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Effect of Different Sensitization Technique on the Photoconversion Efficiency of CdS Quantum Dot and Cdse Quantum Rod Sensitized TiO Solar Cells 2

Diego Esparza, Isaac Zarazúa, Tzarara Lopez-Luke, Andrea Cerdán, Ana Isabel Sánchez-Solís, Alejandro Torres-Castro, Ivan Mora-Sero, and Elder De La Rosa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01525 • Publication Date (Web): 01 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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The Journal of Physical Chemistry

Effect of Different Sensitization Technique on the Photoconversion Efficiency of CdS Quantum Dot and CdSe Quantum Rod Sensitized TiO2 Solar Cells Diego Esparza1, Isaac Zarazúa1, Tzarara López-Luke1, Andrea Cerdán-Pasarán1, 2, Ana Sánchez-Solís1, Alejandro Torres-Castro3, Ivan Mora-Sero4 and Elder De la Rosa1*. 1

Nanophotonics and Advanced Materials Group, Centro de Investigaciones en Óptica, A.P 1-948, Leon, Gto. 37150, Mexico. 2

Universidad de Guanajuato, Campus Guanajuato, División de Ciencias Naturales y

Exactas, Departamento de Ingeniería Química, Noria Alta s/n, Guanajuato, Gto. 36050, México. 3

Universidad Autónoma de Nuevo León, FIME UANL A.P. 126-F, Monterrey, NL, 66450 México.

4

Photovoltaics and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, 12071 Castello, Spain.

* Corresponding author: Nanophotonics and Advanced Materials Group, Centro de Investigaciones en Óptica, A.P 1-948, Leon, Gto. 37150, Mexico +52 4774414200 (phone), +52 4774414209 (Fax). E-mail: [email protected]. 1 ACS Paragon Plus Environment


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Abstract The procedure employed for the sensitization of mesoporous photoanodes affects strongly the final performance of sensitized devices, especially when semiconductor Quantum Dots and Quantum Rods are used as sensitizers. In this work the effect of three different sensitizing methods in the final cell performance was analyzed. The TiO2 films were sensitized with CdS QDs grown by successive ionic layer adsorption and reaction, SILAR, and with CdSe Quantum Rods deposited by electrophoretic and pipetting methods. Several configurations of the sensitizers and combinations of sensitization methods were tested. 4% photoconversion efficiencies were obtained for TiO2 electrodes sensitized with CdS and CdSe by electrophoretic and pipetting respectively, while for the sensitizer with both techniques the efficiency was 4.7%. This high efficiency is mainly due to the high fill factor (60 %) and the photocurrents (13.1 mA/cm2) obtained by the correct combination of near infrared and visible light photoabsorption, the better CdSe QRs distribution in the TiO2 film and a passivation of the TiO2 nanocrystals. Electrochemical impedance measurements has been analyzed and discussed in detail providing a detailed analysis of recombination resistance and charge transport processes. These parameters have been correlated with the cell performance. KEYWORDS: CdS Quantum Dots, CdSe Quantum Rods, SILAR method, electrophoretic deposition, pipetting methods.

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1. Introduction Quantum Dot Sensitized Solar Cells (QDSSC) is an active research topic because they are promising candidates to obtain high efficiencies, since their theoretical maximum efficiency is 44%1; thus, they could be competitive for different photovoltaic tecnologies2-4 as dye sensitized solar cells (DSSCs)5-6. The QDSSC typically consist of TiO2 Nanocrystals (NCs) acting as a highly porous wide bandgap semiconductor for electron collection, and quantum dots (QDs) adsorbed onto the surface of TiO2 NCs acting as sensitizers to harvest solar light. The QDs take advantage of physical processes favored in the quantum confinement regimen as multiple exciton generation, high extinction coefficients and tunable absorption band through size control, resistance to oxidation, and thermal stability710 11

. Different QDs have been used as sensitizer (CdSe, CdTe, CdS, PbS, PbSe, Bi2S3, and

InP)12-24 prepared from solution methods at low cost. In order to obtain higher efficiencies CdS and CdSe QDs have been widely studied9, 25-31

, due to their high quantum efficiency in the visible region (up to 60 % at 450 and up to

80% at 600 nm, respectively), obtaining photoconversion efficiencies up to 2.3% and 5.4 % respectively 25, 32-34. The mixing of different kinds of QDs as sensitizers has been reported as a promising procedure to obtain high efficiencies by expanding the absorption wavelength region for light harvesting35-38 , increase electron injection, and also enhance the charge transport39. In particular, CdS and CdSe have been combined by different methods in various architectures, like ternary compounds as CdSeS (η=4.05%)

40-41

, CdS/CdSe

deposited by Chemical bath deposition (η=3.5%)42 and by SILAR (η=5.21%)14


43

. Other


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method used for TiO2 sensitization is electrophoretic deposition (E) reaching η=1.7% for CdSe QDs44 and 3.2% for CdSeS QDs41. One key factor in many deposition methods is the addition of inorganic ligands to enhance electronic transport and successfully passivate surface defects in colloidal QD, playing an important role mediating electron transfer reactions in the surface chemistry of colloidal QDs45-46. In this paper, it is reported the synthesis and photovoltaic characterization of TiO2 films sensitized with CdS QDs prepared by SILAR (S) and colloidal CdSe QRs sensitized by electrophoresis (E) and pipetting (P). Combination of these sensitization processes as TiO2/CdS(S)/CdSe(EP) produces a substantial improvement in the photoconversion efficiency of η = 4.7 %, with respect to individual cell (TiO2/CdSe(E)) with η = 2.1 %. The increase of the short current circuit is resulting from enlarging the absorption from the 380550 nm (CdS(S)) to the 380-680 (CdS(S)/CdSe(E)) region with the correct proportion of the QDs nanoparticles sized. And the increase in FF produced by a reduction in the recombination processes due to the synergetic interaction and superficial passivation of the films with the ligands used for the CdSe (P). 2. Experimental Section. 2.1 Quantum Dot Solar Cells (QDSCs) preparation Materials.TiO2 Paste (WER2-0 Reflector) and TiO2 Paste (DSL 18NR-T) were obtained from DYESOL, Titanium (IV) isopropoxide(97%), Acetylacetone (>99%), technical-grade trioctylphosphine (TOP 90%), trioctylphosphine oxide (TOPO 99%), poly(ethylene glycol) (PEG average Mn of ca. 10000 g/mol), sodium sulfide (Na2S 99%), Cadmium oxide (CdO 99%), selenium powder (Se 99%), 1-Tetradecylphosphonic acid (TDPA 99%), Cadmium 4 ACS Paragon Plus Environment

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acetate dehydrate (Cd(CH3COO)22H2O), Zinc acetate dihydrate (Zn(CH3COO)22H2O), Sodium hydroxide (NaOH) and 3-Mercaptopropionic acid (MPA) (C3H6O2S 99%) were obtained from Sigma-Aldrich. Sulphur (S), and Sodium sulphide (Na2S9H2O) were obtained from KARAL and Fluorine-doped tin oxide (FTO) from MTI(TEC-15). TiO2 film Preparation. Fluorine-doped tin oxide (FTO) (MTI, TEC-15) glasses were cleaned with water, acetone and ethanol in an ultrasonic bath for 15 min each before use. The photoelectrodes were made by three different TiO2 layers stacked one on the top of the other. 1) Compact layer: A solution of titanium (IV) Isopropoxide (0.2 M) using acetylacetone/ethanol (1:1 V:V) deposited by spray pyrolysis over an FTO and sintered at 450 °C for 30 min to obtain a 150 nm layer. 2) Transparent layer: TiO2 paste, (DSL 18NRT, 20 nm average particle size) deposited on FTO glass by Doctor Blade method obtaining a 6 µm thick layer, and 3) Scattering layer (opaque): a 9 µm layer is obtained by doctor blade depositing Wer2-O Reflector paste (400 nm particle size). All films were sintered for 30 min at 450 °C to obtain a good electrical contact between nanoparticles. Synthesis of CdSe Colloidal QRs: High-quality CdSe QRs were synthesized based in the protocol of Peng et al. 47, wherein CdO is used as the Cd precursor and TDPA and TOPO are the ligands and coordinating solvents, respectively. The synthesis was performed in airfree conditions, wherein 0.05 g (∼0.39 mmol) of CdO, 0.3 g (∼1.1 mmol) of TDPA, and 4 g of TOPO were heated to 110 °C, degassed under vacuum and then heated to 320 °C under a nitrogen flow19. A SeTOP (0.7% by weight) solution was obtained by adding 0.026 g of Se powder with 4.25 mL of TOP inside of a glove box and stirring for 1 h to ensure complete dissolution of the Se powder. After reaching 320 °C, the Cd-TDPA-TOPO solution was cooled to 270 °C prior to the injection of SeTOP. Under nitrogen flow, 3 mL 5 ACS Paragon Plus Environment


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of SeTOP was injected, which resulted in the lowering of the temperature to 260 °C. The temperature was then increased to 280 °C to facilitate particle growth. Aliquots were removed and probed to track nanocrystallite growth via UV-Vis absorption spectroscopy and photoluminescence (PL) spectroscopy. The CdSe solution was cooled and removed from action flask at around 80 °C and dissolved into ∼10 mL of toluene. The QDs in toluene were then cleaned twice through a precipitation and decantation regime using methanol and centrifugation at 3000 rpm, the QDs were ultimately dissolved in toluene prior to their use as a sensitizer. Films sensitization: In order to study the effect of different technique deposition of QDs, four different devices were fabricated. (1) TiO2/CdS(S)/ZnS(S) was prepared by SILAR method (Figure 1a). The TiO2 electrode is introduced in 0.05 M Cd(CH3COO)22H2O dissolved in ethanol and 0.05 M Na2S in methanol:water (V:V = 1:1) as Cd2+and S2sources respectively. A single SILAR cycle consisted of 1 min dip-coating the TiO2 electrode into the cadmium acetate and subsequently into the sodium sulfide solutions, also during 1 min. Between each dipping step in precursor solutions, the electrodes were thoroughly rinsed by immersion in the corresponding solvent (ethanol and methanol/H2O (V:V = 1:1), respectively) in order to remove the excess of precursor. Seven SILAR cycles were done to obtain a uniform coverage of the TiO2 NPs with CdS QDs. To enhance the photovoltaic performance ZnS is deposited by SILAR in order to passivate CdS surface and reduce the recombination of electrons in the TiO2 to the polysulfide electrolyte48-50. ZnS passivation was obtained by using 0.1 M of Zn(CH3COO)22H2O and 0.1 M of Na2S both dissolved in water as Zn2+ and S2- sources respectively. The films were dipped for 1 min/dip in the solutions during 2 SILAR cycles. (2) TiO2/CdS(S)/CdSe(E)/ZnS(S) was fabricated


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as following: After the CdS QDs sensitization, CdSe QRs were deposited by electrophoresis (see Figure 1 b). The TiO2/CdS(S) cell was placed face to face with an FTO at a distance of 0.2 cm and immersed in a cuvette with 2.5 mL of colloidal CdSe QRs dispersed in toluene. 200 DC volts were applied with the TiO2 films in the positive terminal of the power supply. After 105 min the CdSe QRs are uniformly distributed on the surface of the TiO2 NPs. Finally, ZnS QDs were deposited by 2 SILAR cycles as was mentioned above. (3) TiO2/CdS(S)/CdSe(P)/ZnS(S) was prepared by adding CdSe-MPA film by pipetting method (Figure 1c). In this case, the TiO2/CdS(S) films were masked with a tape with a circular hollow of 6.0 mm in diameter. Then, one drop (20 µL) of CdSe QRs colloids was pipetted onto the TiO2/CdS(S) film surface and after 15 min one drop of MPA:methanol (3:10 V:V) solution (20 µL) was pipetted on the decorated film. This process was repeated 10 times. Then, ZnS QDs were added as was explained above. (4) TiO2/CdS(S)/CdSe(EP)/ZnS(S), in this case such device was prepared adding CdSe QRs using both techniques, first electrophoresis method and then by pipetting onto TiO2/CdS(S) at the same conditions explained before. ZnS QDs were added as was explained above.



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Figure 1. Schematic diagram of different techniques used for deposition of QDs and QRs in TiO2 film. a) CdS QDs deposited by SILAR method, b) CdS(S)/CdSe(E), where CdS QDs were deposited by SILAR and CdSe QRs by electrophoretic method, c) CdS(S)/CdSe(P), where CdS QDs were deposited by SILAR and CdSe QRs by pipetting method. Counter electrode manufacture & cell assembling: Cu2S counter electrodes were obtained by immersing brass foil in a HCl solution (38% in volume) at 90 °C for 1 hour, then they were sulfated by adding a drop (20 µl ) of polysulfide electrolyte. The composition of polysulfide electrolyte was 1.0 M Na2S, 1.0 M S and 0.1 M NaOH dissolved in distilled water. The solar cells were constructed by assembling the Cu2S counter electrode and sensitized TiO2 film electrode with a binder clip separated by a Scotch spacer. Then, polysulfide electrolyte was filled inside the cell.


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2.2 Characterization Transmission Electron Microscopy (TEM) was obtained from a FEI- Titan 80-300 kV microscope equipped with ultra-stable Schottky field emitter gun. The samples were suspended in toluene at room temperature and dispersed with ultrasonic agitation. Then, an aliquot of the solution was dropped on a 3mm diameter lacey carbon copper grid. Field Emission Scanning Microscopy (FESEM) images were obtained from a JEOL JSM-7800F microscope and electron dispersion spectroscopy (EDS) analysis was done with an Oxford Instruments. The UV-Vis absorption spectra of colloidal CdSe QRs were measured by transmittance and substrates were measured by diffuse reflectance in the range from 360 nm to 800 nm using an Agilent Technologies Cary Series UV-Vis-NIR spectrophotometer (Cary 5000) and an integrating sphere of 60 mm. The current density curves were measured with a reference 600 Gamry potensiotat scanning from 0 to 600 mV at 100 mV/S. The samples were illuminated with an Oriel Sol 3A solar simulator while measuring current. The light intensity was adjusted employing a NREL calibrated Si solar cell having KG-2 filter for one sun light intensity (100 mW/cm2). Incident photon to current efficiency (IPCE) spectra were measured with a monochromator (Newport model 74125). Electrochemical Impedance Spectroscopy (EIS) measurements were carried out by applying a small voltage perturbation (10 mV rms) at frequencies from 100 kHz to 0.1 Hz for different forward bias voltages in dark conditions. Experimental results were fitted with the ZView software. 3. Results and Discussion 3.1 Morphological Characterization



