Dimethylammonium,cation,stabilizes,all-inorganic,perovskite,solar,cells

Zuolin Zhang, Mengjia Li, Jie Sun, Cong Chen, , Jiangzhao Chen , and Liming Ding,

1State Key Laboratory of Reliability and Intelligence of Electrical Equipment, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300401, China

2Key Laboratory of Optoelectronic Technology & Systems (MoE), College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China

3Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China

4Macao Institute of Materials Science and Engineering (MIMSE), Macau University of Science and Technology, Macau 999078, China

Facing the poor environmental stability of traditional methylammonium or formamidinium-based lead halide perovskites, scientists turn their attention to inorganic lead halide perovskites (ILHPs) with narrow bandgaps, excellent thermal stability and reduced ion migration compared to their organic/inorganic counterparts[1−4].Up to now, the PCEs for ILHP solar cells exceed 21%[5].Especially, the preferred black ILHP (e.g.CsPbI3) with the smallest bandgap of ~1.7 eV and single-halide composition for avoiding phase separation is crucial for high-performance single-junction solar cells and can be applied in tandem devices as the top cells[6,7].However, small Cs+(167 pm) in CsPbI3with a tolerance factor close to 0.8 is unsuitable for the 3D PbI3–framework[8].The mismatched size of cations will induce lattice strain and the perovskite spontaneously transforms to undesired non-photoactive yellow phase (δ-phase, like NH4CdCl3) (Fig.1(a))[9,10].Therefore, improving lattice symmetry and reducing lattice strain are the strategies for inhibiting the phase transition of ILHPs.Reducing the lattice strain by increasing the specific surface area of all-inorganic perovskite crystals to make nanocrystals can improve the phase stability.This usually requires the introduction of long-chain alkyl amines (e.g.oleic ammonia) in the precursor solution, which is widely used in quantum-dot solar cells.Lutheret al.demonstrated that phase-stable colloidal CsPbI3quantum dots gave a PCE of 13.43% due to large contribution of surface energy[11,12].The disadvantage is the negative effect of many grain boundaries between nanocrystals and organic ligands to carrier transport, which seems to be more suitable for electroluminescent devices rather than solar cells.Therefore, much efforts have been devoted to improving the phase stability of black-phase ILHPs at room temperature to obtain high-performance PSCs[13].At present, the effective strategy is cation engineering, including the use of additives DMAI and hydriodic acid (HI).

Previously, adding HI to the precursor to produce HI·PbI2or the so-called hydrogen lead iodide (HPbI3) precursor is a widely-used approach to stabilize blackα-CsPbI3[14].Snaithet al.[15]first added HI to CsPbI3precursor solution to reduce the annealing temperature from 310 to 100 °C and obtained uniform and stable black-phase ILHPs[16].Then, Zhaoet al.reported stabilized black CsPbI3by a low-temperature deposition and by using pre-synthesized PbI2·xHI, which was made from DMF, PbI2, and HI.Combined with larger ethylenediammonium cation,α-CsPbI3solar cells gave a PCE of 11.8%[17].DMA+byproducts (Fig.1(b)) resulted from the chemical reaction of HI and DMF in the precursor solution.Almost all efficient and stable ILHP solar cells have incorporated HI, HPbI3or DMA+in their precursor solutions[18−21].Later, Yuanet al.used HPbX3(X = I, Br) as the precursor combined with phenylethylammonium cation to make low-dimensionalα-CsPbI3, yielding a PCE of 12.4% with enhanced stability[22].HPbI3made by dissolving HI and PbI2in DMF could increase the PCE to 17%[23,24].However, the mechanism for HI or HPbI3to improve the device performance has been unclear.It was argued that the A-site of the prepared ILHPs mixed with organic cation DMA+, thus yielding a stable black organic–inorganic perovskite phase.Whether DMA+exists, the content of DMA+in the lattice, the properties for DMA+/Cs+mixed perovskite phase, and even the phase evolution process during thermal treatment still need deep investigation.

Kanatzidiset al.[25]compared the films made with HI and DMAI additives by nuclear magnetic resonance (NMR) and found that both films exhibited the characteristic peaks of DMA+, so they thought that DMA+entered the A-site of perovskite.Qiet al.[26]used highly-sensitive time-of-flight secondary ion mass spectrometry to prove DMA+existence in perovskite.Further studies indicated that DMA+was generated by the hydrolysis of DMF accelerated by HI.Liuet al.[27]analyzed the products of HI-accelerated DMF hydrolysis by using in-situ thermogravimetry Fourier transform infrared spectroscopy, and the spectra showed two peaks corresponding to DMA+volatilization and PbI2decomposition.Both HI and HPbI3additives helped to produce DMA+in perovskite, leading to high PCE and stability.Seoket al.[18]also used DMAI additives and obtained PCE over 20% by continuous dripping of methylammonium chloride solution.Liuet al.[28]used HPbI3additive combined with a surface treatment to increase PCE to 20.8%.Menget al.directly introduced DMAPbI3and achieved a PCE over 20%[19].By using DMAAc solvent, they developedDMAPb(I, Ac)3intermediate to eliminate Cs4PbI6intermediate phase, yielding a PCE over 21% (Fig.1(c))[21].Panget al.[29]recently modified DMAI-additive fabrication route by using CsI in excess to obtain black phaseγ-CsPbI3perovskite at~100 °C.Subsequent dipping of the film into isopropanol solution realized the stoichiometric balance between DMAPbI3and Cs4PbI6intermediates.Songet al.indicated that vacuumassisted thermal annealing is effective for controlling the morphology and crystallinity of DMA+-based CsPbI3films to obtain PCE exceeding 20.06% along with enhanced stability(Figs.1(d) and 1(e))[1].DMA+assists low-temperature crystallization of black phase CsPbI3through forming intermediates.The enhanced phase stability and photovoltaic performance in HI or DMA+modified ILHP solar cells is mainly attributed to lattice regulation, e.g., change of lattice parameters, relaxation of lattice strain, oscillatory variation of lattice structure,etc.

Fig.1.(Color online) (a) Schematic structures for CsPbI3 phases.Reproduced with permission[15], Copyright 2015, Royal Society of Chemistry.(b) Structures for FA (top) and DMA (bottom) cations.Reproduced with permission[25], Copyright 2018, Nature Publishing Group.(c) Schematic for crystal structure evolution from precursor solution to γ-CsPbI3 and possible chemical interaction.Reproduced with permission[21], Copyright 2022, Wiley.(d) Schematic for vacuum-assisted thermal annealing of CsPbI3 film.(e) The conversion of 1D DMAPbI3 film to 3D CsPbI3 through cations exchange between Cs+ and DMA+.Reproduced with permission[1], Copyright 2022, Wiley.

Fig.2.(Color online) (a) TGA curves for DMAI, DMAPbI3 powders and the powder scratched from the precursor film of CsI+PbI2+DMAI.(b) GIWAXS for β-CsPbI3 films.Reproduced with permission[32], Copyright 2019, AAAS.(c) Schematic for black-phase CsPbI3 formation by using DMAI additive.Reproduced with permission[33], Copyright 2019, Wiley.(d) Schematic for 2-TBA condensation reaction (top) and 3-TBA without condensation reaction (bottom); schematic for DMA+ removal by H2O from the condensation reaction.Reproduced with permission[36], Copyright 2022,American Chemical Society.

Introducing DMA+into ILHPs contradicts the original intention of making all-inorganic PSCs.The excess DMA+prefers to exist at the top surface of the final perovskite film in an inhomogeneous spatial distribution[30].The local heterogeneity in composition at the film surface can cause heterogeneous film morphology and increase structural defects throughout the film[31], thus deteriorating device performance.How to remove the organic component to obtain phase-pure all-inorganic perovskite is a challenge.Some researches showed that increasing annealing temperature and changing annealing environment could help to remove DMA+components.Zhaoet al.[32]indicated that annealing at 210 °C for 5 min was enough to eliminate DMA+component to obtainβ-CsPbI3with good orientation (Fig.2(a), Fig.2(b)).They also found that DMAI residues in the films could deteriorate device performance.Combining DMAI with phenyltrimethylammonium chloride (PTACl), they achieved a PCE of19.03% (Fig.2(c))[33].Decreasing annealing temperature to 180 °C while simultaneously extending annealing time could also remove DMA+, but the device performance was damaged due to the created defects in films[34,35].Recently, Liuet al.[36]developed a low-temperature strategy to remove the residual DMA+via2-thiopheneboric acid oligomerization at the surface.2-Thiopheneboric acid joins in the condensation reaction, releasing H2O molecules, which purge DMA+from CsPbI3film surface at ~100 °C (Fig.2(d)).Meanwhile, the remaining boro-thiophene trimer moiety acts as a Lewis base anchoring group to passivate Pb0defects.

In short, DMA+additive helps ILHP solar cells to realize>21% PCEs[20,21,37,38].Several groups also tried other additives (e.g., zwitterion, ionic liquids, and zinc-based Lewis acid)to interact with perovskite crystals to inhibit intrinsic phase transition.We should try different approaches to develop phase-stable all-inorganic perovskites.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (62004058, U21A2076, 21701041,52071048), Natural Science Foundation of Hebei Province(F2020202022), the Open Fund of the State Key Laboratory of Integrated Optoelectronics (IOSKL2020KF09), State Key Laboratory of Reliability and Intelligence of Electrical Equipment (EERI-PI20200005), the Support Plan for Overseas Students to Return to China for Entrepreneurship and Innovation (cx2020003), the Fundamental Research Funds for the Central Universities (2020CDJ-LHZZ-074), and Natural Science Foundation of Chongqing (cstc2020jcyj-msxmX0629).L.Ding thanks the National Key Research and Development Program of China (2022YFB3803300), the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02), and the National Natural Science Foundation of China (21961160720).