Meng-Lin Tsai

Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures

Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.

Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments

Few-layered MoS2 as Schottky metal-semiconductor-metal photodetectors (MSM PDs) for use in harsh environments makes its debut as two-dimensional (2D) optoelectronics with high broadband gain (up to 13.3), high detectivity (up to ~10(10) cm Hz(1/2)/W), fast photoresponse (rise time of ~70 μs and fall time of ~110 μs), and high thermal stability (at a working temperature of up to 200 °C). Ultrahigh responsivity (0.57 A/W) of few-layer MoS2 at 532 nm is due to the high optical absorption (~10% despite being less than 2 nm in thickness) and a high photogain, which sets up a new record that was not achievable in 2D nanomaterials previously. This study opens avenues to develop 2D nanomaterial-based optoelectronics for harsh environments in imaging techniques and light-wave communications as well as in future memory storage and optoelectronic circuits.

Above-11%-efficiency organic-inorganic hybrid solar cells with omnidirectional harvesting characteristics by employing hierarchical photon-trapping structures.

Hierarchical structures consisting of micropyramids and nanowires are used in Si/PEDOT:PSS hybrid solar cells to achieve a power conversion efficiency (PCE) up to 11.48% with excellent omnidirectionality. The structure provides a combined concepts of superior light trapping ability, significant increase of p-n junction areas, and short carrier diffusion distance, improving the photovoltaic characteristics including short-circuit current density, fill factor, and PCE. The enhancement of power generation is up to 253.8% at high incident angles, showing the outstanding omnidirectional operation ability of hybrid cells with hierarchical Si surfaces. This properly designed hierarchical-structured device paves a promising way for developing low-cost, high-efficiency, and omnidirectional solar applications in the future.

Realizing high-efficiency omnidirectional n-type Si solar cells via the hierarchical architecture concept with radial junctions.

Hierarchical structures combining micropyramids and nanowires with appropriate control of surface carrier recombination represent a class of architectures for radial p-n junction solar cells that synergizes the advantageous features including excellent broad-band, omnidirectional light-harvesting and efficient separation/collection of photoexcited carriers. The heterojunction solar cells fabricated with hierarchical structures exhibit the efficiency of 15.14% using cost-effective as-cut Czochralski n-type Si substrates, which is the highest reported efficiency among all n-type Si nanostructured solar cells. We also demonstrate the omnidirectional solar cell that exhibits the daily generated power enhancement of 44.2% by using hierarchical structures, as compared to conventional micropyramid control cells. The concurrent improvement in optical and electrical properties for realizing high-efficiency omnidirectional solar cells using as-cut Czochralski n-type Si substrates demonstrated here makes a hierarchical architecture concept promising for large-area and cost-effective mass production.

Solar-Blind Photodetectors for Harsh Electronics

We demonstrate solar-blind photodetectors (PDs) by employing AlN thin films on Si(100) substrates with excellent temperature tolerance and radiation hardness. Even at a bias higher than 200 V the AlN PDs on Si show a dark current as low as ~ 1 nA. The working temperature is up to 300°C and the radiation tolerance is up to 1013 cm−2 of 2-MeV proton fluences for AlN metal-semiconductor-metal (MSM) PDs. Moreover, the AlN PDs show a photoresponse time as fast as ~ 110 ms (the rise time) and ~ 80 ms (the fall time) at 5 V bias. The results demonstrate that AlN MSM PDs hold high potential in next-generation deep ultraviolet PDs for use in harsh environments.

Efficiency Enhancement of Silicon Heterojunction Solar Cells via Photon Management Using Graphene Quantum Dot as Downconverters

By employing graphene quantum dots (GQDs), we have achieved a high efficiency of 16.55% in n-type Si heterojunction solar cells. The efficiency enhancement is based on the photon downconversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. The short circuit current and the fill factor are increased from 35.31 to 37.47 mA/cm(2) and 70.29% to 72.51%, respectively. The work demonstrated here holds the promise for incorporating graphene-based materials in commercially available solar devices for developing ultrahigh efficiency photovoltaic cells in the future.

Designing Efficient Solar‐Driven Hydrogen Evolution Photocathodes Using Semitransparent MoQxCly (Q = S, Se) Catalysts on Si Micropyramids

Silicon micropyramids with n(+) pp(+) junctions are demonstrated to be efficient absorbers for integrated solar-driven hydrogen production systems enabling significant improvements in both photocurrent and onset potential. When conformally coated with MoSx Cly , a catalyst that has excellent catalytic activity and high optical transparency, the highest photocurrent density for Si-based photocathodes with earth-abundant catalysts is achieved.

Toward Efficient and Omnidirectional n-Type Si Solar Cells: Concurrent Improvement in Optical and Electrical Characteristics by Employing Microscale Hierarchical Structures

We demonstrated that hierarchical structures combining different scales (i.e., pyramids from 1.5 to 7.5 μm in width on grooves from 40 to 50 μm in diameter) exhibit excellent broadband and omnidirectional light-trapping characteristics. These microscaled hierarchical structures could not only improve light absorption but prevent poor electrical properties typically observed from nanostructures (e.g., ultra-high-density surface defects and nonconformal deposition of following layers, causing low open-circuit voltages and fill factors). The microscaled hierarchical Si heterojunction solar cells fabricated with hydrogenated amorphous Si layers on as-cut Czochralski n-type substrates show a high short-circuit current density of 36.4 mA/cm(2), an open-circuit voltage of 607 mV, and a conversion efficiency of 15.2% due to excellent antireflection and light-scattering characteristics without sacrificing minority carrier lifetimes. Compared to cells with grooved structures, hierarchical heterojunction solar cells exhibit a daily power density enhancement (69%) much higher than the power density enhancement at normal angle of incidence (49%), demonstrating omnidirectional photovoltaic characteristics of hierarchical structures. Such a concept of hierarchical structures simultaneously improving light absorption and photocarrier collection efficiency opens avenues for developing large-area and cost-effective solar energy devices in the industry.

High-Performance a-Si/c-Si Heterojunction Photoelectrodes for Photoelectrochemical Oxygen and Hydrogen Evolution

Amorphous Si (a-Si)/crystalline Si (c-Si) heterojunction (SiHJ) can serve as highly efficient and robust photoelectrodes for solar fuel generation. Low carrier recombination in the photoelectrodes leads to high photocurrents and photovoltages. The SiHJ was designed and fabricated into both photoanode and photocathode with high oxygen and hydrogen evolution efficiency, respectively, by simply coating of a thin layer of catalytic materials. The SiHJ photoanode with sol-gel NiOx as the catalyst shows a current density of 21.48 mA/cm(2) at the equilibrium water oxidation potential. The SiHJ photocathode with 2 nm sputter-coated Pt catalyst displays excellent hydrogen evolution performance with an onset potential of 0.640 V and a solar to hydrogen conversion efficiency of 13.26%, which is the highest ever reported for Si-based photocathodes.

Photon management in nanostructured solar cells

The unique geometry and intriguing physical properties of nanostructure-based solar cells gives them great potential to achieve the goals of cost-effectiveness and high-efficiency. With nanostructured solar cells it is expected to be possible to break the Shockley–Queisser limit. This potential has driven widespread research and development in photon management to enhance light absorption over the past decade. However, efficiency is not proportional to light absorption. Nowadays, researchers are starting to address this issue. A thorough understanding of the advantages and the scope of the application of each photon management scheme is critical to finding a breakthrough for this predicament. In this review, we present the theorems and describe recent progresses in primary photon management schemes for nanostructures, including antireflection, light scattering, and resonance (e.g., metallic resonance, dielectric resonance, and photonic crystals). The antireflection effect allows more light to enter the solar cell. Light scattering enhances the interaction between the light and the nanostructure, extending the light propagation paths in the devices. Resonance effects can redirect and precisely confine the light to the region where efficient photoelectric conversion efficiency occurs. Finally, we discuss the challenges of nanostructured solar cells, and indicate potential routes to overcome the performance-limiting problems.