Hybrid organic and inorganic solar cell based on a cyanine dye and quantum dots

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 166–174

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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Hybrid organic and inorganic solar cell based on a cyanine dye and quantum dots

T

Mostafa F. Abdelbara, , Tarek A. Fayedb, Talaat M. Meazc, Thiyagu Subramanid, Naoki Fukatad, El- Zeiny M. Ebeidb,e ⁎

a

Nanoscience & Nanotechnology Institute, Kafrelsheikh University, Kafrelsheikh, Egypt Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt c Physics Department, Faculty of Science, Tanta University, Tanta, Egypt d International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan e Misr University for Science and Technology (MUST), 6th of October City, Egypt b

ARTICLE INFO

ABSTRACT

Keywords: Aqueous quantum dots Monomethine cyanine dyes Quenching Energy transfer Co-sensitization Hybrid solar cells

The combination of organic dyes with quantum dots (QDs) in solar cells can lead to more improvement in photovoltaic performance for such types of solar cells. Due to the quantum confinement, the photoluminescence (PL) of QDs can be tuned to match the absorption spectra of applied organic dyes. Herein, we introduce a new hybrid solar cell based on monomethine cyanine dye co-sensitized by CdS and CdTe/CdS core shell. The cyanine dye tends to aggregate on the surface of colloidal QDs due to electrostatic attraction followed by energy and electron transfer from QDs to cyanine dye. Electrophoretic deposition method was carried out to assist the cyanine dye loading on the surface of TiO2 thin film and the highest power conversion efficiency was 0.89% in case of cyanine dye co-sensitized by CdTe/CdS with open circuit voltage of 0.78 V and a photocurrent of 1.80 mA/ cm2.

1. Introduction Green sources of energy become an imperative issue for all countries due to the global warming resulting from continuous increasing of carbon dioxide level. Dye-sensitized solar cells (DSSCs) have attracted much attention since the first implementation by Grätzel et al. in 1991 [1]. Due to the ease and low production cost, this type of solar cells competes the silicon-based one. The different types of sensitizers such as merocyanines [2], cyanine [3], squaraine and phthalocyanine dyes [4], were recently applied but the power conversion efficiency is still limited. The quantum efficiency for this type of solar cells is limited by the absorption of the applied dye. The dye plays a crucial role in designing efficient DSSCs as it should capture as much incident light as possible by optimization of the absorption strength (molar extinction coefficient) and overlap of the absorption with the solar spectrum (i.e., the absorption spectral width). Simultaneously, the dye should inject the photogenerated electron into the semiconductor oxide in a time domain faster than the time of electron- hole recombination and the time for the oxidized dye to be regenerated by the redox electrolyte. The higher absorptivity, broad absorption spectra and photostability are the most important factors that determine the efficiency of the

⁎

organic dyes in photovoltaic applications. Due to the lake of presence for organic sensitizer which can achieve such requirements to higher photovoltaic performance, an alternative approach to capture the light over a wide range of absorption spectra by co-sensitization using multiple dyes has been studied and verified with increased light harvesting properties [5–8]. Another way for co-sensitization principle is to use QDs due to its broad absorption spectra, higher extension coefficient and sufficient photostability. Due to the large Bohr radius (7.3 nm) for CdTe which allows the PL tuning under this size, it is a promising candidate for most relevant research. The photostability and photophysical properties of CdTe QDs can be improved by growing a shell of CdS, ZnS, or ZnSe on CdTe core [9]. CdS is the best candidate shell for CdTe for two reasons; the first is that its band gap (2.5 eV) is higher than CdTe (1.5 eV) and secondly for the lattice parameter matching with CdTe Core (96.4%) [9]. Now, one of the research objectives is to find a good combination between organic and inorganic materials as hybrid architectures to increase the efficiency of DSSCs. However, the outcome efficiency has not reached to ruthenium dyes- based solar cells (11%). The efficiency of hybrid solar cells is significantly depending on the interaction taking place between the organic and inorganic parts. There is a great

Corresponding author. E-mail address: [email protected] (M.F. Abdelbar).

https://doi.org/10.1016/j.jphotochem.2019.01.023 Received 8 July 2018; Received in revised form 23 January 2019; Accepted 24 January 2019 Available online 28 January 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 166–174

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challenge to improve the hybrid organic/inorganic photovoltaic materials. The study of interaction between different nanoparticles and organic dyes plays a crucial role to develop new sensitizers for such type of solar cells. In this article, we introduce a detailed photophysical study on the interaction between water soluble thioglycolic acidcapped QDs and thiazole orange monomethine dye for the application in hybrid solar cells.

