is directly related to its carrier concentration (n) and carrier mobility (μ) by σ = neμ where e is the elemental charge. The carrier mobility is given by / µ τ = * e m with τ being the carrier relaxation time and m* the carrier effective mass. [5] The optical transparency (T opt ) is exponentially proportional to both optical absorption coefficient (α) and film thickness (d), via T opt ∝ e -αd . [6] The T opt of a thin film can be optimized by choosing the material with weak absorption or decreasing the film thickness. Accordingly, the strong optical absorption due to the direct interband transition usually characterizes the optical bandgap.A typical approach to TCOs is starting from wide bandgap oxides (e.g., In 2 O 3 , SnO 2 , and ZnO) and making them conducting by doping, which is successful in developing a series of high-performance n-type TCOs such as Sn-doped In 2 O 3 (ITO), Al-doped ZnO (AZO), and F-doped SnO 2 (FTO). [7][8][9][10] The conduction band minimum (CBM) and valence band maximum (VBM) of these parent oxides are primarily consisted of the unoccupied ns 0 orbital and fully occupied oxygen 2p orbital, respectively. The highly dispersive CBM of ns orbital gives small (high) m* (μ) and excellent n-type conductivity by donor doping. [1,9] However, a straightforward transformation of the n-type TCOs to p-type TCOs via acceptor doping is very difficult. The localized oxygen 2p orbital at the VBM leads to large (low) hole m* (μ), which becomes the fundamental limitation to obtain highly mobile holes at the VBM. Furthermore, it is difficult to introduce hole carriers due to the high formation energy of the native acceptors as well as the self-compensation effect (easily formed native donors annihilate holes). [11,12] Engineering the VBM by alloying the oxygen 2p orbitals with the d or s orbitals of metal cations, i.e., "chemical modulation of the valence band" (CMVB) proposed by Kawazoe et al., has been used to solve this conundrum and promoted the development of a number of p-type TCOs. [13][14][15][16][17][18][19][20][21][22][23] High-throughput material screening has also identified several highly promising p-type TCOs displaying small (high) hole m* (μ). [24][25][26] Despite the improvement of hole mobility, these p-type TCOs still exhibit low σ, several orders of magnitude lower than that of n-type TCOs, primarily due to the doping bottleneck in increasing the hole concentration by intrinsic defects (cation vacancies or interstitial oxygen) and/or the solubility limit of the acceptors. Transparent conducting oxides (TCOs) are a unique series of materials combining electrical conductivity with optical transparency. Most of the commercially available TCOs are of the n-type. The performance of p-type TCOs, however, is far behind their n-type TCO counterparts primarily due to the doping bottleneck and low hole mobility. Herein, a Mott-Hubbard insulator of LaVO 3 is proposed as the starting point for exploring p-type TCOs. Substitution of La 3+ by Sr 2+ can introduce high hole carrier concentration at th...

Transparent conducting oxides (TCOs), combining the mutually exclusive functionalities of high electrical conductivity and high optical transparency, lie at the center of a wide range of technological applications. The current design strategy for n-type TCOs, making wide bandgap oxides conducting through degenerately doping, obtains successful achievements. However, the performances of p-type TCOs lag far behind the n-type counterparts, primarily owing to the localized nature of the O 2p-derived valence band (VB). Modulation of the VB to reduce the localization is a key issue to explore p-type TCOs. This Perspective provides a brief overview of recent progress in the field of design strategy for p-type TCOs. First, the introduction to principle physics of TCOs is presented. Second, the design strategy for n-type TCOs is introduced. Then, the design strategy based on the concept of chemical modulation of the valence band for p-type TCOs is described. Finally, through the introduction of electron correlation in strongly correlated oxides for exploring p-type TCOs, the performance of p-type TCOs can be remarkably improved. The design strategy of electron correlation for p-type TCOs could be regarded as a promising material design approach toward the comparable performance of n-type TCOs.

Bismuth ferrite (BiFeO3) has recently become interesting as a room‐temperature multiferroic material, and a variety of prototype devices have been designed based on its thin films. A low‐cost and simple processing technique for large‐area and high‐quality BiFeO3 thin films that is compatible with current semiconductor technologies is therefore urgently needed. Development of BiFeO3 thin films is summarized with a specific focus on the chemical solution route. By a systematic analysis of the recent progress in chemical‐route‐derived BiFeO3 thin films, the challenges of these films are highlighted. An all‐solution chemical‐solution deposition (AS‐CSD) for BiFeO3 thin films with different orientation epitaxial on various oxide bottom electrodes is introduced and a comprehensive study of the growth, structure, and ferroelectric properties of these films is provided. A facile low‐cost route to prepare large‐area high‐quality epitaxial BFO thin films with a comprehensive understanding of the film thickness, stoichiometry, crystal orientation, ferroelectric properties, and bottom electrode effects on evolutions of microstructures is provided. This work paves the way for the fabrication of devices based on BiFeO3 thin films.