Effect of residual catalyst on solar cells made of a fluorene-thiophene-benzothiadiazole copolymer as electron-donor: A combined electrical and photophysical study Nadia Camaioni a,⇑ , Francesca Tinti a , Lorenzo Franco b , Marianna Fabris b , Antonio Toffoletti b , Marco Ruzzi b , Luciano Montanari c , Lucia Bonoldi c , Andrea Pellegrino d , Anna Calabrese d , Riccardo Po d,⇑ a Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, via P. Gobetti 101, I-40129 Bologna, Italy b Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy c Centro Ricerche Divisione Refining and Marketing, ENI S.p.A, via Maritano 26, 20097 San Donato Milanese, Italy d Centro Ricerche per le Energie non Convenzionali, Istituto ENI-Donegani, ENI S.p.A., via Fauser 4, 28100 Novara, Italy article info Article history: Received 30 September 2011 Received in revised form 30 November 2011 Accepted 11 December 2011 Available online 10 January 2012 Keywords: Palladium catalyst Charge transport Trap states Electron Spin Resonance Solar cells abstract The effects of residual catalyst impurities (palladium) on the hole mobility of a fluorene- thiophene-benzothiadiazole copolymer (poly{[4 0 -(9,9-bis(2-ethylhexyl)fluoren-2-yl)-2 0 ,1 0 ,3 0 - benzothiadiazole-7,7 0 -diyl]-co-[2 0 -(9,9-bis(2-ethylhexyl)fluoren-2-yl)thien-7,5 0 -diyl]}) (PFB-co-FT), as well as on its photovoltaic and photophysical response when blended with [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), are investigated. Two samples of the copolymer, only differing for the Pd content (9 and 3360 ppm), are considered and compared. The transport of positive carriers is characterized by a lower mobility and a higher dispersion in the Pd-rich PFB-co-FT sample. The photovoltaic parameters of PFB-co-FT:PCBM solar cells show a significant dependence on the residual catalyst impurities, attributed to a different concentration of trap states. Variations in charge mobility and trapping induced by impuri- ties was confirmed also by ESR (Electron Spin Resonance) experiments: an increased concen- tration of trapped charges in the presence of a higher level of metal impurities was revealed by light induced ESR, while the variation of polaron mobility correlates with the lifetime var- iation of the photogenerated PCBM triplet state detected by time-resolved ESR. All experi- mental evidences point to a strong effect of Pd impurities on the transport properties of charge carriers. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The performance of the so-called bulk-heterojunction (BHJ) solar cells [1], made of an interpenetrated network of a conjugated polymer as electron-donor (D) and a solu- ble fullerene derivative as electron-acceptor (A), under- went to a significant improvement during the recent years. This has been mainly achieved by the combination of improved material engineering [2] and fine control of the nanoscale morphology of the D/A blend [3]. Following the approach of the energy level engineering, novel donor materials with enhanced electronic properties have been prepared, leading to solar cell with efficiencies exceeding 8% [4]. To further improve the power conversion efficiency of BHJ polymer/fullerene solar cells, much efforts are required to understand the factors limiting the device performance and, among them, the transport properties of charge carriers in the interpenetrated D/A blends are attracting greater and greater attention with respect to the past [5]. 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.12.005 ⇑ Corresponding authors. E-mail addresses: camaioni@isof.cnr.it (N. Camaioni), riccardo.po@eni. com (R. Po). Organic Electronics 13 (2012) 550–559 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel