Low Temperature Aqueous Electrodeposited TiO x Thin Films as Electron Extraction Layer for Efficient Inverted Organic Solar Cells Kim Hai Wong,* ,†,‡ Chad William Mason, †,⊥ Sappani Devaraj, †,∥ Jianyong Ouyang,* ,§ and Palani Balaya* ,† † Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore ‡ Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Singapore 117574, Singapore § Department of Material Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore * S Supporting Information ABSTRACT: Organic solar cells based on poly(3-hexylthiophene) and [6,6]- phenyl-C 61 -butyric acid methyl ester were fabricated with electrodeposited TiO x electron extraction layers 5−180 nm thick. Electrodeposition under ambient conditions is an attractive, facile and viable approach to prepare metal oxide interfacial layers. The TiO x films obtained displayed a linear relationship between thickness and deposition time when fabricated under ambient conditions using an aqueous air stable peroxotitanium precursor. The precursor solution was prepared from titanium isopropoxide using a chelate process, which allowed water to be used as solvent due to considerably decreased sensitivity of the precursor solution towards hydrolysis. Highly conformal TiO x films, typically observed with vacuum deposition techniques, were obtained on the indium tin oxide substrate upon electrogeneration of OH − ions using H 2 O 2 additive. Conversely, significantly rougher films with spherical growths were obtained using NO 3 − additives. Low temperature annealing at 200 °C in air was found to greatly improve purity and O stoichiometry of the TiO x films, enabling efficient devices incorporating the electrodeposited TiO x to be made. Using MoO x as the hole extraction layer, the maximum power conversion efficiency obtained was 3.8% (V oc = 610 mV; J sc = 10.6 mA/cm 2 ; FF = 59%) under simulated 100 mW/cm 2 (AM1.5G) solar irradiation, whereas an efficiency of 3.4% was achieved with fully solution processed interfacial layers comprising the electrodeposited TiO x films and a surfactant-modified PEDOT:PSS hole extraction layer. KEYWORDS: Electrodeposition, inverted organic photovoltaic, titanium oxide, thin film, solution processed, morphology ■ INTRODUCTION Organic photovoltaics (OPVs) represent an important class of solar technology that utilizes conjugated organic molecules to capture and convert solar energy into electricity. 1 Current interest in OPV research is strong due to the commercialization potential of this technology as a low cost alternative to mainstream solar cell technologies. Portable electronics featuring lightweight and flexible OPVs have been demon- strated in recent years using scalable cost effective techni- ques. 2−5 Charge selective interfacial layers are critical components in OPVs that provide an intermediary energy step between the bulk heterojunction (BHJ) and a metallic contact, so that a charge selective pathway to the external circuit for one of the two charge carriers in the photoactive layer is formed. 6−8 Typically, the transparent tin-doped indium oxide (ITO) substrate is modified with a hole extraction layer (HEL) so that ITO assumes the role of the anode where holes are extracted (Figure 1). By depositing an electron extraction layer (EEL) on ITO, however, the substrate is modified into the device cathode, resulting in a reversed current flow and a device architecture known as the inverted OPV (iOPV) (Figure 1). iOPVs exhibit distinct improvements in lifetime over conven- tional OPVs because (1) low work function metals that oxidize readily, such as Ca typically used in metalizing conventional OPVs, are avoided and (2) the acidic poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PE- DOT:PSS) HEL is not in direct contact with ITO, thus preventing PEDOT:PSS-induced etching that is known to be detrimental to device lifetime. 9−13 Metal oxide interfacial layers are robust films with the desirable optoelectronic properties and ambient stability required for fabrication of efficient and stable iOPVs. Although both vacuum and wet deposition processes for metal oxide EEL have been reported in literature, the latter stands out because metal oxides, such as TiO x , are readily formed through sol−gel or nanoparticle approaches. 12−26 Electrodeposition is an attractive route for facile and potentially scalable fabrication Received: November 18, 2013 Accepted: January 31, 2014 Published: January 31, 2014 Research Article www.acsami.org © 2014 American Chemical Society 2679 dx.doi.org/10.1021/am405193r | ACS Appl. Mater. Interfaces 2014, 6, 2679−2685