Journal of Nanomaterials

Volume 2014 (2014), Article ID 372381, 5 pages

http://dx.doi.org/10.1155/2014/372381

Research Article

CdTeO₃ Deposited Mesoporous NiO Photocathode for a Solar Cell

Chuan Zhao, Xiaoping Zou, and Sheng He

Beijing Key Laboratory for Sensor, Ministry-of-Education Key Laboratory for Modern Measurement and Control Technology, Research Center for Sensor Technology, School of Applied Sciences, Beijing Information Science and Technology University, Jianxiangqiao Campus, Beijing 100101, China

Received 25 January 2014; Accepted 26 April 2014; Published 20 May 2014

Academic Editor: Marinella Striccoli

Copyright © 2014 Chuan Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Semiconductor sensitized NiO photocathodes have been fabricated by successive ionic layer adsorption and reaction (SILAR) method depositing CdTeO₃ quantum dots onto mesoscopic NiO films. A solar cell using CdTeO₃ deposited NiO mesoporous photocathode has been fabricated. It yields a photovoltage of 103.7 mV and a short-circuit current density of 0.364 mA/cm². The incident photon to current conversion efficiency (IPCE) value is found to be 12% for the newly designed NiO/CdTeO₃ solar cell. It shows that the p-type NiO/CdTeO₃ structure could be successfully utilized to fabricate p-type solar cell.

1. Introduction

Quantum dot sensitized solar cell (QDSC) has been widely researched as the third generation solar cell for its low production cost and high theoretical power conversion efficiency [1–4]. However, the power conversion efficiency of QDSCs is still very low. For improving the power conversion efficiency of QDSCs, the concept that theoretical efficiency of tandem solar cells will be higher than that of constituent single-junction solar cell was firstly proposed by He et al. in 2000 [5], the tandem solar cell is a series connection of n-type solar cell based on TiO₂ and p-type solar cell based on NiO [5–7]. Nevertheless, the current is decided by the lower current of n-type solar cell and p-type solar cell, so the current of the tandem solar cell has been hampered so far by the poor current of the available p-type solar cell, resulting in the fact that the high theoretical power conversion efficiency has not been realized [8]. Photocurrent matching is an essential prerequisite for the realization of highly efficient tandem solar cell [8]. In order to realize high theoretical power conversion efficiency of tandem solar cell, photocurrent matching must be realized by improving the performance of p-type solar cell.

In 2009, Rhee et al. reported a new p-type NiO/Cu₂S solar cell prepared by depositing p-type Cu₂S sensitizer on the mesoporous NiO electrode for the first time, and short-circuit current of 260–360 μA and open-circuit voltage of 91–95 mV were eventually obtained [9]. In 2013, Safari-Alamuti et al. cascaded CdS/CdSe sensitizers onto mesoporous NiO films by successive ionic layer adsorption and reaction (SILAR) method. Finally, the short-circuit current density of 0.87 mA/cm², open-circuit voltage of 86 mV, and conversion efficiency of 0.02% were achieved [10]. In this paper, CdTeO₃ quantum dots (QDs) are deposited on NiO film by SILAR method. After assembling the film in solar cell, it yielded a photovoltage of 103.7 mV and short-circuit current density of 0.364 mA/cm². Finally, the power conversion efficiency of 0.0185% was achieved. Compared to the NiO/Cu₂S solar cell and the NiO/CdS/CdSe cosensitization solar cell, the open-circuit voltage of NiO/CdTeO₃ solar cell has huge improvement and the power conversion efficiency of NiO/CdTeO₃ (single sensitization) solar cell is very close to that of NiO/CdS/CdSe cosensitization solar cell.

2. Experimental Details

2.1. Preparation

NiO paste was produced by mixing slurry of 15 g NiO nanopowder in ethanol with 50 mL 10 wt% ethanolic ethyl cellulose solution and 100 mL terpineol and then ethanol was slowly removed by using rotary evaporation [8]. NiO mesoporous films were prepared by screen printing NiO paste on F-doped SnO₂ oxide (FTO) glass substrates, followed by sintering it at 450°C for 30 min.

SILAR method was used to deposit CdTeO₃ QDs onto the NiO photocathodes. The synthesized films were firstly dipped into the Cd(NO₃)₂ ethanol solution (0.01 M) for 5 min, rinsed with absolute ethanol and then dipped in the Na₂TeO₃ aqueous solution (0.01 M) for another 5 min, and rinsed again with deionized water.

The QDs-sensitized NiO electrode is assembled face-to-face with a platinized counter electrode using 60 μm thick thermoplastic frames. 1.0 M LiI and 0.1 M I₂ are dissolved into acetonitrile as electrolyte [11]. The working area of the solar cell is 0.25 cm².

