**5.7. Quantum dot-dye sensitized solar cells**

Interesting results have been reported by some investigators who studied the incorporation of a layer of PbS quantum dots in thin film solar cells, by direct growth of PbS quantum dots on nanostructured TiO2 electrodes [27]. Deposition of a transition metal oxide (n-type) layer on grown layer of PbS quantum dots to act as hole extractor layers [46] or employing a graded

Several methods have been employed to prepare TiO2 thin layer. We prepared nanostructured thin films following the procedure detailed in [9, 34]. In this method, a suspension of TiO2 is prepared by adding 9 ml of nitric acid solution of PH 3-4 (in ml increment) to 6 g of colloidal P25 TiO2 powder in mortar and pestle. To get a white free flow-paste, we added 8 ml of distilled water (in 1 ml increment) during the grinding process. Finally, a drop of transparent surfactant is added in 1 ml of distilled water to ensure uniform coating and adhesion to the transparent conducting electrode. It was found that the ratio of the nitric acid solution to the colloidal P25 TiO2 powder is a critical factor for cell performance. If the ratio exceeds certain threshold value, the resulting film becomes too thick and has a tendency to peel off. On the other hand, a low

Doctor blade technique was employed by depositing the TiO2 suspension uniformly on a cleaned (rinsed with ethanol) conductive plate. The TiO2 film was allowed to dry for few minutes and then annealed at approximately 450 °C (in a well-ventilated zone) for about 15 minutes to form a nanoporous TiO2 layer as shown in Figure 20. The conductive plate is then allowed to cool slowly to room temperature. This is a necessary condition to remove stresses and avoid cracking of the glass or peeling off the TiO2 layer. Once the TiO2 nanocrystalline layer is prepared, it is coated with colloidal QDs. The counter electrode is coated with graphite that Acts as a catalyst in regenerating quantum dots. Both the photo-and counter electrodes are clamped together and drops of electrolyte are applied to fill the clamped cell. The electro‐ lyte used is iodide electrolyte (0.5 M potassium iodide mixed with 0.05 M iodine in water free

measurements of the open circuit voltage and short circuit current have been performed under direct illumination from a solar simulator producing 1 sun illumination. UV or IR cut-off filters have been used to eliminate carrier generation from TiO2 layer and to impede cell overheat‐ ing.No antireflection coatings on the photoelectrode have been applied. Figure 21 shows an example of the obtained I-V characteristics of PdS quantum dots (size 3.2 nm) sensitized cell

As it's the case with dye sensitized solar cells, quantum dots sensitized solar cells light harvesting efficiency could be enhanced via efficient light management [48] by increasing light scattering effect [49]. The distance traveled by transmitted photons in the cell is increases by multiple scattering and hence get highly probable by the sensitizer. Surface plasmons reso‐ nance effect in the cell also has been suggested [50]. Another approach that is effective in enhancing photovoltaic effect is the reduction of the charge recombination by controlling transparent-conducting-oxide/electrolyte interface such that injected electrons in photoelec‐

/I3-couple). The

ethylene glycol) containing a redox couple (traditionally the iodide/tri-iodide I-

trode are excluded from recombining with the redox couple in electrolyte.

ratio reduces appreciably the efficiency of light generation.

with power conversion efficiency =1.8 %.

recombination layer [47].

324 Solar Cells - New Approaches and Reviews

Incorporation of dye with quantum dots as sensitizer of wide bandgap semiconductor has attracted attention of many research groups. For example, we found that some natural dyes are enhancing power conversion efficiency, while some others are not. Figure 22 shows an example of the I-V characteristics of a first round study of assembled cells illuminated with a collimated beam from a hot filament lamp. In cell preparation we followed the same strategy described in section 5.6 such that after coating the photoelectrode with PbS quantum dots it was soaked in dye for an hour. Then, the electrode was rinsed with deionized water and ethanol. After that the cell is assembled and tested. The dye used was extracted from a pomegranate.

New configuration based on quantum dot-dye bilayer-sensitized solar cells has been demon‐ strated by Zaban and co-workers [51]. The bi-sensitizer layer cell is made up of a nanocrys‐ talline TiO2/CdS QD+amorphous TiO2/N719 dye. The main aim was to providea configuration having increased charge-separation efficiency by slowing the interfacial charge recombination processes that resulted in 250% increase in cell efficiency compared to a QD monolayer cell.

The configuration investigated by Zaban and co-workers [52] established on making colloidal quantum dots (CdSe/CdS/ZnS core/shell/shell quantum dots) that serve as antennas. Via nonradiative energy transfer, absorbed light is funneled to the charge separating dye molecules (SQ02 dye molecules). The colloidal quantum dot donors are incorporated into the solid TiO2 photoelectrode resulting in high energy transfer efficiency as well assubstantial improve‐

Figure 22. I-V<\$%&?>characteristics<\$%&?>of<\$%&?>typical<\$%&?>assembled<\$%&?>quantum<\$%&?>dotdye<\$%&?>sensitized<\$%&?>solar<\$%&?>cell.<\$%&?>Quantum<\$%&?>dots<\$%&?>average<\$%&?>size<\$%&?>of<\$%&?>2.4<\$%&?>n **Figure 22.** I-V characteristics of typical assembled quantum dot-dye sensitized solar cell. Quantum dots average size of 2.4 nm and pomegranate dye extract used as sensitizers of TiO2 nanoporous layer.

ment of the cell stability. In this approach the processes of light absorption can be separated from charge carrier injection. Therefore, this approach enablesoptimization of each independ‐ ently. Time resolved luminescence measurements relate the significant contribution of the QDs to the spectral response of the cell in the presence of the dye to Förster resonance energy transfer from the QDs to the dye molecules. m<\$%&?>and<\$%&?>pomegranate<\$%&?>dye<\$%&?>extract<\$%&?>used<\$%&?>as<\$%&?>sensitizers<\$%&?>of<\$%&?>TiO2<\$%&?>n anoporous<\$%&?>layer. New<\$%&?>configuration<\$%&?>based<\$%&?>on<\$%&?>quantum<\$%&?>dot-dye<\$%&?>bilayersensitized<\$%&?>solar<\$%&?>cells<\$%&?>has<\$%&?>been<\$%&?>demonstrated<\$%&?>by<\$%&?>Zaban<\$%&?>and<\$ %&?>co-workers<\$%&?>[51].<\$%&?>The<\$%&?>bi-

The efficiency of solar cells can be enhanced by combining quantum dots with some dye and used as a sensitizer. We suggest doing further investigations in order to understand QD-dye system. The performance of dye sensitized or quantum dots solar cells can be increased by optimizing preparation technique, using different types of electrolyte, utilizing different nanostructures (e.g., rods, stars), and replacing TiO2 with other types of wide bandgap semiconductors such as zinc oxide ZnO. sensitizer<\$%&?>layer<\$%&?>cell<\$%&?>is<\$%&?>made<\$%&?>up<\$%&?>of<\$%&?>a<\$%&?>nanocrystalline<\$%&?>Ti O2/CdS<\$%&?>QD<\$%&?>+<\$%&?>amorphous<\$%&?>TiO2/N719<\$%&?>dye.<\$%&?>The<\$%&?>main<\$%&?>aim<\$% &?>was<\$%&?>to<\$%&?>providea<\$%&?>configuration<\$%&?>having<\$%&?>increased<\$%&?>chargeseparation<\$%&?>efficiency<\$%&?>by<\$%&?>slowing<\$%&?>the<\$%&?>interfacial<\$%&?>charge<\$%&?>recombinatio n<\$%&?>processes<\$%&?>that<\$%&?>resulted<\$%&?>in<\$%&?>250%<\$%&?>increase<\$%&?>in<\$%&?>cell<\$%&?>effic iency<\$%&?>compared<\$%&?>to<\$%&?>a<\$%&?>QD<\$%&?>monolayer<\$%&?>cell.

#### **6. Conclusion** The<\$%&?>configuration<\$%&?>investigated<\$%&?>by<\$%&?>Zaban<\$%&?>and<\$%&?>coworkers<\$%&?>[52]<\$%&?>established<\$%&?>on<\$%&?>making<\$%&?>colloidal<\$%&?>quantum<\$%&?>dots<\$%&?>(

olecules.

