- Nano Express
- Open Access
Effect of Polyethylene Glycol on the NiO Photocathode
© The Author(s). 2017
- Received: 7 June 2017
- Accepted: 5 August 2017
- Published: 17 August 2017
In this study, a uniform nanoporous NiO film, with a thickness of up to 2.6 μm, was prepared using polyethylene glycol (PEG). The addition of PEG significantly decreased the cracks in the NiO film and prevented the peeling of the NiO film from a fluorine-doped tin oxide substrate. The NiO cathode was prepared using CdSeS quantum dots (QDs) as the sensitizer, with an optimized photoelectric conversion of 0.80%. The optimized QD-sensitized NiO films were first assembled with the TiO2 anode to prepared QD-sensitized p–n-type tandem solar cells. The open circuit voltage was greater than that obtained using the separated NiO cathode or TiO2 anode.
- Nickel oxide
- Polyethylene glycol
- Quantum dots
- p–n-Type tandem solar cells
Solar energy demonstrates potential as the primary source of energy in the future because of its cleanliness, high power, rapid processing, and wide availability [1, 2]. Since the development of solar cells in the past 30 years, sensitized solar cells have become efficient devices for the utilization of solar energy. However, these studies focus on n-type solar cells, which are based on a sensitized n-type photoanode, e.g., TiO2, ZnO, and SnO2 [2–6]. The short-circuit current density was greater than 15 mA cm−2, and the photoelectric conversion efficiency was approximately 13% . He et al. have reported the use of p–n-type tandem dye-sensitized solar cells (DSSCs) , which possibly afford a higher open-circuit voltage (OCV) and photoelectric conversion efficiency. Nakasa et al. have reported an OCV of 0.918 V by the combination of merocyanine NK-2684-sensitized NiO and TiO2 photoanode . Nattestad et al. have reported a decrease in the charge recombination of the NiO photocathode by the optimization of donor–acceptor dyes and achieved an absorbed photon-to-electron conversion efficiency greater than 90% over a spectral range of 400–500 nm , with an open-circuit voltage of 1079 mV. This value is the highest value reported thus far for p–n-type tandem DSSCs.
To obtain higher photocurrents comparable to n-type photoanodes, one way is preparing new p-type cathode [10, 11]. Another way is to prepare thick mesoporous photocathodes which are preferable for adsorbing a large amount of dye molecules. Some attempts have been made to improve the thickness of NiO films; however, the photocurrent density generated is still an order of magnitude less than that observed for n-type DSSCs, and thick films often suffer from poor mechanical stability. Wu et al. have prepared NiO films by the hydrothermal method and improved their properties by the optimization of the film thickness and specific surface area . Qu et al. have fabricated layered NiO films from wrinkled porous NiO nanosheets and reported significantly improved photocurrent and photovoltage . Zhang et al. have improved the photovoltage by the application of highly crystalline NiO . Powar et al. have obtained a high photocurrent of 7.0 mA cm−2 using nanostructured NiO micro-balls as active materials for the photocathode . Sumikura et al. have prepared nanoporous NiO films by the hydrolysis of NiCl2 in a water/ethanol mixed solution using a series of polyethyleneoxide–polypropyleneoxide–polyethyleneoxide (PEO–PPO–PEO) triblock copolymers as the template . They investigated the effects of the PEO–PPO–PEO template in detail. Li et al. have adopted the preparation method used by Sumikura et al. and prepared thick NiO films by a two-step doctor blading method . They obtained a record incident photon-to-current efficiency (IPCE) of 64% and a short circuit current (J SC ) of 5.48 mA cm−2. However, the photoelectric conversion efficiency of the p-type NiO electrode is maintained between 0.02 and 0.3% using different dyes. In this experiment, precursor solutions of NiO were prepared using F108 (polyethyleneoxide–polypropyleneoxide–polyethyleneoxide (PEO–PPO–PEO) triblock co-polymers, MW: ca. 14,600) as the template following Sumikura et al.’s method. Polyethylene glycol (PEG; MW: ca. 20,000) was added into the precursor solution, and its effects on the NiO film were investigated in detail. Finally, a p–n-type quantum dot (QD)-sensitized tandem solar cells were also assembled.
A precursor solution of NiO was prepared according to a previously reported method . First, anhydrous NiCl2 (1 g) and F108 (1 g) were dissolved in a mixture of deionized water (3 g) and ethanol (6 g). Second, the solution was left to rest for 3 days. Third, a specific content of polyethylene glycol (MW of 20,000) was added into the NiO precursor solution. Next, the mixture was stirred for 4 h and centrifuged at 8000 rad/min. The PEG content was controlled at 0.03, 0.075, 0.15, and 0.3 g. The above solution was deposited on a fluorine-doped tin oxide (FTO) glass substrate by the doctor blading method and dried at room temperature. The films were sintered at 400 °C for 30 min under air. CdSeS QDs were prepared by hot-injection synthesis according to previous experiments reported by our group . The prepared NiO films were sensitized by CdSeS QDs by the electrophoretic method using a mixed acetonitrile/toluene (1:2.5 v/v) solution by applying a DC of 50 V for a specific time. TiO2 films were co-sensitized with CdS/CdSe using the conventional successive ionic layer adsorption and reaction (SILAR) method . QD-sensitized TiO2 films were used as the anode instead of CuS to assemble p–n-type QD-sensitized solar cells.
The morphology of the NiO films was examined using a JSM-7001F field-emission scanning electron microscope (FE-SEM). Photocurrent density–voltage (J–V) characteristics were measured using a Keithley 2440 source meter under AM 1.5G illumination from a Newport Oriel solar simulator with an intensity of 1 Sun.
Polyethylene glycol (PEG) was used to prepare NiO films. The addition of PEG significantly decreased the cracks in the NiO films. A uniform 2.6-μm-thick nanoporous NiO film was prepared. The optimized photoelectric conversion efficiency was 0.80%. The optimized quantum dot-sensitized NiO film was first assembled with the TiO2 anode to prepared QD-sensitized p–n-type tandem solar cells. The open-circuit voltage (OCV) was greater than that exhibited by the separated NiO cathode or TiO2 anode. The TiO2(down)/NiO(up) tandem solar cells afforded a total photoelectric conversion of 0.43%, with an OCV, short circuit current density, and fill factor of 0.594 V, 2.0 mA cm−2, and 0.36, respectively.
This study was supported by the Natural Science Foundation of China (Nos. U1404202 and 21403056), Program for Science and Technology Innovation Talents in Universities of Henan Province (18HASTIT031), and the Young Core Instructor Foundation from the Education Commission of Henan Province (2014GGJS-028).
ZC, WK, XJ, JC, and SD carried out the experiment; SL and ZC analyzed the data and finished figures in the manuscript; SL modified the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interest regarding the publication of this paper.
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