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CdS QDs and CdSe QRs: Figure 2a shows the typical TEM image of CdS QDs sensitized TiO2 nanoparticles prepared by SILAR method. The average size of CdS QDs is 3 nm, presenting a high coverage of QDs onto TiO2 NP surface. Figure 2b shows the TEM image of colloidal CdSe QRs showing an average size of 10 nm large and 5 nm width. Therefore, the CdS nanoparticles and CdSe nanorods are in the regime of the quantum confinement11.


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Figure 2. Transmission Electronic Microscopy (TEM) Images of, a) TiO2/CdS(S) where CdS(S) QDs were deposited by SILAR method showing an average size of 3 nm and b) Colloidal CdSe QRs showing an average size of 10 nm large and 5 nm width.

The SEM images of TiO2, TiO2/CdS(S)/ZnS(S), TiO2/CdSe(E)/ZnS and TiO2/CdSe(P)/ZnS are shown in Figure 3. The TiO2 transparent layer deposited by Doctor Blade method without sensitizers, forms a homogenous high porous film with nanoparticle average size of 20 nm, see Figure 3a. When the films are sensitized by SILAR or electrophoresis, the TiO2 nanocrystals are uniformly covered by QDs and QRs, and filling partially the pores as is shown in Figure 3b-c. The figure 3d shows the TiO2 films sensitized with CdSe by pipetting method and the pore filling is more clearly observed.



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Figure 3. Scanning Electronic Microscopy (SEM) images of different configuration. a) TiO2 transparent layer, b) TiO2/CdS(S)/ZnS(S), where CdS(S) were deposited by SILAR method, c) TiO2/CdSe(E)/ZnS(S) where CdSe(E) were deposited by electrophoretic method and d) TiO2/CdSe(P)/ZnS where CdSe(P) were deposited by pipetting method. In all cases ZnS(S) were deposited by SILAR method.

The cross-section SEM images and EDS analyses of CdSe (EP) sensitized TiO2 device are displayed in figure 4. In Figure 4a all layers composing the device are observed. From right to left, the first film observed is the FTO followed by the TiO2 compact layer (190 nm), the transparent layer (9 µm) composed of 20 nm TiO2 nanoparticles, and finally the opaque layer (8µm) composed of 200 nm TiO2 nanoparticles. The Cd and Se atoms distributions detected by EDS mappings are shown in Figure 4b and 4c, respectively. EDS characterization highlight the uniform distribution of CdSe QRs inside of the mesoporous TiO2 films increasing the concentration in transparent layer, which is consistent with the higher superficial area of the nanoparticles in this layer. Distribution plot of Cd2+ atoms along different TiO2 films sensitized by pipetting and electrophoresis methods are presented in Figure 4d. Notice that pipetting produces a concentration profile that decreases along the transparent TiO2 layer, obtaining lower concentration close to the FTO substrate. However, electrophoretic method produces an inverse profile with high QRs concentration close to the substrate. This is expected because pipetting depends on the natural diffusion of QRs while in the electrophoretic process sensitizers are attracted to the FTO electrode. Therefore, the highest concentration is obtained when combined both pipetting and electrophoretic method producing an adequate gradient of QDs along TiO2 being higher close to FTO.


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The effective coverage of CdSe QRs on TiO2 surface was estimated according to reference 27. Considering CdSe QRs as a monodispersed rectangle particle with 10 nm large and 5 nm wide, and the amount of cadmium and selenium measured by EDS mapping dependent on the sensitizing method, it was calculated the total surface area of QRs particle 39 nm2/mol, 49 nm2/mol and 53 nm2/mol for P, E and EP, respectively. In addition, considering TiO2 spherical particle of 20 nm of diameter a surface area of 180 nm2/mol was calculated. Therefore, by dividing the surface areas (CdSe QRs/ TiO2), the coverage of TiO2 surface was obtained being 22% for electrophoretic, 24% for pipetting and 30% for the combination of both techniques due to the increase of active material. In conclusion, the density of CdSe QRs depends of the sensitizing technique and the combinations of both techniques increase the coverage of TiO2 surface by CdSe QRs. This result is consistent with the distribution plot of Cd2+ atom (see Figure 4d).



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Figure 4. a) Cross-section SEM images of TiO2/CdSe QRs films sensitized by Electrophoretic and pipetting method with TiO2 compact layer (190 nm), active layer (9µm) and scattering layer (8 µm); b) and c) EDS mapping of Cd2+ and Se2- atoms respectively by Electrophoretic and pipetting method, and d) Cd2+ distribution through TiO2 films as a function of deposition method. 3.2 Optical Characterization. The absorption spectra of different configurations of sensitized TiO2 are shown in Figure 5. The shoulder in the absorption spectra centered at 450 nm is associated to CdS deposited by SILAR process51. The introduction of CdSe QRs produces an additional absorption band in the red region centered at 625 nm. The red shift of the absorption peak, compared to colloidal QRs, is probably resulting of the agglomeration produced during the film preparation. Interestingly, such red band is stronger when deposited by pipetting than deposited by electrophoresis. An increment in the overall absorption band was observed when QRs were introduced by both, E and P, techniques. This increment in the absorption coefficient confirms the concentration increase of sensitizer QRs and is expected to improve the photovoltaic conversion efficiency of all devices with this configuration. The ZnS band gap is about 3.6 eV which corresponds to an absorption band at 320 nm50 and does not influence significantly the absorption in the visible range. See supporting information for optical characterization of colloidal QRs.


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Figure 5. Absorption spectra of CdS QDs and CdSe QRs sensitized TiO2 layers by TiO2/CdS(S)/CdSe(E)/ZnS(S), different techniques, TiO2/CdS(S)/ZnS(S), TiO2/CdS(S)/CdSe(P)/ZnS(S) and TiO2/CdS(S)/CdSe(EP)/ZnS(S), where S stand for SILAR, E for Electrophoretic, and P for pippeting. 3.3 Electrochemical Characterization. The current density-voltage (J-V) profiles for TiO2 films sensitized with CdS QD and CdSe QR, TiO2/CdS(S)/ZnS(S), TiO2/CdSe(E)/ZnS(S) and TiO2/CdSe(P)/ZnS(S), are displayed in the SI (Figure S2a ). The calculated values for different sensitization techniques are listed in the Supporting information (SI) Table S1. The TiO2/CdSe(E)/ZnS configuration present a Jsc= 7.4 mA/cm2 and Voc= 504 mV resulting on a photoconversion efficiency of η =2.1%. Meanwhile, when CdSe QRs are deposited by pipetting (TiO2/CdSe(P)/ZnS) a Jsc=8.4 mA and Voc=564 mV was obtained that results on an efficiency of η=2.9%. The IPCE curve of the TiO2/CdS/ZnS, TiO2/CdSe(E)/ZnS and TiO2/CdSe(P)/ZnS are presented in Figure S2b. There, it is observed that the origin of the photocurrent is different according to the composition of the sensitizer. TiO2/CdS(S) photoelectrodes have higher quantum efficiency (QE=80% at 450 nm) but in a narrow spectral range that decay rapidly for λ > 550 nm due to the larger bandgap (Eg) of CdS in comparison with CdSe. Devices sensitized 15 ACS Paragon Plus Environment


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with CdSe QRs, either by electrophoresis or pipetting methods, have lower QE (60% at 450 nm) but relatively constant up to 630 nm (EQE=40%) which correspond to the absorption of CdSe QRs. This extra 80 nm of photogeneration region provide enough electrons to obtain almost the same photocurrent than that provided by CdS QDs despites the lower quantum efficiency. The combination of CdS QD prepared by SILAR with colloidal CdSe QR to sensitize TiO2 improves the photoconversion efficiency of solar cell devices. Figure 6a shows the J-V curve for devices sensitized in 4 configurations TiO2/CdS(S)/ZnS, TiO2/CdS(S)/CdSe(E)/ZnS, TiO2/CdS(S)/CdSe(P)/ZnS and TiO2/CdS(S)/CdSe(EP)/ZnS, and Table 1 shows the Jsc, Voc, FF and η of such samples calculated from the J-V curves. The addition of CdSe QRs increases the Jsc from 8.7 to 12 mA/cm2 (38%) for pipetting, and to 13.4 mA/cm2 (54%) for electrophoresis co-sensitized compared to CdS QDs single sensitized TiO2 device. These results induce an increment of 50% for the combination of both techniques suggesting that an excess of QRs could produce a shielding effect where not all sensitizer harvest the impinging light. Such increase in the photocurrent is the result of the absorption bandwidth enlargement (∼100 nm compared to CdS) as is confirmed from the IPCE displayed in figure 6b. CdSe QRs also increases the Voc with an overall increment of 6% when sensitizer is introduced by both techniques (EP), although pipetting method produces a strong increase larger than 15.6% (from 542 mV to 626 mV). This variation suggests an upward shifting of the Fermi level as a result of the interaction between CdS QDs and CdSe QRs and it depends on the deposition method. Interestingly, the FF decay from 53.5% for TiO2/CdS(S) to 49% for TiO2/CdS(S)/CdSe(E) that represent a relative loss of 9%. However, FF presents a dramatic increase of ∼14% when CdSe QRs are introduced 16 ACS Paragon Plus Environment