The paste was prepared by a ball-milling method at room temperature. 4 g of mesoporous titania powder was ball-milled with 5 ml of ethylene glycol for 2 h, then 12.6 g of citric acid was added and ballmilling continued for another 2 h. 0.16 g hydroxyethylcellulose, 0.5 ml distilled water and 0.5 ml ethanol were added and ball-milled product for another 2 h. The film was prepared using the paste which was casted on the conducting glass by doctor-blade method. The cyanine dye Cy (1 mM) was loaded on titania film using electrophoretic deposition method. The working electrode was connected to negative terminal of power supply using cupper connector with 1 cm separation distance between the two electrodes. The applied voltage was optimized at 15 V for different time intervals to avoid the film degradation and dye bleaching. The QDs were loaded after cyanine dye deposition using drop casting of 5 mg QDs in 1 ml deionized water. Iodide/ triiodide electrolyte was prepared according to procedure [14]. Briefly, 0.133 g lithium iodide (0.1 M) and 0.676 g 4-t-butylpyridine (0.5 M) were mixed in 10 ml acetonitrile and stirred for 2 h in an ice bath. 1.59 g 1,2dimethyl-3-propylimidazolium iodide (0.6 M) was added and the mixture was further stirred for 2 h. 0.126 g iodine was added and the mixture was stirred for 4 h and stored at 5 °C. Platinum- coated conducting counter electrode was prepared by spin coating of 1 mM hexachloroplatinic acid solution on 2 × 2 cm2 FTO glass, dried at 100 °C and calcinated at 500 °C for 1 h.

2. Experimental cyanine dye was prepared according to the literature using microwave assisted solvent-free synthesis [10] according to Scheme 1. In brief, benzothiazolium salt (2 mmol) and 4-methylquinolinium salt (2 mmol) with a few drops of triethylamine mixed in a glass conical flask. The mixture was subjected to microwave irradiation with 260 W for 5 min. After cooling and washing with diethyl ether, orange to yellowish orange precipitate was obtained. 2.1. 1-methyl-4-((3-methylbenzo[d]thiazol-2(3 H)-ylidene)methyl) quinolin-1-ium (Cy) Yellowish orange crystals, yield: 87%; m.p.: 272–273 °C; IR(KBr): ν = 1465 (SH), 1519, 1616 cm−1 (C = C, C = N); 1H NMR (DMSO-d6): δ = 1.39 (t, j = 7.0 Hz, 3H, CH3), 4.15 (s, 3H, CH3), 4.62 (q, j = 6.8 Hz, 2H, CH2), 6.89 (s, 1H, =CH), 7.32–8.76 (m, 10H, Ar-H); 13C NMR: δ = 12.2 (CH3), 40.9 (CH2), 42.3 (CH3), 87.1 (=CH), 107.8, 112.6, 118.1, 122.8, 123.9, 124.4, 125.4, 126.9, 128.2, 133.1, 137.9, 139.4, 144.8, 148.6 (Ar-C), 158.8, (NCS).

2.4. Apparatus Steady state electronic absorption spectra were recorded on a Shimadzu double beam UV–vis-NIR scanning spectrophotometer (UV3101 PC). The quenching studies were performed by steady-state fluorescence spectra which were recorded using a Perkin-Elmer LS 50B Scanning Spectrofluorometer. Fluorescence lifetime has been recorded by utilizing a high performance fluorescence lifetime spectrometer FluoTime 300 (PicoQuant, Germany). Lifetimes were evaluated with software FluoFit attached to the equipment. PET CT50AAA solar simulator with an AM1.5 G spectrum was used for the current–voltage solar cell characterization. The intensity was calibrated to 100 mW/cm2 by using a standard silicon reference cell.

