Mutual doping of conjugated polyelectrolytes and polyoxometalate towards high-performance hole transporting materials for organic optoelectronic devices

The lack of high-performance hole transporting layer (HTL) materials has remained a long-standing issue that severely restricts the performance of organic optoelectronic devices. In the development of HTLs, to obtain adequate p-doping property, most conjugated polyelectrolytes (CPEs)-based HTL materials have to be constructed with complicated chemical structures such as cyclopentadithiophene (CPDT) and indacenodithiophene (IDT). Realizing a high doping effect in CPEs that involves simple structures and easy synthesis is essentially significant in developing HTL materials. Herein, based on a simple benzene-thiophene unit, we synthesize a series of CPEs with obvious p-type self-doping effect, sufficient WF and tunable optoelectronic properties for serving as high-performance HTLs in organic optoelectronic devices. By treating the CPE with polyoxometalate (POM), the doping density of CPE could be further enhanced, and the resulting "mutually doped" CPE PBT-Cl:POM exhibited a high WF of 5.25 eV and a good electric conductivity of 5.3 × 10−3 S/m. The PBT-Cl:POM demonstrates an outstanding ability to reduce the energy barrier at the anode/active layer interface, which effectively solves the problem of energy loss during hole transport in organic optoelectronic devices. The organic solar cell (OSC) modified by PBT-Cl:POM exhibits a high PCE of 18.3 %, which is higher than that of the PEDOT:PSS device (PCE = 17.8 %). For organic light-emitting diodes (OLEDs), the device with PBT-Cl:POM exhibits a comparable luminous efficiency to the PEDOT:PSS device but with a significantly reduced turn-on voltage from 4.2 to 3.2 V.