A new method to disperse CdS quantum dot-sensitized TiO2 nanotube arrays into P3HT:PCBM layer for the improvement of efficiency of inverted polymer solar cells
© Li et al.; licensee Springer. 2014
Received: 14 March 2014
Accepted: 24 April 2014
Published: 16 May 2014
We report that the efficiency of ITO/nc-TiO2/P3HT:PCBM/MoO3/Ag inverted polymer solar cells (PSCs) can be improved by dispersing CdS quantum dot (QD)-sensitized TiO2 nanotube arrays (TNTs) in poly (3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) layer. The CdS QDs are deposited on the TNTs by a chemical bath deposition method. The experimental results show that the CdS QD-sensitized TNTs (CdS/TNTs) do not only increase the light absorption of the P3HT:PCBM layer but also reduce the charge recombination in the P3HT:PCBM layer. The dependence of device performances on cycles of CdS deposition on the TNTs was investigated. A high power conversion efficiency (PCE) of 3.52% was achieved for the inverted PSCs with 20 cyclic depositions of CdS on TNTs, which showed a 34% increase compared to the ITO/nc-TiO2/P3HT:PCBM/MoO3/Ag device without the CdS/TNTs. The improved efficiency is attributed to the improved light absorbance and the reduced charge recombination in the active layer.
Polymer solar cells (PSCs) have gained great interest because of their low cost, flexibility, and abundant availability[1–7]. So far, the high power conversion efficiency (PCE) of PSCs is achieved by bulk heterojunction (BHJ) PSCs composed of electron-donating polymers and electron-accepting fullerides. Although significant progress has been made on the improvement of the PCE of PSCs in recent years, the efficiency of the PSCs is still lower than their inorganic counterparts, such as silicon and CIGS. The main factors limiting the efficiency of the PSCs are the low light absorption efficiency due to the narrow absorption band of the absorption spectra of the polymers and the charge recombination in the devices due to the low charge transport efficiency in the electron-donating and electron-accepting materials. To overcome these problems, many efforts have been made on improving the absorption spectra and charge carrier mobility of the photovoltaic materials for higher PCE[10–13]. Some inorganic nanostructure materials with high light absorption of the visible spectrum and the near infrared spectral range are dispersed in to the polymer:fulleride layer to increase the light absorption such as CdS[14, 15], CdSe, PbS, Sb2S3, and FeS2[19, 20]. In addition, some inorganic materials with high charge carrier mobility, such as ZnO and TiO2, are used to increase the charge transport efficiency and reduce the charge recombination[21–23]. Specially, because the ordered TiO2 nanotube arrays (TNTs) possess outstanding charge transport properties, the TNTs are used to reduce the charge recombination in the PSCs and therefore improved the efficiency as reported recently. It is worthy to note that most of these materials are synthesized in advance through complicated chemical method and then dispersed in active layers. Of which, usually, only one type of these inorganic nanostructure materials is dispersed in active layer. However, there are few reports on which two types of inorganic nanostructure materials are compactly combined and dispersed in active layers.
This report focuses on the synthesis of the CdS quantum dot (QD)-sensitized TiO2 nanotube arrays (CdS/TNTs) in a simple way (chemical bath deposition (CBD)) and dispersion in active layers. CdS QDs help light absorption to produce more excitons and also help to form the interface of CdS/P3HT with P3HT in the P3HT:PCBM layer so that more excitons are separated. TNTs are able to make prompt transfer of the excitons produced by light absorption of CdS QDs. Excitons are separated efficiently enough to reduce the charge recombination. Meanwhile, TNTs are used to form the interface of TNTs/P3HT with P3HT in the active layer and also enhance the separation of excitons. Therefore, CdS/TNTs synthesized using the CBD method and dispersed in P3HT:PCBM layer not only increase the light absorption but also reduce the charge recombination. It is known that few studies on the synthesis of CdS/TNTs using the CBD method to enhance PSCs' PCE are reported.
The result shows that after the CdS/TNTs are dispersed in the P3HT:PCBM layer, the light absorption of the active layer is greatly improved, and the charge recombination is largely controlled. Comparing to the device without CdS/TNTs, the efficiency of the device with CdS/TNTs mentioned above increases by 34%, which fully proves the reasonability of this reported method.
Fabrication of TNTs
Highly ordered and vertically oriented TNTs were prepared by anodization of Ti (titanium foil, 0.25-mm thickness, 99.7% purity; Sigma-Aldrich, St. Louis, MO, USA) sheets in an electrolyte consisting of 0.25 wt.% ammonium fluoride (NH4F) (98 + % purity; Sigma-Aldrich) and 0.5 wt.% distilled (DI) water in ethylene glycol (EG) (C2H6O2, 99.0% purity; Sigma-Aldrich) at 40 V for 8 h. A detailed experimental procedure has been described in our previous paper. After anodization, the samples were washed with DI water to remove the occluded ions and dried in a N2 stream. Finally, the samples were annealed at 450°C for 2 h with a heating rate of 5°C min-1 at ambient conditions.
Synthesis of CdS-coated TNTs
CdS as an inorganic photon absorption material was deposited on TNTs by sequential CBD. Briefly, the as-prepared TNTs were successively immersed in four different beakers for about 40 s each: beakers contained a 50 mM cadmium chloride (CdCl2) (98.0%; Sigma-Aldrich) aqueous solution and a 50 mM sodium sulfide nonahydrate (Na2S) (98.0% purity; Sigma-Aldrich) aqueous solution, respectively, and the other two contained DI water to wash the samples to remove the excess of each precursor. The CBD process was performed by dipping the prepared TNTs in CdCl2 aqueous solution, rinsing it with DI water, dipping it in Na2S aqueous solution, followed by a further rinsing with DI water. The two-step dipping procedure is considered as one CBD cycle. After several cycles, the sample became yellow. In this study, 10, 20, and 30 cycles of CdS deposition were performed (denoted as CdS(10), CdS(20), and CdS(30), respectively). The as-prepared samples were dried in a N2 stream. The TNT sample after n cycles of CdS deposition was denoted as CdS(n)/TNTs. Finally, the CdS(n)/TNT powder was peeled off from the Ti sheets by bending them.
Fabrication of devices
Characterization and measurements
Current density-voltage (J-V) characteristics were measured using a computer-programmed Keithley 2400 sourcemeter (Cleveland, OH, USA) under AM1.5G solar illumination using a Newport 94043A solar simulator (Jiangsu, China). The intensity of the solar simulator was 100 mW/cm2. Light intensity was corrected by a standard silicon solar cell. The transmission and reflection spectra were measured using ultraviolet/visible (UV-vis) spectrometer (Cary 5000, Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
Characteristic data of inverted polymer solar cells with different cycles of CdS deposition on TNTs
R s (Ω)
In summary, we demonstrated a new method which significantly improves the solar cells' efficiency which could be obtained via simply dispersing compactly combined CdS/TNTs in an active layer. The CdS/TNTs were synthesized by sequential chemical bath deposition. As a result, a high PCE of 3.52% was achieved for the inverted PSCs with 20 cycles of CdS, which showed a 34% increase compared to conventional P3HT:PCBM devices. We believe that this is a simple but effective method that can be used to improve the efficiency of polymer solar cells.
This work was supported by the National Natural Science Foundation of China (Grant No. 61306019), the Education Department Foundation of Henan Province (Grant No. 14A430022), the Science Foundation of Henan University (Grant No. 2013YBZR049), and Henan University Distinguished Professor Startup Fund.
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