Improvement of organic solar cells using aluminium microstructures prepared in PEDOT:PSS buffer layer by using ultrasonic ablation technique Yasser A.M. Ismail a, ⁎, Naoki Kishi b , Tetsuo Soga b a Third Generation Solar Cells Laboratory, Department of Physics, Faculty of Science, Al-Azhar University, Asyut 71121, Egypt b Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555, Japan abstract article info Article history: Received 22 January 2016 Received in revised form 22 July 2016 Accepted 1 August 2016 Available online 03 August 2016 In the present work we introduce an improvement of organic solar cell efficiency by using aluminium microstruc- tures (Al MSs) dispersed in poly(3,4-ethylene dioxythiophene)-blend-poly(styrene sulfonate) (PEDOT:PSS) buffer layer. Al microstars (in the range of 5–16 μm) in addition to Al nanoparticles (Al NPs) have been prepared by new ultrasonic ablation technique through the application of ultrasonic irradiation upon Al thin film im- mersed in PEDOT:PSS solution. In the beginning, we found that the PEDOT:PSS solution completely dissolves Al thin film before the applying of ultrasonic irradiation as a result of acidic nature of the PEDOT:PSS solution. The action of PEDOT:PSS on the Al film tends to produce Al microcircles, which are dissociated into Al microstars after the applying of ultrasonic irradiation. The short circuit current density, fill factor, and, consequently, power conversion efficiency of the investigated solar cells have been improved by the incorporation of Al microstars, which can facilitate the transport of charge carriers, specifically holes, to be transferred along the axis of the Al microstars in the buffer layer of the solar cell. We found that the increase in ultrasonication time applied upon the solution of PEDOT:PSS-Al microstars diminishes the length of Al microstars and reduces the number of Al microstars' arms. Therefore, further increase in the ultrasonication time tends to decrease the device perfor- mance parameters due to the deficit of charge carriers to be transferred in the buffer layer matrix. The Al microstars prepared by ultrasonic ablation technique are pure and free of surfactants and passivation layers that are inevitably present on the surface of the chemically synthesized metal MSs and NPs. The new ultrasonic ablation method is safe (different than laser ablation technique) to prepare metal NPs and MSs in the sensitive organic solvents and solutions which are directly used to prepare organic solar cells. In addition, the new technique enables us to prepare desirable size of Al microstars, which can be suitable for improving organic solar cells and in other different applications. © 2016 Elsevier B.V. All rights reserved. Keywords: Organic solar cells PEDOT:PSS buffer layer Aluminium microstructures Ultrasonic ablation technique 1. Introduction Organic solar cells (OSCs) have many advantages compared to their inorganic counterparts such as low cost, low thermal budget, solution processing, flexible substrates and a very high speed of processing [1]. There are still some challenges that have to be encountered before a commercial breakthrough of these promising devices can take place: power conversion efficiency (PCE) and degradation under outdoor environment. OSCs have already shown high PCEs of 7–10% [2,3], demonstrating a bright future for commercialization of such solar cells. However the conversion efficiency of OSCs still needs necessary improvement to become more economically efficient than inorganic counterparts. Many research groups [4–8] have studied polymer solar cells in order to improve their PCEs. The incorporation of metal nanoparticles (NPs) in different parts of solar cell architecture, whatever in the active layer blend [9,10], in the buffer layer [11–16], or at the interface between buffer layer and indium tin oxide (ITO) electrode [17], is an emerging area of study for improv- ing conversion efficiency of OSCs. Metallic NPs that their diameters match or seem smaller than the wavelength of light can be employed as subwavelength antennas, through excitation of the localized surface plasmon resonance (LSPR). Through this resonance the plasmonic near-field is coupled to the photoactive layer, increasing its effective ab- sorption cross-section and thus exciton dissociation for producing charge carriers. In addition, relatively larger diameter metal NPs can be used as effective subwavelength scattering elements that couple and trap freely propagating plane waves from the sun into the photoactive layer. Hence, the incorporation of metallic NPs in different parts of the solar cell architecture offers the possibility of enhanced ab- sorption and correspondingly enhanced photogeneration of charge car- riers by both scattering and LSPR [18]. Broadband plasmonic absorption Thin Solid Films 616 (2016) 73–79 ⁎ Corresponding author. E-mail address: Yasser_ami@yahoo.com (Y.A.M. Ismail). http://dx.doi.org/10.1016/j.tsf.2016.08.001 0040-6090/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf