Modifying the NW cross section into the rear-opening crescent, the resonant characteristics are affected significantly to show stronger peaks but with less number of hot spots i. Nevertheless, the front-opening crescent modulation to the NW cross-section leads to substantially enhanced absorption performance in both the increased number of field hot spots as well as the increased field intensity.
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The physical reason lies in that the continuously varying crescent tips allows a nearly perfect impedance match between air and the cell, greatly suppressing the reflection and strengthening the optical antenna effect. Therefore, the high performance of the front-opening crescent a-Si:H NW is from both the impedance match as well as the strengthened cavity resonance.
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However, under TM incidence, the situation changes significantly. This is because the TM illumination has the electric field along the radial direction, along which the NW cavity is too small to support sufficient cavity resonances. Therefore, we could not see the distinct patterns in Fig. In this case, the impedance match plays an important role in coupling the incident light into the NW cavity; therefore, the front-opening crescent configuration has the born benefit in enhancing the absorption performance of SNSCs as displayed in Fig.
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Electric field patterns. Normalized electric field patterns corresponding to the peak wavelengths in Fig. Under TE incidence, performance is slightly recovered under backward incidence, but the J ph value is far lower than that with normal incidence. Therefore, high optical performance can be sustained under a very wide range of the incident angle by employing the front-opening crescent cross-sectional design for SNSCs.
Photocurrent densities. The insets are the schematic diagrams under various cell configurations. Therefore, the strong sensitivity of the absorption performance on the NW size facilitates the control and optimization of the optical response. Absorption spectra and electric field patterns.
The electric field patterns of the resonant modes are inserted into the figure. In the above optical simulation, we assumed a perfect internal quantum efficiency i. To have a comprehensive and actual evaluation on the a-Si:H SNSCs, we perform a detailed electrical simulation [ 26 , 31 — 35 ]. The classical electrical parameters of a-Si:H are used in this model, e. The rest parameters including intrinsic carrier concentration, carrier mobility, lifetime, and recombination coefficients can be found in reference [ 35 ].
Furthermore, two electrodes that are shown in the inserted device schematic in Fig. In this study, we neglect the effect of the collector contacts since the NW length is far beyond than its diameter, leaving a very small portion covered by the electrodes. EQE spectra and I - V characteristics. The device schematic and the tabulated electrical parameters are inserted in a and b , respectively. Apparently, realistic carrier transportation process would bring recombination loss for incidences under all wavelengths, leading EQE to be slightly lower than that of Q abs.
As a result, photocurrent density see the inserted table in Fig. Finally, it is necessary to examine the detailed electronic response of the SNSCs under crescent design since the device has an increased surface area which might lead to a higher surface recombination.
The crescent SNSCs exhibits a relatively larger photocurrent loss, revealing a higher carrier recombination.
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To further explore the recombination processes, Fig. It is obvious that the circular SNSCs have a stronger bulk recombination due to the larger device volume; however, the crescent SNSCs exhibit much higher photocurrent loss by surface recombination due to the increased surface area. The combined effect by bulk and surface recombinations results in comparable overall photocurrent losses i. Therefore, despite that the cell surface is increased by crescent design, the device electrical performance can be well sustained. Moreover, we would like to indicate that the ideally poignant crescent tips have been used in the previous simulations.
To take account of the manufacturing capabilities, the blunt tips have to be considered [ 27 ]. In our simulations, we have examined the effect of the blunt tips on the absorption performance. It is found that the blunt tips do not qualitatively impact the value of J ph , i. Photocurrent density losses by carrier recombinations. Current density losses arisen from a bulk and b surface recombinations for the circular and optimized front-opening crescent designs, respectively. In summary, we proposed a design of a-Si:H-based SNSCs with front-crescent cross-sectional morphology in order to obtain broadband and strong optical absorption for efficient photoconversion.
By electromagnetic and carrier-transport calculation, we comprehensively evaluated the optoelectronic performance of the crescent a-Si:H SNSCs.
This design has a good tolerance against the change of the incident angle and a high tunability on the optical resonance. Electrical simulation forecasted that the light-conversion efficiency can be up to Nature — Nat Mater — Nano Lett — Opt Lett — Appl Phys Lett J Appl Phys Nanotechnology Opt Express A—A Nano Energy — Khudiyev T, Bayindir M Superenhancers: novel opportunities for nanowire optoelectronics.
Sci Rep Opt Express — Li X, Zhan Y Enhanced external quantum efficiency in rectangular single nanowire solar cells. Phys Rev Lett Palik ED Handbook of optical constants of solids.
Academic Press, Orlando. Accessed 20 Apr Accessed 21 Apr Prog Photovolt Res Appl — Zhan Y, Li X, Li Y Numerical simulation of light-trapping and photoelectric conversion in single nanowire silicon solar cells. If you are the author of this article you do not need to formally request permission to reproduce figures, diagrams etc.
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