pubs.acs.org/NanoLett
Carbon Nanotube and CdSe Nanobelt Schottky Junction Solar Cells Luhui Zhang,† Yi Jia,‡ Shanshan Wang,† Zhen Li,‡ Chunyan Ji,† Jinquan Wei,‡ Hongwei Zhu,‡ Kunlin Wang,‡ Dehai Wu,‡ Enzheng Shi,†,§ Ying Fang,§ and Anyuan Cao*,† †
Department of Advanced Materials and Nanotechnology, College of Engineering, Peking University, Beijing 100871, People’s Republic of China, ‡ Key Laboratory for Advanced Materials Processing Technology and Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China, and § National Center for Nanoscience and Technology, 11 Beiyitiao Street, Zhongguancun, Beijing 100190, People’s Republic of China ABSTRACT Developing nanostructure junctions is a general and effective way for making photovoltaics. We report Schottky junction solar cells by coating carbon nanotube films on individual CdSe nanobelts with open-circuit voltages of 0.5 to 0.6 V and modest power-conversion efficiencies (0.45-0.72%) under AM 1.5G, 100 mW/cm2 light condition. In our planar device structure, the CdSe nanobelt serves as a flat substrate to sustain a network of nanotubes, while the nanotube film forms Shottky junction with the underlying nanobelt at their interface and also makes a transparent electrode for the device. The nanotube-on-nanobelt solar cells can work either in front (nanotube side) or back (nanobelt side) illumination with stable performance in air. Our results demonstrate a promising way to develop large-area solar cells based on thin films of carbon nanotubes and semiconducting nanostructures. KEYWORDS Solar cell, Schottky junction, carbon nanotube, CdSe nanobelt, photovoltaics
D
evelopment of nanomaterial-based photovoltaic devices could potentially lead to inexpensive energy sources with enhanced power-conversion efficiency.1 To this end, many semiconducting nanomaterials and innovative structures have been explored, such as core-shell Si nanowires, CdSe/CdTe nanorods, polymer/ ZnO hybrid nanowire, nanowelded single-walled nanotubes, and three-dimensional nanopillar arrays.2-6 Formation of a junction structure by controlled doping or combining different semiconductors (e.g CdTe/CdS) is essential for promoting charge separation and making photovoltaics with improved performance. CdSe and carbon nanotubes (CNTs) are considered as two most important nanomaterials with interesting electronic and photonic properties.7 Recently, CNTs have been incorporated into polymer solar cells as fillers or transparent conductive electrodes owing to their high carrier mobility and the ability to create large interface area with polymer matrix.8,9 Most intriguingly, CNTs can form Schottky contacts or heterojunctions with many conventional semiconductors, such as Si, GaAs, and ZnS, with evident rectifying behavior and photovoltaic effects.10-13 Solar cells based on heterojunctions of CNTs and n-type Si wafer have shown efficiencies up to 7.4%.11 CdSe nanostructures have been studied intensively owing to their merits of suitable bandgap (1.74 eV) for visible light absorption, tailored structure and morphology, and potential applications in photovoltaic and optoelectronic devices.14-20
For example, colloidal CdSe and CdTe nanorods are constructed into bilayer nanocrystal solar cells with power conversion efficiencies approaching 3%.3 In addition, photoelectrochemical solar cells incorporating CdSe and CdS quantum dots have been studied.14,15 Perhaps the most appealing property is that CdSe nanoparticles can be grafted onto CNTs to form hybrid structures with tunable morphology and strong electronic interaction.21-24 In the CdSe-CNT hybrid, effective charge transfer process occurs under light excitation, resulting in electron injection from CdSe to CNTs and a decrease of hole concentration in the CNT network.24 Electron transfer has been observed in similar hybrid systems (e.g., CdS-CNT).25 This electron flow establishes a built-in electric field across the CdSe-CNT interface, which is the central process in producing photovoltaic effects. Here, we utilized the interface between CdSe and CNTs to construct effective Schottky junctions and solar cells with efficiencies up to 0.72%. Although electronic interaction between CdSe and CNTs has been studied extensively, solar cells based on this type of hybrids have not been fabricated yet. We synthesized CdSe nanobelts and transferred thin CNT films onto the nanobelt surface to make solar cells in planar configuration in which the CNT film also serves as transparent electrode. Control experiments show that the presence of CdSe-CNT junction is essential for generating photovoltaic effects. We synthesized neat, macroscopic nanobelts with widths of 50 to 150 µm and lengths of a few millimeters, while keeping long-range structural order by chemical vapor deposition (CVD) method (Figure 1a-c). Previously, various CdSe structures such as nanowires, nanosaws, and nanobelts have
* To whom correspondence should be addressed. E-mail:
[email protected]. Received for review: 05/28/2010 Published on Web: 08/17/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl101888y | Nano Lett. 2010, 10, 3583–3589
FIGURE 1. Growth and characterization of CdSe nanobelts. (a,b) SEM images of individual, thin, flexible CdSe nanobelts grown from a silicon substrate coated with a gold catalyst film. (Inset in b) A flexible section of CdSe nanobelt with thickness of 300 at 1 V, ideality factors (N) in the range of 3 to 5, and series resistances of 100 to 300 Ω · cm2. In comparison, p-n junction made of a core-shell Si nanowire has a N-value of 4.52, which can be further improved to N ) 1.96 by inserting an intrinsic layer.2 The N-value of CNT-CdSe diodes are slightly larger than CNT-Si heterojunction solar cells (N ) 2.62-3.68).11 We think the series resistance mainly comes from the undoped CdSe nanobelt that has limited carrier mobility. Appropriate doping of CdSe (e.g., by Cd, In) could potentially increase the electron mobility28,29 and further reduce the series resistance. The solar cells are tested in the light under AM 1.5G condition at a calibrated intensity of 100 mW/cm2. The cells can be illuminated from both front and back sides. In front illumination, incident light passes through the porous CNT film and reaches the junction area. From the back side, light directly reaches the CdSe nanobelt in which electron-hole pairs are produced. A typical solar cell in front illumination 3585
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FIGURE 2. Fabrication and characterization of CdSe and CNT Schottky junctions. (a) Schematic illustration of a nanobelt (deposited on a glass substrate) covered by a CNT film on one end and contacted by Al electrode on the other end. Arrows show the flow direction of charge carriers (holes and electrons) when the device is illuminated. The overlapped area between the CNT film and CdSe nanobelt makes the interface for hole-electron separation and acceleration toward different directions. (b) Band diagram of the CNT-CdSe interface shows the formation of Schottky junction at contact. (c) Optical image of a CNT-CdSe-Al device consisting of a 3 mm long nanobelt, a transparent CNT film covering about two-thirds of the nanobelt length, and a 100 nm thick Al electrode on the left. The dashed line shows the edge of CNT film, which covers the whole area on the right side of dashed line. The CNT film tends to bunch slightly to form dark strands during transfer, however, most of the overlapped area between the CNT film and CdSe nanobelt is uniform. (d) Microscopic illustration of the interface area between the CNT film and the nanobelt, where holes are directed along the CNT network and electrons are transported through CdSe to the other side. (e-g) SEM images of the device showing a highly pure, uniform CNT network coated tightly around the CdSe nanobelt on the top surface (e) and near the nanobelt side edges (f,g), as indicated in (c).
efficiency (η) of 0.59% (Figure 3b). The current density is calculated based on the junction area (0.59 mm2) where the
shows an open-circuit voltage (Voc) of 0.58 V, a short-circuit current density (Jsc) of 3.5 mA/cm2, and a power conversion © 2010 American Chemical Society
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FIGURE 3. Current-voltage characteristics of CNT-CdSe junction solar cells. (a) Dark J-V curve of a typical cell showing rectifying characteristics. (b) J-V curves recorded when the cell is illuminated from the front (CNT film) or back (CdSe nanobelt) sides, with Voc of 0.58 and 0.57 V, Jsc of 3.5 and 2.9 mA/cm2, and efficiency of 0.59 and 0.49%, respectively. (c) J-V curves of the same cell tested in (b) and recorded in air during a time period of 10 days. Inset, this cell maintains an efficiency of >0.45% over the time. (d) A summary of cell efficiencies in front and back illuminations for devices with different junction areas. Inset, plot of the photocurrent as a function of CNT-CdSe junction area.
