Flexible carbon nanotube/mono-crystalline Si thin-film solar cells
© Sun et al.; licensee Springer. 2014
Received: 20 July 2014
Accepted: 7 September 2014
Published: 20 September 2014
Flexible heterojunction solar cells were fabricated from carbon nanotubes (CNTs) and mono-crystalline Si thin films at room temperature. The Si thin films with thickness less than 50 μm are prepared by chemically etching Si wafer in a KOH solution. The initial efficiency of the thin-film solar cell varies from approximately 3% to 5%. After doping with a few drops of 1 M HNO3, the efficiency increases to 6% with a short-circuit current density of 16.8 mA/cm2 and a fill factor of 71.5%. The performance of the solar cells depends on the surface state and thickness of Si thin films, as well as the interface of CNT/Si. The flexible CNT/Si thin-film solar cells exhibit good stability in bending-recovery cycles.
Low-cost, high-efficiency, and flexible solar cells have attracted great attention due to the increasing energy demands [1–4]. Currently, crystalline Si-based solar cells dominate the photovoltaic market because of their relatively high module efficiency, high stability, and well-established process technology. Mono-crystalline Si thin films with a thickness less than 50 μm are thus expected for making a high-efficiency solar cell, so as to reduce the materials cost. However, it is difficult to manufacture crystalline Si thin films in large quantity by using traditional wire cutting process due to their brittleness. Recently, mono-crystalline Si ribbons and thin films with 2 ~ 50 μm in thickness were fabricated through a wet chemical etching process from bulk Si wafers [5–8]. Flexible mono-crystalline Si solar cells were thus fabricated by making p-n junction through high-temperature doping. Up to now, the efficiency of these crystalline Si-based flexible solar cells is approximately 6% to 13.7% [5–8], which is higher than that of conjugated polymer cells and amorphous Si thin-film tandem solar cells [3, 4, 9] but still lower than that of commercial bulk Si solar cell modules. Furthermore, these Si thin-film solar cells are still required to make p-n junction through doping at high temperature, which is a costly process and also increases the risk of damage to the Si thin films.
Recently, we developed a heterojunction solar cell by combining carbon nanotubes (CNTs) and bulk Si wafer . The CNT/Si heterojunction solar cell has aroused great interests because of its simple process and relatively high efficiency [11–17]. We have successfully improved the efficiency of the solar cells from an initial value of approximately 6% to 13.8% by HNO3 doping and further to 15% by coating with a thin layer of nano-TiO2 particles [11–13]. The CNT/Si solar cells with high efficiency above 10% were also fabricated in other groups, showing the promising photovoltaic devices with a simple structure [14–18]. Most of these researches focused on improving the efficiency by chemical doping, improving the alignment of CNTs, or using p-type semiconducting tubes. Amorphous Si thin films were also used to construct CNT/Si cells in order to cut down the materials cost . Due to the high recombination rate of the electron-hole pairs within the amorphous Si thin film, the CNT/Si cells made from amorphous Si have very poor efficiency. In order to reduce the electron transport distance and recombination rate of electron-hole pairs within Si side, cut down the materials cost, as well as meet the future demands of flexible energy devices, we fabricate flexible CNT/Si solar cells with moderate efficiency from ultrathin mono-crystalline Si films.
Results and discussion
The Si thin films were fabricated by wet chemical etching in a KOH solution from bulk Si wafers using the similar method reported in a recent paper . During the etching, a large amount of gas was released from the Si surface. The etching rate reaches to approximately 180 μm/h when the temperature is kept at 90°C, which is faster than that of the recent report . The thickness of the Si thin films is well controlled by the etching parameters, such as concentration of KOH, temperature, and etching time. Figure 1b shows an optical image of a Si thin film with a thickness of approximately 20 μm. The Si thin film can be bent to almost 180° by tweezers, showing a good flexible characteristic. Actually, the Si films are flexible and bendable when the thickness is less than approximately 50 μm. The inset of Figure 1b shows an optical image of a CNT/Si thin-film solar cell. Because of anisotropic etching, the Si thin films have pyramid-like texture on their surface (Figure 1c), which increases the roughness of the Si surface significantly. However, it is hard to observe such texture under SEM when the Si thin films are covered by CNTs (Figure 1d). It indicates that not all of the CNTs contact with the Si surface and some CNT bundles might bridge over of the pyramid-like peaks of the Si thin films.To further evaluate the surface roughness of the Si thin film, we performed WLI measurement to the different samples. Figure 1e is a WLI profile of the starting Si wafer, showing a roughness of 5 to 8 nm. For a Si thin film with a thickness of 20 μm, the roughness increases evidently to approximately 80 nm, which is more than ten times larger than that of the starting Si wafer (see Figure 1f). Thus, there are large amounts of dangling bonds on the rough Si surface. Although some wrinkles of the CNT film with a height of approximately 250 nm are also detected in the WLI profile, the roughness decreases evidently when the Si thin film is covered by a CNT film (see Figure 1g).
