Design of deterministic light-trapping structures for thin silicon heterojunction solar cells.

We optically designed and investigated two deterministic light-trapping concepts named "Hutong" (wafer thickness dependent, patch-like arrangement of "V" grooves with alternating orientations) and "VOSTBAT" (one directional "V" grooves at the front and saw-tooth like structures at the back) for the application in emerging thin silicon heterojunction (SHJ) solar cells. Calculated photocurrent density (Jph) (by weighting the spectrally resolved absorptance with AM1.5g spectrum and integrating over the wavelength) showed that both the Hutong and VOSTBAT structures exceed the Lambertian reference and achieved Jph of 41.72 mA/cm2 and 41.86 mA/cm2, respectively, on 60 µm thin wafers in the case of directional, normal incidence.

A boron-decorated melon-based carbon nitride as a metal-free photocatalyst for N2 fixation: a DFT study.

On the basis of the electron "acceptance-donation" concept, a boron decorated melon-based carbon nitride (CN) is studied as a metal-free photocatalyst to efficiently reduce N2 to NH3 under visible light irradiation. The results revealed that a boron-interstitial (Bint)-decorated melon-based CN has an outstanding N2 reduction capacity through the enzymatic mechanism with a rather low overpotential (0.32 V). The excellent efficiency and selectivity of Bint-decorated melon-based CN in N2 reduction reaction (NRR) are attributed to the concentrated spin polarization on the B atom, the significant enhancement of visible and infrared light absorption, and the effective inhibition of the competitive hydrogen evolution reaction (HER). Importantly, B-doped melon-based CN has been successfully synthesized in the experiments, so obtaining Bint-decorated melon is promising, while proton transfer from the -NH2 group in CN to the B atom surely will affect the functionality of the catalyst through deactivation of the N2 adsorption site. Our study provides a novel single atom metal-free photocatalyst with high efficiency for NRR, which is conducive to the sustainable synthesis of ammonia.

How does the B,F-monodoping and B/F-codoping affect the photocatalytic water-splitting performance of g-C3N4?

By means of density functional theory (DFT) computations, we investigated the electronic and optical properties of B,F-monodoped and B/F-codoped graphitic carbon nitride (g-C3N4) to explore the doping effects on the photocatalytic performance of g-C3N4. It is found that F atom addition plays a key role in stabilizing the surface of g-C3N4 and facilitating B atom substitution into g-C3N4. Among the different doping strategies, only B/F-codoping does not have localized states in the midgap, which act as recombination centers for the photogenerated electron-hole pairs. All the doping strategies in this study can improve the utilization ratio of visible light for the g-C3N4 photocatalyst. Considering the relationship of overpotential of water redox reaction over g-C3N4 and band edge positions with respect to the water redox potentials, only the F-doped and B/F-codoped g-C3N4 satisfy the criterion for overall water splitting. In other words, the B/F-codoping strategy not only meets the demands of no recombination centers and enhances the visible light utilization ratio, but also satisfies the need of overpotential. Thus, B/F-codoped g-C3N4 is expected to be a promising photocatalyst for overall water splitting under visible light.

N,F-Monodoping and N/F-codoping effects on the electronic structures and optical performances of Zn2GeO4.

First-principles density-functional calculation has been performed to investigate the synergistic effects of N and F doping on the photocatalytic properties of Zn2GeO4. Our results indicate that the presence of F facilitates the introduction of N by reducing the formation energy significantly. As N and F is codoped into Zn2GeO4, the mobility of the charge carriers is more rapid due to the dispersive levels above the valence band. And with the narrowed band gap the optical absorption spectrum red-shifts into the ideal visible-light region. Thus, we propose that the codoping of N and F can be a promising strategy to promote the photocatalytic performances of Zn2GeO4 under visible light.

Why the photocatalytic activity of Mo-doped BiVO4 is enhanced: a comprehensive density functional study.

To explore the origin of the enhanced photocatalytic activity of Mo-doped monoclinic BiVO4, variations of the structures and the electronic properties, as well as the adsorption behavior of water on the (010) surface, introduced by the Mo dopant have been investigated by means of density functional theory computations. For the bulk phase, Mo atoms prefer to substitute the V atoms, which can effectively accelerate the separation of carriers. For the (010) surface, Mo atoms prefer to substitute the Bi atoms at the outermost layer. Mo doping on the surface can result in surface oxygen quasi-vacancies and enhance the exposure of surface Bi atoms, which is confirmed to improve the adsorption of water molecules. Our results demonstrate that the enhanced photocatalytic activity of Mo-doped monoclinic BiVO4 is derived from the facilitated separation of photoinduced carriers and introduced surface oxygen quasi-vacancies.

