Energy Research Institute

The preparation of highly efficient perovskite nanocrystal light-emitting diodes is shown. A new trimethylaluminum vapor-based crosslinking method to render the nanocrystal films insoluble is applied. The resulting near-complete nanocrystal film coverage, coupled with the natural confinement of injected charges within the perovskite crystals, facilitates electron–hole capture and give rise to a remarkable electroluminescence yield of 5.7%. Metal-halide perovskite semiconductors have attracted significant research interest, due to a combination of low-cost solution processability and remarkable performance in optoelectronic devices.1, 2 In 2014, we demonstrated infrared and visible electroluminescence in methylammonium lead halide perovskites, using a charge-confined diode structure to achieve effective radiative recombination.2-4 However, the use of methylammonium halide, which is a chemical combination of gaseous methylamine and hydrogen halide, necessarily limits the thermal stability of these perovskite devices. Replacing methylammonium with inorganic cesium offers the perovskite extra thermal stability up to its melting at ≈500 °C, but makes it more intractable toward solution processing.5 Recently, Protesescu et al. demonstrated the synthesis of cesium lead halide perovskite nanocrystals,6 following the traditional approaches of growing and stabilizing semiconductor particles in the presence of aliphatic ligands.7 These perovskite nanocrystals are highly luminescent and emit over the full visible range, making them ideal candidates for luminescent display applications.6 The synthetic steps are generally straightforward, and the easy control of halide content allows the perovskite bandgaps to be tailored, both by chemical compositions as well as by quantum size effects. So far, perovskite nanocrystals are shown to have color-pure emission, close to unity photoluminescence yield and low lasing thresholds.8 These nanocrystals were also attempted in light-emitting devices, but efficiencies remain modest at 0.12%.9 Here, we show the preparation of highly efficient perovskite light-emitting diodes (PeLED) using solution-processed nanocrystals. We apply a new trimethylaluminum (TMA) vapor-based crosslinking method to render the nanocrystal films insoluble, thereby allowing the deposition of subsequent charge-injection layers without the need for orthogonal solvents. The resulting near-complete nanocrystal film coverage, coupled with the natural confinement of injected charges within the perovskite crystals, facilitate electron–hole capture and give rise to a remarkable electroluminescence yield of 5.7%. Figure 1a shows the device architecture of our perovskite nanocrystal light-emitting diode, and Figure 1b shows the energy-level diagram of the materials within the device stack. Here, our electron-injection layer comprises a film of zinc oxide (ZnO) nanocrystals, directly deposited on an indium tin oxide (ITO)-coated glass substrate.4 The cesium lead halide nanocrystals were solution-coated onto the ZnO film as the emissive layer. Due to the presence of aliphatic ligands on the nanocrystals, the perovskite film remains soluble to organic solvents, which limits the deposition of subsequent charge-injection layers using solution methods. We employed a new TMA vapor-phase crosslinking technique to fix the nanocrystal film in place, thereby enabling us to solution-cast a layer of TFB polymer (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)]) above without washing the nanocrystals off. TFB serves primarily as a hole-injection and electron-blocking layer. A thin, high work-function molybdenum trioxide (MoO3) interlayer and silver electrode were vacuum-thermal evaporated to complete the device. As shown in Figure 1c,d, our perovskite nanocrystal devices show saturated and color-pure emission. We control the perovskite bandgap, primarily by tailoring the halide composition, and achieve electroluminescence across a wide range of the visible spectrum. Our red, orange, green, and blue devices emit at wavelengths of 698, 619, 523, and 480 nm, respectively. All devices exhibit narrow-width emission, with their full-width at half maxima (FWHM) in the range of 17–31 nm. The red and green LEDs were made, respectively, from the pure iodide and pure bromide perovskites, while the orange and blue LEDs were made from mixed-halide perovskites, with CsPbI2.25Br0.75 and CsPbBr1.5Cl1.5 compositions, respectively. The crosslinking of the perovskite nanocrystals was critical toward the realization of our device structure. Traditionally, nanocrystals have been crosslinked, or made insoluble, using ligand exchange techniques, where shorter-chain bi-functionalized organic ligands (e.g., diamines or dithiols) were applied to replace the longer-chain oleyl ligands.11 However, the replacement with smaller ligands often creates cracks and gaps within the nanocrystal film, which could lead to electrical shunts and reduced device performance.12 Closer packing of the nanocrystals could also lead to the self-quenching of the nanocrystals and a lower photoluminescence yield,13 often caused by a more effective Förster resonant energy transfer (FRET) to non-radiative sites.14 Here, crosslinking is achieved by exposing the perovskite nanocrystal film to short pulses of TMA vapor within an enclosed vacuum chamber at room temperature, followed by standing the treated film in ambient air. This creates a well-connected network of hydroxide-terminated aluminum oxide that links the nanocrystals together, hence rendering them insoluble. This vapor-phase technique does not involve ligand exchange processes, and is therefore useful in crosslinking the nanocrystals without altering the original crystal arrangements, thereby allowing much of the film's original structural and electronic properties to be preserved. The photoluminescence spectra of the nanocrystals also remain unchanged after the TMA treatment, hence confirming that the crosslinking has no effect on their optical bandgap. (Note that we use a slightly modified TMA treatment procedure for the blue-emitting nanocrystals—see Figure S1, Supporting Information.) In order to investigate the effectiveness of crosslinking, we varied the exposure of the perovskite nanocrystals to different pulse durations of TMA vapor, and measured the nanocrystal retention upon washing with toluene. The crystal retention ratio was determined using UV–visible absorption spectroscopy. As shown in Figure 2a, the crosslinking was remarkably rapid even at room temperature, and a near complete retention of nanocrystals was achieved in less than 0.5 s of TMA exposure. The inset scanning electron microscopy (SEM) images show a significant wash-off for the non-crosslinked nanocrystals, while the TMA-treated nanocrystals were completely retained after washing. Detailed SEM images and large area scans of the nanocrystal films, upon washing with different organic solvents, are shown in Figure S2 (Supporting Information). Interestingly, we observe a corresponding increase in the photoluminescence quantum efficiency (PLQE) of CsPbI3 and CsPbBr3 with TMA treatment, but a decrease in the case of CsPbBr1.5Cl1.5 (see Figure 2b). This PL enhancement is particularly remarkable for the CsPbI3 nanocrystals, raising its PL by more than a factor of 3, from ≈25% in the untreated film to ≈85% in the TMA-treated film. We note that this is possibly one of the highest PLQEs achieved in a non-core–shell structured nanocrystal thin film. In an attempt to unravel the origins of the PL enhancement, we took high-resolution transmission electron microscopy (HRTEM) images (Figure 2c) and high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure 2d) of the CsPbI3 perovskite nanocrystals before and after the TMA treatment, and further analyzed the distribution of main elements within the samples using electron energy loss spectroscopy (EELS) spectrum imaging (Figure 2e). The nanocrystals show an average size of 19 nm (see Figure S3, Supporting Information), which points to minimal quantum confinement in their electronic bandgap.6 We analyzed the spacing distribution between the nanocrystals in the HRTEM images, and found the average crystal spacing to increase from 1.6 to 1.9 nm upon TMA treatment. We note that a spacing of 1.6 nm between the crystals represents ≈0.55 ligand attachment per unit cell of the perovskite lattice. The spacing distribution analysis is shown in Figure S3 (Supporting Information) and further TEM images of CsPbBr3 perovskite nanocrystals are shown in Figure S4 (Supporting Information). By analyzing the EELS spectrum image using a combination of principal component analysis (PCA), independent component analysis (ICA), and curve-fitting quantification, we were able to map out the distribution of elements, and precisely identify the higher concentrations of aluminum and oxygen elements within the sub-2 nm gaps between the perovskite nanocrystals (Figure 2e). This indicates that the TMA treatment has successfully created an alumina network that covers all areas surrounding and in-between the nanocrystals. The intercalation of alumina among the ligands, therefore, accounts for the small increase in the crystal spacing. While this 0.3 nm increase in spacing may lead to a smaller degree of PL quenching by FRET, it is unlikely to fully account for the threefold enhancement in PL yield. The changes in PL are therefore likely to be chemical in nature, where the introduction of TMA chemically passivates the nanocrystal surface and leads to a reduction in PL-quenching defects.15 To further verify that the aluminum is incorporated between the crystal spacing and not just above the nanocrystal film, we measured the X-ray photoelectron spectra (XPS) of the sample as a function of film depth, and found aluminum to be quite evenly distributed throughout the entire nanocrystal film thickness (see XPS depth profile in Figure S5, Supporting Information). To elucidate the chemical crosslinking mechanism, we measured the infrared (IR) transmittance spectra of the nanocrystal samples before and after the TMA treatment. For comparison, we also measured the IR spectra of the organic ligands, and plotted them on the same graph in Figure 3a. Three distinct changes could be observed in the IR spectra following the TMA treatment. i) A broad peak centered around 3450 cm−1 is produced in all the TMA-treated samples, signally the presence of O–H stretch. This