Spectroscopic Studies of Chromium based p-type Transparent Conducting Oxides
Emma NORTON - Phd Student - Trinity College Dublin
A large range of chromium based p-type TCOs have been reported in the last 5 years. Two corundum lattice structures— MgxCr2−xO3 [1,2] and NixCr2−xO3 —have been shown to be p-type TCOs. Several other reports of high figure of merit (FOM) p-type TCOs also contain chromium in an octahedral coordination. Besides the prototypical MgxCr2−xO3 and NixCr2−xO3,these include LaCrO3:Sr  and spinel structures such as Cr2MnO4:Li . However, all such chromium based TCOs have shown poor mobility, well below the Hall limit, while maintaining comparable conductivity with higher mobility p-type TCOs.
In this study, we present a spectroscopy method, Resonant Valence Band Photoemission Spectroscopy (RVBPES), as an experimental technique to probe the composition of the valence band. RVBPES measurements show that the valence band of the p-type transparent oxides—crystalline MgxCr2−xO3 and nanocrystalline CuxCrO2—show striking similarities to measurements on crystalline CuCrO2:Mg [6, 7]. All films studied show that chromium states compose the top of the valence band, suggesting that the valence-band structure is dominated by the presence of the (Cr-O)6 octahedra. A comparison of the valence band between the best performing p-type, crystalline CuCrO2:Mg, with crystalline MgxCr2−xO3 and nanocrystalline CuxCrO2 shows that the chromium 3d states are fixed irrespective of changes in long-range crystallographic order. This indicates little spatial overlap between adjacent Cr 3d states. This further confirms the conduction mechanism via hopping for chromium based p-type TCOs as the Cr 3d states are localised within the (Cr-O)6 octahedra.  At the same time this explains why films with poor crystalline order can still perform comparable to epitaxial material, as the long range order is less important for a strongly localised valence band state.
Organic solar cells show large improvements in efficiency when buffer layers are employed between the organic layers and the electrodes. Cr2O3 and MgxCr2−xO3 buffer layers were found to increase the open circuit voltage while also increasing the efficiency of the solar cell by a factor of 2-3 when compared to a cell over the absence of a buffer layer. The energy band alignment p- MgxCr2-xO3 atop n-ITO was studied by X-ray photoelectron spectroscopy to provide an insight into the origin of the improvement of the efficiency. To move towards transparent diodes we utilise two approaches: to create p-i-n junction with a high mobility intrinsic layer (MgxIn2-xO3) and creating p-n junctions with a lower carrier concentration on the n-type side.
 L. Farrell et al. Phys. Rev. B 91, 125202 (2015)
 E. Arca et al. Appl. Phys. Lett. 99,111910 (2011)
 N. Uekawa and K. Kaneko, J. Phys. Chem. 100, 4193 (1996)
 K.H.L. Zhang et al. Adv. Mater. 27, 5191 (2015)
 H. Peng et al. Adv. Func. Mater. 23, 5267 (2013)
 Yokobori et al. Phys. Rev. B 87, 195124 (2013)
 E. Norton et al. Phys. Rev. B 93, 115302 (2016)
Date of update June 12, 2016