Limits...
Control of hot-carrier relaxation for realizing ideal quantum-dot intermediate-band solar cells.

Tex DM, Kamiya I, Kanemitsu Y - Sci Rep (2014)

Bottom Line: In the last decade, many works were dedicated to improve the TS-TPA efficiency by modifying the QD itself, however, the obtained results are far from the requirements for practical applications.To reveal the mechanisms behind the low TS-TPA efficiency in QDs, we report here on two- and three-beam photocurrent measurements of InAs quantum structures embedded in AlGaAs.Comparison of two- and three-beam photocurrent spectra obtained by subbandgap excitation reveals that the QD TS-TPA efficiency is improved significantly by suppressing the relaxation of hot TS-TPA carriers to unoccupied shallow InAs quantum structure states.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011 Japan [2] Japan Science and Technology Agency, CREST, Uji, Kyoto, 611-0011 Japan.

ABSTRACT
For intermediate-band solar cells, the broad absorption spectrum of quantum dots (QDs) offers a favorable conversion efficiency, and photocurrent generation via efficient two-step two-photon-absorption (TS-TPA) in QDs is essential for realizing high-performance solar cells. In the last decade, many works were dedicated to improve the TS-TPA efficiency by modifying the QD itself, however, the obtained results are far from the requirements for practical applications. To reveal the mechanisms behind the low TS-TPA efficiency in QDs, we report here on two- and three-beam photocurrent measurements of InAs quantum structures embedded in AlGaAs. Comparison of two- and three-beam photocurrent spectra obtained by subbandgap excitation reveals that the QD TS-TPA efficiency is improved significantly by suppressing the relaxation of hot TS-TPA carriers to unoccupied shallow InAs quantum structure states.

No MeSH data available.


Related in: MedlinePlus

Sample structure.GaAs, AlGaAs, and InAs are shown in gray, green, and blue, respectively. Electrons and holes are shown as red and black spheres, respectively. The epitaxially grown sample structure on the right is connected electrically to obtain the PC (I) by subbandgap photon illumination (red arrow) and the applied bias (U). The magnified region shows the InAs/AlGaAs layer, which consists of disk-like QWIs and pyramidal QDs on top of a 1-ML wetting layer. Carriers are created by the absorption of subbandgap photons in the QWIs and QDs. After upconversion via the Auger or TS-TPA process, the carriers are extracted as current. The InAs/GaAs layer consists only of QWIs. A GaAs/AlGaAs QW was introduced for reference purposes.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3927209&req=5

f1: Sample structure.GaAs, AlGaAs, and InAs are shown in gray, green, and blue, respectively. Electrons and holes are shown as red and black spheres, respectively. The epitaxially grown sample structure on the right is connected electrically to obtain the PC (I) by subbandgap photon illumination (red arrow) and the applied bias (U). The magnified region shows the InAs/AlGaAs layer, which consists of disk-like QWIs and pyramidal QDs on top of a 1-ML wetting layer. Carriers are created by the absorption of subbandgap photons in the QWIs and QDs. After upconversion via the Auger or TS-TPA process, the carriers are extracted as current. The InAs/GaAs layer consists only of QWIs. A GaAs/AlGaAs QW was introduced for reference purposes.

Mentions: The sample structure used in this work is shown in Fig. 1 (see Methods for details). Growing InAs on GaAs or AlGaAs with MBE leads to the formation of pyramidal QDs and flat disk-like QWIs. Because the InAs quantum structures have different heights, the QWI states are situated at high energies, whereas the QD states are found at low energies. The PC is generated upon wavelength selective excitation of different structures and extracted via an applied bias. PC data at room temperature are shown in Figs. 2a,b. Figure 2a shows the PC intensity I(λB1) for single-beam excitation with a variable wavelength λB1 in red and PC intensity I(λB1, λIR) for two-beam excitation with a variable λB1 and fixed λIR = 1550 nm in blue obtained with excitation powers of PB1 = 8 mW and PIR = 1.5 mW, corresponding to excitation power densities of approximately 8 and 1.5 W/cm2, respectively. Details of the optical setup are given in the Methods, Measurement system section. Peaks in the PC spectra were assigned to carrier generation in the QWIs and QDs16. Single-beam experiments on QWIs16 and QDs33 have revealed that the major upconversion mechanisms are Auger34 and thermal processes, respectively.


Control of hot-carrier relaxation for realizing ideal quantum-dot intermediate-band solar cells.

Tex DM, Kamiya I, Kanemitsu Y - Sci Rep (2014)

Sample structure.GaAs, AlGaAs, and InAs are shown in gray, green, and blue, respectively. Electrons and holes are shown as red and black spheres, respectively. The epitaxially grown sample structure on the right is connected electrically to obtain the PC (I) by subbandgap photon illumination (red arrow) and the applied bias (U). The magnified region shows the InAs/AlGaAs layer, which consists of disk-like QWIs and pyramidal QDs on top of a 1-ML wetting layer. Carriers are created by the absorption of subbandgap photons in the QWIs and QDs. After upconversion via the Auger or TS-TPA process, the carriers are extracted as current. The InAs/GaAs layer consists only of QWIs. A GaAs/AlGaAs QW was introduced for reference purposes.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3927209&req=5

f1: Sample structure.GaAs, AlGaAs, and InAs are shown in gray, green, and blue, respectively. Electrons and holes are shown as red and black spheres, respectively. The epitaxially grown sample structure on the right is connected electrically to obtain the PC (I) by subbandgap photon illumination (red arrow) and the applied bias (U). The magnified region shows the InAs/AlGaAs layer, which consists of disk-like QWIs and pyramidal QDs on top of a 1-ML wetting layer. Carriers are created by the absorption of subbandgap photons in the QWIs and QDs. After upconversion via the Auger or TS-TPA process, the carriers are extracted as current. The InAs/GaAs layer consists only of QWIs. A GaAs/AlGaAs QW was introduced for reference purposes.
Mentions: The sample structure used in this work is shown in Fig. 1 (see Methods for details). Growing InAs on GaAs or AlGaAs with MBE leads to the formation of pyramidal QDs and flat disk-like QWIs. Because the InAs quantum structures have different heights, the QWI states are situated at high energies, whereas the QD states are found at low energies. The PC is generated upon wavelength selective excitation of different structures and extracted via an applied bias. PC data at room temperature are shown in Figs. 2a,b. Figure 2a shows the PC intensity I(λB1) for single-beam excitation with a variable wavelength λB1 in red and PC intensity I(λB1, λIR) for two-beam excitation with a variable λB1 and fixed λIR = 1550 nm in blue obtained with excitation powers of PB1 = 8 mW and PIR = 1.5 mW, corresponding to excitation power densities of approximately 8 and 1.5 W/cm2, respectively. Details of the optical setup are given in the Methods, Measurement system section. Peaks in the PC spectra were assigned to carrier generation in the QWIs and QDs16. Single-beam experiments on QWIs16 and QDs33 have revealed that the major upconversion mechanisms are Auger34 and thermal processes, respectively.

Bottom Line: In the last decade, many works were dedicated to improve the TS-TPA efficiency by modifying the QD itself, however, the obtained results are far from the requirements for practical applications.To reveal the mechanisms behind the low TS-TPA efficiency in QDs, we report here on two- and three-beam photocurrent measurements of InAs quantum structures embedded in AlGaAs.Comparison of two- and three-beam photocurrent spectra obtained by subbandgap excitation reveals that the QD TS-TPA efficiency is improved significantly by suppressing the relaxation of hot TS-TPA carriers to unoccupied shallow InAs quantum structure states.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011 Japan [2] Japan Science and Technology Agency, CREST, Uji, Kyoto, 611-0011 Japan.

ABSTRACT
For intermediate-band solar cells, the broad absorption spectrum of quantum dots (QDs) offers a favorable conversion efficiency, and photocurrent generation via efficient two-step two-photon-absorption (TS-TPA) in QDs is essential for realizing high-performance solar cells. In the last decade, many works were dedicated to improve the TS-TPA efficiency by modifying the QD itself, however, the obtained results are far from the requirements for practical applications. To reveal the mechanisms behind the low TS-TPA efficiency in QDs, we report here on two- and three-beam photocurrent measurements of InAs quantum structures embedded in AlGaAs. Comparison of two- and three-beam photocurrent spectra obtained by subbandgap excitation reveals that the QD TS-TPA efficiency is improved significantly by suppressing the relaxation of hot TS-TPA carriers to unoccupied shallow InAs quantum structure states.

No MeSH data available.


Related in: MedlinePlus