confirms hydroxide surface terminations as expected for an AlOx network, and the role of ambient water in the crosslinking process. ii) Another broad peak appears around 630 cm−1, and this is characteristic of vibrational modes in alumina.16 iii) A strong and sharp band appears at 1576 cm−1 (for samples containing oleic acid), which can be assigned to weakened C=O stretching. This is particularly clear in the case of TMA-treated oleic acid. It is likely that the bonding of the carboxylate group to the strongly Lewis acidic aluminum causes the C=O bond to be weakened, thereby giving a lower than usual stretching frequency. Interestingly, the C=O stretching band does not appear in the pristine nanocrystal samples, but only emerges after TMA treatment. This is likely to be due to the coordination of the carboxylate group to the nanocrystals, thereby diminishing its oscillator strength. With this information, we propose a reaction mechanism for the cross-linking process as shown in Figure 3b. The introduction of TMA coordinates and reacts with the carboxylate and amino groups of the ligands, adjacent to the nanocrystals. The subsequent exposure to ambient moisture hydrolyzes the remaining methyl-aluminum to give a network of alumina and aluminum hydroxide that are covalently bonded to the ligands. It is likely that the alumina that is formed next to the crystal surface successfully passivates the surface defects, thereby enhancing the crystals' photoluminescence yield. The strongly reactive TMA appears to be benign toward the iodide and bromide perovskites, but causes some level of damage to the chloride perovskites, hence resulting in a degradation of PL in the latter. To further verify the role of ligands in this crosslinking process, we attempted to wash off TMA-treated oleic acid and oleylamine (i.e., no nanocrystals) with toluene. We show in Figure S6 (Supporting Information) that the ligands were crosslinked and insoluble upon TMA treatment, hence indicating that many of the ligands are chemically incorporated into the alumina network across the nanocrystal film. In order to prove the generality of our crosslinking technique, we treated thin films of CdSe and PbS nanocrystals with TMA, and demonstrate them to be insoluble to organic solvents upon treatment (see Figure S7, Supporting Information). The TMA crosslinking method has enabled us to make high-performance light-emitting devices using these nanocrystalline lead halide perovskites. Figure 4a–d shows the detailed device characteristics of our perovskite nanocrystal LEDs. High luminance levels of 2335 and 1559 cd m−2 were achieved in our green and orange-emitting devices, respectively, at current densities of 831 and 852 mA cm−2. As shown in Figure 4b, charge injection turns on efficiently close to the bandgap voltage of the perovskite. To demonstrate the importance of our crosslinking method, we plotted the current–voltage characteristics of the non-TMA-treated CsPbI3 device, and show that the current density is more than an order of magnitude higher than the crosslinked counterpart, even at similar or lower luminance levels. This is a clear indication of current leakage,3 which is a result of voids created from the wash-off of the perovskite nanocrystals. We achieved remarkable external quantum efficiencies (EQE), as high as 5.7%, in the crosslinked red-emitting CsPbI3 device, more than an order of magnitude higher than the non-crosslinked device (see Figure 4c) and a factor of 50 higher than the previous report.9 Correcting for out-coupling losses,17 this would be equivalent to an internal quantum efficiency of ≈26%. We note that our CsPbI3 device emits at 698 nm, close to the edge of human-eye sensitivity, and therefore gives only a modest luminance level of 206 cd m−2 at a current density of 755 mA cm−2. We observed that the EQE of the nanocrystal devices decrease generally with the widening of the perovskite bandgap. This is in line with the trends of a lower PLQE in our green and blue-emitting materials (see Figure 2b). The less-ideal charge injection into larger bandgap perovskites could also result in the lower device efficiencies. Figure 4d gives a summary of the key performance parameters of our nanocrystal LEDs. During our investigation of the mixed-halide perovskite nanocrystals (CsPbI2.25Br0.75 and CsPbBr1.5Cl1.5), we noticed that their emission red-shifts reversibly during device operation, and return slowly toward their original state after resting (see Figure S8, Supporting Information). This emission shift is not observed in the pure-halide samples. This may be related to previous observations on photoluminescence shifts in mixed-halide perovskites upon photoexcitation, which was suggested to be due to phase segregation into purer halide phases.18 We note that our films consist of spatially separated nanocrystals, and any phase segregation or halide rearrangements could therefore occur only within a small domain of less than 20 nm. We further show in Figure S9 (Supporting Information) that the EL shifts are completely consistent with the PL shifts, thereby confirming that these shifts are a result of intrinsic changes within the perovskites, and not due to a change in the device charge-injection