Hole Selective MoO_x Contact for Silicon Solar Cells

Abstract

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

Transition metal oxides as hole-selective contacts in silicon heterojunctions solar cells

“…These results limit the oxide thickness on the final device to only a few nanometers and justifies the need for an ITO collecting electrode. Ultimately, actual conductivities will depend on the morphology of thermally-evaporated TMOs, which has been reported as completely amorphous for MoO x [20] and nano-crystalline for WO x [30]. Further study of preparation methods and post-deposition treatments could promote oxide crystallinity in order to enhance film conductivities.…”

“…Particularly, heterojunction solar cells based on p-type c-Si (p-Si) and TMOs acting as p-type Back Surface Fields (BSFs) were reported recently [18,19], demonstrating low contact resistivities and efficiencies of 15%. Furthermore, the use of TMOs as p-type emitters in n-type c-Si (n-Si) has also been investigated for MoO 3 [18,20,21] and WO 3 [22], demonstrating a power conversion efficiency of 18.8% for this novel solar cell concept [23].…”

MoS2 P-type Transistors and Diodes Enabled by High Work Function MoOx Contacts

“…MoOx exhibits a high work function potential of up to ~6.6 eV (see Fig. 1a) [16] exceeding those of elemental metals [11].…”

“…Consequently MoOx can be classified as a semiconducting oxide with a metallic defect band. Its workfunction can exceed 6.6 eV, but is known to strongly depend on carbon contamination [16]. For practical applications, MoOx can thus be considered to act as a high workfunction metal with a low density of states at the Fermi level.…”

“…Unlike MoS2, WSe2 has been shown to exhibit a surface Original device characteristics are restored upon placement in vacuum. This observation can be attributed to the sensitivity of the MoOx work function to ambient gas exposure [16]. This behavior is similar to elemental metal contacts to devices, which also show barrier height modulation due to gas exposure, and can be remedied by encapsulating the device [7] [9].…”