**5.6. Quantum dot sensitized solar cells**

**5.4. Polymer-semiconductor structure configuration**

**Figure 15.** Possible p-i-n QD arrangements for solar cells.From [11].

318 Solar Cells - New Approaches and Reviews

**5.5. Depleted heterojunction quantum dots solar cells**

overcome the disadvantages encountered with Schottky cell.

dots [33].

In 2002 Huynh and his coworkers [32] investigated photovoltaic action in hybrid nanorodspolymer solar cells and under Air Mass (A.M.) 1.5 Global solar conditions, a power conversion efficiency of 1.7 % was obtained. This works and others encouraged many groups to investigate incorporation of quantum dots in polymers. Quantum dots are dispersed in organic-semicon‐ ductor polymer matrices. For example, in a hole-conducting polymer such as MEH-PPV (poly(2-methoxy,5-(2-ethyl)-hexyloxy-pphenylenevinylene) disordered arrays of CdSe quantum dots are synthesized. When the cell illuminated, the absorbed photons results in photo-generated carriers. As illustrated in Figure 16 [31], the photo-generated holes are injected into the MEH-PPV polymer phase which is in contact with metallic electrode to facilitate their collection.Sun et al., in 2003 reported that solar power conversion efficiencies of 1.8 % were achieved under AM1.5 illumination fora device containing 86 wt % of quantum

In depleted heterojunction colloidal quantum dot solar cells as detailed in ref. [24] a nano‐ structured wide bandgap semiconductor such as TiO2 and quantum dot film are sandwiched between conductive transparent electrode (glass coated with Tin Oxide SnO2F) and metallic (such as gold) coated electrode (see Figure 17-a).Figure 17-b illustrate the energy band diagram. Although TiO2 has low carrier density (~1016 cm-3) compared to metal, a depletion region in the cells forms due to charge transfer to QD film. And because of high electron density in metal (~ 1022 cm-3), the depletion is negligible on its side of the cell. Depleted heterojunction cell The structure and operation principle of QD sensitized photovoltaic cell is almost identical to dye sensitized cells [9, 34,35] with the exception that now the QDs are the source of current injection. Quantum dots can be produced in situ or more without difficulty adsorbed from a colloidal QD solution. The structure of the photovoltaic cell is shown schematically in Figure 18. In this figure, we distinguish four essential elements of the cell, namely, the conducting and counter conducting electrodes, the nanostructured TiO2 layer, the quantum dot energy levels, and the electrolyte.

The operation of the cell can be described by the following steps and the corresponding process equations:

**1.** Upon absorption of a photon, a quantum dot is excited from the ground state (QDS) to a higher energy state (QDS \* ), as illustrated by Eq.(1) below.

**Figure 17.** (a) Schematic diagram of Depleted-heterojunction Colloidal Quantum Dots Solar Cells, (b) energy band dia‐ gram. From [24].

$$\text{Excitation process:}\,\text{QD}\_{\mathbb{S}} + \text{h.v.} \to \text{QD}\_{\mathbb{S}}\tag{9}$$

Where *QDS* and QDS\* is the quantum dot in its ground state and excited state respectively.

**2.** The absorption process results in the creation of electron-hole pair in the form of exciton. Dissociation of the exciton occurs if the thermal energy exceeds its binding energy.

$$\text{Exaction dissociation} : \text{QD}\_{\text{S}}^{\*} \to \text{e}^{\*} + \text{h}^{\*} \text{(free carriers)}\tag{10}$$

**3.** The excited electron is then injected in the conduction band of the wide bandgap semi‐ conductor nanostructured TiO2 thin film. This process will cause the oxidation of the photosensitizer (The QDs).

$$\text{Injection process}: \text{QD}\_{\text{S}}^{\*} + \text{TiO}\_{2} \rightarrow \text{TiO}\_{2}(\text{e}^{\*}) + \text{QD}\_{\text{S}}^{\*} \tag{11}$$

**4.** The injected electron is transported between the TiO2 nanoparticles, and then gets extracted to a load where the work done is delivered as electrical energy.

$$\text{Energy generation}: \text{TiO}\_2\text{(e}^{-\text{s}}\text{)} + \text{C.E.} \rightarrow \text{TiO}\_2 + \text{e}^{-\text{s}} \text{ (C.E.)} + \text{electrical energy} \tag{12}$$

**Figure 18.** Schematic diagram illustrating the structure and operation of quantum dots-sensitized photovoltaic cell.

Excitation

Where *QDS* and

gram. From [24].

Figure 17

320 Solar Cells - New Approaches and Reviews

QDS

photosensitizer (The QDs).

process

:QDS + h ν → QDS

**2.** The absorption process results in the creation of electron-hole pair in the form of exciton. Dissociation of the exciton occurs if the thermal energy exceeds its binding energy.

**Figure 17.** (a) Schematic diagram of Depleted-heterojunction Colloidal Quantum Dots Solar Cells, (b) energy band dia‐

**3.** The excited electron is then injected in the conduction band of the wide bandgap semi‐ conductor nanostructured TiO2 thin film. This process will cause the oxidation of the

**4.** The injected electron is transported between the TiO2 nanoparticles, and then gets

extracted to a load where the work done is delivered as electrical energy.

*Energy generation* : TiO (e ) + C.E. TiO + e (C.E.) + el 2 2

\* is the quantum dot in its ground state and excited state respectively.

® ( ) \* -\* +\* QD e <sup>S</sup> *Exciton dissociation* : + h free carriers (10)

\* ® -\* <sup>+</sup> QD + TiO TiO (e )+ D S2 2 : <sup>Q</sup> *<sup>S</sup> Injection process* (11)


\* (9)

Where C.E. stands for counter electrode. The counter electrode is identical to the photoelec‐ trode where the nanostructured TiO2 is deposited. The counter electrode is usually coated with a catalyst (graphite).

**5.** Most common electrolytes used in QDSCs are aqueous polysulfide and organic electrolyte with I‐/I3‐redox couple. In some works the liquid electrolyte has been replacedwith solid‐ state hole conductors such as spiro‐OMeDAT andCuSCN [36]. Assuming electrolyte used in the cell contains I- /I3 redox ions, that play the role of electron mediator between the TiO2 photoelectrode and the counter electrode [9]. Therefore, the oxidized photosensitizer states (QDS +) are regenerated by receiving an electron from the oxidized Iion redox mediator, regenerating the ground state (QDS), and Ibecomes oxidized to the oxidized state I3 - (triodide ions).

$$\text{Regeneration of } \mathbf{QDs}: \mathbf{QD}\_S^+ + \frac{3}{2}\mathbf{I}^\cdot \rightarrow \mathbf{QD}\_S + \frac{1}{2}\mathbf{I}\_3^\cdot \tag{13}$$

**6.** The I3 diffuses to the counter electrode and substitutes the internally donated electron with that from the external load and gets reduced back to Iion.

$$\text{Electron capture reaction}: \frac{1}{2}\text{I}\_3^\cdot + \text{e}^\cdot \text{(C.E)} \rightarrow \frac{3}{2}\text{I}^\cdot + \text{C.E.}\tag{14}$$

Overall, generation of electric power in this type of cells causes no permanent chemical transformation.

To enhance electron injection into the conduction band of the TiO2 thin film, one must choose a sensitizer with a proper matching energy gap. Quantum dots can fulfill the necessary energy gap requirement by choosing the ones with the proper size. It is interesting to note that for the QD to effectively accept the donated electron from the redox mediator. Finally, the maximum potential produced by the cell is determined by the energy separation between the electrolyte chemical potential (Eredox) and the Fermi level (EF) of the TiO2 layer, as shown in Figure 18.

The bulk of many research works done on QD synthesized solar cells focused on CdS, CdSe, and CdTe QD as sensitizers [37-39] The choice of these materials follows the success of earlier studies on identifying the morphological and electrolyte effects on their performance and stability [40-41]. Incident Photon to carriers conversion efficiency IPCE is directly related to the product of light harvesting efficiency = 1-10-A(λ) (where A(λ) is the spectral absorbance of the quantum dots sample, electron injection efficiency Φinj (how efficient are electrons injected from excited quantum dot into TiO2 conduction band), and electron collection efficiency Φcoll (how efficent are electrons are collected by the photoelectrode), hence we have:

$$IPCE = LHE \times \Phi\_{inj} \times \Phi\_{coll} \tag{15}$$

Light harvesting efficiency could be affected by both the type and size of quantum dots. For example PdS based quantum dots have broader spectral absorbance than CdS quantum dots. Kamat reported that charge injection from excited CdSe quantum dots into nanostructured TiO2 film can be controlled by varying solution pH as illustrated in Figure 19 "At increasing solution pH, the conduction band of TiO2 shifts 59 mV/pH unit to a more negative potential, thereby decreasing the driving force and thus decreasing the rate of nonradiative electron transfer from excited CdSe. The emission yield and the average emission lifetime increase with increasing pH, thus providing a way to monitor the variation in medium pH." [42].

