Carbon Nanotube-Silicon Nanowire Heterojunction Solar Cells with Gas-Dependent Photovoltaic Performances and Their Application in Self-Powered NO2 Detecting
© The Author(s). 2016
Received: 31 March 2016
Accepted: 1 June 2016
Published: 14 June 2016
A multifunctional device combining photovoltaic conversion and toxic gas sensitivity is reported. In this device, carbon nanotube (CNT) membranes are used to cover onto silicon nanowire (SiNW) arrays to form heterojunction. The porous structure and large specific surface area in the heterojunction structure are both benefits for gas adsorption. In virtue of these merits, gas doping is a feasible method to improve cell’s performance and the device can also work as a self-powered gas sensor beyond a solar cell. It shows a significant improvement in cell efficiency (more than 200 times) after NO2 molecules doping (device working as a solar cell) and a fast, reversible response property for NO2 detection (device working as a gas sensor). Such multifunctional CNT-SiNW structure can be expected to open a new avenue for developing self-powered, efficient toxic gas-sensing devices in the future.
There is a growing requirement of lightweight, energy-saving, integrated devices for various wearable and portable electronic applications. In particular, multifunctional devices combining energy generation and other specific function are highly desired in many applications. To this end, functionalized piezotronic generators and solar cells with additional properties, such as wind speed , UV  and pressure  detecting, have been developed accordingly. In recent years, carbon nanotube (CNT)-silicon heterojunction structure has been considered as one of facile and promising designs for many applications [4–6]. In this structure, CNT films are assembled with silicon wafer to form heterojunction, in which the built-in electric field is generated and driving charge carriers to electrodes. Particularly, owing to its potential prospect for developing high-efficiency and low-cost solar cells, the heterojunction structure has attracted great research interest in both elucidating its working mechanism [7, 8] and improving its power conversion efficiency (η) [9–15]. Some key points in this structure are crucial to the junctions’ performance, including properties of carbon materials (sheet resistance, light transmissivity, etc.), substrate wafer (resistance, energy band gap, light transmissivity, etc.), and their interface (contact state, thickness). To improve the cell efficiency, different strategies focusing on those points have been proposed accordingly. Firstly, chemical doping with volatile oxidants [9–11, 14] was applied to CNT membranes to adjust their Fermi level and carrier concentration. Secondly, an insulator layer (SiOx) with a suitable thickness was introduced at the interface of CNT membrane and silicon wafer to form a metal-insulator-semiconductor (MIS) junction [11, 15]. The existence of this insulator layer significantly suppressed carrier recombination and improved the diode ideality factor of CNT-Si junction. Moreover, surface anti-reflection treatments (PDMS , TiO2 , and MoOx  coating) were also efficient to enhance the light absorption of CNT-Si devices. In addition, silicon nanowire (SiNW) arrays with their unique one-dimensional aligned structure and outstanding electrical properties have exhibited excellent light trapping and carrier-transporting performances . In our previous work, SiNW arrays have been used to assembly heterojunction solar cells with CNT membranes. Electrolyte was also used to fill into the pores between SiNW arrays and CNT membranes to provide additional channels for charge transport . Thus, the cell efficiency increased from 0.092 to 1.29 % after electrolyte infiltration. However, without extra-encapsulation, cell stability was poor due to the gradual evaporation of electrolyte.
In this work, we utilize SiNW arrays to fabricate heterojunction with CNT membranes for reducing light reflection, followed by NO2 gas doping to increase cell performances. In this structure, the large specific surface area of both SiNW arrays and CNT membranes, together with additional channels built by their point-to-line contact, can greatly facilitate gas adsorption and desorption, which will further improve cell performances. To take full advantages of this structure, we also demonstrate the gas-sensitive properties of CNT-SiNW heterojunction structure. Compared with traditional metallic oxide gas sensors working at relatively high temperatures, this CNT-SiNW gas sensor works at room temperature. Particularly, it combines the functions of solar cell and gas sensor, as a result, this CNT-SiNW gas sensor is self-powered (by light), which is more energy-efficient and safer, especially for explosive gases detecting.
Chemical Etching for SiNW Arrays
SiNW arrays used in this study were prepared by a Ag-assisted etching method. n-type (100) silicon wafers with the electrical resistivity of 2~4 Ω cm were cleaned with acetone, ethanol, and piranha solution (H2O2 and H2SO4), followed immersing into HF and AgNO3 mixture solution for 15 min to fabricate SiNW arrays. After that, the as-prepared SiNW arrays were rinsed in deionized water and treated with HF and HNO3 to remove dendrite silver films covered onto SiNW arrays. The height of SiNWs was 300 nm. Then a Ti/Au layer (50 nm) was deposited on the back side of SiNW arrays.
Synthesis of CNT Membranes
High-quality CNT membranes were synthesized by a chemical vapor deposition (CVD) method using xylene as carbon source, ferrocene, and sulfur as catalyst precursor, respectively. The reaction temperature was set at 1160 °C, and CNT membranes were collected onto a piece of nickel foil at the downstream of quartz tube reactor.
Assembly and Test of CNT-SiNW Solar Cells
The as-prepared spiderweb-like CNT membranes were directly lifted up and transferred onto SiNW arrays to construct CNT-SiNW heterojuction devices. The active area for each solar cell was 0.24 cm2. In order to investigate the effects of NO2 modification on cell performances, CNT-SiNW solar cells were sealed into a quartz chamber with a window for light illumination from a solar simulator. The light intensity in the quartz chamber was 80 mW/cm2, which was calibrated by a silicon solar cell. The temperature in the quartz chamber was kept at room temperature. The flow rates of NO2 and N2 in the quartz chamber were adjusted by two mass flow controllers, respectively. Thus, the NO2 concentration could be finely controlled from 0 to 1000 ppm. The I-V data of CNT-SiNW solar cells were recorded by a Keithley 2601 digital source-meter.
Test of CNT-SiNW Gas-Sensing Properties
To test gas-sensing properties, CNT-SiNW solar cells were sealed into the same quartz chamber with some appropriate changes in the devices and equipment. First, a cold light source (LED) was used here to replace the solar simulator. Thus, the thermal effect of solar light source on cell performance could be eliminated. Second, SiNW arrays used to fabricate solar cell here had been stored in air for more than half a year to form a stable interfacial oxide layer. In this case, the non-reversible interfacial oxide layer effect could be avoided during gas-sensitive testing process. Third, the exhaust gas of the quartz chamber was evacuated by a vacuum pump. The V-t data of CNT-SiNW gas sensors were also recorded by the Keithley 2601 digital source-meter.
Results and Discussion
Characteristics of the CNT-SiNW solar cells under AM1.5G, 80 mW/cm2 illumination treated at different NO2 concentrations and exposure time
0 ppm (N2)
NO2 (10 ppm, 30 min)
NO2 (1000 ppm, 30 min)
NO2 (1000 ppm, 60 min)
In summary, we directly assembled CNT membranes with SiNW wafer to form heterojunction for solar cells and gas sensors. The CNT-SiNW heterojunction showed a gas-dependent photovoltaic effect. Thus, the power conversion efficiencies of CNT-SiNW solar cells are up to 8.4 % after NO2 gas doping. The CNT-SiNW heterojunction also demonstrated a self-powered gas detection sensitivity at room temperature. This CNT-SiNW heterojunction-based gas sensor will lead to much more sensitive and simple carbon-based gas sensors in the future.
This work is supported by National Natural Science Foundation of China (No. 51202119 and 51472019) and Tsinghua University Initiative Scientific Research Program.
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