Rapid thermal annealing treatment of ZnO: Al films for photovoltaic applications Zeguo Tang, Hideto Koshino, Shunsuke Sato, Hirokazu Shimizu, Hajime Shirai ⁎ Graduate School of Science and engineering, Saitama University, 255 Shimo-Okubo, Sakuru, Saitama 338-8570, Japan abstract article info Article history: Received 22 August 2011 Received in revised form 28 February 2012 Available online 16 April 2012 Keywords: AZO films; a-Si/c-Si; Heterojunction; RTA We investigated the effect of rapid thermal annealing (RTA) on ZnO: Al (AZO) films prepared at different substrate temperature, the results revealed that the resistivity decreased while crystallinity drastically improved after post annealing treatment. In addition, the amorphous/crystalline silicon heterojunction solar cell was fabricated with AZO films as transparent conductive oxide films. The conversion efficiency of solar cell enhanced significantly after post RTA treatment at 500 °C for 5 min, which due to the improvements of crystallinity, conductivity and transmission of AZO films. The results suggested that RTA treatment was an effective approach to improve the conversion efficiency of heterojunction solar cell. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Transparent conductive oxide (TCO) films which combine high transparence in the visible and near infrared spectral range with a low resistivity have shown their critical role in applications to optoelec- tronic devices [1,2]. At present, impurity-doped ZnO thin films, such as Al- and Ga-doped ZnO, have become increasingly important by recent developments of Si-based thin film solar cells [3,4]. For high-efficiency solar cells on silicon wafers, amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cell draw considerable attention because of its advantages, such as high conversion efficiency reach to 23% [5], low processing temperature (below 250 °C) and the ability to rapidly deposit the amorphous emitter on large areas by PECVD. For high- efficiency a-Si:H/c-Si heterojunction solar cell, the structure of inserted intrinsic a-Si:H films between doped a-Si:H and crystalline silicon is crucial to passivate the silicon surface [6]. In order to realize an effective passivation of the c-Si surface by the a-Si intrinsic layer, low substrate temperature during initiation of a-Si emitter deposition is found to be crucial for reaching high open-circuit voltage (V oc ) in the a-Si/c-Si heterojunction solar cell [7]. Meanwhile, to avoid thermal damnifica- tion of a-Si emitter, the deposition of transparent conductive electrode (AZO) layers must perform at low temperature. It is well known that the substrate temperature of AZO films is a key factor for the resistivity and crystallinity. So the post RTA treatment is essential for the enhancement of conductivity and crystallinity of AZO films. In this work, AZO films are prepared at different substrate temperatures by magnetron sputtering and the post RTA treatment is carried out at 500 °C 5 min. The resistivity and crystallinity are characterized before and after RTA treatment. In addition, a-Si:H/c-Si heterojunction solar cell with AZO film deposited at room temperature as a transparent conductive electrode was fabricated, the performance of solar cell improves significantly after RTA treatment. The structural evolution of AZO film is discussed in detail. 2. Experimental details The AZO films (~1-μm-thickness) were deposited on 25 × 15 mm 2 corning glass by radio frequency (rf) magnetron sputtering from a ceramic AZO target with a fixed Al 2 O 3 content of 2 wt.%. The sputtering depositions were performed at an Ar gas pressure of 0.8 Pa using an rf power of 50 W. The substrate temperature was varied from room temperature to 350 °C. RTA treatment was performed for the as- sputtered AZO films in vacuum for 5 min at annealing temperature of 500 °C. The film thickness was measured by step profiler. The sheet square resistance was determined by a 4-probe tester and corres- ponding resistivity was calculated. The crystallinity of AZO films was characterized by X-ray diffraction (XRD). The heterojunction solar cell consisting of Ag/AZO/p(a-Si)/n(c-Si)/Al was fabricated. We utilized n- type Czochralski (CZ) b100> oriented, 1.0 Ω∙cm, Si wafer as base layer. Before a-Si:H deposition, the c-Si wafer was treated by sequence consisting of (1) RCA1 cleaning at solution of NH 4 OH: H 2 O 2 = 5:1 heated at 80 °C for 10 min, (2) HF dipping (5%, 1 min), (3) RCA2 cleaning at solution of HCl: H 2 O 2 =5:1 heated at 80 °C for 10 min, (4) HF dipping (5%, 1 min), deionized water cleaning for 10 min were performed between above cleaning processes. The substrate is then introduced into the PECVD system and pumped down. A 4 nm thick a- Si:H layer was deposited by conventional 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) using SiH 2 Cl 2 ,B 2 H 6 (diluted to 1% in H 2 ) and H 2 mixture gases. The substrate temperature, working pressure and rf input power were fixed at 250 °C, 200 mTorr and 5 W, respectively. The AZO film (72 nm, obtained from SE fitting results) was deposited by rf magnetron sputtering at room temperature. The AZO Journal of Non-Crystalline Solids 358 (2012) 2501–2503 ⁎ Corresponding author. Tel./fax: + 81 48 858 3676. E-mail address: shirai@fms.saitama-u.ac.jp (H. Shirai). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.03.026 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol