Article pubs.acs.org/JPCC
Effect of Molybdenum Oxide Electronic Structure on Organic Photovoltaic Device Performance: An X‑ray Absorption Spectroscopy Study Kee Eun Lee, Lijia Liu, and Timothy L. Kelly* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK S7N 5C9, Canada S Supporting Information *
ABSTRACT: While molybdenum oxide (MoO3) has been shown to be an effective hole transport layer in organic photovoltaic (OPV) devices, a complete understanding of its electronic behavior has proven elusive. In this work, thin films of substoichiometric molybdenum oxide (MoO3−x) were prepared via thermal evaporation and subjected to a variety of annealing conditions. The films were employed as the hole transport layers in organic photovoltaic devices, and the device performance was found to depend strongly on the annealing conditions: as-prepared MoO3−x films produced poly(3hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester devices with good performance (3.1% power conversion efficiency), while films annealed at higher temperatures or in a reducing atmosphere produced devices with very low efficiencies (≤1%). Through X-ray absorption near-edge structure (XANES) measurements at the Mo L3-edge, we show that while oxygen vacancies present in the as-prepared films may play a key role in hole extraction, extensive reduction of the molybdenum ions leads to more metallic behavior that results in a pronounced drop in device efficiency. These results clearly show that careful control over the MoO3−x stoichiometry is necessary in order to achieve the highest performance in OPV devices and further demonstrate the utility of XANES in correlating OPV device performance to changes in electronic structure.
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INTRODUCTION Organic photovoltaic (OPV) devices are extremely attractive candidates for solar energy applications due to their light weight, mechanical flexibility, and compatibility with roll-to-roll manufacturing processes.1−3 Typical OPV devices consist of a phase-separated blend of an electron donor and an electron acceptor (a bulk heterojunction), which is sandwiched between two electrodes; however, charge-selective interfacial layers are also required in order to obtain the best possible power conversion efficiencies (PCEs).4 This is especially true at the anodic interface, where a hole transport layer (HTL) facilitates hole extraction through better energy level matching with the electron donor and simultaneously blocks the transport of electrons; this substantially reduces unwanted recombination at the electrode surface. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most commonly used HTLs and has been shown to lead to excellent device performance when used with a wide variety of active layer materials. Despite this versatility, a number of studies have implicated the PEDOT:PSS hole transport layer as a major contributor to device degradation, limiting its ultimate utility.5 In order to circumvent this problem, a variety of transition metal oxides6 (such as molybdenum,7−13 nickel,14,15 vanadium,16,17 and tungsten18 oxide) have been explored as replacements for the PEDOT:PSS hole transport layer. In particular, molybdenum oxide (MoO3) has emerged as an excellent alternative to PEDOT:PSS. It is both highly © 2014 American Chemical Society
transparent and relatively easy to process, and MoO3 HTLs have now been prepared using both solution- and vapor-phase deposition techniques. Recent studies have also shown that OPV devices incorporating MoO3 HTLs display efficiencies that are as good or better than cells fabricated using PEDOT:PSS. In their study of poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) devices, Kim et al. reported an 8% improvement in the fill factor (FF) and a 42% reduction in the series resistance when the PEDOT:PSS HTL was replaced with a thermally evaporated MoO3 film;9 similarly, Stubhan et al. reported essentially identical PCEs for P3HT:PC61BM devices prepared using either PEDOT:PSS or solution-processed MoO3 nanoparticles as the HTL.10 Despite these promising results, there has been substantial disagreement in the literature as to the ideal composition of the molybdenum oxide film. Both thermal evaporation and solution deposition processes can introduce oxygen vacancies into the film, creating a substoichiometric material (MoO3−x) with subbandgap states that have been implicated in carrier transport.6,19−23 While some literature reports conclude that increasing the concentration of such states (e.g., through thermal annealing) results in an improvement in device Received: September 4, 2014 Revised: November 2, 2014 Published: November 12, 2014 27735
dx.doi.org/10.1021/jp508972v | J. Phys. Chem. C 2014, 118, 27735−27741
The Journal of Physical Chemistry C
Article
Figure 1. (a) UV−vis absorbance spectra and (b) Tauc plots of MoO3−x thin films on glass substrates, both as-prepared (solid purple squares) and after annealing at 200 °C in air (open blue circles), 200 °C under nitrogen (solid green triangles), 400 °C under nitrogen (open orange squares), and 200 °C under 4% hydrogen in nitrogen (solid red circles). Dashed lines show the linear fits to the high-energy region in the corresponding Tauc plots.
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performance,19 an excess of oxygen vacancies also reduces the transparency of the HTL, leading to a loss of photocurrent. Moreover, while MoO3 is a wide bandgap semiconductor, MoO2 displays metallic behavior; extensive reduction of MoO 3−x films can therefore lead to an increase in recombination at the HTL interface. Since the concentration of these oxygen vacancies depends strongly on the exact processing conditions used (e.g., evaporation rate, annealing temperatures and times) and also on other environmental factors (e.g., relative humidity, aging time), a clear picture has not emerged as to the exact correlation between MoO3−x electronic structure and device performance. X-ray and UV photoelectron spectroscopies have traditionally been used to probe the Mo oxidation state and valence band structure and to provide information about the MoO3−x/ organic semiconductor interface;6 however, the surface sensitivity of the technique makes it difficult to ascertain whether the spectra are truly representative of the bulk material or whether the results are skewed by the effects of surface oxidation or contamination.24 X-ray absorption near-edge structure (XANES) provides an elegant method of directly probing the Mo oxidation state both in the bulk (via fluorescence yield (FLY) measurements) and at the surface (via total electron yield (TEY) measurements). Unlike corelevel XPS in which the oxidation state of element of interest is resolved by peak fitting, XANES yields direct information on the presence of any unoccupied states near the conduction band edge and on the coordination environment of the Mo ions. Here we use XANES measurements at the Mo L3-edge to study MoO3−x films prepared by thermal evaporation and subjected to a variety of different annealing conditions. We then correlate these changes in electronic structure to changes in the efficiency of ITO/MoO3−x/P3HT:PCBM/LiF/Al devices. We show that the thermal evaporation process introduces oxygen vacancies throughout the bulk of the MoO3−x films and that the concentration of these vacancies can be increased by thermal annealing; however, this is found to have a detrimental impact on device performance. These results suggest that while these oxygen vacancies may be related to charge transport in the MoO3−x films, their concentration must be carefully controlled in order to achieve the highest possible PCEs for a given device configuration.
EXPERIMENTAL SECTION Device Fabrication. ITO-coated glass substrates (Delta Technologies, RS = 15−25 ohm/□) were cleaned by ultrasonication in detergent, deionized water, acetone, and isopropanol (20 min each) and blown dry. The substrates were subjected to a UV-ozone treatment for 15 min immediately prior to use. Thin films of MoO3 were thermally evaporated at a rate of 0.1 Å/s at a base pressure of