Ligand Homogenized Br-I Wide-Bandgap Perovskites for Efficient NiOx-Based Inverted Semitransparent and Tandem Solar Cells.

Phase heterogeneity of bromine-iodine (Br-I) mixed wide-bandgap (WBG) perovskites has detrimental effects on solar cell performance and stability. Here, we report a heterointerface anchoring strategy to homogenize the Br-I distribution and mitigate the segregation of Br-rich WBG-perovskite phases. We find that methoxy-substituted phenyl ethylammonium (x-MeOPEA+) ligands not only contribute to the crystal growth with vertical orientation but also promote halide homogenization and defect passivation near the buried perovskite/hole transport layer (HTL) interface as well as reduce trap-mediated recombination. Based on improvements in WBG-perovskite homogeneity and heterointerface contacts, NiOx-based opaque WBG-perovskite solar cells (WBG-PSCs) achieved impressive open-circuit voltage (Voc) and fill factor (FF) values of 1.22 V and 83%, respectively. Moreover, semitransparent WBG-PSCs exhibit a PCE of 18.5% (15.4% for the IZO front side) and a high FF of 80.7% (79.4% for the IZO front side) for a designated illumination area (da) of 0.12 cm2. Such a strategy further enables 24.3%-efficient two-terminal perovskite/silicon (double-polished) tandem solar cells (da of 1.159 cm2) with a high Voc of over 1.90 V. The tandem devices also show high operational stability over 1000 h during T90 lifetime measurements.

Probing the smallest molecular model of MoS2 catalyst: S2 units in the MoS(-/0 (n = 1-5) clusters.

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) calculations are carried out to investigate the electronic and structural properties of a series of monomolybdenum sulfide clusters, MoSn(-/0) (n = 1-5). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). We found that the additional sulfur atoms have a tendency to successively occupy the terminal sites in the sequential sulfidation until the Mo reaches its maximum oxidation sate of +6. After that, the polysulfide ligands (viz., S2 and S3) emerge in the MoS4 and MoS5(-/0) clusters. The MoS4 (C2, (1)A) is predicted to be the ground state and may be used as a neutral model for the sulfur-rich edge sites of the fresh MoS2 catalysts. Molecular orbital analyses are performed to analyze the chemical bonding in the monomolybdenum sulfide clusters and to elucidate their electronic and structural evolution.

Deposition of (WO3)3 nanoclusters on the MgO(001) surface: a possible way to identify the charge states of the defect centers.

Periodic density functional theory calculations have been performed to study the most stable structure of the (WO(3))(3) nanocluster deposited on the MgO(001) surface with three kinds of F(S) centers (F(S)(0), F(S)(+), and F(S)(2+)). Our results indicate that the configuration of (WO(3))(3) cluster, including the cyclic conformation and the heights of three W atoms, and the oxidation states are sensitive to the charge state of the F(S) center. It is interesting that the electron-riched F(S) (0) vacancy on the MgO(001) surface can act as a promoting site to enhance the W-W interaction and the W(3)O(3) cyclic conformation is maintained, while the skeleton of cluster becomes flexible when (WO(3))(3) is adsorbed on the electron-deficient vacancy (F(S)(+) and F(S)(2+)). Accordingly, three F(S)-centers exhibit different arrangements of X-ray photoelectron spectra, the scanning tunneling microscopy images, and the vibrational spectra after depositing (WO(3))(3) cluster. Present results reveal that the (WO(3))(3) cluster may be used as a probe to identify the different F(S) centers on the MgO(001) surface.

Insights into the Si─H Bonding Configuration at the Amorphous/Crystalline Silicon Interface of Silicon Heterojunction Solar Cells by Raman and FTIR Spectroscopy.

In silicon heterojunction solar cell technology, thin layers of hydrogenated amorphous silicon (a-Si:H) are applied as passivating contacts to the crystalline silicon (c-Si) wafer. Thus, the properties of the a-Si:H is crucial for the performance of the solar cells. One important property of a-Si:H is its microstructure which can be characterized by the microstructure parameter R based on Si─H bond stretching vibrations. A common method to determine R is Fourier transform infrared (FTIR) absorption measurement which, however, is difficult to perform on solar cells for various reasons like the use of textured Si wafers and the presence of conducting oxide contact layers. Here, it is demonstrated that Raman spectroscopy is suitable to determine the microstructure of bulk a-Si:H layers of 10 nm or less on textured c-Si underneath indium tin oxide as conducting oxide. A detailed comparison of FTIR and Raman spectra is performed and significant differences in the microstructure parameter are obtained by both methods with decreasing a-Si:H film thickness.

Impermeable Atomic Layer Deposition for Sputtering Buffer Layer in Efficient Semi-Transparent and Tandem Solar Cells via Activating Unreactive Substrate.

Atomic layer deposition (ALD) turns out to be particularly attractive technology for the sputtering buffer layer when preparing the semi-transparent (ST) perovskite solar cells (PSCs) and the tandem solar cells. ALD process turns to be island growth when the substrate is unreactive with the ALD reactants, resulting in the pin-hole layer, which causes an adverse effect on anti-sputtering. Here, p-i-n structured PSCs with ALD SnOx as sputtering buffer layer are conducted. The commonly used electron transportation layer (ETL) PCBM in the p-i-n structured PVK solar cell is an unreactive substrate that prevents the layer-by-layer growth for the ALD SnOx . PCBM layer is activated by introducing reaction sites to form impermeable ALD layers. By introducing reaction sites/ALD SnOx as sputtering buffer layer, the authors succeed to fabricate ST-PSCs and perovskite/silicon (double-side polished) tandem solar cells with power conversion efficiency (PCE) of 20.25% and 23.31%, respectively. Besides, the unencapsulated device with reaction sites maintains more than 99% of the initial PCE after aging over 5100 h. This work opens a promising avenue to prepare impermeable layer for stable PSCs, ST-PSCs, tandem solar cells, and the related scale-up solar cells.

Strain Modulation for Light-Stable n-i-p Perovskite/Silicon Tandem Solar Cells.

Perovskite/silicon tandem solar cells are promising to penetrate photovoltaic market. However, the wide-bandgap perovskite absorbers used in top-cell often suffer severe phase segregation under illumination, which restricts the operation lifetime of tandem solar cells. Here, a strain modulation strategy to fabricate light-stable perovskite/silicon tandem solar cells is reported. By employing adenosine triphosphate, the residual tensile strain in the wide-bandgap perovskite absorber is successfully converted to compressive strain, which mitigates light-induced ion migration and phase segregation. Based on the wide-bandgap perovskite with compressive strain, single-junction solar cells with the n-i-p layout yield a power conversion efficiency (PCE) of 20.53% with the smallest voltage deficits of 440 mV. These cells also maintain 83.60% of initial PCE after 2500 h operation at the maximum power point. Finally, these top cells are integrated with silicon bottom cells in a monolithic tandem device, which achieves a PCE of 26.95% and improved light stability at open-circuit.

Stable Organic Passivated Carbon Nanotube-Silicon Solar Cells with an Efficiency of 22.

The organic passivated carbon nanotube (CNT)/silicon (Si) solar cell is a new type of low-cost, high-efficiency solar cell, with challenges concerning the stability of the organic layer used for passivation. In this work, the stability of the organic layer is studied with respect to the internal and external (humidity) water content and additionally long-term stability for low moisture environments. It is found that the organic passivated CNT/Si complex interface is not stable, despite both the organic passivation layer and CNTs being stable on their own and is due to the CNTs providing an additional path for water molecules to the interface. With the use of a simple encapsulation, a record power conversion efficiency of 22% is achieved and a stable photovoltaic performance is demonstrated. This work provides a new direction for the development of high-performance/low-cost photovoltaics in the future and will stimulate the use of nanotubes materials for solar cells applications.

Phosphorus Catalytic Doping on Intrinsic Silicon Thin Films for the Application in Silicon Heterojunction Solar Cells.

Parasitic absorption and limited fill factor (FF) brought in by the use of amorphous silicon layers are efficiency-limiting challenges for the silicon heterojunction (SHJ) solar cells. In this work, postdeposition phosphorus (P) catalytic doping (Cat-doping) on intrinsic amorphous silicon (a-Si:H(i)) at a low substrate temperature was carried out and a P concentration of up to 6 × 1021 cm-3 was reached. The influences of filament temperature, substrate temperature, and processing pressure on the P profiles were systemically studied by secondary-ion mass spectrometry. By replacing the a-Si:H(n+er with P Cat-doping of an a-Si:H(i) layer, the passivation quality was improved, reaching an iVOC of 741 mV, while the parasitic absorption was reduced, leading to an increase in JSC by ∼1 mA/cm2. On the other hand, the open-circuit voltage and the FF of a conventional SHJ solar cell (with the a-Si:H(n) layer) can be improved by adding a Cat-doping process on the a-Si:H(i) layer, resulting in an increase in FF by 4.7%abs and in efficiency by 1.5%abs.

Development of Conductive SiCx:H as a New Hydrogenation Technique for Tunnel Oxide Passivating Contacts.

Conductive hydrogenated silicon carbide (SiCx:H) is discovered as a promising hydrogenation material for tunnel oxide passivating contacts (TOPCon) solar cells. The proposed SiCx:H layer enables a good passivation quality and features a good electrical conductivity, which eliminates the need of etching back of SiNx:H and indium tin oxide (ITO)/Ag deposition for metallization and reduces the number of process steps. The SiCx:H is deposited by hot wire chemical vapor deposition (HWCVD) and the filament temperature (Tf) during deposition is systematically investigated. Via tuning the SiCx:H layer, implied open-circuit voltages (iVoc) up to 742 ± 0.5 mV and a contact resistivity (ρc) of 21.1 ± 5.4 mΩ·cm2 is achieved using SiCx:H on top of poly-Si(n)/SiOx/c-Si(n) stack at Tf of 2000 °C. Electrochemical capacitance-voltage (ECV) and secondary ion mass spectrometry (SIMS) measurements were conducted to investigate the passivation mechanism. Results show that the hydrogenation at the SiOx/c-Si(n) interface is responsible for the high passivation quality. To assess its validity, the TOPCon stack was incorporated as rear electron selective-contact in a proof-of-concept n-type solar cells featuring ITO/a-Si:H(p)/a-Si:H(i) as front hole selective-contact, which demonstrates a conversion efficiency up to 21.4%, a noticeable open-circuit voltage (Voc) of 724 mV and a fill factor (FF) of 80%.

Exploring the potentials of Ti3N2 and Ti3N2X2 (X = O, F, OH) monolayers as anodes for Li or non-Li ion batteries from first-principles calculations.

The electronic properties and different metal ion (Li, Na, Mg) storage capabilities of the two-dimensional (2D) Ti3N2 monolayer and its Ti3N2X2 derivatives (X = O, F, and OH) as anode materials in rechargeable batteries have been systematically investigated by density functional theory (DFT) computations. Results show that the bare Ti3N2 and terminated monolayers in their most stable configurations are all metallic before and after metal ion adsorption. The relatively low diffusion barriers on the bare Ti3N2 monolayer were also confirmed, which implies faster charge and discharge rates. With respect to storage capacity, a high theoretical capacity of 1874 mA h g-1 can be provided by the Ti3N2 monolayer for Mg due to its multilayer adsorption and two-electron reaction. The existence of functional groups is proven to be unfavorable to metal ion migration and will decrease the corresponding storage capacities, which should be avoided in experiments as much as possible. These excellent performances suggest that the bare Ti3N2 is a promising anode material for Li-ion or non-Li-ion batteries.

In Situ-Doped Silicon Thin Films for Passivating Contacts by Hot-Wire Chemical Vapor Deposition with a High Deposition Rate of 42 nm/min.

Hot-wire chemical vapor deposition was used to deposit in situ-doped amorphous silicon layers for poly-Si/SiOx passivating contacts at a high deposition rate of 42 nm/min. We investigated the influence of a varied phosphine gas (PH3) concentration during deposition on (i) the silicon film properties and (ii) the passivating contact performances. The microstructural film properties were characterized before and after a high-temperature crystallization step to transform amorphous silicon films into polycrystalline silicon films. Before crystallization, the silicon layers become less dense as the PH3 concentrations increase. After crystallization, an increasing domain size is derived for higher PH3 concentrations. Sheet resistance is found to decrease as domain size increased, and the correlation between mobility and domain size was discussed. The performances of the passivating contact were measured, and a firing stable open circuit voltage of 732 mV, a contact resistivity of 8.1 mΩ·cm2, and a sheet resistance of 142 Ω/□ could be achieved with the optimized PH3 concentration. In addition, phosphorous doping tails into the crystalline silicon were extracted to evaluate the Auger recombination of the passivating contact.

Wet-Chemical Preparation of Silicon Tunnel Oxides for Transparent Passivated Contacts in Crystalline Silicon Solar Cells.

Transparent passivated contacts (TPCs) using a wide band gap microcrystalline silicon carbide (µc-SiC:H(n)), silicon tunnel oxide (SiO2) stack are an alternative to amorphous silicon-based contacts for the front side of silicon heterojunction solar cells. In a systematic study of the µc-SiC:H(n)/SiO2/c-Si contact, we investigated selected wet-chemical oxidation methods for the formation of ultrathin SiO2, in order to passivate the silicon surface while ensuring a low contact resistivity. By tuning the SiO2 properties, implied open-circuit voltages of 714 mV and contact resistivities of 32 mΩ cm2 were achieved using µc-SiC:H(n)/SiO2/c-Si as transparent passivated contacts.

Different Atomic Terminations Affect the Photocatalytic Nitrogen Fixation of Bismuth Oxybromide: A First Principles Study.

We have systematically investigated the electronic structures and activation capacities of BiOBr {001} facets with different atomic terminations by means of DFT methods. Our calculations reveal that oxygen vacancies (OVs) give a significant boost in band edges of the O-terminated BiOBr {001} facets, and excess electrons induced by OVs could exceed the reduction potentials of high-energy N2 intermediates. Interestingly, the Bi-terminated BiOBr {001} facets may be good candidates for photocatalytic nitrogen fixation due to the stronger activation ability of N2 molecules comparing with O-terminated BiOBr {001} facets with OVs. Moreover, the Bi-terminated BiOBr {001} facets may tend to yield NH3 instead of N2 H4 .

Enhancing the High-Voltage Cycling Performance of LiNi1/3Co1/3Mn1/3O2/Graphite Batteries Using Alkyl 3,3,3-Trifluoropropanoate as an Electrolyte Additive.

The present study demonstrates that the use of alkyl 3,3,3-trifluoropropanoate, including methyl 3,3,3-trifluoropropanoate (TFPM) and ethyl 3,3,3-trifluoropropanoate (TFPE), as new electrolyte additive can dramatically enhance the high-voltage performance of LiNi1/3Co1/3Mn1/3O2/graphite lithium-ion batteries (3.0-4.6 V, vs Li/Li+). The capacity retention was significantly increased from 45.6% to 75.4% after 100 charge-discharge cycles due to the addition of 0.2 wt % TFPM in the electrolyte, and significantly increased from 45.6% to 76.1% after 100 charge-discharge cycles due to the addition of 0.5 wt % TFPE in the electrolyte, verifying their suitability in this application. Electrochemical impedance spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were employed to study the effect of TFPM and TFPE on cell performance. The data indicates that the improved cycling activity can be ascribed to the participation of TFPM or TFPE in the formation of a thinner cathode/electrolyte interfacial film, thereby enhancing the cell cycling performance owing to a reduced interfacial resistance at high voltage.

A theoretical study on the electronic structures of TiO2: Effect of Hartree-Fock exchange.

The effects of the Fock exchange on the geometries and electronic structures of TiO2 have been investigated by introducing a portion of Hartree-Fock (HF) exchange into the traditional density functional. Our results indicate that the functional with 13% HF exchange can correctly predict the band gap and the electronic structures of rutile TiO2, and such an approach is also suitable to describe the structural and electronic properties of anatase and brookite phases. For the TiO2 (110) surfaces, although the surface relaxations are insensitive to the variation of HF exchange, there are larger effects on the positions of the occupied surface-induced states. When 13% HF exchange is employed, the predicted band gap of the perfect surface and position of defect state of the reduced surface are consistent with the experimental values. Moreover, the electronic structures of TiO2 (110) surface are carefully reexamined by using this hybrid density functional method.