Phosphate Functionalized Ionic Liquid: An Efficient Li⁺ Migration Inhibitor and Hole Extraction Accelerator for Perovskite Solar Cells

Abstract

There is no doubt that the hole transporting layer (HTL) plays an indispensable role in highly efficient devices. [5] The materials used for HTL include: (1) organic polymer molecules, [6,7] such as poly(3,4ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS), poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) and poly(3-hexylthiophen-2,5-diyl) (P3HT); (2) inorganic compound, [8] for instance, CuSCN and NiO X . Besides, small molecule HTL materials have been widely studied due to their designability in molecular structure. [9] The organic 2,2′,7,7′-tetrakis (N,N-dip-methoxyphenylamine)-9,9′spirobifluorene (Spiro-OMeTAD) has been extensively employed as hole transport layer for high-performance devices due to its suitable energy level alignment and great solubility. [10] However, the amorphous characteristic of Spiro-OMeTAD depends its low hole mobility (∼4 × 10 −5 cm 2 V −1 s −1 ). [10,11] The bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) and 4-tert-Butylpyridine (TBP) are always used as the dopant of Spiro-OMeTAD. [10,11] TBP was used to reduce phase separation of Spiro-OMeTAD and Li-TFSI. [10,11] Li-TFSI can enhance the hole mobility of Spiro-OMeTAD by promoting the oxidation process of Spiro-OMeTAD in an ambient atmosphere. [12] However, this oxidation process is always uncontrollable owing to the oxidized Spiro-OMeTAD can be reduced to its initial state during the device operation process, [13] which leads to the instable device. Besides, the hygroscopic properties Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) doped 2,2′,7,7′ -tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is employed as one of the most common hole transport layer. However, the Li + in Li-TFSI is easy to migrate from Spiro-OMeTAD to perovskite and SnO 2 , left the voids/pinholes in perovskite film to accelerate the invasion of oxygen and moisture, and lead to the device degradation. Here, the 1-butyl-3-methyl imidazolium phosphate dibutyl ester ([Bmim] + [DBP] − ) ionic liquid (IL) is prepared and used to modify the perovskite/Spiro-OMeTAD interface in the perovskite solar cells. The coordination interaction between P  O in [Bmim] + [DBP] − IL and Pb 2+ can improve the morphology of perovskite film by passivating the uncoordinated Pb 2+ defects. The interaction between P  O and Li + can suppress the undesired Li + migration. The regulated energy level assignment and ion nature of [Bmim] + [DBP] − IL can accelerate the carrier extraction at perovskite/Spiro-OMeTAD interface. The device based on the [Bmim] + [DBP] − IL interface modification yields a highly efficient photoelectric conversion efficiency of 21.16% and improved stability, outperforming that of control (19.31%) devices under the illumination of 100 mW cm −2 (AM 1.5 G).

Rationally designed hole transporting layer system for efficient and stable perovskite solar cells

“…According to a report by Zhu et al, the tiny Li cations are astatic, and thus readily diffuse from the HTL to adjacent layers, such as the perovskite layer and electron transporting layer (ETL), under bias condition or at high temperature as shown in Figure 2E,F 16 . The accumulation of Li cations at the interface between the perovskite and ETL can affect the performance and long‐term stability of the PSCs 38 . More recent reports also have indicated that Li ion‐based byproducts such as LiOH could water‐uptake, and the migration of these Li ions might cause perovskite degradation 39 .…”

“…16 The accumulation of Li cations at the interface between the perovskite and ETL can affect the performance and long-term stability of the PSCs. 38 More recent reports also have indicated that Li ion-based byproducts such as LiOH could water-uptake, and the migration of these Li ions might cause perovskite degradation. 39 Addressing these limitations of the conventional dopant system, by developing new dopant systems to boost both efficiency and stability, remains a challenging and urgent obstacle to the future commercialization of PSCs.…”

Role of antisolvent temperature and quaternary ammonium cation-based ionic liquid engineering in the performance of perovskite solar cells processed under air ambient conditions

“…37 In addition, Noel et al observed that upon applying [BMIM]BF 4 ionic liquid at the SnO 2 /FA 0.83 MA 0.17 Pb(I 0.83 Br 0.17 ) 3 interface, the n-type nature of the perovskite enhances along with other benefits like interfacial defect passivation. 38 In the literature, most of the ionic liquid modifications in perovskite solar cells have been done with imidazolium cation-based ionic liquids, 36–41 which are relatively more hygroscopic. 42,43 On the other hand, quaternary ammonium cation-based ionic liquids have not been explored in perovskite solar cells, which could have potential for interfacial engineering in PSCs.…”