Crystalline semiconductor solar cells besides possessing low efficiency due to their band gab limit (Shockley-Queisser limit)[53] they are expensive in terms of manufacturing cost per generated Watt of delivered electric power. In single junction bulky semiconductor solar cells, photon of energies less than the band gap are wasted since none of them are absorbed. Moreover, excess energy of those photons with energies greater than the bandgap is wasted as heat as a result of hot-carriers thermalization. A quantum dot is a crystalline semiconductor nanoparticles. Examples of well investigated quantum dots structures are CdS, CdSe, PbS, and PbSe. The operation principle of quantum dots sensitized solar cell is similar to that of the dye CdSe/CdS/ZnS<\$%&?>core/shell/shell<\$%&?>quantum<\$%&?>dots)<\$%&?>that<\$%&?>serve<\$%&?>as<\$%&?>antennas .<\$%&?>Via<\$%&?>nonradiative<\$%&?>energy<\$%&?>transfer,<\$%&?>absorbed<\$%&?>light<\$%&?>is<\$%&?>funneled <\$%&?>to<\$%&?>the<\$%&?>charge<\$%&?>separating<\$%&?>dye<\$%&?>molecules<\$%&?>(SQ02<\$%&?>dye<\$%&?>m olecules).<\$%&?>The<\$%&?>colloidal<\$%&?>quantum<\$%&?>dot<\$%&?>donors<\$%&?>are<\$%&?>incorporated<\$%&? >into<\$%&?>the<\$%&?>solid<\$%&?>TiO2<\$%&?>photoelectrode<\$%&?>resulting<\$%&?>in<\$%&?>high<\$%&?>energy< \$%&?>transfer<\$%&?>efficiency<\$%&?>as<\$%&?>well<\$%&?>assubstantial<\$%&?>improvement<\$%&?>of<\$%&?>the< \$%&?>cell<\$%&?>stability.<\$%&?>In<\$%&?>this<\$%&?>approach<\$%&?>the<\$%&?>processes<\$%&?>of<\$%&?>light<\$ %&?>absorption<\$%&?>can<\$%&?>be<\$%&?>separated<\$%&?>from<\$%&?>charge<\$%&?>carrier<\$%&?>injection.<\$%

> &?>Therefore,<\$%&?>this<\$%&?>approach<\$%&?>enablesoptimization<\$%&?>of<\$%&?>each<\$%&?>independently.<\$ %&?>Time<\$%&?>resolved<\$%&?>luminescence<\$%&?>measurements<\$%&?>relate<\$%&?>the<\$%&?>significant<\$% &?>contribution<\$%&?>of<\$%&?>the<\$%&?>QDs<\$%&?>to<\$%&?>the<\$%&?>spectral<\$%&?>response<\$%&?>of<\$%& ?>the<\$%&?>cell<\$%&?>in<\$%&?>the<\$%&?>presence<\$%&?>of<\$%&?>the<\$%&?>dye<\$%&?>to<\$%&?>Förster<\$%&?> resonance<\$%&?>energy<\$%&?>transfer<\$%&?>from<\$%&?>the<\$%&?>QDs<\$%&?>to<\$%&?>the<\$%&?>dye<\$%&?>m

> The<\$%&?>efficiency<\$%&?>of<\$%&?>solar<\$%&?>cells<\$%&?>can<\$%&?>be<\$%&?>enhanced<\$%&?>by<\$%&?>comb ined<\$%&?>the<\$%&?>quantum<\$%&?>dots<\$%&?>with<\$%&?>some<\$%&?>dyes.<\$%&?>We<\$%&?>suggest<\$%&?>f

> dye<\$%&?>system.<\$%&?>However,<\$%&?>the<\$%&?>performance<\$%&?>of<\$%&?>dye<\$%&?>sensitized<\$%&?>sol ar<\$%&?>cells<\$%&?>or<\$%&?>quantum<\$%&?>dots<\$%&?>solar<\$%&?>cells<\$%&?>can<\$%&?>be<\$%&?>increased< \$%&?>by<\$%&?>optimizing<\$%&?>preparation<\$%&?>technique,<\$%&?>using<\$%&?>different<\$%&?>types<\$%&?>of <\$%&?>electrolyte<\$%&?>,<\$%&?>utilizing<\$%&?>different<\$%&?>nanostructures<\$%&?>(e.g.,<\$%&?>rods,<\$%&?>sta rs),<\$%&?>and<\$%&?>replacing<\$%&?>TiO2<\$%&?>with<\$%&?>other<\$%&?>types<\$%&?>of<\$%&?>wide<\$%&?>bandga

urther<\$%&?>investigations<\$%&?>to<\$%&?>understand<\$%&?>the<\$%&?>interaction<\$%&?>of<\$%&?>QD-

p<\$%&?>semiconductors<\$%&?>such<\$%&?>as<\$%&?>zinc<\$%&?>oxide<\$%&?>ZnO.

sensitized solar cells DSSCs. In a quantum dot, confinement effect arises from size effect when particle size is smaller or comparable to exciton Bohr radius. As the size of the quantum dot decreases its characteristic excitonic beak get blue shifted.

