graphene oxide as efficient modified layer

Journal of Colloid and Interface Science 480 (2016) 49–56

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CdS/CdSe quantum dots and ZnPc dye co-sensitized solar cells with Au nanoparticles/graphene oxide as efficient modified layer Cong Chen, Yu Cheng, Junjie Jin, Qilin Dai ⇑, Hongwei Song ⇑ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China

g r a p h i c a l a b s t r a c t In this work, we present a new design for the development of QDSSCs based on the macroporous ZnO IOs photoanode sensitized by QDs which have light harvesting in the visible region and organic dye in visible/NIR region. Because of the complementary absorption spectral response caused by CdS/CdSe QDs and ZnPc dye, the co-sensitized solar cell can expand the absorption spectra to
750 nm. As a result, the co-sensitized solar cells exhibited a significant improvement in Jsc leading to the overall PCE of 4.01%, which is much better than that of the individual CdS/CdSe (3.08%) or ZnPc (1.88%) sensitizer. We also found that the cell with an Au NPs/GO composite as a modified layer on traditional Cu2S counter electrode shows higher photovoltaic performance (4.60%) which can be attributed to the superior combination of high electro-catalytic activity of Au NPs and the electrical conductivity of the GO network structure. Enhanced stability remains 85% of its original value because ZnPc prevents the degradation of the CdS/CdSe QDs in S2/Sn2 liquid electrolyte and Au NPs/GO composites catalyse the regeneration of QDs and transmission of carriers. We think the present work can provide a simple stepwise co-sensitization method to improve photoelectric response by expanding the absorption spectrum and an efficient way for green chemistry by modifying the counter electrode with Au NPs/GO composites for QDSSCs.

a r t i c l e

i n f o

Article history: Received 2 June 2016 Accepted 30 June 2016 Available online 1 July 2016 Keywords: Quantum dots Zinc phthalocyanine Co-sensitized solar cells Au nanoparticles Graphene oxide

a b s t r a c t Co-sensitization by using two or more sensitizers with complementary absorption spectra to expand the spectral response range is an effective approach to enhance device performance of quantum dot sensitized solar cells (QDSSCs). To improve the light-harvesting in the visible/near-infrared (NIR) region, organic dye zinc phthalocyanine (ZnPc) was combined with CdS/CdSe quantum dots (QDs) for co-sensitized solar cells based on ZnO inverse opals (IOs) as photoanode. The resulting co-sensitized device shows an efficient panchromatic spectral response feature to
750 nm and presents an overall conversion efficiency of 4.01%, which is superior to that of the individual ZnPc-sensitized solar cells and CdS/CdSe-sensitized solar cells. Meanwhile, an Au nanoparticles/graphene oxide (Au NPs/GO) composite layer was successfully prepared to modify Cu2S counter electrode for the co-sensitized solar cells. Reducing the carrier recombination process by GO and catalytic process of Au NPs leads to increased power conversion efficiency(PCE) from 4.01 to 4.60% and sustainable stability remains
85% of its

⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Dai), [email protected] (H. Song). http://dx.doi.org/10.1016/j.jcis.2016.06.076 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

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original value after 60 min light exposure. In this paper, introduction of the organic dyes as co-sensitizer and Au NPs/GO as counter electrode modified layer has been proved to be an effective route to improve the performance of QDSSCs. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction QDs have attracted great attention as a light absorbing material for preparing high efficient and low cost QDSSCs. QDs show several advantages such as size-dependent band gaps, large intrinsic dipole moments and higher extinction coefficients over organic dyes. In addition, their ability to generate multiple excitons with a single photon provides the possibility of high-efficiency QDSSCs due to overcoming the Shockleye-Queisser limit [1,2]. Up to now, the most popular QDSSCs are based on CdS/CdSe as sensitizers and mesoporous TiO2 as photoanode, and the PCE of the solar cells has been increased from less than 1% in 2002 to 5% in 2010 [3,4]. Recently, with the development of the efficient loading of presynthesized core/shell ZnTe/CdSe QDs into photoanode and designed ITO@Cu2S nanowire array counter electrodes, QDSSCs with PCEs of up to 7% have been demonstrated [5–7]. However, QDSSCs still lag far behind dye sensitized solar cells (DSSCs), typically above 12% [8]. The main drawback of CdS and CdSe QDs is the lack of light-harvesting capability in the red region of the visible light QDs. To overcome this problem, more efforts should be put in developing the QDSSCs with more broad and efficient light absorption in the visible/NIR region and faster charge transfer by effective band alignment of sensitizer. Metal phthalocyanines (MPcs) are well-known for their intense absorption in the red/near IR region. Therefore, they are excellent candidates to be used as light absorber to expand the photo-response in QDSSCs [9–14]. Among them, unsymmetrical amphiphilic zinc phthalocyanine possesses strong absorption bands in the visible/NIR region. More importantly, ZnPc can provide an efficient electron transfer from the excited dye to CdX(X = S, Se) QDs due to matched band alignment. Therefore, it is expected that the device performance could be considerably improved by expanding the light absorption utilizing CdX(X = S, Se) QDs and ZnPc dyes as co-sensitizers. It should be highlighted that Miguel et al. recently prepared DSSCs using the ZnPc dye molecules, and the PCE was only 3.5% [15]. The unimpressive PCE is mainly attributed to their strong tendency to aggregate on the semiconductor oxide surface and the lack of electrontransfer directionality [16]. These drawbacks could also be overcome to some extent in the QDs and dye co-sensitized solar cells through effective electron injection from ZnPc to CdS/CdSe rather than direct electron transfer to N-type semiconductor oxide. In addition to the absorber, suitable photoanode material is the precondition for highly efficient QDSSCs. ZnO nanomaterial is another low cost alternative with unique advantages of similar band gap and CB edge to TiO2, the flexibility in morphology control and the possibility of low temperature applications. The ZnO-based QDSSCs can achieve a higher open-circuit voltage (Voc) than that of TiO2 due to its higher carrier mobility [17,18]. Varieties of ZnO nanostructures, including nanorods, nanocombs, nanotubes, nanowires and flower-like structures, have been reported to be applied on solar cells including QDSSCs, DSSCs and perovskite solar cells (PSCs) [19–21]. ZnO IO structure has been considered as an attractive candidate for the QDSSCs, owing to their large specific surface area for loading a large amount of sensitizers (QDs and dyes) and strong scattering effect for capturing a sufficient fraction of photons [22,23]. The macroporous structure may shorten the electronic diffuse distance between the FTO substrate and the sensitizers, thus improving the electron transfer ability from

the sensitizers to the FTO. Moreover, the structure of IOs can increase the light absorption of certain wavelength, which may enhance the device performance of solar cells [24]. The counter electrode is another indispensable component in sensitized solar cells, which contribute to the electron transfer to regenerate the oxidized species in the electrolyte and keep the circuit running [25]. So a lot of research focused on improving efficiency of QDSSCs by introducing modified layer to counter electrode. Graphene material such as GO as a rising star in the carbon family has also been proposed as an electrode modified material for high efficient solar cells due to its excellent electronic, conductive and mechanical properties, superior chemical stability and large surface area. Currently, a lot of attractive work is based on metal NPs and GO composite material with conductive network structures [26,27]. In the meantime, such an attachment of metal NPs onto GO may also prevent the restack and agglomeration of graphene sheets during the reduction process due to the Van der Waals interactions between them [27]. While taking advantage of the surface groups, GO can be effectively combined with Au NPs in an aqueous solvent. In this work, we report a novel kind of QDSSCs, with the application of organic dye ZnPc and CdS/CdSe as co-sensitizers, ZnO IOs as photoanode and Au NPs@ GO as counter electrode modified layer. The spectral response of our QDSSCs expands from 400 to 750 nm, and the optimum PCE is 4.60% in this study. This relatively efficient PCE is comparable with previous dominant results based on CdS/CdSe-QDSSCs [28,29]. The introduction of ZnPc dye as co-sensitizer and Au NPs/GO as counter electrode modified layer has been proved to be an effective approach to improve the performance of QDSSCs. This work paves the way towards the design of new QDSSCs with expanded light harvesting. 2. Experiments 2.1. Measurements Scanning electron microscopy (SEM) images were obtained from a SIRION field-emission scanning electron microscope. The transmission electron microscope (TEM) data were measured on a JEM-2010 with the working voltage of 200 kV. J-V characteristics of the cells were recorded using a Keithley 2400 source meter and a 1.5 AM, 100 mW/cm2 solar simulator lamp. Incident photon to current conversion efficiency (IPCE) was recorded using a computerized setup consisting of Solar Cell Quantum Efficiency—— SolarCellScan100. Absorption spectra were recorded with the Shimadzu UV-1600 Spectrophotometer. Electrochemical impedance spectroscopy (EIS) were performed on a model CHI630E electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China) in the frequency range of 0.1–105 Hz, and the applied bias voltage and AC amplitude were set as the open-circuit voltage of the cells and 10 mV between the counter electrode and the working electrode, respectively. 2.2. Synthesis of ZnO IOs ZnO IOs were synthesized by the sol-gel method using the PMMA latex spheres as the colloidal crystal template according to our previous works (See in supporting information). Specially,

