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Photoelectrochemical water splitting is one of the sustainable routes to renewable hydrogen production. One of the challenges to deploying photoelectrochemical (PEC) based electrolyzers is the difficulty in the effective capture of solar radiation as the illumination angle changes throughout the day. Herein, we demonstrate a method for the angle-independent capture of solar irradiation by using transparent 3 dimensional (3D) lattice structures as the photoanode in PEC water splitting. The transparent 3D lattice structures were fabricated by 3D printing a silica sol–gel followed by aging and sintering. These transparent 3D lattice structures were coated with a conductive indium tin oxide (ITO) thin film and a Mo-doped BiVO4 photoanode thin film by dip coating. The sheet resistance of the conductive lattice structures can reach as low as 340 Ohms per sq for ∼82% optical transmission. The 3D lattice structures furnished large volumetric current densities of 1.39 mA cm−3 which is about 2.4 times higher than a flat glass substrate (0.58 mA cm−3) at 1.23 V and 1.5 G illumination. Further, the 3D lattice structures showed no significant loss in performance due to a change in the angle of illumination, whereas the performance of the flat glass substrate was significantly affected. This work opens a new paradigm for more effective capture of solar radiation that will increase the solar to energy conversion efficiency. Ministry of Education (MOE) National Research Foundation (NRF) Published version This work was supported by the Singapore Ministry of Education (MOE) Tier 2 grant (MOE T2EP50120-00081) and Tier 1 grant (2020-T1-001-147 (RG64/20)). This research was also supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Campus of Research Excellence and Technological Enterprise (CREATE) programme

The discovery of more efficient and stable catalysts for oxygen evolution reaction (OER) is vital in improving the efficiency of renewable energy generation devices. Given the large numbers of possible binary and ternary metal oxide OER catalysts, high-throughput methods are necessary to accelerate the rate of discovery. Herein, Mn-based spinel oxide, Fe10 Co40 Mn50 O, is identified for the first time using high-throughput methods demonstrating remarkable catalytic activity (overpotential of 310 mV on fluorine-doped tin oxide (FTO) substrate and 237 mV on Ni foam at 10 mA cm-2 ). Using a combination of soft X-ray absorption spectroscopy and electrochemical measurements, the high catalytic activity is attributed to 1) the formation of multiple active sites in different geometric sites, tetrahedral and octahedral sites; and 2) the formation of active oxyhydroxide phase due to the strong interaction of Co2+ and Fe3+ . Structural and surface characterizations after OER show preservation of Fe10 Co40 Mn50 O surface structure highlighting its durability against irreversible redox damage on the catalytic surface. This work demonstrates the use of a high-throughput approach for the rapid identification of a new catalyst, provides a deeper understanding of catalyst design, and addresses the urgent need for a better and stable catalyst to target greener fuel. Ministry of Education (MOE) National Research Foundation (NRF) This research was partially supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus of Research Excellence and Technological Enterprise (CREATE) program. This work was partially supported by the Singapore Ministry of Education (MOE2019-T2-1-163), Tier 1 grant (2020-T1-001-147(RG64/20)), Tier 2 grant (MOE T2EP50120-00081), Singapore National Research Foundation-National University of Singapore Postdoc Fellowship, and NUS core SupportC-380-003-003-001.

Tactile technologies that can identify human body features are valuable in clinical diagnosis and human-machine interactions. Previously, cutting-edge tactile platforms have been able to identify structured non-living objects; however, identification of human body features remains challenging mainly because of the irregular contour and heterogeneous spatial distribution of softness. Here, freestanding and scalable tactile platforms of force-softness bimodal sensor arrays are developed, enabling tactile gloves to identify body features using machine-learning methods. The bimodal sensors are engineered by adding a protrusion on a piezoresistive pressure sensor, endowing the resistance signals with combined information of pressure and the softness of samples. The simple design enables 112 bimodal sensors to be integrated into a thin, conformal, and stretchable tactile glove, allowing the tactile information to be digitalized while hand skills are performed on the human body. The tactile glove shows high accuracy (98%) in identifying four body features of a real person, and four organ models (healthy and pathological) inside an abdominal simulator, demonstrating identification of body features of the bimodal tactile platforms and showing their potential use in future healthcare and robotics. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version The project was supported by Singapore Ministry of Education (MOE2019-T2-2-022) and the National Research Foundation, Singapore (NRF) under NRF’s Medium Sized Center: Singapore Hybrid-Integrated Next-Generation μ-Electronics (SHINE) Center funding program.

Semitransparent solar cells are able to capitalize on land scarcity in urban environments by co-opting windows and glass structures as power generators, thereby expanding the capacity of photovoltaics to meet energy needs. To be successful, devices must be efficient, possess good visual transparency, long-term stability, and low cost. Copper zinc tin sulfide is a promising thin-film material that consists of earth-abundant elements. For optical transparency, the usual molybdenum back contact is replaced with a transparent conducting oxide (TCO). However, due to subsequent high-temperature annealing, the TCO degrades, losing conductivity, or forms a poor interface with CZTS. Lower temperatures mitigate this issue but hinder grain growth in CZTS films. Herein, cadmium substitution and silver and sodium doping are used to aid grain growth and improve film quality at lower annealing temperatures. Thin molybdenum is sputtered on TCO to help improve the interface transition postannealing by conversion to MoS2. Rapid thermal processing is used to minimize high-temperature exposure time to preserve the TCO. With these methods, a semitransparent device with a front illumination efficiency of 2.96% is demonstrated. Nanyang Technological University National Research Foundation (NRF) The authors acknowledgethe funding support from NTU-COE Industry Research Collaboration Award 2015 and CREATE Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), which is supported by the National Research Foundation, Prime Minister’s Office, Singapore.

Cation disorder which arises from the size and chemical environment similarity of Cu and Zn is the limiting factor in Cu2ZnSnSxSe4−x (CZTSSe) performance. Cation substitution is one effective way to solve this issue, however, the most commonly reported substitutes, Ag and Cd, are not ideal as they detract from the earth-abundant and non-toxic motivation of CZTSSe. Mn is a promising candidate in comparison with other candidates (e.g. Fe, Ni or Co), because of its oxidation state stability and larger ionic size mismatch with Cu. In this study, Cu2MnxZn1−xSn(S,Se)4 (CMZTSSe) thin film solar cells were prepared by chemical spray pyrolysis and a subsequent selenization process. We study the influence of Mn substitution on the morphological, structural, optical, electrical and device properties. A distinct phase transformation from CZTSSe kesterite to C(M,Z)TSSe stannite is observed at 20% Mn substitution. A high amount of Mn substitution (x ≥ 0.6) is shown to increase the carrier density significantly which introduces more defects and non-radiative carrier recombination as shown by quenched photoluminescence intensity. Consequently, reduction in device performance is observed for these samples. The highest power conversion efficiency is achieved at x ≈ 0.05 with η = 7.59%, Voc = 0.43 V, Jsc = 28.9 mA cm−2 and FF = 61.03%. The improved open circuit voltage (Voc) and fill factor (FF) are attributed to the improved shunt resistance and carrier transport due to low defect density especially at the CdS/CMZTSSe interface. Finally, based on our electrical characterization, a few suggestions to improve the efficiency are proposed. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

Solution‐based thin‐film semiconductors offer a promising path for the mass production of low‐cost solar cells prepared at low temperatures. Thin‐film Cu‐based chalcogenides such as Cu(In,Ga)(S,Se)2 (CIGSSe) and Cu2ZnSn(S,Se)4 (CZTSSe) hold great promise and have been regarded as viable candidates because of the abundance of their constituent elements and environmentally nontoxic nature. This Review summarizes the recent progress in solution‐processed Cu chalcogenides (CuInSe2, Cu(In,Ga)(S,Se)2, Cu2ZnSnS4, Cu2ZnSn(S,Se)4) for thin‐film solar cells, with emphasis on the precursor solution deposited by spray pyrolysis and spin coating. The general aspects, current status, and recent research highlights are introduced and analyzed in detail. Finally, the challenges and future prospects of these solar cells are also discussed. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

