Patterning effects on poly (3-hexylthiophene) organic thin film transistors using photolithographic processes

We explore the effects of conventional photo lithographic patterning of the active layer of poly (3-hexylthiophene) (P3HT) organic thin film transistors (OTFT) on device performance. The performance of the devices was monitored in each step of the patterning process. We successfully developed a patterning process which is compatible with plastic substrates and P3HT as the organic semiconductor. In this process, parylene and atomic layer deposition (ALD) Al₂O₃ were used as capping layers. Al₂O₃ and parylene/P3HT were etched using Al etchant and O₂ plasma reactive ion etching (RIE), respectively. The degradation occurred primarily during the ALD Al₂O₃ deposition and capping layer etching. There was a 30% degradation in mobility, a 1–2× reduction in drive current, and an increase in threshold voltage after the ALD Al₂O₃ deposition. In the capping layer etching, a near 50% degradation in mobility was observed. The patterned devices have a mobility of 0.02 cm²/V s, which is 1000× better than photo lithographically patterned P3HT OTFTs previously reported in the literature, and comparable to un-patterned P3HT devices.     

Atomic-layer-deposited Al2O3 and HfO2 on GaN: A comparative study on interfaces and electrical characteristics

Al₂O₃, HfO₂, and composite HfO₂/Al₂O₃ films were deposited on n-type GaN using atomic layer deposition (ALD). The interfacial layer of GaON and HfON was observed between HfO₂ and GaN, whereas the absence of an interfacial layer at Al₂O₃/GaN was confirmed using X-ray photoelectron spectroscopy and transmission electron microscopy. The dielectric constants of Al₂O₃, HfO₂, and composite HfO₂/Al₂O₃ calculated from the C-V measurement are 9, 16.5, and 13.8, respectively. The Al₂O₃ employed as a template in the composite structure has suppressed the interfacial layer formation during the subsequent ALD-HfO₂ and effectively reduced the gate leakage current. While the dielectric constant of the composite HfO₂/Al₂O₃ film is lower than that of HfO₂, the composite structure provides sharp oxide/GaN interface without interfacial layer, leading to better electrical properties.     

Highly-impermeable Al2O3/HfO2 moisture barrier films grown by low-temperature plasma-enhanced atomic layer deposition

Polymer substrates are essential components of flexible electronic applications such as OTFTs, OPVs, and OLEDs. However, high water vapor permeability of polymer films can significantly reduce the lifetime of flexible electronic devices. In this study, we examined the water vapor permeation barrier properties of Al₂O₃/HfO₂ mixed oxide films on polymer substrates. Al₂O₃/HfO₂ films deposited by plasma-enhanced atomic layer deposition were transparent, chemically stable in water and densely amorphous. At 60 °C and 90% relative humidity (RH) accelerated condition, 50-nm-thick Al₂O₃/HfO₂ had water vapor transmission rate (WVTR) = 1.44 × 10⁻⁴ g m⁻² d⁻¹, whereas single layers of Al₂O₃ had WVTR = 3.26 × 10⁻⁴ g m⁻² d⁻¹ and of HfO₂ had WVTR = 6.75 × 10⁻² g m⁻² d⁻¹. At 25 °C and 40% RH, 50-nm-thick Al₂O₃/HfO₂ film had WVTR = 2.63 × 10⁻⁶ g m⁻² d⁻¹, which is comparable to WVTR of conventional glass encapsulation.     

Understanding the role of organic polar solvent induced nanoscale morphology and electrical evolutions of P3HT:PCBM composite film

We investigated the effect of organic polar solvent on the properties of [6,6]-phenyl-C₇₁-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT):PCBM blend films employed as active layer in organic photovoltaic. The nanoscale morphology and the electrical characteristics of the P3HT:PCBM film can be controlled through organic polar solvent exposure, which exhibited with a short-circuit current density of 8.64 mA/cm², an open circuit voltage of 0.63 V, and a power conversion efficiency of 3.29% under AM 1.5 illumination with a light intensity of 100 mW/cm². By exposing the active layer films to organic polar solvent a favorable phase separation in the P3HT:PCBM films is obtained. The improved power conversion efficiency can be to the high conductivity and high surface area of the P3HT:PCBM layer treated with organic polar solvent.     

Nanoscale Film Thickness Gradients Printed in Open Air by Spatially Varying Chemical Vapor Deposition

Nanoscale films are integral to all modern electronics. To optimize device performance, researchers vary the film thickness by making batches of devices, which is time-consuming and produces experimental artifacts. Thin films with nanoscale thickness gradients that are rapidly deposited in open air for combinatorial and high-throughput (CHT) studies are presented. Atmospheric pressure spatial atomic layer deposition reactor heads are used to produce spatially varying chemical vapor deposition rates on the order of angstroms per second. ZnO and Al₂O₃ films are printed with nm-scale thickness gradients in as little as 45 s and CHT analysis of a metal-insulator-metal diode and perovskite solar cell is performed. By testing 360 Pt/Al₂O₃/Al diodes with 18 different Al₂O₃ thicknesses on one wafer, a thicker insulator layer (≈7.0 nm) is identified for optimal diode performance than reported previously. Al₂O₃ thin film encapsulation is deposited by atmospheric pressure chemical vapor deposition (AP-CVD) on a perovskite solar cell stack for the first time and a convolutional neural network is developed to analyze the perovskite stability. The rapid nature of AP-CVD enables thicker films to be deposited at a higher temperature than is practical with conventional methods. The CHT analysis shows enhanced stability for 70 nm encapsulation films.     

Atomic layer deposited Al2O3 as a capping layer for polymer based transistors

The strong sensitivity of organic/polymeric semiconductors to the exposure to O₂ and H₂O atmospheres makes the use of capping layers mandatory for the realization of stable devices based on such materials. In this paper we explore the realization of inorganic capping layers by atomic layer deposition (ALD) that provides smooth and pinhole-free films with a great potential as passivation layer for organic based devices. We show that the deposition of Al₂O₃ on transistors based on poly-3 hexyltiophene (P3HT) allows to obtain air stable devices. Whereas the growth of Al₂O₃ directly on the P3HT layer leads to a rough interface and significant intermixing between the oxide and the polymer, which results in a deterioration of transistor performances, an interlayer of a poly-alcohol such as poly-vinylphenol interposed between the Al₂O₃ and the P3HT gives a well defined Al₂O₃/polymer interface without degradation of the hole mobility. Transistors capped with Al₂O₃/PVP are very stable in air, with no appreciable differences in the electrical characteristics when measured in vacuum or in air. In addition no significant degradation of the transistors electrical properties was detected even after one month of air exposure.     

Physical properties and electrical characteristics of H2O-based and O3-based HfO2 films deposited by ALD

Ozone (O₃) and H₂O are used as the oxidant to deposit hafnium oxide (HfO₂) thin films on p-type Si (1 0 0) wafers by atomic layer deposition (ALD). The physical properties and electrical characteristics of HfO₂ films change greatly for different oxidants and deposition temperature. Compared with O₃ as the oxidant, HfO₂ films grown with H₂O as the oxidant are more consistent in composition and growth rate. The O₃-based HfO₂ films have lower C impurity and higher concentration N impurity than the H₂O-based HfO₂ films. The impact of the annealing process on the electrical properties and stability of HfO₂ films are also investigated. A width step is observed in the O₃-based HfO₂ C–V curves, which disappears after annealing process. It is because the unstable Hf–O–N and Hf–N bonds in O₃-based HfO₂ films are re-bonded with the non-HfO₂ oxygen after annealing process, and the binding energy of N_1s shifts.     

Enhanced OLED performance upon photolithographic patterning by using an atomic-layer-deposited buffer layer

This study addresses the problem of patterning-induced degradations to organic light-emitting diodes (OLEDs) by using a thin (10 Å) atomic-layer-deposited (ALD) Al₂O₃ film as both an electron-injection layer and a protecting layer for the electroluminescent material, poly[1-methoxy-4-(2′-ethyl-hexyloxy)-2,5-phenylene vinylene] (MEH-PPV). With the ALD Al₂O₃ film, the OLEDs not only withstood an aggressive photolithographic patterning process without any degradation but unprecedentedly showed increased luminous efficiency (by 100%) and lowered turn-on voltage (by 19%) afterward. Although the ALD precursor, trimethylaluminum (TMA), was found to damage the MEH-PPV layer through electrophilic addition to the vinylene groups of MEH-PPV during the deposition of the Al₂O₃ film, its damaging effect was eliminated by pre-treating the MEH-PPV surface with isopropyl alcohol (IPA), whose hydroxyl groups scavenged TMA throughout the ALD process. The performance of the photo-patterned OLEDs was further improved by using a high-conductivity hole-injection layer, which increased accumulation of holes at the EL–buffer interface to enhance electron injection. The method reported herein improves the applicability of photolithography to OLED fabrication, promising to resolve the issue of patterning that has in part impeded OLED’s commercialization.     

Thickness dependent barrier performance of permeation barriers made from atomic layer deposited alumina for organic devices

Organic devices like organic light emitting diodes (OLEDs) or organic solar cells degrade fast when exposed to ambient air. Hence, thin-films acting as permeation barriers are needed for their protection. Atomic layer deposition (ALD) is known to be one of the best technologies to reach barriers with a low defect density at gentle process conditions. As well, ALD is reported to be one of the thinnest barrier layers, with a critical thickness – defining a continuous barrier film – as low as 5–10 nm for ALD processed Al₂O₃. In this work, we investigate the barrier performance of Al₂O₃ films processed by ALD at 80 °C with trimethylaluminum and ozone as precursors. The coverage of defects in such films is investigated on a 5 nm thick Al₂O₃ film, i.e. below the critical thickness, on calcium using atomic force microscopy (AFM). We find for this sub-critical thickness regime that all spots giving raise to water ingress on the 20 × 20 μm² scan range are positioned on nearly flat surface sites without the presence of particles or large substrate features. Hence below the critical thickness, ALD leaves open or at least weakly covered spots even on feature-free surface sites. The thickness dependent performance of these barrier films is investigated for thicknesses ranging from 15 to 100 nm, i.e. above the assumed critical film thickness of this system. To measure the barrier performance, electrical calcium corrosion tests are used in order to measure the water vapor transmission rate (WVTR), electrodeposition is used in order to decorate and count defects, and dark spot growth on OLEDs is used in order to confirm the results for real devices. For 15–25 nm barrier thickness, we observe an exponential decrease in defect density with barrier thickness which explains the likewise observed exponential decrease in WVTR and OLED degradation rate. Above 25 nm, a further increase in barrier thickness leads to a further exponential decrease in defect density, but an only sub-exponential decrease in WVTR and OLED degradation rate. In conclusion, the performance of the thin Al₂O₃ permeation barrier is dominated by its defect density. This defect density is reduced exponentially with increasing barrier thickness for alumina thicknesses of up to at least 25 nm.     

Hybrid photovoltaic structures based on amorphous silicon and P3HT:PCBM/PEDOT:PSS polymer semiconductors

Hybrid thin film photovoltaic structures, based on hydrogenated silicon (Si:H), organic poly(3-hexythiophene):methano-fullerenephenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM) and poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) films, have been fabricated. Organic semiconductor thin films were deposited by spin-coating technique and were exposed to radio frequency plasma enhanced chemical vapor deposition (RF PECVD) of Si:H films at deposition temperature T_d = 160 °C. Different types of structures have been investigated: H1) ITO/(p)SiC:H /P3HT:PCBM/(n) Si:H, H2) ITO/PEDOT:PSS/(i)Si:H/(n) Si:H and H3) ITO/PEDOT:PSS/P3HT:PCBM/(i)Si:H/(n)Si:H. Short circuit current density spectral response and current-voltage characteristics were measured for diagnostic of the photovoltaic performance. The current density spectral dependence of hybrid structures which contains organic layers showed improved response (50–80%) in high photon energy range (hν ≈ 3.1–3.5 eV) in comparison with Si:H reference structure. An adjustment in the absorbing layer thickness and in the contact material for ITO/PEDOT:PSS/(i)Si:H/(n)Si:H structure, resulted in a remarkably high short circuit current density (as large as 17.74 mA/cm²), an open circuit voltage of 640 mV and an efficiency of 3.75%.     

Characterisation of the Al2O3 films deposited by ultrasonic spray pyrolysis and atomic layer deposition methods for passivation of 4H–SiC devices

Al₂O₃ films were deposited using atomic layer deposition (ALD) and ultrasonic spray pyrolysis (USP) methods on p- and n-type Si substrates, n-type 4H–SiC substrates and 4H–SiC diodes for passivation studies. UV exposure in N₂ atmosphere and 5% HF treatment were used as two separate surface preparation procedures prior to Al₂O₃ deposition. The films deposited with USP technique contain a large amount of fixed negative charge and are vulnerable to water incorporation into the material. The Al₂O₃ film prepared by ALD method shows much better uniformity and less negative charge. Decrease of the leakage current in the 4H–SiC diodes is observed after Al₂O₃ passivation using both methods.     

Oriented Growth of Al2O3:ZnO Nanolaminates for Use as Electron‐Selective Electrodes in Inverted Polymer Solar Cells

Atomic layer deposition is used to synthesize Al₂O₃:ZnO(1:x) nanolaminates with the number of deposition cycles, x, ranging from 5 to 30 for evaluation as optically transparent, electron‐selective electrodes in polymer‐based inverted solar cells. Al₂O₃:ZnO(1:20) nanolaminates are found to exhibit the highest values of electrical conductivity (1.2 × 10³ S cm^?1; more than six times higher than for neat ZnO films), while retaining a high optical transmittance (≥80% in the visible region) and a low work function (4.0 eV). Such attractive performance is attributed to the structure (ZnO crystal size and crystal alignment) and doping level of this intermediate Al₂O₃:ZnO film composition. Polymer‐based inverted solar cells using poly(3‐hexylthiophene) (P3HT):phenyl‐C₆₁‐butyric acid methyl ester (PCBM) mixtures in the active layer and Al₂O₃:ZnO(1:20) nanolaminates as transparent electron‐selective electrodes exhibit a power conversion efficiency of 3% under simulated AM 1.5 G, 100 mW cm^?2 illumination.     

High barrier properties of transparent thin-film encapsulations for top emission organic light-emitting diodes

This paper reported a low-temperature thin film encapsulation (TFE) process based on atomic layer deposition Al₂O₃ layer for top-emission organic light-emitting devices (TE-OLEDs). The barrier characteristics of both H₂O-based and O₃-based Al₂O₃ films were investigated. O₃-based Al₂O₃ TFE showed lower water vapor transmission rate (WVTR) of 8.7 × 10⁻⁶ g/m² day and longer continuous operation lifetime of 5 folds compared to the device with H₂O-based Al₂O₃ TFE under identical environmental and driving conditions. Furthermore, the extraction of emitting light of the devices with barrier layer was enhanced compared to the bared one. The theory simulation data were consistent with our experimental results and showed the potential for the design of TFE structures optimized for enhancing light transmission.     

Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlO_x layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlO_x film growth and device longevity is presented. First, AlO_x film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7 nm by 500 cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100 °C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150 °C range. Ultra-thin AlO_x encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000 h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlO_x interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.     

Spatially separated atomic layer deposition of Al2O3, a new option for high‐throughput Si solar cell passivation

A next generation material for surface passivation of crystalline Si is Al₂O₃. It has been shown that both thermal and plasma‐assisted (PA) atomic layer deposition (ALD) Al₂O₃ provide an adequate level of surface passivation for both p‐ and n‐type Si substrates. However, conventional time‐resolved ALD is limited by its low deposition rate. Therefore, an experimental high‐deposition‐rate prototype ALD reactor based on the spatially separated ALD principle has been developed and Al₂O₃ deposition rates up to 1.2 nm/s have been demonstrated. In this work, the passivation quality and uniformity of the experimental spatially separated ALD Al₂O₃ films are evaluated and compared to conventional temporal ALD Al₂O₃, by use of quasi‐steady‐state photo‐conductance (QSSPC) and carrier density imaging (CDI). It is shown that spatially separated Al₂O₃ films of increasing thickness provide an increasing surface passivation level. Moreover, on p‐type CZ Si, 10 and 30 nm spatial ALD Al₂O₃ layers can achieve the same level of surface passivation as equivalent temporal ALD Al₂O₃ layers. In contrast, on n‐type FZ Si, spatially separated ALD Al₂O₃ samples generally do not reach the same optimal passivation quality as equivalent conventional temporal ALD Al₂O₃ samples. Nevertheless, after “firing”, 30 nm of spatially separated ALD Al₂O₃ on 250 µm thick n‐type (2.4 Ω cm) FZ Si wafers can lead to effective surface recombination velocities as low as 2.9 cm/s, compared to 1.9 cm/s in the case of 30 nm of temporal ALD Al₂O₃. Copyright © 2011 John Wiley & Sons, Ltd.     

Fabrication of tunable [Al2O3:Alucone] thin-film encapsulations for top-emitting organic light-emitting diodes with high performance optical and barrier properties

The optical and barrier properties of thin-film encapsulations (TFEs) for top-emitting organic light-emitting diodes (TEOLEDs) were investigated using TFEs fabricated by stacking multiple sets of inorganic–organic layers. The inorganic moisture barrier layers were prepared by atomic layer deposition (ALD) of Al₂O₃ using trimethylaluminum (TMA) and O₃ as precursors and are shown to be efficient barriers against gases and vapors. The organic alucone layers were produced by molecular layer deposition (MLD) using TMA and ethylene glycol as precursors. The [Al₂O₃:Alucone] ALD/MLD films were used because their adjustable inorganic–organic nanolaminate composition allows for the tuning of the optical properties, thereby enhancing their application potential for the design and fabrication of high performance light out-coupling structures for TEOLEDs. By carefully adjusting the relative thickness ratio of the inorganic–organic encapsulation materials, optimized light extraction was achieved and the films not only maintained their high moisture barrier strength but also showed excellent optical performance.     

Adhesion properties of inverted polymer solarcells: Processing and film structure parameters

We report on the adhesion of weak interfaces in inverted P3HT:PCBM-based polymer solar cells (OPV) with either a conductive polymer, PEDOT:PSS, or a metal oxide, molybdenum trioxide (MoO₃), as the hole transport layer. The PEDOT:PSS OPVs were prepared by spin or spray coating on glass substrates, or slot-die coating on flexible PET substrates. In all cases, we observed adhesive failure at the interface between the P3HT:PCBM with PEDOT:PSS layer. The adhesion energy measured for the solar cells made on glass substrates was about 1.8 J/m², but only 0.5 J/m² for the roll-to-roll processed flexible solar cells. The adhesion energy was insensitive to the PEDOT:PSS layer thickness in the range of 10–40 nm. A marginal increase in adhesion energy was measured with increased O₂ plasma power. Compared to solution processed PEDOT:PSS, we found that thermally evaporated MoO₃ adheres less to the P3HT:PCBM layer, which we attributed to the reduced mixing at the MoO₃/P3HT:PCBM interface during the thermal evaporation process. Insights into the mechanisms of delamination and the effect of different material properties and processing parameters yield general guidelines for the design of more reliable organic photovoltaic devices.     

Structural and Electrical Properties of Atomic Layer Deposited Al‐Doped ZnO Films

Structural and electrical properties of Al‐doped ZnO (AZO) films deposited by atomic layer deposition (ALD) are investigated to study the extrinsic doping mechanism of a transparent conducting oxide. ALD‐AZO films exhibit a unique layer‐by‐layer structure consisting of a ZnO matrix and Al₂O₃ dopant layers, as determined by transmission electron microscopy analysis. In these layered AZO films, a single Al₂O₃ dopant layer deposited during one ALD cycle could provide ≈4.5 × 10¹³ cm^?2 free electrons to the ZnO. The effective field model for doping is suggested to explain the decrease in the carrier concentration of ALD‐AZO films when the interval between the Al₂O₃ layers is reduced to less than ≈2.6 nm (>3.4 at% Al). By correlating the electrical and structural properties, an extrinsic doping mechanism of ALD‐AZO films is proposed in which the incorporated Al atoms take oxygen from the ZnO matrix and form doubly charged donors, such as oxygen vacancies or zinc interstitials.     

Design and synthesis of indole-substituted fullerene derivatives with different side groups for organic photovoltaic devices

A series of indole-substituted fulleropyrrolidine derivatives with different side groups on a pyrrolidine rings, including methyl (OIMC60P), benzyl (OIBC60P), 2,5-difluoroinebenzyl (OIB2FC60P), and 2,3,4,5,6-pentafluoroinebenzyl (OIB5FC60P), have been synthesized and used as electron acceptor in the active layer of polymer-fullerene solar cells to investigate the effect of various substitute groups on the electronic structures, morphologies, and device performances. Optical absorption, electrochemical properties and solubility of the fullerene derivatives have been explored and compared. The inverted photovoltaic devices with the configuration ITO/ZnO/Poly(3-hexylthiophene)(P3HT):[60]fullerene derivatives/MoO₃/Ag have been prepared including the reference cell based on the P3HT: methyl [6,6]-phenyl-C61-butylate (PCBM) blend films. All the devices properties were measured in air without encapsulation. We also investigated the effect of the thermal annealing on the crystallinity and morphology of the active layer and the device performance. The device based on the blend film of P3HT and OIBC60P showed a power conversion efficiency of 2.46% under illumination by AM1.5G (100 mW/cm²) after the annealing treatment at 120 °C for 10 min in air.     

Soft breakdown in irradiated high-κ nanolaminates

In this paper the electrical characteristics of different atomic layer deposited (ALD) high permittivity dielectric films (Al₂O₃ and Al₂O₃/HfO₂ nanolaminates) subjected to ion irradiation (25 MeV oxygen ions and 10 MeV protons) are evaluated. The capacitance-voltage (C-V) and current-voltage (I-V) characterization show that high-κ nanolaminates are more tolerant to radiation than the Al₂O₃ layers, but suffer radiation soft breakdown (RSBD) events. The main variation on the electrical characterization could be interpreted as a gradual decrease of the dielectric constant and/or as an increase of the series resistance of the device.