Kulbinder K. Banger

Low-temperature, high-performance solution-processed metal oxide thin-film transistors formed by a ‘sol–gel on chip’ process

At present there is no ‘ideal’ thin-film transistor technology for demanding display applications, such as organic light-emitting diode displays, that allows combining the low-temperature, solution-processability offered by organic semiconductors with the high level of performance achievable with microcrystalline silicon1. N-type amorphous mixed metal oxide semiconductors, such as ternary oxides Mx1My2Oz, where M1 and M2 are metals such as In, Ga, Sn, or Zn, have recently gained momentum because of their high carrier mobility and stability2, 3 and good optical transparency, but they are mostly deposited by sputtering. So far no route is available for forming high-performance mixed oxide materials from solution at low process temperatures <250 °C. Ionic mixed metal oxides should in principle be ideal candidates for solution-processable materials because the conduction band states derived from metal s-orbitals are relatively insensitive to the presence of structural disorder and high charge carrier mobilities are achievable in amorphous structures2. Here we report the formation of amorphous metal oxide semiconducting thin-films using a ‘sol–gel on chip’ hydrolysis approach from soluble metal alkoxide precursors, which affords unprecedented high field-effect mobilities of 10 cm2 V−1 s−1, reproducible and stable turn-on voltages Von≈0 V and high operational stability at maximum process temperatures as low as 230 °C.

Electronic Structure of Low‐Temperature Solution‐Processed Amorphous Metal Oxide Semiconductors for Thin‐Film Transistor Applications

The electronic structure of low temperature, solution-processed indium–zinc oxide thin-film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm2 V−1 s−1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution-processed oxide thin films as a function of annealing temperature and environment using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop-off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub-bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution-processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy-induced donor levels.

High Performance, Low Temperature Solution-Processed Barium and Strontium Doped Oxide Thin Film Transistors.

Amorphous mixed metal oxides are emerging as high performance semiconductors for thin film transistor (TFT) applications, with indium gallium zinc oxide, InGaZnO (IGZO), being one of the most widely studied and best performing systems. Here, we investigate alkaline earth (barium or strontium) doped InBa(Sr)ZnO as alternative, semiconducting channel layers and compare their performance of the electrical stress stability with IGZO. In films fabricated by solution-processing from metal alkoxide precursors and annealed to 450 °C we achieve high field-effect electron mobility up to 26 cm2 V–1 s–1. We show that it is possible to solution-process these materials at low process temperature (225–200 °C yielding mobilities up to 4.4 cm2 V–1 s–1) and demonstrate a facile “ink-on-demand” process for these materials which utilizes the alcoholysis reaction of alkyl metal precursors to negate the need for complex synthesis and purification protocols. Electrical bias stress measurements which can serve as a figure of merit for performance stability for a TFT device reveal Sr- and Ba-doped semiconductors to exhibit enhanced electrical stability and reduced threshold voltage shift compared to IGZO irrespective of the process temperature and preparation method. This enhancement in stability can be attributed to the higher Gibbs energy of oxidation of barium and strontium compared to gallium.

Stability investigations of inverted organic solar cells with a sol-gel processed ZnSrO or ZnBaO electron extraction layer

Stability of organic photovoltaic devices is a limiting factor for their commercialization and still remains a major challenge whilst power conversion efficiencies are now reaching the minimum requirements. The inverted organic solar cell architecture shows promising potential for improving significantly the cells working lifetime however, often when solution processed ZnO is used as electron extraction layer (EEL), a light soaking step is required before the device reaches a non-permanent maximum performance. Here we show that by doping ZnO with Sr or Ba using sol-gel processing the light-soaking step is circumvented. In a model poly [3-hexylthiophene] (P3HT): [6, 6]-Phenyl C60 butyl acid methyl ester (PCBM) system we obtain EQE 55% before UV exposure for ZnSrO or ZnBaO EELs as compared to 10% for undoped ZnO EEL. We have investigated the origin of this improvement by comparing the response to UV light of doped and undoped ZnO. Characterization includes electrical conductivity and x-ray photoemission spectroscopy studies on thin films, current-voltage experiments and electroabsorption (EA) spectroscopy to probe the built-in field in the devices. We will discuss how the results obtained and in particular the higher effective built-in field in doped ZnO devices (1.5V) compared to a ZnO device (0.5V) can help interpret the mechanism behind the device performance improvement with Sr and Ba doping of ZnO.

Semi-transparent photovoltaic devices for smart window applications

Photovoltaic devices integrated into smart windows can provide power necessary to tune the transparency of the windows for energy savings and environmental control. Both wide-bandgap polymer and oxide thin-films can provide visibility necessary through windows as well as voltage to drive the window to vary its transparency. Mini-modules including polymer bulk heterojunction solar cells have been fabricated and the open-circuit voltage up to 7.7 V was obtained from a 6cm × 6cm-size module which contains two banks of the cells connected in parallel, and each bank has 14 cells connected in series. The rectifying heterojunctions of Cu2O/i-ZnO/AZO were also successfully demonstrated. The presence of i-ZnO is critical to reduce the significant tunneling current caused by considerable band offset and the inherent lattice mismatch at the junction interface.

Characterization of deposition parameters in aerosol assisted chemical vapor deposition of CuInS/sub 2/ from a single-source precursor

Alloys of Cu(In:Ga)(S:Se)/sub 2/ have shown high potential as thin film photovoltaic absorbers due to their high absorption coefficients, near ideal band gaps, and good electrical properties. Efforts have been lead to create easily decomposing organometallic single-source precursors (SSP) to produce films at temperatures below 400/spl deg/C. Along with that, the SSP (PPh/sub 3/)/sub 2/Cu(SEt)/sub 2/In(SEt)/sub 2 /has been shown to deposit CuInS/sub 2/ films with good optical, morphological, and electrical properties via aerosol-assisted chemical vapor deposition (AACVD). Presented here are studies aimed to understand how certain deposition parameters can be used to optimize the AACVD process. Parameters included in this study are temperature of the deposition zone, substrate location within the reactor, and concentration of the SSP in solution. Deposition control has produced films with four distinct morphologies, varying in density, adhesion, smoothness, and color.