To control the process of cation exchange (CE) in a multielemental system, a detailed understanding of structural changes at the microscopic level is imperative. However, the synthesis of a multielemental system has so far relied on the CE phenomenon of a binary system, which does not necessarily extend to the higher-order systems. Here, direct experimental evidence supported by theoretical calculations reveals a growth model of binary Cu–S to ternary Cu–Sn–S to quaternary Cu–Zn–Sn–S, which shows that cations preferentially diffuse along a specific lattice plane with the preservation of sulfuric anionic framework. In addition, we also discover that, unlike the commonly accepted structure (P63mc), the metastable crystal structure of Cu–Zn–Sn–S phase possesses fixed Sn occupancy sites. By revealing the preferential nature of cations diffusion and growth mechanism, our work provides insight into controlling the stoichiometry and phase purity of novel multielemental materials. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version We acknowledge financial support from National Research Foundation (NRF), Singapore, through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) and Nanomaterials for Energy and Water Management (SHARE NEW) CREATE program. L.H.W. thanks the funding support from Singapore Ministry of Education, Tier 2 (2016-T2-1-030). S.L. acknowledges the funding support from Singapore Ministry of Education Tier 1 (107/15). H.Z. thanks the funding support from U.S. DOE BES Materials Sciences and Engineering Division Under Contract No. KC22ZH. X.Y.L. thanks the funding support from Singapore Ministry of Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1- 043) grants.. The work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Fiona Doyle for lending us her synthetic laboratory in University of California Berkeley (UCB), Song Chengyu and Karen Bustilo for their help and assistance on TEM, and Matthew P. Sherburne for nanoparticle growth discussion.

Potassium (K) post-treatment on CIGSSe has been shown to yield the highest efficiency reported to date. However, very little is known on the effect of K doping in CZTSSe and the mechanism behind the efficiency improvement. Here we reveal the mechanism by which K enhances the charge separation in CZTSSe. We show that K accumulates at the CdS/CZTSSe, passivating the recombination at the front interface and improving carrier collection. K is also found to accumulate at the CZTSSe/Mo interface and facilitates the diffusion of Cd into the absorber which affects the morphology and grain growth of CZTSSe. As revealed by the C–V, external quantum efficiency, and color J–V test, K doping significantly increases the carrier density, improves carrier collection, and passivates the front interface and grain boundaries, leading to the enhancement of Voc and Jsc. The average power conversion efficiency has been promoted from 5% to above 7%, and the best 7.78% efficiency has been achieved for the 1.5 mol % K-doped CZTSS...

Inorganic hole-transport layers (HTLs) are widely investigated in perovskite solar cells (PSCs) due to their superior stability compared to the organic HTLs. However, in p–i–n architecture when these inorganic HTLs are deposited before the perovskite, it forms a suboptimal interface quality for the crystallization of perovskite, which reduces device stability, causes recombination, and limits the power conversion efficiency of the device. The incorporation of an appropriate functional group such as sulfur-terminated surface on the HTL can enhance the interface quality due to its interaction with perovskite during the crystallization process. In this work, a bifunctional Al-doped CuS film is wet-deposited as HTL in p–i–n architecture PSC, which besides acting as an HTL also improves the crystallization of perovskite at the interface. Urbach energy and light intensity versus open-circuit voltage characterization suggest the formation of a better-quality interface in the sulfide HTL–perovskite heterojunction. The degradation behavior of the sulfide-HTL-based perovskite devices is studied, where it can be observed that after 2 weeks of storage in a controlled environment, the devices retain close to 95% of their initial efficiency. National Research Foundation (NRF) Accepted version This research was funded by the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program. The authors would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their XPS/UPS facilities. They would also like to thank Dr. Gonzalo Carrasco from Earth Observatory of Singapore, NTU, for his assistance in carrying out ICPMS.

Recently, considerable attention in the development of Cu2ZnSnS4 (CZTS)-based thin-film solar cells has been given to the reduction of antisite defects via cation substitution. In this Letter, we report the substitution of copper atoms by silver, incorporated into the crystal lattice through a solution processable method. We observe an increase in open-circuit voltage (VOC) by 50 mV and an accompanying rise in device efficiency from 4.9% to 7.2%. The incorporation of Ag is found to improve the grain size, enhance the depletion width of the pn-junction, and reduce the concentration of antisite defect states. This work demonstrates the promising role of Ag in reducing the VOC deficit of Cu-kesterite thin-film solar cells.

Cation substitution has been extensively used to improve the fundamental optoelectronic properties and the photovoltaic performance of kesterite solar cells, and some of the most promising results have been obtained by substituting zinc with cadmium. Structurally, the positive effects of Cd have been attributed to the expected increase in the formation energy of defects such as CuZn + ZnCu due to the larger ionic radius of Cd2+ as compared to Zn2+. However, ab initio calculations using density functional theory (DFT) showed similar formation energies for CuZn + ZnCu in Cu2ZnSnS4 and CuCd + CdCu in Cu2CdSnS4. Further, in this report, it is shown that Cd does not directly substitute the zinc lattice sites (2d Wyckoff positions) in the Cu2ZnSnS4 structure, but rather, a two-way cation restructuring due to the continuous transformation of the structure from kesterite to stannite leads to Cu replacing Zn, and Cd occupying the Cu sites (2a Wyckoff positions) in the partially Cd-substituted Cu2Zn1−xCdxSnS4. Hence, the structural reasons for the beneficial effects of Cd need to be reinterpreted. Here, using computational model based on cluster expansion (fitted on DFT data), Monte-Carlo simulations, and differential scanning calorimetry, it is shown that Cu2CdSnS4 has less structural disorder than Cu2ZnSnS4 even if the thermodynamic point defect formation energy calculated using diluted point-defect models for disorder-inducing CuZn + ZnCu and CuCd + CdCu defects in these two materials is predicted to be similar. This difference in the structural disorder is due to a sharp order-disorder transformation in Cu2ZnSnS4 at about 530 K, and a continuous order-disorder transformation in Cu2CdSnS4 throughout the range of processing temperatures. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version S. H., J. M. R. T., and L. W. acknowledge nancial support from National Research Foundation (NRF), Singapore, through the Nanomaterials for Energy and Water Management (SHARE NEW) CREATE programme, MOE Tier 2 (MOE2016-T2-1- 030S). W. C., G.-M. R., and G. H. acknowledge support from the F.R.S.-FNRS. W. C., G.-M. R., and G. H. acknowledge access to various computational resources: the Tier-1 supercomputer of the F´ed´eration Wallonie-Bruxelles funded by the Walloon Region (grant agreement No. 1117545), and all the facilities provided by the Universit´e catholique de Louvain (CISM/UCL) and by the Consortium des ´Equipements de Calcul Intensif en F´ed´eration Wallonie Bruxelles (C´ECI). M. G. and V. I acknowledge support by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Spanish Ministry of Science, Innovation and Universities under the IGNITE (ENE2017-87671-C3-1-R), and by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the “Generalitat de Catalunya” (Ref. 2017 SGR 862).

A non-ideal buffer-absorber interface due to interface recombination is one of the limitations in Cu2ZnSnS4 (CZTS) solar cells. An absorber gradient using partial cation-substitution is one possible solution to this issue by reducing interface recombination sites and improving band alignment. Partial substitution of Zn with Mn is a potentially attractive concept owing to its abundance as well as similar size and electronic properties compared to Zn. In this study, Cu2MnxZn1-xSnS4 (CMZTS) thin film were fabricated by a sol-gel spin coating technique to understand the effect of Mn on the device performance. X-ray diffraction (XRD) and Raman analysis confirm Mn substitution for Zn in the crystal lattice of Cu2ZnSnS4 (CZTS) with no secondary phases detected for x ≤ 0.4. Enhancement of short circuit current (Jsc) in Cu2Mn0.15Zn0.85SnS4 has led to improved efficiency in comparison with CZTS. Since external quantum efficiency (EQE) indicates the short wavelength collection as the main reason for the improved Jsc, we designed a double layered structure of CZTS as bottom layer and CM0.15Z0.85TS as top layer. With this structure, we obtained our best performance cell with power conversion efficiency (PCE) of 5.73% and Jsc of 17.86 mA/cm2. The improved Jsc is attributed to increased depletion width and higher activation energy of the limiting recombination mechanism as shown by capacitance-voltage and temperature dependent current-voltage measurements. Our study concludes that CM0.15Z0.85TS top layer improves the interface quality of the p-n junction and may be an alternative method to improve the performance of CZTS solar cells. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

Controlling sub-10 nm ligament sizes and open-shell structure in nanoporous gold (NPG) to achieve strained lattice is critical in enhancing catalytic activity, but it remains a challenge due to poor control of reaction kinetics in conventional dealloying approach. Herein, a ligament size-controlled synthesis of open-shell NPG bowls (NPGB) through hetero-epitaxial growth of NPGB on AgCl is reported. The ligament size in NPGB is controlled from 6 to 46 nm by varying the hydroquinone to HAuCl4 ratio. The Williamson-Hall analysis demonstrates a higher lattice strain in smaller ligament size. In particular, NPGB with 6 nm (NPGB 6) ligament size possess the highest strain of 15.4 × 10(-3) , which is nearly twice of conventional 2D NPG sheets (≈8.8 × 10(-3) ). The presence of high surface energy facets in NPGBs is also envisaged. The best electrocatalytic activity toward methanol oxidation is observed in NPGB 6 (27.8 μA μg(-1) ), which is ≈9-fold and 3-fold higher than 8 nm solid Au nanoparticles, and conventional NPG sheets. The excellent catalytic activity in NPGB 6 is attributed to the open-shell structure, lattice strain, and higher electro-active surface area, allowing efficient exposure of catalytic active sites to facilitate the methanol oxidation. The results offer a potential strategy for designing next generation electrocatalysts.

