Copper oxide based nanostructures for improved solar cell efficiency A. Bhaumik a , A. Haque a , P. Karnati a , M.F.N. Taufique a , R. Patel b , K. Ghosh a, ⁎ a Department of Physics, Astronomy and Materials Science, Missouri State University, Springfield, MO 65897, USA b Centre for Applied Science and Engineering, Missouri State University, Springfield, MO 65897, USA abstract article info Available online xxxx Keywords: Solar cell Nanostructures Nanotechnology Photovoltaics Resurgence of copper oxide based thin film solar cells demands exclusive methods of integrating various layers with superior constituents for increased solar-electric conversion efficiency. Exceedingly optically active nano- structured phase mixture of copper oxides was synthesized by an energy efficient hydrothermal process. Com- prehensive structural and optical studies of these nanostructured copper oxides reveal its efficacy as a unique solar cell material. Excellent solar cell characteristics have been observed when these nanopowders are integrat- ed with ZnO/CuO based thin films. X-ray diffraction, Raman micro-scattering, scanning electron microscopy, en- ergy dispersive X-ray spectroscopy, UV–vis spectroscopy, atomic force microscopy, and optoelectronic measurements were employed to characterize these unified electronic devices. Solar cell measurements indicate a considerable increase in short circuit current density (J sc ) and open circuit voltage (V oc ) in the fabricated nano- structure powder-thin film hybrid solar cell devices. The solar cell efficiency of these nanopowder-thin film de- vices is found to be 2.88%. The physics behind this enrichment of solar cell properties has also been elucidated in the study. Exhaustive Raman spectroscopic and photoluminescence studies prove that multi-phonon scattering may play a major role for this enhancement. This integration of nanostructures with thin film solar cells can evolve to a new direction in photovoltaic technology. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Developing high-end solar cell devices by implementing inexpen- sive nanostructured materials and unique processing techniques is ur- gently required to sustain the ever demanding energy need. Copious new concepts for solar-electric energy conversion have been reported which challenge traditional photovoltaic (PV) devices based on the physics of semiconductor p–n junction diode [1]. Semiconductor mate- rials used in solar cell technology are predominantly governed by their energy band gap, optical properties, and charge carrier diffusion length [2,3]. Highly efficient solar cell design necessitates optically active pho- tovoltaic layers to enable nearly complete light absorption thereby in- creasing the rate of electron–hole pair generation. Considerable efforts have been made to develop state-of-the art nanostructured materials that absorb better at long wavelengths, so far with little success [4,5]. An option to increase the absorption of visible solar spectrum is by in- creasing the film thickness to improve the optical density. However, this leads to detrimental effects of exceeding the electron diffusion length through the nanoparticle network [6]. The ever demanding in- creased solar cell efficiency and reduced material consumption urgently require a decrease in active cell region thickness while preserving high optical absorption. Copper oxide based semi-conductors are widely studied as photo- voltaic materials [7], owing to its abundance and suitable optical prop- erties for solar cell applications. These materials provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from band gap energies of 2.1 eV to the infrared at 1.4 eV, i.e. right into the middle of the maximum efficiency for solar- cell applications. Cupric oxide (CuO) and cuprous oxide (Cu 2 O) are being widely used as a p-type semiconductor for designing solar cells [8,9]. There are innumerous examples of copper oxide based PV devices reported in literature, often prepared by using low-cost, solution-based methods [10–12]. Predominantly, all of these devices are either bulk or thin-film bilayer cells, and these types of cells suffer from the fact that the optimal material length scales for optical absorption and carrier extraction are contrary to one another. The use of nanomaterials repre- sents a general approach to reduce both cost and size thereby improving efficiency in photovoltaic cells [13,14]. The physical mechanism under- lying high external quantum efficiencies for photoluminescence in low dimensional materials is mainly due to the quantum confinement of ex- citons in a nanometer-scale crystalline structure [15,16]. As photons are absorbed by the material and charge carriers are produced, the average diffusion time (τ d ) from the bulk of the material to the surface has been shown to follow Eq. (1): τ d ¼ r 2 Dπ 2 ð1Þ Thin Solid Films xxx (2014) xxx–xxx ⁎ Corresponding author. E-mail address: kartikghosh@missouristate.edu (K. Ghosh). TSF-33745; No of Pages 8 http://dx.doi.org/10.1016/j.tsf.2014.09.056 0040-6090/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Please cite this article as: A. Bhaumik, et al., Copper oxide based nanostructures for improved solar cell efficiency, Thin Solid Films (2014), http:// dx.doi.org/10.1016/j.tsf.2014.09.056