Omnidirectional Harvesting of Weak Light Using a Graphene Quantum Dot-Modified Organic/Silicon Hybrid Device

Despite great improvements in traditional inorganic photodetectors and photovoltaics, more progress is needed in the detection/collection of light at low-level conditions. Traditional photodetectors tend to suffer from high noise when operated at room temperature; therefore, these devices require additional cooling systems to detect weak or dim light. Conventional solar cells also face the challenge of poor light-harvesting capabilities in hazy or cloudy weather. The real world features such varying levels of light, which makes it important to develop strategies that allow optical devices to function when conditions are less than optimal. In this work, we report an organic/inorganic hybrid device that consists of graphene quantum dot-modified poly­(3,4-ethylenedioxythiophene) polystyrenesulfonate spin-coated on Si for the detection/harvest of weak light. The hybrid configuration provides the device with high responsivity and detectability, omnidirectional light trapping, and fast operation speed. To demonstrate the potential of this hybrid device in real world applications, we measured near-infrared light scattered through human tissue to demonstrate noninvasive oximetric photodetection as well as characterized the device’s photovoltaic properties in outdoor (<i>i</i>.<i>e</i>., weather-dependent) and indoor weak light conditions. This organic/inorganic device configuration demonstrates a promising strategy for developing future high-performance low-light compatible photodetectors and photovoltaics

High-Performance <i>a</i>‑Si/c-Si Heterojunction Photoelectrodes for Photoelectrochemical Oxygen and Hydrogen Evolution

Amorphous Si (<i>a</i>-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 NiO_<i>x</i> as the catalyst shows a current density of 21.48 mA/cm² 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

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/cm² 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

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

Monolayer MoS₂ Heterojunction Solar Cells

We realized photovoltaic operation in large-scale MoS₂ monolayers by the formation of a type-II heterojunction with p-Si. The MoS₂ monolayer introduces a built-in electric field near the interface between MoS₂ and p-Si to help photogenerated carrier separation. Such a heterojunction photovoltaic device achieves a power conversion efficiency of 5.23%, which is the highest efficiency among all monolayer transition-metal dichalcogenide-based solar cells. The demonstrated results of monolayer MoS₂/Si-based solar cells hold the promise for integration of 2D materials with commercially available Si-based electronics in highly efficient devices

Si Hybrid Solar Cells with 13% Efficiency <i>via</i> Concurrent Improvement in Optical and Electrical Properties by Employing Graphene Quantum Dots

By employing graphene quantum dots (GQDs) in PEDOT:PSS, we have achieved an efficiency of 13.22% in Si/PEDOT:PSS hybrid solar cells. The efficiency enhancement is based on concurrent improvement in optical and electrical properties by the photon downconversion process and the improved conductivity of PEDOT:PSS via appropriate incorporation of GQDs. After introducing GQDs into PEDOT:PSS, the short circuit current and the fill factor of rear-contact optimized hybrid cells are increased from 32.11 to 36.26 mA/cm² and 62.85% to 63.87%, respectively. The organic–inorganic hybrid solar cell obtained herein holds the promise for developing photon-managing, low-cost, and highly efficient photovoltaic devices

Realizing High-Efficiency Omnidirectional n‑Type Si Solar Cells <i>via</i> 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

Shape-Dependent Light Harvesting of 3D Gold Nanocrystals on Bulk Heterojunction Solar Cells: Plasmonic or Optical Scattering Effect?

In the work, mechanisms behind various 3D nanocrystals enhanced performance of bulk heterojunction solar cells were studied comprehensively. Four types of gold nanoparticles (NPs) with distinctly different shapes and great uniformity were designed and synthesized, including cubes, rhombic dodecahedra (RD), edge- and corner-truncated octahedra (ECTO), and triangular plates, to systematically probe their influences on photovoltaics. RD and triangular plates show a higher growth rate, while slower growth favors cubes and ECTO formation by controlling the reduction agent and capping ion amount. NPs with increasing corners and proper size of cross-section induce stronger near-field coupling and far-field scattering in P3HT:PC₆₁BM-based active layers. Both finite-difference time-domain simulation and UV–visible absorption spectra firmly support that RD exhibit the strongest localized surface plasmon resonance and optical scattering. With optimized conditions, a high power conversion efficiency exceeding 4% was reproducibly achieved