Article pubs.acs.org/IECR
Enhancing the Conversion Efficiency of Semiconductor Sensitized Solar Cells via the Cosensitization of Dual-Sized Quantum Dots Jun-Guo Song,†,§ Xin Song,§ Tao Ling,*,† Xi-Wen Du,*,† and Shi Zhang Qiao*,‡ †
Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, P. R. China ‡ ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Australia ABSTRACT: Dual-sized CdS quantum dots were employed as light harvesters of solar cells to achieve high power conversion efficiency. Chemical bath deposition and annealing treatment were adopted to produce quantum dots with different sizes. The strategy of cosensitization by dual-sized CdS quantum dots was found effective to improve the performance of solar cells by forming a type-I band structure. Our work indicates that a reasonable configuration of quantum dots is crucial on boosting the performance of solar cells.
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INTRODUCTION Since 1991, dye-sensitized solar cells (DSSCs) have received extensive attention because of their high efficiency and relatively inexpensive fabrication procedure compared with conventional inorganic solar cells.1−5 Recently, semiconductor nanocrystals, also known as quantum dots (QDs), have been suggested and tested as a next-generation sensitizer with advantages over molecular dyes.6−9 Due to the quantum confinement effect, the optical properties and the band gap of QDs can be adjusted by changing their size.10−13 It is also possible to generate multiple electron−hole pairs per photon through the impact ionization effect.14 Another advantage of the QD sensitizers over conventional dyes is their high extinction coefficient, which is known to reduce the dark current and increase the overall efficiency of solar cells.8,9 To improve the power conversion efficiency (PCE) of solar cells, a cosensitization structure has been designed, where two kinds of QDs with different band gaps were assembled onto a large-band gap semiconductor, such as TiO2 or ZnO.15,16 The cosensitization structure shows superior capability on accelerating the separation of electron−hole pairs and enhancing the PCE.15,16 It was reported that the PCE of semiconductor sensitized solar cells can be readily tuned by controlling the size and then the band gap of QDs.17 Furthermore, if QDs with the same composition and different size were produced and combined to form a cosensitization structure, they may coordinate to absorb the incident light at different wavelength ranges, and more importantly, they can construct a reasonable band structure for the electron injection. In our early research,18,19 CdS QDs with different size and band gap have been obtained by varying annealing parameters, which makes it possible to construct a cosensitization structure. On the other hand, photoanode, the substrate for loading QDs, can influence the performance of solar cells seriously. ZnO exhibits many advantages as a photoanode material, such as a wide band gap (3.37 eV),20 higher electron mobility, lower charge recombination,21 and controllable morphologies.22−26 © 2012 American Chemical Society
Herein, we choose aligned ZnO nanowires (NWs) as the photoanode to provide a large surface area for the adsorption of QDs and the efficient transportation of photogenerated carries. In the present work, we utilize dual-sized CdS quantum dots as light harvesters of solar cells to achieve high power conversion efficiency. Dual-sized CdS QDs are grown successfully on ZnO NWs through a chemical bath deposition (CBD) method and sequential annealing process. The results indicate that the strategy of cosensitization by dual-sized CdS quantum dots is effective to improve the performance of solar cells by forming a type-I band structure. Our work indicates the reasonable configuration of quantum dots is crucial on boosting the performance of solar cells.
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EXPERIMENTAL SECTION The synthesis of ZnO NW arrays in this work was similar to that described in reference 27. ZnO seed layer was first deposited on the glass substrate with a fluorin-doped tin oxide (FTO) layer by a dip-coating method with a coating solution prepared by zinc acetate dissolved in a 2-methoxyethanol-MEA solution and finally annealed at 500 °C for 1 h. Then ZnO nanowire arrays were grown in a precursor aqueous solution composed of 0.025 mol/L Zn(NO3)2 and (CH2)6N4 in 1:1 molar ratio; the reaction was conducted at 90 °C for 8 h. Finally, the resulting nanowire arrays were rinsed with distilled water and annealed at 500 °C for 1 h. CdS QDs were synthesized by using a CBD method, which involves dipping the ZnO arrays in 0.1 mol/L Cd(NO3)2 ethanol solution for 5 min, rinsing with ethanol, and then dipping for another 5 min in a 0.1 mol/L Na2S methanol solution, and rinsing again with methanol. The two-step dipping procedure is considered as one CBD cycle. Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 10074
January 12, 2012 February 26, 2012 February 29, 2012 March 2, 2012 dx.doi.org/10.1021/ie300109u | Ind. Eng. Chem. Res. 2012, 51, 10074−10078
Industrial & Engineering Chemistry Research
Article
Three samples were prepared with different CBD cycles and annealing treatment. Sample S1 was ZnO NWs with QDs by 2cycle CBD, sample S2 was ZnO NWs with QDs by 5-cycle CBD and annealing treatment, and sample S3 was sample S2 with additional 2-cycle CBD. The experimental details are shown in Table 1. Table 1. Parameters for Preparing Different Samples sample
first CBD
annealing
second CBD
S1 S2 S3
2 cycles 5 cycles 5 cycles
N/A 400 °C 400 °C
N/A N/A 2 cycles
The morphologies of samples were observed by a Hitachi S4800 field emission scanning electron microscope. The detailed structures were analyzed using a FEI Tecnai G2 F20 transmission electron microscope (TEM) with a field-emission gun operating at 200 kV. The absorption spectra were acquired in a Hitachi U3100 UV spectrometer. Rigaku D/max 2500v/pc XRD was adopted to identify the phase structure of the products. To construct solar cells, the photoanodes were assembled and sealed with a thin transparent hot-melt 25 μm thick Surlyn ring to the counter electrodes (Pt-coated FTO glass). The iodide-based electrolyte was injected into the interelectrode space from the counter electrode side through a predrilled hole. The effective area of the cell is 0.4 cm2. The photocurrent− voltage characteristics were detected with a Keithley model 2611 digital source meter under a simulated AM 1.5 G solar irradiation with a light intensity of 100 mW/cm2.
Figure 1. Characterizations of bare ZnO NWs: (a) SEM image; (b) TEM image; (c) HRTEM image; (d) EDX spectrum; and (e) XRD pattern.
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RESULTS AND DISCUSSION The characterization results of the bare ZnO NWs are shown in Figure 1. Figure 1a shows a SEM image of ZnO NWs with a diameter about 200 nm and a length about 2 μm. TEM (Figure 1b) and HRTEM images (Figure 1c) of ZnO NWs indicate a perfect single crystal structure. The chemical composition of ZnO nanowires is determined by energy-dispersive X-ray analysis (EDX) (Figure 1d). In the XRD pattern of ZnO NW arrays (Figure 1e), the diffraction peaks labeled by circles can be indexed to (100), (002), (101), (102), (110), (103), (112), and (201) planes, respectively, of wurtzite-phase ZnO (JCPDS 89-1397). The strong (002) peak suggests that ZnO NWs grew with their c-axis perpendicular to the FTO surface. TEM images of samples S1 and S2 are shown in Figure 2. CdS QDs can be observed on the surface of ZnO NWs after the CBD process and annealing treatment. The CdS QDs by 2circle CBD show ultrafine size of several nanometers, while those with 5 circles and annealing treatment are much bigger (more than 10 nm). The size of CdS QDs depends on CBD cycles as well as annealing treatment. The CBD cycles determine the total amount of CdS deposited on ZnO naowires, and the annealing treatment promotes the ripening and coalescence of fine nanocrystals. Sample S1 was obtained by CBD for only 2 cycles and without annealing; the total amount of CdS material is very limited and nanocrystals cannot grow up in absence of annealing treatment, thus CdS QDs are fine and rare. In contrast, sample S2 was prepared by 5 CBD circles and annealing treatment at 400 °C; the large amount of CdS material and the rapid repining during annealing treatment facilitate the growth of CdS QDs, inducing large nancrystals, which agrees with the previous reports.28,29
Figure 2. TEM images of sample S1 (a) and sample S2 (b); the insets show low magnification images of the ZnO NWs with CdS QDs. EDX spectrum (c) and XRD pattern (d) of sample S2.
EDX analysis demonstrates that the QDs are composed of Cd and S elements (Figure 2c). The crystal structure of assynthesized CdS QDs is determined by XRD. As shown in Figure 2d, new peaks emerge in addition to ZnO and FTO substrate, which can be indexed as the wurtzite structure of CdS (JCPDS file 41-1049). In TEM image of sample S3 (Figure 3), two layers of CdS QDs are coated onto ZnO NWs through the CBD−annealing− CBD process. The HRTEM images in Figure 3b, c, and d reveal that fine CdS QDs are attached to big ones, and both of them form a compact shell on the ZnO nanowire. The formation of dual-sized QDs in sample S3 can be explained by the same theory for the fine QDs in S1 and the large QDs in S2, and the 10075
dx.doi.org/10.1021/ie300109u | Ind. Eng. Chem. Res. 2012, 51, 10074−10078
Industrial & Engineering Chemistry Research
Article
Figure 4. (a) UV−vis absorption spectra of three samples, and (b) (αhν)2 versus hν plot for determining band gap of QDs in samples S1 and S2.
Figure 3. TEM images of sample S3: (a) low magnification image, (b) enlarged image of the area in the black frame of (a), (c) and (d) HRTEM images corresponding to the areas labeled by the frames in (b).
two step process for sample S3 allows the sequential growth of large QDs and fine QDs on ZnO nanowires. UV−vis absorption spectra of the three samples are shown in Figure 4. The absorption in visible range (560−375 nm) originates from CdS QDs, while the intensive absorption in UV range (
