ReviewCoordination modulated crystallization and defect passivation in high quality perovskite film for efficient solar cells
Graphical abstract
Introduction
The organic–inorganic hybrid perovskite solar cells (OIHPs), as the new member of the third generation solar cells, have become the most viable competitor to conventional silicon solar cells in terms of cost and power conversion efficiency (PCE) among numerous emerging photovoltaic technologies. The formula of regular perovskite material is ABX3, where A represents a monovalent organic or inorganic cation such as methylammonium (MA+), formamidinium (FA+) or Cs+, B represents a divalent metal cation like Pb2+, Sn2+, or Ge2+, and X represents a halide anion such as I−, Br−, or Cl− [1], [2], [3], [4], [5], [6], [7]. The general perovskite crystal structure can be regarded as a network framework consisting of [BX6]4− octahedrons connected at the vertices in three-dimensional space, with the B site at center. A-site ions located at the [BX6]4− framework interstices [8], [9]. Goldschmidt’s empirical tolerance factor (t) was widely utilized to estimate the structural stability of perovskite [10]. Since the A sites or X sites ions in hybrid organic–inorganic perovskite can be replaced by molecular ions, the tolerance factor is modified to the following equation:where rB is the radius of B-site metal ion, rAeff is the effective radius of the A-site cation (the rAeff can be calculated by rAeff = rmass + rion, rmass is the distance between the center of mass of the molecule and the atom farthest from the center of mass, excluding hydrogen atoms, rion is the corresponding ionic radius of this atom), rXeff and hXeff is the effective radius and height of X-site molecular ions, respectively [11]. A stable perovskite structure usually has a t value ranging from 0.81 to 1.01, which indicates such hybrid coordination compounds are elemental-adjustable and have intriguing properties such as strong light absorption with a suitable band gap, a high extinction coefficient, long carrier diffusion length and a bipolar charge transport, thus leading to a preeminent certified PCE record up to 25.2% [12], [13], [14].
A primary prerequisite for high efficiency PSCs is a uniform and dense perovskite film with high crystal crystallinity, full coverage, large grain size and favorable grain orientation. Surface defect passivation [15], [16], [17], [18], [19], microstructure regulation [20], component engineering [21], [22], additive engineering [23], [24], [25] and solvent engineering [26], [27] have been widely utilized to improve the quality of perovskite film by slowing the crystallization of perovskite or modulating trap states in perovskite film in order to achieve better photovoltaic performance. Meanwhile, many advanced thin film deposition techniques have been developed to deposit high quality perovskite film, such as one-step spin-coating [28], two-step sequential deposition [29], [30], [31], double source co-evaporation [32], flash-evaporation printing [33], vapor deposition [34] and slot-die deposition [35]. The solution deposition methods are most widely used to fabricate perovskite films due to their low cost and the simplicity of manufacturing processes. The early one-step spin-coating method dissolves precursors as PbX2 (I−, Br−, Cl−) and methylammonium iodide (MAI) or formamidinium iodide (FAI) in a single solvent such as N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), 1,4-butyrolactone (γ-GBL), but the deposited films exhibit poor surface coverage with substantial pinholes and cracks, which eventually leads to poor device performance. In 2013, Grätzel et al. fabricated uniform perovskite films with a high PCE over 15% with a reproducible and controllable method named two-step sequential deposition method [31]. This method successfully resolved the problem of poor surface coverage in the one-step method, but suffers from the possible incomplete conversion of PbI2 when deposited on a planar substrate. To deposit high-quality perovskite film, Soek and co-workers introduced a solvent engineering strategy to obtain extremely uniform and dense perovskite films through forming of the MAI-PbI2-DMSO intermediate phase after using a mixture solvents of DMSO and γ-GBL [36]. Afterwards, forming intermediates by coordination has been widely used to assist perovskite crystal growth. The strong-coordination molecule DMSO is generally used to form PbI2-DMSO or MAI-PbI2-DMSO complexes in perovskite precursor solution.
Perovskites deposited through solution method are commonly polycrystalline, which means considerable structural disorders occur such as grain boundaries, uncoordinated Pb2+ ions on the surface, and vacancy defects [37]. Typically, charge carrier diffusion is limited on these trap states, which induce nonradiative recombination loss and reduce the charge carrier lifetime and thus affect the photoluminescence yield [38]. Moreover, the vacancy defects provide a pathway for ion migration and thus cause hysteresis behavior, and the grain boundary and surface defects as the photoluminescence quenching sites induce photogenerated carrier loss [39], [40], [41].
Suppressing these trap states in solution-processed perovskite film plays a critical role in improving the thermodynamic properties of perovskite films and related PSCs [42], [43], [44]. Additive engineering has been proven to be an effective way to control the nucleation and crystallization of perovskite films [45], [46], [47]. These additives are expected to reduce the probability of shunting, charge carrier recombination, and the trap state density during the device fabrication process [48], [49]. The lone pair of electrons from atoms (e.g. S and O atoms) in additives could bond with Pb2+ in perovskite to regulate the crystallization process and passivate the grain boundary defects. Additionally, surface modification is another effective way to suppress the uncoordinated surface ions and passivate the as-formed defects. Here, we systematically reviewed the coordination engineering in the formation of perovskite from precursor solution by crystallization regulation and trap states passivation. In particular, the coordination interaction of N/S/O atoms in additives with PbI2 is highlighted. The long-term stability and hysteresis behavior for perovskite photovoltaics are also discussed with the coordination effect. Finally, some open questions towards the fundamental understanding, better utilization of coordination interaction for high quality perovskite films and efficient PSCs and other related optoelectronic devices are outlined.
Section snippets
Solvent intermediates
Polar aprotic solvents such as DMF [50], [51], DMSO [52], [53], γ-GBL [54], and N-methyl pyrrolidone (NMP) [55], [56] are widely utilized as a dissolving agent in perovskite precursor solution, which contains a strong electronegative polar group with C=O or S=O and thus may coordinate with PbI2 to form a stable intermediate phase such as PbI2-DMSO or MAI-PbI2-DMSO. The O/N in active carbonyl, acyl group and cyano group in solvent molecules are the key to react with Pb. These intermediate phases
Effect of coordination on defects passivation
The defects in perovskite film always have a negative influence on the optoelectronic property of perovskite film and related PSC device performance. Usually, defects induced carrier recombination center and caused non-radiative recombination, and thus deteriorated the attainable Voc and fill factor (FF) of the PSC devices. The defects exist on the surface or grain boundaries of perovskite crystals such as uncoordinated Pb2+ ions and uncoordinated halides, which may cause deep level traps.
Effect of coordination on hysteresis behavior
Photocurrent hysteresis is another problem that hinders the continuous power output and therefore commercial application of PSCs. Hysteresis behavior means that the current density–voltage (J-V) curve of PSCs will vary with the scanning direction and scanning rate during the performance test. This phenomenon complicates device characterization. Therefore, a lot of research works have been carried out to develop feasible and general strategy to eliminate the hysteresis phenomena [137], [138],
Effect of coordination on Long-term stability
Coordination interaction with perovskite can also significantly improve the humidity and thermal stability of perovskite films and related PSC devices. The related device performance and stability are summarized in Table 2. It’s well known that the grain boundaries are considered as a main channel for water invasion into perovskite crystals. The coordinated molecules may anchor to the surface of perovskite and form hydrophobic layer or block grain boundaries to avoid moisture permeation. In
Conclusions and outlook
Tremendous efforts have been made to achieve long-term stable, high efficiency and hysteresis-free PSCs, which is critical to the transition from academic research to commercial application. Various trap states and grain boundaries in the bulk and at the interface are considered as the main limiting factor for efficient and stable running of PSCs. Therefore, it’s in urgent need to develop reliable method to fabricate device with excellent integrated performance. In this Review, we
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.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China NSFC (51702038) and the Recruitment Program for Young Professionals. P. Dong acknowledges George Mason University for startup support. L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720) for financial support.
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