[Paper Review] Low Hole Effective Mass p-type Transparent Conducting Oxides: Identification and Design Principles

Geoffroy Hautier, Anna Miglio+3 more|arXiv (Cornell University)|Apr 22, 2013

Copper-based nanomaterials and applications65 references617 citations

TL;DR

This study identifies non-Cu-based p-type transparent conducting oxides with exceptionally low hole effective masses (down to 0.27 m₀) and wide band gaps using high-throughput DFT and GW calculations on 3,052 oxides. It reveals two novel design principles: (1) hybridization of (n-1)d¹⁰ ns² cations (e.g., Sn²⁺, Sb³⁺) with oxygen, and (2) mixed anionic systems (e.g., O/S, O/P, O/Cl) to delocalize valence bands, enabling high mobility p-TCOs beyond traditional Cu-based materials.

ABSTRACT

The development of high performance transparent conducting oxides (TCOs) is critical to many technologies from transparent electronics to solar cells. While n-type TCOs are present in many devices, current p-type TCOs are not largely commercialized as they exhibit much lower carrier mobilities, due to the large hole effective masses of most oxides. Here, we conduct a high-throughput computational search on thousands of binary and ternary oxides and identify several highly promising compounds displaying exceptionally low hole effective masses (up to an order of magnitude lower than state of the art p-type TCOs) as well as wide band gaps. In addition to the discovery of specific compounds, the chemical rationalization of our findings opens new directions, beyond current Cu-based chemistries, for the design and development of future p-type TCOs.

Motivation & Objective

• To overcome the fundamental challenge of high hole effective mass in p-type transparent conducting oxides (TCOs), which limits their mobility and commercialization.
• To identify new, non-toxic, earth-abundant oxide materials with low hole effective masses and wide band gaps suitable for high-performance p-TCOs.
• To move beyond the long-dominant Cu-based chemistry by discovering alternative chemical systems with superior electronic properties.
• To establish new, chemically rational design principles for future p-TCO development based on electronic structure analysis.

Proposed method

• Conducted high-throughput density functional theory (DFT) calculations on 3,052 binary and ternary oxides using VASP with PBE functional and PAW pseudopotentials.
• Computed electronic band structures and effective masses using non-self-consistent band structure runs and Boltztrap for tensor averaging on a 8,000 k-point grid.
• Applied the one-shot GW method with ABINIT to obtain more accurate band gaps, using LDA for initial DFT and Godby-Needs plasmon pole approximation.
• Performed structural relaxations using AFLOW and used materials from the Materials Project database based on ICSD structures.
• Analyzed band character via wavefunction projections to identify orbital contributions (e.g., O 2p, S 3p, B 2p) and understand hybridization effects.
• Screened for compounds with hole effective mass <1.5 m₀ and band gap >2.5 eV, focusing on non-toxic, abundant elements.

Experimental results

Research questions

• RQ1 Which oxides exhibit exceptionally low hole effective masses while maintaining wide band gaps, enabling high mobility p-type TCOs?
• RQ2 What chemical and electronic factors enable low hole effective mass in oxides beyond Cu-based delafossites?
• RQ3 How do mixed anionic systems (e.g., O/S, O/P, O/Cl) influence valence band dispersion and effective mass?
• RQ4 Can (n-1)d¹⁰ ns² cations (e.g., Sn²⁺, Sb³⁺, Bi³⁺) form low-effective-mass p-TCOs through hybridization with oxygen p-orbitals?
• RQ5 What are the design principles for p-TCOs that avoid the limitations of oxygen p-orbital localization and high effective mass?

Key findings

• The compound B6O exhibits the lowest hole effective mass of 0.27–0.28 m₀, approaching the best electron effective masses in n-type oxides.
• Sb4Cl2O5 shows a hole effective mass of 0.37 m₀ and a wide band gap of 3.6 eV, making it a top candidate for high-performance p-TCOs.
• K2Sn2O3 has a very low hole effective mass of 0.27–0.28 m₀ but a relatively small band gap of 2.4 eV, which can be increased by Na substitution.
• ZrOS and HfOS exhibit larger band gaps (>4.5 eV) and moderate hole effective masses (~0.6–0.8 m₀), enabling full visible transparency.
• The study identifies two new design principles: (1) hybridization of (n-1)d¹⁰ ns² cations (e.g., Sn²⁺, Sb³⁺) with oxygen, and (2) mixed anionic systems with more delocalized p-orbitals (e.g., S²⁻, P³⁻, Cl⁻).
• These principles enable hole effective masses up to an order of magnitude lower than state-of-the-art p-TCOs, such as CuAlO2, and open new chemical spaces beyond Cu-based materials.