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 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 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.
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].
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)
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.
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).