2.2. Aqueous synthesis of QDs CdS nanocrystals were prepared according to previously reported method [11]. The thioglycolic acid was used as a source of sulfur and capping agent at the same time. To do that, H2O2 was used to decompose TGA and release the sulfur in basic medium. The molar ratio of Cd2+/TGA/H2O2 was 0.01: 1.32: .0.33. The size of QDs increases with time when the mixture was refluxed at 100 °C. The preparation of CdTe/CdS nanocrystals was carried out according to previously reported method [12]. (0.0190 g) Te powder and 0.028 g sodium borohydride were added to 8 mL deionized water with stirring to prepare NaHTe solution. Then, CdCl2·2.5H2O (0.1370 g) and TGA (80 μL) were dissolved in 150 mL distilled water, and the pH was adjusted to 12.0 using 1 mol L−1 NaOH solution. NaHTe solution was injected into cadmium chloride solution under N2 flow over 30 min. The molar ratio of Cd2+/TGA/Te was fixed at 1:2:0.25. The mixture was refluxed at 100 °C under open air condition with a condenser for 1 h. To form a shell from CdS, 0.0045 g of thioacetamide was added and the solution was refluxed for another 30 min at 100 °C. The thioglycolic acidcapped QDs were precipitated from the reaction vessels to remove unreacted ions by the addition of excess isopropanol and the precipitate was collected by centrifugation (4000 rpm) and further washed by isopropanol, dried and stored at 4 °C for further use.

3. Results and discussion The absorption and emission spectra of CdS QDs are presented in Fig. 1. There is a red shift in both spectra during the growth of nanocrystals. The emission peaks at 464 nm, 478 nm, 490 nm and 508 nm are blue shifted compared to bulk CdS at 650 nm. The fluorescence quantum yields for prepared CdS was 19%. CdS QDs had sufficient stability and fluorescence spectrum for stored samples (4 °C), almost had no change with time. Fig. 1.c show the PL spectra of as prepared CdS QDs and PL after 10 days at 4 °C. It is clearly observed that there is no change in PL spectrum which elucidates the QDs stability. The calculated size using Eq. (1) was 2.8 nm according to excitonic peak at 380 nm. The extinction coefficient for the prepared size was 229,947 LM−1.

CdS: D = ( 6.6521x10 8)

2.3. Solar cell fabrication Nanocrystalline TiO2 was prepared by a modified sol-gel method [13]. A volume 62.5 mL of distillated water and 20 mL of glacial acetic acid were mixed into a 500 mL round bottom flask under ice bath. A mixture of 2.5 mL of 2-propanol and 9.25 mL of titanium (IV) butoxide was then dropped slowly to acetic acid solution over a 60–70 min period under vigorous stirring. Then, the obtained transparent colloid was refluxed for 8 h under vigorous stirring at 80 °C in a hot water bath. The resulting gel was autoclaved under hydrothermal condition and then cooled down to room temperature slowly. The resulting suspension was filtered and the precipitate was then washed several times with distilled water. Finally, the product was dried for 24 h at 100 °C.

CdS :

=

3

+ (1.9557x10 4)

2

(9.2352x10 2)

+ (13.29)

(1)

21536(D )2.3

(2)

Fig. 2 presents the absorbance changes during CdS shell formation, the absorption spectra show a new CdS excitonic peak above CdTe excitonic one. This shell resulted in a useful effect in the surface modification as shown in Fig. 2.a which indicates the relative enhancement of CdTe/CdS emission compared with CdTe core only. The nucleation process of CdTe is also continued during the shell growth so that a red shift (30 nm) has been observed in PL spectra by increasing the growth time as shown in Fig. 2.b. We also noticed a successive decrease in PL intensity of CdTe/CdS with CdS growth after 40 min but 167

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Scheme 1. Microwave synthesis of Cy.