Growth or synthesis methods of quantum dots are well established. In quantum dots, the rate of energy dissipation is significantly reduced, and the charge carriers are confined within a minute volume, thereby increasing their interactions and enhancing the probability for generating multiple excitons due to hot carries mechanism. There are many proposed quantum dot solar cells configurations.

The functional principle of QD-sensitized solar cell is the same as that of DSSC. The difference is that the dye in DSSC is replaced with quantum dots. This class of third generation solar cell is promising and recently attracting considerable attention. Operation principles, and per‐ formance limitations are well understood and many solutions have been proposed to enhance cell efficiency.

#### Figure 22. I-V<\$%&?>characteristics<\$%&?>of<\$%&?>typical<\$%&?>assembled<\$%&?>quantum<\$%&?>dot-**Acknowledgements**

m<\$%&?>and<\$%&?>pomegranate<\$%&?>dye<\$%&?>extract<\$%&?>used<\$%&?>as<\$%&?>sensitizers<\$%&?>of<\$%&?>TiO2<\$%&?>n New<\$%&?>configuration<\$%&?>based<\$%&?>on<\$%&?>quantum<\$%&?>dot-dye<\$%&?>bilayer-The author is greatly indebted to prof. Dr. Shawqi Aldallal for his kind support and encour‐ agements. His valuable suggestions are highly appreciated. Special thanks to Miss. Fatema Aljaboori for here assistance in obtaining the I-V characteristic presented in Figure 21.

#### sensitizer<\$%&?>layer<\$%&?>cell<\$%&?>is<\$%&?>made<\$%&?>up<\$%&?>of<\$%&?>a<\$%&?>nanocrystalline<\$%&?>Ti **Author details**

ment of the cell stability. In this approach the processes of light absorption can be separated from charge carrier injection. Therefore, this approach enablesoptimization of each independ‐ ently. Time resolved luminescence measurements relate the significant contribution of the QDs to the spectral response of the cell in the presence of the dye to Förster resonance energy transfer

**Figure 22.** I-V characteristics of typical assembled quantum dot-dye sensitized solar cell. Quantum dots average size of

**Without Dye**

**0 50 100 150 200 250 300 350**

**Voltage (mV)**

**With Dye**

dye<\$%&?>sensitized<\$%&?>solar<\$%&?>cell.<\$%&?>Quantum<\$%&?>dots<\$%&?>average<\$%&?>size<\$%&?>of<\$%&?>2.4<\$%&?>n

sensitized<\$%&?>solar<\$%&?>cells<\$%&?>has<\$%&?>been<\$%&?>demonstrated<\$%&?>by<\$%&?>Zaban<\$%&?>and<\$

O2/CdS<\$%&?>QD<\$%&?>+<\$%&?>amorphous<\$%&?>TiO2/N719<\$%&?>dye.<\$%&?>The<\$%&?>main<\$%&?>aim<\$%

workers<\$%&?>[52]<\$%&?>established<\$%&?>on<\$%&?>making<\$%&?>colloidal<\$%&?>quantum<\$%&?>dots<\$%&?>(

&?>Therefore,<\$%&?>this<\$%&?>approach<\$%&?>enablesoptimization<\$%&?>of<\$%&?>each<\$%&?>independently.<\$ %&?>Time<\$%&?>resolved<\$%&?>luminescence<\$%&?>measurements<\$%&?>relate<\$%&?>the<\$%&?>significant<\$% &?>contribution<\$%&?>of<\$%&?>the<\$%&?>QDs<\$%&?>to<\$%&?>the<\$%&?>spectral<\$%&?>response<\$%&?>of<\$%& ?>the<\$%&?>cell<\$%&?>in<\$%&?>the<\$%&?>presence<\$%&?>of<\$%&?>the<\$%&?>dye<\$%&?>to<\$%&?>Förster<\$%&?> resonance<\$%&?>energy<\$%&?>transfer<\$%&?>from<\$%&?>the<\$%&?>QDs<\$%&?>to<\$%&?>the<\$%&?>dye<\$%&?>m

