Band Gap Of Crystalline Silicon Research Articles

Silicon interface passivation studied by modulated surface photovoltage spectroscopy

We demonstrate that the modulated surface photovoltage spectroscopy (modulated SPS) technique can be applied to investigate interface states in the bandgap, i.e. interface passivation, of crystalline silicon coated with a downshift layer such as hydrogenated aluminum nitride with embedded terbium ions by suppressing straylight with a cut-off filter. Different hydrogen contents influence the surface photovoltage spectra at photon energies below the bandgap of crystalline silicon. Modulated SPS reveals that at higher hydrogen content there is a lower signal and, thus, a lower density of surface defect states. Our experiments show that modulated SPS can become a powerful tool for characterizing defect states at interfaces which cannot be easily studied by other methods.

Photon upconversion utilizing energy beyond the band gap of crystalline silicon with a hybrid TES-ADT/PbS quantum dots system.

The recent introduction of inorganic semiconductor quantum dots (QDs) as triplet sensitizers for molecular semiconductors has led to significant interest in harvesting low energy photons, which can then be used for photon upconversion (PUC), via triplet-triplet annihilation (TTA). A key goal is the harvesting of photons from below the bandgap of crystalline silicon 1.12 eV (≈1100 nm) and their upconversion into the visible region. In practice, the systems demonstrated so far have been limited to harvesting photons with energies above 1.2 eV (≈1 μm), due to two reasons: firstly the need to use transmitter ligands which allow efficient energy harvesting from the QD but introduce an energy loss of larger than 200 meV in transmission from the QD to the annihilator, and secondly due to the use of molecules such as tetracene which cannot accept smaller energy than 1.2 eV. Here, we introduce a new strategy to overcome these difficulties by using a low energy triplet annihilator that also harvests excitations efficiently from QDs. Specifically, we show that 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT, triplet energy of 1.08 eV: ca. 1150 nm) functions as a triplet annihilator (20% TTA efficiency) while also rapidly extracting triplet excitons from lead sulfide (PbS) QDs with a rate constant of k = ca. 2 × 10-8 s-1 with an excitation at 1064 nm. This rate is consistent with an orbital overlap between TES-ADT and PbS QDs, which we propose is due to the thiophene group of TES-ADT, which enables a close association with the PbS surface, allowing this system to function both as annihilator and transmitter. Our results pave the way for the design of triplet annihilators that can closely associate with the QD surface and harvest low energy excitons with minute losses in energy during the TET process, with the ultimate goal of efficiently utilizing photon energy beyond the bandgap of crystalline silicon.

Tb3+/Yb3+ doped aluminosilicate phosphors for near infrared emission and efficient down-conversion

The spectral mismatch between incident solar photons and the bandgap of crystalline silicon in solar cells results in major energy losses. Photons with energies lower than the bandgap are lost due to transparency, while those with energies above the bandgap value will be absorbed, generating electron-hole pairs, but consuming only the energy Eg, with the excess being lost as heat through thermalization. Despite intensive research to obtain efficient down-conversion (DC) materials for frequency shifting layers in c-Si cells, their development so far involves techniques which are not cost effective. Taking this into account, aluminosilicate phosphors co-doped with the Tb3+/Yb3+ ion pair have been synthesized by sol-gel (SG) processing, a low cost technique which allows scalability and is especially useful to obtain films. The molar composition 90 SiO2-10 AlO1.5 was chosen as host to investigate if DC based on the Tb3+/Yb3+ pair leads to efficient quantum cutting in such material. We demonstrate the potential of this approach to obtain near infrared DC emission in aluminosilicate matrix with a conversion efficiency of up to 158%, with potential to improve the performance of low bandgap photovoltaic solar cells.

Comprehensive Ab Initio Study of Electronic, Optical, and Cohesive Properties of Silicon Quantum Dots of Various Morphologies and Sizes up to Infinity

We present a comprehensive and integrated model-independent ab initio study of the structural, cohesive, electronic, and optical properties of silicon quantum dots of various morphologies and sizes in the framework of all-electron “static” and time-dependent density functional theory (DFT, TDFT), using the well-tested B3LYP and other properly chosen functional(s). Our raw ab initio results for all these properties for hydrogen-passivated nanocrystals of various growth models and sizes from 1 to 32 A are subsequently fitted, using power-law dependence with judicially selected exponents, based on dimensional and other plausibility arguments. As a result, we can not only reproduce with excellent accuracy known experimental and well-tested theoretical results in the regions of overlap but also extrapolate successfully all the way to infinity, reproducing the band gap of crystalline silicon with almost chemical accuracy as well as the cohesive energy of the infinite crystal with very good accuracy. Thus, our r...

