Inﬂuence of the electro-optical properties of an α-Si:H single layer on the performances of a pin solar cell I. Crupi a, ⁎, F.S. Ruggeri a, 1 , A. Grasso b , F. Rufﬁno a , G. Catania b , A.M. Piro b , S. Di Marco b , S. Mirabella a , F. Simone a , F. Priolo a a MATIS, CNR-IMM and Dipartimento di Fisica ed Astronomia, Università di Catania, Via Santa Soﬁa 64, Catania, I-95123, Italy b R&D, STMicroelectronics Stradale Primosole 50, Catania, I-95121, Italy abstract article info Article history: Received 12 September 2011 Received in revised form 24 January 2012 Accepted 26 January 2012 Available online 1 February 2012 Keywords: Hydrogenated amorphous silicon Conductivity Activation energy Optical gap Quantum efﬁciency Thin ﬁlm solar cell We analyze the results of an extensive characterization study involving electrical and optical measurements carried out on hydrogenated amorphous silicon (α-Si:H) thin ﬁlm materials fabricated under a wide range of deposition conditions. By adjusting the synthesis parameters, we evidenced how conductivity, activation en- ergy, electrical transport and optical absorption of an α-Si:H layer can be modiﬁed and optimized. We ana- lyzed the activation energy and the pre-exponential factor of the dark conductivity by varying the dopant- to-silane gas ﬂow ratio. Optical measurements allowed to extract the absorption spectra and the optical bandgap. Additionally, we report on the temperature dependence of the activation energy to satisfy the Meyer–Neldel rule. Finally, the inﬂuence of the individual ﬁlms parameters upon the ﬁnal performances of a single junction pin α-Si:H have been studied. The measurements show how a more than doubled enhance- ment in energy conversion efﬁciency can be obtained in an α-Si:H solar cell with a proper selection of syn- thesis conditions. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Since the report of a hydrogenated amorphous silicon (α-Si:H) junction in 1976 [1], the α-Si:H material, with a large tailorable band- gap, easy dopability and high optical absorption coefﬁcient, has attracted a great deal of worldwide interest for photovoltaic (PV) ap- plications, also thanks to the chance of large areas deposition at very low cost, as required for PV market [2]. In the meantime, over the past decade, hydrogenated amorphous silicon has generated great interest as a key component in the development of next-generation multi- junction PV devices [3,4]. In fact, although the single-junction α-Si: H solar cell suffers from low conversion efﬁciency, α-Si:H based tandem cell is one of the most promising thin ﬁlm candidates for the advancement of photovoltaic technology due to high attained efﬁciency and low materials costs. Thus, after years of research and technology development, there is still great interest in improving materials for stable high-performance α-Si:H solar cells [5–9]. In order to obtain solar cell as efﬁcient as possible, there are many interdependent parameters to consider. Most importantly, each layer in the typical pin structure must have appropriate electrical and opti- cal properties. This work presents and discusses a systematic investigation on the effects of synthesis parameters on the behavior of a homojunction α-Si:H pin solar cell under solar simulator and monochromatic illumination. The measurements show improved re- sults as the hydrogen content in the intrinsic layer increases. In par- ticular, a more than doubled enhancement in energy conversion efﬁciency (η) was observed in the solar cell in which the intrinsic layer has the highest hydrogen content. Moreover, the paper outlines the temperature dependence of the average energy of the conduction electrons and the Fermi level in our amorphous ﬁlms and contributes to a body of work analyzing the Meyer–Neldel (MN) correlation [10]. 2. Sample preparation and experimental details In order to optimize the pin diodes, a series of doped and undoped α- Si:H ﬁlms were synthesized on soda-lime substrates (which are used in commercial solar cells) in a radio frequency plasma enhanced chemical vapor deposition (RF PECVD) system at 280 °C. The p layers were depos- ited using silane (SiH 4 ) and trimethylboron, B(CH 3 ) 3 (abbreviated as TMB), as gas sources with different ﬂow ratios of [SiH 4 /TMB] (5:1, 3:1, 2:1), and dilution ratio of [H 2 /SiH 4 ] of 1:1.5. The n-doped ﬁlms were syn- thesized using SiH 4 and phosphine, PH 3 , as gas sources with different ﬂow ratios of [SiH 4 /PH 3 ] (6:1, 3:1, 2:1), and a ﬁx [H 2 /SiH 4 ] ratio of 1:0.6. The undoped α-Si:H layers were deposited by the decomposition of pure SiH 4 with ﬂow rates of 12 and 100 sccm (sccm denotes standard cubic centimeters per minute) and dilution ratio of 10:1 to study the effect of Thin Solid Films 520 (2012) 4036–4040 ⁎ Corresponding author. Tel.: + 39 0953785396; fax: + 39 0953785231. E-mail address: Isodiana.Crupi@ct.infn.it (I. Crupi). 1 Present address: IPSB-LPMV BSP, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.01.044 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf