Effect of light induced degradation on electrical transport and charge extraction in polythiophene:Fullerene (P3HT:PCBM) solar cells S. Khelifi a,n , E. Voroshazi c , D. Spoltore b , F. Piersimoni b , S. Bertho b , T. Aernouts c , J. Manca b , J. Lauwaert d , H. Vrielinck d , M. Burgelman a a Department of Electronics and Information Systems (ELIS), University of Gent, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium b Institute of Materials Research, University of Hasselt, Wetenschapspark 1, B-3590 Diepenbeek, Belgium c Imec, Organic Photovoltaics, Polymer & Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Belgium d Department of Solid State Sciences, University of Gent, Krijgslaan 281-S1, B-9000 Gent, Belgium article info Article history: Received 10 June 2013 Received in revised form 30 August 2013 Accepted 4 September 2013 Available online 16 October 2013 Keywords: Organic photovoltaics P3HT:PCBM Traps Illumination aging Characterization Modeling abstract We investigate the photodegradation in inert atmosphere of (poly 3-hexylthiophene:[6,6]-phenyl-C61- butyric acid methyl ester) (P3HT:PCBM) heterojunction solar cells under continuous illumination using advanced electrical characterization and a device modeling tool. Our results indicate that different failure mechanisms contribute to the performance loss. The first 250 h of illumination induced p-type doping and recombination related to traps in the blend which mainly decreases the short-circuit current and the efficiency of the cells. Device modeling and simulation allowed us to prove that increased p-type doping of the blend provoke the loss in the short-circuit current and the quantum efficiency by simultaneous reduction of charge carrier mobility and the electric field together with a shrink of the space charge region. Transmission electron microscopy (TEM) measurements reveal a change in the blend morphology upon long illumination times manifested by phase segregation in the blend. The reduction in the open- circuit voltage is reported to be related to a slight reduction of the charge transfer energy (CT) upon 700 h of illumination aging. The final failure mechanism was a rapid drop in the fill factor which occurs upon 1000 h of illumination and manifested by the appearance of an S-shape J–V characteristic. This failure mechanism is linked to the reduction of charge extraction caused by a reduced surface recombination velocity at the contacts. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Organic solar cells (OSCs) based on P3HT:PCBM (poly 3-hex- ylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester) are widely investigated in terms of device stability and degradation [1]. Among the failure mechanisms, photodegradation of the active layer forms the most complex stability problem, since most of the organic polymers are unstable under UV illumination when exposed to ambient atmosphere. This process is further compli- cated in the presence of oxygen, water as these stress factors can cause both photo-oxidation of the polymer as well as degradation at the interfaces of the device. In a study of photo and thermo-oxidation of poly (3-hexylthio- phene), Manceau et al. [2], showed that modifications of the chemical structure of poly (3-hexylthiophene) (P3HT) occur dur- ing the degradation which results in the formation of numerous oxidized species affecting the properties of the materials. In a recent study by Hintz et al. [3], where samples were degraded under controlled atmospheric conditions (dry synthetic air and relative humidity o2%), it was shown that illumination with wavelength close to the absorption maximum of the polymer leads to a break in the pi-conjugated system, and the primary attack takes place at the polymer backbone. Besides chemical degradation of the compounds, trap formation can be a major source of instability and deteriorate charge carrier mobility and exciton diffusion length [4]. Polymers exhibit a reversible p-type doping whose density increases upon exposure to air or moisture [5] due to charge reaction with oxygen in dark and under light exposure [6–10]. Degradation in OSCs depends not only on the organic materials but also on the device architecture. Current device lifetime is not limited by photo-oxidation of the polymer or the fullerene but by the failure at the photo-active layer/electrode interfaces. Indeed, Reese et al. [11], reported that P3HT:PCBM active layers under prolonged illumination of 1000 h at one sun intensity and under inert conditions, without additional layers exhibited no photo- oxidation, in contrast to complete devices where the photodegra- dation is caused by charge collection layers and interfaces [12]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.09.010 n Corresponding author. Tel.: þ32 9 264 8953. E-mail address: samira.khelifi@elis.ugent.be (S. Khelifi). Solar Energy Materials & Solar Cells 120 (2014) 244–252