Asymmetric back contact nanograting design for thin c-Si solar cells

Texturing of silicon wafers with periodic gratings in submicron scale is one of effective light trapping routes especially for thin crystalline silicon solar cells. The grating can be very effective for trapping of photons near band gap of crystalline silicon when its period is suitably chosen. The asymmetric gratings provide higher light trapping efficiency than symmetric ones because the asymmetric one suppress the escape of internally reflected light in the silicon wafers. In this study, we conceptually show that asymmetric grating can be fabricated by simple stacking of a dielectric layer onto the symmetric silicon grating structures. Optical simulations were performed to calculate optical absorptions in thin crystalline silicon of a 5 μm thickness with symmetric and asymmetric grating structures. We demonstrate that with the asymmetric grating structures combined with highly efficient antireflection coating, optical absorptions in the thin silicon wafers can reach over 97% of the Lambertian absorption limit.

Cooperative downconversion and near infrared luminescence of Tm3+/Yb3+ codoped Calcium Scandate phosphor

An efficient near infrared (NIR) quantum cutting in CaSc2O4: Tm3+/Yb3+ phosphor has been demonstrated by the visible and NIR spectra properties as well as the decay curves of Tm3+: (1)G(4) level. Upon excitation of Tm3+: (1)G(4) level with a blue photon at 466 nm, Yb3+: F-2(5/2) level can emit two NIR photons around 1000 nm through cooperative energy transfer (ET) from Tm3+ to Yb3+ with the maximum energy transfer efficiency is 71%, and highest theoretical quantum efficiency reaches 171%, close to the limit of 200%. Because the energy of Yb3+ transition is matched well with band gap of crystalline silicon, the excellent downconversion luminescence property indicates that CaSc2O4: Tm3+/Yb3+ is a promising oxide material for increasing the conversion efficiency of crystalline silicon solar cells. (C) 2013 Elsevier Ltd. All rights reserved.

Characterization of the Mobility Gap in μc-Si:H Pin Devices

ABSTRACTFor the mobility gap of hydrogenated micro-crystalline silicon (μc-Si:H) a value near 1.1 eV is commonly found, similar to the bandgap of crystalline silicon. However, in other studies mobility gap values have been reported to be in the range of 1.48-1.59 eV. Indeed, for accurate modeling of μc-Si:H solar cells it is paramount that key parameters like the mobility gap are accurately determined. In this work we will discuss a method to determine the mobility gap of μc-Si:H using the dark current activation energy of μc-Si:H pin devices, and apply this method to μc-Si:H solar cells with varying crystalline volume fraction. We found the mobility gap is around 1.2 eV to 1.26 eV for μc-Si:H solar cells with a crystalline volume fraction between 50 % and 70 %. For a highly crystalline solar cell we found a mobility gap of 1.07 eV.

Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation

We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF6) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 μm thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV.

The Electronic Structure of One-Dimensional (1-D), 2-D, and 3-D Silicon Clusters

The electronic structure of one-dimensional (1-D), 2-D, and 3-D silicon clusters have been calculated using the Discrete Variational (DV)- X α Molecular-Orbital method. The calculated results are discussed with respect to the effect of the size of the cluster on the energy level of molecular orbitals. The energy-gap ( E g ) between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) decreases with the increase of cluster size and the number of dimensions. It is found that including silicon 3 d orbitals as basis sets decreases the E g value. The results also show that the components of silicon 3 d orbitals in the unoccupied levels near LUMO are over 50 per cent. The calculated results predict that the E g value will be close to the band gap of crystalline silicon when a 3-D cluster contains more than 1000 silicon atoms.

Hydrogenated Si clusters: Band formation with increasing size.

The electronic structures of various Si clusters of different sizes (with hydrogenated surfaces) are evaluated using a nearest-neighbor empirical tight-binding Hamiltonian which describes well the band structure and fundamental band gap of crystalline silicon. The largest cluster contains 3109 Si atoms and 852 H atoms, has a diameter of 49 \AA{}, and has both a normalized Si density of states and a band gap very close to those of crystalline Si.

Simultaneous measurement of the two-photon coefficient and free-carrier cross section above the bandgap of crystalline silicon

We report what is to our knowledge the first simultaneous measurement of the two-photon absorption coefficient and the free-carrier cross section above the bandgap in a semiconductor. This is also the first observation of two-photon absorption of 1 μm radiation in single-crystal Si at room temperature in a regime where a two-photon stepwise process involving indirect absorption followed by free-carrier absorption is usually dominant. A critical pulsewidth (and fluence) is established below (and above) which two-photon absorption cannot be neglected. Pulses that range from 4 to 100 ps in duration are then used to isolate the irradiance-dependent two-photon absorption from the fluence-dependent free-carrier absorption. We obtain an indirect two-photon absorption coefficient of 1.5 cm/GW and extract a free-carrier cross section of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5 \times 10^{-18}</tex> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> by using a simple technique that does not require a knowledge of the actual carrier density.