properties during operation. In view of the emission shifts in mixed-halide systems, bandgap tuning of pure-halide perovskites via quantum size effect may emerge as the more successful strategy for consumer display applications, where stringent requirements on emission stability are demanded. We have now successfully departed from the traditional core–shell nanocrystal approach, and shown that highly efficient electroluminescence could be realized in semiconductor nanocrystals using a simple TMA crosslinking technique. The remarkable ability of alumina to double as a surface passivating agent further makes the TMA treatment an attractive technique for the fabrication of quantum-dot optoelectronic devices. Indeed, given their intensely luminescent properties, versatile color tuning, coupled with an enhanced thermal stability, these all-inorganic lead halide perovskites may quickly emerge as strong contenders in the color display industry. Materials: TFB was provided by Cambridge Display Technology (CDT) and was used as received. All other chemicals were purchased from Sigma–Aldrich, and were used as received. Synthesis of CsPbX3 (X =Cl, Br, I) Nanocrystals: Perovskite nanocrystals were synthesized using previously reported procedures.6 Cs2CO3 (0.814g, 99.9%) was loaded into 100 mL three-neck flask along with octadecene (ODE) (30 mL, 90%) and oleic acid (OA) (2.5 mL, 90%), and the mixture was dried for 2 h at 120 °C under N2. The solution temperature was then lowered to 100 °C. ODE (75 mL), oleylamine (OLA) (7.5 mL, 90%), and dried OA (7.5 mL) and PbX2 (2.82 mmol) such as PbI2 (1.26 g, 99.99%), PbBr2 (1.035 g, 99.99%), PbCl2 (0.675g, 99.99%), or their mixtures were loaded into a 250 mL three-neck flask and dried under vacuum for 2 h at 120 °C. After complete solubilization of the PbX2 salt, the temperature was raised to 170 °C and the Cs-oleate solution (6.0 mL, 0.125 m in ODE, prepared as described above) was quickly injected. After 10 s, the reaction mixture was cooled in an ice-water bath. For CsPbCl3 synthesis, 1 mL of trioctylphosphine (TOP) (97%) was added to solubilize PbCl2. The nanocrystals were precipitated from solution by the addition of equal volume anhydrous butanol (BuOH) (99%) (ODE:BuOH = 1:1 by volume). After centrifugation, the supernatant was discarded and the nanocrystals were redispersed in anhydrous hexane (99%) and precipitated again with the addition of BuOH (hexane:BuOH = 1:1 by volume). These were redispersed in hexane. The nanocrystal dispersion was filtered through a 0.2 μm poly(tetrafluoroethylene) (PTFE) filter and diluted to 10 mg mL−1 in hexane before use. Synthesis of ZnO Nanocrystals: Colloidal ZnO nanocrystals were synthesized by a solution-precipitation process according to previously reported zinc was in by at room A solution of hydroxide in mL) was then added within After 2 the mixture was and with and mL) was added to the and a ZnO nanocrystal dispersion with a of mg The dispersion was filtered with a μm filter before use. of Perovskite Nanocrystals: A layer deposition was used for the vapor-phase crosslinking treatment. TMA was purchased from The deposition temperature was at 1 °C, which is also the ambient room The chamber was 0.2 before the process. TMA was applied in short pulses at a of The of TMA was by the of TMA pulses between and 20 The of pulse was to 1 s, hence a 10 s for achieved with 10 1 s A mg mL−1 dispersion of ZnO nanocrystals in was onto an glass at for s, followed by at 100 °C for 10 in a to give a nm film. A 10 mg mL−1 perovskite nanocrystal dispersion in hexane was at for 20 s in to give a nm film. The films were to in for and were into the chamber for crosslinking treatment. the crosslinking a solution of TFB in mg was on of the perovskite nanocrystals at for s in the to give a nm thin film. and layers were deposited by thermal in a high vacuum of The devices were by voltage characteristics were measured using a The was measured using a centered over the light-emitting in cd m−2 was on the emission spectrum of the the function and on the of the The external quantum efficiency was a emission spectra were measured using a In the PL the nanocrystal films were in a and were using a nm The and the emission were measured and using a for the of PL quantum UV–visible absorption of the nanocrystal films was measured using an were prepared on using same parameters as in device were measured by an were onto a and measured in the range of TEM was out on CsPbI3 and CsPbBr3 nanocrystals that were on thin TEM The TEM samples were analyzed in an at The images were using a The EELS spectra were using a with and spectra were over different of the using a 2 nm size and a dispersion of 1 per The was 0.2 s for spectra and 10 for and were and respectively. The were by and The by of the was using All the EELS analysis was using the SEM images were using the in a The voltage was at 10 This was by the and Cambridge for is for from the Cambridge and Cambridge and for and from the under from the under from the in on at The this are at The of the was on after As a to our and this by the materials are and may be for but are not or from than be to the The is not for the content or of any by the than be to the corresponding for the

Al 2 O 3 is a versatile high-κ dielectric that has excellent surface passivation properties on crystalline Si (c-Si), which are of vital importance for devices such as light emitting diodes and high-efficiency solar cells. We demonstrate both experimentally and by simulations that the surface passivation can be related to a satisfactory low interface defect density in combination with a strong field-effect passivation induced by a negative fixed charge density Qf of up to 1013 cm−2 present in the Al2O3 film at the interface with the underlying Si substrate. The negative polarity of Qf in Al2O3 is especially beneficial for the passivation of p-type c-Si as the bulk minority carriers are shielded from the c-Si surface. As the level of field-effect passivation is shown to scale with Qf2, the high Qf in Al2O3 tolerates a higher interface defect density on c-Si compared to alternative surface passivation schemes.

Abstract The electrochemically stable and relatively high conductive room temperature molten salts (RTMS) have been obtained with the use of small ammonium cations such as methoxymethyltrimethylammonium and bis(trifluoromethylsulfonyl)imide. The RTMS showed high conductivity (4.7 mS cm−1 at 25 °C) which is the highest value of all the ammonium based RTMS reported so far.

Copper electrodes have been shown to be selective toward the electroreduction of carbon dioxide to ethylene, carbon monoxide, or formate. However, the underlying causes of their activities, which have been attributed to a rise in local pH near the surface of the electrode, presence of atomic-scale defects, and/or residual oxygen atoms in the catalysts, etc., have not been generally agreed on. Here, we perform a study of carbon dioxide reduction on four copper catalysts from -0.45 to -1.30 V vs. reversible hydrogen electrode. The selectivities exhibited by 20 previously reported copper catalysts are also analyzed. We demonstrate that the selectivity of carbon dioxide reduction is greatly affected by the applied potentials and currents, regardless of the starting condition of copper catalysts. This study shows that optimization of the current densities at the appropriate potential windows is critical for designing highly selective copper catalysts.

The review addresses the prospects of global hydrogen energy development. Particular attention is given to the design of materials for sustainable hydrogen energy applications, including hydrogen production, purification, storage, and conversion to energy. The review highlights the key role of oxide-supported metal or alloy nanoparticles as catalysts in the hydrogen production via the conversion of natural gas or alcohols. An alternative approach is the pyrolysis of hydrocarbons giving hydrogen and carbon. The direct production of high-purity hydrogen can be performed using electrolysis or membrane catalysis. Apart from conventional hydrogen storage methods such as the compression and liquefaction, the hydrogen alloy absorption and chemical conversion to liquid carriers (ammonia and toluene cycles) are considered. Fuel cells, containing catalysts and proton-conducting membranes as the key components, are used for hydrogen energy generation. Binary platinum alloys or core – shell structures supported on carbon or oxides can be employed to facilitate the oxygen electroreduction and CO electrooxidation in low-temperature fuel cells. High conductivity and selectivity are provided by perfluorinated sulfonic acid membranes. The high cost of the latter materials dictates the development of alternative membrane materials. A crucial issue in high-temperature fuel cells is the necessity of reducing the operating temperature and ohmic losses. This problem can be solved by designing thin-film materials and replacing oxygen-conducting ceramic membranes by proton-conducting membranes. The bibliography includes 290 references.

This review focuses on monolithic 2-terminal perovskite-silicon tandem solar cells and discusses key scientific and technological challenges to address in view of an industrial implementation of this technology. The authors start by examining the different crystalline silicon (c-Si) technologies suitable for pairing with perovskites, followed by reviewing recent developments in the field of monolithic 2-terminal perovskite-silicon tandems. Factors limiting the power conversion efficiency of these tandem devices are then evaluated, before discussing pathways to achieve an efficiency of >32%, a value that small-scale devices will likely need to achieve to make tandems competitive. Aspects related to the upscaling of these device active areas to industry-relevant ones are reviewed, followed by a short discussion on module integration aspects. The review then focuses on stability issues, likely the most challenging task that will eventually determine the economic viability of this technology. The final part of this review discusses alternative monolithic perovskite-silicon tandem designs. Finally, key areas of research that should be addressed to bring this technology from the lab to the fab are highlighted.

Abstract The effect of the viscosity of room temperature molten salt (RTMS) electrolyte has been investigated on the performance of dye-sensitized solar cell (DSSC). Both the short circuit photocurrent and conversion efficiency are increased with decreasing the viscosity of RTMS as in the case of conventional electrolytes. The conversion efficiency of 2.1% observed for the cell of EMIm-F·2.3HF is the highest value reported for all the DSSC consisted of RTMS.

Abstract The lattice defects in the bulk and on the surface of the halide perovskite layer serve as trap sites and recombination centers to annihilate photogenerated carriers, determining the performance and stability of perovskite optoelectronic devices. Herein, the previously reported surface defects passivation engineering is extended to a full defects passivation strategy through stereoscopically introducing the cysteamine hydrochloride (CSA‐Cl) in the bulk and on the surface of perovskites. First‐principle density functional theory (DFT) calculations are employed to theoretically verify the multiple defects passivation effect of the CAS‐Cl on the perovskite. The perovskite layer with full defects passivation exhibits superior carrier dynamics as revealed by femtosecond transient absorption due to the reduced defect density determined by a highly sensitive photothermal deflection spectroscopy technique. Consequently, a high efficiency approaching 21% is achieved for the inverted planar perovskite solar cells (PVSCs). More importantly, the CAS‐Cl passivated PVSCs exhibit operation in air, which will be beneficial for the in situ device test for understanding the photophysics involved. This work provides a promising strategy to reduce the defects in both the perovskite bulk and surface for superior optoelectronic properties, facilitating the development of highly efficient and stable PVSCs and other optoelectronic devices.

Abstract The world record device efficiency of single‐junction solar cells based on organic–inorganic hybrid perovskites has reached 25.5%. Further improvement in device power conversion efficiency (PCE) can be achieved by either optimizing perovskite films or designing novel device structures such as perovskite/Si tandem solar cells. With the marriage of perovskite and Si solar cells, a tandem device configuration is able to achieve a PCE exceeding the Shockley–Queisser limit of single‐junction solar cells by enhancing the usage of solar spectrum. After several years of development, the highest PCE of the perovskite/Si tandem cell has reached 29.5%, which is higher than that of perovskite‐ and Si‐based single‐junction cells. Here, in this review, we will (1) first discuss the device structure and fundamental working principle of both two‐terminal (2T) and four‐terminal (4T) perovskite/Si tandem solar cells; (2) second, provide a brief overview of the advances of perovskite/Si tandem solar cells regarding the development of interconnection layer, perovskite active layer, tandem device structure, and light management strategies; (3) third, discuss the challenges and opportunities for further developing perovskite/Si tandem solar cells. This review article, on the one hand, provides a comprehensive understanding to readers on the development of perovskite/Si tandems. On the other hand, it proposes various novel applications that may bring such tandems into the market in a near future.

Abstract Symmetric and aliphatic trialkylsulfonium cation form room temperature molten salts (RTMS) with bis(trifluoromethylsulfonyl)imide (TFSI). In particular RTMS based on triethylsulfonium shows low melting point (−35 °C) and the lowest viscosity (30 mPs at 25 °C) of all the TFSI based RTMS as reported so far. The conductivity of 7.1 mS cm−1 at 25 °C is the highest of all the non-chloroaluminate RTMS based on aliphatic onium cations.

Abstract The electrocatalytic CO 2 reduction reaction (CO 2 RR) can dynamise the carbon cycle by lowering anthropogenic CO 2 emissions and sustainably producing valuable fuels and chemical feedstocks. Methanol is arguably the most desirable C 1 product of CO 2 RR, although it typically forms in negligible amounts. In our search for efficient methanol‐producing CO 2 RR catalysts, we have engineered Ag‐Zn catalysts by pulse‐depositing Zn dendrites onto Ag foams (PD‐Zn/Ag foam). By themselves, Zn and Ag cannot effectively reduce CO 2 to CH 3 OH, while their alloys produce CH 3 OH with Faradaic efficiencies of approximately 1 %. Interestingly, with nanostructuring PD‐Zn/Ag foam reduces CO 2 to CH 3 OH with Faradaic efficiency and current density values reaching as high as 10.5 % and −2.7 mA cm −2 , respectively. Control experiments and DFT calculations pinpoint strained undercoordinated Zn atoms as the active sites for CO 2 RR to CH 3 OH in a reaction pathway mediated by adsorbed CO and formaldehyde. Surprisingly, the stability of the *CHO intermediate does not influence the activity.

Abstract Radio frequency (RF) magnetron sputtering was used to deposit tungsten disulfide (WS 2 ) thin films on top of soda lime glass substrates. The deposition power of RF magnetron sputtering varied at 50, 100, 150, 200, and 250 W to investigate the impact on film characteristics and determine the optimized conditions for suitable application in thin-film solar cells. Morphological, structural, and opto-electronic properties of as-grown films were investigated and analyzed for different deposition powers. All the WS 2 films exhibited granular morphology and consisted of a rhombohedral phase with a strong preferential orientation toward the (101) crystal plane. Polycrystalline ultra-thin WS 2 films with bandgap of 2.2 eV, carrier concentration of 1.01 × 10 19 cm −3 , and resistivity of 0.135 Ω-cm were successfully achieved at RF deposition power of 200 W. The optimized WS 2 thin film was successfully incorporated as a window layer for the first time in CdTe/WS 2 solar cell. Initial investigations revealed that the newly incorporated WS 2 window layer in CdTe solar cell demonstrated photovoltaic conversion efficiency of 1.2% with V oc of 379 mV, J sc of 11.5 mA/cm 2 , and FF of 27.1%. This study paves the way for WS 2 thin film as a potential window layer to be used in thin-film solar cells.

Despite the fact that perovskite solar cells (PSCs) have a strong potential as a next-generation photovoltaic technology due to continuous efficiency improvements and the tunable properties, some important obstacles remain before industrialization is feasible. For example, the selection of low-cost or easy-to-prepare materials is essential for back-contacts and hole-transporting layers. Likewise, the choice of conductive substrates, the identification of large-scale manufacturing techniques as well as the development of appropriate aging protocols are key objectives currently under investigation by the international scientific community. This Review analyses the above aspects and highlights the critical points that currently limit the industrial production of PSCs and what strategies are emerging to make these solar cells the leaders in the photovoltaic field.

Abstract As emerging eco‐friendly alternatives to traditional Cd/Pb‐based quantum dots (QDs), InP/ZnSe(S) core/shell QDs have demonstrated huge potential in light‐emitting technologies. So far, these QDs have been rarely employed in solar energy conversion applications due to their type‐I band structure offering limited photo‐induced charge carrier separation and transfer. Here, a controllable Cu shell doping approach is reported to engineer the optoelectronic properties of InP/ZnSe core/shell QDs and realize high performance and stable solar‐driven photoelectrochemical (PEC) hydrogen evolution. As compared to the pristine InP/ZnSe QDs, the Cu‐doped core/shell QDs exhibit enhances the photo‐induced electron transfer rate due to the capture of photo‐generated holes via Cu impurity states in the shell, leading to improved photocurrent density and long‐term durability in as‐fabricated InP/ZnSe:Cu QDs‐PEC devices under standard one sun illumination. The results indicate that the doping of Cu in the shell has a preeminent effect on the optoelectronic properties of the core/shell QDs and may open up new avenues to tailor eco‐friendly core/shell QDs for high performance and stable solar energy conversion.

Metal halide perovskites have demonstrated rich photophysics and remarkable potential in photovoltaic and electroluminescent devices. However, the photoactivity of perovskite semiconductors in chemical processes remains relatively unexplored. Here, a general approach toward the synthesis of luminescent perovskite-polymer nanocomposites is reported, whereby perovskite nanocrystals are used as photoinitiators in the polymerization of vinyl monomers. The white-light illumination of a perovskite-monomer mixture triggers a free-radical chain-growth polymerization process, giving rise to high molecular weight polymers of ≈200 kDa. The in situ growth of polymer chains from the perovskite crystal surface allows the formation of individually dispersed nanocrystal cores within an encapsulating polymer matrix, and leads to a significant threefold enhancement in photoluminescence quantum yield. This photoluminescence enhancement is attributed to the spatial separation of the perovskite nanocrystals and hence the deactivation of energy transfer to dark crystals. The resulting perovskite-polymer nanocomposites exhibit excellent stability against moisture and are shown to be useful as functional downconversion phosphor films for luminescent displays and lighting.

Abstract Two‐dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub‐bandgap photodetection by employing excitonic effects in 2D Ruddlesden–Popper‐type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two‐photon absorption (D‐2PA) coefficients as giant as 0.2–0.64 cm MW − 1 . 2D RPP‐based sub‐bandgap photodetectors show excellent detection performance in the near‐infrared (NIR): a two‐photon‐generated current responsivity up to 1.2 × 10 4 cm 2 W −2 s −1 , two orders of magnitude greater than InAsSbP‐pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered‐RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D‐2PA coefficients (β [001] /β [011] = 2.4, 70% larger than the ratios reported for zinc‐blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 can be utilized for three‐photon photodetection in the NIR region.

