Inorganic Solar Cells Based on Electrospun ZnO Nanofibrous Networks and Electrodeposited Cu2O
© Zhang et al. 2015
Received: 13 October 2015
Accepted: 23 November 2015
Published: 1 December 2015
The nanostructured ZnO/copper oxide (Cu2O) photovoltaic devices based on electrospun ZnO nanofibrous network and electrodeposited Cu2O layer have been fabricated. The effects of the pH value of electrodeposition solution and the Cu2O layer thickness on the photovoltaic performances have been investigated. It is revealed that the pH value influences the morphology and structure of the Cu2O layer and thus the device performances. The Cu2O layer with an appropriate thickness benefits to charge transfer and light absorption. The device prepared at the optimal conditions shows the lowest recombination rate and exhibits a power conversion efficiency of ~0.77 %.
Inexpensive solar cells that can be synthesized from solutions on various low-cost substrates are of particular interest for distributed electricity generation [1–3]. A series of advantages such as material abundance, low toxicity, and high stability are identified for these “ultra-low-cost” cells [4, 5]. Obviously, all-oxide photovoltaics have these potential advantages. For example, copper oxide (Cu2O) has been recognized as one of the promising photovoltaic materials due to its abundance, high absorption coefficient, low-cost fabrication, and high theoretical power conversion efficiency (PCE) of ~20 % . In fact, various techniques such as electrodeposition, sputtering, and thermal oxidation of metallic Cu sheet were once used for fabricating Cu2O films for photovoltaic devices [7–9]. Among them, electrodeposition is easily down-scaled and can produce extremely uniform films on conductive substrates, allowing an attractive potential to synthesize inexpensive Cu2O photovoltaics on a variety of supporting substrates with minimal energy input . To date, the Cu2O photovoltaic layers were used in a number of heterojunction solar cells [11–16].
ZnO as a wide-bandgap semiconductor (approximately 3.3 eV) with high electron mobility has been found to be the most stable and efficient. Fortunately, ZnO nanostructures with different morphologies can be cheaply synthesized by solution method [17–19]. A combination of Cu2O and ZnO to fabricate heterojunction solar cells has thus been receiving attention recently. Nevertheless, ZnO/Cu2O heterojunction solar cells fabricated by magnetron-sputter deposition only show a PCE of 0.24 % . Solution-processed 3D ordered Al-doped ZnO (AZO)-ZnO/Cu2O solar cells based on patterned ZnO nanorod arrays and electrodeposited Cu2O films demonstrate a maximum PCE of 1.52 % . These exciting results indicate that one can improve the performance of ZnO/Cu2O solar cells by taking advantage of nano-heterojunctions. A high efficiency of 6.1 % was reported for Cu2O-based heterojunction solar cells prepared by thermally oxidizing copper sheets . With respect to the cells based on high-energy-cost copper oxidation, the electrodeposited and sputtered ZnO/Cu2O cells show worse performances. The important reason is the lower minority carrier transport length and shorter collection length for photo-generated charges in the Cu2O layer.
In order to enhance the interfacial areas in ZnO/Cu2O structure and reduce the minority carrier collection length, ZnO nanostructures were once extensively used [18, 20–22]. However, the efficiency of the nanostructure cells is lower than that of the best bilayer planar cells. This is due to the incompatibility between the short length required for good charge collection and the longer length for the formation of full built-in potential (V bi) for inhibition of recombination . In order to balance the charge collection and conformation of V bi length, it is necessary to explore new strategy to engineer the ZnO/Cu2O interface and nanostructures.
Electrospinning technique provides a simple, cost-effective, and template-free process with potentials to prepare various materials with one-dimensional (1D) nanostructures for large-scale production [22, 24]. High-surface-area 1D nanofibrous networks have the potentials to improve the performances of solar cells because of increased interface area and improved charge carrier collection. The as-prepared electrospun nanofibers were already used in various solar cells [19, 25]. To our best knowledge, implication of ZnO nanofibers (ZnO-NFs) in ZnO/Cu2O solar cell devices has not yet been reported so far. On the other hand, an open-circuit voltage (V oc) of 1.2 V was reported for Cu2O device fabricated by oxidizing Cu sheets in a controlled atmosphere , while the reported V oc for electrodeposited bilayer cells only ranges from 0.19 to 0.59 V [9, 27, 28]. These results suggest that the V oc may be sensitive to the electrodeposition processes. In order to improve the performance of cells based on ZnO nanostructure and electrodeposited Cu2O, the parameters in the electrodeposition process needs be optimized.
Based on these considerations, inorganic solar cells with electrodeposited Cu2O layer and electrospun ZnO nanofibrous networks are fabricated in this work. Here, ZnO nanofibrous networks are employed to electrodeposit Cu2O, so that effective radial junctions across the interfaces can be formed, benefiting to rapid charge separation and transport [21, 29]. The key issue here is to design a continuous nanofiber heterojunction with a sufficient interfacial area for the efficient charge transfer. Though the effect of pH value on the performance of ZnO/Cu2O solar cells has been reported previously [30, 31], the electrodeposition processes impact the electrical performance significantly. Thus, the effects of the pH value and the duration on the performances of ZnO-NFs/Cu2O and relative mechanism have been further investigated. Subsequently, we investigate the performances and underlying device physics of these inorganic solar cells based on the ZnO nanostructures. It is found that the recombination rate decreases dramatically at the optimized process and an efficiency of 0.77 % is demonstrated.
Materials and Methods
The pH value of electrodeposition solution was adjusted with saturated NaOH aqueous solution from 8.0 to 13.0. The deposition temperature was fixed at 60 °C. The deposition voltage was set at −0.3 V versus Ag/AgCl reference electrode. The thickness of Cu2O layer was controlled by the growth time, which was changed from 10 to 60 min. In order to reduce interfacial defects and improve the interfacial contact between the ZnO nanofibrous networks and Cu2O layer, ZnO powder was added to the buffered solution to protect the ZnO from etching during the electrodeposition . After the Cu2O deposition, the as-prepared samples were rinsed thoroughly with deionized water and dried in air and then annealed in a glove-box filled with high-purity N2 at 100 °C for 120 min. Finally, an Ag electrode was evaporated on the sample surface through a shadow mask under a vacuum of 10−4 Pa. To this stage, the solar cells were fabricated with the standard in-plane size of 2 mm × 4 mm. The morphology, dimensions, and crystallinity were characterized by transmission electron microscopy (TEM, F2010, Japan), scanning electron microscopy (SEM, JEOL 5700, Japan), and X-ray diffraction (XRD, X’Pert PRO, Cu Ká radiation), respectively.
The photovoltaic performances of these inorganic solar cell devices were characterized using a Keithley 2400 source meter under an illumination of 100 mW cm−2 (Newport 91160, 150 W solar simulator equipped with an AM1.5G filter). The radiation intensity was calibrated by a standard silicon solar cell (certified by NREL) as the reference. The external quantum efficiency (EQE) and the UV-vis absorption spectra were measured using a standard EQE system (Newport 66902). The electrochemical impedance spectroscopy (EIS) measurements in the device configuration were performed on the Zahner Zennium electrochemical workstation in the dark. A 30 mV AC sinusoidal signal source was employed over the constant bias with the frequency ranging from 1 Hz to 4 MHz. The obtained impedance data were fitted with a proper equivalent circuit by Scribner Associate Z-View software. The Mott-Schottky (M-S) measurements were carried out by a standard electrochemical measurement at a 1-kHz frequency with AC amplitude of 20 mV in a 0.1 M Na2SO4 solution in dark. A Pt foil serves as the counter electrode, and the Ag/AgCl (statured with 1 M KCl solution) electrode was used as reference electrode.
Results and Discussion
We have fabricated the cells with planar ZnO structure instead of ZnO nanofibrous networks for comparison. Additional file 1: Figure S2 shows the J-V curves for the reference cell and the present cell. The reference device yields a J sc of 3.17 mA/cm2, a V oc of 0.316 V, a FF of 0.328, and a PCE of 0.328 %. It can be seen that the ZnO nanofibrous networks do benefit to higher J sc and thus higher PCE. It was reported that alkaline solution for depositing Cu2O does not erode the ZnO-NFs . The higher-than-optimal pH value yields the lower PCE, mainly attributed to the decreases in V oc and FF. It is known that V oc is not only controlled by the band alignment of Cu2O and ZnO-NFs but also the heterojunction quality [22, 38, 44].
Subsequently, we compare the Nyquist plots for the cell at fixed bias voltage of −0.3 V (close to V oc) and the results are presented in Fig. 13b. The solid lines in Fig. 11a, b are the fits of experimental data using the model in the panel of Fig. 13d. For more accurate fitting, the CPE is used instead of the ideal capacitance C to account for spatial inhomogeneities that can be induced by defects and impurities at the interface. The measured Nyquist plots can indeed be fitted well. Figure 13c shows the fitted values of R rec as a function of the growth time. By fitting the curve with the equivalent circuit, the parameters for ZnO-NFs/Cu2O junction are extracted. As shown in Fig. 13c, the value of R rec increases in the first 30 min, and then decreases. It indicates that the recombination rate decreases upon the increasing growth time in the first stage because the recombination rate is inversely proportional to R rec . This will benefit for the charge transfer. When the growth time is longer than 30 min, the recombination rates increase. This result is consistent with the variation tendency of the V oc and FF as a function of the growth time. These results confirm the fact that the growth time of ~30 min is the optimized one for the cells. Compared with the other cells, the better performance can be attributed to the effective charge transfer between the ZnO nanofibrous networks and Cu2O films and the higher absorption . Figure 13d shows the equivalent circuit, where R s is included due to contacts and wire.
A series of inorganic solar cells based on electrospun ZnO nanofibrous networks and electrodeposited Cu2O films have been fabricated. We have demonstrated that controlling the pH value and growth time during the deposition of Cu2O layers can improve the performance of the ZnO-NFs/Cu2O cells significantly. The underlying mechanism is intrinsically related to the increased light absorption and balanced charge transfer at the ZnO-NFs/Cu2O heterojunctions. The measured PCE value of the as-prepared ZnO-NFs/Cu2O inorganic solar cell reaches up to 0.77 %.
We acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51431006, 51203045, 61271127, 51472093, 21303060,61574065), Program for Pearl River Star (Grant No. 2012 J2200030), International Science & Technology Cooperation Platform Program of Guangzhou (No. 2014 J4500016), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), and Program for Changjiang Scholars and Innovative Research Team in University (IRT13064).
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