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Surface passivation has been proven as an effective strategy to improve power conversion efficiency and stability of perovskite solar cells. However, the rationale of choosing appropriate passivator is still not clear yet, especially the binding interaction of different molecules with perovskite beneath. Here, we elucidated the binding interaction of two judiciously chosen molecules, fluorinated phosphonic acid and its corresponding phosphonate salt, and monitor the extent of interaction between these molecules and perovskite interface. Higher photoluminescence in perovskite film treated with phosphonate salt was observed which possesses stronger binding interaction with perovskite. The effect on photovoltaic performance and its stability were also examined. The corresponded device shows enhancement in conversion efficiency from 18.27% to 19.44%. Furthermore, water contact angle of treated perovskite film exceeding 110.9° was demonstrated which allows super hydrophobic nature, enabling excellent long term stability of devices by retaining over 90% of their initial efficiency after 90 days.

MATERIALS FOR HUMANITY (MH 21)

Toward Efficient and Stable Perovskite Photovoltaics with Fluorinated Phosphonate Salt Surface Passivation

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non-exposed feature. In this presentation, the buried mysteries in full device stacks will be unveiled by a series of dedicated techniques including the lift-off strategy, the in-situ mapping spectroscopy, and the cross-sectional high-resolution microscopy. By establishing the microstructure–property relations, the basic losses at the contact interfaces are systematically presented, which are induced by both the sub-microscale extended imperfections and lead-halide inhomogeneities. Furthermore, a brand new mechanism for the most popular ammonium-halide post-treatment is explored and called molecule-assisted microstructural reconstruction. The losses can be considerably mitigated by the use of the passivation-molecule-assisted microstructural reconstruction, which unlocks the full potential for improving device performance.

Buried interfaces in halide perovskite photovoltaics

The CsPbI2Br perovskite solar cells based on carbon electrode (CE) are considered to be low-cost and stable devices. Nevertheless, the insufficient contact and energy level mismatch in the interface of CsPbI2Br/CE hinder the hole extraction and limit the enhancement of cell efficiency. Herein, a carbon black (CB) interlayer was introduced between the CsPbI2Br and CE. [1] The hole extraction was facilitated due to the larger contact area and suitable energy band alignment in the CsPbI2Br/CB interface. Moreover, the carbon based CsPbI2Br solar cells maintained over 90% of original efficiency after constant 85 oC heating for 2000 hours, showing excellent thermal stability of carbon based CsPbI2Br solar cells. Further investigation indicated the diffusion of CB nanoparticles from CE or CB layer to CsPbI2Br film after a certain period of time. We disclosed the formation of a CB-CsPbI2Br bulk heterojunction structure due to the carbon diffusion, which resulted in an efficiency enhancement. As a result, a high efficiency of 13.13% was achieved for carbon based CsPbI2Br solar cells. This work indicates that the diffusion of CB nanoparticles in CB-contained perovskite solar cells is universal and inevitable, although this kind of diffusion results in the enhancement of the cell efficiency.

Carbon black interlayer facilitates hole extraction and improves the efficiency of carbon based CsPbI2Br solar cells

Surface imperfections created during fabrication of halide perovskite (HP) films could induce formation of various defect sites that affect device performance and stability. In this work, all‐organic surface modifiers consisting of alkylammonium cations and alkanoate anions are introduced on top of the HP layer to passivate interfacial vacancies and improve moisture tolerance. Passivation using alkylammonium alkanoate does not induce formation of low‐dimensional perovskites species. Instead, the organic species only passivate the perovskite's surface and grain boundaries, which results in enhanced hydrophobic character of the HP films. In terms of photovoltaic application, passivation with alkylammonium alkanoate allows significant reduction in recombination losses and enhancement of open‐circuit voltage. Alongside unchanged short‐circuit current density, power conversion efficiencies of more than 18.5 % can be obtained from the treated sample. Furthermore, the unencapsulated device retains 85 % of its initial PCE upon treatment, whereas the standard 3D perovskite device loses 50 % of its original PCE when both are subjected to ambient environment over 1500 h.

Effects of All-organic Interlayers Surface Modifiers on Perovskite Solar Cells Efficiency and Stability

Halide perovskite-based solar cell (PSC) has emerged as the fastest photovoltaic material to have reached the power-conversion efficiency (PCE) of larger than 25%. Despite unprecedented growth of research interest to study the fundamental aspects as well as to optimize interfacial layers in solar cells to achieve high-performing PSCs, moisture stability still remains as roadblock to get attention PSCs deserve for commercialization and mass production. To obtain first degree of protection, there have been reports on using additives into perovskite precursors and applying protective layers directly on perovskite active materials. For instances, hydrophobic molecules such as functionalized aliphatic compounds, fullerenes, and ionic liquids have been reported to increase the moisture-resistance of PSCs when used as additives in perovskite precursor solution. Separately, hydrophilic polymers such as poly(ethylene glycol) and poly(acrylic acid) were found to maintain device stability despite their water-absorbing nature. However, the beneficial effect of amphiphilic organic molecules has been scarcely discussed. In our study, novel amphiphilic corannulene-pentakis-(triethyleneglycol-monomethyl ether)-sulfone (Cor-TEG) was utilized to explore the mutual effect of both hydrophobic corannulene and hydrophilic side chains (ethylene glycol oligomers) in increasing the moisture resistance of PSCs. It is found that the Cor-TEG additive modulated the perovskite crystal growth to form large and columnar grains with reduced defects and enhanced photocarrier extraction. Notably, the Cor-TEG modified perovskite devices showed improved stability under humid Singapore ambient air, retaining 87% of initial PCE for two weeks of exposure.

Amphiphilic additive for moisture-resistant perovskite solar cells

Lead-free halide perovskite semiconductors are necessary due to the atmospheric instability and lead toxicity associated with the 3D lead halide perovskites. However, a stable lead-free perovskite with an ideal bandgap (1.2-1.4 eV) for photovoltaics is still missing. In this work, we synthesized organic-inorganic gold halide double perovskites ((CH3NH3)2Au2X6, X = Br, I) through a solution-processed route that offers an ideal direct bandgap for photovoltaic applications. Density functional theory calculations confirm the direct nature of bandgap nature with reasonable absorption coefficients in the visible range and excellent charge transport properties. In addition, the Au-halide perovskites show high chemical stability and photoresponse. These combined properties demonstrate that Au-based halide perovskites can be a promising group of compounds for optoelectronic applications.

Direct-bandgap Mixed-valence Organic-inorganic Gold Perovskite as Visible Light Absorber

Lithium ion batteries have seen rapid development in recent years, accompanied by increasing industrial interest in electrification. Nonetheless, several issues remain, including insufficient capacity, poor safety and high raw material costs. This review utilises the periodic table to rationalise popular developments in alternative battery chemistries aimed at addressing these issues, including sodium ion, multivalent ion as well as various alloying and conversion electrodes such as lithium-silicon and metal anodes, and sulfur and oxygen cathodes. The major advantages and challenges faced by each chemistry are briefly discussed, along with the strategies being adopted to overcome these challenges and state-of-the-art developments in industry.

Alternative Chemistries to Lithium-Ion for High Energy Batteries

Energy – one of the major crises humanity is facing in today’s modern life. There is a great interest in developing micro-scaled devices that can generate electricity using ambient radiofrequency (RF) energy without the need of any bias or magnetic field. We addressed this problem and used the intrinsic quantum property of the Weyl semimetal to convert the RF energy into electricity. The whole process is wireless and there is no need for any external electrical bias (battery-free) and magnetic field.
In this work, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. The NLHE is present only when the electric field is applied along the axis of the mirror plane (a-axis of the crystal). We also observe a temperature-induced sign inversion of NLHE in TaIrTe4. Finally, based on the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless RF rectification with zero external bias and magnetic field. As shown in Fig. 1(b), an incident electric field along a-axis (Ea) generates a rectified voltage along b-axis (Vb), whereas, there is no rectification when the incident electric field is along b-axis (Eb) in consistent with the symmetry constraints of NLHE. This work opens a door to realizing room temperature applications based on the NLHE in Weyl semimetals.