Electrochemical reduction of CO (2) to value-added chemicals and fuels is a promising strategy to sustain pressing renewable energy demands and to address climate change issues. Direct observation of reaction intermediates during the CO (2) reduction reaction will contribute to mechanistic understandings and thus promote the design of catalysts with the desired activity, selectivity, and stability. Herein, we combined in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy and ab initio molecular dynamics calculations to investigate the CORR process on Cu single-crystal surfaces in various electrolytes. Competing redox pathways and coexistent intermediates of CO adsorption (*CO atop and *CO bridge ), dimerization (protonated dimer *HOCCOH and its dehydrated *CCO), oxidation (*CO 2 − and *CO 3 2− ), and hydrogenation (*CHO), as well as Cu-O ad /Cu-OH ad species at Cu-electrolyte interfaces, were simultaneously identified using in situ spectroscopy and further confirmed with isotope-labeling experiments. With AIMD simulations, we report accurate vibrational frequency assignments of these intermediates based on the calculated vibrational density of states and reveal the corresponding species in the electrochemical CO redox landscape on Cu surfaces. Our findings provide direct insights into key intermediates during the CO (2) RR and offer a full-spectroscopic tool (40–4,000 cm −1 ) for future mechanistic studies.

Abstract We present n ‐type bifacial solar cells with a rear interfacial SiO x / n + :poly‐Si passivating contact (‘monoPoly’ cells) where the interfacial oxide and n + :poly‐Si layers are fabricated using an industrial inline plasma‐enhanced chemical vapor deposition (PECVD) tool. We demonstrate outstanding passivation quality with dark saturation current density ( J 0 ) values of approximately 3 fA/cm 2 and implied open‐circuit voltage ( iV oc ) of 730 mV at 1‐sun conditions after firing in an industrial belt furnace. Using a simple solar cell process flow that can be easily adapted for mass production, a peak cell efficiency of 22.8% with a cell open circuit voltage ( V oc ) of 696 mV is achieved on large‐area, screen‐printed, Czochralski‐silicon ( Cz‐ Si) solar cells using commercial fire‐through metal pastes.

The power conversion efficiency of organic photovoltaic cells depends crucially on the morphology of their donor–acceptor heterostructure. Although tremendous progress has been made to develop new materials that better cover the solar spectrum, this heterostructure is still formed by a primitive spontaneous demixing that is rather sensitive to processing and hence difficult to realize consistently over large areas. Here we report that the desired interpenetrating heterostructure with built-in phase contiguity can be fabricated by acceptor doping into a lightly crosslinked polymer donor network. The resultant nanotemplated network is highly reproducible and resilient to phase coarsening. For the regioregular poly(3-hexylthiophene):phenyl-C61-butyrate methyl ester donor–acceptor model system, we obtained 20% improvement in power conversion efficiency over conventional demixed biblend devices. We reached very high internal quantum efficiencies of up to 0.9 electron per photon at zero bias, over an unprecedentedly wide composition space. Detailed analysis of the power conversion, power absorbed and internal quantum efficiency landscapes reveals the separate contributions of optical interference and donor–acceptor morphology effects. The conversion efficiency of organic solar cells depends on the shape of the interface between their donor and acceptor components. Liuet al. demonstrate a scalable method using crosslinked polymer networks to fabricate the finely interpenetrating structures needed to achieve near-perfect internal quantum efficiency.

As synthetic antioxidants that are widely used in foods are known to cause detrimental health effects, studies on natural additives as potential antioxidants are becoming increasingly important. In this work, the total phenolic content (TPC) and antioxidant activity of Ficus carica Linn latex from 18 cultivars were investigated. The TPC of latex was calculated using the Folin-Ciocalteu assay. 1,1-Diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and ferric ion reducing antioxidant power (FRAP) were used for antioxidant activity assessment. The bioactive compounds from F. carica latex were extracted via maceration and ultrasound-assisted extraction (UAE) with 75% ethanol as solvent. Under the same extraction conditions, the latex of cultivar 'White Genoa' showed the highest antioxidant activity of 65.91% ± 1.73% and 61.07% ± 1.65% in DPPH, 98.96% ± 1.06% and 83.04% ± 2.16% in ABTS, and 27.08 ± 0.34 and 24.94 ± 0.84 mg TE/g latex in FRAP assay via maceration and UAE, respectively. The TPC of 'White Genoa' was 315.26 ± 6.14 and 298.52 ± 9.20 µg GAE/mL via the two extraction methods, respectively. The overall results of this work showed that F. carica latex is a potential natural source of antioxidants. This finding is useful for further advancements in the fields of food supplements, food additives and drug synthesis in the future.