This PDF file contains the front matter associated with SPIE Proceedings Volume 11097, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Achieving ohmic contacts is key to maximizing the performance of the organic semiconductor in field-effect transistors. The same is true also for organic solar cells, and light-emitting diodes. Recently, by tuning the work function of hole collection layers, and electron collection layers, in fine steps across the Fermi-level pinning threshold of the model photoactive layers in solar cells, which are particularly sensitive to charge extraction contact resistance, we obtain direct evidence for a non-ohmic to ohmic transition associated with strong suppression of resistivity at the contact, when the electrode work function crosses a second threshold beyond the onset of Fermi-level pinning. Detailed current balance analysis reveals this transition to be a fundamental feature of charge transfer kinetics, relevant to both injection and collection, at disordered semiconductor interfaces. Similar considerations apply also to field-effect transistor contacts. Using an enhanced transmission-line model, we show that it is possible to separate out contact resistance from carrier mobility effects, including both their current density and carrier density dependences, from measured IV curves, enabling the characterization of the true carrier mobility in field-effect transistors.

Over the past decade, amorphous metal oxide semiconductors have attracted great interest as low-cost alternatives to thin film transistors (TFTs) due to their high electron mobility, high optical transparency, good environmental/thermal stability, and processing versatility. In contrast, realization of high performance transparent p-type oxide semiconductors remain intangible with urgent industrial demands. In this work, we report solution-processed inorganic p-type copper iodide (CuI) and oxide thin film transistors (TFTs). The spin coated CuI film showed high mobility over 2 cm2 V-1 s-1 by proper treatments such as optimization process condition and applying dopant. Transparent complementary inverters composed of p-type CuI and n-type indium gallium zinc oxide TFTs are demonstrated. We also introduce our recent results of perovskite transistors.

We report efficient interface engineering to realize high-performance fully inkjet-printed organic thin-film transistor (OTFTs) and their applications. By depositing an organo-compatible end-functionalized polystyrene (PS) layer between organic semiconducting layers and inkjet-printed metallic or conductive polymer contacts, simultaneously, well-ordered crystals of organic semiconductors and reduced contact resistance were delivered, resulting in much improved carrier transport and carrier injection abilities. These results may be attributed to the introduction of organo-compatible surface properties that requires less activation energy for carrier transport and carrier injection. This methodology is compatible to high-speed and low-temperature printing technologies, therefore it can pave a promising route for realizing high-performance printed thin-film devices including TFTs, sensors, and light-emitting device applications.

We developed a simple method to improve the stability of organic field-effect transistors (OFETs) with bilayer gate dielectrics. The bilayer gate dielectric comprises an amorphous fluoropolymer (CYTOP) layer and an Al₂O₃-HfO₂ nanolaminate (NL) grown by the atomic layer deposition (ALD) technique. In the OFETs with bilayer gate dielectrics, two aging mechanisms exist, and they cause the shifts of threshold voltage in opposite directions during long-term operation. By engineering the bilayer gate dielectric, the effects of these two mechanisms can compensate, leading to devices with remarkable operational stability that is comparable or superior to that of commercial inorganic counterparts. The NL grown by ALD shows excellent encapsulation property and improves the environmental stability of the OFETs. The devices are tested by exposing the devices to high temperature and high moisture conditions (i.e., the standard 85/85 condition, meaning 85°C and 85% relative humidity). The results of OFETs with CYTOP/NL bilayer gate dielectrics are presented and compared to those OFETs with Al₂O₃ gate dielectrics.

Organic thin-film transistors (OTFTs) are a key technology for enabling novel electronics such as flexible displays, low-cost sensors, or printed RFID tags. Device mobility is the primary figure of merit for OTFTs, but a low contact resistance is critical to achieving marketable performance. The energy level mismatch at the electrode/semiconductor interface hinders charge injection, limiting device performance. This issue has been addressed through chemical treatments with self-assembled monolayers, insertion of metal oxide interlayers, and doping. Here we combine these treatments with modified electrode deposition and post-deposition processing and evaluate the impact on the device properties. Specifically, we alter the contact deposition rate and flame anneal the electrode surface in bottom contact/top gate OTFTs based on the polymer semiconductor indacenodithiophene-co-benzothiadiazole (C16IDT-BT). Tuning the deposition rate leads to larger, flatter grains of gold, increasing the degree of order within the SAM at its surface and creating high work function channels that enhance charge injection. We achieved contact resistances of 200 Ωcm, boosting device mobility up to 10 cm2V-1s-1, a factor of three improvement over previous C16IDT-BT devices in this geometry with the same gate dielectric. We found that flame annealing is effective for further optimizing the gold contact surfaces, increasing grain size by an order of magnitude over those in as-deposited films. Here, a butane torch was passed directly over the contacts and substrate for a short period of time (5 minutes). We determined the impact on device characteristics, including mobility, on/off ratio, subthreshold swing, and threshold voltage.

Post-deposition semiconductor dewetting is the transformation of a (nearly) closed organic-semiconductor monolayer into separated individual islands of multilayer height [1]. We have recently observed this phenomenon in both ultra-thin (1-3 nm) and thin (25-40 nm) films of the small-molecule semiconductor dinaphthothienothiophene (DNTT) [2,3]. Since the gate-field-induced carrier channel is located in close vicinity to the semiconductor-dielectric interface, the accelerated pace of dewetting of ultra-thin semiconductor films is relevant to the performance and stability of organic thin-film transistors (TFTs). We have therefore fabricated bottom-gate, bottom-contact TFTs based on 2 nm and 25 nm-thick DNTT films. Compared to the relatively stable charge-carrier mobility of 1.1 cm2/Vs for the 25-nm-DNTT TFT, the 2-nm-DNTT TFTs show a sharp decrease from 0.2 cm2/Vs to 0.011 cm2/Vs over 72 hours after fabrication. To stabilize the TFT performance, we have explored strategies to prevent ultra-thin DNTT films from dewetting, including substrate cooling and semiconductor encapsulation, and fabricated stable DNTT TFTs with monolayer semiconductor thickness. Encapsulation with vacuum-deposited polytetrafluoroethene (PTFE) or titanyl phthalocyanine (TiOPc) leads to a relative decrease in mobility by only 12% and 44%, compared to 99.6% for TFTs without encapsulation over 28 hours after device fabrication. [1]T. Breuer et al., ACS Appl. Mater. Interfaces, 9, 8384, (2017). [2]K. Takimiya et al., Sci. Technol. Adv. Mater., 8, 273, (2007). [3]U. Zschieschang et al., Organic Eletronics, 12, 1370, (2011).

Printed electronics is an emergent subject for the low-cost and large-area fabrication of flexible electronic devices. Direct printing of organic thin-film transistors (OTFTs) is a particularly promising fabrication method that can be used in large-scale production at low cost. In this work, we developed “room-temperature printed electronics”, which allows OTFT devices to be printed at room temperature without application of heat. The room-temperature fabrication process never causes any thermal damage to the flexible substrate. Thus the process allows very accurate and high-resolution patterning even on a flexible plastic substrate. We also developed the printing technique using patterned surface wettabilities on surface. Through this process, short-channel OTFTs having 1-um channel length were successfully demonstrated. These results suggest that printed electronics technologies are promising as a core technology for low-cost and high-performance printed electronics.

Small-molecule based organic devices continue to demonstrate the highest mobilities of any organic materials, making them the evident choice for better performing organic devices. Here we study a range of binary charge-transfer crystals where a fraction of electronic charge is transferred from the donor molecule to the acceptor molecule. Due to significant electron-phonon coupling, the vibrational motion in these materials has a strong impact on the electronic characteristics. To quantify this polaronic effect, we have completed the measurements of resonance Raman and absorption spectra of the charge-transfer excitations and performed quantum-chemical calculations on a range of binary organic crystals with the same acceptor and a range of donor molecules. Comparison of the reorganization energies of intermolecular vs intramolecular phonons in these materials helps us understand the relative contribution of the two major electron-phonon coupling mechanisms: local vs non-local. We find that small variations in the donor molecule and thereby the resultant crystal structure can have a large impact on the predominant electron-phonon coupling mechanism. Understanding the reasons for these variations is important in selecting and designing materials with suitable characteristics for the next generation of electronic devices.

The electrical performance of organic thin-film transistors (OTFTs) continues to improve, but the effect of electromagnetic radiation on the device performance is still unclear. OTFTs made with solution-processed 5, 11 bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) in a bottom-gated bottom-contact configuration were fabricated on SiO2 gate dielectric and the interaction of visible light with the semiconducting layer was studied. Monochromatic illumination (λ = 532 nm) that matches the highest absorption band of crystalline diF-TES ADT was used to generate a large number of carriers during device operation. We observe that the OTFTs showed an efficient photocurrent response when incident light of intensity ranging from 1 to 11 µW/µm2 was focused at the center of the channel. Over this range, transfer characteristic curves shifted by up to +14 V as illumination was increased. At an intensity of 1 µW/µm2, the ratio of the number of photons absorbed (and thus excitons generated) to the number of holes measured at the electrode was approximately equal to one. With a five-fold increase of the illumination intensity, we found that the ratio of the excitons generated to the measured charge carriers was an order of magnitude less indicating that the effects of trapping in OFETs has a stronger impact at higher incident power.

The emergence of hybrid organic inorganic perovskites (HOIPs) perovskites, with their higher charge carrier mobilities and tunable bandgap, coupled with low-cost processability, has opened new avenues of research in optoelectronic applications. Various devices based on HOIPs, including photovoltaics, light-emitting diodes, transistors or photodetectors have been reported. Here, we explore the photocurrent properties in heterostructures consisting of small molecule semiconductors and HOIPs. Bilayer devices can use bandgap alignment to improve carrier separation and collection. The HOIP CH3NH3PbI3-xClx was the primary photoactive layer and electron transport layer, while the hole transport layer was 2,8-Difluoro-6,13-Bis(triisopropylsilylethynyl) anthradithiophene (diF-TIPS-ADT). We found that the responsivity of the bilayer device was six times greater than neat perovskite film, reaching values as high as 5.1 A/W, and detectivity more than doubled, to 2.9*10^11 Jones. We further tuned the interfacial processes by altering the microstructure of the organic semiconductor layer and evaluated the photocurrent response. We found that treatment of the gold contacts with (2,3,4,5,6)-Pentafluorothiophenol (PFBT) resulted in large grains within the organic semiconductor. Consequently, responsivity was over 15 times greater than that of the neat perovskite film, reaching values of 13.7 A/W, and detectivity improved by 8 times to 8.4*10^11 Jones. Grazing-incidence X-ray diffraction measurements indicate that diF-TIPS-ADT is preferentially oriented with its (001) plane parallel to the substrate in both sample types. The observed performance improvement thus cannot originate from differences in the interfacial coupling related to molecular orientation. Instead, we believe differences in trapping at grain boundaries is responsible for the changes in charge generation efficiency.

Polythiophenes are among the most studied organic semiconductors and they have found widespread use as the active material in organic field-effect transistors and as the electron donor component in organic photovoltaic devices. Here, we investigate how subtle structural modifications of archetypical polythiophenes such as poly(3-hexylthiophene) (P3HT) affect the optical, structural and electrical properties of the resulting polymer thin films. We furthermore discuss the implications of these structural modification for binary systems incorporating molecular dopants.

Solution-processed organic field-effect transistors (OFETs) have been an attractive research topic due to their applicability to future electronic devices. Several issues like high quality material synthesizing, and patterning process are firstly considered to commercialize the OFETs. Especially, the patterning issues are strongly related with the crosstalk and performance of device in solution-processed OFETs. Therefore, we report the analyzation of micropatterned 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) crystals using capillary force lithography (CFL) by UV curable polyurethane acrylate (PUA) templates to investigate the lateral confinements effect on the crystallization. More aligned and free-grain boundary crystalline morphology was observed in narrow patterned width condition, and the electrical performance was also more improved than those of wide pattern width condition (0.58 cm²/(V∙s) in 100 μm and 1.66 cm²/(V∙s) in 5 μm). These consequences demonstrate that the lateral confinements caused by CFL can induce the highly aligned TIPS-pentacene crystals having advanced charge transport path way. Therefore, CFL, one type of soft lithography, should have a considerable potential to facilitate the commercialization of solution-processed OFETs, and to ensure the reproducibility of the crystallinity and the orientation.