Electrospun ZnO Nanowire Plantations in the Electron Transport Layer for High-Efficiency Inverted Organic Solar Cells Naveen Kumar Elumalai, †,‡,§,|| Tan Mein Jin, †,‡,§ Vijila Chellappan,* ,§ Rajan Jose,* ,⊥ Suresh Kumar Palaniswamy, # Sundaramurthy Jayaraman, || Hemant Kumar Raut, ‡,§,|| and Seeram Ramakrishna ‡,|| ‡ National University of Singapore, Singapore 117576 § Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602 ⊥ Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Kuantan, Pahang, Malaysia # Environmental & Water Technology, Centre of Innovation, Ngee Ann Polytechnic, Singapore || Center for Nanofibers and Nanotechnology, National University of Singapore, Singapore * S Supporting Information ABSTRACT: Inverted bulk heterojunction organic solar cells having device structure ITO/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) /MoO 3 /Ag were fabricated with high photoelectric conversion efficiency and stability. Three types of devices were developed with varying electron transporting layer (ETL) ZnO architecture. The ETL in the first type was a sol-gel-derived particulate film of ZnO, which in the second and third type contained additional ZnO nanowires of varying concentrations. The length of the ZnO nanowires, which were developed by the electrospinning technique, extended up to the bulk of the photoactive layer in the device. The devices those employed a higher loading of ZnO nanowires showed 20% higher photoelectric conversion efficiency (PCE), which mainly resulted from an enhancement in its fill factor (FF). Charge transport characteristic of the device were studied by transient photovoltage decay and charge extraction by linearly increasing voltage techniques. Results show that higher PCE and FF in the devices employed ZnO nanowire plantations resulted from improved charge collection efficiency and reduced recombination rate. KEYWORDS: renewable energy materials, charge transport layers, inverted polymer solar cells, heirarchical structures, carrier lifetime, electrospinning ■ INTRODUCTION Organic solar cells (OSCs) have attracted considerable interest as a potential renewable energy device because they can be prepared at lower cost, with lower environmental load and flexibility, and are lightweight compared to the conventional silicon solar cells. 1,2 The photoactive layer of OSCs consists of a p-conjugated polymer as an electron donor and a fullerene derivative as an electron acceptor. A large number of stable conjugated polymers at atmospheric conditions are proposed for efficient and stable OSCs. 3-6 Because of their acceptable hole and electron conductivities, desirable optical absorption cross-section, and atmospheric stability, the poly3-hexylthio- phene (P3HT): l-3-methoxycarbonylpropyl-L-phenyl-6,6 meth- anofullerene (PCBM) conjugate has emerged as one of the popular choices to build high-efficiency OSCs. Recent advances in developing new organic active layer materials and device- processing techniques have led to improvement in photo- electric conversion efficiencies (PCE) exceeding ∼10%. 7,8 The P3HT:PCBM photoactive layer is sandwiched between two electrodes comprising an indium tin oxide (ITO) coated with poly(3,4-ethylenedioxythiophene)-poly(styrene sulfo- nate) (PEDOT:PSS) (anode) and a low work function metal, typically Al and Ag (cathode) in OSCs. However, operational stability of the above devices is poor under ambient conditions for practical applications because of many complex processes in the photoactive as well as other functional layers. 9 To address the stability issues, inverted OSCs (IOSCs) 10-14 are proposed. In IOSCs, electrons are collected at the ITO side by coating it with an n-type metal oxide semiconductor (MOS), which serve Received: April 16, 2013 Accepted: September 12, 2013 Published: September 12, 2013 Research Article www.acsami.org © 2013 American Chemical Society 9396 dx.doi.org/10.1021/am4013853 | ACS Appl. Mater. Interfaces 2013, 5, 9396-9404