Recent Progress of Solution‐Processed Copper‐based p‐Channel Thin‐Film Transistors

Abstract

unstoppable march of today's technology, it is of great demand to develop alternative and inexpensive processing technologies for fabricating high-performance p-channel TFTs. Ink-jet printing, by which semiconductor materials can be deposited on flexible substrates such as polymer films or paper, is one such promising alternative. [5][6][7] However, due to the difficulty in formulating the printable ink precursors of inorganic materials compared to organic ones, printing technology is generally uncommon in metal oxides TFTs, particularly in p-type channels. [8,9] Another obstacle facing this technology is how to improve the purity and densification of the metal-oxide-metal (M-O-M) framework in the solution-processed thin films at low temperatures. To resolve this problem and allow device flexibility, novel approaches based on manipulating the metal salt precursor, solvents, and postdeposition annealing methods had been established. [10,11] Stability, or reproducibility under ambient conditions of humidity and temperature, [12] is a third challenge facing solution-based oxide electronic devices. These roadblocks are common for all solution-based metal oxide thin films, yet p-type metal oxide TFTs in particular, face another essential defiance which is the lack of appropriate hole mobility. The most critical issues that solutionbased p-type metal oxide TFTs need to resolve are summarized schematically in Figure 1a.The difficulty in achieving p-type metal oxides with analogous performance to the n-type counterparts originates from the entirely different configurations of their conduction band minimum (CBM) and valence band maximum (VBM). Mainly, in metal oxides the CBM consists of metal s orbitals, offering a small electron effective mass and thus high mobility. In contrast, the VBM is principally composed of localized oxygen 2p orbitals, resulting in a large hole effective mass and accordingly low mobility. [13] Therefore, reducing the localization behavior of the VBM, via an engineered modification, is required to obtain suitable p-type oxide materials. [14] Up until now, only a few p-type oxide semiconductors (copper oxide, [15] tin monoxide, [16] and nickel oxide [17] ) were reported as active channel materials for TFTs. Their VBMs were inherently modified by the hybridization of metal (M) s states with oxygen (O) 2p states which result in delocalized VBMs and hence good holes mobility. [18] Copper oxide (Cu x O), with its native hybridization of Cu 3d and O 2p near the VBM, is one of the most promising holetransporting oxide semiconductors. [19] The copper oxide system (CuO) has two stable phases; cubic Cu 2 O (cuprous oxide) and The development of well-performing p-type semiconductors is essential in pushing transparent electronics to the next frontier. Copper oxide (Cu x O) is a potentially attractive hole-transport material for such applications because of its native p-type semi-conductivity, abundant availability, non-toxic nature, and low production cost. Solution-based processing methods offer many other...

Copper(I) Iodide Thin Films: Deposition Methods and Hole-Transporting Performance

“…Inorganic transparent semiconductors are required in solar cells and other devices, like thin-film transistors (TFTs), light-emitting diodes (LEDs), and smart sensors [1][2][3][4][5][6]. Solar cells produce electric power from solar energy and are a promising option to meet the energy demand with high efficiency.…”

“…Hence, there is a considerable need to find materials that possess low resistivity for electrical conduction and high transparency for solar cells, TFTs, and LEDs. This has led to extensive research on compounds, such as In 2 O 3 , ZnO, SnO 2 , and complex oxides [4,[15][16][17][18][19]. Despite extensive research, most of these materials discovered thus far display n-type conductivity, and there are few viable choices for p-type materials with high hole carrier density and/or mobility.…”

Enhanced Performance of P-Channel CuIBr Thin-Film Transistor by ITO Surface Charge-Transfer Doping

“…Among the different candidates, a transparent p-type semiconductor copper iodide (CuI), including I 5p orbital with large spatial spread and displaying high intrinsic hole mobility over 40 cm 2 V –1 s –1 , has great potential to fabricate high-performance p-channel TFTs . Usually, the CuI film used in p-channel TFTs is easily obtained by solution methods at low postannealing temperatures (<150 °C). − ,− Liu et al obtained the first solution-processed CuI TFT with a low on/off current ( I ON / I OFF ) ratio of 5 × 10 2 at room temperature . The low I ON / I OFF ratio of CuI TFT is mainly derived from the lower formation energy of Cu vacancies, resulting in excessive holes formation in CuI film. , Kwon et al reported CuI TFT with optimized morphology based on high- k ( k ∼ 108) sodium-embedded alumina (Na-doped Al 2 O 3 ), showing a hole mobility of 21.6 cm 2 V –1 s –1 and a high I ON / I OFF ratio of 10 5 .…”