Here we have reported the synthesis of Cu2O nanoparticles from copper acetate under microwave irradiation at a low power of 180 W, followed by calcinations at higher temperature of 500 °C, in the presence of three reducing agents viz oxalic acid, ascorbic acid and potassium hydroxide. Oxalic acid gave spherical-shaped aggregates of Cu2O nanoparticles which varied from 2 to 5 µm having the particle size less than 100 nm. Ascorbic acid also produced spherical-shaped nanoparticles, below 100 nm. Potassium hydroxide gave cotton ball-like structures of Cu2O nanoparticles, having particle size less than 50 nm. A current–voltage (I-V) performance of Cu2O nanoparticles, grapheme oxide (GO) and titanium dioxide (TiO2) nanoparticles-based bulk heterojunction cell was further investigated. Thin films of nanocomposites viz GO/Cu2O and GO/Cu2O/TiO2 taken in 1:1 and 1:1:1 ratios, respectively, were prepared. The highest efficiency of 1.41 % was observed in case of thin film of GO/Cu2O/TiO2 resulting from Jsc value of 3.83 mAcm−2, Voc value of 0.594 V and fill factor (FF) of 0.62.
Solar energy production is fast becoming a vital source of renewable energy being developed as an alternative to traditional fossil fuel-based sources of power. One of the primary challenges to the full-scale implementation of solar energy remains the expensive cost associated with the construction of photovoltaic modules and certain toxic elements in some thin-film solar cells. The principal photovoltaic (PV) material on the market today is silicon, and although silicon-based solar cells are comprised of a very abundant element, their large-scale production is hampered by the high cost of processing and refining, which sets the average electricity cost from a silicon solar cell well above that which comes from coal- or gas-burning power plants. Although there are solar cells reported with very high efficiencies (>25 %), these cells are all constructed in a laboratory-scale setting, often requiring rare or expensive materials and/or methods. If a material is truly to become a marketable option for the photovoltaic industry, some considerations must be given to the expense of its synthesis, manufacture, processing and construction into a device, in addition to its ultimate PV efficiency.
Among the inorganic materials proposed as a PV alternative to silicon, cuprous oxide (Cu2O) is one of the most extensively studied with investigations stretching back more than 20 years. Cuprous oxide (Cu2O) is a non-toxic p-type semiconductor with direct band gap of 2.0 eV, which makes it a promising material for photocatalysis [1–3], lithium ion batteries , magnetic storage media , gas sensors  and conversion of CO to CO2 . Several methods have been developed for the preparation of cuprous oxide, including thermal reduction, sonochemical reduction, metal vapor synthesis and chemical reduction [8–10]. TiO2, an n-type semiconductor with a bandgap energy of 3.0–3.2 eV, has also attracted a great deal of attention in solar energy applications due to its relatively high efficiency, easy fabrication, low cost, low toxicity and long-term stability [11–14]. It is established that Cu2O with absorption in visible light spectrum, associated with the favorable band energy alignment with TiO2, leads to an efficient pathway for charge transfer. Moreover, low cost of TiO2 and Cu2O provides the feasibility to build large-scale photovoltaic devices.
McFarland et al. created a Cu2O/TiO2 heterojunction thin film and observed a photoresponse in a photoelectrochemical cell . Li et al. prepared core–shell Cu2O/TiO2 solar cell with an efficiency ~0.01 % . Later on, Zainun et al. reported about Cu2O/TiO2 heterojunction solar cells made by using electrochemical deposition (ECD) . They demonstrated simple and low-cost solar cells using ECD methods, where the efficiencies were found to be lower (less than 0.1 %). The highest efficiency of ~2 % for Cu2O solar cells has been obtained by using the high-temperature annealing method and an expensive vacuum evaporation technique . Besides, efficient heterojunction solar cells of Cu2O with ZnO fabricated by electrodeposition and photochemical deposition methods have been also investigated and reported [19, 20]. A solar conversion efficiency of 2 % was reported for Cu2O–ZnO solar cells ; however, it is still one order of magnitude lower than the theoretical limit of Cu2O solar cells (20 %). Recently, graphene-based functional materials are being used as transparent window/counter electrodes, interface layers, hole/electron transport materials and as buffer layers to retard charge recombination in organic photovoltaics. It was revealed that the incorporation of graphene could increase the short-circuit current density and photoelectric conversion efficiency [21, 22]. There are several examples of Cu2O-based PV devices reported in the literature, often prepared by using low-cost, solution-based methods. [23–26].
In continuation of our work on the synthesis of metal nanoparticles for solar applications [27–29], here we report the synthesis of Cu2O nanoparticles using different reducing agents, viz oxalic acid, ascorbic acid and potassium hydroxide under microwave irradiation at lower energy, followed by calcinations at higher temperature. The nanoparticles so formed were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction pattern (XRD). In this study, we have also investigated the current–voltage (I-V) performances of Cu2O/TiO2/GO-based bulk heterojunction solar cells for their suitable application in solar cells.
Materials and methods
Copper acetate (99.0 %) was purchased from S. d fine –chem. Ltd. Oxalic acid (99.5 %), ascorbic acid (99.0 %) and potassium hydroxide were purchased from Merck. Doubled-distilled water was used throughout the experiment.
Synthesis of Cu2O nanoparticles using oxalic acid
Copper acetate and oxalic acid in 1:1 ratio were taken in diethylene glycol and mixed thoroughly. The mixture was subjected to vigorous stirring for about 30 min followed by sonication, until a clear solution was obtained. The resultant solution was subjected to microwave irradiation at low power of 180 W. It was observed that light gray-colored precipitate was formed within a minute. On cooling, the thick precipitate obtained was centrifuged, filtered, washed and dried in hot air oven at 60 °C for 4–5 h. The dried product was further calcined at 500 °C for 30 min in a muffle furnace. A brown-black-colored precipitate was obtained, which was again filtered and finally dried at hot air oven at 100 °C for 3–4 h.
Synthesis of Cu2O nanoparticles using ascorbic acid
1:1 ratio of copper acetate and ascorbic acid was taken in diethylene glycol and mixed thoroughly as mentioned earlier. The mixture was continuously stirred for about 30 min and then sonicated for 20–30 min. After sonication, the solution was kept in microwave oven at 180 W. After 2 min, a light brown-colored precipitate was observed which was then centrifuged, filtered, washed and dried in hot air oven. Finally, the dried product was subjected to heating at 500 °C for 30 min. After calcinations, a brown-black-colored precipitate was observed which was filtered. Finally, it was dried in hot air oven for 3–4 h.
Synthesis of Cu2O nanoparticles using potassium hydroxide
Copper acetate and potassium hydroxide were taken in 1:1 ratio in diethylene glycol, and the mixture was vigorously stirred for about 30 min followed by sonication until the mixture gives a clear solution. As mentioned earlier, the solution was subjected to microwave irradiation at 180 W which gave light gray-colored precipitate within a minute. The precipitate was centrifuged, filtered and dried and then calcined at 500 °C for 30 min in muffle furnace. A brown-black-colored precipitate so obtained was filtered, washed and dried for 3–4 h.
Preparation of thin films of Cu2O nanocomposites and current–voltage (I-V) measurements
To study the current–voltage performance, 1:1 and 1:1:1 ratios of thin films of nanocomposites of graphene oxide (GO)/Cu2O and GO/Cu2O/TiO2 were prepared. The mixtures were sonicated for 30 min followed by stirring at 60 °C for 1 h. Sonication was further continued for 3 h with simultaneous stirring. The resultant slurry was spread on indium tin oxide (ITO)-coated glass substrate by doctor blade method, and thin films obtained were dried at 50 °C–60 °C for 1 h in hot air oven. The photovoltaic measurements of thin films were taken using a PGSTAT 101 solar simulator with an irradiance of 100 mWcm−2. The current–voltage characteristics of the cell were measured by applying external potential bias to the cell and measuring the generated photocurrent. Parameters such as short-circuit photocurrent (Jsc), open-circuit photovoltage (Voc), fill factor (FF) and efficiency of the solar cell (h) were measured for all the thin films.
Results and discussion
Characterization of Cu2O nanoparticles
The formation of Cu2O nanoparticles from copper acetate has been carried out under microwave irradiation at a low power of 180 W, followed by calcinations at higher temperature of 500 °C, in the presence of three reducing agents viz oxalic acid, ascorbic acid and potassium hydroxide. In case of oxalic acid as reducing agent, the SEM image (Fig. 1) shows many spherical-shaped Cu2O nanoparticles having the particle size less than 100 nm. The diffraction peaks of Cu2O nanoparticles obtained from oxalic acid were observed at 2θ value of 36.6° and 59.8° (Fig. 2). The most intense peak was observed at 36.6°, which corresponded to the plane (111) and a small peak at 59.8° corresponded to the (220) plane matched with the literature value [30, 31]. Formation of Cu2O nanoparticles using ascorbic acid showed spherical-shaped nanoparticles as shown in SEM image (Fig. 3). The size of the nanoparticles was found to be below 100 nm. Figure 4 showed the diffraction peaks of Cu2O nanoparticles, obtained from ascorbic acid at 2θ values of 36.3° representing (111) plane. The SEM image in Fig. 5 shows cotton ball-like structures of Cu2O nanoparticles obtained from copper acetate and potassium hydroxide having size of the particles less than 50 nm. In Fig. 6, the XRD peak of Cu2O nanoparticles, obtained from potassium hydroxide, is observed at 2θ value of 37.1° representing (111) plane.
Current–voltage (I-V) performances of nanocomposites
To study the I-V performances of nanocomposites, Cu2O nanoparticles, obtained from oxalic acid, were used, as oxalic acid was found to give better yield of nanoparticles. Figure 7 shows the I-V curves of the thin films. The details of measured parameters are presented in Table 1. The table shows that GO as the best reference (type-a) gave a Jsc value of 3.21 mAcm−2, Voc value of 0.551 V and fill factor (FF) value of 0.43, resulting in a power conversion efficiency (PCE) value of 0.76 %. For GO/Cu2O (type-b) thin film, an increased PCE value of 1.05 % was observed, resulting from an enhancement of Jsc value (3.60 mAcm−2) and FF (0.51). However, its efficiency was still lower. It is well known that ion diffusion, which diffused in conduction band of TiO2, increases the transport speed of electrons in nanocrystalline TiO2 network and enhances the short current of solar cells. So, TiO2 was incorporated in order to increase the charge transfer. As seen, thin film of GO/Cu2O/TiO2 (type-c) showed the increase in Jsc value up to 3.83 mAcm−2, Voc value up to 0.594 V and FF of 0.62 which further increased the PCE value up to 1.41 %. The observed enhancement in the Jsc value could be attributed to the improved electron transport resulting from the increase in more number of nanoparticles. This indicated a higher mobility of charges resulting from both Cu2O and TiO2 nanoparticles induced into the GO layer . The highest PCE value obtained from type-c nanocomposite was also attributed to a stronger coupling effect due to the close distance between the Cu2O–TiO2 nanoparticles and the active layer of GO.
The authors have synthesized Cu2O nanoparticles using oxalic acid, ascorbic acid and potassium hydroxide as reducing agent without using any kind of capping agent. Reducing agent-mediated size and shape controlling of the Cu2O nanoparticles is the novelty of the present investigation. Nanocomposites of Cu2O with TiO2 and grapheme oxide have been synthesized taking different ratios by very simple method of mechanical stirring and ultrasonication. Current–voltage (I-V) performance of Cu2O nanoparticles-based bulk heterojunction cell was further investigated. After doping with graphene oxide, the I-V graphs of Cu2O/TiO2/GO exhibited more efficiency. Thus, the increase in the efficiencies may due to the role played by graphene oxide.
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Cite this article
Debbarma, M., Sutradhar, P. & Saha, M. Synthesis of Cu2O nanoparticles and current–voltage measurements (I-V) of its nanocomposites. Nanotechnol. Environ. Eng. 1, 6 (2016). https://doi.org/10.1007/s41204-016-0006-3
- Cu2O nanoparticles
- Graphene (GO)
- I-V performances