High Performance n-Type Polymer Semiconductors for Printed Logic Circuits
Solution processable polymer semiconductors open up potential applications for radio-frequency identification (RFID) tags, flexible displays, electronic paper and organic memory due to their low cost, large area processability, flexibility and good mechanical properties. An inverter, providing the ‘NOT’ logic function, is a fundamental unit in these applications. A complementary inverter, which consists of both p-type and n-type transistors, offers low power consumption, higher voltage gain, and stronger immunity against noise, and thus is most widely studied and used for integrated circuits. A high-performance complementary logic circuit requires p-type and n-type transistors with high and balanced hole and electron mobilities. However, stable n-type polymer semiconductors are rare and the current level of electron mobility of n-type polymers remains much lower than that of p-type polymers. In this work, a number of high-performance n-type polymer semiconductors were successfully developed using several approaches. In Chapter 2, a new polymer semiconductor with a record high electron mobility value of 6.30 cm2V-1s-1 in OTFT devices was developed. This polymer contains a key new electron-accepting building block, DBPy, which consists of a DPP core flanked with two 2-pyridinyl substituents. The 2-pyridinyl substituents can effectively alleviate its steric interaction with the DPP core, resulting in a highly coplanar structure; meanwhile, the relatively electron deficient characteristic of pyridine helped to reduce the LUMO energy level. Our results demonstrate that DBPy is a highly promising new building block for polymer semiconductors for ambipolar complementary logic. In Chapter 3, a DBPy-thieno[3,2-b]thiophene copolymer with a high ambipolar performance was synthesized. The source and drain electrodes of the OTFT devices were modified with a thin layer of polyethyleneimine (PEI) to convert the polymer into a unipolar n-type semiconductor, achieving an electron mobility as high as 2.38 cm2V-1s-1. This opens a door to ambipolar polymers to be used as n-type polymers for complementary circuits. In Chapter 4, the surface modification of source and drain electrodes by an ultrathin layer (~ 2-5 nm) of polyethyleneimine (PEI) on source and drain electrodes utilized in Chapter 3, is further demonstrated to be an effective and universal way to convert other ambipolar polymers and even a p-type semiconductor into unipolar n-type semiconductors in OTFTs. In Chapter 5, another general approach to converting ambipolar and even p-type polymer semiconductors into unipolar n-type semiconductor materials for high performance OTFTs was discovered by doping these polymers with a small amount of PEI. The PEI dopant was uniformly distributed throughout polymer semiconductors even when the blend films were annealed at high temperatures up to 200ºC. This general doping method is a significant step forward for making complementary circuits using ambipolar and even p-type polymer semiconductors in n-channel OTFTs. In the Conclusion and Outlook, the fabrication of complementary inverters based on both p-type and n-type OTFTs using the general approach developed in Chapter 5 is discussed. The inverters demonstrate low power consumption, high voltage gain and strong robustness to noise.