RSC Advances PAPER View Article Online View Journal | View Issue Study on the efficient PV/TE characteristics of the self-assembled thin films based on bismuth Cite this: RSC Adv.,2017,7,6735 telluride/cadmium telluride Vivekanandan Raman,a Dinah Punnoose,a Pari Baraneedharan,b Sunkara Srinivasa Rao,a Chandu V. V. M. Gopi,a Venkatesh S,c S Brahadeeswarand and Hee-Je Kim*a Solar radiation has a spectral distribution ranging from short wavelengths (200–800 nm, visible part) to long wavelengths (800–3000 nm, heat part). Cadmium telluride and bismuth telluride are well known photovoltaic (PV) and thermoelectric (TE) materials, respectively. CdTe converts light energy into electricity and Bi2Te3 converts heat into electricity. To effectively use the entire solar spectrum for energy conversion, a new type of solar cell based on a Bi2Te3/CdTe composite in a core/shell structure was designed and prepared using a wet chemical method. X-ray diffraction and high-resolution Creative Commons Attribution 3.0 Unported Licence. transmission electron microscopy confirmed the formation of Bi2Te3/CdTe in a core/shell structure with Received 11th November 2016 high crystallinity. Bi Te nanoparticles function as built-in nanoscale electron generators to convert heat Accepted 29th December 2016 2 3 into electricity and CdTe functions as a photovoltaic cell. The efficiency of the thin film solar cell device DOI: 10.1039/c6ra26638j was found to be 2.5% at room temperature and 4.8% when exposed to sunlight. The combined PV and www.rsc.org/advances TE modules resulted in an overall power conversion efficiency of 4.8%. 1. Introduction Seebeck coefficient and can generate electricity, even at room temperature.2 Considerable efforts have been made to combine This article is licensed under a Solar energy is the cleanest and the most abundant renewable both PV and TE technologies to maximally utilize the entire energy source available. CdTe, CuIn(Ga)Se2, and c-Si are solar spectrum. These hybrid technologies that combine the advanced materials that are used as absorber materials in solar modules of PV and TE cannot solve the problems of solar energy cells.1 Although these materials have high efficiency, the lack of synergy utilization at the material level because the functions of Open Access Article. Published on 20 January 2017. Downloaded 10/3/2021 5:00:10 AM. abundance, high toxicity levels, and high production costs are PV and TE modules are independent of each other. On the other problems that have prompted researchers to search for new hand, it will be benecial if both the PV and TE functions can be materials and/or newer and easier synthesis routes. Therefore, achieved in a single material. If this single material can be continuous efforts are being made to nd new, cost-effective prepared in a nanostructure, it will be more efficient. Nano- photovoltaic materials with good conversion efficiency. Solar particles of CdTe have good photoluminescence properties and radiation has a spectral distribution from the short wavelength have been used in the production of solar cells. CdTe nano- range, 200–800 nm (light part), to the long wavelength range, particles are prepared via aqueous and non-aqueous synthesis 800–3000 nm (heat part). Better control of light scattering and routes. Compared to non-aqueous synthesis, aqueous synthesis electronic transport through a semiconductor is crucial for is more reproducible, cheaper, and less toxic. In addition, the 3 improving the solar efficiency. Photovoltaic cells (PV), such as prepared samples are more water-soluble and bio-compatible. CdTe, can only convert the short wavelengths of solar energy To prepare nanorods, a seed-mediated growth technique was into electricity and the remaining long wavelength solar radia- performed, in which the precursor was added dropwise and the 4,5 tion is waste heat. Thermoelectric materials (TE), such as capping agent assisted in the growth of nanorods. This study Bi2Te3, convert the waste heat into electricity. Bi2Te3 has a high reports the integration of photovoltaic CdTe and thermoelectric Bi2Te3 materials into a single photoelectrode in the form of a core/shell nanostructure. Interestingly, this material showed aDepartment of Electrical and Computer Engineering, 2, Busandaehak-ro 63 beon-gil, a conversion efficiency of 4.8% when exposed to sunlight. Geumjeong-gu, Busan 46241, Korea. E-mail: [email protected] Core/shell nanorods have been produced by a layer-by-layer b Alternative Energy and Nanotechnology Laboratory, Indian Institute of Technology self-assembly method. This is an effective technique for the Madras, Chennai, 600036, India construction of well-dened nanostructures. Hengguo Wang cDepartment of Physics, Indian Institute of Science, Bangalore, India dDepartment of Physics, Anna