Junhui Liang

A thin-film silicon based photocathode with a hydrogen doped TiO2 protection layer for solar hydrogen evolution

Photoelectrochemical (PEC) devices for solar water splitting require not only high solar to hydrogen conversion efficiency but also high chemical stability in strong acidic or alkaline electrolytes for long-term operation. Titanium dioxide (TiO2) has been considered as a highly promising protection layer to achieve high chemical stability for solar water splitting devices, especially for silicon based monolithic photovoltaic electrochemical (PV–EC) systems, while there is a trade-off relationship between activity and stability in these devices: the high charge transport barrier at the PV (silicon based thin film solar cells)/TiO2 interface and the high ohmic loss in TiO2 films hinder the device performance, especially when a thick TiO2 protection layer (preferred to enhance the chemical stability in the electrolyte) is used. Herein, we show that a hydrogen doped TiO2 protection layer can break this traditional trend to increase the activity without deteriorating the stability, when thick protection layers are employed to ensure stability. We demonstrated significant performance enhancement in hydrogenated amorphous silicon/silicon germanium (a-Si:H/a-SiGe:H) photocathodes through this approach. On one hand, the H-doping can shift up the Fermi level and reduce the electron transport barrier at the interface of the PV/TiO2 protection layer. On the other hand, the higher carrier density via H-doping leads to the enhancement of electron transport in TiO2 films and a shorter depletion layer barrier. Thus, the H-doping results in a higher photocurrent output at 0 V vs. reversible hydrogen electrode (RHE), indicating the high potential of the H-doped TiO2 protection layer for achieving stable and efficient monolithic solar water splitting devices.

Compound Homojunction:Heterojunction Reduces Bulk and Interface Recombination in ZnO Photoanodes for Water Splitting

Photoelectrochemical water splitting is far more efficient thanks to the novel ZnOSe/ZnO/BZO thin-film photoanodes fabricated in this work. A novel structure is developed for simultaneously suppressing the charge recombination in the ZnO bulk and at the semiconductor-electrolyte interface. This structure achieves a five-fold enhancement in water-splitting performance, compared to that of pristine ZnO photoanodes, when illuminated using visible light.

Hydrogenated TiO2 Thin Film for Accelerating Electron Transport in Highly Efficient Planar Perovskite Solar Cells

Abstract Intensive studies on low‐temperature deposited electron transport materials have been performed to improve the efficiency of n‐i‐p type planar perovskite solar cells to extend their application on plastic and multijunction device architectures. Here, a TiO2 film with enhanced conductivity and tailored band edge is prepared by magnetron sputtering at room temperature by hydrogen doping (HTO), which accelerates the electron extraction from perovskite photoabsorber and reduces charge transfer resistance, resulting in an improved short circuit current density and fill factor. The HTO film with upward shifted Fermi level guarantees a smaller loss on V OC and facilitates the growth of high‐quality absorber with much larger grains and more uniform size, leading to devices with negligible hysteresis. In comparison with the pristine TiO2 prepared without hydrogen doping, the HTO‐based device exhibits a substantial performance enhancement leading to an efficiency of 19.30% and more stabilized photovoltaic performance maintaining 93% of its initial value after 300 min continuous illumination in the glove box. These properties permit the room‐temperature magnetron sputtered HTO film as a promising electron transport material for flexible and tandem perovskite solar cell in the future.

Molybdenum-supported amorphous MoS3 catalyst for efficient hydrogen evolution in solar-water-splitting devices

We report molybdenum (Mo) metal-supported amorphous molybdenum sulfide (a-MoS3) catalysts with a porous and nanostructure nature, which exhibit excellent catalytic activity for the hydrogen evolution reaction (HER) in wired solar-water-splitting devices. Mo-supported a-MoS3 catalysts were prepared by wet chemically synthesizing a-MoS3 nanoparticles at room-temperature and then loading with Earth-abundant and scalable Mo metals sputtered at low temperature (100 °C). Electrochemical studies and applications in wired photoelectrochemical/photovoltaic (PEC–PV) solar-water-splitting devices reveal that the HER performance of wired PEC–PV solar-water-splitting devices can be efficiently enhanced with the proposed highly conductive Mo-supported a-MoS3 catalysts by enlarging the electrochemically active areas, accelerating the electron transport to active sites, and improving the charge transfer at the catalysts/electrolyte interfaces. The low-temperature preparation of highly active Mo-supported a-MoS3 catalysts paves the way to integrating them into various high-performance PV devices to develop highly efficient, scalable, low-cost, and monolithic PEC–PV solar-water-splitting devices.

Highly wettable and metallic NiFe-phosphate/phosphide catalyst synthesized by plasma for highly efficient oxygen evolution reaction

Many catalysts exhibit a high overpotential with a current density of 10 mA cm−2 for the oxygen evolution reaction (OER). High conductivity, wettability and active sites play key roles for a highly efficient OER catalyst. Here, we report a NiFe foam starting material treated with a two-step process using plasma-enhanced chemical vapor deposition (PECVD) in the presence of PH3, CO2 and H2, to form phosphate (Pi) and phosphide (P) groups on the foam, and forming NiFePi/P. The self-supported material combines conductivity, wettability with active sites, and is used directly as a working electrode for excellent oxygen evolution in alkaline solutions. Significantly, the strong synergistic effect between the phosphate and phosphide lead to a change in the surrounding electronic environment of metal ions that contributes to the increase in active sites, while improving the wettability and metallic nature of the catalyst, both of these result in an enhanced OER performance. This new material and design strategy appear to represent an intriguing advance that is likely to be of considerable interest to other researchers in the field.

A catalyst-free amorphous silicon-based tandem thin film photocathode with high photovoltage for solar water splitting

Nowadays, efficient production of storable clean hydrogen from abundant and sustainable solar energy is increasingly being identified as an essential route to realize the future sustainable hydrogen energy. Here we demonstrate a p-type amorphous silicon carbon (a-SiC:H) protected amorphous silicon/amorphous silicon germanium (a-Si/a-SiGe) tandem photocathode that is highly promising to realize a stable, large-scale, and efficient solar water splitting device. Our studies show that by electrically lossless coating a p-type a-SiC:H protection layer on an a-Si/a-SiGe tandem thin film solar cell with a preceding n-type narrow-gap μc-Si:H layer to improve the electron transfer, a high photocurrent onset potential can be achieved for the protected a-Si/a-SiGe tandem photocathode. In comparison to reported intrinsic a-SiC:H protection layers and n-type layers in a-Si/a-SiGe tandem cells, the proposed p-type a-SiC:H protection layer shows a better hydrogen evolution reaction (HER) catalytic activity, which is comparable to amorphous molybdenum sulfide (a-MoS3) catalyzed unprotected a-Si/a-SiGe tandem photocathodes even without any HER catalyst. Combined with the hybrid photoelectrode concept, this stable photocathode with high photovoltage is highly promising to form a wireless, highly stable, and efficient monolithic solar water splitting device for hydrogen production.

Periodically textured metal electrodes: large-area fabrication, characterization, simulation, and application as efficient back-reflective scattering contact-electrodes for thin-film solar cells

The integration of periodic back reflectors into thin-film solar cells offers the potential to accurately control the scattering behavior and improve the absorption enhancement in active layers, thereby overcoming the inherent performance limitations imposed by their poor light absorption and carrier collection. Periodically textured metal electrodes were therefore fabricated using a unique sauna-like method, and were investigated both experimentally and theoretically. In this way, we confirm the effectiveness of tuning the geometric parameters and the corresponding surface morphology on enhancing the diffraction behavior and light absorption through rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) simulation. Furthermore, the periodically textured metal electrodes produced by this unique fabrication process provide a means of enhancing absorption in the long wavelength range, thus opening a new way to further improve the performance of thin-film solar cells.

Electron transport layer driven to improve the open-circuit voltage of CH3NH3PbI3 planar perovskite solar cells

Suitable electron transport layers are essential for high performance planar perovskite heterojunction solar cells. Here, we use ZnO electron transport layer sputtered under oxygen-rich atmosphere at room temperature to decrease the hydroxide and then suppress decomposition of perovskite films. The perovskite films with improved crystallinity and morphology are achieved. Besides, on the ZnO substrate fabricated at oxygen-rich atmosphere, open-circuit voltage of the CH3NH3PbI3-based perovskite solar cells increased by 0.13 V. A high open-circuit voltage of 1.16 V provides a good prospect for the perovskite-based tandem solar cells. The ZnO sputtered at room temperature can be easily fabricated industrially on a large scale, therefore, compatible to flexible and tandem devices. Those properties make the sputtered ZnO films promising as electron transport materials for perovskite solar cells.摘要电子传输层对于N-I-P型平面钙钛矿太阳电池的性能至关重要. ZnO薄膜由于其高载流子迁移率在钙钛矿太阳电池中被广泛应用,但是薄膜内部羟基的存在影响了电池性能. 本文在磁控溅射沉积ZnO薄膜的过程中引入富氧环境来抑制ZnO中羟基的生成, 进而抑制钙钛矿薄膜的分解, 从而获得具有较高结晶质量的均匀致密的钙钛矿薄膜. 基于富氧环境下制备的ZnO作为电子传输层的钙钛矿太阳电池开压增加了0.13 V. 1.16 V的高开路电压对钙钛矿太阳电池在叠层电池中的应用提供了较好的发展前景. 此外, 室温磁控溅射制备的ZnO可以实现大面积工业化生产, 且适用于柔性和叠层器件. 该研究表明ZnO在太阳电池领域具有潜在应用.

Perovskite/silicon-based heterojunction tandem solar cells with 14.8% conversion efficiency via adopting ultrathin Au contact*

A rising candidate for upgrading the performance of an established narrow-bandgap solar technology without adding much cost is to construct the tandem solar cells from a crystalline silicon bottom cell and a high open-circuit voltage top cell. Here, we present a four-terminal tandem solar cell architecture consisting of a self-filtered planar architecture perovskite top cell and a silicon heterojunction bottom cell. A transparent ultrathin gold electrode has been used in perovskite solar cells to achieve a semi-transparent device. The transparent ultrathin gold contact could provide a better electrical conductivity and optical reflectance-scattering to maintain the performance of the top cell compared with the traditional metal oxide contact. The four-terminal tandem solar cell yields an efficiency of 14.8%, with contributions of the top (8.98%) and the bottom cell (5.82%), respectively. We also point out that in terms of optical losses, the intermediate contact of self-filtered tandem architecture is the uppermost problem, which has been addressed in this communication, and the results show that reducing the parasitic light absorption and improving the long wavelength range transmittance without scarifying the electrical properties of the intermediate hole contact layer are the key issues towards further improving the efficiency of this architecture device.

Activity enhancement via borate incorporation into a NiFe (oxy)hydroxide catalyst for electrocatalytic oxygen evolution

The oxygen evolution reaction (OER) is a key process in electrocatalysis and is critical for achieving the cost-effective conversion of renewable electricity to chemicals and fuels. However, the high overpotential (η) originates from poor charge-transfer ability and low catalytic activity may lead to high power consumption. Herein, we alleviate these issues by introducing borate into the NiFe (oxy)hydroxide framework. Our density functional theory (DFT) calculations demonstrated that the borate could be efficiently adsorbed onto the Ni/NiFe (oxy)hydroxide surface. Microscopically, the adsorbed borate can induce a favorable electronic structure for the Ni active sites. Meanwhile, the macroscopic charge-transfer ability of this synthesized catalyst has been dramatically increased. Hence, the catalytic performance of this material is improved compared with its NiFe counterpart: we achieved a higher OER activity with an ultralow η of only 230 mV at 10 mA cm−2 on a glassy carbon electrode (GCE) and of 200 mV at 10 mA cm−2 on Ni foam in alkaline medium. Moreover, this borate mediated NiFe (oxy)hydroxide is very stable: no appreciable degradation is observed after more than 110 hours of operation.