Material properties of microcrystalline silicon for solar cell application Czang-Ho Lee n , Myunghun Shin, Mi-Hwa Lim, Jun-Yong Seo, Jung-Eun Lee, Hee-Yong Lee, Byoung-June Kim, Donguk Choi Development Group, LCD Division, Samsung Electronics, Yongin City, Gyeonggi-do, South Korea article info Article history: Received 2 November 2009 Accepted 2 February 2010 Available online 4 March 2010 Keywords: Microcrystalline silicon Amorphous silicon Plasma-enhanced chemical vapor deposition Solar cell abstract The paper reviews the material requirements of microcrystalline silicon (mc-Si) in terms of the device operation and configuration for thin film solar cells and thin film transistors (TFTs). We investigated the material properties of mc-Si films deposited by using 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) from a conventional H 2 dilution in SiH 4 . Two types of intrinsic mc-Si films deposited at the high pressure narrow electrode gap and the low pressure wide electrode gap were studied for the solar cell absorption layers. The material properties were characterized using dark conductivity, Raman spectroscopy, and transmission electron microscope (TEM) measurements. The mc-Si quality and solar cell performance were mainly determined by microstructure characteristics. Solar cells adopting the optimized mc-Si film demonstrated high stability with no significant changes in solar cell performance after air exposure for six months and subsequent illumination for over 300 h. The results can be explained that low ion bombardment and high atomic hydrogen density under the PECVD condition of the high pressure narrow electrode gap produce high-quality mc-Si films for solar cell application. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Microcrystalline silicon (mc-Si) films have great potential for solar cells [1,2] and thin film transistors (TFTs) [3,4]. This stems from their better optoelectrical properties [1–4], and higher stability against the light and electrical stress [5,6] compared to amorphous silicon (a-Si) films. Plasma-enhanced chemical vapor deposition (PECVD) is widely used to deposit mc-Si films on large-area glass substrate [7]. It is well known that atomic hydrogen dissociated by H 2 -diluted SiH 4 plasma plays an important role in making such crystalline Si thin films at low temperatures (30–350 1C) [8]. The PECVD process for mc-Si growth is similar for both solar cell and TFT applications [1–4]. At the point of view of the device operation and structure, characteristics of mc-Si films optimized for solar cell and TFT applications are different. It is important to control the microstructure of Si phase because nanometer-size crystallites embedded in an amorphous phase are directly related to device behavior [1–4]. Especially, in a-Si/mc-Si tandem solar cells, the crystalline phase of mc-Si films must be carefully adjusted for the deposition process of 2–3 mm thickness, where the bottom mc-Si thickness is usually more than ten times of mc-Si films in TFT application [1,2]. In this paper, we discuss at first the mc-Si material require- ments in terms of the device operation and configuration. It is followed by the characteristics of mc-Si films deposited under two deposition regimes and their application to solar cells. Finally, we show the stability of mc-Si solar cells against ambient atmosphere exposure and subsequent illumination. 2. lc-Si requirements for the device operation and configuration The microstructural phase of Si evolves as the film grows during the deposition in PECVD [1,2]; after an initial amorphous incubation layer, nucleation starts to form crystal grains, then grains coalesce to be columnar structure. The microstructure characteristics correspond to the incubation and the coalescence of the grains, which mainly depend on deposition conditions, and result in device performance. In TFTs, carrier (electron or hole created by positive or negative gate bias, respectively) is laterally accumulated near the interface of gate dielectric/active channel layer and flows along a 10–100 mm-length channel from source to drain when a drain voltage is applied (Figs. 1(a) and (b)). Hence, main interests of the TFT performance are to improve the gate dielectric/mc-Si interface integrity and to obtain the crystal qualities such as large grain size and effective grain boundary passivation in the active channel with only 50–100 nm thickness, therefore demanding high crystallinity mc-Si films [3,4,6]. In PIN solar cells, optically generated carriers (both electron and hole) in the intrinsic absorption layer separate vertically towards n-layer for electron and towards p-layer for hole, due to the internal electric field generated by the dope p- and n-layers. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.021 n Corresponding author. Tel.: + 82 31 209 6149. E-mail address: czangho.lee@samsung.com (C.-H. Lee). Solar Energy Materials & Solar Cells 95 (2011) 207–210