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by combining both electrophoresis (E) and pipetting (P) method. A reduction or enhancement on the FF is associated to an increase or a decrease of the recombination charge. Therefore, these experimental results suggest that a homogenous distribution of CdSe QRs along the TiO2/CdS, as the one obtained when combined electrophoresis and pipetting method (see figure 4d), promote a major mobility and then reduces the recombination charge. This confirms that none electrophoresis neither pipetting method alone but both methods combined produces the major enhancement on the performance of the solar cell device. In fact, the photoconversion efficiency for CdS/CdSe is higher than CdS alone in all cases. But, when combining electrophoresis and pipetting the photoconversion efficiency was 4.6% that represent an increment of 84% compared to CdS QD single sensitized. Table 1. Photovoltaic parameters Jsc, Voc, FF, and η extracted from J-V curves of the QDs and QRs sensitized TiO2 solar cells with different configuration and combined techniques (SILAR, Electrophoresis and Pipetting). SAMPLE

Jsc(mA/cm2)

Voc(mV)

FF(%)

η(%)

TiO2/CdS(S)/ZnS(S)

8.7

542

53.5

2.5

TiO2/CdS(S)/CdSe(E)/ZnS(S)

13.4

589

49

3.9

TiO2/CdS(S)/CdSe(P)/ZnS(S)

12.0

627

53.4

4.0

TiO2/CdS(S)/CdSe(EP)/ZnS(S)

13.1

575

60.9

4.7



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Figure 6. J−V (a) and IPCE curves (b) of TiO2/CdS(S)/ZnS(S), TiO2/CdS(S)/CdSe(E)/ZnS(S), TiO2/CdS(S)/CdSe(P)/ZnS(S) and TiO2/CdS(S)/CdSe(EP)/ZnS(S) devices. The working electrode areas were 0.125 cm2. A Cu2S counter electrode was used, and aqueous 1 M Na2S, 1M S and 0.1M NaOH served as the electrolyte. The performance of the solar cells was measured under AM 1.5 radiation with an incident power of 100 mW/cm2.

A detailed analysis of the IPCE spectra displayed in Figure 6b show that the addition of CdSe QRs effectively increases the light harvest region with no reduction of the external quantum efficiency (EQE) corresponding to the CdS light absorption band. Both CdS and CdS/CdSe-sensitized TiO2 solar cells exhibit strong photoconversion response over the 18 ACS Paragon Plus Environment

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visible light range with EQE larger than 75%. However, CdS sensitized electrode decay faster below 70% after 510 nm to be completely extinguished at 700 nm. The extended queue observed in CdS sample extending practically close to 700 nm could be probably due to the trap states at the band gap52.

For CdS/CdSe co-sensitized electrode such decay start after 550 nm for QRs deposited by pipetting and after 575 nm for those deposited by electrophoresis. Indeed, for CdSe introduced by both electrophoresis and pipetting decay start after 610 nm. This mean, the combination of CdS with CdSe introduced by electrophoresis and pipetting extend the light harvest region more than 230 nm, from 380 to 610 nm with an EQE of 70% being completely extinguished at 700 nm. Such synergistic effect is clearly observed by comparing the IPCE between single CdSe QRs (see figure 5b) with CdS/CdSe cosensitized TiO2. This is the reason of the Jsc and FF enhancement and explains the strong increment of photoconversion efficiency. The near infrared absorbance of CdSe QRs nanocrystals is due to the large particle size, increasing the photoabsorption range obtaining a maximum photoconversion efficiency of 4.7 %. The Jsc enhancement (66 %) adding CdSe, is by using together CdS/CdSe sensitized TiO2. To understand the dynamics of electrons, the band-edge levels of TiO2, CdS, CdSe and CdS/CdSe electrodes were determined from Mott-Schottky plots53 and the band gap was obtained from the absorption spectra as is shown in Figure 7. The calculated TiO2 conduction band (CB) is -4.03 eV, and when is sensitized with CdS QDs and CdSe QRs the CB increase to -3.83 eV and -3.22 eV, respectively. This results on an increase of the electron injection efficiency due to the cascade mechanism as described in figure 7. Finally,



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it is observed that the CdS/CdSe CB is -3.49 eV due to the Fermi-level alignment

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39

showing the CB above of the TiO2 CB obtaining a favorable electron injection and increase of Voc from 542 mV to 575 mV.

Figure 7. Energy diagrams level for TiO2, CdS, CdSe and CdS/CdSe QDs and QRs. The CB position was determined from Mott-Schottky plots.

3.4 Electrochemical impedance Electrochemical impedance spectroscopy (EIS) measurements were carried out to explain the trends observed in the solar cell parameters when CdS QDs and CdSe QRs are combined in terms of the internal physical processes. The transmission line model shown in Figure 8 was used to fit the impedance measurements51, 54. There, Rs stand for the series resistance and is related to the FTO and the wires resistance, Rt is the transport resistance and is related with the resistance of TiO2 to the electron flux, Zd is the Warburg element showing the Nernst diffusion of the electrolyte; Rc and Cc are the charge-transfer resistance and double-layer capacitance at the counter electrode. Rrec and Cµ are the recombination 20 ACS Paragon Plus Environment

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resistance and the chemical capacitance respectively. Figure 10 shows Cµ, Rrec and Rt extracted from the EIS measurements under dark conditions of the TiO2/CdS(S)/ZnS and TiO2/CdS(S)/CdSe/ZnS sensitized solar cells. Cµ is plotted as a function of Vf=Vapp-Vseries which is the voltage drop at the active electrode obtained as the applied voltage, Vapp, corrected by the voltage drop due to series resistance (Vseries)51, 54. The chemical capacitance is proportional to the density of states (DOS) in the band gap, which in turn increases exponentially with the proximity to the conduction band (CB). Therefore, an increase in Cµ reflects the upward displacement of the Fermi level (and then the CB). As Rrec and Rt are exponentially dependents on the electron levels density (i.e. the distance of Fermi level to the CB), it is defined a new potential in order to remove the effect of the CB shift such that measurements are at the same equivalent CB position, defined as: ܸ௘௖௕ ൌ ܸி െ Δܸ௜

(4),

Figure 8. Transmission line in the diffusion–recombination model used to fit the impedance measurements. Rrec is the charge-transfer resistance of the charge recombination process between electrons in the TiO2 film and the electrolyte; Cµ is the chemical capacitance of the TiO2 film; Rt is the transport resistance of the electrons in the TiO2 film; Zd is the Warburg element showing the Nernst diffusion of the electrolyte; Rc and Cc are the charge-transfer resistance and double-layer capacitance at the counter electrode. 21 ACS Paragon Plus Environment


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Where ecb means equivalent conduction band and Δܸ௜ is the voltage shift of the capacitance respect the sample used as reference, the TiO2/CdS/ZnS films, see Figure 9a. After the voltage correction Cµ overlaps as is shown in Figure 9b. Comparing Rrec and Cµ for the cells, it is possible to understand the origin in the Voc variation51, 54. Higher Voc observed in multi-sensitized cells (EP) could be due to three processes: (1) An increase in the light absorption. (2) A shift in the TiO2 CB, and/ or (3) a change in the recombination rate. An increase in the photo absorption will result in an increase in Jsc and producing consequently an increase of Voc. The second effect, a shift in the TiO2 CB, produces a movement of the TiO2 electron quasi-Fermi level which determines the Voc, then an upward of the TiO2 CB will mean an increase in the Voc. This upward displacement of the conduction band is identified by a shift to higher potentials of Cµ, as the 25 mV shift observed

for

TiO2/CdS(S)/CdSe(E)/ZnS(S)

sample

in

comparison

with

TiO2/CdS(S)/ZnS(S) sample in Figure 9a. But this is not the case of the EP samples that presents no shift respect the reference sample. The third effect, the recombination rate, can be also determined with EIS by analyzing the Rrec. An increase of this parameter indicates a reduction in the electron recombination processes from the TiO2 CB to acceptor states either in the electrolyte or the sensitizers55 56. Comparing TiO2/CdS(S)/CdSe(E)/ZnS(S) and TiO2/CdS(S)/ZnS(S) samples in Figure 9c, it is observed that the multi-sensitized cell have lower Rrec indicating an increase in the electron recombination and then a reduction of the Voc. Therefore, the Voc is increased by an increment in the photogeneration and a shift in the

TiO2

CB

when

CdSe

QRs

were

added

by

electrophoresis

(TiO2/CdS(S)/CdSe(E)/ZnS(S)). And at the same time, it is reduced by an increase in the recombination processes that result in the increase of 47 mV observed in the J-V curve. In 22 ACS Paragon Plus Environment

