Solution processing of back electrodes for organic solar cells with inverted architecture Y. Galagan a,n , S. Shanmugam a , J.P. Teunissen a , T.M. Eggenhuisen a , A.F.K.V. Biezemans b , T. Van Gijseghem c , W.A. Groen a,d , R. Andriessen a a Holst Centre - Solliance, P.O. Box 8550, 5605KN Eindhoven, The Netherlands b Energy Research Centre of the Netherlands (ECN) - Solliance, High Tech Campus 21, 5656AE Eindhoven, The Netherlands c Agfa-Gevaert NV, Septestraat 27, 2640 Mortsel, Belgium d Faculty of Aerospace Engineering, Delft University, Kluyverweg 1, 2629 HS Delft, The Netherlands article info Article history: Received 14 March 2014 Received in revised form 12 May 2014 Accepted 5 July 2014 Keywords: Organic solar cell Inkjet printing PEDOT:PSS Printed back electrode Inverted cell architecture Solution processing abstract Solution processing of the electrodes is a big challenge towards scaling up and R2R processing of organic solar cells. Inkjet printing is a non-contact printing method, it can be realized by solution processing at ambient condition and provides freedom of shape in the electrode pattern. The inkjet and screen printing of the back electrode in organic solar cells with inverted architecture is investigated. To this purpose, different types of PEDOT:PSS hole transport layers were combined with ambient processed inkjet printed or evaporated Ag electrodes. The thickness of the PEDOT:PSS layer is the main factor determining the performance of the devices. A too thin layer of EL-P5015 PEDOT:PSS allows penetration of the solvents from the Ag ink, which leads to degradation of the devices and a drop in performance. To overcome this effect, a 1200nm thick layer of EL-P5015 is required. Alternatively, an aqueous based coating formulation of PEDOT:PSS (S305), can be combined with either inkjet printed or screen printed electrode at significantly reduced thickness down to 40 nm. At the same time, the performance of these devices was comparable to devices with an evaporated electrode. Reducing the thickness of PEDOT:PSS layer from 1200 nm to 40 nm is a very efficient way to reduce the manufacturing costs of OPV devices. In addition, combining of thin PEDOT:PSS layer with an inkjet printed Ag grid structure allows manufacturing of semi-transparent devices. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Organic photovoltaics (OPVs) attract a lot of attention due to their low cost potential which can be achieved via all-solution roll- to-roll (R2R) processing [1–7]. Towards all-solution processing, a lot of effort has been put into the development of an alternative for ITO as a front electrode [8–16]. However, the back electrode also has to be produced from solution. PEDOT:PSS as a hole transport layer followed by the deposition of Ag is a typical back electrode in organic solar cells with the inverted configuration. Although it is well accepted that this electrode can be solution processed, often Ag is still deposited by thermal evaporation. Alternative solution processing deposition methods for Ag back electrodes are spray (aerosol) coating [7,17–20], inkjet printing [21–23], flexographic [21] or screen printing [21,24,25]. A substantial limitation in the solution processing of the back electrode is the requirement of sintering at high temperature and under ambient conditions. These conditions can lead to degradation of the PEDOT:PSS and photo-active layers. Screen-printing in ambient condition is the most explored so far in both lab scale devices and R2R production. While only limited sintering time was needed (3 min at 140 1C) to yield high conductivity, the majority of defects was caused by an interaction of the solvents from the screen print Ag ink with the underlying layers [22]. Moreover, screen printing is a contact method, which may also lead to an increasing number of defects in underlying layers. To protect underlying layers from aggressive effects of solvents and to avoid the mechanical stress from the screen printing process, the thickness of the PEDOT:PSS layer is signifi- cantly increased to provide a kind of ‘buffer layer' protecting the photoactive layer [24,25]. Inkjet printing is a non-contact technique and might require thinner PEDOT:PSS layers, which is a positive aspect in terms of the final cost of OPV. Previously, an inkjet printed back electrode has been successfully demonstrated with a 800 nm thick PEDOT:PSS layer in organic solar cells with an inverted cell configuration [22]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.07.007 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ31 40 4020447; fax: þ31 40 4020699. E-mail address: yulia.galagan@tno.nl (Y. Galagan). Solar Energy Materials & Solar Cells 130 (2014) 163–169