The<\$%&?>efficiency<\$%&?>of<\$%&?>solar<\$%&?>cells<\$%&?>can<\$%&?>be<\$%&?>enhanced<\$%&?>by<\$%&?>comb ined<\$%&?>the<\$%&?>quantum<\$%&?>dots<\$%&?>with<\$%&?>some<\$%&?>dyes.<\$%&?>We<\$%&?>suggest<\$%&?>f

dye<\$%&?>system.<\$%&?>However,<\$%&?>the<\$%&?>performance<\$%&?>of<\$%&?>dye<\$%&?>sensitized<\$%&?>sol ar<\$%&?>cells<\$%&?>or<\$%&?>quantum<\$%&?>dots<\$%&?>solar<\$%&?>cells<\$%&?>can<\$%&?>be<\$%&?>increased< \$%&?>by<\$%&?>optimizing<\$%&?>preparation<\$%&?>technique,<\$%&?>using<\$%&?>different<\$%&?>types<\$%&?>of <\$%&?>electrolyte<\$%&?>,<\$%&?>utilizing<\$%&?>different<\$%&?>nanostructures<\$%&?>(e.g.,<\$%&?>rods,<\$%&?>sta rs),<\$%&?>and<\$%&?>replacing<\$%&?>TiO2<\$%&?>with<\$%&?>other<\$%&?>types<\$%&?>of<\$%&?>wide<\$%&?>bandga

urther<\$%&?>investigations<\$%&?>to<\$%&?>understand<\$%&?>the<\$%&?>interaction<\$%&?>of<\$%&?>QD-

p<\$%&?>semiconductors<\$%&?>such<\$%&?>as<\$%&?>zinc<\$%&?>oxide<\$%&?>ZnO.

The efficiency of solar cells can be enhanced by combining quantum dots with some dye and used as a sensitizer. We suggest doing further investigations in order to understand QD-dye system. The performance of dye sensitized or quantum dots solar cells can be increased by optimizing preparation technique, using different types of electrolyte, utilizing different nanostructures (e.g., rods, stars), and replacing TiO2 with other types of wide bandgap

%&?>co-workers<\$%&?>[51].<\$%&?>The<\$%&?>bi-

Crystalline semiconductor solar cells besides possessing low efficiency due to their band gab limit (Shockley-Queisser limit)[53] they are expensive in terms of manufacturing cost per generated Watt of delivered electric power. In single junction bulky semiconductor solar cells, photon of energies less than the band gap are wasted since none of them are absorbed. Moreover, excess energy of those photons with energies greater than the bandgap is wasted as heat as a result of hot-carriers thermalization. A quantum dot is a crystalline semiconductor nanoparticles. Examples of well investigated quantum dots structures are CdS, CdSe, PbS, and PbSe. The operation principle of quantum dots sensitized solar cell is similar to that of the dye

iency<\$%&?>compared<\$%&?>to<\$%&?>a<\$%&?>QD<\$%&?>monolayer<\$%&?>cell.

The<\$%&?>configuration<\$%&?>investigated<\$%&?>by<\$%&?>Zaban<\$%&?>and<\$%&?>co-

from the QDs to the dye molecules.

anoporous<\$%&?>layer.

2.4 nm and pomegranate dye extract used as sensitizers of TiO2 nanoporous layer.

**0.0**

**0.1**

**0.2**

**0.3**

**Photocurrent (mA)**

326 Solar Cells - New Approaches and Reviews

**0.4**

**0.5**

**0.6**

semiconductors such as zinc oxide ZnO.

olecules.

**6. Conclusion**

&?>was<\$%&?>to<\$%&?>providea<\$%&?>configuration<\$%&?>having<\$%&?>increased<\$%&?>charge-Khalil Ebrahim Jasim

separation<\$%&?>efficiency<\$%&?>by<\$%&?>slowing<\$%&?>the<\$%&?>interfacial<\$%&?>charge<\$%&?>recombinatio n<\$%&?>processes<\$%&?>that<\$%&?>resulted<\$%&?>in<\$%&?>250%<\$%&?>increase<\$%&?>in<\$%&?>cell<\$%&?>effic Department of Physics, College of Science, University of Bahrain, Kingdom of Bahrain
