Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat Improving electrical conductivity and its thermal stability of Al-doped ZnO polycrystalline flms using ultrathin Al flm as a passivation layer Hoa T. Dao a , Hisao Makino a,b,∗ a Graduate School of Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami, Kochi, 782-8502, Japan b Research Institute, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami, Kochi, 782-8502, Japan ARTICLEINFO Keywords: Al-doped ZnO Transparent conductive oxide Thermal stability Conductivity Al flm Passivation ABSTRACT Aluminum-doped ZnO (AZO) has been considered as a prospective material for application as transparent electrodes in solar cells. In this application, an improvement of the Hall mobility is desired to achieve the high conductivity, because an increase in the carrier concentration results in a decrease in transmission at near- infrared wavelengths. However, the achievement of high Hall mobility is still a challenge. Another challenge associatedwithAZOisthelowthermalstabilityoftheelectricalproperties,whichlimitstheapplicationofAZO. This limitation originates from Zn desorption and migration of oxygen from the environment into the AZO flm. It has been shown that using an ultrathin Al capping layer on an AZO flm can greatly improve its thermal stability.AnimprovementintheHallmobilitywasobtainedforanAlcappedAZO flmafterannealinginN 2 gas at 400°C. The Hall mobility reached 49.1cm 2 /V, which is close to the theoretical limit of the mobility for an AZOpolycrystallineflm,withamoderatecarrierconcentrationof2.8×10 20 cm −3 ,ensuringhightransparency in the visible and near-infrared ranges. The obtained results are due to the passivation efect of the AlO x flm formed by natural oxidation of Al flm. Zn desorption was suppressed and the migration of O from the en- vironment was reduced, leading to an improvement in the thermal stability. Once Zn desorption is prevented, improvementofthecrystallineperfectionduetothermalannealingcancontributetoanimprovementintheHall mobility. 1. Introduction Transparent conductive electrodes (TCEs) are essential components in optoelectronic devices such as touch panel displays, light emitting diodes and solar cells [1–5]. In these applications, the TCEs must have highelectricalconductivityandhighopticaltransparencyinthevisible (Vis) and near-infrared (NIR) regions. To date, tin-doped indium oxide (ITO) and fuorine-doped tin oxide (FTO) are most used materials in commercial market of transparent electrodes. However, their high production cost, toxicity, and limited resource have raised the demand for the development of alternative TCEs candidates. Many interests have been paid on several type of TCEs, including conductive polymers, carbon nanotube (CNT) and graphene based TCEs, metal thin flm, metal nanowires (NWs) and nanoparticles (NPs) based flms, metal meshes, MXenes based TCEs, and other transparent conductive oxide (TCOs) flms [6–11]. The TCEs based on conductive polymers, CNTs, graphene, metal thin flms, metal NWs and NPs, MXeneandtheirhybridscanhavehighfexibilitywithsignifcantlylow resistance [7,13], for instance, graphene/silver NWs and graphene/ metal (Au, Cu, Al) grid hybrids have low sheet resistance (R S ) of ~10–20 Ω/sq and transmittance of ~75–90% in Vis range [12], or MXene/Ag NWs hybrid have a R s of ~26Ω/sq and an average trans- mittance of ~83% in Vis range [13]. However, in addition to their strong trade-of between conductivity and transparency, complicated and low-yield fabrication processes are cause of difculties for in- dustrial production so far [7,14]. Moreover, their instabilities in en- vironment and high temperature conditions are big concerns [14]. Accordingly,TCOsbasedTCEsarestillfavoredinapplicationsthatneed high temperature processes. In recent years, studies on TCOs is continuously increasing, espe- cially on heavily doped n-type ZnO thin flms due to their high con- ductivityandtransparency,withvarietyoffabricationmethodssuchas spin-coating [15], dip-coating [16], spray pyrolysis [17], atomic layer deposition (ALD) [18,19], chemical vapor deposition (CVD) [20], and magnetron sputtering (MS) [21,22]. High yield and enlargeable pro- ductions make these materials suitable for commercial market. Beside studiesonsinglemetaldopedZnO[23–27],intensivestudieshavebeen carried out on co-doped ZnO [17,28–34] and on multilayers of metal https://doi.org/10.1016/j.solmat.2019.110159 Received 10 June 2019; Received in revised form 22 August 2019; Accepted 31 August 2019 ∗ Corresponding author. Graduate School of Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami, Kochi, 782-8502, Japan. E-mail addresses: 216006c@gs.kochi-tech.ac.jp (H.T. Dao), makino.hisao@kochi-tech.ac.jp (H. Makino). Solar Energy Materials and Solar Cells 203 (2019) 110159 0927-0248/ © 2019 Elsevier B.V. All rights reserved. T