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.     

Performance Improvements in Conjugated Polymer Devices by Removal of Water‐Induced Traps

The exploration of a wide range of molecular structures has led to the development of high‐performance conjugated polymer semiconductors for flexible electronic applications including displays, sensors, and logic circuits. Nevertheless, many conjugated polymer field‐effect transistors (OFETs) exhibit nonideal device characteristics and device instabilities rendering them unfit for industrial applications. These often do not originate in the material's intrinsic molecular structure, but rather in external trap states caused by chemical impurities or environmental species such as water. Here, a highly efficient mechanism is demonstrated for the removal of water‐induced traps that are omnipresent in conjugated polymer devices even when processed in inert environments; the underlying mechanism is shown, by which small‐molecular additives with water‐binding nitrile groups or alternatively water–solvent azeotropes are capable of removing water‐induced traps leading to a significant improvement in OFET performance. It is also shown how certain polymer structures containing strong hydrogen accepting groups will suffer from poor performances due to their high susceptibility to interact with water molecules; this allows the design guidelines for a next generation of stable, high‐performing conjugated polymers to be set forth.     

Roll‐to‐Roll Cohesive,Coated, Flexible,High‐Efficiency Polymer Light‐Emitting Diodes Utilizing ITO‐Free Polymer Anodes

This paper reports solution‐processed, high‐efficiency polymer light‐emitting diodes fabricated by a new type of roll‐to‐roll coating method under ambient air conditions. A noble roll‐to‐roll cohesive coating system utilizes only natural gravity and the surface tension of the solution to flow out from the capillary to the surface of the substrate. Because this mechanism uses a minimally cohesive solution, the roll‐to‐roll cohesive coating can effectively realize an ultra‐thin film thickness for the electron injection layer. In addition, the roll‐to‐roll cohesive coating enables the fabrication of a thicker polymer anode film more than 250 nm at one time by modification of the surface energy and without wasting the solution. It is observed that the standard sheet resistance deviation of the polymer anode is only 2.32 Ω/□ over 50 000 bending cycles. The standard sheet resistance deviation of the polymer anode in the different bending angles (0 to 180°) is 0.313 Ω/□, but the case of the ITO‐PET is 104.93 Ω/□. The average surface roughness of the polymer anode measured by atomic force microscopy is only 1.06 nm. Because the surface of the polymer anode has a better quality, the leakage current of the polymer light‐emitting diodes (PLEDs) using the polymer anode is much lower than that using the ITO‐PET substrate. The luminous power efficiency of the two devices is 4.13 lm/W for the polymer anode and 3.21 lm/W for the ITO‐PET. Consequently, the PLEDs made by using the polymer anode exhibited 28% enhanced performance because the polymer anode represents not only a higher transparency than the ITO‐PET in the wavelength of 560 nm but also greatly reduced roughness. The optimized the maximum current efficiency and power efficiency of the device show around 6.1 cd/A and 5.1 lm/W, respectively, which is comparable to the case of using the ITO‐glass.     

11.2% All‐Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

All‐polymer solar cells (all‐PSCs) that contain both p‐type and n‐type polymeric materials blended together as light‐absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of small molecular‐based organic solar cells. In this work, the interfacial stability is studied by using highly stable all‐polymer solar cell as a platform. It is found that the thermally deposited metal electrode atoms can diffuse into the active layer during device storage, which consequently greatly decreases the power conversion efficiency. Fortunately, the diffusion of metal atoms can be slowed down and even blocked by using thicker interlayer materials, high‐glass‐transition‐temperature interlayer materials, or a tandem device structure. Learning from this, homojunction tandem all‐PSCs are successfully developed that simultaneously exhibit a record power conversion efficiency over 11% and remarkable stability with efficiency retaining 93% of the initial value after thermally aging at 80 °C for 1000 h.     

High‐Performance All‐Polymer Solar Cells Enabled by an n‐Type Polymer Based on a Fluorinated Imide‐Functionalized Arene

A novel imide‐functionalized arene, di(fluorothienyl)thienothiophene diimide (f‐FBTI2), featuring a fused backbone functionalized with electron‐withdrawing F atoms, is designed, and the synthetic challenges associated with highly electron‐deficient fluorinated imide are overcome. The incorporation of f‐FBTI2 into polymer affords a high‐performance n‐type semiconductor f‐FBTI2‐T, which shows a reduced bandgap and lower‐lying lowest unoccupied molecular orbital (LUMO) energy level than the polymer analog without F or with F‐functionalization on the donor moiety. These optoelectronic properties reflect the distinctive advantages of fluorination of electron‐deficient acceptors, yielding “stronger acceptors,” which are desirable for n‐type polymers. When used as a polymer acceptor in all‐polymer solar cells, an excellent power conversion efficiency of 8.1% is achieved without any solvent additive or thermal treatment, which is the highest value reported for all‐polymer solar cells except well‐studied naphthalene diimide and perylene diimide‐based n‐type polymers. In addition, the solar cells show an energy loss of 0.53 eV, the smallest value reported to date for all‐polymer solar cells with efficiency > 8%. These results demonstrate that fluorination of imide‐functionalized arenes offers an effective approach for developing new electron‐deficient building blocks with improved optoelectronic properties, and the emergence of f‐FBTI2 will change the scenario in terms of developing n‐type polymers for high‐performance all‐polymer solar cells.