Preferred Growth Direction by PbS Nanoplatelets Preserves Perovskite Infrared Light Harvesting for Stable, Reproducible, and Efficient Solar Cells

Abstract

Due to its impressive properties such as a high absorption coefficient, long diffusion length and low exciton dissociation energy, [2] the perovskite has been exploited in several applications, among them light amplifiers and lasers, [3-,5] light emitting diodes (LEDs), [6-8] and photovoltaic devices. [9,10] The halide perovskite structure, ABX 3 [11] allows multiple atom combinations, making it very versatile. In addition, it can be synthesized at a low temperature permitting a relatively easy synthesis by a broad range of methods. The monovalent cation A (formamidinium [FA], [12] methylammonium [MA], [13] or cesium [14]) is located in the cage of BX 6 4− octahedra, where B is the metal (commonly, lead or tin) and X a halide or combination of them. The most studied hybrid-perovskite materials are MAPbI 3 and FAPbI 3. Both compounds result in a perovskite structure, despite the difference in the tolerance factor (0.95 and 1.03, respectively, and the same octahedral factor. [15,16] MAPbI 3 is intrinsically phase stable at ambient Formamidinium-based perovskite solar cells (PSCs) present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. The surface chemistry of PbS has been used to improve the formamidinium black phase stability. Here, the use of PbS nanoplatelets with (100) preferential crystal orientation is reported, to potentiate the repercussion on the crystal growth of perovskite grains and to improve the stability of the material and consequently of the solar cells. As a result, a vertical growth of perovskite grains, a stable current density of 23 mA cm −2 , and a stable incident photon to current efficiency in the infrared region of the spectrum for 4 months is obtained, one of the best stability achievements for planar PSCs. Moreover, a better reproducibility than the control device, by optimizing the PbS concentration in the perovskite matrix, is achieved. These outcomes validate the synergistic use of PbS nanoplatelets to improve formamidinium long-term stability and performance reproducibility, and pave the way for using metastable perovskite active phases preserving their light harvesting capability.

Electrodeposited PEDOT:PSS-Al2O3 Improves the Steady-State Efficiency of Inverted Perovskite Solar Cells

“…For this purpose, a ~2 nm Al layer was thermally evaporated on PEDOT:PSS-Cl and annealed at 200° C. The thickness (16–18 nm) and homogeneity of the resulting PVD bilayer suggest that the electrochemical process offers similar quality and film thickness control as PVD. Regarding the perovskite layer, cross-sectional SEM images show similar monolithic vertical grains for all variations ( Figure S3 ), which is beneficial for cell performance and stability [ 41 ].…”

“…For this purpose, a ~2 nm Al layer was thermally evaporated on PEDOT:PSS-Cl and annealed at 200 • C. The thickness (16-18 nm) and homogeneity of the resulting PVD bilayer suggest that the electrochemical process offers similar quality and film thickness control as PVD. Regarding the perovskite layer, cross-sectional SEM images show similar monolithic vertical grains for all variations (FigureS3), which is beneficial for cell performance and stability[41].By AFM imaging, it was observed that the ITO substrate has a root mean square (RMS) roughness of 1.7 nm (FigureS5). After electrodepositing the HTMs, the RMS roughness increased by a similar value for all variations (around 3 nm), even for the inorganic layer by PVD.…”

Boosting Long-Term Stability of Pure Formamidinium Perovskite Solar Cells by Ambient Air Additive Assisted Fabrication

“…Another additive with several advantages is embedded PbS quantum dots (QDs) into the perovskite thin films. 46 − 49 The chemi-structural match of PbS QDs 28 or nanoplatelets (NPLs) 50 with perovskite (MAPI or FAPI) crystal phase results in the generation of seed-like nucleation regions for the bulky-epitaxial growth of high-quality perovskite absorber with enhanced optoelectronic properties and improved long-term stability. 28 , 50 − 53 Although the huge interest of FAPI PSCs advancement in the stabilization of the FAPI solar cells, and the pieces of evidence that NMP improves its stability, the foundation of the improvement of the formamidinium stabilization was not yet analyzed, 23 , 26 especially beyond low RH in real ambient conditions including medium-high RH fabrication conditions and how these conditions could affect the final device stability.…”

“…Different concentrations were investigated as previous works pointed out the performance of the solar cell is influenced by both PbS QDs size and embedded PbS QDs concentration. 28 , 50 The resultant perovskite films are methodically characterized, Figure S6 . There are no significant differences in terms of structural and optical properties over the concentration of added PbS QDs.…”

The Contribution of NMR Spectroscopy in Understanding Perovskite Stabilization Phenomena

“…In this case, the effectiveness of the –NH 3 + group in reducing lattice defects is combined with the size increase in both perovskite grains and crystals and with the formation of an intermediate SN–PbI 2 adduct triggered by the –SH group, thus making these multifunctional derivatives very promising for perovskite stability improvement. In perspective, the use of sulfur-based organic additives will be considered for metastable perovskite stabilization, also due to the great interest created by the use of external stabilizer additives such as PbS quantum dots [ 72 ] or nanoplatelets [ 77 ], which are able to epitaxially propagate the crystal lattice parameter while creating chemical bonds between the Pb and S atoms.…”