www.afm-journal.de FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3978 www.MaterialsViews.com wileyonlinelibrary.com Adv. Funct. Mater. 2014, 24, 3978–3985 The new lifetime record approximately doubled the previous lifetime record that was observed in P3HT:PC 60 BM OPV devices. [6] Degradation in encapsulated polymer solar cells cannot be attributed to any one mechanism; [8–12] but, the different mechanisms of degradation in polymer solar cells can be classified into three general categories. The first category is light-induced burn-in degradation. This degradation is characterized by an expo- nential drop of about 20% of the initial efficiency and most of it occurs in the first 200 hours. The burn-in is found to be caused by photo-induced traps and is inde- pendent of the electrodes and the amount of injected current. [13,14] Two theories that attempt to explain the degradation include cross-linking [15] and light-induced breaking of C–H bonds. [16] The second cat- egory of degradation is long term degradation which is charac- terized by a slow, linear degradation. Of all of the degradation categories, the least is known about long-term degradation. A third category is thermal burn-in and is characterized by an exponential drop in efficiency that stabilizes over time. The highest solar cell temperature that solar cells are exposed to for a significant amount of time under solar illumination is 65 °C; this is the standard temperature used for testing thermal deg- radation. [17] Thermal degradation appears to be related to the interface. For example, PBDTTPD-based solar cells with power conversion efficiencies (PCE) of 7.3% suffer from thermal deg- radation and the loss in performance was shown to be restored by peeling off and reapplying the electrode. [18] To maximize the long-term performance of solar cells, all three of the degrada- tion categories need to be addressed. In this paper we gener- alize the cause and solution of thermal burn-in for several polymer-fullerene systems. We show that thermal burn-in is caused by a less than 4 nm layer of polymer adhering to the back contact, where the back contact refers to the contact that is applied after the polymer-fullerene bulk-heterojunction (BHJ) film is processed. The polymer adhesion occurs at the glass transition temperature ( T g ) of the polymer-fullerene blend. If the T g of the polymer-fullerene blend is higher than 65 °C then Electron Barrier Formation at the Organic-Back Contact Interface is the First Step in Thermal Degradation of Polymer Solar Cells I. T. Sachs-Quintana, Thomas Heumüller, William R. Mateker, Darian E. Orozco, Rongrong Cheacharoen, Sean Sweetnam, Christoph J. Brabec, and Michael D. McGehee* Long-term stability of polymer solar cells is determined by many factors, one of which is thermal stability. Although many thermal stability studies occur far beyond the operating temperature of a solar cell which is almost always less than 65 °C, thermal degradation is studied at temperatures that the solar cell would encounter in real-world operating conditions. At these temperatures, movement of the polymer and fullerenes, along with adhesion of the polymer to the back contact, creates a barrier for electron extraction. The polymer barrier can be removed and the performance can be restored by peeling off the electrode and depositing a new one. X-ray photoelectron spectroscopy measurements reveal a larger amount of polymer adhered to electrodes peeled from aged devices than electrodes peeled from fresh devices. The degradation caused by hole-transporting polymer adhering to the electrode can be suppressed by using an inverted device where instead of electrons, holes are extracted at the back metal electrode. The problem can be ultimately eliminated by choosing a polymer with a high glass transition temperature. DOI: 10.1002/adfm.201304166 I. T. Sachs-Quintana, W. R. Mateker, D. E. Orozco, R. Cheacheroen, S. Sweetnam, Prof. M. D. McGehee Department of Materials Science and Engineering Stanford University Stanford, CA 94305, USA E-mail: mmcgehee@stanford.edu T. Heumüller, C. J. Brabec Institute of Materials for Electronics and Energy Technology (I-MEET) Friedrich-Alexander-University Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen, Germany C. J. Brabec Bavarian Center for Applied Energy Research (ZAE Bayern) Haberstrasse 2a 91058 Erlangen, Germany 1. Introduction As the power conversion efficiency (PCE) of solution-process- able, organic photovoltaics (OPVs) exceeds 10%, [1,2] the question of long-term stability becomes the next barrier to commerciali- zation. [3–5] The record lifetime for a polymer OPV device is 6.2 years and was observed in glass encapsulated devices based on the polymer-fullerene blend of PCDTBT and PC 70 BM. [6,7]