The effect of the dielectric end groups on the positive bias stress stability of N2200 organic field effect transistors

Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,[1,2,3] as well as fabrication-induced defects.[4,5] However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.[6,7] In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S).[8] We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.[1,9,10] The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.[11] It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage

Trap Healing for High-Performance Low-Voltage Polymer Transistors and Solution-Based Analog Amplifiers on Foil.

Solution-processed semiconductors such as conjugated polymers have great potential in large-area electronics. While extremely appealing due to their low-temperature and high-throughput deposition methods, their integration in high-performance circuits has been difficult. An important remaining challenge is the achievement of low-voltage circuit operation. The present study focuses on state-of-the-art polymer thin-film transistors based on poly(indacenodithiophene-benzothiadiazole) and shows that the general paradigm for low-voltage operation via an enhanced gate-to-channel capacitive coupling is unable to deliver high-performance device behavior. The order-of-magnitude longitudinal-field reduction demanded by low-voltage operation plays a fundamental role, enabling bulk trapping and leading to compromised contact properties. A trap-reduction technique based on small molecule additives, however, is capable of overcoming this effect, allowing low-voltage high-mobility operation. This approach is readily applicable to low-voltage circuit integration, as this work exemplifies by demonstrating high-performance analog differential amplifiers operating at a battery-compatible power supply voltage of 5 V with power dissipation of 11 µW, and attaining a voltage gain above 60 dB at a power supply voltage below 8 V. These findings constitute an important milestone in realizing low-voltage polymer transistors for solution-based analog electronics that meets performance and power-dissipation requirements for a range of battery-powered smart-sensing applications.The authors gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) through the Centre for Innovative Manufacturing in Large Area Electronics (CIMLAE, program grant EP/K03099X/1) and the project Integration of Printed Electronics with Silicon for Smart sensor systems (iPESS). V.P. also acknowledges financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Collaborative Innovation Center of Suzhou Nano Science and Technology

High-mobility, trap-free charge transport in conjugated polymer diodes

Charge transport in conjugated polymer semiconductors has traditionally been thought to be limited to a low mobility regime by pronounced energetic disorder. Much progress has recently been made in advancing carrier mobilities in field-effect transistors through developing low-disorder conjugated polymers. However, in diodes these polymers have to date not shown much improved mobilities, presumably reflecting the fact that in diodes lower carrier concentrations are available to fill up residual tail states in the density of states. Here, we show that the bulk charge transport in low-disorder polymers is limited by water-induced trap states and that their concentration can be dramatically reduced through incorporating small molecular additives into the polymer film. Upon incorporation of the additives we achieve space-charge limited current characteristics that resemble molecular single crystals such as rubrene with high, trap-free SCLC mobilities up to 0.2 cm2/Vs and a width of the residual tail state distribution comparable to kBT.We gratefully acknowledge financial support the Engineering and Physical Sciences Research Council (EPSRC) through a Programme Grant (EP/M005141/1). M.N. acknowledges financial support from the European Commission through a Marie-Curie Individual Fellowship (EC Grant Agreement Number: 747461)

Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions.

Thermal vibrations and the dynamic disorder they create can detrimentally affect the transport properties of van der Waals bonded molecular semiconductors. The low-energy nature of these vibrations makes it difficult to access them experimentally, which is why we still lack clear molecular design rules to control and reduce dynamic disorder. In this study we discuss the promising organic semiconductors rubrene, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothio-phene and 2,9-di-decyl-dinaphtho-[2,3-b:20,30-f]-thieno-[3,2-b]-thiophene in terms of an exceptionally low degree of dynamic disorder. In particular, we analyse diffuse scattering in transmission electron microscopy, to show that small molecules that have their side chains attached along the long axis of their conjugated core are better encapsulated in their crystal structure, which helps reduce large-amplitude thermal motions. Our work provides a general strategy for the design of new classes of very high mobility organic semiconductors with a low degree of dynamic disorder.S.I. acknowledges funding from the EPSRC, the Winton Programme for the Physics of Sustainability and the Cambridge Home and EU scholarship scheme (CHESS). G. S. acknowledges postdoctoral fellowship support from the Wiener-Anspach Foundation. We acknowledge the support of Nippon Kayaku in providing the materials C8-BTBT and C10-DNTT. We acknowledge Dr John Morrison for synthesis of TMTES-P and Marie Beatrice for her work that resulted in the thin-film structure of TMTES-P. We acknowledge Audrey Richard and Christian Ruzié for the synthesis of ditBu-BTBT and diTMS-BTBT.This is the final version of the article. It first appeared from Nature Publishing Group via https://doi.org/10.1038/ncomms1073

High operational and environmental stability of high-mobility conjugated polymer field-effect transistors through the use of molecular additives.

Due to their low-temperature processing properties and inherent mechanical flexibility, conjugated polymer field-effect transistors (FETs) are promising candidates for enabling flexible electronic circuits and displays. Much progress has been made on materials performance; however, there remain significant concerns about operational and environmental stability, particularly in the context of applications that require a very high level of threshold voltage stability, such as active-matrix addressing of organic light-emitting diode displays. Here, we investigate the physical mechanisms behind operational and environmental degradation of high-mobility, p-type polymer FETs and demonstrate an effective route to improve device stability. We show that water incorporated in nanometre-sized voids within the polymer microstructure is the key factor in charge trapping and device degradation. By inserting molecular additives that displace water from these voids, it is possible to increase the stability as well as uniformity to a high level sufficient for demanding industrial applications.We gratefully acknowledge financial support from Innovate UK (PORSCHED project) and the Engineering and Physical Sciences Research Council though a Programme Grant (EP/M005141/1). I.N. acknowledges studentship support from FlexEnable Ltd. K.B. gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft (BR 4869/1-1). B.R., M.K.R., and J.L.B. thank the financial support from King Abdullah University of Science and Technology (KAUST), the KAUST Competitive Research Grant program, and the Office of Naval Research Global (Award N62909-15-1-2003 );This is the author accepted manuscript. The final version is available from Nature Publishing Group via https://doi.org/10.1038/nmat478

Short contacts between chains enhancing luminescence quantum yields and carrier mobilities in conjugated copolymers

Abstract: Efficient conjugated polymer optoelectronic devices benefit from concomitantly high luminescence and high charge carrier mobility. This is difficult to achieve, as interchain interactions, which are needed to ensure efficient charge transport, tend also to reduce radiative recombination and lead to solid-state quenching effects. Many studies detail strategies for reducing these interactions to increase luminescence, or modifying chain packing motifs to improve percolation charge transport; however achieving these properties together has proved elusive. Here, we show that properly designed amorphous donor-alt-acceptor conjugated polymers can circumvent this problem; combining a tuneable energy gap, fast radiative recombination rates and luminescence quantum efficiencies >15% with high carrier mobilities exceeding 2.4 cm2/Vs. We use photoluminescence from exciton states pinned to close-crossing points to study the interplay between mobility and luminescence. These materials show promise towards realising advanced optoelectronic devices based on conjugated polymers, including electrically-driven polymer lasers

Limits for Recombination in a Low Energy Loss Organic Heterojunction

Donor–acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (V$_{OC}$) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PC$_{61}$BM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10$^{-2}$ cm$^{2}$ V$^{-1}$ s$^{-1}$), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the V$_{OC}$ is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.SMM, RHF, MKR, SAA, and JLB acknowledge support from the KAUST Competitive Research Grant Program. MKR, SAA, and JLB also acknowledge generous support of their work by KAUST and the Office of Naval Research Global (Award N62909­15­1­2003); they thank the KAUST IT Research Computing Team and Supercomputing Laboratory for providing computational and storage resources. NAR, MW, TQN, and GCB acknowledge support from the Department of the Navy, Office of Naval Research (Award Nos. N00014-14-1-0580 and N00014-16-1-25200. AS would like to acknowledge the funding and support from the India-UK APEX project. HLS acknowledges support from the Winton Programme for the Physics of Sustainability. MN and HS gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council though a Programme Grant (EP/M005141/1)

Decoupling Charge Transport and Electroluminescence in a High Mobility Polymer Semiconductor.

Fluorescence enhancement of a high-mobility polymer semiconductor is achieved via energy transfer to a higher fluorescence quantum yield squaraine dye molecule on 50 ps timescales. In organic light-emitting diodes, an order of magnitude enhancement of the external quantum efficiency is observed without reduction in the charge-carrier mobility resulting in radiances of up to 5 W str(-1) m(-2) at 800 nm.We gratefully acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) through a programme grant EP/M005143/1. We would like to thank the Doctoral Training Centre in Plastic Electronics EP/G037515/1. K. B. acknowledges financial support by the Deutsche Forschungsgemeinschaft (BR-4869/1-1). The group at Würzburg would like to acknowledge support from the Deutsche Forschungsgemeinschaft (DFG Research Unit FOR 1809) and from the SolTech Initiative of the Bavarian State Ministry of Science, Research and the Arts. D. H. and K.B. would like to thank Dr. Jiři Novak and Jakub Rozbořil (Central European Institute of Technology, Masaryk University, Czech Republic) and Dr. Tom Arnold (Diamond Light Source, Didcot, UK) for assistance during the synchrotron experiment and Diamond Light Source, Didcot, UK for financial support