of the cubic CdTe (6.48 Å) and the cubic CdS phases as a result of CdS shell and incorporating sulfur into growing nanoparticles and this confirms the zinc blende crystal structure for QDs. The Scherrer’s Eq. (3) was applied to calculate the particle size “d” for CdS QDs

it was still higher than the PL intensity of CdTe core. This is due to the surface trap states which have a negative influence on the radiative decay in core shell structures. The fluorescence quantum yield for the prepared CdTe/CdS was 43% compared to core CdTe of 37%. Fig. 3 shows the X- ray diffraction patterns for CdS and CdTe/CdS QDs. The diffraction peaks were strong and sharp confirming that the QDs were well crystallized. Two characteristic peaks were observed, one peak at 26° (111), and another broad band at about 45° resulting from the overlap of (220) and (311) diffractions. The peaks match well with cubic lattice parameter for CdS a = 5.832 Å. For CdTe/CdS, the lattice parameter was 6.15 Å which is intermediate between the value

d = 0.9 λ / β cos θ

(3)

Where β is the full width of XRD peak at half maximum (FWHM), θ is the angle of diffraction and λ is the wavelength of X-ray radiation. The particle size calculated from this method (3 nm) nearly matches the value obtained from UV–Vis absorption method (2.8 nm). The particles were found to be monodisperse with a particle size of around 5 nm as

Fig. 1. (a) Absorption and (b) emission spectra (λex = 380 nm) for different aliquots taken during the synthesis of CdS QDs at 80 0C for time intervals (15, 30, 45 and 60 min), c) emission spectra of as prepared CdS QDs and after 10 days at 4 °C. 168

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Fig. 2. a) Absorption spectra and emission spectra of CdTe QDs and the CdTe/CdS QDs b) Emission spectra of CdTe QDs and CdTe/CdS QDs core shell samples taken at refluxing times at 100 °C for (1) 20, (2) 40, (3) 60 and (4) 80 min.

where I and I0 are the fluorescence intensities in the presence and absence of quencher (cyanine), respectively, KSV is the Stern-Volmer quenching constant and [Q] is the concentration of quencher. The quenching constant can be calculated from the plot of I0/ I vs. [Q]. The second order quenching rate constant kq was calculated from the lifetime value (average value of τ = 26 ns) and KSV where Ksv =Kq τ. The value of kq was calculated as 1.7 × 1012 M−1s−1, which far exceeds the diffusion rate constant in aqueous media indicating a static – type quenching mechanism. [15] Fig. 6. Show the change in the optical absorbance of QDs with the gradual addition of cyanine dye. We noticed a built up in the absorption peak around 480 nm related to the n-π* of cyanine. The large surface and high quantum yield in core\shell structure (43%) compared with CdS (19%) acquire a more surrounded cyanine to efficiently quench the radiative action of CdTe/CdS QDs. This type of electrostatic attraction and sufficient spectral overlap between the absorption of cyanine and the emission of CdTe/CdS leads to an efficient radiative energy transfer (RET). Therefore the combination between organic and inorganic QDs will extend the spectral response and light harvesting in photovoltaic applications. Fig. 7 shows the lifetime decay curves of CdTe/CdS QDs in presence and absence of cyanine dye. The lifetime decay curves are nearly the same suggesting a static quenching mechanism due to ground state complex formation. The volume and radius of quenching spheres can be calculated according to Perrin-model if the distance between the donor and acceptor does not change within the timescale of energy transfer. In this derivation, it is assumed that there is an effective quenching sphere around the donor with radius r. If a quencher molecule is within this sphere, the excited state of the donor is deactivated with unit efficiency. However, if the quencher molecule is outside the quenching sphere, no quenching occurs. It is also assumed in this derivation that neither donor nor acceptor can undergo displacements in space within the excited state lifetime. Eq. (5) expresses the Perrin relationship [16]

Fig. 3. X- ray diffraction patterns for CdS and CdTe/CdS QDs.

Fig. 4. TEM image of a) CdS, b) CdTe and c) CdTe/CdS QDs.

ln

shown in TEM images in Fig. 4. Fig. 5 shows the PL quenching of QDs by cyanine dye (λex = 400 nm). According to the interaction between the fluorophore (QDs) and Quencher (cyanine dye). There are two mechanisms that distinctly explain the PL quenching; the formation of ground state complex (static mechanism) and dynamic collisional mechanism within the lifetime of the fluorophore. This phenomenon can be described by the well-known Stern-Volmer equation (Eq. (4)).

