Bofei Liu

Transparent double-period electrode with effective light management for thin film solar cells

Aiming to significantly enhance solar cell efficiency through light management, we designed and fabricated transparent electrodes with a double-period structure. After demonstrating a significant increase of quantum efficiency in short wavelengths with a small-period self-textured ZnO : B, and in long wavelengths with large-period texture-etched ZnO : Al, respectively, we made the double-period transparent solar cell electrode by integrating self-textured ZnO : B and texture-etched ZnO : Al on μc-Si : H solar cells, using conventional physical and chemical vapor deposition techniques. Our investigation indicates that the double-period electrode can play an important role towards improving light absorption and trapping, thus effectively increasing the efficiency and reducing the cost of electricity from the solar cells.

High efficiency and high open-circuit voltage quadruple-junction silicon thin film solar cells for future electronic applications

Conversion of clean and renewable solar energy into electricity with photovoltaic (PV) devices, based on earth-abundant silicon elements to meet increasing global energy demands and environmental sustainability, has motivated various potential industrial and domestic applications. In addition to large-scale electricity production of market-dominant crystalline silicon PVs, the unique properties of silicon-based thin-film solar-cells (TFSCs) make them very attractive as affordable clean and safe energy devices. Herein, with large-scale and mature plasma-enhanced chemical vapor deposition (PECVD) process that can efficiently fabricate high-performing a-SiC:H, a-SiGe:H, a-Si:H, and μc-Si:H single- and various multi-junction TFSCs, we report a highly-efficient and flexibly tunable monolithic quadruple-junction silicon TFSC with a high photovoltage above 3.0 V and power conversion efficiency of 15.03% (NREL measured 14.58%). Our proposed high-voltage silicon TFSCs, with excellent performance, can further enrich the toolbox for functional photoelectrical devices and inspire possible future applications as highly promising power supply sources in charging electronics, splitting and disinfecting water, powering household electronic devices, solar to CO2 reduction, and other possible applications.

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.

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.

Two-dimensional high efficiency thin-film silicon solar cells with a lateral light trapping architecture

Introducing light trapping structures into thin-film solar cells has the potential to enhance their solar energy harvesting as well as the performance of the cells; however, current strategies have been focused mainly on harvesting photons without considering the light re-escaping from cells in two-dimensional scales. The lateral out-coupled solar energy loss from the marginal areas of cells has reduced the electrical yield indeed. We therefore herein propose a lateral light trapping structure (LLTS) as a means of improving the light-harvesting capacity and performance of cells, achieving a 13.07% initial efficiency and greatly improved current output of a-Si:H single-junction solar cell based on this architecture. Given the unique transparency characteristics of thin-film solar cells, this proposed architecture has great potential for integration into the windows of buildings, microelectronics and other applications requiring transparent components.

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.

Mechanism insight into the effect of I/P buffer layer on the performance of NIP-type hydrogenated microcrystalline silicon solar cells

A simulation and experimental study on the effect of the buffer layer at the I/P interface on the performance of NIP-type hydrogenated microcrystalline silicon (μc-Si:H) single-junction solar cells is presented. Device-quality hydrogenated amorphous silicon (a-Si:H) material as a buffer layer at the I/P interface obviously improves the performance of NIP-type μc-Si:H single-junction solar cells. In addition to the well-known mechanism that an a-Si:H I/P buffer layer can reduce the recombination current density at I/P interfaces, the optically and electrically calibrated simulations and supporting experimental results in this study illustrate that the performance improvement also originates from the mitigation of the electric screening effect due to the reduced defect density at the I/P interfaces, which reinforces the bulk electric field. Integrating an optimized hydrogen profiling strategy and adding a-Si:H I/P buffer layer yielded an initial efficiency of 9.20% for μc-Si:H single-junction solar cells wi...

Ti/Co-S catalyst covered amorphous Si-based photocathodes with high photovoltage for the HER in non-acid environments

Making highly efficient photoelectrodes for a photoelectrochemical (PEC) water splitting reaction is vitally important for bringing solar/electrical-to-hydrogen energy conversion processes into reality. Active and stable catalysts in the photoelectrode are the key component in PEC devices. Although most hydrogen evolution catalysts exhibit excellent performance in acid solutions, earth abundant oxygen evolution catalysts are generally unstable. Therefore, the elaborate design of highly active and stable photocathodes possessing great onset potential in non-acid environments is imperative. Here, we report that magnetron sputtering and electrochemical-deposition preparation methods are compatible with photo-absorbers for depositing titanium (Ti)/cobalt–sulfide (Co–S) catalyst on amorphous silicon/amorphous silicon (a-Si/a-Si) tandem solar cells, realizing a stable, low-cost, and efficient photocathode for water splitting in non-acid solutions. This photocathode exhibits a high photocurrent onset potential of 1.78 V vs. reversible hydrogen electrode (RHE) in alkaline and neutral electrolyte, with a photocurrent of 6.34 mA cm−2 at 0 V vs. RHE. The facile preparation of this highly active a-Si/a-Si/Ti/Co–S photocathode paves the way to integrate them into various oxygen evolution catalysts to develop highly efficient, low-cost, and monolithic PEC solar water splitting devices.

Conductive layer protected and oxide catalyst-coated thin-film silicon solar cell as an efficient photoanode

Photovoltaic–photoelectrochemical (PV-PEC) water splitting based on silicon (Si) is very promising because of its broad visible light absorption, earth abundance and high carrier mobility. However, its practical application is hindered by its poor performance in the oxygen evolution reaction (OER) and poor stability in electrolytes. Here, we introduced a conductive indium tin oxide (ITO) protective layer for superior interface charge transfer and developed a synergistic strategy by combining the protective layer with a NiOx catalyst layer for favorable kinetic overpotential. Thus, we delivered a photocurrent density as high as 7.64 mA cm−2 at 1.23 V versus RHE on the double junction microcrystalline silicon (μc-Si : H) solar cell PEC device in 1 M NaOH electrolyte. This current density is 37 times higher than that of the device with only NiOx under the same conditions. The μc-Si : H solar cell with ITO and NiOx layers has a maximum efficiency of 2% at an applied bias of 0.85 V vs. RHE. Our work will open up new opportunities for designing and preparing high-performance and non-noble PEC water splitting devices.