C. Chen et al. / Journal of Colloid and Interface Science 480 (2016) 49–56

the mono-dispersed PMMA spheres were firstly synthesized using our previously published method which can be found in supporting information. Then the PMMA template was self-assembled into highly ordered colloidal arrays through the vertical deposition process on FTO substrate which was dealt with H2SO4/H2O2 aqueous solution in advance to make it hydrophilic [22,30]. Then the photonic crystals were sintered for 40 min at 120 °C to enhance their physical strength. Zinc ion precursor was prepared by dissolving Zn(NO3)26H2O in ethanol. Then, an appropriate amount of citric acid and a small quantity of tetraethyl orthosilicate were added to the above zinc solution as chelating agent and stirring for 1 h. The prepared precursor solution was used to infiltrate into the interstices of the templates under the capillary force from one side of the PMMA template. After sufficient infiltration, the template was dried in air overnight. Then, the template was heated to 500 °C at a rate of 1 °C/min and kept for 3 h to remove the original PMMA template, and the ZnO IOs macroporous structure was obtained.

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Five cycles were employed to obtain a suitable amount of CdSe QDs on the films. Then part of these CdS/CdSe coated electrodes were immersed in a solution of the 0.1 mM ZnPc (Zinc,[29H,31Hphthalocyaninato(2(-))-N29,N30,N31,N32]-(SP-4-1)-,TCI Co.,Ltd) in N,N-Dimethylformamide for 12 h to adsorb the dye molecules. Finally, QDSSCs were fabricated by clamping an Au NPs/GO/Cu2S/ FTO glass plate onto a CdS/CdSe + ZnPc coated photoanode and filling the capillary space with the electrolytes (1 M Na2S + 1 M S,18 MX deionized water). For comparison, the ZnO IOs were also immersed in ZnPc solution to prepare ZnPc sensitized DSSCs assembled by I/I 3 electrolyte and Pt counter electrode with previous method [36]. Among the co-sensitized system, Kamat et al. have proved that the S2/S2 n redox couple is the best choice for most liquid junction QDSSCs as it provides redox couple in the regeneration of QDs and desirable stability during irradiation [37]. I/I3 will reduce hole regeneration and anodic corrosion of the QDs. We adopted S2/S2 as electrolyte for the reason that n CdS/CdSe as the main component to generate carrier in the co-sensitized devices.

2.3. Preparation of Au NPs/GO composites for counter electrode Au NPs were assembled on the GO surface through reduction of chloroaurate with reducing agent in GO suspension. Specially, 4 mg GO was dispersed in 50 mL deionized water under magnetic stirring at room temperature for 1 h, then 15 mg HAuCl4 was added (we chose a relatively optimized Au concentration in this study according to our previous work about Au/GO) [31]. The resulting suspension was aged for 30 min to promote the interaction of gold ions with the GO surface. Next, 5 mL sodium citrate aqueous solution (the mass ratio of sodium citrate to deionized water is 0.02) was added under magnetic stirring for 30 min. Then the solution was kept at 80 °C for 2 h. Finally, the composites were obtained by washing and centrifuge 3 times with deionized water to remove the free Au NPs. We used radio frequency magnetron sputtering (RFMS) method to prepare Cu2S films on FTO substrates. According to our previous result [32], we have applied the sputtered Cu2S on FTO as counter electrode rather than traditional method by in immersing Cu plate in Na2S + S (S2/S2 n ) solution. The function of Cu2S is a good electro-catalyst for the polysulfide redox reaction [33]. The biggest advantage of magnetron sputtering for Cu2S is that the film coated on substrate can maintain the consistent composition of Cu2S target (China New Metal, 99.99%). But the traditional solution method is hard to determine final composition of Cu2-XS. After sputtering, the Au NPs/GO was coated on Cu2S/FTO glass by spin coating at 1000 rpm in ambient air. The final Au NPs/GO modified Cu2S was collected after keeping at 60 °C in an oven for 12 h. Detailed sputtering conditions and processes are described in supporting information. 2.4. Solar cell fabrication CdS/CdSe QDs were coated on ZnO IOs/FTO photoanode by using the successive ionic layer adsorption and reaction (SILAR) method following the procedure reported previously [29,34,35]. ZnO IOs /FTO substrate was first immersed into 0.05 M Cd(NO3)2 in methanol and then into 0.05 M Na2S in methanol for 2 min each. The two-step dipping procedure is termed as one SILAR cycle. Following each immersion, the substrate was rinsed in pure methanol for 2 min to remove excessive precursors and then was dried before the next dipping to finish one QD coating cycle. Six cycles were employed to obtain a suitable amount of CdS QDs on the films. In the case of CdS/CdSe QDs, the initial CdS/ZnO IOs /FTO photoanode was then immersed into an solution of 0.1 M Cd(CH3COO)2 in methanol for 2 min and then the film was immersed into a solution of 0.1 M Se2 for 2 min which was prepared by the reaction of diluted SeO2 in ethanol with NaBH4.

3. Results and discussion 3.1. Characterization A schematic illustration of the formation process and structure of the co-sensitized solar cell is shown in Fig. 1(a), the specific preparation process are described in the experimental section. The SEM image of the PMMA opals template can be seen in Fig. S1, and the average diameter of the PMMA spheres is 390 nm. In Fig. 1(b), a long-range ordered hexagonal closepacked arrangement of IOs in a three-dimensional structure can be observed. It demonstrates a high degree of ordered IOs structure. The macroporous structure of ZnO IOs facilitates the entrance and adsorption of the sensitizers. The large surface area of ZnO IOs also offers more chance for the sensitizers to absorb on surface. The top-view SEM image also presents the ZnO IOs with a center-tocenter distance of 300 nm, which is about 30% smaller than the original size of the PMMA template due to the shrinkage of sphere diameters during calcination. Fig. 1(c) shows the cross sectional SEM image of the ZnO IOs on FTO, a macroporous structure with 30–50 layers can be observed. The thickness of the macroporous structure is 8–10 lm, which is sufficient to ensure the effective absorption of sensitizer. In Fig. S2, XRD analysis is used to demonstrate the presence and component of ZnO IOs macroporous structure. The black line shows the standard XRD peaks ZnO (JCPDS number #36-1451). The red line shows the XRD diffraction peaks for bare ZnO IO films on FTO after 500 °C annealing treatment. This implies that the presence of pure ZnO IOs because of the complete consistency with the standard XRD diffraction peaks. Fig. 1(d) shows the TEM image of the Au NPs/GO and the Au NP size distribution. The Au NPs have a size distribution of 10–18 nm with an average size of 14 nm exhibited by Gauss fitting. Fig. S3(a) shows the TEM image of the GO sheets. The GO sheets with uniform evenness exhibit flake-like shape and are few flexible wrinkled. The smooth and planar surface indicate that GO sheets can provide a high surface to volume ratio, and the 2D structure is beneficial to Au NP loading. In order to further demonstrate the dispersion of Au NPs on GO, Fig. S3(b) shows the Au NPs on the graphene surface with large area. 3.2. Photovoltaic performances According to the order of device structure design, the effect of the co-sensitization mechanism on the performance of the device was discussed firstly. Fig. 2(a) shows the absorption spectra of

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Fig. 1. (a) A schematic illustration of the formation process and structure of the co-sensitized solar cell. (b) Side-view image of the IO sample on FTO and (c) ZnO IOs with long-range ordered hexagonal close-packed arrangement of IOs in a three-dimensional structure. (d) TEM image of Au NPs/GO composites, the inset shows the average size of Au NPs by Gauss fitting.

Fig. 2. (a) UV–visible absorption spectra of six-cycle CdS/CdSe, ZnPc and six-cycle CdS/CdSe + ZnPc, the inset figure shows the photo of samples with three different kinds of sensitizer. (b) J-V curve for the devices based on CdS/CdSe, ZnPc and co-sensitizers. (c) IPCE spectra of the devices based on CdS/CdSe, ZnPc and co-sensitizers. (d) Schematic representation of the band alignment of CdS/CdSe and ZnPc co-sensitized ZnO IOs photoanode.

