Lisha Bai

Effective light trapping in thin film silicon solar cells from textured Al doped ZnO substrates with broad surface feature distributions

We used a multi-step process to make aluminum-doped ZnO (AZO) films with a wide range distribution of textures for light trapping in thin film silicon solar cells, which includes AZO deposition, HCl etching, AZO re-deposition, and HCl re-etching. The large features created by the first etching provide an effective light trapping for long wavelength light; the small features from the second etching enhances the short wavelength light trapping. Microcrystalline silicon solar cells deposited on the above-mentioned AZO show an improved photocurrent density without loss in the open-circuit voltage and fill factor, resulting in an overall increase in efficiency by 14.64%.

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.

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.

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...

Triple-functional n-type microcrystalline silicon oxide layers in hydrogenated amorphous silicon/microcrystalline silicon tandem solar cells

A novel tunnel recombination junction (TRJ) consisted of n type hydrogenated microcrystalline silicon oxide (n-μc-SiO_x:H) layer and p type hydrogenated nanocrystalline silicon oxide (p-nc-SiOx:H) layer was proposed in hydrogenated amorphous silicon/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cell. The absence of n-μc-Si:H compared to conventional n-μc-SiO_x:H/n-μc-Si:H/p-nc-SiO_x:H TRJ reduced parasitic absorption. Meanwhile, the new TRJ indicated an ohmic contact, which is suitable for the tandem solar cell. The application of the new TRJ significantly improved the short-circuit current (J_sc) of bottom cell. Moreover, n-μc-SiOx:H layer functioned as intermediate reflector layer to ensure high J_sc of top cell. Initial conversion efficiency of optimized a-Si:H/μc-Si:H tandem solar cell with novel TRJ based on as-grown MOCVDZnO: B (BZO) substrate reached up to 12.99%.

Textured transparent conductive B/Al doped ZnO films utilizing reactive magnetron sputtering

High-quality textured transparent conductive oxide (TCO) is significant to the exceptional performance of solar cells. In this study, textured B/Al doped ZnO (ZnO:B/Al) films were first reported utilizing reactive RF magnetron sputtering from an intrinsic ZnO ceramic target in a B2H6–Ar gas mixture. Elaborate thermal treatment was conducted for film optimization, as the temperature dependency of the B2H6 source was quite sensitive. A compound structure with a 200 nm thick Al capping layer and 2000 nm thick ZnO:B main layer was proposed to further improve the annealing performance. The initial resistivity and mobility of the reactive sputtered ZnO:B/Al film were 4.8 × 10−4 Ω cm and 32.1 cm2 V−1 s−1 respectively. A remarkable surface texture with root-mean-square higher than 170 nm developed after a chemical etching procedure, resulting in average light scattering over 89% and total transmittance higher than 86% in the whole spectrum. These optical properties illustrate prominent progress in TCO application in thin film silicon (TFS) solar cells. Preliminary microcrystalline silicon solar cells deposited on textured ZnO:B/Al films indicate 0.6 mA cm−2 enhancement in short-circuit current density compared to conventional ZnO:Al front contacts, indicating potential application in high-efficiency TFS solar cells.