I0 / I = 1 +KSV [Q]

I0 = VN0 [Q] I

(5)

Where V is the volume of the quenching sphere in cubic centimeters No is Avogadro’s number; [Q] is the molar concentration of the quencher. I A plot of ln I0 versus [Q] demonstrates linearity with slope equal to VNo from which the V values were calculated and given in Table 1. When the cyanine molecules bind with the TGA capped QDs, the equilibrium between the bound and free molecule can expressed by Eq. (6) [17].

log(

(4) 169

Io

I I

) = log K + n log[Q]

(6)

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Fig. 5. The quenching effect of cyanine on the steady state fluorescence spectrum of a) CdS b) CdTe/CdS QDs (λex = 400 nm).

Fig. 6. The absorption of a) CdS QDs and b) CdTe\CdTe QDs with gradual addition of cyanine dye.

*

Where “K” and “n” are the binding constant and the number of binding sites respectively. The values of n and K can be calculated from the plot of log (I0 −I)/I versus log [Q]. The corresponding results are shown in Table 1. The calculated binding sites were nearly unity as a result of the interaction between negatively charged carboxylic group of thioglycolic acid on the QDs surface and the positively charged cyanine dye. The critical transfer distance (R0) between the donor (QDs) and acceptor (cyanine) is the distance at which the efficiency of radiative energy transfer (E) is 50%. The value of E and R0 could be calculated from Eqs. (7)–(9) at distance d [18]:

E=

R 06

R 06 + d6

R 06 = 8.8 × 10 E=1 Fig. 7. Fluorescence lifetime decay curves for CdTe/CdS QDs in the absence of and presence of cyanine dye.

(7) 25K 2n 4

(8)

J

I0 I

(9) 2

where K is the orientation factor (K = 2/3 for random alignment), n is the refractive index of the medium, φ is the QDs fluorescence quantum

170

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Table 1 Stern–Volmer constants (Ksv), quenching volume (V), quenching radius (r), binding constant (K), binding sites (n), critical transfer distance (R0), distance between donor and acceptor (d), energy transfer efficiency(E) for donor (QDs) and acceptor (cyanine) systems. QD

KSV×105 M−1

V cm3

r nm

K×105 M−1

n

R0 (nm)

d (nm)

E

CdS CdTe/CdS

1.36 0.443

17.3 × 10−16 5.65 × 10−17

34.58 23.81

15.48 2.13

1.19 1.14

2.99 2.53

4.00 3.23

0.15 0.18

Table 2 Oxidation potentials of the donor, the excited state energies of the sensitizer (Es) and free energy changes (ΔGet) for the electron transfer reactions of donor (QDs) and acceptor (cyanine) systems. QD

(OX ) E1/2

Es

ΔGet

CdS CdTe/CdS

+1.60 +1.02

3.26 2.58

−0.78 −0.68

Eg = 1242 /

By using cyclic voltammetry technique shown in Fig. 9.The HOMO and LUMO energy level for cyanine were calculated to be -5.530 eV and -3.520 eV respectively, which shows the gap of 2.010 eV between HOMO and LUMO. This gap is consistent with the calculated value from optical absorption onset (2.16 eV) using Eq. (10). The oxidation and the reduction potential for CdTe/CdS QDs were +1.02 and - 0.7 eV respectively with peak to peak separation (Eg = 1.72 eV) which is nearly the band gap energy calculated from the onset of the tail of fluorescence peak at 680 nm (Eg = 1.80 eV). This electronic configuration of both donor and acceptor clarify the electron transfer feasibility which participates side by side with the energy transfer mechanism. The free energy changes (ΔGet) for the electron transfer process are given in Table 2. They give an indication about the feasibility for electron transfer from QDs to cyanine dye which is a crucial criterion in photovoltaic applications. The free energy changes can be calculated from the Rahm–Weller expression (13). [19]

Fig. 8. The overlap between cyanine dyes absorption and CdS, CdTe/CdS emission spectra.

yield and J is the overlap integral between the QDs fluorescence and cyanine absorption spectra (Fig. 8). Herein, n = 1.336 and φ = 0.19, 0.43 for CdS and CdTe/CdS QDs respectively. According to Eqs. (7)–(9), we calculated the Förster critical distance and the energy transfer efficiency experimentally from the QDs emission intensity in the absence (I0) and presence of the cyanine dye, normalized to the same donor concentration (Table 1). R0 and d are found to be within the normal range calculated in similar systems [17–19]. To interpretate the proper mechanism we also calculate the energy levels for both cyanine and CdTe/CdS using the following empirical Bredas et al equations [20]:

E(HOMO) = - e[Eonset ox + 4.4]

(10)

E(LUMO) = - e[Eonset red + 4.4]

(11)

(12)

onset

(ox ) Get = E1/2

(red ) E1/2

Es + C

(13)

(OX ) (red ) WhereE1/2 is the oxidation potential of the donor, E1/2 is the reduction potential of the acceptor (for cyanine), Es is the excited state energy of the sensitizer as calculated from the intersection between absorption and emission spectra for QDs, and C is the Columbic term. Since one of the species is neutral and the solvent used is polar in nature, the Columbic term is neglected [17]. The calculated ΔGet values are negative. Therefore, the electron transfer mechanism is thermodynamically favorable. [21,22] The XRD patterns of the TiO2 nanoparticles prepared by sol-gel route are shown in Fig. 10. The nanoparticles showed crystalline nature with 2θ peaks at 25.25° (101), 2θ = 37.8° (004), 2θ = 47.9° (200), 2θ = 53.59° (105) and 2θ = 62.36° (204). The preferred orientation corresponding to the (101) plane is observed. All the diffraction peaks were in good agreement with the reference anatase phase of TiO2 data. Crystallite size was calculated by Debye Scherrer’s equation as 7 nm. Well-dispersed nanoparticles can be clearly observed in the TEM and SEM images (Fig. 11 a and b). These particles are agglomerates of very small relatively monodisperse particles of about 10 nm in size. Fig. 11. C shows the surface morphology of TiO2 photoanode. The deposited layer possesses nanostructured morphology. The titania layer showed good homogeneity over the active area without cracks. This uniform and ordered morphology of TiO2 electrode gives a probable opportunity for the photons to interact with adsorbed cyanine molecules and increase the light harvesting efficiency. The cross -section

Fig. 9. Cyclic voltammogram of cyanine dye and CdTe/CdS QDS. 171

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followed by a small current (2–5 μA). This small current was not enough to cause film degradation during the deposition time. Fig. 12 shows the titania film after applying 15 V for different time intervals. The red color intensity increases with time during 20 min. After 20 min, the film began to deteriorate. The amount of dye loaded on the titania film was measured spectrophotometrically after the reverse release in alkaline ethanolic solution. Fig. 13 reveals a good linearity between the adsorbed dye and EPD time. Pre-synthesized QDs was casted after cyanine dye loading for hybrid structure solar cell. The combination between QDs and cyanine increases the cross-sectional absorption in the film. Drop casting method was used for QDs casting. The thioglycolic acid on surface of QDs participates in the electrostatic attraction and increases the loading extent. The band gap tunability of different QDs was used to match the energy level between QDs and cyanine. The effect of different QDs on the cyanine- sensitized solar cells was studied. The cells were prepared with traditional sandwich type configuration with platinum counter electrode deposited on FTO glass. A mask with a window area of 1 cm2 was applied on the TiO2 film side to define the active area of the cells. The photovoltaic characteristics of these devices (measured under standard illumination conditions, namely AM 1 G) are summarized in Table 3. The fill factor (FF) and

Fig. 10. XRD pattern of TiO2 nanoparticles synthesized via sol-gel route.

view of TiO2 electrode showed a layer of 14.5 μm thickness. Based on the ionic properties of cyanine EPD method (Fig. 12) was used for cyanine deposition on the nanostructured titanium dioxide film. FTO specimen with active area of (1 cm2) was immersed in an ethanolic solution of Cy (1 mM). A small voltage was applied to drive the positively charged dye to the cathodic electrode. This process was

Fig. 11. a) SEM micrographs, b) TEM Image of TiO2 nanoparticles synthesized via sol-gel route, (c) Top SEM micrographs and (d) Cross-sectional micrographs of TiO2 nanoparticles on FTO glass substrate.

