Photovoltaic Conversion Enhancement of CdSe ... - ACS Publications

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Photovoltaic Conversion Enhancement of CdSe Quantum Dot-Sensitized TiO2 Decorated with Au Nanoparticles and P3OT ngeles Chavez,§ and I. Zarazua,† E. De la Rosa,*,† T. Lopez-Luke,† J. Reyes-Gomez,‡ S. Ruiz,† C. A || Jin Z. Zhang Centro de Investigaciones en Optica, A.P. 1-948 Leon, Gto. 37150, Mexico Facultad de Ciencias, Universidad de Colima, Colima, Col. 28045, Mexico § Instituto Mexicano del Petroleo, Ciudad Mexico D.F, 07730, Mexico Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States †

)

‡

ABSTRACT: Novel composite titanium dioxide (TiO2) films of 10 μm thickness were prepared and characterized with an emphasis on evaluating their photovoltaic properties. The films contained 13 nm TiO2 nanocrystals (NCs) with anatase crystalline phase deposited on FTO substrates, composited with CdSe quantum dots (QDs), gold nanoparticles (Au NPs), and poly(3-octylthiophene) (P3OT) in different configurations and combinations. With the introduction of Au NPs or P3OT into the TiO2/QDs films, the photocurrent increased to ∼85% and ∼150%, respectively, while the photoconversion efficiency increased by ∼167% and ∼177%, respectively. An interesting synergistic effect was observed when Au NPs and P3OT were used in conjunction. The configuration of TiO2/Au/QDs/P3OT film exhibited a photocurrent of 906 μA (an enhancement of ∼285%) and a photoconversion efficiency of 0.661% (an enhancement of ∼600%) compared to that of TiO2/QDs films. Such significant enhancement is attributed to the ability of the Au NPs to facilitate charge separation and improve electron injection as well as P3OT’s ability to inject electrons and enhance hole transport. These properties, when combined with the QD’s strong photoabsorption in the visible, led to the overall increase in photocurrent generation, fill factor, and consequently photoconversion efficiency.

1. INTRODUCTION Photovoltaic cells have attracted significant attention recently since incident solar energy on the Earth is 104 times more powerful than other energy sources as well as being environmentally friendly.1 The most common solar cells, silicon cells, have a photoconversion efficiency of 20%.2 However, the manufacturing process is expensive and involves the use of toxic chemicals. In the past decade, nanomaterials have emerged as a new and promising alternative for harvesting solar energy.36 There are three types of solar cells based on nanostructured materials: i) organic cells based on molecules or semiconducting polymers sensitized with a carbon nanomaterial,711 ii) inorganic cells based on inorganic semiconductor nanomaterials sensitized with another semiconductor or metallic nanoparticles,1214 and iii) hybrid cells based on a mixture of organicinorganic nanostructures combining the best properties of each component and providing a possible synergetic effect as a result of this combination.1524 The use of nanomaterials for solar cells could reduce production costs due to the low synthesis temperature and deposition methods,1 although the photoconversion efficiency (η) for such devices is still generally much lower than that of the best silicon-based solar cells. The advantages of nanostructured solar cells include the possibility to manipulate factors such as the shape and size of the particles to improve the conversion efficiency, making them commercially feasible.3,19,2529 One key factor to increase conversion efficiency is to improve both electron and hole mobility to avoid recombination. r 2011 American Chemical Society

One of the most studied hybrid-type nanostructured solar cells is the Gr€atzel or dye sensitized solar cell (DSSC).2937 DSSCs typically consist of TiO2 NCs acting as a highly porous, wide bandgap semiconductor for electron collection, and dye molecules adsorbed onto the TiO2 NCs surface acting as sensitizers to harvest solar light. Nanostructured TiO2 cells have many features that can affect the photoconversion efficiency. As particle sizes increase, surface defects and grain boundaries are reduced, and consequently the charge carrier recombination decreases, resulting in increased fill factor.3 However, the sensitizer-metal oxide interface area is reduced relative to volume, resulting in a photocurrent decrease. The largest photoconversion efficiency of DSSC reported in literature is 11%,2 obtained with a ruthenium-based dye.19,21,27,29,33,35,38 Unfortunately, this type of solar cell is costly because of the high cost of the ruthenium dye. An alternative to DSSC is the utilization of quantum dots (QDs), e.g., CdSe, CdTe, CdS, PbS, PbSe, Bi2S3, and InP,13,14,25,34,3944 as sensitizers to replace the expensive ruthenium dyes. QDs have large extinction coefficients in the visible region and, after bandgap excitation, undergo charge separation, injecting electrons to the conduction band of the metal oxide Received: August 12, 2011 Revised: October 5, 2011 Published: October 13, 2011 23209

dx.doi.org/10.1021/jp207744n | J. Phys. Chem. C 2011, 115, 23209–23220

The Journal of Physical Chemistry C

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(MO) according to the following equations13 QDs þ hv f QDs ðe þ hÞ

ð1Þ

QDs ðe þ hÞ þ TiO2 f QDs ðhÞ þ TiO2 ðeÞ

ð2Þ

Thus, the charge separation is improved by coupling QDs with metal oxide with appropriate energetics. The driving force for electron transfer between QDs and TiO2 is dictated by the energy difference between the conduction band edges. For example, the conduction band edge for CdSe QDs varies from 0.8 to 1.57 V vs NHE for a size range from ∼7.5 to 3 nm,45 while the conduction band edge of TiO2 is located at 0.5 V vs NHE.46 Therefore, it is possible to increase the driven force for electron transfer by controlling the crystallite size of QDs, which in turn can improve the conversion efficiency. Recently, a synergistic effect was reported when QD sensitization was combined with nitrogen-doping (N-doping) TiO2, in which N-doping was proposed to facilitate the hole transport while QDs acted as sensitizers.39 N-doping into the TiO2 lattice resulted in a red-shift of the electronic absorption and enhanced photocurrent response relative to undoped TiO2 films.39,47,48 In that report, CdSe QDs linked to TiO2:N NCs significantly increased the photocurrent and photoconversion efficiency of the films compared to standard TiO2:N films without QD sensitization.39 QD-sensitized TiO2 solar cells have been reported to have quantum efficiency (QE) as high as 12%,39,49 but a photoconversion efficiency (η) of only 4.44% has been reported recently.50 Thus, for such a configuration, it is necessary to improve the electron injection efficiency to increase η and take advantage of the QD’s strong photoabsorption in the visible. Another alternative to DSSCs is the use of a hole-conducting polymer for the regeneration of the dye complex. Thiophene derivatives seem to be the best option for this purpose.21 Although possessing a longer useful life due to polymer photostability, this configuration has a lower efficiency than DSSCs with a liquid electrolyte. Thiophene has been widely used in organic solar cells for making heterojunctions with fullerene derivatives (acceptor), and a photoconversion efficiency as high as 5% was reported for a configuration with donoracceptor.5154 The extremely low electron mobility of the polymer is compensated by the fast electron transfer to fullerene or another molecule.5557 QDs have also been used as electron acceptors, resulting in an improved charge separation because of the favorable energy differences in the conductive band edges and higher electron mobility. In this configuration, QDs are used as the electron transporter, whereas thiophene is used as an effective hole transporter, resulting in a photoconversion efficiency of 1.7%.58 More recently, thiophene combined with TiO2 NCs as an electron conducting layer was found to improve the conversion efficiency up to 3.9%.16 The photoresponse of semiconductors has also been improved with the presence of metallic NPs such as gold (Au) and silver (Ag), although the mechanism of the observed enhancement is not completely understood.5963 It has been suggested that such NPs act as electron traps to help separate the photogenerated charges and subsequently improve interfacial charge transfer.6265 Studies have shown a shift in the Fermi level to a higher negative level by doping the semiconductor with metal NPs. This shift enhances the efficiency of the interfacial charge transfer process.66 In this case, a 40% improvement of hole transfer efficiency from the semiconductor film to the

electrolyte has been reported.64 Alternatively, metallic NPs have been suggested to act as sensitizers in TiO2 NCs and zinc oxide (ZnO) nanowires, similar to organic dyes, based on the photoresponse and the absorption spectra of metallic NPs.63,67 In such a case, the electron injection from Au NPs to TiO2 competes effectively with electron relaxation within Au NPs. Enhancement of photoconversion efficiency has also been reported in Si thin films in the presence of Au NPs. In this case, the proposed mechanism was based on the absorption enhancement of Si thin film via the excitation of the surface plasmon resonance (SPR).68,69 However, for larger particle sizes, the dominant mechanism is based on the increment of light scattering and absorption of the Si-thin film.7072 Furthermore, plasmonassisted current generation for single crystal TiO2 and Au nanorods using visible to near IR light has been reported recently.73 Similarly, a strong increase in photoconversion efficiency with QD-sensitized Au-TiO2 NP composite have been found which was attributed to enhanced absorption of QDs caused by increased scattering of light by the Au NPs.59 In this work, we report, for the first time, a systematic characterization of TiO2 films composited with poly-3-octylthiophene (P3OT), CdSe QDs, and Au NPs in different configurations. The objective is to improve the overall photoconversion efficiency by extracting more efficiently the electrons from the QDs with the help of Au NPs and by increasing the hole transport with the presence of P3OT. A detailed study of the structural, optical, and photoelectrochemical properties was carried out to gain new insight into the underlying mechanism. While the Au NPs increased the electron injection rate from CdSe QDs to TiO2, P3OT injected electrons by absorbing the remaining visible light not absorbed by the QDs and increased the hole transport. In conjunction, Au NPs and P3OT improved the regeneration of QDs and facilitated the overall photocurrent generation of the sensitized TiO2 NC films.

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2-Sensitized Films. Materials. Titanium isopropoxide (IV), pluronic F127, cadmium oxide (CdO, 99.5%), 1-tetradecylphosphonic acid (TDPA, 99%), trioctylphosphine oxide (TOPO, 99%), cetyl trimethylammonium bromide (CTAB), selenium powder (Se 99%), sodium borohydride (NaBH4), thioglicolic acid (TGA 99%), and poly-3-octylthiophene (P3OT) were obtained from Sigma Aldrich. A 105 M P3OT/toluene solution was prepared and stored in a dark environment to be used as a sensitizer. TiO2 Film Preparation. TiO2 NCs were prepared by a solgel method. Briefly, 3.75 mL of titanium isopropoxide (IV) and pluronic F127 were added to a solution of 5 mL of HNO3 in 50 mL of absolute ethanol (EtOH) and 2.5 mL of H2O. The mixture was stirred for one hour. The synthesis process was carried out in a glovebox with a N2 atmosphere. The mixed solution was transferred to a Teflon autoclave. The hydrothermal treatment was carried out at 70 C during 12 h outside the glovebox. The resulting xerogel was washed three times with EtOH and annealed for one hour at 550 C. The TiO2 powder was suspended in EtOH and deposited on an FTO substrate by the doctor blade method.50 CdSe QD Synthesis. To synthesize high-quality CdSe QDs, CdO was used as the Cd precursor, while 1-tetradecylphosphonic acid (TDPA, 99%) and trioctylphosphine oxide (TOPO, 99%) were used as the surfactants. The synthesis was performed in an 23210

dx.doi.org/10.1021/jp207744n |J. Phys. Chem. C 2011, 115, 23209–23220

The Journal of Physical Chemistry C air-free reaction environment according to the procedure proposed by Robel et al.,13 where 0.05 g (0.39 mmol) of CdO, 0.3 g (1.1 mmol) of TDPA, and 4 g (10.3 mmol) of TOPO were heated to 110 C, degassed under vacuum, and then heated to 300 C under a nitrogen flow.13,39 A Se-TOP (0.7 wt %) solution was prepared in an inert atmosphere by adding 0.026 g of Se powder with 4.25 mL of TOP which was stirred for 1 h to ensure complete dissolution of the Se powder. After reaching 300 C, the CdTDPA-TOPO solution was cooled to 270 C prior to the injection of the Se-TOP. Under nitrogen flow, 3 mL of Se-TOP was injected, thereby lowering the reaction temperature to 260 C. The temperature was subsequently increased to 280 C to facilitate particle growth. Aliquots were extracted and measured via UVvis absorption spectroscopy to track QD particle growth. The CdSe solution was cooled and removed from the reaction flask at ∼80 C and dissolved into 10 mL of toluene. The CdSe was cleaned twice through a precipitation and decantation process using methanol and centrifugation at 3000 rpm. The QDs were finally redissolved in toluene prior to their use as a sensitizer. The

Figure 1. Schematic representation of TiO2 NCs/Au NPs/CdSe QDs/ P3OT film for photoelectrochemical characterization.

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resulting CdSe QDs were in the strong confinement size regime ∼4.5 nm. Au NPs Synthesis. Au NPs were synthesized by reacting 10 mL of a 0.2 M solution of cetyl trimethylammonium bromide (CTAB) with 5 mL of 0.5 mM solution of HAuCl4. The mixture was stirred in an ice bath to which 0.60 mL of 0.01 M NaBH4 was added, resulting in a brown solution. The solution was stirred for 2 h to obtain 5.5 nm particle size which was verified via TEM and UVvis absorption spectroscopy. The final obtained solution was stored in a dark environment to avoid light exposure for later use. Preparation of TiO2 Composite Film. Au, CdSe QDs, and P3OT were linked to TiO2 NCs thin films in eight different configurations: TiO2/QDs, TiO2/QDs/P3OT, TiO2/P3OT/ QDs, TiO2/QDs/Au, TiO2/Au/QDs, TiO2/QDs/Au/P3OT, TiO2/P3OT/QDs/Au, and TiO2/Au/QDs/P3OT, as shown in Figure 1. Thioglicolic acid (TGA, HSCH2COOH) was used as a molecular linker for QDs and Au NPs. TiO2 has a strong affinity to the carboxylate group of the linker molecules, while the sulfur atom from the TGA binds strongly to both QDs and Au NPs through surface interaction (Figure 2a and 2b). TiO2 films were dried on a hot plate at 100 C for 1 h to remove excess of H2O. They were then immersed in a TGA solution diluted to 70% in the presence of CH3Cl for 12 h in a nitrogen atmosphere glovebox. Consequently, they were immersed in toluene to remove the excess of TGA and immersed in a solution containing CdSe QDs, P3OT, or Au NPs for 12 h inside the glovebox. The second and third component layers were deposited in a similar way. From this, only the first component was strongly linked to TGA, while the subsequent components were introduced as uniform layers. The resulting films of 1.61 cm2 of active area were stored in a nitrogen-filled glovebox and not exposed to light prior to photoelectrochemical (PEC) characterization. Effect of Au NP Concentration. TiO2 films decorated with three different concentrations of Au NPs were prepared by the immersion technique in order to evaluate the effect of

Figure 2. Schematic representation of TiO2 NCs decoration. a) Functionalization with TGA (R = CH2CO). b) Decoration with Au NPs. 23211


The Journal of Physical Chemistry C concentration. The original solution (with a concentration of 1.0 1014 Au NPs/mL) was diluted in water to 50 and 25 vol% of the original composition. 2.2. Characterization. Raman spectra were measured by exciting the TiO2 NC powder with a HeNe laser, with a power of 60 mW and a wavelength of 632.8 nm, focusing the beam with a 40x microscope objective. The spectra were collected from 200 to 1000 cm1. A JEOL model JEM-1200EX transmission electron microscope (TEM) was used to verify the TiO2 NC size. The composition was measured by electron energy loss spectroscopy (EELS), and the crystalline phase was studied by electron diffraction pattern (EDP). UVvis absorption spectra were measured with a PerkinElmer spectrophotometer (Lambda 900). The TiO2 spectra were measured via reflectance in pellets of compact powder, scanning between 200 to 800 nm using an integrating sphere of 60 mm. Electrochemical measurements were made in a threeelectrode cell using the sample as the working electrode which

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was illuminated with 31.83 mW/cm2 from a halogen lamp. A saturated calomel electrode (SCE) was used as the reference electrode, while a platinum (Pt) wire was used as the counterelectrode and sodium sulfate (Na2S) as the electrolyte using a potentiostat Gamry Reference 600. The electrolyte was purged with N2 for ten minutes, although a small amount of O2 could have remained in solution and played the role of electron scavenger, the amount of hole scavenger should be much higher. The excess of O2 in the electrolyte could induce loss of electrons by recombination with O2 and thus may affect the conversion efficiency. Open-circuit voltage (Voc) decay measurements were conducted following the procedure described by Paulose et al.27 Measurements were performed by monitoring the Voc transient during relaxation from an illuminated quasi-equilibrium state obtained after 10 min of light illumination, to the dark equilibrium state 10 min after illumination was stopped. Photocurrent measurements were collected by linear sweep voltammetry by changing the voltage from 0.5 to 0.5 V vs SCE. Incident photon-to-current conversion efficiency (IPCE) measurements were conducted for TiO2/QDs, TiO2/Au/QDs, and TiO2/Au/QDs/P3OT films with a 1000 W Xe lamp (Oriel arc lamp model 66021 and power supply 68820) aligned into a monochromator (Oriel model 77200 1/4 m) scanning from 350 to 600 nm. A Na2S solution (1 M) served as the hole scavenger source to maintain the stability of the QDs. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying a sinusoidal voltage between (10 mV from the equilibrium potential, varying the sinusoidal frequency between 1 102 and 1 105 Hz. EIS measurements were fit with the Boukamp Equivalent Circuit software.

3. RESULTS Figure 3. Raman spectrum of TiO2 NCs powder annealed at 550 C for one hour.

3.1. Structural and Morphological Characterization. The Raman spectrum of TiO2 NCs powder in Figure 3 shows the

Figure 4. a) Typical TEM image, b) electron diffraction pattern (EDP), c) size distribution, and d) EELS spectrum of TiO2 NCs. 23212



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Figure 5. TEM image of a) CdSe QDs and b) AuNPs and size distribution of c) CdSe QDs and d) AuNPs.

well-defined characteristic peaks of anatase crystalline phase at 395 cm1, 513 cm1, and 635 cm1, indicative of a high degree of crystallinity.74 The morphology of the TiO2 NCs is shown in Figure 4a, while Figure 4b shows the electron diffraction pattern (EDP) with diffraction lines corresponding to the (1,0,1), (0,0,4), (2,0,0), and (2,1,1) planes of anatase TiO2. The average crystallite size was 13 nm in diameter, and the size distribution is shown in Figure 4c. The composition of the NCs powder was studied by using EELS, and the resulting spectrum is shown in Figure 4d which displays peaks centered at 450, 465, and 535 eV that correspond to titanium L3 and L2 and oxygen K energy levels, respectively. Figure 5 displays the TEM images of both QDs and Au NPs as well as the size distribution with average sizes of 4.5 and 5.5 nm for CdSe and Au NPs, respectively. 3.2. Optical Absorption. The absorption spectra of TiO2 NCs, P3OT, CdSe QDs, and Au NPs are shown in Figure 6a. The characteristic absorption peaks for each component are centered at 338 nm (3.67 eV), 459 nm (2.7 eV), 552 (2.24 eV), and 517 nm (2.40 eV), respectively. The absorption spectrum of Au NPs is characteristic of the surface plasmon resonance (SPR) expected for such size of NPs.75 The absorption of the TiO2 NC film decorated with the QDs, Au NPs, and P3OT was likewise studied, and Figure 6a shows the absorption spectrum of the composite film TiO2/Au/QDs/P3OT. It presents two welldefined bands corresponding to TiO2 NCs (338 nm) and CdSe QDs (552 nm). The slope from 400 to 550 nm is attributed to combined absorption from P3OT and QDs, and the absorption between the 600 to 700 nm was observed in all Au NPsdecorated films. This long red-shifted absorption tail confirms

the presence of Au NPs and suggests possible agglomeration during the film deposition. When agglomeration occurs, the absorption band becomes characteristically weaker and broader. Thus, the slope from 400 to 550 nm also contains the absorption of Au NPs. The first derivative of the absorption spectrum was calculated to estimate the threshold bandgap of each sample. The resulting curves exhibit peaks centered at 379 nm (3.27 eV), 502 nm (2.47 eV), and 567 nm (2.18 eV) associated with the bandgap of TiO2, P3OT, and QDs respectively, as shown in Figure 6b. These spectra show an obvious red-shift of the absorption edge toward the visible light region compared to TiO2 NCs, which is useful for photoconversion using visible light. 3.3. Electrochemical Characterization. Photoconversion Measurements. The current densityvoltage (J-V) profiles for TiO2 NC films decorated with Au NPs, QDs, and P3OT fabricated with different configurations were obtained and are shown in Figure 7. Before such characterization, the electrolyte behavior was studied by cyclic voltammetry with platinum as the working electrode, while the oxidation peak at 0.214 V vs SCE was observed. This oxidation voltage was taken as a reference (V = 0) to evaluate all films under study. The measured shortcircuit current density (Jsc) and open-circuit voltage (Voc) are summarized in Table 1. The fill factor (FF) and photoconversion efficiency (η) were calculated by using eqs 3 and 476 FF ¼ η¼ 23213

Pmax 100 Jsc Voc

Pmax Jsc Voc 100 ¼ FF Pi Pi

ð3Þ ð4Þ



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Figure 7. Characteristic J-V curve of TiO2 NCs films decorated with a) CdSe QDs, b) CdSe QDs/Au NPs, c) P3OT/CdSe QDs/Au NPs, and d) Au NPs/CdSe QDs/P3OT. Measurements were performed in a three-electrode cell, a saturated calomel electrode as a reference, Pt as a counter electrode and the sample under study as a work electrode, and Na2S as electrolyte using the measured oxidation potential 0.214 V Vs SCE. A halogen lamp with 31.83 mW/cm2 was used for illumination. Figure 6. a) Absorption spectra of Au NPs, P3OT, CdSe QDs, TiO2 NCs, and TiO2/Au/QDs/P3OT film and b) first derivative of absorption spectra of P3OT, CdSe QDs, and TiO2 NCs.

