Ultrathin TaOx film based photovoltaic device Pawan Tyagi ⁎ Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, 40506, USA abstract article info Article history: Received 31 August 2010 Received in revised form 24 November 2010 Accepted 24 November 2010 Available online 2 December 2010 Keywords: Photovoltaic cell Tantalum oxide Tunnel junction Atomic defects p–n junction Application of the economical metal oxide thin-film photovoltaic devices is hindered by the poor energy efficiency. This paper investigates the photovoltaic effect with an ultrathin tantalum oxide (TaOx) tunnel barrier, formed by the plasma oxidation of a pre-deposited tantalum (Ta) film. These ~ 3 nm TaOx tunnel barriers showed approximately 160 mV open circuit voltage and 3–5% energy efficiency, for varying light intensity. The ultrathin TaOx (~ 3 nm) could absorb approximately 12% of the incident light radiation in 400– 1000 nm wavelength range; this strong light absorbing capability was found to be associated with the dramatically large extinction coefficient. Spectroscopic ellipsometry revealed that the extinction coefficient of 3 nm TaOx was ~0.2, two orders higher than that of tantalum penta oxide (Ta 2 O 5 ). Interestingly, refractive index of this 3 nm thick TaOx was comparable with that of stochiometeric Ta 2 O 5 . However, heating and prolonged high-intensity light exposure deteriorated the photovoltaic effect in TaOx junctions. This study provides the basis to explore the photovoltaic effect in a highly economical and easily processable ultrathin metal oxide tunnel barrier or analogous systems. Published by Elsevier B.V. 1. Introduction The realization of thin film photovoltaic devices offers a lucrative option for the replacement of costly silicon based solar cells [1]. A thin film solar cell should be able to absorb sufficient light radiation. More importantly, it should be endowed with a mechanism to separate the photo-energy created electron-hole pairs [1], akin to built-in potential [2,3] in the p–n junction solar cells. Several forms of thin film solar cells, involving thin film silicon, cadmium telluride (CdTe), Copper indium gallium selenide (CuInSe), and dye sensitized solar cells have been employed. In these solar cells, typical thickness of photovoltaic materials ranges from few nm to tens of μm. Thin film metal oxide based photovoltaic cells [4] are promising candidates for the environment friendly and economical harvesting of solar energy [5]. Devices based on ~100 nm thick semiconducting cuprous oxide (Cu 2 O) exhibited a photovoltaic effect [5,6]. The mechanism of charge separation in Cu 2 O was attributed to the dissimilar metal contacts with different work functions [6], and the presence of atomic defects [7,8]. The maximum ~2% solar cell efficiency was achieved with Cu 2 O based solar cells, which is well below the theoretically predicted value of 20% for this system [9]. In order to enhance the light absorption capability, generally a thick Cu 2 O (~ 100 nm) [8] was utilized. However, thick Cu 2 O also possesses large population of atomic defects [7]; a higher defect density can annihilate the photo-generated electron-hole pairs before they are separated and transferred to the opposite electrodes [9]. A better approach to make the metal oxide based solar cells is to reduce the thickness of metal oxide film to a few nm, yet managing to keep the light absorbance reasonably high. Utilization of 2–3 nm thick photoactive layers has already been demonstrated. Gratzel cells or Dye Sensitized Solar Cell (DSSC) utilized only a dye molecules monolayer to produce ~8% energy conversion efficiency [10]. This monolayer of dye molecules absorbs a significant part of incident radiation. It is logical to think that if a 2–3 nm thick tunnel barrier, equivalent to DSSC's molecular monolayer thickness, possess a high density of photoactive defects then significant light radiation can be absorbed. In this analogy defects in tunnel barrier will be equivalent to individual dye molecules. Additionally, if this tunnel barrier band diagram can be modified to yield a skewed band diagram similar to p– n junction based solar cells or metal-insulator-semiconductor (MIS) solar cell then a net photovoltaic effect can be realized. Graded density of dopants/defects has been observed to produce band banding in tunnel junctions and solar cells [1,11]. The tunnel junctions utilizing aluminum (Al) electrodes and 3 nm alumina (AlOx) have exhibited ~ 0.9 eV difference in the barrier heights at two Al/AlOx interfaces [1]. This study suggested that an inhomogeneous oxidation can alone produce significant band banding, similar to p–n junction solar cells [1,11]; no photovoltaic effect was reported with the Al/AlOx/Al system. A tantalum oxide (TaOx) tunnel barrier is particularly promising to observe photovoltaic effect. X-ray photoelectron emission (XPS) studies of TaOx, grown by the thermal oxidation of tantalum (Ta) metal, exhibited the graded distribution of Tantalum ions (Ta + ) Thin Solid Films 519 (2011) 2355–2361 ⁎ Present address: Department of Civil and Mechanical Engineering, University of the District of Columbia, Washington, DC, 20008, USA. E-mail address: ptyagi@udc.edu. 0040-6090/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.tsf.2010.11.039 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf