Investigation on the Mechanism of Radical Intermediate Formation and Moderate Oxidation of Spiro-OMeTAD by the Synergistic Effect of Multisubstituted Polyoxometalates in Perovskite Solar Cells

Homogenization and Rapid Oxidation of Spiro-OMeTAD with Ionic Liquids for Efficient Perovskite Solar Cells

Spiro-OMeTAD is widely recognized as the most effective hole transport layer (HTL) for n-i-p perovskite solar cells (PSCs), which typically requires doping with LiTFSI to overcome its low inherent conductivity. However, the doping takes a prolonged oxidation (≈24 h) in an ambient atmosphere, hindering the commercial development. Moreover, the aggregation of LiTFSI leads to poor conductivity and accelerated degradation of the HTL, which are often ignored. This study introduces the long-chain ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OMIMTFSI) as a multifunctional additive to mitigate the aggregation of LiTFSI and promote the oxidation of Spiro-OMeTAD simultaneously. The strong electrostatic interactions between OMIM+ and LiTFSI, coupled with the dispersion effect of OMIM+ in chlorobenzene, effectively hamper the aggregation of LiTFSI, beneficial for uniform doping and enhanced conductivity. The OMIM+ also facilitates rapid oxidation of Spiro-OMeTAD by attracting lone pair electrons from the triphenylamine group. As a result, the power conversion efficiency of PSCs processed in air is significantly improved from 21.48% to 24.04% with enhanced stability, maintaining over 80% of initial values after storing in air for 1360 h or under light and heat treatment for 500 h. This strategy provides valuable insights of designing lithium salt-doped Spiro-OMeTAD for efficient and stable PSCs.

Radical p-Doping Spiro-OMeTAD for Efficient, Stable and Self-Healing Flexible Perovskite Solar Cells

Spiro-OMeTAD is the primary hole transport material (HTM) for high-efficiency and stable flexible perovskite solar cells (FPSCs). However, the slow oxidation rate and susceptibility to film cracking under stress in Spiro-OMeTAD lead to reduced device stability and efficiency. In this paper, a multi-functional novel self-healing nitroxide radical monomer, 4-[[5-(1,2-dithiolane-3-yl)-1-oxopentyl]amino]-2,2,6,6-tetramethylpiperidin-1-oxyl (DT-TEMPO), has been introduced to address these challenges. DT-TEMPO, on one side, enhances the hole mobility and conductivity by p-doping Spiro-OMeTAD, while boosting the charge transfer process from perovskite to Spiro-OMeTAD with an optimized energy level alignment on the other side. Additionally, DT-TEMPO endows a self-healing capability to Spiro-OMeTAD through the introduction of dynamic breaking and reconstructing disulfide bond. The optimized perovskite solar cells achieve impressive power conversion efficiencies, 25.69% on rigid substrates (certified 25.30%), 21.23% on rigid mini-modules, and 24.19% on flexible substrates. Remarkably, the FPSCs with DT-TEMPO retain over 90% of their initial efficiency even after 20 000 bending cycles (r = 6 mm) and recover to ≈95% of their initial value through the self-healing process.

A selective oxidative depolymerization of larch lignin to ethyl vanillate by multifunctional catalysts combining alkaline ionic liquid and polyoxometalates with hydrogen peroxide

Ethyl vanillate (EV) is an important component of flavors and fragrances and has been widely used in the food, pharmaceutical, and cosmetic industries. The highly selective preparation of EV from lignin, the most abundant monophenolic compound in nature, is a great challenge in the field of lignin depolymerization. In this study, the multi-active catalysts from alkaline ionic liquid and polyoxometalates were constructed, which were characterized by acidity, alkaline and oxidizing ability. They can efficiently cleave the CC and CO bonds of larch lignin in an ethanol/water system at 160 °C for 10 h with H2O2 as an oxidizing agent. Vanillin and EV were main products with the phenolic yield of 10.14 wt% and the selectivity of EV was up to 83.63 %. The depolymerization of phenolic and non-phenolic lignin model compounds demonstrated that the cleavage of β-O-4 and Cα-Cβ bonds contributed to the selective preparation of EV in the catalytic system. The catalyst can be recycled up to 4 times with excellent stability and recoverability. This study provides a new approach for the production of EV, which is promising in the high-valued utilization of larch waste.

"Double-Use" Strategy for Improving the Photoelectrochemical Performance of BiVO4 Photoanodes Using a Cobalt-Functionalized Polyoxotungstate

Doping and surface-modification are well-established strategies for the performance enhancement of bismuth vanadate (BiVO4) photoanodes in photoelectrochemical (PEC) water splitting devices. Herein, a "double-use" strategy for the development of high-performance BiVO4 photoanodes for solar water splitting is reported, where a molecular cobalt-phosphotungstate (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) is used both as a bulk doping agent as well as a surface-deposited water oxidation cocatalyst. The use of CoPOM for bulk doping of BiVO4 is shown to enhance the electrical conductivity and improve the charge separation efficiency, resulting in the enhancement of the maximum applied-bias photoconversion efficiency (ABPE) by a factor of ∼18 to 0.54% at 0.87 V vs. RHE, as compared to pristine BiVO4 (0.03% at 1.04 V vs. RHE). The ratio of W/Co on the surface of the photoanode is related to the activity and stability. In addition, modification of CoPOM-doped BiVO4 with CoPOM as a surface cocatalyst enhances the hole extraction and improves the water oxidation kinetics, resulting in the overall enhancement of the ABPE to 0.79% (at 0.82 V vs. RHE), i.e., by a factor of ∼26 with respect to pristine BiVO4. This study establishes the "double-use" strategy involving CoPOMs as an effective, straightforward, and easily scalable approach for the development of high-quality photoanodes for solar water splitting and highlights the future potential of utilizing well-designed polyoxometalates as precursors for the synthesis of energy materials.

Iodonium Initiators: Paving the Air-free Oxidation of Spiro-OMeTAD for Efficient and Stable Perovskite Solar Cells

To date, perovskite solar cells (pero-SCs) with doped 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) hole transporting layers (HTLs) have shown the highest recorded power conversion efficiencies (PCEs). However, their commercialization is still impeded by poor device stability owing to the hygroscopic lithium bis(trifluoromethanesulfonyl)imide and volatile 4-tert-butylpyridine dopants as well as time-consuming oxidation in air. In this study, we explored a series of single-component iodonium initiators with strong oxidability and different electron delocalization properties to precisely manipulate the oxidation states of Spiro-OMeTAD without air assistance, and the oxidation mechanism was clearly understood. Iodine (III) in the diphenyliodonium cation (IP+ ) can accept a single electron from Spiro-OMeTAD and forms Spiro-OMeTAD⋅+ owing to its strong oxidability. Moreover, because of the coordination of the strongly delocalized TFSI- with Spiro-OMeTAD⋅+ in a stable radical complex, the resulting hole mobility was 30 times higher than that of pristine Spiro-OMeTAD. In addition, the IP-TFSI initiator facilitated the growth of a homogeneous and pinhole-free Spiro-OMeTAD film. The pero-SCs based on this oxidizing HTL showed excellent efficiencies of 25.16 % (certified: 24.85 % for 0.062-cm2 ) and 20.71 % for a 15.03-cm2 module as well as remarkable overall stability.

Efficient electrocatalytic CO2 reduction to ethanol through the proton coupled electron transfer process of PVnMo(12-n) (n = 1, 2, 3) over indium electrode

The multistep proton-coupled electron transfer (PCET) processes are beneficial for products distribution and selectivity of the electrocatalytic CO2 reduction reaction (CO2RR), which are affected by the nature of the catalyst and electrolyte at electrode-electrolyte interface. Polyoxometalates (POMs) are electron regulators of PCET processes, which can catalyze CO2RR effectively. Accordingly, the commercial indium electrodes are combined in this work with a series of Keggin-type POMs (PVnMo(12-n)O40)(n+3)-, n = 1, 2, 3) to process CO2RR with Faradaic efficiency toward ethanol reaching 93.4% at -0.3 V (vs. RHE). The cyclic voltammetry and X-ray photoelectron spectroscopy results reveal the activation of CO2 molecules by the first PCET process of the VⅤ/Ⅳ in POM. Subsequently, the PCET process of MoⅥ/Ⅴ results the oxidation of the electrode, causing the loss of In0 active sites. Electrochemical in-situ infrared spectroscopy confirms the weak adsorption of *CO at the later stage of electrolysis due to the oxidation of the In0 active sites. The indium electrode in PV3Mo9 system retains more In0 active sites owing to the highest V-substitution ratio, thereby ensuring a high adsorption ratio of *CO and CC coupling. In sum, the regulation of the interface microenvironment by POM electrolyte additives can be used to boost the performance of CO2RR.

Dendrimer Modification Strategy Based on the Understanding of the Photovoltaic Mechanism of a Perovskite Device under Full Sun and Indoor Light

The wide-band-gap inorganic CsPbI₂Br perovskite material provides a highly matched absorption range with the indoor light spectrum and is expected to be used in the fabrication of highly efficient indoor photovoltaic cells (IPVs) and self-powered low-power Internet of Things (IoT) sensors. However, the defects that cause nonradiative recombination and ion migration are assumed to form leakage loss channels, resulting in a severe impact on the open-circuit voltage (V_OC) and the fill factor (FF) of IPVs. Herein, we introduce poly(amidoamine) (PAMAM) dendrimers with multiple passivation sites to fully repair the leakage channels in the devices, taking into account the characteristics of IPVs that are extremely sensitive to nonradiative recombination and shunt resistance. The as-optimized IPVs demonstrate a promising PCE of 35.71% under a fluorescent light source (1000 lux), with V_OC increased from 0.99 to 1.06 V and FF improved from 75.21 to 84.39%. The present work provides insight into the photovoltaic mechanism of perovskites under full sun and indoor light, which provides guidance for perovskite photovoltaic technology with industrialization prospects.