ZnO IOs/FTO substrate with different sensitizers including CdS/ CdSe QDs and ZnPc dye molecules. CdS/CdSe QDs on ZnO IOs show an absorption peak around 400–650 nm, and the onset wavelength is at 700 nm corresponding the band gap of
1.7 eV for CdS/CdSe [38], which is in agreement with the previous reports [29,39]. ZnPc

sensitized photoanode shows a limited absorption around 550– 750 nm. It should be noted that the dye loading time is 12 h in the present QDSSCs. Actually, the devices with different dye loading time were also prepared in this work. 12 h dye loading time results in the best device performance in this study. Too much

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dye adsorption will affect the diffusion length of electrons, and also lead to the decomposition of CdS/CdSe QDs in solution. The comparison of PCE values are shown in Fig. S4. The absorption spectrum of CdS/CdSe/ZnPc exhibits a very broad absorption band from 400 to 750 nm. The optical absorption is desirable for solar light harvesting after loading ZnPc on the CdS/CdSe photoelectrodes, leading to improved device performance. CdS/CdSe, ZnPc and co-sensitized solar cells were assembled and their photovoltaic performances were investigated by J-V test station under 1 sun (100 mW/cm2 AM1.5G solar illumination). An optimized SILAR process of six cycles of CdS QDs and five cycles of CdSe QDs were employed to fabricate devices, which exhibit the optimum results in this study. The device performance of QDSSCs based on CdS/CdSe, ZnPc and CdS/CdSe /ZnPc are shown in Fig. 2 (b) and Table 1. CdS/CdSe based device exhibits a PCE of 3.08%, the current density (Jsc) of 10.9 mA/cm2, fill factor (FF) of 48.7%, Voc of 0.58 V, which is comparable with the published results [4,21,29]. When only ZnPc was used as a sensitizer, the device exhibits a relatively low device performance (Jsc = 6.37, FF = 43.4% and PCE = 1.88%), but a relatively high Voc of 0.68 V is obtained. It is reported that the ZnPc dye as a single sensitizer showed relative low device performances, which could not be comparable to N719 because of strong aggregation on the TiO2 surface and the lack of directionality in the excited state of the ZnPc according to the literature [15,40,41]. The device with cosensitizer of QDs and dye has a better performance (Jsc = 11.3 mA/cm2 FF = 53.7% PCE = 4.01%) than the device sensitized by QDs. The higher Jsc and FF of the device are attributed to more photoelectrons generated by more visible/NIR absorbers. The PCE of the device based on ZnO IOs as photoanode is also compared with that of the conventional TiO2 nanoparticles and ZnO nanoparticles in Fig. S5. The CdS/CdSe + ZnPc sensitized device based on macroporous ZnO IOs demonstrate a Jsc of 11.3 mA/cm2 while the TiO2 nanoparticles and ZnO nanoparticles show 12.1 and 11.0 mA/cm2 respectively. Although TiO2 has a higher Jsc and Voc, the PCE of device is still lower than that of ZnO IOs, which can be attributed to the more uniform coating of sensitizers into the ZnO IO structure. Fig. 2(c) shows the IPCE of QDSSCs based on CdS/CdSe, ZnPc and CdS/CdSe/ZnPc. ZnPc-sensitized solar cell shows IPCE only in the range of 600–750 nm corresponding to ZnPc Q-band absorption. The lower IPCE in the ZnPc-sensitized cell due is consistent with lower Jsc in the J-V measurements. The IPCE of CdS/CdSesensitized solar cell mainly concentrated in the range of 400–650 nm, while it is quite low in NIR region. The expanded photoelectric response can be observed in the visible/NIR region up to 750 nm by introducing ZnPc in CdS/CdSe QD based devices. The device based on co-sensitizers shows two peaks around 450 and 670 nm, which confirms the contribution of both the CdS/CdSe QDs and ZnPc dye molecules to the overall photocurrent. The maximum IPCE observed for the CdS/CdSe/ZnPc cell is about 57% at 450 nm, higher than that of ZnO /CdS/CdSe (
47%). Hence, both the ZnPc and QDs contribute to the IPCE at
450 nm. The IPCE of the co-sensitized device is significantly improved as compared to the dye or QD-sensitized devices. Therefore, a much higher Jsc and better performance could be obtained from a co-sensitized solar cell.