where Pmax is the maximum power (J V product) observed from the current densityvoltage curve for each device, and Pi is the incident light power density (31.83 mW/cm2, in our case). All calculated values for different configurations are displayed in Table 1. Values reported here are measured and calculated with an error lower than 5%. The results clearly show that all multidecorated systems present photocurrent and photoconversion efficiency higher than that observed for the TiO2/QDs film. With the introduction of Au NPs and P3OT, the photocurrent increases by ∼85% and ∼150%, while the photoconversion efficiency increases by ∼167% and ∼177%, respectively. The improvement demonstrates the importance of using properly configured material architectures (TiO2-QDs-Au and TiO2QDs-P3OT). The enhancement with the introduction of Au NPs is a little higher than that reported recently by the Zhang group, which was obtained using a different configuration involving blending of TiO2 NC, Au NPs, and CdSe QDs in large colloidal spheres.59 The different photocurrent and similar photoconversion efficiency in our experimental results suggests a stronger effect on FF of Au NPs than P3OT, as confirmed by the calculated data shown in Table 1. As the FF parameter is related with the ratio of charge carriers extracted from the device to those photogenerated in the active material, an increase in FF indicates an increase of the transport efficiency caused by either a reduction of the leak current or reduction in resistance. Thus, Au NPs enhance charge separation and transport and reduce charge recombination which is in good agreement with results reported by others.62,65 To our knowledge, this is the first reported result using thiophene combined with TiO2 NCs and CdSe QDs to improve photovoltaic properties. P3OT enhances the photocurrent by contributing to the electron injection as well as hole transport. Therefore, one may expect an increase in photoconversion efficiency when Au NPs and P3OT are used in conjunction. Indeed, the highest FF (38.1%) was obtained from the TiO2/Au/ QD/P3OT configuration that represents an increase of ∼95% compared to that obtained for TiO2/QDs film (19.5%). Such a

configuration shows an increase of photocurrent of ∼285% (906 μA), resulting in an increment of ∼600% on the photoconversion efficiency (0.661%). This strong enhancement is a result of the synergistic effect of the combined use of both P3OT and Au NPs. Detailed analysis of each configuration will be performed and described later. Discharge Decay Rate. Under open-circuit conditions and visible irradiation, electrons accumulate within the nanostructured semiconductor films, shifting the apparent Fermi level to negative potentials, as is observed in Figure 8. Once the illumination is stopped, the accumulated electrons are slowly discharged by the recombination with holes trapped in the composite and because dissolved oxygen in the electrolyte scavenges electrons. The decay rate of such discharge is related to the electron response time of the system by the expression27 τn ¼

kb T dVoc 1 Ø ø e dt

ð5Þ

where kbT is the thermal energy, e is the elementary charge, and dVoc/dt is the derivative of the transient open circuit voltage. The minimum response time of each system τnmin obtained by applying eq 5 to photovoltage discharge in Figure 8 is given in Table 1. Both the Au NPs and P3OT components promote a fast response time, but the location of the Au NPs critically affects the response. Longer response times were obtained for the TiO2/ QDs/Au/P3OT film, indicating less recombination in this system, likely because of the high energy barrier originated from the dipole effect between metalorganic interface obstructing the carrier transport, as was suggested recently.65 The minimum response times were obtained from TiO2/Au/QDs/P3OT film (Table 1). IPCE Characterization. Incident photon to current conversion efficiency (IPCE) provides a measure of how many electrons are photogenerated for each incident photon at a given wavelength (λ) and can be determined from the short-circuit photocurrent (Jsc), using the expression25 IPCE% ¼

1240 Jsc ðAjcm2 Þ 100 λðnmÞ I inc ðWjcm2 Þ

ð6Þ

where Iinc stand for the incident light power. The calculated IPCE for TiO2/QDs, TiO2/Au/QDs, and TiO2/Au/QDs/P3OT films 23214



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Table 1. Photocurrent (Jsc) Measured at 0.214 V vs SCE, Photovoltage (Voc), Field Factor (FF), Photoconversion Efficiency (η), and Response Time (τ) of Multidecorated TiO2 Films under 31.83 mW/cm2 of Light Powera sample

τnmin, s Jsc, μA 3 cm2 Voc, mV FF, %

η, %

TiO2/QDs

3.9

236

675

19.5

0.098

TiO2/QDs/P3OT

2.8

584

631

23.5

0.272

TiO2/P3OT/QDs

2.5

503

637

20.3

0.206

TiO2/QDs/Au

4.5

437

597

32.1

0.262

TiO2/Au/QDs

2.2

401

531

28.9

0.194

TiO2/QDs/Au/P3OT

6.1

260

526

28.1

0.120

TiO2/P3OT/QDs/Au TiO2/Au/QDs/P3OT

2.7 1.7

609 906

601 610

34.3 38.1

0.394 0.661

a

All values reported here are measured and calculated with an error lower than 5%.

are shown in Figure 9. All samples have a peak around 365 nm, attributed to the strong photoabsorption of TiO2 NCs (Figure 6a), combined with the absorption of FTO (360 nm). For the TiO2/QDs film, the photocurrent generation starts at 590 nm with shoulders at 550 and 512 nm and a weak peak at 445 nm, that corresponds to the absorption of CdSe QDs (Figure 9a). The TiO2/Au/QDs film showed reduced IPCE (Figure 9b), down to about a third of that obtained for the TiO2/ QDs film, with bands at 365 nm associated with TiO2, 518 nm associated with Au NPs, and a weak band at 550 nm associated with the QDs. The enhancement of the band at 518 nm suggests a possible photosensitization role due to the Au NPs, in agreement with results reported by Tian et al.63,67 However, the observed decrease in the IPCE values is likely the result of dominant trapped photogenerated electrons as suggested previously62,65 as well as light scattering which reduces the excitation of QDs. The photosensitizer or electron trapping effect dominance depends on the interaction between TiO2 and Au NPs as suggested recently,59 which, in turn, is dependent on the film and linking mechanism between both nanoparticles. Thus, the photosensitizing effect observed in our case suggests a strong interaction between TiO2 and Au NPs and contributes to the electron injection, while the dominant electron trapping effect helps to separate the photogenerated charges, improving the interfacial charge transfer. The combined effects result in an increase of overall conversion efficiency. A 68% increase of IPCE was observed for films prepared with the TiO2/Au/QDs/ P3OT configuration, as compared to that of the TiO2/QDs film. The IPCE plot shows a well-defined broad band from 455 to 480 nm that could be attributed to P3OT, confirming its contribution to the photocurrent generation directly by absorbing light. Additionally, an enhanced band associated with QDs confirming the role of P3OT in the hole regeneration and a weak band associated with Au NPs was observed. Therefore, the strong increase of IPCE from 580 nm to the UV confirms the synergistic effect, especially considering the enhancement is larger than the addition of contribution of each component in the composite. The relatively low IPCE values can be attributed to the fact that most of the important parameters were not yet optimized, such as electrons and holes trapped at the interface, internal and surface defects of each component that promote recombination, light scattering, etc. The electron scavenging by the remnant O2 in the electrolyte could also partially contribute to the reduced IPCE.

Figure 8. Photovoltage discharge of TiO2 films decorated with a) CdSe QDs/P3OT, b)Au NPs/CdSe QDs/P3OT, c) CdSe QDs, and d) P3OT/CdSe QDs/Au NPs. These values, in combination with eq 3, were used to calculate the response time (τ) of each sample under study.

Electrochemical Impedance. Electrochemical impedance spectroscopy (EIS) measurements were carried out to study the resistance behavior of different film components. Figure 10 shows the equivalent circuit used to fit the impedance measurements. R1 is related to the resistance of FTO electrode, R2 is the resistance of TiO2/sensitizers interface, and C1 is its capacitance. R3 and C2 are the resistance and capacitance of the sensitizers/ electrolyte interface, outside the TiO2 pores, and W is the Warburg impedance generated by the resistance to the ion movement in the electrolyte. The TiO2/QDs film has a high internal resistance, with R2 = 80 KΩ as shown in Figure 11, due to the interface between the nanoparticles. The introduction of the Au NPs into the TiO2/ Au/QDs configuration reduces strongly the internal resistance to R2 = 4.13 KΩ and the external resistance R3 from 127 KΩ to 11.19 KΩ (Figure 11). In this case, Au NPs were imbedded into the film deeper than QDs, increasing strongly the electron transfer from QDs to TiO2. However, direct interaction of QDs with electrolyte probably increases electron scavenging by the remnant O2 dissolved in the electrolyte. The introduction of P3OT into the TiO2/Au/QDs/P3OT configuration induces a slight increase in the internal and external resistance, R2 = 8.06 KΩ and R3 = 24.86 KΩ, respectively. In this case, the increase of R2 suggests a partial penetration of P3OT into TiO2 film, blocking the electron injection from QDs. This explanation is based on the consideration that P3OT is not as good a conductor as Au NPs and that its conduction band edge is higher than that of CdSe QDs. In addition, because most of the P3OT is between QDs and the electrolyte where P3OT acts as insulator, it is also reasonable to expect an increase of the external resistance. Thus, EIS analysis confirms the important role of Au NPs in reducing the internal resistance and thereby improving the electron injection as well as the role of P3OT in the electron injection and hole recovering.

4. DISCUSSION With the results described above, it is possible to gain a better understanding of the physical mechanism involved in the photoconversion efficiency for each configuration under study. Among all the composite films studied here, the TiO2/QDs films show the lowest photoconversion efficiency (0.098%). This is still higher than that obtained for pristine TiO2 due to the QD’s ability to photosensitize TiO2.49 In the TiO2/QDs film, photogenerated electrons in the conduction band (CB) of the CdSe 23215



Figure 9. Incident photon to current conversion efficiency (IPCE) determined from the photocurrent at different excitation wavelength and by using eq 6, for different configuration. a) TiO2 NCs/CdSe QDs, b) TiO2 NCs/Au NPs/CdSe QDs, and c) TiO2 NCs/Au NPs/CdSe QDs/P3OT. Inset shows a zoom-out of the same samples. Notice that curve a and b are amplified 5X for visualization purpose.

Figure 10. Equivalent circuit based on electrochemical impedance spectroscopic (EIS) of the sensitized TiO2 NCs films. R1 stands for the resistance of FTO electrode, R2 (C1) stands for the resistance (capacitance) on the interface between TiO2 and sensitizer, R3 (C2) stands for the resistance between sensitizer and electrolyte, and W stands for the impedance of the electrolyte.

QDs are injected into the CB of TiO2 NCs due to the energy difference between their band edges, as shown in Figure 12a. Once in the CB of TiO2, electrons can be transported to the FTO substrate. However, QDs have significant electron losses mediated by electronhole radiative and nonradiative recombination processes, which limit the FF to 19.5%. Furthermore, the TiO2/QDs configuration presents a relatively high resistance at the interface between TiO2 NCs and CdSe QDs, reducing the electron injection efficiency and consequently the FF and photocurrent. An increase of photocurrent (584 μA/cm2) and FF (23.5%) was observed with the addition of P3OT to form the TiO2/QDs/ P3OT configuration, although the FF parameter is still small in comparison to other configurations studied here. The increase in photocurrent is attributed to the photoabsorption of P3OT that has a broad absorption band and acts to complement the absorption of CdSe QDs. When relevant energy levels are considered, as shown in Figure 12b, it is likely that electrons which are photogenerated in P3OT are first injected to QDs because of the energy difference between the CB of P3OT and CB edge of the CdSe QDs. These electrons are subsequently transported and injected, along with electrons directly photogenerated in the CdSe QDs, to TiO2 NCs and then to the FTO substrate. However, not all electrons generated contribute to the photocurrent. In fact, a limitation with this configuration is the inefficient electron injection of QDs due to significant surface recombination that explains the relative low value of FF. Nevertheless, holes in QDs in this configuration are injected to P3OT

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(and then filled by oxidizing the electrolyte), improving the electron injection as confirmed by the small increment of FF. When the order of sensitizer is switched to have the TiO2/ P3OT/QDs configuration, the photocurrent (503 μA/cm2) and FF (20.3%) were found to be lower than those obtained from TiO2/QDs/P3OT. Such a reduction is because many electrons generated in QDs do not reach the TiO2 effectively due to the CB energy level of P3OT being higher than CB edge of QDs, as shown in Figure 12c. An additional reason is from the reduction of the QD absorption, considering that irradiation was first absorbed by P3OT. Therefore, the increase of photocurrent and conversion efficiency (0.272%) is likely a result of contribution of electron injection from P3OT, and the reduction of photocurrent, FF, and photoconversion efficiency (0.206%) is possible due to reduced contribution from QDs. The addition of Au NPs instead of P3OT in TiO2/QDs/Au configuration increases the FF to 32.1%. This is the result of improved hole injection from the QDs to the electrolyte due to the better conduction properties of Au NPs (Figure 12d). Au NPs likely improved the regeneration rate of QDs that in turn increase the photoabsorption and photocurrent in comparison to TiO2/QDs films. This increase is strong enough to give a conversion efficiency of 0.262%, similar to that obtained with P3OT (Table 1). However, the photocurrent is relatively low for this configuration (437 μA/cm2) compared to sample using P3OT because Au NPs, unlike P3OT, do not contribute to the photogeneration process. Furthermore, a stronger decrease in photocurrent (401 μA/cm2), FF (28.9%), and photoconversion efficiency (0.194%) was observed when Au NPs were placed between TiO2 and QDs to form the TiO2/Au/QDs configuration (Table 1, Figure 12e). In this case, electron injection from QDs to TiO2 may be reduced due to the electron trap and the scattering of incident light by Au NPs, as indicated by the IPCE results discussed above. Overall, the FF obtained with the introduction of Au NPs is higher than that obtained with bare TiO2/QDs and with P3OT in any configuration. This is explained by the ability of Au NPs to extract electrons from QDs and inject it into TiO2 electrode and is in good agreement with results recently reported.77 In summary, the use of P3OT contributed substantially to increasing the photogeneration of charge carriers but maintained a relatively low FF due to the low electron injection efficiency. The use of Au NPs improves the electron injection rate, resulting in a higher FF but does not contribute (or contribute in a minor proportion) to the photogeneration of charge carriers and may actually reduce it somewhat due to electron trapping and scattering of the incident light. The combination of Au NPs and P3OT in the right configuration was found to improve the photoconversion efficiency by enhancing the FF as well as the photocurrent. When Au NPs are linked between QDs and P3OT (TiO2/QDs/Au/P3OT), a strong reduction of photocurrent (260 μA/cm2) and photoconversion efficiency (0.12%) was observed compared to TiO2/QDs/Au and TiO2/QDs/P3OT films. This configuration can be visualized as a couple of cells with opposite electron flux. In this case, QDs and P3OT photogenerate charge carriers in a similar way to that in the TiO2/QDs/ P3OT configuration, but Au NPs act as a charge sink capturing holes from both sides, as shown in Figure 12f. This is corroborated by the strong increase of τn (6.1 s) that suggest a reduction in the charge carriers flux to the electrolyte. Furthermore, the low conversion efficiency suggests a low flux of electrons through 23216



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Figure 11. Calculated resistance (R2 and R3) of the equivalent circuit for TiO2 NCs/CdSe QDs, TiO2 NCs/AuNPs/CdSe QDs, and TiO2 NCs/Au NPs/CdSe QDs/P3OT samples.

Figure 12. Schematic representation of the energy levels in different configurations of sensitized TiO2 films. Blue and red lines indicate possible paths of electron and holes, respectively.

TiO2 as a result of reduced QDs regeneration. In addition, the excess of holes in Au NPs induces oxidation, as has been reported previously,65 changing its role to a charge sink.

A strong increase of photocurrent (609 μA/cm2) and FF (34.3%) was observed with the configuration of TiO2/P3OT/ QDs/Au. In this case, the photoconversion efficiency (0.394%) 23217


The Journal of Physical Chemistry C represents an increase of ∼91% when compared to similar configuration but without Au NPs. The enhancement is attributed to the improvement of QDs regeneration by Au NPs. However, QDs are not used in its optimal efficiency because their CB edge is lower than that of P3OT (Figure 12g). Thus, the limitation of this configuration is that P3OT is not a good electron conductor and QDs are not good hole conductor resulting in a resistance to the charge carriers flux and low performance of the configuration. The strongest improvement of both photocurrent (906 μA/ cm2) and FF (38%) was obtained with the TiO2/Au/QDs/ P3OT configuration (Figure 12h), taking advantage of the positive characteristics of each component. This configuration shows a photoconversion efficiency of 0.661%, which represents an increase of ∼600% when compared with samples sensitized only with QDs. In this configuration, P3OT absorbs part of the incident light and photogenerates electrons that can be efficiently injected into the CB of the QDs and then to TiO2. Moreover, P3OT is very efficient in transporting holes from QDs to the electrolyte, facilitating its regeneration that should increase overall photoabsorption. Meanwhile, Au NPs improve the charge carrier transport, electrons from QDs, resulting in a strong improvement of photocurrent. When all is combined, this produces a system with optimal energy levels for efficient charge carrier generation, transfer, and transport and thereby high overall photoconversion efficiency. The observed decrease of Voc with the introduction of Au NPs and P3OT suggest a shift of the band edge to positive potential as a result of the coupling between the Fermi energy levels (EF) of each component in the heterostructure. The strongest effect was observed with the introduction of Au NPs between TiO2 and QDs. In this case, the EF from the Au NPs is below the conduction band edge of TiO2 (which explains the electron trapping role) and shifts the conduction band edge of QDs to a positive potential. The overall result is a strong decrease of Voc. The effect of P3OT is lower, suggesting a much better coupling in the EF level. The strong effect of Au NPs can be reduced by the presence of P3OT shifting Voc to negative potential. However, when Au NPs is situated between QDs and P3OT, Voc was significantly reduced. According to IPCE results, Au NPs strongly affect the photoabsorption of QDs due to the electron trapping effect and light scattering. However, FF and EIS measurements indicate that Au NPs considerably reduce the resistance to the charge carriers flux, resulting in an increase of photoconversion efficiency. This effect was confirmed by evaluating the conversion efficiency for a different concentration of Au NPs on the TiO2/Au films. Experimental results show an increase of photoconversion efficiency with the increase of Au NPs loading, 0.074%, 0.096%, and 0.104% for 0.26 1014, 0.52 1014, and 1.04 1014 nanoparticles/mL, respectively. The gain in photocurrent is attributed to the high conductivity of Au NPs, allowing a better electric contact between TiO2 and the electrolyte. Higher concentration of Au NPs covers more efficiently the TiO2 surface and enhances the electric contact. EIS measurements confirm that the internal resistance of the film (R2) decreases from 91.48 KΩ to 33.59 KΩ with the increased loading of Au NPs. A linear relation with negative slope between FF and R2 was found, indicating that these resistance affect the charge carriers flux.

5. CONCLUSIONS The photovoltaic properties of CdSe QD-sensitized TiO2 NC films decorated with Au NPs and P3OT in different configurations

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have been systematically studied and analyzed. It was demonstrated that all multidecorated systems show higher photoconversion efficiency than TiO2/CdSe QDs or TiO2/Au NPs. The presence of Au NPs improves the charge separation and thereby photocurrent generation. The introduction of P3OT contributes to electron injection as well as to hole transport. Most significantly, the combined use of Au NPs and P3OT synergistically improves the photoconversion efficiency. The strong enhancement of ∼285% on Jsc and ∼600% on photoconversion efficiency with the optimum configuration of TiO2/Au/QDs/P3OT being the result of the fast electron extraction by Au NPs from QDs and the electron injection from P3OT accompanied with hole conduction. The composite systems based on CdSe QDs, Au NPs, P3OT, and TiO2 NCs, when constructed in the appropriate configuration, have been demonstrated to be promising for photovoltaic and possibly other optoelectronics applications. However, the optimal configuration reported here can be improved to increase IPCE and photoconversion efficiency in order to make such configuration more reliable for practical applications.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by CONACyT ref.134111, CONCyTEG ref. 09-04-K662-072, and 09-04-K662-055. I. Zarazua and S. Ruiz acknowledge CONACyT for the PhD scholarship. J.Z.Z. is grateful to the Basic Energy Sciences Division of the U.S. DOE (DE-FG02-ER46232) for support. We thank A. Garcia de la Rosa for helpful discussions and Damon Wheeler for proofreading the manuscript. ’ REFERENCES (1) Kamat, P. V. Meeting the clean energy demand: nanostructure architectures for solar energy conversion. J. Phys. Chem. C 2007, 111, 2834–2860. (2) Green, M.; Emery, K.; Hishikawa, Y. Solar cell efficiency tables (V. 35). Prog. Photovoltaics 2010, 18, 144–150. (3) Nanostructured Materials for Solar Energy Conversion, 1st ed.; Soga, T., Ed.; Elsevier Publication: Amsterdam, The Netherlands, 2006. (4) Gregg, B. Excitonic solar cells. J. Phys. Chem. B 2003, 107, 4688– 4698. (5) Lokhande, C. D.; Park, B.-O.; Park, H.-S.; Jung, K.-D.; Joo, O.-S. Electrodeposition of TiO2 and RuO2 thin films for morphologydependent applications. Ultramicroscopy 2005, 105, 267–274. (6) Smestad, G. P.; Spiekermann, S.; Kowalik, J.; Grant, C. D.; Schwartzberg, A. M.; Zhang, J.; Tolbert, L. M.; Moons, E. A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices. Sol. Energy Mater. Sol. Cells 2003, 76, 85–105. (7) Saunders, B. R.; Turner, M. L. Nanoparticlepolymer photovoltaic cells. Adv. Colloid Interface Sci. 2008, 138, 1–23. (8) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design rules for donors in bulkheterojunction solar cells towards 10% energy-conversion efficiency. Adv. Mater. 2006, 18, 789–794. (9) Brabec, C. J.; Carvino, A.; Menissener, D.; Aariciftci, N. S.; Formherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Origin of the open circuit voltage of plastic solar cells. Adv. Funct. Mater. 2001, 11, 374–380. (10) Lee, S. W.; Lee, H. J.; Choi, J. H.; Koh, W. G.; Myoung, J. M.; Hur, J. H.; Park, J. J.; Cho, J. H.; Jeong, U. Periodic array of 23218


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