Fabrication of flexible CdTe solar modules with monolithic cell interconnection J. Perrenoud n , B. Schaffner, S. Buecheler, A.N. Tiwari Laboratory for Thin Films and Photovoltaics, Empa, Swiss Federal Laboratories for Materials Science and Technology, ¨ Uberlandstr. 129, 8600 D¨ ubendorf, Switzerland article info Available online 16 December 2010 Keywords: Polyimide CdTe Solar module Monolithic interconnection Laser scribing abstract CdTe solar cells and modules have been manufactured on polyimide (PI) substrates. Aluminum doped zinc oxide (ZnO:Al) was used as a transparent conductive oxide (TCO) front contact, while a thin high resistive transparent layer of intrinsic zinc oxide (i-ZnO) was used between the front contact and the CdS layer. The CdS and CdTe layers were evaporated onto the ZnO:Al/i-ZnO coated PI films in a high vacuum evaporation system followed by a CdCl 2 activation treatment and a Cu–Au electrical back contact deposition. In some cases prior to the cell deposition, the PI film was coated with MgF 2 on the light facing side and the effects on the optical and electrical properties of TCO and solar cells were investigated. The limitations on current density of solar cells due to optical losses in the PI substrate were estimated and compared to the experimentally achieved values. Flexible CdTe solar cells of highest efficiencies of 12.4% and 12.7% were achieved with and without anti-reflection MgF 2 coating, respectively. Laser scribing was used for patterning of layers and monolithically interconnected flexible solar modules exhibiting 8.0% total area efficiency on 31.9 cm 2 were developed by interconnection of 11 solar cells in series. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The lowest production cost per Watt achieved with CdTe solar modules on glass substrates by First Solar [1] is an indicator of the promising potential of the CdTe based solar cells. A variety of deposition methods have been used for processing CdTe solar cells [2]. Chemical robustness, ease for stoichiometry control and high growth rate ( 41 mm/min) deposition possibilities are some of the attractive features of the CdTe for solar cell applications. The highest efficiency of 16.5% has been reported for CdTe solar cells developed with the close space sublimation (CSS) method on glass substrates coated with a bi-layer of stannates as transparent contact [3]. Such solar cells are processed at high substrate temperature 4600 1C [4]. However, low deposition temperature ( o450 1C) processes have also yielded high efficiency solar cells on glass substrates[5,6]. Such low temperature processes have also been applied for the development of flexible CdTe solar cells in substrate [7] and superstrate [8–12] configurations on polyimide (PI) films (see also review paper [13]). While the CdTe technology on glass substrates has progressed to a high level of manufacturing, the development of flexible CdTe technology has just started. The advantages of high speed deposition of CdTe/CdS and adapting those low temperature processes for roll-to-roll manufacturing could lead to reduction in production costs. Fig. 1 shows a schematic of the roll-to-roll deposition concept. In continuation of our ongoing work on flexible CdTe solar cells we substituted ITO by ZnO:Al and achieved better performance in terms of adhesion and solar cell efficiency. The present paper discusses further studies on the development of flexible CdTe solar cells and monolithically interconnected mini-modules where laser scribing is used for patterning of the layers. In order to further improve the efficiency we analyzed the optical losses due to PI absorption and reflection and their consequences to limit the achievable current density in solar cells. The TCO resistivity and its influence on the FF of cells with different geometry were investigated in order to design a CdTe mini-module. Finally a CdTe mini-module was produced using laser scribing for patterning. 2. Experimental CdTe/CdS solar cells in superstrate configuration were grown on PI substrates. The flexible PI films were mounted in molybdenum frames (substrate holder) to simplify the handling during proces- sing. On some samples a 100 nm thick MgF 2 coating was applied on the light facing side of the PI film by e-beam evaporation as a first process step. Radio frequency magnetron sputtering at 300 1C substrate temperature was used for the growth of aluminum doped zinc oxide (ZnO:Al) transparent conducting oxide (TCO) front contact layer followed by a highly resistive transparent layer 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.2010.11.019 n Corresponding author. E-mail address: julian.perrenoud@empa.ch (J. Perrenoud). Solar Energy Materials & Solar Cells 95 (2011) S8–S12