LMGP Seminar - 15/06/2016 - Emma NORTON

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— Mg_xCr_2−xO₃ [1,2] and Ni_xCr_2−xO₃ [3]—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 Mg_xCr_2−xO₃ and Ni_xCr_2−xO₃,these include LaCrO₃:Sr [4] and spinel structures such as Cr₂MnO₄:Li [5]. 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 Mg_xCr_2−xO₃ and nanocrystalline Cu_xCrO₂—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 Mg_xCr_2−xO₃ and nanocrystalline Cu_xCrO₂ 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. [7] 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 Mg_xCr_2−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- Mg_xCr_2-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 (Mg_xIn_2-xO₃) and creating p-n junctions with a lower carrier concentration on the n-type side.

[1] L. Farrell et al. Phys. Rev. B 91, 125202 (2015)

[2] E. Arca et al. Appl. Phys. Lett. 99,111910 (2011)

[3] N. Uekawa and K. Kaneko, J. Phys. Chem. 100, 4193 (1996)

[4] K.H.L. Zhang et al. Adv. Mater. 27, 5191 (2015)

[5] H. Peng et al. Adv. Func. Mater. 23, 5267 (2013)

[6] Yokobori et al. Phys. Rev. B 87, 195124 (2013)

[7] E. Norton et al. Phys. Rev. B 93, 115302 (2016)