Fig. 12. Schematic representation for the electrophoretic deposition cell and the effect of time on the electrophoretic deposition of cyanine dye (applied voltage = 15 V). 172

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Fig. 13. (a) Absorption spectra of desorbed dye extracted by alkaline ethanolic solvent at different electrophoretic deposition time and (b) The amount of dye loaded on titania by electrophoretic method as a function of time.

Table 3 The photovoltaic performances for different types of DSSCs based on cyanine dyes and QDs/cyanine hybrid sensitizers. Code

Isc (mA)

Voc (V)

Iopt (mA)

Vopt (V)

FF

η

TiO2/Cy TiO2/Cy/CdS TiO2/Cy/CdTe/CdS

1.43 1.51 1.80

0.72 0.74 0.78

1.26 1.33 1.54

0.53 0.55 0.58

0.648 0.654 0.636

0.67 0.73 0.89

overall conversion efficiency (η) were calculated as follows: FF = (Iopt × Vopt)/ (Isc × Voc), ɳ = (Iopt × Vopt)/Pin

(14)

where Iopt and Vopt are the current and voltage at the maximum output power point, respectively. Isc and Voc are the short-circuit current and open-circuit voltage, respectively. Pin is 100 mW/cm2 in the experimental setup. The cells sensitized by hybrid QDs/cyanine materials showed an improvement in the total photovoltaic parameter in comparison with the cyanine dye ones as shown in Tabel 3. The enhancement in Isc produced by CdS QDs and CdTe/CdS QDs addition were found to be 2.72% and 12.58% which are higher than cyanine dye alone. This is attributed, among other factors, to the high extinction coefficients of QDs with better photon harvesting and increased photovoltage response. The extinction coefficients values of the studied QDs increase in the order CdTe/CdS > CdS as previously described. The matching between band gap structure of QDs and energy state for organic dyes is yet an important factor for improving such hybrid solar cells. The improved values of Isc reflected the enhanced incident photon to current efficiency by the QDs with higher rates of electron transfer to cyanine dye. The cell performance showed an enhancement also in the open-circuit voltage Voc and total cell efficiency (η) with respect to the cyanine solar cell (Voc = 0.72 V, η = 0.67%) (Fig. 14). The result of photovoltaic performance in Table 3 reflects the impact of radiative energy transfer between QDs and cyanine dye in the output current. This radiative energy transfer can transfer excitation energy by the radiative decay of semiconductor nanocrystals and subsequent reabsorption of the emitted photons by cyanine dye. Moreover the absorption of this hybrid system extended to a broader range of the solar spectrum which is important in sensitizing materials in photovoltaic application. The enhancement in both absorption besides the energy transfer will also leads to much number of carriers that can be injected in TiO2 layer. This radiative energy transfer is largely affected by the spectral overlap between donor and acceptor which is higher in

Fig. 14. I –V curves of solar cells based on the cyanine dyes alone and cyanine dyes/QDs hybrid sensitizers.

case of CdTe/CdS than CdS QDs. This form of efficiency enhancement derived from energy transfer in hybrid based silicon solar cell is recently reported in previous work in our group [23,24]. Fig. 15 shows the mechanism of sensitization in the present hybrid solar cell. The electronic configuration in the three layer demonstrates the availability of efficient electron transfer from the conduction band of CdS and CdTe/CdS QDs to the LUMO level of cyanine then to TiO2 nanoparticles. The radiative decay of semiconductor QDs is accompanied by reabsorption of emitted photons by under layer cyanine dye which dedicate the radiative energy transfer mechanism to push the excitation towards cyanine dye and maximize the carrier concentration. The thermodynamically favored electron transfer from QDs to cyanine in Table 2 adds another explanation for the enhancement occurred in photovoltaic performance. According to the Gibbs free energy for electron transfer from CdS to cyanine which is higher than in case of CdTe/CdS QDs (Table 2), and so the probability of electron transfer will be higher in case of CdS than CdTe/CdS QDs. But as the resulted photovoltaic performance is not in accordance with electron transfer mechanism so radiative transfer becomes the predominant mechanism due to the higher energy transfer efficiency in case of CdTe/CdS than CdS QDs. 173

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Fig. 15. Schematic band diagram and the mechanism of working for current hybrid solar cells.

4. Conclusion

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