Photovoltaic parameters of CNT/Si solar cells made from Si thin film with different thicknesses
Si thickness (μm)
20 (acid doping)
The photovoltaic performance of the CNT/Si thin-film solar cells depends not only on the interface of heterojunction but also on the thickness of the Si thin film. Figure 2b shows the light J-V curves of the CNT/Si solar cells made from Si thin film with various thicknesses. The photovoltaic parameters of the CNT/Si thin-film solar cells obtained from light J-V curves are given in Table 1. It is clear that both Voc and Jsc decrease when the thickness of Si thin film reduces. When the thickness of Si films reduces from 235 to 20 μm, the Voc decreases from 540 to 472 mV, while the Jsc decreases slowly from 19 to 16.6 mA/cm2. It is well known that if the surface passivation is not good enough, Voc decreases with decreasing the wafer roughness because of surface recombination. The roughness increases as the etching time extends. So, it needs to reduce surface recombination rate by polishing Si surface so as to improve the efficiency.
Figure 2c shows external quantum efficiency (EQE) profiles of two CNT/Si thin-film solar cells with Si thicknesses of 20 and 100 μm, respectively. The EQE spectrum of the 20-μm-thick Si film cell is lower than that of the 100-μm-thick Si film, especially in the long-wavelength region (>700 nm), which is in good agreement with the light J-V curves. The reduction of EQE derives from increasing of light transmission in ultrathin Si film. It needs to be noted that the EQE of the CNT/Si solar cell 20-μm-thick Si films is higher than that of ultrathin Si cell with a thickness of 8.9 μm . As an indirect bandgap semiconductor, it needs large thickness for crystalline Si film to completely absorb the light. It indicates that the Jsc of CNT/Si solar cells is also affected by optical absorption in crystalline Si thin film. It might be the main reason for the significant drop of Jsc from 16.2 to 11 mA/cm2 when the thickness of Si thin film reduces from 20 to 5 μm.Figure 2d shows the reflection spectra of the Si thin film and CNT/Si solar cells, respectively. The reflectance of the Si thin film is almost the same as that of the starting Si wafer at visible region but higher than that at infrared region (>1,100 nm). Figure 2d also shows a sharp increase of the reflectance at the wavelength above approximately 1,100 nm, corresponding to the bandgap of Si and CNT/Si. The bandgap of Si thin film shows a blueshift slightly to the original Si wafer according to the reflection spectra. When the Si thin film is covered by the CNT film, the reflectance declined evidently, which indicates that CNTs can enhance the light absorption at visible region.
We also tested J-V curves of a CNT/Si thin-film solar cell under several bending and recovery cycles. An original CNT/Si solar cell (with an initial efficiency of η = 1.52%) is first bended to 30° and then recovered to original state. Figure 3d shows the light J-V curves under three bending-recovery cycles. For the first bending, the efficiency of the cell decreases from 2.5% to 1.4%. After the first bending, the light J-V curves almost overlap with each other in the three bending-recovery cycles, showing a good reliability of the flexible CNT/Si solar cells in bending and recovery cycles.
In summary, flexible mono-crystalline Si thin films with a thickness varying from 5 to 50 μm were fabricated by wet chemical etching of bulk wafers in a KOH solution. The flexible CNT/Si heterojunction solar cells are made at room temperature from the CNT films and mono-crystalline Si thin film. The efficiency of the CNT/Si thin-film solar cells depends on the surface and thickness of Si thin films, as well as the interface of heterojunction. The CNT/Si thin-film solar cells show an initial efficiency of approximately 3% to 5% and reaches to 6% by HNO3 doping. The CNT/Si thin-film solar cells show a good stability in bending-recovery cycles.
HS received his BS degree in Mechanical Engineering from Tsinghua University, China, in 2012. At the moment, he is pursuing his Master's Degree in Materials Science and Engineering at Tsinghua University. His research interests are in solar cells and nanomaterials.
JW received his BS and doctoral degree in Mechanical Engineering from Tsinghua University, China, in 1999 and 2004, respectively. He became a faculty member at the Department of Mechanical Engineering at Tsinghua University since 2006. He transferred to the School of Materials Science and Engineering at Tsinghua University in 2013. He has been working in the field of nanomaterials since 1998 and in the field of solar energy since 2006.
YJ received his BS and doctoral degree in Mechanical Engineering from Tsinghua University, China, in 2005 and 2011, respectively. Currently, he is a faculty member at the College of Materials Science and Engineering, Beijing University of Chemical Technology, China. His research interests are nanomaterials and solar energy.
XC received his BS degree in Mechanical Engineering from Tsinghua University, China, in 1999. At the moment, he is a technician working at the School of Materials Science and Engineering at Tsinghua University, China.
KW transferred from the Department of Mechanical Engineering to the School of Materials Science and Engineering at Tsinghua University in 2013. He is a professor and group leader of the Laboratory of Carbon Nanomaterials.
DW is a professor in the Department of Mechanical Engineering at Tsinghua University, China. He has been working in the field of carbon nanomaterials since 1992 and retired in 1999.
This work was supported by the National Natural Science Foundation of China (51172122), Beijing Nanotechnology Specific Project (Z121100001312002), Tsinghua University Initiative Scientific Research Program (20111080939), and Cooperation Project of Beijing Nova Program (XXHZ201204).
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