University BIT Campus, Tiruchirappalli, Tamilnadu, et al. reported that the LBL technique is facile and relatively India controllable and they con rmed the incorporation of Fe3O4 and This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7,6735–6742 | 6735 View Article Online RSC Advances Paper quantum dots (QDs) into a single structure. The shell produced should be thick enough to not to be fully consumed by CdTe.20 6–9 by this method is meant to be a functional hybrid nanoshell. Bi2Te3/CdTe core/shell nanorods were prepared by a wet The alternating adsorption of oppositely charged materials chemical method. Bismuth(III) nitrate pentahydrate (0.04 mol) gives rise to a layer-by-layer (LBL) self-assembly deposition was placed in 10 mL of double-distilled water (DDW). Tellurium method, through which hydrogen-bonding interactions and (0.06 mol) in 10 mL of DDW was mixed with bismuth nitrate electrostatic forces hold the nanostructures assembly solution and stirred. Subsequently, 0.5 mol of KOH was added 10–12 together. The aim of developing Bi2Te3/CdTe core shell to the abovementioned solution aer 5 min. The CdS-coated nanorods is to harvest electricity from both thermal energy and FTO substrate was then immersed in the prepared solution. light energy produced by the sun. Jie Yang et al. proposed that About 0.04 mol of the reducing agent, NaBH4, was added self-assembled Bi2Te3/CdTe nanosheets showed an increase in dropwise to reduce the bismuth nitrate and tellurium to 21 the thermoelectric performance of Bi2Te3 due to the addition of bismuth telluride nanoparticles. The reducing agent is À CdTe, thus reaching a Seebeck coefficient of À182.2 mVK 1 at oxidized because it loses electrons in the redox reaction. The 300 K, which is slightly higher than that of the bulk Bi2Te3 rst step involves the reduction of Bi(NO3)3$5H2O and tellu- À1 sample, À180.0 mVK . This clearly shows that CdTe will also rium. When H2O is added to Bi(NO3)3$5H2O, salt separates into 3+ À enhance the thermoelectric performance of Bi2Te3. Solar cells Bi and 6NO ions, whereas in the case of Te powder, Te 2À based on nanoparticles have higher solar efficiency because of changes into Te . When NaBH4 is added, these ions form their large surface area.13 Therefore, the increase in the surface clusters. In the second stage, the clusters coalesce to produce area of planar nanorod carpets has been proposed to enhance small particles with radius of 20–50 nm. This is followed by solar light harvesting.14–16 a metastable state, where the particles maintain the constant size for around 5–10 min. Aer 10–20 min, the last coalescence phase takes place to generate nal Bi2Te3 nanoparticles about 2. Experimental section the size of 150–250 nm. 2.1 Materials þ Creative Commons Attribution 3.0 Unported Licence. ð Þ $ ! 3 þ À Bi NO3 3 5H2O Bi 6NO H O All chemicals, such as bismuth(III) nitrate pentahydrate 2 $ (Bi(NO3)3 5H2O), tellurium powder (Te), potassium hydroxide (KOH), polyvinyl alcohol (PVA), cadmium sulphate (CdSO4), and Te ! Te2À sodium borohydride (NaBH4), were purchased from Aldrich and NaBH4 used without further purication. 2.2 Synthesis of Bi Te /CdTe core shell thin lm 2À 3+ 2 3 3Te + 2Bi / Bi2Te3 nanostructures This article is licensed under a The front conductor inuences the conductivity and plays When NaBH is added followed by stirring, nanorods of a large role in the performance of a solar cell. An increase in the 4 Bi Te are formed via a self-assembly mechanism. Herein, in the carrier mobility in the conductive FTO transparent conducting 2 3 Open Access Article. Published on 20 January 2017. Downloaded 10/3/2021 5:00:10 AM. self-assembly mechanism, at the critical concentration, the oxide layer provides lower absorptivity and lower resistivity.17 A molecules start to interact with each other resulting in the FTO is properly cleaned to avoid weak adhesion, whereas nucleation of crystals. Using hydrogen bonding and electrostatic a coating of CdS is deposited on the surface of the FTO. Acid or ionic forces, Bi Te clusters with itself to form nanorods. exposure of the FTO will compromise the uniform deposition of 2 3 When PVA is added, it bonds to Bi Te . PVA plays a critical role in CdS and affect the performance of the solar cell.18 CdS has high 2 3 bonding Bi Te and CdTe. PVA attracts the CdTe nanoclusters band width (2.4 eV) and acts as a windows layer for the cells, 2 3 due to hydrogen bonding and forms core/shell nanorods, as which plays a vital role in observing sunlight. In a clean FTO shown in Fig. 1. A 0.1 g sample of polyvinyl alcohol (PVA) was glass substrate, a three layer coating was deposited. The FTO added to the Bi Te solution under stirring to form bonds with substrate was masked and a 50–100 nm CdS layer was coated by 2 3 the Bi Te nanoparticles.