Comparison of short and long wavelength absorption electron donor materials in C 60 -based planar heterojunction organic photovoltaics Shun-Wei Liu a,⇑ , Wei-Cheng Su b , Chih-Chien Lee b,⇑ , Chi-Feng Lin c , Shih-Chieh Yeh d , Chin-Ti Chen d,⇑ , Jiun-Haw Lee e,⇑ a Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan, ROC b Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10617, Taiwan, ROC c Department of Electro-Optical Engineering, National United University, Miaoli 36003, Taiwan, ROC d Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC e Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei 10607, Taiwan, ROC article info Article history: Received 4 December 2011 Received in revised form 4 June 2012 Accepted 6 June 2012 Available online 28 June 2012 Keywords: Planar heterojunction organic photovoltaics Field-effect transistor Open circuit voltage Fill factor C 60 abstract Buckminsterfullerene, C 60 -based planar heterojunction (PHJ) organic photovoltaics (OPVs) have been created using a short wavelength absorption (k max = 490 nm) electron-donating bis(naphthylphenylaminophenyl)fumaronitrile (NPAFN). NPAFN exhibits a hole mobility greater than 0.07 cm 2 V 1 s 1 as determined by its field-effect transistor. It can be attrib- uted to such hole mobility that enables a thin layer (<10 nm) NPAFN in PHJ OPV, ITO/ NPAFN/C 60 /bathocuproine/Al. Because of the low lying HOMO energy level (5.75 eV) of NPAFN and relatively high ionization potential ITO (5.58 eV), such OPVs exhibit a very high open circuit voltage of 1.0 V, relatively high fill factor of 0.60, and a relatively high shunt resistance of 1100 X cm 2 , which all compensate for a relatively low short circuit current of 3.15 mA cm 2 due to the short absorption wavelength and inferred short exciton diffusion length of NPAFN. Altogether, NPAFN OPVs display a power conversion efficiency (g PC ) of 2.22%, which is better than other long wavelength absorption materials in similar PHJ OPVs, such as pentacene (k max 670 nm, HOMO 5.12 eV, g PC 1.50%) and copper phthalo- cyanine (k max 624, 695 nm, HOMO 5.17 eV, g PC 1.43%). Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction Since Tang’s report on the first thin-film organic photo- voltaic (OPV) based on small molecules in a donor-accep- tor planar heterojunction (PHJ) configuration, extensive research efforts have been devoted to the development of more efficient materials or device architecture [1–6]. Due to the limitation of the so-called ‘‘diffusion length bottle- neck’’ [7], PHJ OPVs may not be the optimum device struc- ture for short circuit currents (J SC ), and hence power conversion efficiency (g PC ). However, compared with bulk heterojunction (BHJ), planar-mixed heterojunction (PM-HJ), or p (or n)-doped hole (or electron) transporting layer OPVs, PHJ OPVs have a simple structure and more reproducible photocurrent–photovoltage (J–V). Therefore, PHJ OPVs are a useful platform for the testing and charac- terizing of materials for OPVs. In order to overcome the inadequate J SC , long wavelength (k max near or longer than 600 nm) and strong absorption metallophthalocyanines (MPc), boron subphthalocyanine chloride (SubPc), or dicy- anovinyl-appended oligothiophenes (DCV5T and DCV6T) are the most desirable for converting broad spectral solar energy into electrical power [8–22]. Other long wavelength absorption polycyclic aromatic hydrocarbons (PAHs), such as pentacene, tetraphenyldi- benzoperiflanthene (DBP), diindenoperylene (DIP), are also ideal choices [23–29]. Such PAHs often have high charge carrier mobility (such as 0.1 cm 2 V 1 s 1 of DIP) [29] and are beneficial to hole transport of dissociated donor 1566-1199/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.006 ⇑ Corresponding authors. E-mail addresses: swliu@mail.mcut.edu.tw (S.-W. Liu), cclee@mail. ntust.edu.tw (C.-C. Lee), cchen@chem.sinica.edu.tw (C.-T. Chen), jhlee@ cc.ee.ntu.edu.tw (J.-H. Lee). Organic Electronics 13 (2012) 2118–2129 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel