Mario Caironi

Organic Light Detectors: Photodiodes and Phototransistors

While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.

Downscaling of self-aligned, all-printed polymer thin-film transistors

Printing is an emerging approach for low-cost, large-area manufacturing of electronic circuits, but it has the disadvantages of poor resolution, large overlap capacitances, and film thickness limitations, resulting in slow circuit speeds and high operating voltages. Here, we demonstrate a self-aligned printing approach that allows downscaling of printed organic thin-film transistors to channel lengths of 100-400 nm. The use of a crosslinkable polymer gate dielectric with 30-50 nm thickness ensures that basic scaling requirements are fulfilled and that operating voltages are below 5 V. The device architecture minimizes contact resistance effects, enabling clean scaling of transistor current with channel length. A self-aligned gate configuration minimizes parasitic overlap capacitance to values as low as 0.2-0.6 pF mm(-1), and allows transition frequencies of fT = 1.6 MHz to be reached. Our self-aligned process provides a way to improve the performance of printed organic transistor circuits by downscaling, while remaining compatible with the requirements of large-area, flexible electronics manufacturing.

Charge injection in solution-processed organic field-effect transistors: physics, models and characterization methods.

A high-mobility organic semiconductor employed as the active material in a field-effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics. In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll-to-roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics. With the exception of electrolyte-gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution-processed and where the most common architectures are adopted, are ∼10 kΩ·cm for transistors with a field effect mobility in the 0.1-1 cm(2)/Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case-by-case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low-cost, large-area printed circuits.

17.6% stabilized efficiency in low-temperature processed planar perovskite solar cells

We present here a planar perovskite solar cell with a stabilized power conversion efficiency (PCE) of 17.6% at the maximum power point and a PCE of 17% extracted from quasi-static J–V with an open-circuit voltage of 1.11 V. Such excellent figures of merit can be achieved by engineering a solution-processed electron buffer layer that does not require high temperature steps. A compact thin film of perovskite absorber is grown onto a PCBM-based electron extraction layer by implementing a novel two-step procedure which preserves the soluble organic interlayer during the deposition of successive layers. We demonstrate that efficient charge extraction is the key for high steady state efficiency in perovskite solar cells with a highly integrable architecture.

High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers

Adv. Mater. 2010, 22, 2371–2375 2010 WILEY-VCH Verlag G Field-effect transistors (FETs) based on conjugated polymers and small molecules have been of extensive fundamental and practical interest for more than two decades. In terms of fundamental charge transport properties organic semiconductors have been recently shown to be intrinsically ambipolar, i.e., able to accumulate and transport both holes and electrons within the same material under suitable biasing conditions and device configurations. The discovery of the intrinsic ambipolar charge transport properties in common semiconducting polymers was made possible by the understanding of the crucial role played by electronegative trapping groups in the dielectric, such as hydroxyl groups on the surface of a SiO2 gate dielectric. [10] Ambipolar charge transport is not only of fundamental, but also of practical interest as it enables the realization of novel device architectures such as complementary-like voltage inverters with a single organic semiconductor as well as ambipolar light-emitting field-effect transistors (LFETs). Here we report the general observation of ambipolar charge transport characteristics in a series of regioregular polyselenophene-based polymers. Compared to the well-studied polythiophenes, which appear among the most promising solution processable organic semiconductors, polyselenophenes were recently developed as analogue systems providing several advantages over their predecessors. The highest occupied molecular orbital (HOMO) of polythiophenes has little contribution from the sulfur heteroatom, whereas the lowest unoccupied molecular orbital (LUMO) has significant electron density on the heteroatom. Polyselenophenes were initially developed as promising alternatives to polythiophenes for solar cell applications, mainly because of their reduced optical band gaps and their enhanced photostability due to the lower lying LUMO. For FET applications we expect the hole transport to be similar to that of polythiophenes, while the lower lying LUMO in polyselenophenes should result in improved electron transport due to enhanced electron injection from metal electrodes and lower susceptibility of electrons to trap states and oxidation. The regioregular polyselenophenes investigated in this work were: (1) poly(3,300-di-n-alkylterselenophene) (PSSS) of three different alkyl side-chains, namely PSSS-C10, PSSS-C8, and PSSS-C6; and (2) poly(3-octyl)selenophene (P3OS) (Fig. 1). We employed identical top-gate, bottom contact (TGBC) configurations with gold source-drain electrodes for all polymers. For ambipolar FETs the TGBC device configuration offers several advantages over a bottom-gate/bottom-contact (BGBC) configuration: (i) the freedom to select different gate dielectrics to minimize irreversible charge trapping at the semiconductordielectric interface and to act as encapsulation for the FET channel, and (ii) a lower contact resistance due to reduction of current-crowding effects. PSSS is the selenium analogue of the previously reported poly(3,300-dialkylterthiophene) (PTT) with a ‘‘spaced-out’’ distribution of the alkyl side-chains along the polymer backbone. 28] PTT was reported to readily self-assemble into a threedimensional lamellar p-stacking arrangement with an ‘‘edge-on’’

Charge Injection Engineering of Ambipolar Field-Effect Transistors for High-Performance Organic Complementary Circuits

Ambipolar π-conjugated polymers may provide inexpensive large-area manufacturing of complementary integrated circuits (CICs) without requiring micro-patterning of the individual p- and n-channel semiconductors. However, current-generation ambipolar semiconductor-based CICs suffer from higher static power consumption, low operation frequencies, and degraded noise margins compared to complementary logics based on unipolar p- and n-channel organic field-effect transistors (OFETs). Here, we demonstrate a simple methodology to control charge injection and transport in ambipolar OFETs via engineering of the electrical contacts. Solution-processed caesium (Cs) salts, as electron-injection and hole-blocking layers at the interface between semiconductors and charge injection electrodes, significantly decrease the gold (Au) work function (∼4.1 eV) compared to that of a pristine Au electrode (∼4.7 eV). By controlling the electrode surface chemistry, excellent p-channel (hole mobility ∼0.1-0.6 cm(2)/(Vs)) and n-channel (electron mobility ∼0.1-0.3 cm(2)/(Vs)) OFET characteristics with the same semiconductor are demonstrated. Most importantly, in these OFETs the counterpart charge carrier currents are highly suppressed for depletion mode operation (I(off) < 70 nA when I(on) > 0.1-0.2 mA). Thus, high-performance, truly complementary inverters (high gain >50 and high noise margin >75% of ideal value) and ring oscillators (oscillation frequency ∼12 kHz) based on a solution-processed ambipolar polymer are demonstrated.

Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation.

A highly efficient, simple, and environmentally friendly protocol for the synthesis of an alternating naphthalene diimide bithiophene copolymer (PNDIT2) via direct arylation polycondensation (DAP) is presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative yield using aromatic solvents. Most critical is the suppression of two major termination reactions of NDIBr end groups: nucleophilic substitution and solvent end-capping by aromatic solvents via C-H activation. In situ solvent end-capping can be used to control MW by varying monomer concentration, whereby end-capping is efficient and MW is low for low concentration and vice versa. Reducing C-H reactivity of the solvent at optimized conditions further increases MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy, which reveals PNDIT2 chains to be fully linear and alternating. This is further confirmed by investigating the optical and thermal properties as a function of MW, which saturate at Mn ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities μsat up to 3 cm(2)/(V·s) are measured using off-center spin-coating, with FET devices made from DAP PNDIT2 exhibiting better reproducibility compared to Stille controls.

Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics

High-mobility semiconducting polymers offer the opportunity to develop flexible and large-area electronics for several applications, including wearable, portable and distributed sensors, monitoring and actuating devices. An enabler of this technology is a scalable printing process achieving uniform electrical performances over large area. As opposed to the deposition of highly crystalline films, orientational alignment of polymer chains, albeit commonly achieved by non-scalable/slow bulk alignment schemes, is a more robust approach towards large-area electronics. By combining pre-aggregating solvents for formulating the semiconductor and by adopting a room temperature wired bar-coating technique, here we demonstrate the fast deposition of submonolayers and nanostructured films of a model electron-transporting polymer. Our approach enables directional self-assembling of polymer chains exhibiting large transport anisotropy and a mobility up to 6.4 cm2 V−1 s−1, allowing very simple device architectures to operate at 3.3 MHz. Thus, the proposed deposition strategy is exceptionally promising for mass manufacturing of high-performance polymer circuits.

Fully Inkjet‐Printed Organic Photodetectors with High Quantum Yield

Bulk-heterojunction based organic photodetectors are fabricated by means of drop-on-demand inkjet printing with vertical topology, inverted structure, and small footprint (about 100 μm x 100 μm). Due to optimization of the deposition technique, an external quantum efficiency in excess of 80% at 525 nm and a -3dB bandwidth of a few tens of kHz is achieved.

High Yield, Single Droplet Electrode Arrays for Nanoscale Printed Electronics

In this work we demonstrate two building blocks of a scalable manufacturing technology for nanoscale electronic devices based on direct-write printing: an architecture for high-yield printing of electrode gaps with 100 nm dimension and a low-temperature silver complex ink for integration of organic materials with high conductivity metal interconnects. We use single printed droplets that are made to dewet slowly from each other to allow reliable, high yield patterning even in the presence of certain surface defects.