Single-junction photovoltaic devices exhibit a bottleneck in their efficiency due to incomplete or inefficient harvesting of photons in the low- or high-energy regions of the solar spectrum. Spectral converters can be used to convert solar photons into energies that are more effectively captured by the photovoltaic device through a photoluminescence process. Here, recent advances in the fields of luminescent solar concentration, luminescent downshifting, and upconversion are discussed. The focus is specifically on the role that materials science has to play in overcoming barriers in the optical performance in all spectral converters and on their successful integration with both established (e.g., c-Si, GaAs) and emerging (perovskite, organic, dye-sensitized) cell types. Current challenges and emerging research directions, which need to be addressed for the development of next-generation luminescent solar devices, are also discussed.

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Towards Efficient Spectral Converters through Materials Design for Luminescent Solar Devices.

Nanophase nc-Si/a-SiC films that contain Si quantum dots (QDs) embedded in an amorphous SiC matrix were deposited on single-crystal silicon substrates using inductively coupled plasma-assisted chemical vapor deposition from the reactive silane and methane precursor gases diluted with hydrogen at a substrate temperature of 200 °C. The effect of the hydrogen dilution ratio X (X is defined as the flow rate ratio of hydrogen-to-silane plus methane gases), ranging from 0 to 10.0, on the morphological, structural, and compositional properties of the deposited films, is extensively and systematically studied by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier-transform infrared absorption spectroscopy, and X-ray photoelectron spectroscopy. Effective nanophase segregation at a low hydrogen dilution ratio of 4.0 leads to the formation of highly uniform Si QDs embedded in the amorphous SiC matrix. It is also shown that with the increase of X, the crystallinity degree and the crystallite size increase while the carbon content and the growth rate decrease. The obtained experimental results are explained in terms of the effect of hydrogen dilution on the nucleation and growth processes of the Si QDs in the high-density plasmas. These results are highly relevant to the development of next-generation photovoltaic solar cells, light-emitting diodes, thin-film transistors, and other applications.

Si quantum dots embedded in an amorphous SiC matrix : nanophase control by non-equilibrium plasma hydrogenation

Optimizing the light-emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light-emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2–10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum-confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum-confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light-emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.

Optimum Quantum Yield of the Light Emission from 2 to 10 nm Hydrosilylated Silicon Quantum Dots

Porous-shaped silicon carbide ultraviolet photodetectors on porous silicon substrates

As the most fundamental material for microelectronics, silicon (Si) has bourgeoned in the past more than half a century. However, given the indirect bandgap of Si, the use of Si in optoelectronics is relatively limited due to its mediocre optical absorption and rather poor optical emission. During many years of efforts for extending the use of Si in optoelectronics Si nanocrystals (NCs) that are one type of the most important Si nanostructures have attracted significant attention owing to their remarkable electronic and optical properties. Si NCs are actually crystalline Si nanoparticles, which may be called Si quantum dots if their size is smaller than ∼10 nm. With the manipulation of the size, surface and doping of Si NCs great tunability for the light emission from Si NCs with the quantum yield of more than 60% has been realized. Based on the efficient light emission from Si NCs high-performance Si-NC light-emitting devices have been demonstrated. In the meantime, the efficient light emission from Si NCs has also been utilized for synaptic simulations in neuromorphic computing and down-shifting in photovoltaics. Broadband optical absorption ranging from the ultraviolet to mid-infrared has been recently obtained for Si NCs mainly by taking advantage of doping. This has enabled the use of Si NCs in novel solar cells, photodetectors and optoelectronic synaptic devices. The continuous improvement of the electronic and optical properties of Si NCs has made Si NCs unfading Si materials for optoelectronics, contributing to the development of Si-based optoelectronic integration.

Silicon nanocrystals: unfading silicon materials for optoelectronics

Structural and electronic properties of Si nanocrystals embedded in amorphous SiC matrix

Si quantum dots (QDs) were formed by thermal annealing the hydrogenated amorphous silicon carbide films (a-SiCx:H) with different C/Si ratio x, which were controlled by using a different gas ratio R of methane to silane during the deposition process. By adjusting x and post annealing temperature, the QD size can be changed from 1.4 to 4.2 nm accordingly, which was verified by the Raman spectra and transmission electron microscopy images. Size-dependent electroluminescence (EL) was observed, and the EL intensity was higher for the sample containing small-sized Si QDs due to the quantum confinement effect (QCE). The EL peak energy as a function of the Si QDs size was in good agreement with a modified effective mass approximation (EMA) model. The calculated finite barrier potential of the Si QDs embedded in SiC matrix is 0.4 and 0.8 eV for conduction and valence band, respectively. Moreover, the current-voltage properties and the linear relationship between the integrated EL intensity and injection current i...

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Hydrogenated amorphous Si thin films were prepared by plasma-enhanced chemical vapor deposition technique. As-deposited samples were thermally annealed at various temperatures to obtain nanocrystalline Si. The microstructures and carrier transport behaviors were evaluated during the transition process from amorphous to nanocrystalline structures. Raman scattering spectroscopy and Fourier-transform infrared spectroscopy were used to characterize the changes in microstructures and bonding configurations. It is found that hydrogen is completely effused from the film at the annealing temperature of 600 °C, while crystallization occurs at around 700 °C. The carrier transport characteristics in nanocrystallized films are different from those in the amorphous Si films. The carrier transport in the amorphous silicon films is strongly influenced by the defect states resulting from the effusion of hydrogen. The dual activation energies are found in temperature-dependent conductivity results which can be attributed ...

Evaluation of microstructures and carrier transport behaviors during the transition process from amorphous to nanocrystalline silicon thin films

Periodically nanopatterned Si structures have been prepared by using a nanosphere lithography technique. The formed nanopatterned structures exhibit good anti-reflection and enhanced optical absorption characteristics. The mean surface reflectance weighted by AM1.5 solar spectrum (300–1200 nm) is as low as 5%. By depositing Si quantum dot/SiO2 multilayers (MLs) on the nanopatterned Si substrate, the optical absorption is higher than 90%, which is significantly improved compared with the same multilayers deposited on flat Si substrate. Furthermore, the prototype n-Si/Si quantum dot/SiO2 MLs/p-Si heterojunction solar cells has been fabricated, and it is found that the external quantum efficiency is obviously enhanced for nanopatterned cell in a wide spectral range compared with the flat cell. The corresponding short-circuit current density is increased from 25.5 mA cm−2 for flat cell to 29.0 mA cm−2 for nano-patterned one. The improvement of cell performance can be attributed both to the reduced light loss and the down-shifting effect of Si quantum dots/SiO2 MLs by forming periodically nanopatterned structures.

Light Trapping and Down‐Shifting Effect of Periodically Nanopatterned Si‐Quantum‐Dot‐Based Structures for Enhanced Photovoltaic Properties

Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC multilayers prepared in a plasma-enhanced chemical vapor deposition system. The thickness of amorphous Si layer was designed to be 4 nm, and the thickness of amorphous SiC layer was kept at 2 nm. Transmission electron microscopy observation revealed the formation of Si QDs after 900°C annealing. The optical properties of the Si QDs/SiC multilayers were studied, and the optical band gap deduced from the optical absorption coefficient result is 1.48 eV. Moreover, the p-i-n structure with n-a-Si/i-(Si QDs/SiC multilayers)/p-Si was fabricated, and the carrier transportation mechanism was investigated. The p-i-n structure was used in a solar cell device. The cell had the open circuit voltage of 532 mV and the power conversion efficiency (PCE) of 6.28%.

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Jun Xu

Papers

Structural and electronic properties of Si nanocrystals embedded in amorphous SiC matrix

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Evaluation of microstructures and carrier transport behaviors during the transition process from amorphous to nanocrystalline silicon thin films

Light Trapping and Down‐Shifting Effect of Periodically Nanopatterned Si‐Quantum‐Dot‐Based Structures for Enhanced Photovoltaic Properties

Enhanced photovoltaic property by forming p-i-n structures containing Si quantum dots/SiC multilayers