Quantum Dot Solar Cell (QDS)
The aspect of crystals of Nano of the semiconductor as the blocks of the structure of nanotechnology has opened new traditions to apply them in solar cells of the following generation. The emphases in the new progresses in the operation populate with the points of the quantum of the semiconductor for the slight adaptation of the energy. Three main conducts to apply points of the semiconductor in solar cell include;
- Metal-semiconducting or cell photovoltaic of the joint of Schottky.
- Hybrid solar cell of the Polymer-semiconducting.
- Solar cell sensitized point of Quantum.
The flexion of the excessive control as large as the energies of the bandage proposes new methods to the photo-answer of the regulator and the effectiveness of the photo-conversion of the solar cell. Several policies to maximize the photo-induced processes of the transference of the separation and the electron of the load to refine the global effectiveness of the slight conversion of the energy are available. To indicate the efforts of the investigation that come in the way to the use from nanostructures adapted will be a significant challenge for the progress in solar cells 36 of the following generation .
These last years the solar cells of the colloidal points of the quantum have been the subject of the extensive investigation. A promising alternative existing the solar silicon cells, solar cells of the point of the quantum is between the candidates for the devices photovoltaic of the following generation. The colloidal points of the quantum are attractive in the investigation photovoltaic due to their capacity of the process of the solution that is useful for their integration in the several solar cells. Here, we reviewed the recent one we progressed in the solar cells of I vary point of the quantum that are prepared of colloidal points of the quantum. We discussed the methods of the preparation, the work concepts, the advantages and the disadvantages of the architectures event of the device of I gave ﬀ. The discussed important subjects in this revision include the integration of the colloidal points of the quantum in: The solar cells of Schottky, the exhausted solar cells of heterojunction, the solar cells of the extremely fine shock absorber, the solar cells organic-inorganic hybrids, the solar cells of heterojunction in bulk and the point of the quantum sensitized the solar cells. The revision organizes according to the principle of work and the architecture of the devices photovoltaic .
These last years, the inorganic crystals of semiconducting Nano of the (also known like puntea Quantum, QDs) have been to sees promising it pave cell [3, 4] of the following generation. QDs have been explored due to its size and competitive dependent absorption [5-9].
The colloidal points of the quantum have been used in solar cells of heterojunction in bulk also. [10, 11] hand reposed recent progress in this field. Due to the great research interests in one-dimensional (1D) structures highlighted integration of colloidal nanoparticles in Nano rods and nanowires based to pave cells In addition , has reviewed a wide range of applications of quantum dots in optoelectronic devices such as photo detectors, light emitting diodes and briefly about quantum dot solar cells.
CdSe quanta dots have been prepared with diﬀerent sizes and exploited as inorganic dye to sensitize a wide band gap TiO2 thin films for QDs solar cells. The synthesis is based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and permits controlled growth of macroscopic quantities of Nano crystallites. XRD, HRTEM, UV-visible, and PL were used to characterize the synthesized quantum dots. The results showed CdSe quantum dots with sizes ranging from 3 nm to 6 nm which enabled the control of the optical properties and consequently the solar cell performance. Solar cell of 0.08% performance under solar irradiation with a light intensity of 100 mW/cm2 has been obtained. CdSe/TiO2 solar cells without and with using Mercator propionic acid (MPA) as a linker between CdSe and TiO2 particles despite a Voc of 428 mV, Jsc of 0.184 mAcm−2, FF of 0.57, and η of 0.05% but with linker despite a Voc of 543 mV, Jsc of 0.318 mAcm−2, FF of 0.48, and η of 0.08%, respectively .
A high surface area pn-heterojunction between TiO2 and an organic p-type charge transport material (spiro-OMeTAD) was sensitized to visible light using lead sulphide (PbS) quantum dots. PbS quantum dots were formed in situ on a Nano crystalline TiO2 electrode using chemical bath deposition techniques. The organic whole conductor was applied from solution to form the sensitized heterojunction. The structure of the quantum dots was analysed using HRTEM technique. Ultrafast laser photolysis experiments suggested the initial charge separation to proceed in the sub picosecond time range. Transient absorption laser spectroscopy revealed that interfacial charge recombination of the initially formed charge carriers is much faster than in comparable dye-sensitized systems. The sensitized heterojunction showed incident photon-to-electron conversion efficiencies (IPCE) of up to 45% and energy conversion efficiencies under simulated sunlight AM1.5 (10 mW/cm2) of 0.49% .
Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photo-generated carriers to produce higher photo-voltages or higher photocurrents. The former effect is based on mini band transport and collection of hot carriers in QD array photo electrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photo=electrodes comprising QD arrays, (2) QD-sensitized Nano-crystalline TiO2, and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times .
Finally, in recent years it has been proposed and experimentally verified in some cases , that the relaxation dynamics of photo-generated carriers may be markedly affected by quantization effects in the semiconductor (i.e., in semiconductor quantum wells, quantum wires, QDs, superlatives, and nanostructures). That is, when the carriers in the semiconductor are confined by potential barriers to regions of space that are smaller than or comparable to their de-Broglie wavelength or to the Bohr radius of exactions in the semiconductor bulk, the relaxation dynamics can be dramatically altered; specifically the hot carrier cooling rates may be dramatically reduced, and the rate of impact ionization could be-come competitive with the rate of carrier cooling.
Semiconductors such as CdS, CdSe, CdTe, CuInS2, Cu2S, PbS, PbSe, InP, InAs, Ag2S, Bi2S3 and Sb2S3 have been synthesized as QDs and deposited onto wide-band gap nanostructures as sensitizers. Depending on their size, these materials can absorb photons over a broad spectral range or within a confined window of the solar spectrum. Especially CdS, CdSe and PbS have been used to investigate the operating principles of QDSCs. The limited number of materials listed indicates that much room remains for materials scientists to identify new semiconductors which can be applied as QD sensitizers. Table 1 lists the most common material compositions used for QDSCs. Due to the many publications in later years, the table only includes a fraction of the work done in this field, in order to highlight articles that contain a detailed description of QD deposition and methods for solar-cell fabrication.
CdSe quantum dots (QDs) with different organic linker molecules are attached to ZnO nanowires (NWs) to study the luminescence dynamics and the electron tunnelling from the QDs to the nanowires in time-resolved photoluminescence (PL) and photoconductivity measurements. The PL transients of the QD luminescence indicate two different recombination channels: the direct recombination inside the QD core and the recombination via QD surface defect states. After linking the QDs to the ZnO NW surface, photo-induced electron tunnelling from an excited state of the QD into the conduction band of the nanowire becomes visible by a clear decrease of the PL decay time. Efficient electron tunnelling is confirmed by a strong enhancement of the photocurrent through the functionalized nanowires in which the tunnelling rate can be controlled by using different organic linker molecules .
Properties of QDs
During the decade last colloidal QDs they have been integrated in the types event of I gave ﬀ of solar cells for example: The exhausted solar cells of heterojunction of the solar cells of Schottky extremely rarefy the solar cells of hybrid polymer of the cells of the shock absorber, the solar cells of hetero-junction inorganic-organic and the solar cells sensitized point of the quantum (QDSSCs) [54-62]. Figure 1.4 demonstrations used the typical architectures of the device in the solar cells of QDs.
CdSe QDs, capsuled with trioctylphosphine (COVER), was prepared by a solvo-thermal route that allows control of the size. Briefly, the selenium reacts with microstate of cadmium in toluene in the presence of acid oleic and of the COVER. The reaction happens in ◦C 180 in a sealed sterilizer. Changing the time of reaction, diverse sizes of the points of the quantum were obtained. The like-prepared colloidal solutions were characterized by a PC of Shimadzu UV-2401. In the case of electrochemical measures of the photo, the points of the quantum of CdSe were purified previously by the ethanol precipitation, the isolation by the centrifugalization and the movement, three times. Picture 1.5 demonstrates the phantoms of absorption of the three dispersions of QD with diverse sizes of QD used in this study. The QDs was synthesized on 2, 5, 5 and 15 H. The changes in the wavelength of the absorption limit reflect the confinement of quantization of the size, the tip that it changes of position to lower wavelengths whereas the size of QD diminishes. Comparing the maximum of the absorption limit (495, 517 and 536 nanometre) corresponding to the first transition exciton with the curves disclosed by Peng and the fellow workers, the diameters of QD could be obtained (2, 3, 2, 5 and 2, 8 nanometre, respectively) [18, 19].
Figure 1.5: UV–vies absorption spectrum of colloidal CdSe QDs, informed with three diverse reaction periods, 15, 5 and 2.5 h, and dissimilar quantum dot sizes. Adapted from .
Electronic Properties of QDs
Slowed hot electron freezing in InP QDs has been practical by the research exploration assembly at NREL . For QDs, one mechanism for breaking the phonon bottleneck that is predicted to slow carrier cooling in QDs and hence allow fast cooling is an Auger process. Here a hot electron can give its excess kinetic energy to a thermalized hole via an Auger process, and then the hole can then cool quickly because of its higher effective mass and more closely spaced quantized states. This effect was first shown for CdSe QDs; it has now also been shown for InP QDs, where a fast hole trapping species (Na biphenyl) was found to slow the electron cooling to about 7 ps. This is to be compared to the electron cooling time of 0:3 PS for passivized InP QDs without a whole trap present and thus where the holes are in the QD core and able to undergo an Auger process with the electrons [21-23].
QDs are very trifling semiconductor Nano crystals with a size analogous to the Bohr radius of an exciton. For utmost semiconductors, the Bohr radius of an exciton is in the series of 1-10 nm: for instance, it is 4.2 nm for Si, 3.1 nm for CdS, 6.1 nm for CdSe, and 2.2 nm for ZnO. Yet, the Bohr radius of the exciton is actually huge for more or less semiconductors: it is 20.4 nm for PbS, 46 nm for PbSe, and 67.5 nm for InSb. Owing to the facet effect, the performance of electrons in QDs varies from that in the agreeing bulk substantial, which is baptised as the ‘quantum confinement effect’. A semiconductor with a bigger exciton Bohr radius means that the QDs made from the factual may reach a strong quarantine effect more simply. For the reason that the quantum confinement effect, the band gap energy (Eg) of QD rises with the reduction of particle dimensions. Eg can be explained by Eg 8 1/r2, wherever r is the radius of QD. The surge of Eg earnings that additional energy will be desired so as to be engrossed by the QD. So the variety of optical fascination wavelengths of QD can be altered by governing the size of QD. Such a piece of QDs with tuneable Eg has directed to their submissions in light-emitting diodes (LEDs) for full-colour presentations, and in QD-sensitized solar cells for the group of optical absorption at favourite wavelengths [23-30].
Ref. (36) has established that the conduction band energy (Ecb) of QD would go up to the in height energy level by lessening particle size because of the quantum confinement effect, which is very important to solar cells. Fig. 1.6 (a) shows the Ecb of bulk PbS is 4.74 eV, which is inferior than that of TiO2 (4.21 eV). The electrons in the conduction band of bulk PbS is hard to hedge into the conduction band of TiO2. The Eg of PbS can be enlarged by dropping its particle size, as shown in Fig. 1.6 (b), chief to the conduction band minimum move to higher energy. Once the conduction band energy of PbS ties that of TiO2, the electrons separated from exciton couples by photons can simply handover from the conduction band in PbS interested in the conduction band in TiO2. So the quantum confinement effect is the essential ailment for the building of QDSCs. For QDSCs, smaller QDs are favourite so as to attain more QDs adsorbed on the photo electrode film. Smaller QDs have also established an advanced electron injection degree than their larger matching part.
Fig. 1.7: (a) The requirement of the electron transfer rate on the energy and (b) an outline exemplifying the attitude of electron transmission from two dissimilar CdSe quantum dots hooked on a TiO2 nanoparticle .
Figure 1.7: demonstrate the necessity of the electron transmission rate fixed on the energy alteration among the conduction bands and the cipher of electron transmission from two altered-sized CdSe QDs into a TiO2 nanoparticle . It can be realised that the electron allocation rate obviously surges with lessening QD size in the CdSe TiO2 system. For QDs, the surge of band gap would be predictable to consume constructive conduction band energies for vaccinating electrons obsessed by a photo electrode.
But, the rise of Eg directs that single extraordinary energy photons can be engrossed by the QD, foremost to the absorption wavelength control of the QD blue swing as exposed in Fig. 1.8: Too minor QDs will pointer to too much stumpy the optical absorption on behalf of the photo electrodes, which has bad influences on the solar cells. Thus, the top solar-to-electricity alteration effectiveness can be gotten by improving the band energy assembly of QDs to contest the oxide film and find an extensive optical raptness wavelength .
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