2.2. Characterizations

SEM images were taken by a scanning electron microscopy (SEM, S-4300). X-ray diffraction (XRD) patterns were measured by Bruker D8 Focus diffractometer. Energy dispersive X-ray spectroscopy (EDS) image was recorded by EDS system. Inductively coupled plasma mass spectrometry (ICP-MS, IRIS Intrepid II XSP) was used to analyze the exact composition and content of metal elements. The ultraviolet-visible-near infrared (Uv-Vis-Ni) spectroscopy was recorded in the range of 300–2500 nm by using the Cary 5000 spectrometer (US Varian). The - curves of samples were measured under illumination (100 mW/cm²) using a 350 W sun simulator (Oriel). The incident photon-to-current conversion efficiency (IPCE) spectra of the samples were measured with an IPCE measuring system.

3. Results and Discussion

Figure 1(a) shows SEM image of NiO film, and we can easily find that the surface morphology of NiO film is rough. Furthermore, the corresponding EDS spectrum of NiO film is shown in inset of Figure 1(a), which demonstrated the film containing Ni and O elements. The NiO film was clearly evidenced from the XRD patterns showed in Figure 1(b). The typical X-ray diffraction pattern peaks at 2θ values of NiO film are approximately 37.4°, 43.3°, and 62.9°, which correspond to (111), (200), and (220) crystalline planes of NiO, respectively.

(a)

(b)

Figure 1: (a) SEM image of NiO film and inset in (a) is the corresponding EDS spectrum and (b) XRD pattern of NiO film.

Figure 2(a) shows SEM image of the NiO film deposited with CdTeO₃ QDs. The thickness of the film is about 2.5 μm. Figure 2(b) shows SEM image of the top view of NiO film deposited with CdTeO₃ QDs. From Figure 2(b), we can readily find that CdTeO₃-sensitized NiO film is made up of porous nanoparticles. Figure 2(c) shows the EDS patterns, and we can readily determine that the film contains O, Ni, Cd, and Te elements. In order to obtain the exact composition of the NiO mesoporous film which is deposited with CdTeO₃ QDs, ICP-MS was used to analyze the content of metal elements and the mole ration of CdTeO₃ and NiO is about 1 : 57.5 by calculation. Figure 2(d) is the XRD patterns of the NiO mesoporous film deposited with CdTeO₃ QDs. The X-ray diffraction patterns of FTO, NiO, and CdTeO₃ can be found from X-ray diffraction pattern (corresponding with orange square, black diamond, and blue triangle, resp.). The typical X-ray diffraction pattern peaks at 2θ values of CdTeO₃ QDs are approximately 28.8° and 33.1° (corresponding with blue triangle, resp.), which correspond to (111) and (200) crystalline planes of CdTeO₃, respectively.

(a)

(b)

(c)

(d)

Figure 2: SEM images of the (a) cross-section, (b) top view of the NiO mesoporous film deposited with CdTeO₃ QDs, (c) EDS spectrum, and (d) XRD patterns of the NiO mesoporous film deposited with CdTeO₃ QDs.

Figure 3 shows the UV-Vis-Ni absorption spectra of NiO mesoporous film and CdTeO₃ QDs deposited NiO mesoporous film. As we can see from the UV-Vis-Ni absorption spectra, the scope of absorption wavelength is from 300 to 2500 nm, but the absorption intensity of NiO film is obviously higher than CdTeO₃ QDs-sensitized NiO film. That is because the color of NiO film is gray which is darker than the milk-white color of CdTeO₃. On one hand, the absorbing ability of gray NiO is stronger than milk-white CdTeO₃, CdTeO₃ QDs that when deposited onto NiO film leads to reflection increase and light absorption decrease. On the other hand, NiO film is mesoporous structure, so there are many pores on its surface of NiO film. This structure is good for improving the light absorption. Nevertheless, after CdTeO₃ QDs are deposited onto NiO film, the pores on the surface of NiO film were covered with the CdTeO₃ QDs, so the quantity of pores is decreased, eventually leading to absorption decrease. Finally, the absorption intensity of NiO film is obviously higher than that of CdTeO₃ QDs-sensitized NiO film.

Figure 3: UV-Vis-Ni absorption spectrum of NiO mesoporous film and CdTeO₃ QDs deposited NiO mesoporous film.

The CdTeO₃ QDs-sensitized NiO electrode was used as the photoelectrode in the thin sandwich-type cell with counter electrode Pt-coated FTO, spacer film, and electrolyte solution. CdTeO₃ QDs are deposited onto NiO by SILAR method, and the SILAR cycles are 2, 4, 6, 8, and 10, respectively. The - curves are shown in Figure 4, and the corresponding parameters are listed in Table 1. It can be easily seen that as the SILAR deposition cycles increase (except the SILAR cycles which are eight and the NiO solar cell), values of the short-circuit current density (), fill factor (FF), and conversion efficiency () of CdTeO₃-sensitized solar cells all decrease. When the SILAR cycles are 2, the conversion efficiency is 0.0185%. It is the highest efficiency that the CdTeO₃-sensitized solar cell could achieve in terms of performance, the corresponding short-circuit current density is 0.364 mA/cm², and the open-circuit voltage () is 103.7 mV (the highest open-circuit voltage). This is because when the SILAR cycles exceed 2, the deposited CdTeO₃ QDs increase along the thickness direction and the aggregation of CdTeO₃ QDs and the bad coverage of NiO film appeared. Moreover, the gap between NiO nanoparticles decreases, which is difficult for electrolyte to cover the surface of NiO, so the electrolyte cannot contact completely with CdTeO₃ which are deposited on the surface of NiO. This increases recombination probability of photogenerated holes and electrons, so the performance of solar cells decreases [12].

Table 1: Parameters obtained from the measurement of CdTeO₃-sensitized solar cells.

Samples
(mV)
(mA/cm²)
FF
(%)

Only NiO
60.7
0.463
0.388
0.0109

CdTeO₃ (2)
103.7
0.364
0.490
0.0185

CdTeO₃ (4)
99.1
0.309
0.454
0.0139

CdTeO₃ (6)
96.9
0.318
0.425
0.0131

CdTeO₃ (8)
100.6
0.285
0.460
0.0132

CdTeO₃ (10)
96.6
0.256
0.457
0.0113

Figure 4: - curves of CdTeO₃-sensitized solar cells with different deposition cycles.

From Figure 4 and Table 1, we can readily find that the open-circuit voltage (about 100 mV) of CdTeO₃-sensitized solar cell is obviously higher than the open-circuit voltage (about 60 mV) of NiO solar cell (without CdTeO₃ QDs sensitizer). This can be explained as follows: the open-circuit voltage of NiO solar cell is decided by the different values between the quasi-Fermi level of the NiO and the redox potential of electrolyte (see Figure 5(a)). However, the open-circuit voltage of CdTeO₃-sensitized solar cell is decided by the different values between the quasi-Fermi level of the photocathode (CdTeO₃ QDs-sensitized NiO) and the redox potential of electrolyte (see Figure 5(b)), and the quasi-Fermi level of the photocathode moves down causing the different values to increase, and, finally, the open-circuit voltage increases.

(a)

(b)

Figure 5: The structures and energy levels of (a) NiO solar cell and (b) CdTeO₃-sensitized solar cell.

The IPCE spectrum of QDSC with 2 SILAR cycles at different incident light wavelengths was shown in Figure 6. Compared to the spectral response range of the UV-Vis-Ni absorption, the spectral response range of the IPCE spectra is only 350 to 430 nm and the maximum IPCE closes to 12%. That is because NiO is a large band gap (3.6–4.0 eV) semiconductor with gray color [13–15], which leads to the fact that the absorbed light cannot be converted to electricity completely, so the light of long wavelength cannot be utilized to generate holes but used to generate heat. That also reveals the reason why the short-circuit current density of QDSCs is so low. From the images of absorption intensity of NiO film and CdTeO₃ QDs-sensitized NiO film in Figure 3, we know that the short-circuit current density of QDSCs is lower than NiO solar cell. This is because the absorption intensity of CdTeO₃ QDs-sensitized NiO film is inferior to NiO film obviously.

Figure 6: IPCE of QDSC with two SILAR cycles.

4. Conclusions

In summary, SILAR method was used for fabricating CdTeO₃ QDSCs. We found that when the deposition cycles are 2, the solar cell yields a photovoltage of 103.7 mV and a short-circuit current density of 0.364 mA/cm² and the conversion efficiency of 0.0185% is achieved. The results in this work demonstrate that the p-type NiO/CdTeO₃ structure could be successfully utilized to fabricate p-type solar cell.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was partially supported by Key Project of Beijing Natural Science Foundation (3131001), Key Project of Natural Science Foundation of China (91233201 and 61376057), Key Project of Beijing Education Committee Science & Technology Plan (KZ201211232040), State 863 Plan of MOST of China (2011AA050527), Beijing National Laboratory for Molecular Sciences (BNLMS2012-21), State Key Laboratory of Solid State Microstructures of Nanjing University (M27019), State Key Laboratory for Integrated Optoelectronics of Institute of Semiconductors of CAS (IOSKL2012KF11), State Key Laboratory for New Ceramic and Fine Processing of Tsinghua University (KF1210), Key Laboratory for Renewable Energy and Gas Hydrate of Chinese Academy of Sciences (y207ka1001), Beijing Key Laboratory for Sensors of BISTU (KF20131077208), and Beijing Key Laboratory for Photoelectrical Measurement of BISTU (GDKF2013005).

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