Effect of Rubidium Incorporation on the Structural, Electrical, and Photovoltaic Properties of Methylammonium Lead Iodide-Based Perovskite Solar Cells

High Performance Inverted RbCsFAPbI3 Perovskite Solar Cells Based on Interface Engineering and Defects Passivation

Lead halide-based perovskites solar cells (PSCs) are intriguing candidates for photovoltaic technology due to their high efficiency, low cost, and simple fabrication processes. Currently, PSCs with efficiencies of >25% are mainly based on methylammonium (MA)-free and bromide (Br) free, formamide lead iodide (FAPbI3)-based perovskites, because MA is thermally instable due to its volatile nature and Br incorporation will induce blue shift in the absorption spectrum. Therefore, MA-free, Br-free formamidine-based perovskites are drawing huge research attention in recent years. The hole transporting layer (HTL) is crucial in fabricating highly efficient and stable inverted p-i-n structured PSCs by enhancing charge extraction, lowering interfacial recombination, and altering band alignment, etc. Here, this work employs a NiO_x /PTAA bi-layer HTL combined with GuHCl (guanidinium hydrochloride) additive engineering and PEAI (phenylethylammonium iodide) passivation strategy to optimize the charge carrier dynamics and tune defects chemistry in the MA-free, Br-free RbCsFAPbI3-based perovskite absorber, which boosts the device efficiency up to 22.78%. Additionally, the device retains 95% of its initial performance under continuous 1 sun equivalent LED light illumination at 45 °C for up to 500 h.

Role of Alkali Cations in Stabilizing Mixed-Cation Perovskites to Thermal Stress and Moisture Conditions

Perovskite solar cells (PSCs) based on organic-inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA_1-xCs_xPbI₃ and (FA_1-xCs_x)_1-yRb_yPbI₃ (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs⁺ and Rb⁺ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA_1-xCs_xPbI₃ by providing a source of I^- that fills iodine vacancy sites in the perovskite lattice, while Rb⁺ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.

The Progress of Additive Engineering for CH3NH3PbI3 Photo-Active Layer in the Context of Perovskite Solar Cells

Methylammonium lead triiodide (CH3NH3PbI3/MAPbI3) is the most intensively explored perovskite light-absorbing material for hybrid organic–inorganic perovskite photovoltaics due to its unique optoelectronic properties and advantages. This includes tunable bandgap, a higher absorption coefficient than conventional materials used in photovoltaics, ease of manufacturing due to solution processability, and low fabrication costs. In addition, the MAPbI3 absorber layer provides one of the highest open-circuit voltages (Voc), low Voc loss/deficit, and low exciton binding energy, resulting in better charge transport with decent charge carrier mobilities and long diffusion lengths of charge carriers, making it a suitable candidate for photovoltaic applications. Unfortunately, MAPbI3 suffers from poor photochemical stability, which is the main problem to commercialize MAPbI3-based perovskite solar cells (PSCs). However, researchers frequently adopt additive engineering to overcome the issue of poor stability. Therefore, in this review, we have classified additives as organic and inorganic additives. Organic additives are subclassified based on functional groups associated with N/O/S donor atoms; whereas, inorganic additives are subcategorized as metals and non-metal halide salts. Further, we discussed their role and mechanism in terms of improving the performance and stability of MAPbI3-based PSCs. In addition, we scrutinized the additive influence on the morphology and optoelectronic properties to gain a deeper understanding of the crosslinking mechanism into the MAPbI3 framework. Our review aims to help the research community, by providing a glance of the advancement in additive engineering for the MAPbI3 light-absorbing layer, so that new additives can be designed and experimented with to overcome stability challenges. This, in turn, might pave the way for wide scale commercial use.

Lattice Strain Relaxation and Grain Homogenization for Efficient Inverted MAPbI3 Perovskite Solar Cells

The lattice strain of a perovskite film is vital to the controllable growth and charge transport in perovskite solar cells (PSCs). In this work, a lead chloride (PbCl₂) assisted crystallization (LCAC) protocol is introduced for releasing the strain across the interface of a NiO_x/perovskite, which induces a preferred (h00) crystal plane growth and grain homogenization. PSCs with LCAC show a facilitated charge extraction and suppressed nonradiative recombination. Thanks to the controlled film growth and strain-released interface, the inverted MAPbI₃ (MA = methylammonium) PSC devices with LCAC deliver a power conversion efficiency (PCE) over 20% with a short-circuit current density (J_sc) of 23.60 mA cm^-2, which is obviously higher than that of the control device with a PCE of 18.36% and a J_sc of 21.74 mA cm^-2. Meanwhile, the LCAC devices maintain 80% of their initial efficiency after being exposed to an ambient atmosphere with a relative humidity of 40% over 1000 h in the dark.

Advent of alkali metal doping: a roadmap for the evolution of perovskite solar cells

Metal–halide hybrid perovskites have prompted the prosperity of the sustainable energy field and simultaneously demonstrated their great potential in meeting both the growing consumption of energy and the increasing social development requirements.

Compositional Engineering Study of Lead-Free Hybrid Perovskites for Solar Cell Applications

Hybrid organic-inorganic perovskite solar cells (HOIPs), especially CH₃NH₃PbI₃ (MAPbI₃), have received tremendous attention due to their excellent power conversion efficiency (25.2%). However, two fundamental hurdles, long-term stability and lead (Pb) toxicity, prevent HOIPs from practical applications in the solar industry. To overcome these issues, compositional engineering has been used to modify cations at A- and B-sites and anions at the X-site in the general form ABX₃. In this work, we used the density functional theory (DFT) to incorporate Rb, Cs, and FA at the A-site to minimize the volatile nature of MA, while the highly stable Ca²⁺ and Sr²⁺ were mixed with the less stable Ge²⁺ and Sn²⁺ at the B-site to obtain a Pb-free perovskite. To further enhance the stability, we mixed the X-site anions (I/Br). Through this approach, we introduced 20 new perovskite species to the lead-free perovskite family and 7 to the lead-containing perovskite family. The molecular dynamic (MD) simulations, enthalpy formation, and tolerance and octahedral factor study confirm that all of the perovskite alloys we introduced here are as stable as pristine MAPbI₃. All Pb-free perovskites have suitable and direct band gaps (1.42-1.77 eV) at the Γ-point, which are highly desirable for solar cell applications. Most of our Pb-free perovskites have smaller effective masses and exciton binding energies. Finally, we show that the introduced perovskites have high absorption coefficients (10⁵ cm^-1) and strong absorption efficiencies (above 90%) in a wide spectral range (300-1200 nm), reinforcing their significant potential applications. This study provides a new way of searching for stable lead-free perovskites for sustainable and green energy applications.

Metal Cations in Efficient Perovskite Solar Cells: Progress and Perspective

Metal halide perovskite solar cells (PVSCs) have revolutionized photovoltaics since the first prototype in 2009, and up to now the highest efficiency has soared to 24.2%, which is on par with commercial thin film cells and not far from monocrystalline silicon solar cells. Optimizing device performance and improving stability have always been the research highlight of PVSCs. Metal cations are introduced into perovskites to further optimize the quality, and this strategy is showing a vigorous development trend. Here, the progress of research into metal cations for PVSCs is discussed by focusing on the position of the cations in perovskites, the modulation of the film quality, and the influence on the photovoltaic performance. Metal cations are considered in the order of alkali cations, alkaline earth cations, then metal cations in the ds and d regions, and ultimately trivalent cations (p- and f-block metal cations) according to the periodic table of elements. Finally, this work is summarized and some relevant issues are discussed.

Compositional Engineering for Thermally Stable, Highly Efficient Perovskite Solar Cells Exceeding 20% Power Conversion Efficiency with 85 °C/85% 1000 h Stability

Perovskite solar cells have received great attention because of their rapid progress in efficiency, with a present certified highest efficiency of 23.3%. Achieving both high efficiency and high thermal stability is one of the biggest challenges currently limiting perovskite solar cells because devices displaying stability at high temperature frequently suffer from a marked decrease of efficiency. In this report, the relationship between perovskite composition and device thermal stability is examined. It is revealed that Rb can suppress the growth of PbI₂ even under PbI₂ -rich conditions and decreasing the Br ratio in the perovskite absorber layer can prevent the generation of unwanted RbBr-based aggregations. The optimized device achieved by engineering perovskite composition exhibits 92% power conversion efficiency retention in a stress test conducted at 85 °C/85% relative humidity (RH) according to an international standard (IEC 61215) while exceeding 20% power conversion efficiency (certified efficiency of 20.8% at 1 cm² ). These results reveal the great potential for the practical use of perovskite solar cells in the near future.

Synthesis and Characterization of Multiple-Cation Rb(MAFA)PbI3 Perovskite Single Crystals

We synthesized multiple-cation Rb(MAFA)PbI₃ perovskite single crystals for the first time. The effect of Rb⁺ substitution was systemically investigated, and the addition of 1.5 M 5% RbI was the optimum condition to obtain high-quality Rb(MAFA)PbI₃ single crystals. Lattice shrinkage occurred in the Rb(MAFA)PbI₃ single crystal because of the small ionic radius of Rb⁺, resulting in blue-shifted absorption and photoluminescence (PL) peaks. The 1.5 M 5% RbI-added (MAFA)PbI₃ single crystal showed the longest carrier lifetime of 18.35 ns, exhibiting the highest photoresponse than other crystals. We believe that this work will provide a basic insight into the mixed-cation perovskite single crystals for the future optoelectronic applications.

Hydrophobic Polystyrene Passivation Layer for Simultaneously Improved Efficiency and Stability in Perovskite Solar Cells

The major restraint for the commercialization of the high-performance hybrid metal halide perovskite solar cells is the long-term stability, especially at the infirm interface between the perovskite film and organic charge-transfer layer. Recently, engineering the interface between the perovskite and spiro-OMeTAD becomes an effective strategy to simultaneously improve the efficiency and stability in the perovskite solar cells. In this work, we demonstrated that introducing an interfacial polystyrene layer between the perovskite film and spiro-OMeTAD layer can effectively improve the perovskite solar cells photovoltaic performance. The inserted polystyrene layer can passivate the interface traps and defects effectively and decrease the nonradiative recombination, leading to enhanced photoluminescence intensity and carrier lifetime, without compromising the carrier extraction and transfer. Under the optimized condition, the perovskite solar cells with the polystyrene layer achieve an enhanced average power efficiency of about 19.61% (20.46% of the best efficiency) from about 17.63% with negligible current density-voltage hysteresis. Moreover, the optimized perovskite solar cells with the hydrophobic polystyrene layer can maintain about 85% initial efficiency after 2 months storage in open air conditions without encapsulation.

Rubidium as an Alternative Cation for Efficient Perovskite Light-Emitting Diodes

Incorporation of rubidium (Rb) into mixed lead halide perovskites has recently achieved record power conversion efficiency and excellent stability in perovskite solar cells. Inspired by these tremendous advances in photovoltaics, this study demonstrates the impact of Rb incorporation into MAPbBr₃-based light emitters. Rb partially substitutes MA (methyl ammonium), resulting in a mixed cation perovskite with the formula MA_{(1- x)}Rb _xPbBr₃. Pure MAPbBr₃ crystallizes into a polycrystalline layer with highly defective sub-micrometer grains. However, the addition of a small amount of Rb forms MA_{(1- x)}Rb _xPbBr₃ nanocrystals (10 nm) embedded in an amorphous matrix of MA/Rb Br. These nanocrystals grow into defect-free sub-micrometer-sized crystallites with further addition of Rb, resulting in a 3-fold increase in exciton lifetime when the molar ratio of MABr/RbBr is 1:1. A thin film fabricated with a 1:1 molar ratio of MABr/RbBr showed the best electroluminescent properties with a current efficiency (CE) of 9.45 cd/A and a luminance of 7694 cd/m². These values of CE and luminance are, respectively, 19 and 10 times larger than those achieved by pure MAPbBr₃ devices (0.5 cd/A and 790 cd/m²). We believe this work provides important information on the future compositional optimization of Rb⁺-based mixed cation perovskites for obtaining high-performance light-emitting diodes.