Room Temperature Nonlinear Hall Effect and Wireless RF Rectification in Weyl Semimetal TaIrTe4

Atomically thin Bi2O2Se has emerged as a new member in two-dimensional (2D) materials with ultra-high carrier mobility and excellent air-stability, showing great potential for electronics and optoelectronics. In addition, its ferroelectric nature renders ultra-low thermal conductivity, making it a perfect candidate for thermoelectrics. In this work, we experimentally investigate the thermoelectric performance of 2D Bi2O2Se over a wide temperature range (20 - 300 K). We observe a gate-tunable transition from polar optical phonon (POP) scattering to piezoelectric scattering, which facilitates the capacity of drastic mobility engineering in 2D Bi2O2Se. Consequently, high power factor of more than 400 uWm-1K-2 over an unprecedented temperature range (80 – 200 K) is achieved, corresponding to the persistently high mobility arising from the highly gate-tunable scattering mechanism. This finding provides a new avenue for maximizing thermoelectric performances by changing scattering mechanisms and carrier mobility over a wide temperature range.

Gate-tunable Polar Optical Phonon to Piezoelectric Scattering in Few-layer Bi2O2Se for High Performance Thermoelectrics

Metal lead halide perovskites are promising candidates for optoelectronic applications due to their high absorption coefficient, thin emission line width, solution processability, high photoluminescence efficiency, and wide color tunability. When nanostructured these perovskites offer the additional advantage of strong quantum confinement.1 In this pioneering work, triple cation perovskite Csx(MA0.17FA0.83)1−xPbBr3 (x = 0–0.15) nanocrystals were synthesised. These proved to have superior phase and environmental stability, less sensitivity to the processing conditions so high reproducibility, and high efficiency when deployed in LEDs and solar cells.2 Although these of lead halide perovskites show excellent properties they contain toxic lead. This motivated the study of stable and lead-free cesium copper iodides as optoelectronic materials.3 By synthesising these compounds as nanocrystals using a hot-injection route the phase and composition were prescribed such that Cs3Cu2I5 nanoplates gave blue emission and CsCu2I3 nanorods yield yellow emission. By mixing these components pure white emission from lead-free metal halide nanocrystals was demonstrated for the first-time. 
References:
1. J. Butkus, P. Vashishtha, K. Chen, J. K. Gallaher, S. K. Prasad, D. Z. Metin, G. Laufersky, N. Gaston, J. E. Halpert and J. M. Hodgkiss, Chemistry of Materials, 2017, 29, 3644-3652.
2. P. Vashishtha, S. A. Veldhuis, S. S. H. Dintakurti, N. L. Kelly, B. E. Griffith, A. A. M. Brown, M. S. Ansari, A. Bruno, N. Mathews, Y. Fang, T. White, S. G. Mhaisalkar and J. V. Hanna, Journal of Materials Chemistry C, 2020, DOI: 10.1039/D0TC02038A.
3. P. Vashishtha, G. V. Nutan, B. E. Griffith, Y. Fang, D. Giovanni, M. Jagadeeswararao, T. C. Sum, N. Mathews, S. G. Mhaisalkar and J. V. Hanna, Chemistry of Materials, 2019, 31, 9003-9011.



Metal Halide Nanocrystals for Optoelectronics

The search for batteries with higher energy and power densities than the current Li–ion batteries is a high priority research area in the continuing development of energy storage systems. Among the various possibilities, lithium–sulfur (Li–S) batteries have drawn a strong following because of the high theoretical specific capacity (1675 mAh g–1) and low cost (<$150 ton–1) of sulfur. The actual energy density of Li–S batteries as implemented to date is however notably lower than the theoretical value because of several technical challenges, such as the dissolution of reaction intermediate products (polysulfides), the polysulfide shuttle between the sulfur cathode and lithium anode, and the need for excess electrolyte. The use of catalysts to accelerate the polysulfide conversion to their end discharge product (Li2S) can inhibit the loss of polysulfides to the electrolyte and has the appeal of a root–cause solution. While a great deal of attention has been focused on the search for new polysulfide catalysts, relatively few have focused on strategies which can maximize the performance of catalysts developed to date. This talk introduces several strategies which can improve the catalytic activity and the sustainable use of catalysts in lithium–sulfur batteries.

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