Table 1 Photovoltaic parameters of the solar cells with different sensitizers. Samples

Jsc (mA/cm2)

Voc (v)

FF (%)

PCE (%)

ZnPc CdS/CdSe CdS/CdSe + ZnPc

6.37 10.9 11.3

0.68 0.58 0.66

43.4 48.7 53.7

1.88 3.08 4.01

The energy level diagrams of ZnO, CdS, CdSe and ZnPc are shown in Fig. 2(d), which is based on previous work reported in literatures [29,41,42]. It can be observed that both the conduction band (CB) and valence band (VB) positions of the three materials are in the order of ZnO < CdS < CdSe < ZnPc, which is benefit for the electrons transfer from the HOMO states of sensitizers to ZnO. The CB of CdS/CdSe QDs and HOMO states of ZnPc dyes are higher than that of ZnO (4.0 eV). This indicates that there is sufficient driving force for the electron injection from the sensitizers to ZnO. Moreover, ZnPc has a higher LOMO position (5.2 eV) to VB of the CdS (6.15 eV) and CdSe (5.4 eV) to ZnO photoanode (7.2 eV) which can result in a higher driving force for the hole transfer. The driving force for electron transfer at the sensitizers/ semiconductor oxide interface is dependent on the CB edge difference. A larger CB difference for CdS/CdSe/ZnPc causes larger driving force and increased charge injection efficiency compared to that of CdS/CdSe QDs [41,43], therefore, leading to the better performance of the devices based on CdS/CdSe/ZnPc. S2/S2 n redox couple is used commonly in the regeneration of QDs by scavenging the holes in QDSSCs. The working principle of our devices is discussed below. þ

CdS=CdSe þ hv ! CdS=CdSeðe þ h Þ 

CdS=CdSeðe Þ þ ZnO ! CdS=CdSe þ ZnOðe Þ þ

ð1Þ ð2Þ

ZnPc þ hv ! ZnPcðe þ h Þ

ð3Þ

ZnPcðe Þ þ ZnO ! ZnPc þ ZnOðe Þ

ð4Þ

ZnPcðe Þ þ CdS=CdSe ! ZnPc þ CdS=CdSeðe Þ þ

CdS=CdSeðh Þ þ S2 ! CdS=CdSe þ S2 n

ð5Þ ð6Þ

Electron-hole pairs will be generated when the CdS/CdSe QDs are irradiated (reaction 1) [44]. Considering the energy level diagrams of CdS/CdSe higher than that of ZnO, the electrons are subsequently transferred from the CB of CdS/CdSe to the CB of ZnO (Eq. (2)). At the same time, a portion of the dye adsorbed on the surface of the ZnO IOs could produce electrons, which are directly injected to ZnO IOs (Eqs. (3) and (4)). The electrons produced by the other portion of the dye adsorbed on the surface of the CdS/CdSe are transported to ZnO IOs indirectly by injection to the QDs first (Eq. (5)), which contributes to the overall photocurrent generation by efficient electron transport [44]. During the photo-irradiation of CdS/CdSe in the presence of the sulfide electrolyte, it is possible to expect one interfacial reaction (reactions (5)) dominate after the initial charge separation (reaction (1)–(4)). In the electrolyte, the holes are collected by the S2/S2 n redox couple (reaction 6). Finally, all the photo-generated electrons are ultimately transferred to counter electrode through the external circuit. 3.3. EIS analysis EIS measurements were conducted to study the internal resistance, recombination properties and charge-transfer kinetics of the QDSSCs [35]. Fig. 3 shows the EIS data obtained using Nyquist diagrams to analyze the internal resistance in QDSSCs (plots of Z00 vs Z0 are the real and imaginary parts of the impedance, respectively). The Nyquist curves of the EIS results contain typically two semicircles which are fitted by the equivalent circuit (inset in Fig. 3(a)), and the fitted values are listed in Table 2. Usually, Rs represents the series resistance, which is mainly influenced by the sheet resistance of the substrate and electrical contact at the interface of FTO-glass and ZnO IOs [45]. The small semicircles at high frequency are assigned to the charge-transfer resistance (R1) which represents the impedance with charge transfer at the electrolyte/counter electrode interfaces. The two systems (CdS/ CdSe and CdS/CdSe/ZnPc) have adopted the same Cu2S as counter electrode and S2/S2 n as electrolyte in sandwiched structure, but

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Fig. 3. (a) Nyquist plots and (b) Bode plots of EIS spectra recorded under AM 1.5 conditions at an applied bias of 0.6 V for QDSSCs based on the photoanodes prepared with different sensitizers. The inset in Fig. 3(a) displays the corresponding equivalent circuit.

3.4. Device performance with the addition of Au NPs/GO

Table 2 Fitting results of the Nyquist plot and Bode plot. Samples

Rs (X)

R1 (X)

R2 (X)

fmax (Hz)

s (ms)

CdS/CdSe CdS/CdSe + ZnPc

12.4 13.1

97.8 74.1

77.2 158.8

56.23 3.162

2.83 50.36

the existence of ZnPC will affect the catalysis of Cu2S for S2/S2 n . The limited decrease of R1 value in CdS/CdSe/ZnPc on behalf of the reduced internal impedance at counter electrode and means the transmitting pathway is more unimpeded to the hole. This implies that the existence of ZnPC actually promotes the transfer of carriers (hole) at the electrolyte /counter electrode interface. The big semicircles in the middle-frequency are assigned to the charge-transfer resistance (R2) which represents the impedance related to the charge transfer process at ZnO IOs /sensitizers/electrolyte interfaces. The higher R2 implies a lower recombination of carriers at the interfaces. R2 values for the devices based on CdS/ CdSe, CdS/CdSe/ZnPc are 77.2 X and 158.8 X, respectively. The increased R2 for the device based on co-sensitizers leads to better performance [46,47]. Therefore, the existence of ZnPc reduces the internal resistance at electrolyte/counter electrode interfaces of the co-sensitized solar cells (decrease of R1), on the other hand, it also reduces the carrier recombination at ZnO IO /sensitizers/electrolyte interfaces (increase of R2). Fig. 3(b) shows bode plots of QDSSCs based on CdS/CdSe and CdS/CdSe/ZnPc at an applied bias of Voc and a frequency range from 101 to 106 Hz under the illumination of one Sun. The lifetime of electrons (s) can be estimated according to the following equation [48]:

s ¼ 1=2pf max where fmax is the maximum frequency of the mid-frequency peak. The fmax value for ZnO/CdS/CdSe/ZnPc is 3.162 Hz, smaller than that for ZnO/CdS/CdSe (56.23 Hz). Therefore, the electron lifetime is calculated to be 2.83 ms for CdS/CdSe based devices which are shorter than that of CdS/CdSe/ZnPc (50.36 ms). The longer electron lifetime for ZnO/CdS/CdSe/ZnPc indicates more effective suppression of the recombination between photoelectrons in the CB of ZnO and the holes in electrolyte. The collective effect of CdS/CdSe and ZnPc on ZnO IOs photoanode leads to higher efficiency of solar cells. Based on the above discussion, the ZnPc dye could play three different roles in the co-sensitized solar cells. (i) ZnPc as sensitizer can produce electrons to promote a much higher Jsc and better performance for co-sensitized solar cells. (ii) The dye molecules adsorbed on the surface of CdS/CdSe can also contribute electrons transport to ZnO via CdS/CdSe. (iii) ZnPc can also act as an effective hole transporting material due to its higher LOMO state edge than the VB of QDs. That also inhibits recombination of electrons and holes in the interior of the solar cell [37].

In order to further explore the feasibility of improving solar cell photovoltaic response, Au NPs/GO as the electrode modified layer was also introduced. Fig. 4(a) and Table 3 show the characteristics of QDSSCs fabricated using different counter electrodes including the Au NPs/GO/Cu2S/FTO, GO/Cu2S/FTO and Cu2S/FTO. In Table 3, it can be observed that the Jsc increases for the device based on Au NPs/GO/Cu2S/FTO compared with those based on the GO/ Cu2S/FTO and Cu2S/FTO. The increased Jsc results in highly efficiency of 4.60%, which is 15% increase compared to that of device based on Cu2S/FTO, and is about 10% increase compared to that of the GO/Cu2S/FTO-based QDSSCs. The large surface area and favourable electro-catalytic properties of Au NPs/GO lead to an improved cell performance and a decreased resistance to charge transport in the QDSSCs. Interestingly, the Jsc of the QDSSCs prepared using GO/Cu2S/FTO is lower than that of the QDSSCs based on Au NPs/GO/Cu2S/FTO. This mainly is the result of Au NPs could promote the transformation of S2 and S2 n . By combining the advantages of Au NPs and GO, high charge transfer rate and high specific surface area at the interface could be achieved, leading to the high performance observed in our experiments. The incorporation of Au NPs/GO materials has been shown to have an increased charge transfer rate, decreased internal resistance and decreased diffusion resistance, which is correlated to favourable electrocatalytic properties of Au NPs [49]. That is, a faster S2/S2 n reduction rate caused by Au NPs/GO/Cu2S/FTO composites can accelerate the QD regeneration. Therefore, the diffusion resistance and QD regeneration are limited by the relatively low catalytic activity of GO/Cu2S/FTO, leading to lower Jsc values. 3.5. The photo-stability of device with different structure Because of the existence of liquid electrolyte, stability has always been the focus and difficulty for attention of sensitized solar cells. A long-term stability test of a 25 solar cell was carried out for over 60 min under the illumination of AM 1.5G, 100 mW cm2. When testing the CdS/CdSe solar cells without ZnPc as co-sensitizer and Au NPs/GO composite as a modified layer,the devices shows the poor stability because of the degradation of the CdS/CdSe QDs in S2/S2 n liquid electrolyte [50]. The average PCE fell to 50% of the original after 15 min and down to 15% of the original after 60 min. The black line in Fig. 4(b) shows the stability was improved when ZnPc was introduced. ZnPc act as an effective carrier transporting material prevents direct contact of CdS/CdSe QDs and S2/S2 electrolyte. Meanwhile, ZnPc as con sensitizer can produce electrons to boost the electrons transport to ZnO via CdS/CdSe. When Au NPs/GO composite as a modified layer assembled to the co-sensitized devices, the stability was

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Fig. 4. (a) J-V curves of the typical co-sensitized solar cell Au NPs/GO/Cu2S composites as counter electrode, the inset figure shows the changing range of PCE from devices with different Cu2S, GO/Cu2S and Au NPs/GO/Cu2S composites. (b) Photo-stability test of a 25 solar cells. Red line shows the stability of CdS/CdSe devices without ZnPC as cosensitizer and Au NPs/GO composite as a modified layer (S1). Black line show shows the stability of co-sensitized devices without Au NPs/GO composite as a modified layer (S2). Green line shows the stability of co-sensitized devices with Au NPs/GO composite as a modified layer (S3). Testing devices under illumination of one sun (AM 1.5G, 100 mW cm2) reached to 60 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3 Photovoltaic parameters of the QDSSCs with the addition of Au NPs/GO composites as counter electrode modified layer. Samples

Jsc (mA/cm2)

Voc (v)

FF (%)

PCE (%)

GO/Cu2S/FTO Au NPs/GO/Cu2S/FTO

11.5 11.9

0.67 0.67

54.5 57.7

4.20 4.60

enhanced greatly, the PCE was very stable during the first 30 min, and then gradually decreased down to
85% of its original value in the next 30 min. More importantly, the overall stability of the device is improved. This can be concluded that a faster S2/S2 reduction rate caused by Au NPs/GO/Cu2S/FTO n composites can accelerate the QD regeneration. The catalytic properties of Au NPs and electrical conductivity GO corporately provide a unique and innovative way for higher PCE with remarkable stability which is much better than what has been reported by Zaban et al. [50]. 4. Conclusions In summary, we present a new design for the development of QDSSCs based on the macroporous ZnO IOs photoanode sensitized by QDs which have light harvesting in the visible region and organic dye in visible /NIR region. Because of the complementary absorption spectral response caused by CdS/CdSe QDs and ZnPc dye, the co-sensitized solar cell can expand the absorption spectra to
750 nm. As a result, the co-sensitized solar cells exhibited a significant improvement in Jsc leading to the overall PCE of 4.01%, which is much better than that of the individual CdS/CdSe (3.08%) or ZnPc (1.88%) sensitizer. We conclude that the use of ZnPc is a good match to regenerate CdS/CdSe by capturing photo-generated holes while concurrently serving as a sensitizer in the visible/NIR region. Meanwhile, we also found that the cell with an Au NPs/GO composite as a modified layer on traditional Cu2S counter electrode shows higher photovoltaic performance (4.60%) which can be attributed to the superior combination of high electro-catalytic activity of Au NPs and the electrical conductivity of the GO network structure. Enhanced stability remains
85% of its original value after 60 min light exposure because ZnPc prevents the degradation of the CdS/CdSe QDs in S2/S2 liquid n electrolyte and Au NPs/GO composites catalyse the regeneration of QDs and transmission of carriers. We think the present work can provide a co-sensitization method to improve photoelectric response by expanding the light harvesting and an efficient way to enhance the photovoltaic performance of QDSSCs by modifying the counter electrode.

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