Kongkanand and co-workers has investigated the effect of quantum dots size on charge injection rate [43]. They found that smaller-sized CdSe quantum dots show greater charge injection rates and also higher IPCE at the excitonic band.Interestingly, Larger particles have better absorption in the visible region, on the other hand,it cannot inject electrons into TiO2 as effectivelyas smaller-sized CdSe quantum dots. It has been found that surface treatments can strongly influence charge transfer, recombination, and transport processes of photogenerated electrons and holes in QDSCs [44]. Figure 20 schematically shows that transport of electrons through the nonporous electrode is dominated by diffusion [45]. A path taken by an electron is not simply a straight line. An electron undergoes through a crisscross path before reaching


2 2 *Electron capture reaction* (14)

= ´F ´F *inj coll IPCE LHE* (15)

3 : 1 3 I + e (C.E) I + C.E.

Overall, generation of electric power in this type of cells causes no permanent chemical

To enhance electron injection into the conduction band of the TiO2 thin film, one must choose a sensitizer with a proper matching energy gap. Quantum dots can fulfill the necessary energy gap requirement by choosing the ones with the proper size. It is interesting to note that for the QD to effectively accept the donated electron from the redox mediator. Finally, the maximum potential produced by the cell is determined by the energy separation between the electrolyte chemical potential (Eredox) and the Fermi level (EF) of the TiO2 layer, as shown in Figure 18.

The bulk of many research works done on QD synthesized solar cells focused on CdS, CdSe, and CdTe QD as sensitizers [37-39] The choice of these materials follows the success of earlier studies on identifying the morphological and electrolyte effects on their performance and stability [40-41]. Incident Photon to carriers conversion efficiency IPCE is directly related to the product of light harvesting efficiency = 1-10-A(λ) (where A(λ) is the spectral absorbance of the quantum dots sample, electron injection efficiency Φinj (how efficient are electrons injected from excited quantum dot into TiO2 conduction band), and electron collection efficiency Φcoll

Light harvesting efficiency could be affected by both the type and size of quantum dots. For example PdS based quantum dots have broader spectral absorbance than CdS quantum dots. Kamat reported that charge injection from excited CdSe quantum dots into nanostructured TiO2 film can be controlled by varying solution pH as illustrated in Figure 19 "At increasing solution pH, the conduction band of TiO2 shifts 59 mV/pH unit to a more negative potential, thereby decreasing the driving force and thus decreasing the rate of nonradiative electron transfer from excited CdSe. The emission yield and the average emission lifetime increase with

Kongkanand and co-workers has investigated the effect of quantum dots size on charge injection rate [43]. They found that smaller-sized CdSe quantum dots show greater charge injection rates and also higher IPCE at the excitonic band.Interestingly, Larger particles have better absorption in the visible region, on the other hand,it cannot inject electrons into TiO2 as effectivelyas smaller-sized CdSe quantum dots. It has been found that surface treatments can strongly influence charge transfer, recombination, and transport processes of photogenerated electrons and holes in QDSCs [44]. Figure 20 schematically shows that transport of electrons through the nonporous electrode is dominated by diffusion [45]. A path taken by an electron is not simply a straight line. An electron undergoes through a crisscross path before reaching

increasing pH, thus providing a way to monitor the variation in medium pH." [42].

(how efficent are electrons are collected by the photoelectrode), hence we have:

transformation.

322 Solar Cells - New Approaches and Reviews

**Figure 19.** Illustration of how charge injection from excited CdSe quantum dots into nanostructured TiO2 film can be controlled by varying solution pH. From [42].

the photoelectrode. The electron diffusion length, *Ln* is related to electron lifetime (*τn*) and diffusion coefficient (*Dn*) as:

$$L\_n = \sqrt{D\_n \tau\_n} \tag{16}$$

**Figure 20.** Illustrate a path taken by an electron after being injected into nanoporous TiO2 layer (Not to scale). The nanoporous TiO2 layer has been imaged using scanning electron microscope SEM.

Cell conversion efficiency is affect by morphology of the photoelectrode [43]. Tube type is more advantageous to the fast electronic conduction, due to shorter diffusion path comparedwith the particle type. For example, OTE / TiO2 (Nanoparticles)/CdSe: 0.6 % versus Ti / TiO2 Nanotubes /CdSe: 0.7 %.

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 recombination layer [47].

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 ratio reduces appreciably the efficiency of light generation.

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 ethylene glycol) containing a redox couple (traditionally the iodide/tri-iodide I- /I3-couple). The 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 with power conversion efficiency =1.8 %.

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

**Figure 21.** I-V characteristics of PbS quantum dot sensitized solar cell. Quantum dots have on the average radius of 3.2 nm.
