| Compared with silicon-based solar cells, thin film solar sells have many advantages, such as lightweight, low cost, flexible and could be used on the curved surfaces of many buildings and instruments, which makes it as a main and promising development direction of future solar energy application. CuInS2 (CIS) and relatedâ… -â…¢-â…¥2 chalcopyrite compounds have garnered considerable interest due to their myriad benefits, including large absorption coefficients, adjustable band gap, low toxicity and long-term stability. Hence, based on CuInS2 film as the absorber layer of solar cells, the studies were carried out in this paper as follows:In the first part, an in-situ growth of CuInS2 films on a Cu foil for flexible solar cell fabrication was developed by a facile solvothermal synthetic route, where the ethylene glycol was used as a solvent, InCl3·4H2O and thioacetamide were used as reactants, respectively. The CuInS2 film obtained at 180℃for 16 h by treating Cu foils in ethylene glycol solution containing InCl3·4H2O (0.025 M) and thioacetamide (0.05 M) in the autoclave is composed of a great quantity of potato chips shaped CuInS2 nanosheet arrays with a high density. These nanosheets are well aligned with a thickness of-200 nm, assembled and intermeshed with each other, forming a continuous net-like flat film. The effects of solvothermal temperature, time and concentration on the morphology and phase of CuInS2 films are investigated. Solvothermal temperature has no obvious effect on the morphology of the final films, but higher temperature is favorable for the growth of CuInS2 films with higher crystallinity. Reactant concentration plays a significant role in controlling the morphology of CuInS2 films; if InCl3·4H2O concentration is relatively low (<0.042 M), single-layered CuInS2 films can be produced, which are composed of high ordered potato chips shaped nanosheets, otherwise, it prefers to form a double-layered film, which the lower layer is similar CuInS2 ordered nanosheets while the upper layer is composed of ball shaped superstructures.A possible mechanism of the CuInS2 films is also investigated. The formation processes of the CuInS2 films occure via following steps:an initial nucleation at the Cu foil, the subsequent vertical growth, homogeneous nucleation in the solution, Ostwald ripening, self-assemble in the solution, and deposition on the Cu foil. From a chemical reaction point of view, the involved chemical reactions for the fabrication of the CuInS2 nuclei can be formulated as follows:Firstly, since Cu element on the surface of the Cu foil can be easily oxidized to Cu2+ ions at high temperature while oxygen was simultaneously reduced, Cu2+ ions are released from the Cu foil into the solution. Then, these Cu2+ ions are reduced to Cu+ ions by ethylene glycol which acts both as the reaction solvent and reductant (4Cu2-+HO-CH2-CH2-OHâ†'4Cu++4H++CHO-CHO). It has been revealed that thioacetamide can react with water to release H2S (CH3CSNH2+H2Oâ†'CH3CONH2+H2S), and then H2S decomposes to give S2- ions at the proceeding temperature (180℃) (H2Sâ†'2H-+S2-). With a continuous supply of Cu from the substrate. S2- from thioacetamide, and In3- from InCl3·4H2O, CuInS2 nuclei occurs via a heterogeneous process and then in-situ growth carries out on the Cu foil.The typical diffuse reflection spectra show that all these CuInS2 films present a strong and borad adsorption band ranging between 400 and 800 nm and possess direct band gap energy of 1.48 eV, which is appropriate for the absorption of solar spectra. Photocurrent density/voltage characteristics of the resulting solar cells were measured under standard AM 1.5 solar illumination with an intensity of 80 mW/cm2. The solar cell based on single-layered CuInS2 film exhibits open-circuit voltage (Voc) of 268 mV, short-circuit current density (Jsc) of 4.00 mA/cm2, and fill factor (FF) of 0.56, yielding an overall solar-to-electric energy conversion efficiency (n) of 0.75%. But, the conversion efficiency from the solar cells based on double-layered CuInS2 film is 0.33%.In the second part, firstly, the TiO2 layer was prepared on the conductive glass (the indium tin oxide ITO) via a roll to roll deposition technique. And then this conductive glass with TiO2 layer was solvothermally treated in ethylene glycol solution containing InCl3·4H2O, CuSO4·5H2O and thioacetamide, leading to the CuInS2 film in-situ growth on or in the TiO2 layer. Finally, the solar cell device with a structure of conductive glass/TiO2/CuInS2/P3HT/Au was made in which as-deposited CuInS2 serves a sensitizer onto mesoporous-TiO2 and P3HT acts as a hole conductor. Researches show that such hybrid solar cells have big positive spread current and small reverse current drift, suggesting the typical characteristics of p-n heterojunction, i.e., one-way electrical conductivity. Taking the sample obtained by treating the substrate in ethylene glycol solution containing InCl3·4H2O (0.03 M) for example, the as-fabricated solar cell exhibits open-circuit voltage (Voc) of 218 mV, short-circuit current density (Jsc)of 2.8 mA/cm2, and fill factor (FF) of 0.26, yielding an overall solar-to-electric energy conversion efficiency (n) of 0.2%. Further improvement of the conversion efficiency can be expected by optimizing the morphology, structure and composition of the CuInS2 films, as well as the device fabrication technique.In the third part, using copper wire (the diameter of-60μm) as substrate, we present a simple solvothermal route for in-situ growth of CuInS2 thin films on the flexible copper wire. It is found that the optimizing solvothermal conditions for copper wire circumventing breakage and cracks is treating the copper wire in an ethylene glycol solution at 60℃for 8 h in an autoclave. Research results suggest that the grown CuInS2 film has a good stability or a considerably strong adhered force with the Cu substrate. No obvious cracks were found in the following processes of making solar cells. The n-type junction partner CdS or ZnS was deposited on CuInS2 film by using a chemical bath approach. After that, three magnetron sputtering processes were utilized to deposit layers of intrinsic ZnO, transparent conductive indium tin oxide (ITO) and Ni-Al. The as-fabricated solar cell with a structure of Cu wire/CuInS2/CdS//-ZnO/ITO/Ni-Al exhibits big positive diffusion current, almost none reverse drift current, suggesting the typical characteristics of p-n heterojunction. Such a wire-like solar cell is lightweight, flexible and could realize the treatment of weaving, which widely broaden its applications, having attractive prospects in the future. |
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