Structural/morphological monitoring approach to stability and durability issues of photoactive films for organic solar cells B. Paci a,⇑ , A. Generosi a , D. Bailo a,b , R. Caminiti b , R. de Bettignie c , V. Rossi Albertini a a ISM-CNR-Area di Ricerca di Tor Vergata, Via del Fosso del Cavaliere 100, 00133 Roma, Italy b Dip. to di Chimica, Università ‘La Sapienza’, P.le A. Moro 5, 00185 Roma, Italy c Composants Solaires CEA INES-RDI, SaVoie Technolac, BP 332, 50 Avenue du lac Léman, 73377 Le Bourget Du Lac, France article info Article history: Received 29 October 2010 In final form 2 February 2011 Available online xxxx abstract Here we report on the effect of light on surface and bulk morphology, structural and optical properties of polymer/fullerene blends used for organic solar cells. The crystallization kinetics of the organic film mol- ecules was followed in real time during illumination, by time-resolved X-ray diffraction, applied in-situ, jointly with atomic force microscopy. The occurrence of possible phase separation of the two components was detected. This latter hypothesis was confirmed by Fourier Transform Infrared spectroscopy experi- ments. As a result of such cross monitoring, a comprehensive picture of the chemical/physical process concurring to the devices photo-active layers aging was gained. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction At present, plastic photovoltaic (PV) cells are among the most interesting candidates for environmental-friendly and low-cost conversion of solar energy. Enhanced photovoltaic conversion effi- ciency was obtained using bulk heterojunctions, in which the ac- tive element is a blend of an electron donor and an electron acceptor organic material. The basic mechanism allowing the remarkable performances of such devices is photoinduced ultra- fast electron transfer from a conjugated polymer to fullerene [1]. Notable improvements in Power Conversion Efficiencies (PCE) [2,3] were obtained using P3HT (poly(3-hexyl thiophene)) blended with methano-fullerene [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM), i.e. the system object of the present study. Noteworthy, an encouraging raise in lifetime (up to 10 000 h) was obtained, pro- vided that contact with environmental oxygen and moisture is pre- vented [4,5]. However, for commercialization of organic PV devices to take place, a substantial boost of their performances is still required, efficiency and lifetime remaining the major aspects to address. A breakthrough in this direction may be obtained via the optimiza- tion of the structural/morphological properties of the photo-active component of the cell. Several studies focused on this aspect and it is now clear that the photovoltaic properties strongly depend on the structure and morphology of P3HT and PCBM nanodomains [6]. It follows that the polymer–fullerene matrix nanoscale charac- teristics [7–10] and the molecular order of the two components, strongly influencing the film charge transport properties, are key aspects to control [11–13]. In particular, polymer regularity and film molecular packing are believed to play a crucial role, on both the optical and electrical properties of bulk heterojunction films [14–16]. Despite this, topographical and structural investigations are often limited to the results of ex-situ experiments. Conversely, in-situ studies, allowing the real time degradation of the cell com- ponents to be monitored during exposition to light, would be essential to understand the aging mechanisms limiting organic de- vices durability. With this goal, the present Letter reports the results of the joint in-situ morphological/structural studies of P3HT:PCBM bulk het- erojunctions. Time-resolved Energy Dispersive X-ray Diffraction (EDXD) was applied, in synergy with Atomic Force Microscopy (AFM) experiments, to perform ‘non-perturbative and non- destructive’ investigations. In the ED mode [17,18], the patterns are collected using a polychromatic beam and carrying out an en- ergy scan of the diffracted radiation by means of an energy sensi- tive detector. Indeed, the diffracted intensity is a function of the momentum transferred from the X-rays to the sample electrons. The momentum transferred, in turn, depends on both the reflection angle and the energy of the X-rays. The adoption of the ED mode allows for a simplified experimental geometry, kept unchanged during data collection. The stationary geometry of the apparatus turns out to be particularly suitable for in situ time-resolved exper- iments, as in the present case. Indeed, once the X-ray beam path is set, the irradiated portion of the sample does not change during the whole observation time, therefore allowing to collect information on a selected component of the device. Reversely, the variable geometry required in the conventional laboratory Angular Disper- sive X-ray Diffraction technique would induce a change of both the 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.02.011 ⇑ Corresponding author. E-mail address: barbara.paci@ism.cnr.it (B. Paci). Chemical Physics Letters xxx (2011) xxx–xxx Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett Please cite this article in press as: B. Paci et al., Chem. Phys. Lett. (2011), doi:10.1016/j.cplett.2011.02.011