FIGURE 4. Diverse cell structures and comparison. (a) Illustration of seven different structure solar cells in which the CdSe nanobelts are contacted by electrode pairs such as CNT-Al, CNT-ITO, CNT-Ag, CNT-Au, Au-Al, Ag-Al, Al-Al, respectively. (b) J-V characteristics of solar cells with CNT-Al, CNT-ITO, CNT-Ag, CNT-Au electrodes. (c) I-V curves of devices with Ag-Al and Au-Al electrode pairs showing negligible Voc and Isc under 1-sun illumination. (d) I-V curves of a CdSe nanobelt with Al electrodes on both ends, showing more than 4000 times increase in the current flow under incident light with intensity of 100 mW/cm2 (versus in the dark).
area (0.72 mm2) for back-side efficiency. In this way, the lower-bound efficiencies for the device in Figure 3b are 0.48 and 0.42% in front and back illuminations, respectively. Currently, the efficiencies of our CdSe-CNT Schottky junction solar cells are relatively lower compared with previously reported structures based on CdSe-CdTe or CdS-CdTe heterojunctions (3-6%),3,6 photoelectrochemical cells sensitized by CdSe quantum dots (up to 4.2%),14,15 and CdSe nanostructure-polymer hybrids (1-3.6%).16-19
CdSe surface is covered by the CNT film and charge separation takes place. Back illumination results in a similar Voc of 0.57 V and a slightly reduced efficiency of 0.49%. In back illumination, a small portion of CdSe nanobelt on top of the Al electrode is blocked from incident light, resulting in reduced photoconductivity near that position. Since the photocurrent also generated outside of the junction area, we also used the entire nanobelt area (0.79 mm2) to calculate the front-side efficiency, and the total area minus Al contact © 2010 American Chemical Society
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The CdSe-CNT solar cells show excellent environmental stability over a long period. When they are stored in air for ten days, the cell efficiency remains above 0.45% and has gradually increased to 0.64% in 10 days (Figure 3c). The slight increase of efficiency might be related to the hole-doping of CNTs by oxygen adsorption. This result shows that the physical contact between CNT films and CdSe nanobelts is very stable, as evidenced by our SEM characterization (Figure 2e). By controlling the nanobelt length covered by the CNT film, the CNT-CdSe junction area can be changed in the range of 0.04-0.6 mm2. Solar cells with different junction areas show efficiencies of 0.46-0.72% in front illumination, and the efficiency remains stable upon 15 times increase of the junction area (Figure 3d). If the entire CdSe nanobelt area is used for calculation, the front-side cell efficiencies are in the range of 0.28-0.48% (Supporting Information Table S1). Consistently, all devices show higher efficiency in front illumination in which incident light passes through the CNT film. The change of current density (rather than open-circuit voltage) has caused different efficiencies in the front or back illumination conditions. Furthermore, the measured photocurrent increases from 1 to 20 µA with increasing junction area, suggesting that the junction area directly contributes to the device performance (inset of Figure 3d). Our solar cell structure offers great flexibility in choosing metal electrodes to serve as the counterpart to CNT films. We have replaced the Al electrode with several other conventional metals such as Au, Ag, and indium tin oxide (ITO), as illustrated in Figure 4a. These solar cells show Voc values of 0.44-0.57 V and efficiencies of 0.15-0.48% under the same illumination condition (Figure 4b). In comparison, the cell with the Al electrode shows the highest Voc (0.61 V) and efficiency (0.71%). The results indicate that as long as the CdSe-CNT Schottky junction is established at one end of the nanobelt, active cell devices can be made by using metal electrodes with different work functions at the other end. Metals with lower work functions (Al, Ag) tend to form ohmic contact with CdSe nanobelts, and solar cells containing these metal electrodes also show higher Voc (>0.55 V) and efficiencies (>0.45%) than devices using higher work function materials (Au, ITO) (Voc < 0.5 V and η < 0.25%) (Figure 4b). In addition, control experiments were carried out based on different electrode pairs such as Ag-Al and Au-Al to contact CdSe nanobelts at two ends. These devices show very small Jsc and Voc values under illumination (Jsc < 0.1 mA/ cm2, Voc < 0.14 V), indicating that connecting a CdSe nanobelt by asymmetric electrodes does not necessarily result in an active solar cell (Figure 4c). With symmetric electrodes (Al) at both ends, the CdSe nanobelt shows remarkable increase (>4000 times) in the current flow when placed in the light (versus in the dark) (Figure 4d). Enhanced photoconductivity of CdSe nanobelts is a favorable factor for fast charge carrier transport in solar cells. The results of control experiments indicate that the presence of a CNT film, © 2010 American Chemical Society
and the resulting CNT-CdSe Schottky junction is therefore the essential component of our solar cells. To summarize, we demonstrated Schottky junction solar cells by coating CNT films on individual CdSe nanobelts, with open-circuit voltages of 0.5 to 0.6 V. Although the cell efficiency (
