Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Bifacial CIGS solar cells grown by Low Temperature Pulsed Electron Deposition M. Mazzer a, ⁎ , S. Rampino a , G. Spaggiari a,b , F. Annoni a,b , D. Bersani b , F. Bissoli a , M. Bronzoni a , M. Calicchio a , E. Gombia a , A. Kingma a,b , F. Pattini a , E. Gilioli a a CNR-IMEM, Parco Area delle Scienze 37/a, 43124 Parma, Italy b Department of Physics University of Parma, via G.P. Usberti, 7/a, 43124 Parma, Italy ARTICLE INFO Keywords: CIGS Pulsed electron deposition TCO Bifacial solar cells ABSTRACT In this paper we report on the single stage deposition of CuIn x Ga 1-x Se 2 (CIGS)-based bifacial solar cells on glass coated with Fluorine-doped Tin Oxide (FTO) or Indium Tin Oxide (ITO) by single-stage low-temperature (250 °C) pulsed electron deposition (LTPED). We show that the mechanism of Sodium incorporation during the low-temperature deposition of CIGS on both FTO and ITO leads to the formation of a stable n+/p+ ohmic tunnel junction and photovoltaic efficiencies exceeding 14% can be obtained without any intentional bandgap grading of CIGS. The significant degradation of the cell fill factor with decreasing CIGS thickness is found to be related to the presence of craters left behind by micro-fragments of CIGS target, which are weakly incorporated in the film during the LTPED growth and removed during the subsequent process steps. Evidence is also presented that the low-temperature deposition of CIGS on ITO leads to the formation of a Ga-rich CIGS layer at the interface and to an unintentional compositional grading propagating towards the active region of the solar cells. The defects associated with this grading may be responsible for the loss in FF and Voc with respect to the cells deposited on FTO and Mo back contacts. 1. Introduction Photovoltaics is expected to play an increasingly important role in the global effort to meet the environmental targets set by the interna- tional agreements to tackle climate change. The impressive progress made by the photovoltaic industry in the last 7–8 years in terms of production capacity and cost reduction has fuelled the progress of new installations particularly in countries where a feed-in tariff scheme was adopted. On the technological front the most visible result has been the steady increase in the efficiency of solar cells and modules but no real breakthrough has yet been achieved in terms of product innovation as the market is still dominated by conventional flat panels with no significant differentiation between crystalline Silicon and thin film technologies. Among the most interesting innovations being explored and devel- oped by the research community bifacial solar cells (BFSCs) can play an important role in the take-off of building integrated photovoltaics [1] as they can make use of radiation illuminating either or both the front and the back side of a solar cell. This property makes BFSCs very promising for the integration in a built environment where vertical architectural elements are illuminated from different sides during the day and where albedo is an important component of the overall illumination. BFSCs have been fabricated on a number of different cell archi- tectures, as polycrystalline silicon [2,3], dye-sensitized devices [4], perovskite thin-films [5], kesterite-based structures [6] and SnS solar cells [7]. In BFSCs based on thin films the light absorbing layer is sand- wiched between two transparent electric contacts one of which can be a standard n-type transparent conducting oxide (TCO) like ZnO:Al while the other must be either a p-type TCO or a very highly doped n-type TCO behaving essentially as a metal. CuInGaSe 2 (CIGS) is an ideal candidate as light absorbing layer in a BFSC because of the high photovoltaic conversion efficiency (21.7%) [8] achieved by this material in standard thin film solar cells with this typical architecture: ZnO:Al/i-ZnO/CdS/CIGS/Mo/Glass. An obvious solution to make a CISG-based BFSC is to replace the opaque Mo back contact with a highly conductive TCO ( > 10 21 carriers per cm 3 ) like Fluorine-doped Tin Oxide (FTO) or Indium Tin Oxide (ITO). Some studies [9,10] have shown that good ohmic CIGS/FTO and http://dx.doi.org/10.1016/j.solmat.2016.10.048 Received 31 August 2016; Received in revised form 13 October 2016; Accepted 26 October 2016 ⁎ Corresponding author. E-mail address: massimo.mazzer@cnr.it (M. Mazzer). Solar Energy Materials & Solar Cells xx (xxxx) xxxx–xxxx 0927-0248/ © 2016 Elsevier B.V. All rights reserved. Available online xxxx Please cite this article as: mazzer, m., Solar Energy Materials & Solar Cells (2016), http://dx.doi.org/10.1016/j.solmat.2016.10.048