Moisture and Oxygen Enhance Conductivity of LiTFSI‐Doped Spiro‐MeOTAD Hole Transport Layer in Perovskite Solar Cells

Abstract

pinholes with an average diameter of ≈135 nm. These pinholes form channels that wiggle within the spiro-MeOTAD fi lm, and are thought to accelerate diffusion of gas molecules from ambient air (e.g., H 2 O and O 2 ) into the perovskite layer in perovskite-based solar cells. [ 8c ] They also facilitate the outward diffusion of chemical elements/compounds, such as LiTFSI, which is hygroscopic. Therefore, ambient air exposure results in a re-distribution of LiTFSI dopants across the spiro-MeOTAD fi lm. [ 8c ] Efforts have been made to better understand environmental effects on LiTFSI-doped spiro-MeOTAD fi lms. [ 8c , 11b , 12,14 ] However, a clear understanding of the LiTFSI dopant re-distribution process and resulted effects is still lacking. It is crucial to study LiTFSI-doped spiro-MeOTAD charge dynamics under controlled environmental conditions to understand the overall current-voltage behavior of perovskite solar cells. [ 15 ] We investigated the effects of environmental H 2 O vapor with a relative humidity (RH%) of 90% at room temperature (RT) of 25 ºC that is generated by letting a stream of dry N 2 gas fl ow through an H 2 O bubbler, high purity dry O 2 gas (>99.5%), and ambient air (RH 50%, RT 25 ºC) on electrical properties of hole-only devices of LiTFSI-doped spiro-MeOTAD fi lms, using mercury drop electrode I -V measurements. We also examined chemical and electronic properties using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS), respectively.XPS measurments were performed to determine the main component in ambient air that drives the re-distribution of LiTFSI dopants across spiro-MeOTAD fi lms upon ambient air exposure ( Figure S1, Supporting Information). LiTFSI-doped spiro-MeOTAD fi lms were exposed to controlled environments of H 2 O vapor (RH 90%, RT 25 ºC) and dry O 2 gas. Afterward, the chemical states of gas-exposed fi lms were studied using XPS. In F-1s and S-2p regions ( Figure 1 a,b,d,e), as-prepared fi lms showed small peaks at ≈688.7 eV and ≈169.4 eV in the binding energy (BE) scale, respectively, and were assigned to the TFSI molecule ( Figure S1, Supporting Information). [ 16 ] Gradual increases in both F-1s and S-2p peaks were observed when samples were exposed to H 2 O vapor. By contrast, changes in XPS peak intensities for F-1s and S-2p were not observed when samples were exposed to O 2 . This observation reveals that re-distribution of LiTFSI within the doped spiro-MeOTAD fi lm to reach the top surface is mainly driven by H 2 O vapor exposure.Unlike ambient air exposure, [ 8c ] with H 2

Gas-solid reaction based over one-micrometer thick stable perovskite films for efficient solar cells and modules

“…While the PCEs of the MAPbI 3 (AS) devices decreased to 90% of their initial performance after approximately 300 h. An initial increase of PCEs for both samples is observed in the first 20 h of the stability test. A similar phenomenon was also observed previously 55,62 , and was ascribed to the effects such as elevated temperature (approximately 46 °C), light or field induced ion movement, light-induced traps formation, interfacial charge accumulation 55 , or spiro-OMeTAD conductivity variation 63 . In addition, both devices showed a fast, exponential decay region after reaching the highest performance points, which is followed by a slower linear decay 64 .…”

Graphene–Perovskite Solar Cells Exceed 18 % Efficiency: A Stability Study

“…[35][36][37] Conings et al [28] found significant MAPbI 3Àx Cl x degradation at 85 8C, even under inert conditions. [40] On the other hand, spiro-OMeTAD [35,36,41,42] tends to crystallize owing to segregation of the lithium bis-trifluoromethanesulfonimide (LiTFSI) additive under air exposure, inducing pinholes in the film, [43] which can negativelya ffect the contact between the perovskite and the HTL. [38] On one hand, strategies to improve the stabilityo ft he perovskite activel ayer could be the replacement of MAPbI 3 with am ixed halide MAPbI 3Àx Br x with large bandgap [39] or the substitutiono fM A + with formamidinium (FA + ), for the realization of FAPbI 3 perovskite film.…”

Low‐Cost TiS2 as Hole‐Transport Material for Perovskite Solar Cells

“…Nevertheless, inclusion of these two components signifies a major bottleneck toward low‐cost commercialization, as their preparation requires expensive purification processes and they exhibit poor long‐term stability . Furthermore, to overcome poor material conductivity and hole mobility, lithium and/or cobalt salts are usually incorporated into the film, which has a detrimental effect on device stability normally attributed to their reactivity and highly hygroscopic behavior. Therefore, despite their remarkable power conversion efficiency, PSC commercialization is hindered by the use of expensive hole‐transport materials and poor device stability.…”