Enhanced solar energy conversion in Au-doped, single-wall carbon nanotube-Si heterojunction cells
© Chen et al.; licensee Springer. 2013
Received: 14 March 2013
Accepted: 26 April 2013
Published: 10 May 2013
The power conversion efficiency (PCE) of single-wall carbon nanotube (SCNT)/n-type crystalline silicon heterojunction photovoltaic devices is significantly improved by Au doping. It is found that the overall PCE was significantly increased to threefold. The efficiency enhancement of photovoltaic devices is mainly the improved electrical conductivity of SCNT by increasing the carrier concentration and the enhancing the absorbance of active layers by Au nanoparticles. The Au doping can lead to an increase of the open circuit voltage through adjusting the Fermi level of SCNT and then enhancing the built-in potential in the SCNT/n-Si junction. This fabrication is easy, cost-effective, and easily scaled up, which demonstrates that such Au-doped SCNT/Si cells possess promising potential in energy harvesting application.
Photovoltaic devices based on nanomaterials may be one kind of next-generation solar cells due to their potential tendency of high efficiency and low cost . Among them, carbon nanotube (CNT), possessing one-dimensional nanoscale structure, high aspect ratios, large surface area , high mobility , and excellent optical and electronic properties, could be beneficial to exciton dissociation and charge carrier transport, which allow them to be useful in photovoltaic devices [4–8]. In recent years photovoltaic devices and photovoltaic conversion based on the heterojunctions of CNT and n-type silicon have been investigated [9–12]. In those devices, electron–hole pairs are generated in CNT under illumination and are separated at the heterojunctions. This means that the CNT acts as the active layer of the cells for exciton generation, charge collection, and transportation, while the heterojunction acts for charge dissociation. The conductivity and transparency of the single-wall carbon nanotube (SCNT) films are two important factors for fabricating the higher performance of SCNT/n-Si solar cell. Kozawa had found that the power conversion efficiency (PCE) strongly depended on the thickness of the SCNT network and showed a maximum value at the optimized thickness . Li had found that photovoltaic conversion of SCNT/n-silicon heterojunctions could be greatly enhanced by improving the conductivity of SCNT . Therefore, the efficiency of the solar cells for SCNT/n-Si is directly related to the property of SCNT film. Recently, doping in CNT has been employed to improve the performance of their cells [15–17]. Saini et al. also reported that the heterojunction of boron-doped CNT and n-type Si exhibited the improved property due to boron doping . Bai et al. found that the efficiency of Si-SCNT solar cells is improved to 10% by H2O2 doping . Furthermore, it was reported that higher performance SCNT-Si hybrid solar cells could be achieved by acid doping of the porous SCNT network . It is believed that the doping of CNT and the reduced resistivity are in favor of the charge collection and prevention of carriers from recombination, so the PCE of the CNT-based solar cells can be enhanced.
In this paper, we prepared a SCNT film on a n-Si substrate by an electrophoretic method, and then doping the SCNT by a simple method in a HAuCl4·3H2O solution at room temperature [21, 22], to improve the PCE as the result of improved conductivity and increased density of carriers. In this experiment, it was found that p-type doping due to Au could shift down the Fermi level and enhanced the work function of SCNT so that the open circuit voltage was increased. It was also found that the conversion efficiency of the Au-doped SCNT cells was significantly increased compared with that of pristine SCNT/n-Si cells.
SCNT of 95% purity with an outer diameter of 1 to 2 nm and lengths of 1 to 3 μm were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences, (Chengdu, Sichuan, China). In the experiments, 1 to 3 mg of SCNT were added into 50 ml of analytically pure isopropyl alcohol in which Mg(NO3)2·6H2O at a concentration of 1 × 10−4 M was dissolved. This solution was subjected to the high-power tip sonication for 2 h. A small part of the solution was diluted in 200 ml of isopropyl alcohol and then placed in a sonic bath for about 5 h to form SCNT electrophoresis suspension.
Constructing the homogeneous semitransparent SCNT network is the first step for fabricating SCNT/n-Si photovoltaic conversion cell. So SCNT film was prepared by the method of electrophoretic deposition (EDP) . A piece of n-type silicon wafer (cathode) and a stainless-steel plate (an anode) were immersed into the SCNT electrophoresis suspension at room temperature. The two electrodes were kept in parallel with a gap of 1 cm. The deposition was carried out for 10 min by applying a constant DC voltage of 100 V. After the EDP and drying in air, the SCNT film on the Si wafer was put into a diluted nitric acid solution to remove possible surviving Mg(OH)2 on the surface.
The morphology of SCNT network before and after doping was characterized by field emission scanning electronic microscope (FESEM) and transmission electronic microscope (TEM). The Raman spectra were measured with a laser Raman spectrophotometer. The excitation wavelength of the Ar ion laser was 514.5 nm. An ultraviolet–visible spectrometer (Varian Cary 100; Varian Inc., Palo Alto, CA, USA) was used to study the absorption of the SCNT film. The resistance of SCNT film was measured by a four-point probe method. The carrier density and mobility for the pristine SCNT film and doping film were measured with a Hall effect measurement system (Bio-Rad Corp. Hercules, CA, USA). An Oerlikon external quantum efficiency (EQE) measurement system (Oerlikon Co., Pfaffikon, Switzerland) was used to obtain the EQE of solar cells. The characteristics of cell performance were measured under the standard conditions (1 sun, AM 1.5 Global spectrum), using a Berger Flasher PSS 10 solar simulator (Berger Lichttechnik GmbH & Co. KG, Pullach im Isartal, Germany).
Results and discussion
In formula (1), M means the molar ratio of the a-C and the SCNT, and M a-C + MpureSWCNTs =1, ID/IG are the ratios of the intensities of D band and G band.
The ID/IG value of commercial SCNT calculated from the Raman spectrum as shown in Figure 3 is about 0.70. Usually, the pure SCNT has very small ID/IG value and could be assumed as 0.01 [24–26]. Meanwhile, the value of ID/IG for a-C is similar to that of multiwall CNT (MCNT) and about 1.176 . Thus, the calculated concentration ratio of amorphous carbon and SCNT is about 5.26%. It is obvious that the commercial SCNT is highly pure with little amorphous carbon.
In order to better understand the effect of Au doping on the carrier density and mobility of the SCNT, Hall effect measurements were performed for the SCNT film deposited on a glass substrate at room temperature. The Hall effect measurements revealed that the SCNT networks were all p-types conductivity before and after Au doping. After doping, an average carrier density for the SCNT film increased from 5.3 × 1018 to 1.4 × 1020 cm−3. This enhanced carrier density is advantageous for SCNT/n-Si photovoltaic devices because p doping and the reduced resistivity are in favor of charge collection and preventing carriers from recombination. The gold-hybridization SCNT can provide more charge transport paths, resulting in improved cell PCE more than three folds. Recent studies showed that doping also decreased the tunneling barrier between SCNT and concluded that this is the major fact in the overall film resistance [45–47]. So the devices series resistance (Rs) dropped from 218 Ω (or 8.72 Ω·cm2) in the SCNT/Si cell to 146 Ω (or 5.84 Ω·cm2) in the gold-hybridization SCNT-Si cell.
Photovoltaic characteristics of SCNTs-Si solar cell for SCNT immersion in Au solution at different times
Rs (Ω cm2)
Carrier density (cm−2)
5.2 ± 0.05
0.38 ± 0.02
18 ± 0.01
0.36 ± 0.06
8.72 ± 0.01
5.3 × 1018
7.2 ± 0.04
0.45 ± 0.01
26 ± 0.01
0.84 ± 0.04
7.5 ± 0.02
7.9 × 1019
7.65 ± 0.06
0.50 ± 0.02
30 ± 0.02
1.15 ± 0.05
5.84 ± 0.01
7.46 ± 0.05
0.47 ± 0.01
31 ± 0.01
1.09 ± 0.04
5.65 ± 0.02
1.3 × 1021
7.1 ± 0.02
0.46 ± 0.02
30 ± 0.01
0.98 ± 0.01
5.63 ± 0.02
1.5 × 1021
In summary, the photovoltaic performance of SCNT-Si heterojunction devices can be significantly improved by doping Au nanoparticles on the wall of SCNT. In the experiments, the PCE, open circuit voltage, short-circuit current density, and fill factor of the devices reached to 1.15%, 0.50 V, 7.65 mA/cm2, and 30% from 0.36%, 0.38v, 5.2, and 18%, respectively. The improved conductivity and the enhanced absorbance of active layers by Au nanoparticles are mainly the reasons for the enhancement of the PCE. It is believed that the photovoltaic conversion efficiency can be further improved by optimizing some factors, such as the density of SCNT, the size and shape of Au nanoparticles, and efficient electrode design.
The authors would like to appreciate the financial supports of 863 project no. (2011AA050517), the Fundamental Research Funds for the Central Universities, and the financial support from Chinese NSF Projects (no. 61106100).
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