In the last few decades, there has been much emphasis placed on the development of solar harvesting materials. Multi-component (Cu-(M)-Sn-S) materials which is made up of naturally abundant components are highly promising as solar harvesting materials. However to realize an efficient solar harvesting device from these materials, several challenges such as: producing single phase material, controlling appropriate carrier concentration with good carrier transport properties, suitable bandgap, matching band energy level, doping control and minimal intrinsic defect states need to be overcome. These fundamental properties of the materials can be controlled if there is understanding of the material formation mechanism, kinetics and thermodynamics. The objective of this thesis is to investigate the synthesis of pure CMTS; Cu2-M-Sn-S4 (M=Zn, Fe, Mn, Co), based semiconductors as potential solar harvesting materials for both solar cell and solar fuel applications. From these studies, crucial growth steps have been pinpointed and conflicting growth kinetics have been solved via a proposed reaction pathway. In the initial stage of the study, the synthesis of Cu2ZnSnS4 (CZTS) nanoparticles was used as a model system to establish general reaction conditions for the synthesis of CMTS nanoparticles. Three key research foundations were laid. Firstly, Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time to be a sensitive characterization tool for unambiguous characterization of phases present in as-synthesized nanoparticles. This resolves the inability to differentiate mixed compositional phase (e.g., CZTS, CTS and ZnS) by powder X-ray diffraction (XRD) and conventional Raman spectroscopy. Secondly, the growth of CZTS is established to proceed strictly via formation of Cu2-xS nuclei, followed by diffusion of Sn4+ to form Cu3SnS4 and lastly diffusion of Zn2+ to form Cu2ZnSnS4 phase. Lastly, the reaction kinetics of Sn4+ ion were identified to be the limiting factor for CZTS nanoparticles’ purity. It is demonstrated that by carefully controlling the reactivity of organometallic ligand complex, it is possible to achieve balance in the reaction kinetics allowing one to synthesize CZTS phase without any observable binary/ternary phase from XRD and SERS. Following on, using the knowledge gained from the first study, a model study is presented to highlight the complexity of phase transition via cation exchange to form multi-component sulfide nanomaterials (Cu29S16 to Cu4SnS4 to Cu2ZnSnS4). Using transmission electron microscopy (TEM) and XRD, direct evidence indicates that the phase transformation of binary Cu29S16 to ternary Cu4SnS4 proceeds via sequential three steps reaction processes involving two intermediate phases (P121n1 Cu31S16 and P121c1 Cu32S16) while preserving the anionic framework of Cu29S16. In addition, the ex-situ growth study of CZTS from CTS reveals a negative domino cationic exchange growth effect resulting in precipitation of parasitic phases. Furthermore, the use of HR-HAADF imaging with Z-contrast capability enables the visualization of Sn fixed occupancy sites in our as-synthesized quaternary CZTS nanoparticles. This confirms that the crystal does not follow the commonly accepted P63mc. Lastly, using the knowledge gained from the first two studies, Cu2-M-Sn-S4 (M= Fe, Mn, Co) nanoparticles synthesis were carried out based on reaction conditions obtained from the initial study to identify conflicting growth mechanism. It is revealed that Cu2(M)SnS4 nanoparticles proceed via two different reaction pathways. Growth of Cu2(Mn)SnS4 nanoparticles is similar to Cu2(Zn)SnS4 nanoparticles where ternary Cu-Sn-S phase is the intermediate phase. On the other hand, the growth of Cu2(Co)SnS4 and Cu2(Fe)SnS4 proceeds via an intermediate ternary Cu-(M)-S phase. XRD characterization of as-synthesized Cu2(M)SnS4 nanoparticles revealed CZTS and CMTS to crystallize in hexagonal crystal structure while CFTS and CCTS crystallize in zinc blende crystal structure. In addition, as a proof of concept, the as-synthesized chemically treated CZTS and CMTS nanoparticles are drip-casted to form thin film solar cells. We demonstrated an encouraging power conversion efficiency of 1.16% (CZTS) and 0.078% (CMTS). In the three hypothesis proposed for this thesis, the first hypothesis on SERS having the capability to characterize nanomaterials unambiguously is proven to be true. For the second hypothesis that the growth of CZTS nanoparticle proceeds via nucleation of Cu-S phase, followed by formation of ternary Cu-Sn-S phase is proven to be accurate. Lastly, for the third hypothesis that the growth of Cu2(M)SnS4 (M= Mn, Co and Fe) nanoparticles follows the growth of CZTS nanoparticles, Cu2MnSnS4 is proven to follow the growth of CZTS nanoparticles while Cu2(M)SnS4 (M= Co and Fe) show interesting growth phenomena which prompt the need for further investigations. Doctor of Philosophy (IGS)

To alleviate the limitations of pure sulfide Cu2ZnSnS4 (CZTS) thin film, such as band gaps adjustment, antisite defects, secondary phase and microstructure, Cadmium is introduced into CZTS thin film to replace Zn partially to form Cu2Zn1−xCdxSnS4 (CZCTS) thin film by low-cost sol–gel method. It is demonstrated that the band gaps and crystal structure of CZCTS thin films are affected by the change in Zn/Cd ratio. In addition, the ZnS secondary phase can be decreased and the grain sizes can be improved to some degree by partial replacement of Zn with Cd in CZCTS thin film. The power conversion efficiency of CZTS solar cell device is enhanced significantly from 5.30% to 9.24% (active area efficiency 9.82%) with appropriate ratio of Zn/Cd. The variation of device parameter as a function of Zn/Cd ratio may be attributed to the change in electronic structure of the bulk CZCTS thin film (i.e., phase change from kesterite to stannite), which in turn affects the band alignment at the CZCTS/buffer interface and the charge separation at this interface.