perovskite solar cells
Nat. Commun.
(2019) - et al.
Europium-doped CsPbI2Br for stable and highly efficient inorganic perovskite solar cells
Joule
(2019) - et al.
Thermodynamically stabilized β-CsPbI3–based perovskite solar cells with efficiencies >18%
Science
(2019)
The role of dimethylammonium iodide in CsPbI3 perovskite fabrication: additive or dopant?
Angew. Chem. Int. Ed.
Anharmonicity and disorder in the black phases of cesium lead iodide used for stable inorganic perovskite solar cells
ACS Nano
Bandgap-tunable cesium lead halide perovskites with high thermal stability for efficient solar cells
Adv. Energy Mater.
Efficient and stable inorganic perovskite solar cells manufactured by pulsed flash infrared annealing
Adv. Energy Mater.
Light processing enables efficient carbon-based, all-inorganic planar CsPbIBr2 solar cells with high photovoltages
ACS Appl. Mater. Interfaces
Hole transport layer free inorganic CsPbIBr2 perovskite solar cell by dual source thermal evaporation
Adv. Energy Mater.
CsPb0.9Sn0.1IBr2 based all-inorganic perovskite solar cells with exceptional efficiency and stability
J. Am. Chem. Soc.
Goldschmidt-rule-deviated perovskite CsPbIBr2by barium substitution for efficient solar cells
Nanomater. Energy
Intermolecular exchange boosts efficiency of air-stable, carbon-based all-inorganic planar CsPbIBr2 perovskite solar cells to over 9%
Adv. Energy Mater.
Extrinsic ion distribution induced field effect in CsPbIBr2 perovskite solar cells
Small
Interface modulator of ultrathin magnesium oxide for low-temperature-processed inorganic CsPbIBr2 perovskite solar cells with efficiency over 11%
Solar RRL
Bifunctional dye molecule in all‐inorganic CsPbIBr2 perovskite solar cells with efficiency exceeding 10%
Solar RRL
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These authors contribute equally.