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contrast, a negative shift of 50 mV was observed in Cµ and at the same time an increase in Rrec

were

obtained

when

the

CdSe

QRs

was

added

by

pipetting

(TiO2/CdS(S)/CdSe(P)/ZnS(S)). This, combined with the increase of IPCE, indicates that for such samples Voc also increases by an increase in photogereration. But contrary to the sample sensitized by electrophoresis, it is reduced by downshift of the CB and increased by a reduction in the recombination processes resulting in a higher increase of Voc (85 mV). Finally,

when

the

CdSe

QRs

were

deposited

by

both

methods

(TiO2/CdS(S)/CdSe(EP)/ZnS(S)), Cµ is almost the same than for TiO2/CdS/ZnS cells indicating that the TiO2 CB is keep in the same level. Downwards and upwards shift produced in P and E samples, respectively, is compensated producing no final shift in the CB of EP sample. Rrec is reduced which means a downshift of the electron quasi-Fermi level by an increase in recombination processes that would result in a Voc decrease. However, the photogeneration in this sample is as high as in TiO2/CdS(S)/CdSe(E)/ZnS(S) samples compensating the effect of Rrec an resulting in a similar increase of Voc (33mV).



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Figure 9. Impedance spectroscopy characterization of the TiO2/CdS(S)ZnS(S), TiO2/CdS(S)/CdSe(P)/ZnS(S) and TiO2/CdS(S)/CdSe(E)/ZnS(S), TiO2/CdS(S)/CdSe(EP)/ZnS(S) . a) and b) Chemical capacitance, Cµ. c) Recombination resistance Rrec, d) Transport resistance Rt as function of Final voltage Vf and inset Rt as a function of voltage equivalent conduction band Vecb.

One

important

parameter

to

obtain

the

high

efficiency

of

the

TiO2/CdS(S)/CdSe(EP)/ZnS(S) samples is the FF. Variations in this parameter can be due to a change in the Transport resistance (Rt)33, 54. It has been previously reported that Rt can vary depending on the sensitization process

54

. Analyzing the Rt behavior in the VF

convention, it could be observed that the addition of CdSe QRs by electrophoresis (TiO2/CdS(S)/CdSe(E)/ZnS) result on an increase of Rt indicating a reduction in the


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electron transport which result in the reduction of the FF observed in Table 1. When the CdSe QRs are added by pipetting the Rt decreases being a little lower resulting in almost the same FF. Finally, when both methods are used together (EP), it is obtained a reduction in Rt resulting in the meaningful increase of FF obtained for this sample. When Rt is analyzed in the ecb convention (inset of Figure 9d), the behavior is preserved by all samples except TiO2/CdS(S)/CdSe(P)/ZnS, this suggest that probably the use of inorganic ligand successfully passivate surface defects in colloidal QD enhancing the charge transport. This highlight the important role of inorganic ligand mediating electron transfer reactions in the surface chemistry of colloidal QDs 45-46. This indicates that in this particular case the improvement in Rt is due to the CB upward shift instead to a real electron transport resistance reduction. While for the TiO2/CdS(S)/CdSe(EP)/ZnS there is a real improvement of the electron transport. 4. Conclusions It was studied the effect of different sensitization method on the photoconversion efficiency using several methods of deposition, SILAR (S), electrophoresis (E) and pipetting (P) of colloidal Cadmium Selenide (CdSe) quantum Rod (QR) on Cadmium Sulphide (CdS) quantum dots (QDs) deposited by SILAR. The photoconversion efficiency of TiO2 sensitized with CdSe deposited by electrophoresis (CdS(S)/CdSe(E)/ZnS(S)) and pipetting (CdS(S)/CdSe(P)/ZnS(S)) was approximately a significant 4.0%, and the combination of both methods (E) and (P), (CdS(S)/CdSe(EP)/ZnS(S)) produces an increases η to 4.7%. J-V curves and impedance analysis indicate that the higher efficiency is due to the photocurrents (13.1 mA/cm2) obtained by the correct combination of near infrared and



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visible light photoabsorption of the Quantum Dots and Quantum Rods and the increase of FF obtained by reducing the transport resistance in the solar cells. Acknowledgment We acknowledge financial support from CONACYT through grant 134111, the UCMEXUS program grant 00007, CIO-UGTO 2013-2014 and the CEMIE-Solar (04002) consortium. D. Esparza, and A. Cerdán acknowledge scholarship from CONACYT, I. Zarazua acknowledge CONACYT for the postdoctoral fellow and thanks to Maria Christian Albor for SEM and EDS analysis. Supporting Information Available UV-Vis and photoluminescence spectra of the colloidal quantum rods and the current density vs voltage curves for CdS QD and CdSe QR sensitized TiO2 films in different configurations: TiO2/CdS(S)/ZnS(S), TiO2/CdSe(E)/ZnS(S) and TiO2/CdSe(P)/ZnS(S). This information is available free of charge via the Internet at http://pubs.acs.org.


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