Low-temperature synthesis of multilayer graphene/amorphous carbon hybrid films and their potential application in solar cells
© Cui et al.; licensee Springer. 2012
Received: 5 July 2012
Accepted: 28 July 2012
Published: 11 August 2012
The effect of reaction temperature on the synthesis of graphitic thin film on nickel substrate was investigated in the range of 400°C to 1,000°C. Amorphous carbon (a-C) film was obtained at 400°C on nickel foils by chemical vapor deposition; hybrid films of multilayer graphene (MLG) and a-C were synthesized at a temperature of 600°C, while MLG was obtained at temperatures in excess of 800°C. Schottky-junction solar cell devices prepared using films produced at 400°C, 600°C, 800°C, and 1,000°C coupled with n-type Si demonstrate power conversion efficiencies of 0.003%, 0.256%, 0.391%, and 0.586%, respectively. A HNO3 treatment has further improved the efficiencies of the corresponding devices to 0.004%, 1.080%, 0.800%, and 0.820%, respectively. These films are promising materials for application in low-cost and simple carbon-based solar cells.
KeywordsGraphene Amorphous carbon Temperature effect Nickel foil Solar cell HNO3 treatment
Graphene has attracted widespread attention due to its unique band structure and fascinating electronic, optical, chemical, and mechanical properties [1–4]. Hybrid structures of graphene with other carbon materials such as carbon nanotubes (CNTs) [5, 6] and amorphous carbon (a-C)  could combine advantages of the constituent structures and find applications in many areas. For example, composites of a-C and graphene sheets exhibit attractive catalytic performance for the hydrolysis of cellohexaose . Multilayer graphene (MLG) oxide and a-C hybrid films were synthesized by incorporating MLG oxide into a-C matrix . The hybrid films show good electrical, mechanical, and tribological properties, with a sheet resistivity of approximately 100 Ω cm, Young’s modulus of 171 GPa, and elastic recovery of 81.4% . In addition, heterojunction solar cells based on carbon materials such as a-C films and graphene have attracted much attention . Their interesting optical properties, chemical inertness, and low cost make a-C films as potential candidate materials for solar cells [10, 11]. Indeed, carbon films were the earliest carbon materials partially replacing silicon (Si) in Si-based solar cells , but their poor electrical conductivity hinders their practical application . Excellent conductivity, good transparency, and high hole transport mobility make graphene as a promising candidate in photovoltaic devices [9, 13, 14]. Graphene/n-Si Schottky-junction solar cells have been assembled and demonstrated power conversion efficiencies up to 1.5% , but the fabrication process of the device is relatively complex because graphene easily cracks . Some of the above-mentioned shortcomings can be overcome by a graphene/a-C hybrid structure. Unfortunately, the reported method to prepare graphene/a-C hybrid structure involves several rigorous processing steps, including the fabrication of graphene oxide using expandable graphite, dissolving the graphene oxide into methanol, and electrolysis deposition of the methanol solution . Therefore, in situ graphene/a-C hybrid structure fabrication is highly desirable.
The synthesis of carbon nanomaterials (carbon nanotubes, graphene, carbon films, etc.) at a relatively low temperature is crucial for their practical applications [16, 17]. Firstly, a low temperature could simplify the growth process , and it is more convenient, cost-effective, and environment-friendly . Secondly, a low-temperature process is important for their electronic device applications . For example, complementary metal-oxide semiconductor (CMOS) technology is widely used in transistors. In CMOS technology, an oxide layer serves as an insulator between the transistor gate and the channel . In this sense, synthesis of carbon nanomaterials directly on certain substrates (e.g., oxide , nickel [13, 21]) is crucial. However, many substrates are vulnerable to heating ; thus, low-temperature synthesis of carbon nanomaterials would be attractive.
In this work, hybrid films of MLG and a-C were prepared at a relatively low temperature of 600°C. When the temperature exceeded 800°C, MLG is obtained. Schottky-junction solar cells based on n-type Si and 400°C, 600°C, 800°C, and 1,000°C samples demonstrate efficiencies of 0.003%, 0.256%, 0.391%, and 0.586%, respectively. After a HNO3 treatment, the efficiencies of the corresponding solar cells have further increased to 0.004%, 1.080%, 0.800%, and 0.820%, respectively. Our work has opened up a new avenue for the production of low-cost carbon-based solar cells in the future.
Synthesis of MLG
The experimental setup is similar to that for N-doped carbon films (N-CFMs) described in our previous work , except that H2 is used in this work. In addition, the substrate is also different from that of N-CFMs. Nickel (Ni) foil instead of copper foil is used as substrate for graphene growth. The Ni foil is mounted in the center of the quartz tube reactor and gradually heated up to the pre-determined temperature of 400°C, 600°C, 800°C, or 1,000°C in 100 min under the protection of an Ar flow of 300 ml/min. When the temperature reaches the set value, the Ni foil is further annealed for 30 min to homogenize the crystal grains and to remove the oxidation layers under a reducing atmosphere of Ar (2,000 ml/min) mixed with H2 (100 ml/min). Then, acetonitrile (CH3CN) is introduced into the reactor at a feed rate of 20 μl/min for 2 min under the reducing atmosphere of Ar (2,000 ml/min) and H2 (100 ml/min). Afterwards, the Ni foil is moved to the low-temperature region of the quartz reactor to achieve a fast cooling rate. To obtain freestanding films, the as-grown samples are treated in a mixed solution of 0.5 M FeCl3 and 0.5 M HCl. After being rinsed in distilled water for several times, freestanding films could be collected for further characterizations and device fabrication.
Characterization of samples
The morphologies of as-synthesized films were characterized using a transmission electron microscope (TEM, JEOL-2010, Akishima-shi, Japan). Raman spectra were obtained on a microscopic confocal Raman spectrometer (Renishaw RM 2000, Wotton-under-Edge, UK) with a 514.5-nm laser line. Optical transmission spectra were taken using a UV-2450 UV/Vis optical spectrometer (Shimadzu Corporation, Kyoto, Japan). Sheet resistances (Rs) of the samples were measured using a four-probe resistivity test system.
Solar cell device assembly
Heterojunction solar cells were assembled by covering the films synthesized at 400°C, 600°C, 800°C, and 1,000°C onto an n-type Si wafer with a square window of 3 mm × 3 mm surrounded by insulating silicon dioxide, and the detailed procedure of the cell assembling can be found in our previous report . The assembled solar cells were evaluated with a solar simulator (at AM 1.5, Newport, Irvine, CA, USA) and a Keithley 2400 SourceMeter (Cleveland, OH, USA).
The HNO3 treatment was carried out by exposing the as-synthesized films to HNO3 fumes. The assembled solar cell was placed above a vial containing fuming HNO3 (65 wt.%) for 60 s.
Results and discussion
Different synthesizing temperatures resulted in different films. The reason could be explained as follows: The CVD growth of graphene on a transition metal consists of three stages: (1) precursor molecules collide with the metal surface; (2) carbon precursor molecules dehydrogenate and form active carbon species; and (3) active carbon species coalesce, nucleate, and grow to graphene . In stage 1, the temperatures do not have a significant effect on the reaction because the adsorption energies of organic precursors on the metal surface are very small (approximately 0.02 eV) . At stage 2, the activation energy of liquid organic precursor dehydrogenation is approximately 1.5 eV ; thus, a high temperature favors the formation of active carbon species. In stage 3, the high temperature is also beneficial for the graphene growth. At a low temperature, active carbon species lack the mobility to form crystalline carbon, and a more disordered form of carbon is formed .
Photovoltaic properties of C/Si heterojunction solar cells
The power conversion efficiencies of the above-mentioned solar cells are very low, and our previous report had shown that a HNO3 treatment could enhance the efficiencies of CNT/n-Si solar cells . To the best of our knowledge, HNO3 treatment on MLG/a-C hybrid films had not been investigated yet. We believed that a HNO3 treatment may have a similar effect on the above-mentioned solar cells. After the HNO3 treatment, the efficiencies of the corresponding solar cells have improved to 0.004%, 1.080%, 0.800%, and 0.820%, respectively, as shown in Table 1. There are three main reasons for the efficiency improvement. Firstly, HNO3 doping could enlarge the work function of MLG ; thus, a higher Voc is obtained after HNO3 treatment (Table 1). Secondly, HNO3 modification enhances the sheet conductance of the films, leading to a larger Jsc. The Rs of the 400°C sample decreases from 9,564 to 8,572 Ω/sq, that of the 600°C sample decreases from 1,864 to 1,032 Ω/sq, that of the 800°C sample decreases from 346 to 282 Ω/sq, and that of the 1,000°C sample decreases from 262 to 208 Ω/sq. Thirdly, a HNO3 treatment could reduce the internal resistance of the solar cells ; thus, the FF is enhanced (Table 1).
The pristine cell efficiencies of the 800°C and 1,000°C samples are better than that of the 600°C sample, and it is interesting that after HNO3 treatment, the efficiency of the 600°C sample is better than that of the 800°C and 1,000°C samples. The reasons could be explained by the working mechanism of our Schottky-junction solar cells as follows: Firstly, 600°C, 800°C, and 1,000°C films serve as a transparent electrode for light, and the 600°C sample possesses better light transmittance. Compared with the 800°C and 1,000°C samples, more light reaches to the Schottky-junction interface of the 600°C sample, generating much more electron–hole pairs in the 600°C sample. Secondly, 600°C, 800°C, and 1,000°C films also serve as charge transport path. The electrical conductivity of the 600°C film is much poorer than that of 800°C and 1,000°C ones; thus, the electron–hole pairs could not be effectively separated and transported, resulting in a lower Jsc and power conversion efficiency. Thirdly, after HNO3 treatment, the electrical conductivities of 600°C, 800°C, and 1,000°C films are all improved, enhancing charge transport and Jsc. The 600°C sample generates much more electron–hole pairs, so the magnitude of the increase in Jsc is much larger than that associated with the 800°C and 1,000°C samples (Table 1), resulting in a better efficiency.
In summary, the temperature effect on the synthesis of graphitic thin film on Ni foil was investigated. It was observed that temperature was critical in the production of MLG and α-C films. At 400°C, a-C film was obtained on Ni foil by CVD. Hybrid film of MLG and a-C could be synthesized at 600°C, while MLG was obtained at 800°C and above. Schottky heterojunction solar cells based on 400°C, 600°C, 800°C, and 1,000°C samples and n-type Si demonstrated power conversion efficiencies of 0.003%, 0.256%, 0.391%, and 0.586%, respectively. HNO3 modification has improved the efficiencies by enhancing the sheet conductance and work functions of the as-synthesized samples. After the HNO3 treatment, the efficiencies of 400°C, 600°C, 800°, and 1,000°C devices could be increased to 0.004%, 1.080%, 0.080%, and 0.820%, respectively. Now, low-cost carbon-based window-layer materials have been synthesized and their efficiency can certainly be further improved. Also, carbon-based solar cells are easy to assemble. As such, the commercial production of this type of solar cells holds promise for the future.
We are grateful to the financial supports from the National Natural Science Foundation of China (grant nos. 50902080 and 50632040) and Guangdong Province Innovation R&D Team Plan (grant no. 2009010025).
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