Preparation of Quantum Dot Solar Cell – QDs

Preparation of Quantum Dot Solar Cell – QDs

Preparation of QDs

In this configuration, the QDs are formed into an ordered 3-D array with inter-QD spacing sufficiently small such that strong electronic coupling occurs and mini bands are formed to allow long-range electron transport; the QD array is placed in the intrinsic region of a p⁺–I–n⁺ structure. The QD collection is a 3-D equivalence to a 1-D super lattice and the mini band assemblies moulded therein. The delocalized quantized 3-D mini band shapes could be anticipated to sluggish the carrier cooling and certificate the transference and assemblage of hot transporters at the corresponding p and n links to harvest an advanced photo potential in a PV cell or else in a photo electrochemical cell where the 3-D QD range is the photo electrode. Also, influence ionization strength predictable to happen in the QD arrays, improving the photocurrent. Nevertheless, hot electron carriage gathering and impact ionization cannot happen instantaneously; they are equally high-class and only single of these procedures can be present in an agreed system [1].

Significant development has been complete in opening 3-D groups of both colloidal and epitaxial II– VI and III–V QDs. The previous have been shaped through evaporation and crystallization of colloidal QD solutions comprising an unvarying QD size circulation; manifestation of QD solids from bigger size supplies hint to close-packed QD solids, but with a high gradation of disorder. Regarding the final, selections of epitaxial QDs have been designed by sequential epitaxial deposition of QD films; after the primarily layer of epitaxial QDs is fashioned, consecutive layers have a tendency to form with the QDs in each layer associated on topmost of each other. Theoretical and experimental studies of the assets of QD arrays are now further down way. Chief matters are the flora of the electronic states as a role of inter-dot detachment, range directive vs. malady, QD direction and shape, surface states, surface structure, and superficial chemistry. Transport belongings of QD arrays are also of serious reputation, and they are in examination [2].

Metal oxides of wide band-gap semiconductors such as TiO₂ and ZnO have been the most often used porous electrodes in QDSSCs. Morphologies of these metal oxides have been actively explored in order to achieve maximum power conversion efficiencies from these devices. For example, nanowires of metal oxides were used in order to increase the conductivity of the films. Sensitization of metal oxides with Nano crystals have been focused on four approaches:

1. QDs have been grown directly onto TiO₂ electrodes using chemical bath deposition (CBD) procedure.
2. Successive ion-layer adsorption and reaction (SILAR) procedure.
3. QDs have been attached to metal oxide surfaces through bi-functional linkers.
4. QDs have been linked to metal oxides using physisorption process.

Chemical bath deposition is the earliest method applied for deposition of semiconductor QDs on metal oxide surfaces. This method gained popularity because of its simplicity and possibility to apply it in large scale production. However, despite its simplicity in general chemical bath deposition does not allow precise control over the particle size distribution and spectral properties of QDs. In recent years chemical bath deposition method has transformed into a more successful technique called successive ion-layer adsorption and reaction method (SILAR). Simultaneously, another method that gained also wide recognition is the linker assisted self-assembly. In this procedure pre-synthesized Nano crystals are adsorbed on metal oxides surface by using molecular linkers that have various functional groups [3].

Moreover, QDs can be solution processed and could be an alternative to commonly employed sensitizer molecules [96]. Figure 1.9(a) indicates the cell building of a QDSC, which entails of an extensive-band-gap meso-porous oxide film resilient electron hole pairs that are swiftly unglued into electrons and holes at the border among the Nano crystalline oxide and QDs. The electrons hedge obsessed by the oxide film, and the holes are free by redox pairs in the electrolyte. Figure 1(b) demonstrate photo persuaded charge transfer methods retaining S² /Sn² as the redox pair (7):

1. Charge injection from an excited QD into TiO₂.
2. Transport of electrons to the collecting electrode surface.
3. Hole transfer to the redox couple.
4. Regeneration of the redox couple.
5. Recombination of electrons as of QD and the oxidized practice of the redox couple.
6. Interfacial recombination of electrons from TiO₂ and the oxidized form of the redox couple.

Kamat group workings and testified that the electron allocation between QD and TiO₂ was an ultrafast progression with a frequency constant of the command of 10¹⁰ 10¹¹ s ¹, which was quicker than that of hole handover (10⁷ 10⁹ s ¹). So the recombination wounded developed a foremost influence in preventive the global efficiency [4-7].

Fig. 1.9: (a) Representation diagram of the structure of a QDSC; and (b) representation diagram of photo induced charge transfer process. Adapted from [8].

In a distinctive progression for the production of QDSC photo-electrodes, QDs can be familiarised by means of two methods:

• In situ growth straight from precursor solutions.
• Adsorption of pre-synthesized QDs with or without a bi-functional linker.

Nevertheless, the QDSCs shaped by the last method have comparatively low adaptation efficiency, mostly due to the trouble in attaining adequate attention of QDs [9]. The former includes chemical bath deposition (CBD) [10] and successive ionic layer absorption and reaction (SILAR) [10,11].

