www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2594 www.MaterialsViews.com wileyonlinelibrary.com Adv. Funct. Mater. 2012, 22, 2594–2605 Jacek J. Jasieniak, Jason Seifter, Jang Jo, Tom Mates, and Alan J. Heeger* 1. Introduction The development of appropriate electron and hole charge blocking and transport layers is paramount to the fabrication of more efficient solution processed optoelectronic devices such as organic photovoltaic cells (OPVs) and light-emitting diodes (OLEDs). [1–2] The role of such layers is to create asymmetrical interfaces with respect to charge injection and collection. In OPVs, [1] this asymmetry enables reduced recombination of photogenerated electrons and holes at the electrodes, while in OLEDs, [2] it enables specific control of the spatial distribution and density of carriers in a device. The importance of such modifying layers in governing the interfacial and bulk electronic properties in these devices highlights that their degradation will have an adverse influence on device performance. Thus, only materials with suitable electronic properties and long term stability can be considered useful for the development of such layers. The original works on OPVs [3–5] and OLEDs [6,7] utilized bare indium tin oxide (ITO) electrodes as the anode and low work function metals (e.g., Ca, Ba or Al) as the cathode. It was quickly realized that in these proof-of-concept devices, the low work function and interfacial instability of the ITO were limiting factors for achieving high device performance. [8,9] The deposi- tion of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), a highly conductive polymer, as an inter- layer on the ITO, was subsequently shown to result in a more stable interface. [10,11] Advantageously, the higher work function of the PEDOT:PSS (5.2 eV) compared to ITO (4.8 eV) was also found to facilitate better hole injection into poly(2-methoxy- 5-(2 ′ -ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) (ioni- zation energy at 5.1 eV), [10,12] thus permitting higher external quantum efficiencies for electroluminescence to be realized. Maximizing the internal electric field through the formation of ohmic contacts at the anode and cathode enables the highest device performances to be achieved. [2] For the majority of donor materials used in OPVs, PEDOT:PSS has been found to possess a sufficiently high work function to meet this requirement. [13] This factor is exemplified through recent reports of optimized thieno[3,4-b]-thiophene/benzothiophene/benzothiophene:[6,6]- phenyl C 71 butyric acid methyl ester (PTB7:PC 71 BM) bulk het- erojunction OPVs that exhibited certified photoconversion effi- ciencies exceeding 8.3% based on this anode configuration. [14] Despite the promising electrical characteristics of OPVs that use PEDOT:PSS, its high acidity and hygroscopicity have also now been associated with long term device instability. [8,15,16] These factors cooperatively act to degrade the metallic cath- odes [17] and de-dope the polymer layer itself. [18] Finding a material with the necessary electronic properties to replace PEDOT:PSS is thus a current challenge that needs to be resolved. Transition metal oxides are a class of material that offers the necessary tunability in their electronic properties and intrinsic stability towards oxidation to be considered highly attractive for organic electronic applications. [19] The use of high work func- tion transition metal oxides for anode modifications was first reported by Tokito et al., [20] who demonstrated that the oper- ating voltages of OLEDs were significantly reduced through the use of evaporated MO x (where M = vanadium, molybdenum, or ruthenium and x is the oxygen stoichiometry) layers as ITO A Solution-Processed MoO x Anode Interlayer for Use within Organic Photovoltaic Devices A simple, solution-processed route to the development of MoO x thin-films using oxomolybdate precursors is presented. The chemical, structural, and electronic properties of these species are characterized in detail, within solu- tion and thin-films, using electrospray ionization mass spectrometry, grazing angle Fourier transform infrared spectroscopy, thermogravimetric analysis, atomic force microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. These analyses show that under suitable deposi- tion conditions the resulting solution processed MoO x thin-films possess the appropriate morphological and electronic properties to be suitable for use in organic electronics. This is exemplified through the fabrication of poly(3- hexylthiophene):[6,6]-phenyl C 61 butyric acid methyl ester (P3HT:PC 61 BM) bulk heterojunction (BHJ) solar cells and comparisons to the traditionally used poly(3,4-ethyldioxythiophene)/poly(styrenesulfonate) anode modifying layer. DOI: 10.1002/adfm.201102622 Dr. J. J. Jasieniak, J. Seifter, Dr. J. Jo, Prof. A. J. Heeger Center for Polymer and Organic Solids University of California Santa Barbara, CA, 93106, USA E-mail: ajhe@physics.ucsb.edu Dr. J. J. Jasieniak CSIRO Materials Science and Engineering Clayton, Victoria, 3168, Australia Dr. T. Mates Materials Research Laboratory University of California Santa Barbara, CA, 93106, USA