Browsing by Author "Kechiantz, A. M."

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Band alignment and conversion efficiency in Si/Ge type-II quantum dot intermediate band solar cells
(2007-10-10)
Kechiantz, A. M.
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Kocharyan, L. M.
;
Kechiyants, H. M.
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應用化學系分子科學碩博班
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Institute of Molecular science
The concept of the intermediate band (IB) solar cells (SC) offers the promise of achieving 63% conversion efficiency devices. The effect of the type II band alignment in the quantum dot (QD) IB SCs on the above percentage is analyzed and the potential of the Ge/Si system for fabrication of the type II QD IB SC is discussed. Also, it is shown that the increase of the sunlight concentration leads to the rise of the potential barrier around QDs and the concentration of S-x approximate to 700 can induce the epsilon(L) = 0.2 eV height barrier in the Ge/Si system, making this a significant result. Furthermore, the increase of the sunlight concentration leads to the separation of the quasi-Fermi levels from the confined states and also leads to the decrease of the recombination activity in QDs. The two-photon absorption in QDs increases rapidly and dominates over recombination at the moderate concentration. As the contributions of QDs to both the photo- and dark currents in the type II QD IB SC are evaluated it is shown that, compared to the conventional Si SCs, the type II Ge QD IB Si SCs can generate about 25% higher photocurrent and conversion efficiency.
Carrier capture and relaxation in charged quantum dots triggered by vibrational relaxation
(2008-01-01)
Kechiantz, A. M.
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Sun, K. W.
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應用化學系分子科學碩博班
;
Institute of Molecular science
Local polarization and the strain of the crystal lattice around the charged quantum dot in semiconductors must vibrate to relax when the mobile carrier is captured by the dot and makes it uncharged. Triggered vibrations induce the sudden perturbation of electronic states in the quantum dot. The Hamiltonian of electron interaction with a local vibrating field and carrier capture time are calculated. It is shown that the principle of uncertainty has a strong influence on this effect. The calculated values of electron capture time are on a sub-picosecond timescale at a high pumping intensity, and are strongly dependent on the scale of quantum confinement. The calculations fit well with photoluminescence rising times tau = 0.45ps [K. Gundogdu, K.C. Hall, T.F. Boggess, D.G. Deppe, O.B. Shchekin, Appl. Phys. Lett. 85 (2004) 4570.] and tau = 1.7ps [K.W. Sun, A. Kechiantz, B.C. Lee, C.P. Lee, Appl. Phys. Lett. 88 (2006) 163117.] which are observed in the experiments on p-doped InAs quantum dots. (C) 2007 Elsevier B.V. All rights reserved.
Highly active nanocrystalline TiO(2) photoelectrodes
(2008-03-19)
Paronyan, Tereza M.
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Kechiantz, A. M.
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Lin, M. C.
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應用化學系
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應用化學系分子科學碩博班
A simple method for the fabrication of highly photoactive nanocrystalline two-layer TiO(2) electrodes for solar cell applications is presented. Diluted titanium acetylacetonate has been used as a precursor for covering SnO(2):F (FTO) films with dense packed TiO(2) nanocrystallites. The nanoporous thick TiO(2) film follows the dense packed thin TiO(2) film as a second layer. For the latter, amorphous TiO(2) nanoparticles have been successfully synthesized by a sol-gel technique in an acidic environment with pH < 1 and hydrothermal growth at a temperature of 200 degrees C. The acidic nanoparticle gel was neutralized by basic ammonia and a TiO(2) gel of pH 5 was obtained; this pH value is higher than the recently reported value of 3.1 ( Park et al 2005 Adv. Mater. 17 2349 - 53). Highly interconnected, nanoporous, transparent and active TiO2 films have been fabricated from the pH 5 gel. SEM, AFM and XRD analyses have been carried out for investigation of the crystal structure and the size of nanoparticles as well as the surface morphology of the films. Investigation of the photocurrent-voltage characteristics has shown improvement in cell performance along with the modification of the surface morphology, depending on pH of the TiO(2) gel. Increasing the pH of the gel from 2.1 to 5 enhanced the overall conversion efficiency of the dye-sensitized solar cells by approximately 30%. An energy conversion efficiency of 8.83% has been achieved for the cell (AM1.5, 100 mWcm(-2) simulated sunlight) compared to 6.61% efficiency in the absence of ammonia in the TiO(2) gel.
Strong barrier effect on the conversion efficiency of solar cells with buried type-II quantum dots
(2007-06-01)
Kechiantz, A. M.
;
Sun, K. W.
;
Kechiyants, H. M.
;
Kocharyan, L. M.
;
應用化學系
A model of a quantum dot (QD) buried solar cell is described. The cell takes advantage of the generation of an additional photocurrent by a two-photon excitation of electrons from the valence band into the conduction band via the confined ( intermediate) state. Since the intermediate states are active recombination centres suppressing the open-circuit voltage and the conversion efficiency either by increasing the dark-current or by arresting the quasi-Fermi level of mobile carriers, a barrier layer promoting the separation of the quasi-Fermi levels is built in around the QDs. Conditions for the separation of the quasi-Fermi levels and the activation of the two-photon generation of mobile carriers were found. Under these conditions the photocurrent and the conversion efficiency of the Ge QD buried Si solar cell exposed to concentrated sunlight must be approximately 25% larger than that of conventional Si solar cells.