And it has been exposed to achieve healthier than the concluding when being accepted to collect QDSCs. The CBD is a comparatively humble process to credit QDs and nanoparticle films, and it holds many rewards, for instance constant yielding besides uniform and decent reproducibility. The growing of QDs sturdily be subject to on the progress environments, for example the size of deposition, composition and temperature of the solution, and features of the mesoporous films. The SILAR method is founded on consecutive reactions on the superficial oxides. So a thin film can be grown up layer by layer. Figure 1.10 indicates the graphic drawing of the creation procedure of CdS and CdSe QD co-sensitized solar cells. The QDSCs assembled with a TiO₂ mesoporous film, CdS CdSe QDs, a polysulfide electrolyte, and a Cu₂S counter-electrode show a great power adaptation efficiency of 4.62% [12, 13].

Fig. 1.10: Draught of the formation of CdS CdSe QDs and the J V arch of QDSCs. Adapted from [14].

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradioactive exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

Handling of energy and electron transfer procedures in a light collecting assemblage is an imperative standard to copycat usual photosynthesis. Peoples currently flourished in successively accumulating CdSe quantum dot (QD) and squaring dye (SQSH) on TiO₂ film and pair up energy and electron transfer developments to create photocurrent in an amalgam solar cell. When involved unconnectedly, both CdSe QDs and SQSH vaccinate electrons into TiO₂ below noticeable near-IR treatment. But CdSe QD if connected to TiO₂ with SQSH linker contributes in an energy transmission process.

By energy adaptation competences in nonstop growth, quantum dot sensitized solar cells (QDSCs) are presently beneath an accumulative attention, but here is an absenteeism of a comprehensive model for these expedients. To amass the newest expansions in this generous of cells so as to achieve tall efficiency QDSCs, demonstrating the presentation. Zen’s coating and earlier growth of CdS were examined. Polysulfide electrolyte and Cu₂S counter electrodes were cast-off to deliver advanced photocurrents and fill factors, FF. Incident photon-to-current competence peaks as extraordinary as 82%, beneath complete 1 sun illumination, were gotten, which almost overwhelms the photocurrent curb commonly pragmatic in QDSCs [15].

References:

1. J. Nozik, unpublished manuscript, 1996.
2. B. Murray, C.R. Kagan, M.G. Bawendi, Annu. Rev. Mater. Sci. 30 (2000) 545.
3. Sugawara (Ed.), Semiconductors and Semimetals, Vol. 60, Academic Press, San Diego, 1999.
4. Nakata, Y. Sugiyama, M. Sugawara, in: M. Sugawara (Ed.), Semiconductors and Semimetals, Vol. 60, Academic Press, San Diego, 1999, p. 117
5. Yum, J.-H., Choi, S.-H., Kim, S.-S., Kim, D.-Y., Sun, Y.-E., 2007. CdSe quantum dots sensitized TiO₂ electrodes for photovoltaic cells. J. Korean Electrochem. Soc. 10, 257–261.
6. Lee, W., Kang, S.H., Min, S.K., Sung, Y.-E., Han, S.-H., 2008b. Co-sensitization of vertically aligned TiO₂ nanotubes with two different sizes of CdSe quantum dots for broad spectrum. Electrochem. Commun. 10, 1579–1582.
7. Toyoda, T., Uehata, T., Suganuma, R., Tamura, S., Sato, A., Yamamoto, K., Kobayashi, N., Shen, Q., 2007. Crystal growth of CdSe quantum dots adsorbed on nanoparticle, inverse opal, and nanotube TiO₂ photoelectrodes characterized by photoacoustic spectroscopy. Jpn. J. Appl. Phys. 46, 4616–4621.
8. Baker, D., Kamat, P., 2009. Photosensitization of TiO₂ nanotubes with CdS quantum dots: Particulate versus tubular support architectures. Adv. Funct. Mater. 19, 805–811.
9. Chen, S., Paulose, M., Ruan, C., Mor, G.K., Varghese, O.K., Kouzoudis, D., Grimes, C.A., 2006. Electrochemically synthesized CdS nanopar-ticle-modified TiO₂ nanotube-array photoelectrodes; preparation, characterization, and application to photoelectrochemical cells. J. Photochem. Photobiol. A 177, 177–184.
10. Chen, J., Wu, J., Lei, W., Song, J.L., Deng, W.Q., Sun, X.W., 2010a. Co-sensitized quantum dot solar cell based on ZnO nanowires. Appl. Surf. Sci. 256, 7438–7441.
11. Chang, C.-H., Lee, Y.-L., 2007. Chemical bath deposition of CdS quantum dots onto mesoscopic TiO₂ films for application in quan-tum-dot-sensitized solar cells. Appl. Phys. Lett. 91, 053503.
12. Gime´nez, S., Mora-Sero´, I., Macor, L., Guijarro, N., Lana-Villarreal, T., Go´mez, R., Diguna, L.J., Shen, Q., Toyoda, T., Bisquert, J., 2009. Improving the performance of colloidal quantum-dot-sensitized solar cells. Nanotechnology 20, 295204.
13. Blackburn, J.L., Selmarten, D.C., Ellingson, R.J., Jones, M., Micic, O., Nozik, A.J., 2005. Electron and hole transfer from indium phosphide quantum dots. J. Phys. Chem. B 109, 2625–2631.
14. Simurda, M.S., Ne˘mec, P., Formananek, P., Ne˘mcova, I., Maly´, P., 2006. Morphology of CdSe films prepared by chemical bath deposition: The role of substrate. Thin Solid Films, 71–75.
15. Bang, J.H., Kamat, P.V., 2009. Quantum dot sensitized solar cells. A tale of two semiconductor nanocrystals: CdSe and CdTe. ACS Nano 3, 1467–1476.

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